lateral and temporal variability of bottom currents near

Lateral and temporal variability of bottom

currents near the Pen Duick Escarpment

Elke Vangampelaere

2nd Master in the Marine and Lacustrine science

Academic year: 2009-2010

Promotor

Dr. David Van Rooij

Guidance Lies De Mol

1

'Not to be cited without prior reference to the

promotor/supervisor of the thesis'

2

Table of Content

1. Abstract .............................................................................................................................. 4

2. Introduction ........................................................................................................................ 4

3. Regional setting .................................................................................................................. 5

3.1 Geology ....................................................................................................................... 5

3.2 Hydrography ................................................................................................................ 6

3.2.1 Surface water masses ........................................................................................... 7

3.2.2 Intermediate water masses ................................................................................... 7

3.2.3 Deep water masses ............................................................................................... 8

3.3 Palaeoceanography ...................................................................................................... 8

4 Material and methods ......................................................................................................... 9

4.1 Site survey ................................................................................................................... 9

4.2 Cores .......................................................................................................................... 10

4.3 Analysis ..................................................................................................................... 11

4.3.1 Non-Destructive analyses ................................................................................... 12

4.3.2 Destructive analyses ........................................................................................... 12

5. Data description and interpretation .................................................................................. 14

5.1 MD08-3227 ............................................................................................................... 15

5.1.1 Terrigenous proxies ............................................................................................ 15

5.1.2 Biogenic proxies ................................................................................................. 16

5.2 MD04-2806 ............................................................................................................... 16



5.3 MD08-3213 ............................................................................................................... 18



5.4 MD08-3223 ............................................................................................................... 20



5.5 MD08-3228 ............................................................................................................... 21



5.6 Time conversion ........................................................................................................ 23

6. Discussion ........................................................................................................................ 25

6.1 Chronostratigraphy .................................................................................................... 25

6.2 Bottom current variability ......................................................................................... 25

6.3 Effect on cold-water corals ........................................................................................ 29

7. Conclusion ........................................................................................................................ 30

8. Acknowledgements .......................................................................................................... 31

9. References ........................................................................................................................ 31

4

1. ABSTRACT

The Gulf of Cadiz is known for its complex hydrodynamic setting and the occurrence of cold-

water corals (CWC). Most of the research is performed on the Iberian shelf due to the

presence of the Mediterranean Outflow Water (MOW). Along the Northern European margin

this water mass has an influence on the existence of the CWC as it is a water mass with high

amounts of suspended material. Though CWC can also reside outside the MOW region, the

Pen Duick Escarpment (PDE) off the coast of Larache is one of such regions. In this region

the dominant water masses are the Antarctic Intermediate Water (AAIW) and the North

Atlantic Central Water (NACW). The sediment particles are mainly in suspension due to

interaction with the topographic relief and water masses, creating internal tides. Between the

water masses an interface provides lateral sediment transport. However, these interactions and

the paleoceanography of AAIW and NACW on the Moroccan shelf are poorly known. The

objective of this study is to obtain a paleoceanographic understanding of water mass dynamics

from the Last Glacial Maximum (LGM) till now in the El Arraiche mud volcano field, more

specifically on the PDE. Based on grain-size, X-Ray Fluorescence (XRF) and Multi-Scan

Core Logger (MSCL) core scanning analyses, the variation in water mass strength can be

reconstructed and the suspension of the sediment based on Mass Accumulation Rates (MAR).

In addition, the foraminifera assemblage will provide an estimate regarding the

contemporaneous Sea Surface Temperatures (SST) relating to past climate changes. The

analyses show that the overall MAR is lower during interglacial periods and high during

glacial times. The MAR away from the PDE is much higher than near and on the PDE

verifying that the PDE has regional hydrodynamics superimposed on the climatic variations.

This reduced amount of MAR near the PDE is favourable for CWC growth as they will not be

covered by sediment. Though, not all places near the PDE are favourable for CWC like the

location between the drift mound and the PDE as sediment transport is limited by the flanks

on both sides. CWC normally grow in elevated wide open spaces where the catchment of

sediment particles and erosion of the seafloor is increased. MD08-3228 at a plateau behind the

PDE offers the best location for coral growth, based on Mass Accumulation Rates as it

provides enough sediment to build up the mound, but not to cover the corals. There is

however a period of mass-wasting, which is negative to coral growth. Overall the demise in

CWC from the LGM is due to a different hydrodynamic regime that provides a less suitable

environment during glacial periods than before the LGM.

Key words: Gulf of Cadiz, paleoceanography, Pen Duick Escarpment, cold-water corals,

water masses

2. INTRODUCTION

Many studies, including hydrographical (Baringer and Price, 1999; Criado-Aldeanueva et al.,

2006a), paleoceanographic (Cayre et al., 1999a; Ferreira et al., 2008) as well as

sedimentological (Ambar et al., 2002; Baas et al., 1997; Hernandez-Molina et al., 2006), have

5

been carried out within the northern part of the Gulf of Cadiz due to the presence of the

Mediterranean Outflow Water (MOW) and its complex interaction with the seafloor. After

exiting the Strait of Gibraltar the MOW turns north and follows the Iberian margin. The

hydrography on the Moroccan shelf is different as the main water masses are North Atlantic

Central Water (NACW) and Antarctic Intermediate Water (AAIW). MOW only reaches the

Moroccan shelf as a meddy, a vortex that is shed from the MOW due to the bathymetry and

outline of the shore line (Carton et al., 2002).

In 2002, on the R/V Belgica, cold-water coral (CWC) mounds were discovered in the El

Arraiche mud volcano field on the Moroccan margin, more specifically on the Pen Duick

Escarpment (PDE), Renard Ridge, Vernadsky Ridge and Al Idrisi Ridge (Foubert et al.,

2008). These ridges occur in water depths from 200 m to 700 m and are covered at the top by

several mounds with a height of 60 m. The mounds on the PDE are the highest and best

developed. In front of the PDE a drift mound is formed by the erosion of passing water

masses covered with some smaller mounds (Foubert et al., 2008). The mounds are build up

by CWC eg. L. pertusa, M. oculata, and Dendrophyllia spp that represent 90% of the total

CWC biomass (Wienberg et al., 2009). These ridges are perfect substrates for the initial CWC

growth as they are elevated, for more food passing trough, and hard, for the initial settling

(Foubert et al., 2008). The suspension of material and organic particles is due to 1) the lateral

transport of sediment at a depth of 500 m – 700 m at the interface of NACW and AAIW that

are influenced by internal tides (McCave and Hall, 2002) and 2) the sporadic occurrence of

meddies (Foubert et al., 2008; Wienberg et al., 2009) providing input of nutrients (Mienis et

al., 2007). CWC were already present in the Gulf of Cadiz for over 300 ka. However the

overall growth started at 48 ka and ended with the onset of the Holocene. There is no

restriction to glacial-interglacial cycles for CWC abundance (Wienberg et al., 2009), though

the conditions during the glacial periods appear to be more favourable (Foubert et al., 2008).

During the Younger Dryas CWC bloomed for the last time, after 12 ka there was a quick

demise of CWC due to a rapid increase of ocean water temperature (Wienberg et al., 2009).

Because changes in water masses influences CWC growth a detailed study of the

palaeoceanography is necessary to reconstruct the life and death struggle of CWC in the Gulf

of Cadiz. There is however no palaeoceanographic information available for the region of the

PDE. Hence this study will focus on the oceanographic variations between the Last Glacial

Maximum till the present day. Physical properties will allow the interpretation of the

surrounding area on water mass variability. This will help to determine the evolution of the

PDE bottom currents and the demise of CWC in the Holocene. Micropaleontological analyses

will provide information on the climatic changes based on Sea Surface Temperatures (SST).

3. REGIONAL SETTING

3.1 Geology

The Gulf of Cadiz is located west of the Strait of Gibraltar (Fig. 1A) with a complex

geological history. The main geodynamic setting is the subduction of the African plate under

the European plate resulting in a compressive regime. The westward drift of the Gibraltar Arc

(Betic-Rif) formed an accretionary wedge. Increased subsidence in the Tortonian formed the

6

Allochthonous Unit of the Gulf of Cadiz (AUGC) (Foubert et al, 2008; Van Rooij et al,

subm.), which was deposited in the centre of the Gulf of Cadiz (Casas-Sainz and de Vicente,

2009; Medialdea et al., 2009; Medialdea et al., 2004). The AUGC holds thermogenic/biogenic

gasses (Medialdea et al., 2009) fuelling the mud volcanoes and the seeps in the region. The

pathways for the gasses were formed during the Triassic as diapires rose and formed cracks in

the AUGC (Medialdea et al., 2009).

The study area, the El Arraiche mud volcano field (Fig. 1B), has a different tectonic regime

than the compressional-transpressional regime in the Gulf of Cadiz. The El Arraiche mud

volcano field consists of eight mud volcanoes not located near the AUGC (Van Rensbergen et

al., 2005b), but in the Pliocene basin that covers anticline ridges where gas is stored and can

migrate via fractures in the sediment (Van Rensbergen et al., 2005a). The regime is

extensional forming lystric faults during the Pliocene. These lystric faults are expressed as

parallel ridges. Two of them are called Vernadsky ridge and Renard ridge. Renard ridge is

accompanied by a steep escarpment called, Pen Duick Escarpment (PDE). The PDE rises 100

- 200 m above the sea floor (Foubert et al., 2008) and has a NW-SE direction. The cliff is 65

m high with a slope angle of 20°. On top of the PDE mounds of 15 m to 60 m are formed by

fossil cold-water corals (Foubert et al., 2008; Van Rooij et al., submitted) and at the foot of

the cliff several smaller mounds have been observed.

3.2 Hydrography

The present day hydrography in the Gulf of Cadiz is complex due to the addition of the

Mediterranean Outflow Water to the water masses coming from the Northern Atlantic Ocean.

Fig. 1: A: map of the location of the El Arraiche mud volcano field in the Gulf of Cadiz. B: a 3D image of the El Arraiche mud

volcano field with the prominent sea floor structures (Foubert et al, 2008).

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3.2.1 Surface water masses

There are two different water masses in the surface area of the Gulf of Cadiz. The first one is

North Atlantic Central Water (NACW) from the surface to 600 m. The characteristics of the

first 100 m changed due to atmospheric interactions and is called North Atlantic Surface

Water (NASW), where the salinity is 36.4 psu and the temperature 16°C (Criado-Aldeanueva

et al., 2006a). NACW has a temperature from 11°C to 17°C and a salinity of 35.6 psu to 36.5

psu. The NACW flows between 100 m and 600 m from the Atlantic Ocean east where a small

part flows through the Strait of Gibraltar (Criado-Aldeanueva et al., 2006a; Criado-

Aldeanueva et al., 2006b; Machín et al., 2006) and the rest mixes intensely with the

Mediterranean Outflow Water (MOW) near the Strait of Gibraltar to form the Modified

Atlantic Water (MAW) or Atlantic Inflow (AI) water or it turns south (Baringer and Price,

1999; Johnson et al., 2002). In the central part of the Gulf of Cadiz the NACW undergoes

almost no mixing with the surrounding waters and remains pure (Criado-Aldeanueva et al.,

2006b).

The second water mass is the Shelf Water (SW) flowing from east to west on the shelf. It

originates from the NASW but is influenced by river input and processes on the shelf, hence

the salinity is lower (35.9 – 36.5 psu) and the temperature higher (14 – 18 °C) (Criado-

Aldeanueva et al., 2006a; Criado-Aldeanueva et al., 2006b).

3.2.2 Intermediate water masses

Between 600 m and 1500 m there are two intermediate water masses present but highly

variable in space as both do not cover the entire Gulf of Cadiz, but divide it into a north and

south part.

The shallowest water mass is Antarctic Intermediate Water (AAIW) at a depth range of 600 to

1000 m with a low salinity of 35.5 psu (Knoll et al., 2002; Machín et al., 2006).

The other water mass is Mediterranean Outflow Water (MOW) that flows from the Strait of

Gibraltar into the Gulf of Cadiz at a depth of 1500 m (Baringer and Price, 1999) and turns

north along the Iberian shelf. (García et al., 2009; Llave et al., 2006). As it follows the Iberian

shelf it splits up (Fig. 2) in different cores due to friction, bathymetry and the Coriolis force

(García et al., 2009):

Shallow Core (MS): is located in the upper most regions of 600 m (Ambar et al.,

2008)

Upper Core (MU): is the continuation of the MOW at a depth range of 800 m. The

temperature is 3.7°C and the salinity is 37.07 psu with a speed of 46 cm/s (Hernandez-

Molina et al., 2006)

Lower Core (ML): goes south after the split up with a speed of 25 cm/s and descends

to 1200 m. The temperature is around 13.7°C and has a salinity of 37.47 psu.

(Hernandez-Molina et al., 2006).

The MOW cannot reach the PDE as the escarpment is too shallow (Van Rooij et al.,

submitted), though due to irregularities in bathymetry the MOW generates vortices, called

meddies. These meddies can move south (meddy Isabelle) and transport MOW water to the

Moroccan shelf and interacts with the NACW (Carton et al., 2002). The formation of these

8

meddies is more intense in colder periods as the stratification of the water column is minimal

and the turbulence high within the MOW (Ambar et al., 2008).

The source of the MOW is the Mediterranean Sea, where two water masses are formed, one is

the West Mediterranean Deep Water formed in the Gulf of Lion and the Adriatic Sea, the

other is Levantine Intermediate Water formed in the Levantine Basin near Rhodes (Baringer

and Price, 1999; Cacho et al., 2000). The variation in forming theses water masses is climate

related as the westerlies, that blow stronger during colder periods, cause cooling and

evaporation, changing the density of the surrounding water (Cacho et al., 2000).

3.2.3 Deep water masses

Beneath the MOW the North Atlantic Deep Water (NADW) resides. It is located at a depth of

>1500 m and has a temperature of 5°C and a salinity around 34.95 - 35.2 psu (Hernandez-

Molina et al., 2006). It comprises two different water masses: the Labrador Sea Water (LSW),

formed in the Labrador Sea and responsible for severe reduction of salinity in the MOW, and

the Iceland-Scotland Overflow Water from the North Atlantic Ocean (McCave and Hall,

2002).

3.3 Palaeoceanography

The present day Mediterranean Outflow started to form with the opening of the Strait of

Gibraltar at 5.5 Ma. Following this event a Contourite Depositional System (CDS) was

formed at around 4.2 Ma that holds significant information on climate change from the Early

Pliocene on. These climatic changes control the strength of the MOW and thus affects the

depositional regime of the CDS (García et al., 2009; Llave et al., 2006). In the CDS there are

Fig. 2: Flow movement of the five branches of the MOW. MU (Upper Core), IB (Intermediate Branch), PB (Primary Branch),

SB (Southern Branch), ML (Lower Core) (Hernandez-Molina et al, 2006). The red square represents the location of the study

area.

9

several discontinuities all related to prominent climate coupled sea level drops: the Late-

Messinian (5.5 Ma), the Lower Pliocene Revolution (LPR) (4.2 Ma), the Upper Pliocene

Revolution (UPR) (2.4 Ma) and the Middle-Pleistocene at about 900-1200 ka. The sea level

drops are correlated with climatic changes from warm periods to cold periods. They leave

strong imprints on the sedimentation regime as an unconformity surface (Hernandez-Molina

et al., 2006). The bathymetry near the PDE is formed by a contourite drift that eroded a moat

at the foot of the steep cliff of the PDE. During a severe sea level drop this moat was filled up

by mass wasting during the LPR. The PDE stopped evolving at 1.8 Ma but the drift

intensified during the MPR and created the final bathymetry today (Van Rooij et al.,

submitted).

In the Late-Pleistocene the sedimentation was influenced by glacial-interglacial stages

controlled by eccentricity and precession. The MOW was submitted to these changes as the

source waters WMDW and LIW changed (Llave et al., 2006). In glacial times the westerlies

blew stronger inducing more evaporation in the Mediterranean leading to saltier waters in the

upper most layers. This resulted in the formation of deeper and stronger WMDW and LIW

that induced a stronger MOW (Cacho et al., 2000) leading to changes in sediment deposition.

The stronger MOW winnowed the finer sediments creating a coarse grained contourite

deposition in the CDS (Llave et al., 2006). The time line of this study falls within the Marine

Isotope Stages (MIS), MIS2 and MIS 1. MIS 1 begins at 0 ka and covers the entire Holocene

till 12 ka, the onset of the Younger Dryas and MIS 2. MIS 2 contains several glacial and

interglacial cycles like Heinrich event 1 and the Last Glacial Maximum (LGM) (Toucanne et

al., 2007; Voelker et al., 2006). The cold LGM period is characterized by a strong and

continuous MOW (Schonfeld and Zahn, 2000). From then on there is an abrupt warming

called Termination I (LaViolette, 2005). This weakened the deep flow and strength of the

MOW as more melt water flowed into the Mediterranean decreasing the surface water salinity

and resulting in more buoyant MOW flowing at 800 m instead of 1200 m (Hernandez-Molina

et al., 2006). The Younger Dryas (12.7-11.5 ka) proceeded the sudden warming of the Pre-

Boreal warming and was characterized by an intensified MOW that sank to 1700 m until the

warming at 7.5 ka (Baas et al., 1997; Hernandez-Molina et al., 2006). At 5.5 ka, during the

Holocene Optimum, the present day hydrography was established (Schonfeld and Zahn,

2000). These changes in MOW strength are seen in the grain-size distribution as during colder

periods the grain-size is coarser than during warmer periods (Toucanne et al., 2007).

4 MATERIAL AND METHODS

4.1 Site survey

The bathymetric data were collected during the R/V Marion Dufresne 169 cruise using a

Thomson SeaFalcon 11 multibeam echosounder at 12kHz. The beams cover an area of 60 m²

in an average water depth of 625 m. In addition a subbottom profiler at 3.5 kHz provides an

image of the upper 50 m of the sediment in high resolution (Van Rooij et al., submitted).

The seismics were recovered during two cruises. The first set was acquired during the R/V

Pelagia 64PE253 cruise with towed guns (5 s, 100 bar). The receiver was a 24 channel

streamer with 10 hydrophones per channel spaced 1 m from each other. The processing was

10

exicuted with 30 Hz and 700 Hz filter, using the RadexPro (DECO geophysical) program.

The second dataset comes from the R/V Belgica GENESIS “Pen Duick” cruise, where high

resolution seismic profiles were recorded with a SIG sparker at 8 kHz (2 s, 500 J). During the

measurements the ship sailed on electrical engines to reduce the noise in the data hence only

the basic processing was used (Van Rooij et al., submitted).

4.2 Cores

Three gravity cores, MD08-3213, MD08-3223 and MD08-3228, and three Calypso piston

cores, MD08-3227, MD04-2806 and MD04-2809, were used for this study. MD04-2806 and

MD04-2809, were retrieved during the R/V Marion Dufresne -140 Privilege cruise in 2004

(Turon et al., 2004) and the other four cores were taken during the R/V Marion Dufresne -

169 Microsystems cruise in 2008 (Van Rooij et al., 2008). All six cores were taken off-mound

as these provide the best continuous record of sedimentation. The cores were taken at

different depths to provide a temporal as well as a spatial variability of the currents. Based on

the seismic profiles from the R/V Belgica GENESIS “Pen Duick” the region can be split up

into four areas (Fig. 3): the drift region, the depression, the Pen Duick Escarpment and the

plateau that can be used as background.

The geographic locations of the cores are shown in Table 1 and the visual location on Fig. 4,

with the high resolution seismic data that shows the penetration of the core and the possible

layering that is recorded in the core. Based on the subbottom profiler data (3.5 kHz) the

location of the cores have been chosen (Fig. 4).

Table. 1: The coordinates of all the cores used for this project and there specific region

Core n° Latitude Longitude Recovery Depth Region MD08-3213G 35°18.55’N 6°49.09’W 5.93 m 660 m Drift

MD08-3223G 35°18.78’N 6°48.45’W 3.83 m 632 m Depression

MD08-3227 35°16.28’N 6°47.89’W 33.2 m 642 m Drift

MD08-3228G 35°19.77’N 6°45.65’W 3.435 m 561 m Plateau

MD04-2806 35°17.60’N 6°49.56’W 24.63 m 684 m Drift

MD04-2809 35°17.55’N 6°4.14’W 15.13 m 539 m Pen Duick escarpment

Fig. 3: Seismic data from the MD 169 cruise taken perpendicular on the PDE. It shows in the drift region four sedimentary

packages and a slump. U1 and U2 are deformed by tectonics, MW is a massive mass wasting deposition probably due to

tectonics and the drift. U3 covers the slump and U4 is the last deposition is from the drift.

11

4.3 Analysis

Because the cores were taken with different cruises and analyzed by different research groups,

each core underwent a different analyses (Table. 2) before the actual sampling. Some of the

samples have been taken directly on the ship for isotopic analysis of δ18

O and δ13

C. The data

used for this study has all been taken onboard by non destructive measures. The samples were

taken at VLIZ in Oostende where the cores were stored as well.

Table. 2: overview of analyses preformed on the different cores before the analysis for this study. Where L* is the luminescence and

MST the data of the Multi scan core logger.

XRF Petrophysical data Samples

MD04-2809 CORTEX, 2 cm at 20 kHz for 30s MST, photo’s and description

MD04-2806 CORTEX, 2 cm at 20 kHz for 30s MST, photo’s and description At 5 cm for δ18O

MD08-3227 Avaatech XRF, 1 cm at 10 and 30

kHz for 30s

MST, photo’s and description and

L* Selected sampling for biochemical

analysis and at 5 cm for this study

MD08-3213 Avaatech XRF, 1 cm at 10 and 30

kHz for 30s


L* Selected sampling on board and at 5

cm for this study

MD08-3223 Avaatech XRF, 1 cm at 10 kHz for

30s


L* At selected intervals for δ13C and δ18O

At 5 cm for this study

MD08-2338 Avaatech XRF, 1 cm at 10 kHz for

30s


L* At 5 cm for this study

Fig. 4: The location of the cores on a bathymetric map. The sub bottom profiles show a red vertical bar that represents the

core that is taken and the possible sedimentary structures.

12

4.3.1 Non-Destructive analyses

The photos, description and MST were preformed on board. The cores are opened and

photographed, described and measured for luminescence (L*) on the archive half and logged

for p-wave velocity, magnetic susceptibility (MS) and density on the work half.

To measure the MST data a Geotec Multi-Scan Core Logger was used during the R/V Marion

Dufresne -140 Privilege and R/V Marion Dufresne - 169 Microsystems every 2 cm (Turon et

al., 2004). The reflectivity (L*) and chromaticity (a*, b*) of the MD04-2806 and MD04-2809

are measured during the R/V Marion Dufresne -140 Privilege with a Minolta CM-508i at a

distance of 2 cm. The colours, based on Lab and Munsell scales, provide a preliminary

stratigraphic view of layers with more calcium that is produced during warmer periods (Turon

et al., 2004). The reflectivity (L*) and chromaticity (a*, b*) of the other cores were also done

on the R/V Marion Dufresne, but the colour scale was not based on the Munsell scale. The X-

Ray Fluorescence is done by three different detectors (Table. 2). The cores from the R/V

Marion Dufresne -140 Privilege cruise in 2004 are scanned with a CORTEX 1st generation

scanner from MARUM in Bremen with a 20 kV strength and 30s interval (K-Sr) (Van Rooij

et al., submitted). Core MD04-2809 was only scanned from 75 cm deep as the upper part

contained too much corals and debris (Van Rooij et al., 2008). The cores from 2008 were all

scanned by the AVAATECH XRF scanner in NIOZ (The Netherlands). MD08-3227 was

scanned at 10 kV and 30 kV with a 30s interval (Al-Zr). The other cores from 2008 were

scanned with a new AVAATECH detector allowing shorter measuring time (10s). Only the

core MD08-3213 was measured every cm at 10 kV (Al-Rh) and 30 kV (Zn-Bi), MD08-3223

and MD08-3228 cores were scanned every 2 cm with an intensity of 10 kV. The MD08-3223

was already sampled which had a negative impact on the XRF data as much of the sediment

was gone.

4.3.2 Destructive analyses

These analyses were preformed in de sedimentary lag in Ghent University. All cores, except

MD04-2809, have been sampled every 5 cm and the last 90 cm of core MD08-3228 have been

sampled every 10 cm. Core MD04-2809 was not used as the first 75 cm is lost to coral debris

and the origin of the sediment could be foreign as it could come from the top of the Alpha

mound. Not all the samples that were taken were used (Table. 3).

Table. 3: the overview of the taken samples and the used samples.

Total samples Grain size analysis micropaleontological analysis

MD04-2806 81 40 31

MD08-3227 108 54 39

MD08-3213 41 20 28

MD08-3223 41 20 19

MD08-3228 34 22 18

A

13

The samples were 10 cc in volume and split up into 2 cc and 8 cc. All samples were weighted

wet and then dried in an oven at 60°C and weighed dry. The 2 cc was prepared in an acid bath

(10% acitic acid) to destroy the biogenic carbonate material and washed with distilled water.

The grain-size analysis was preformed every 10 cm using the Malvern Mastersizer

2000Hydro from the Marine Biology Section at Ghent University. The output provided

information on the overall grain-size distribution of the samples between 0.01µm – 1000µm.

This data was then analyzed using GRADISTAT. The remaining 8 cc was dissolved in 100 ml

distilled water and sieved wet with mesh sizes of 150 µm and 63 µm. The fraction >150µm

was dried in an oven at 60°C and weighted. The fraction <63 µm remained in an Erlenmeyer

with the rinsing water, which is rested for 1 - 2 days. When the fraction was sufficiently

deposited the rinsing water was pumped out and the sample dried in an oven at 60°C, forming

a mud cake that was weighed and stored for future sortable silt analysis

The micropaleontological study is based on the occurrence of specific foraminifera in a

certain Sea Surface Temperature (SST) range (Fig. 5). Within a SST-boundary one

foraminifera is more dominant than the other as the environment is more suitable for it to

grow (Cayre et al., 1999a; Kucera, 2007). Five species have been chosen from the fraction >

150 µm, providing five different SST ranges. The five species range a temperature going from

polar to tropical respectively (Boersma, 1998; Hemleben et al., 1989; Kucera, 2007):

- Neogloboquadrina pachyderma and Neogloboquadrina incompta (Darling et al.,

2006) (Fig. 6a): thrives in cold waters with temperatures of 6° or less. The N.

incompta used to be known as the right coiling variant of the N. pachyderma and

prefers warmer waters of 9°C to 21°C.

Fig. 5: Temperature distribution of foraminifera species. Underlined are the picked foraminifera (Kucera, 2007)

14

- Globigerina bulloides (Fig. 6b): lives in subpolar to transitional temperatures of 10°C

to 24 °C

- Globorotalia inflata (Fig. 6c): grows in water temperatures of around 18°C and ranges

from 13°C to 24°C, which is in the transition zone.

- Globorotalia truncatulinoides (Fig. 6d): it dwells in regions with temperatures of 18 –

21°C, but it also lives in waters up to 24°C.

- Globigeriniodes ruber white and Globigeriniodes ruber pink (Fig. 6e): prefers

subtropical waters with a temperatures of 18°C to 27°C. The G. ruber pink thrives in

even warmer water with tropical temperatures >24°C.

The counting was preformed every 10 cm, but reduced to 5 cm when an event was seen on the

Ca/Fe XRF data. There were around 50 foraminifera picked in 10 squares going from the left

upper corner to the middle lower square and then to the upper right corner. This provides a

quick, but overall abundance count of the seven species and a fast estimate of the SST.

5. DATA DESCRIPTION AND INTERPRETATION

Ca/Fe data together with foraminifera (SST), Al/Ti (Fe/Ti) and magnetic susceptibility (MS),

provide information on the climate variation. When all curves show an increase they indicate

an interglacial period. Ca/Fe increases as more Ca is formed due to the bloom of the

biological cycle when conditions are warmer (Croudace et al., 2006; Rothwell, 2006). The

SST increases as the climate warms up and will define what foraminifera species will

dominate the upper part of the water column (Kucera, 2007). The increase in Al (Fe) is due to

more input of Al (Fe) by eolean dust transport. This indicates that the continent was dry and

physical erosion could take place. The MS increases when Al (Fe) input increases as more

magnetic elements are deposited (Larrasoana et al., 2008; Yarincik et al., 2000). The grain-

size analyses indicates the variability of the bottom water strength. When the strength of the

bottom currents is high, the finer sediments are winnowed (Foubert et al., 2008; Toucanne et

al., 2007). One of the Statistical parameters is the mode which is the grain-size that occurs the

most in the distribution and is not submitted to outliers like the mean value. The mud (< 3.8

µm), sortable silt (10 µm – 63 µm) and sand (> 63 µm) fraction are a %-volume to the total

volume. A vertical line displays the mean volumetric percentage of the mud, sortable silt and

sand fraction. The skewness of the data will indicate if finer or more coarse grains are present.

The Gaussian distribution indicates that the mode there is an even amount of fine grained

material as coarse grained and is shown in the figures as a vertical line at the zero value. If the

skewness is negative there is an increases in finer grain-sizes and more positive coarser grain-

sizes. If the grain-size distribution is bimodal the skewness data cannot be trusted and this

occurs when a large addition of sand sized grains enters the system.

Fig. 6: images of the foraminifera (a: N. pachyderma or N. incompta; b: G. bulloides; c: G. inflate; d: G. truncatulinoides; e: G.

ruber w/p (source: http://www.emidas.org/))

a b d c e

15

To have a structure four zones have been made, based on the Ca/Fe pattern. As Ca/Fe displays

a similar pattern to δ18

O it can be correlated to a known δ18

O. This subdivision is then based

on paleoclimatological proxies and are indications of glacial and interglacial cycles. Zone 1 is

a period with high Ca/Fe values while zone 2 is characterized by a massive drop in the Ca/Fe

value. Zone 3 has an overall low Ca/Fe value that is suddenly interrupted by a high value

located within zone 4. Because there are terrigenous proxies and biogenic proxies the results

will be split up accordingly. Where the Ca/Fe, foraminifera and luminescence will be

displayed under biogenic as all are related to biological cycle and the MST, Al/Ti and grain-

size will be discussed in the terrigenous part.

5.1 MD08-3227

5.1.1 Terrigenous proxies

Zone 1: the XRF Al/Ti increases from 0 cm to 128 cm and is supported by an increase in MS.

The grain-size analysis shows an increase in mode from 7 µm to 16 µm together with a

decrease in mud and an increase in sortable silt. The sand fraction stays constant till the end of

the zone at 128 cm where an increase of 2%sand fraction and 8% of sortable silt fraction is

noticed. Skewness follows the mud fraction, but increases with an input of 2% sand (Fig 7).

Zone 2: the increase in XRF Al/Ti at 140 cm is accompanied by an increase in MS. The XRF

Al/Ti curve varies more than the MS curve, but both drop at 170 cm and rise at 200 cm. The

mode average is 16 µm and all grain size analysis curves follow the constant trend (Fig. 7).

Zone 3: between 210 cm and 240 cm the MS and the XRF Al/Ti do not correspond and the

MS shows a sudden decrease between 250 cm and 280 cm, which is probably due to a fault in

the measurements. From 280 cm both curves move synchronously downward and at 400 cm

the curves have a negative excursion that is repeated at 480 cm. The mode decreases at 400

Fig. 7: The summary of the XRF and grain size data with mud, silt and sand as percentages.

16

cm followed by an increase of 10 µm at 424 cm. The sortable silt is above the average

throughout the zone (Fig. 7).

Zone 4: within this zone the XRF Al/Ti and MS do not align as the XRFAl/Ti curve decreases

while the MS increases. The mode indicates a sudden input of bigger grains which is

supported by the input of 4% more sand and 8% more sortable silt. This increase has no

influence on the skewness as at the end of zone 1 (Fig. 7).

5.1.2 Biogenic proxies

Zone 1: both XRF Ca/Fe (Fig. 7) and the

amount of warm water foraminifera (Fig.

8) move synchorneously. Within zone 1

both curves have high values and decreases

at 120 cm. At 110 cm G. ruber pink

disappears, but directly reappears in zone 2

(Fig. 8). Although the L* and Ca/Fe are

linked by the Ca amount the L* decreases

throughout zone 1 while the Ca/Fe stays at

a constant value (Fig. 7).

Zone 2: This zone is characterized by a

decrease in Ca/Fe, L* (Fig. 7) and warm

water foraminifera (Fig. 8). In this zone the

G. ruber pink disappears but both G. ruber

white and G. truncatulinoides remain in a

constant abundance.

Zone 3: the decrease that started in all

parameters in zone 2 continuous till 240 cm. During the decrease both G. truncatulinoides and

G. ruber white disappear at 210 cm and at 240 cm respectively (Fig. 8). Both reappear when

the cold water foraminifera decrease. At 360 cm the cold water foraminifera increase in

abundance and G. inflate vanishes briefly at 400 cm when Ca/Fe decreases (Fig 7 & 8). At

410 cm G. ruber white and G. truncatulinoides vanish and when G. bulloides reaches a

maximum at 470 cm G. inflata is replaced by G. truncatulinoides. From this event Ca/Fe

decreases, but the L* increases. At 495 cm G. ruber white and G. inflata return and the warm

water foraminifera increases again (Fig. 8).

Zone 4: is characterized by a sudden increase in warm water foraminifera and as well in XRF

Ca/Fe and L* (Fig. 7&8). Here G. ruber white returns vanishes at 560 cm.

5.2 MD04-2806


Zone 1: Al is replaced by Fe as no Al has been measured. Fe/Ti shows several positive peaks

at 30 cm, 40 cm, 50 cm, 60 cm and 75 cm and these sharp excursions are not measured in the

MS. But the overall rise in MS from 30 cm is also seen in the Fe/Ca without the excursions.

Fig. 8: The variation of all 7 foraminifera species of

all cores. The species are not displayed but the temperature

range is. 0-6°C: N. pachyderma; 9-21°C: N. incompta;

10-24°C: G. bulloides; 13-24°C: G. inflata;

18-24°C: G. truncatulinoides; 18-27°C: G. ruber white;

24-27°C: G. ruber pink

17


The rise in mode at 40 cm implies a 6 µm increase where the mode remains at 13 µm until 82

cm. This rise is accompanied by a decrease in mud and an increases in sortable silt while sand

input remains constant as does the skewness (Fig. 9).

Zone 2: there are two positive excursions of Fe/Ti at 100 cm and at 150 cm which are not seen

in the MS. There are however two peeks in the MS at 120 cm which is positive and at 140 cm

is negative. Overall there is no variation in the Fe/Ti nor in MS during this period, only at 150

cm there is a decrease in XRF Fe/Ti, but not in MS. The mode increases together with the

sortable silt, but at 125 cm there is a 3% increase of sand but not accompanied by sortable silt

(Fig. 9).

Zone 3: the Fe/Ti curve is stable with a positive excursion at 280 cm. However the MS does

not follow the Fe/Ti curve as there are 2 positive excursions at 160 cm and between 180 cm

and 220 cm. After the second positive excursion in MS the two curves remain stable. The

mode remains constant at 18 µm until 235 cm and diminishes to 13 µm at 260 cm. At 270 cm

the plateau of 18 µm is reached again, but at 320 cm it decreases to 8µm. The sand and

skewness show an increase, mud a similar decrease, whereas the sortable silt remains constant

above average except at 260 cm and 320 cm (Fig. 9).

Zone 4: is characterized by a low Fe/Ti but the MS increases at 340 cm. The mode has a high

value in zone 4 but the rise started in zone 3. There is no sudden input of sand in this zone

compared to MD08-3227 but the storable silt increases as well (Fig. 9).

18


Zone 1: this zone is abundant in warm

water foraminifera (60%) and decreases to

40% near zone 2 (Fig. 10). Fe/Ca has the

same high indication as in MD08-3227,

however no L* data was available (Fig. 9).

Zone 2: the cold water foraminifera

abundance are high at 85 cm and 153 cm

(Fig. 10) where Ca/Fe decreases steadily

throughout the zone (Fig. 9). At 113 cm

the warm water foraminifera G. inflata and

G. truncatulinoides increase to 30% while

G. ruber white and pink are constant. At

160 cm topical species and G.

truncatulinoides disappear and 70% of the

total abundance is G. bulloides (Fig. 10).

Zone 3: after 160 cm the warm water

foraminifera increase as G. ruber white

and G. truncatilinoides reappear (Fig. 10) during an increase in Ca/Fe (Fig. 9) till 180 cm

where it reaches a plateau till 240 cm. A strong negative excursion is seen at 280 cm together

with a significant rise in G. bulloides, at 300 cm Ca/Fe decreases while G. bulloides increases

with 80% and at 330 cm G. bulloides decreases to 35% as the Ca/Fe increases (Fig. 9&10).

Zone 4: is a small zone with an decrease warm water foraminifera, but immediately followed

by an increase at 350 cm that coincides with the increases in Ca/Fe (Fig. 9&10).

5.3 MD08-3213


Zone 1: MS rises throughout the zone and Al/Ti follows after 10 cm. The mode rises from 5

µm to 15 µm, without an input of sand but with an increase in sortable silt and decreases in

mud. At 25 cm the mode decreases by 8 µm due to an increase in sortable silt (Fig. 11).

Zone 2: is characterized by a decrease in Al/Ti, but the MS remains constant. The mode

increases till 50 cm depth, followed by a descend of 5 µm in grain-size. The decrease is

accompanied by a negative excursion in skewness and mud and an positive excursion in

sortable silt at 80 cm (Fig. 11).

Zone 3: at 85 cm the Al/Ti increases, but MS has already increased. At 95 cm the MS

decreases while the Al/Ti still increases, this pattern remains until 150 cm, where both curves

show a positive excursion. The mode is at 17 µm grain size the first 30 cm of zone 3 and

decreases to 14 µm at 150 cm and followed by an increase of 5 µm. The other fractions link to

the mode quite well as at the end of zone 3 the sand fraction increases with 2% and the

sortable silt by 4% and remains above average (Fig. 11).

Fig. 10: The variation of all 7 foraminifera species of all cores.

The species are not displayed but the temperature range is. 0-

6°C: N. pachyderma; 9-21°C: N. incompta;




19

Zone 4: Al/Ti and MS increase during this period. The mode has two maxima one at 160 cm

(20 µm) and at 180 cm (18 µm). The sand fraction increases still to 4%, but the excursion of

the skewness does not correlate with the amount of the sand added. The input of sortable silt

is comparable with the other previous cores (Fig. 11).


Zone 1: the first 40 cm the warm water

foraminifera represent 60% of the total

assemblage. Where an increase in tropical

species at 30 cm pushed G. inflata to a

minimum (Fig. 12). Within this zone Ca/Fe

is high and reaches a maximum at 38 cm and

decreases rapidly after that. L* has an overall

negative trend with a negative excursion at

25 cm (Fig. 11) like in MD04-2806 (Fig. 9).

Zone 2: the increase of cold water

foraminifera only starts at 60 cm (Fig. 12),

though the decrease in Ca/Fe and L* began

from the onset of zone 2 (Fig. 11). There is a

steady decrease of G. ruber white and the

disappearance of G. ruber pink. The end of

zone 2 is characterized by an increase in

warm water foraminifera and return of G. ruber pink (Fig. 12).

Zone 3: Ca/Fe is constant with positive excursion at 90 cm and at 110 cm and at 150 cm there

is a minimum, mimicked by L* but at 140 cm (Fig. 11). At 90 cm G. inflata and G.


Fig. 12: The variation of all 7 foraminifera species of all

cores. The species are not displayed but the temperature

range is. 0-6°C: N. pachyderma; 9-21°C: N. incompta;




20

truncatulinoides decreases there is a peek in the G. inflata and G. ruber pink disappears,

though at 110 cm the abundance of G. inflate rises even more. G. bulloides has a maximum of

75% at 135 cm, where deeper the warm water foraminifera increase again (Fig. 12).

Zone 4: Ca/Fe has a positive excursion which is followed by the L* and this zone has a 40%

warm water foraminifera and an increase in the polar species N. pachyderma (Fig. 11&12).

5.4 MD08-3223


Zone 1: Al/Ti has many variations, but overall there is a small decrease where MS also shows

a general decrease, between 10 cm and 15 cm there is a minimum. The mode indicates a rise

of 8 µm in 8 cm and a second rise of 6 µm at 30 cm, followed by a decrease of 6 µm at 48 cm.

The sand fraction increases at 30 cm with 2%, which is observed in the skewness and the

sortable silt fraction is complementary to the other cores as the amount is higher than the

average fraction (Fig. 13).

Zone 2: Al/Ti is higher than zone 1 and is still variable, but MS is lower in comparison. The

mode decreases from 9 µm to 7 µm, as the mud fraction increases and the silt decreases.

There is no addition of sand and the skewness follows the mode again. Here the sortable silt

decreases instead of increasing (Fig. 13).

Zone 3: Al/Ti has strong variations with a negative excursion at 150 cm followed by the MS,

but the positive MS excursion at 170 cm is not visible in Al/Ti. The mode peaks at 90 cm and

140 cm as does the sortable silt which has a lower abundance then the average fraction. At

160 cm a rise of 6% in sand fraction is observed leading to an increase in the skewness (Fig.

13).


21

Zone 4: is characterized by a negative peak of Al/Ti, but it is not reflected in the MS. The

mode rises to 13 µm in zone 4 without addition of any sand fraction. Here the sortable silt is

comparable with the other cores as it increases during zone 4 (Fig.13).


Zone 1: Ca/Fe value is not as outspoken as

the other cores and the L* has an overall

negative trend, though the abundance of the

warm water foraminifera is around 70 %

with 30% G. inflate (Fig. 13&14).

Zone 2: is characterized by an increase in

cold water foraminifera as G. bulloides

increases in numbers and G. ruber pink

vanishes (Fig. 14). This coincides with a

decrease in Ca/Fe, but L* increases till 65

cm and then decreases (Fig. 13).

Zone 3: Ca/Fe rises to a plateau at 80 cm

with a maximum at 95 cm and 140 cm. Both

peaks are mimicked by the L* curve, but

between 140 – 170 cm the Ca/Fe decreases

and the L* increases (Fig. 13). The polar

species N. pachyderma and N. incompta

increase as does G. bulloides. At 135 cm the polar spacies decine and at 150 cm there are a

high number of cold water foraminifera, but N. incompta and G. ruber white disappears for a

short time (Fig. 14).

Zone 4: is characterized by a maximum in Ca/Fe and L* and the polar species decrease

together with the disappearance of N. incompta (Fig. 13&14). The warm water species

increase their abundance with 10%.

5.5 MD08-3228


Zone 1: Al/Ti has a minimum at 20 cm but rises till 55 cm and MS has a similar pattern. The

mode rises from 4 µm to 15 µm and the sand input is 0%. The sortable silt fraction is below

average, the same as in the drift cores (Fig. 15).

Zone 2: Al/Ti stays the same throughout the zone as does the MS. The mode varies between

10 µm and 14 µm and there is no sign of sand input even though the previous cores, except

MD08-3213, have an increase. There is an increase of sortable silt at 50 cm of 10% that is

reflected by an increase in the mode (Fig. 15).

Zone 3: at 110 cm and 150 cm there is a decrease in Al/Ti and MS ad at 150 cm the decrease

is even more rapid than in Al/Ti. At 170 cm Al/Ti increases, but MS continuous to decrease.

The mode is constant around 18 µm until 180 cm were it increases to 24 µm. This increase is


The species are not displayed but the temperature range is.

0-6°C: N. pachyderma; 9-21°C: N. incompta;




22

together with an 6 % increase of sand and 10 % in sortable silt. The skewness shows a

negative trend even though the input of bigger grain sizes (silt and sand) is high (Fig. 15).

Zone 4: Al/Ti decreases together with the MS and increases suddenly at 210 cm. The mode

decreases to a minimum of 4 µm due to the loss of sand and silt input (Fig. 15).


Zone 1: Ca/Fe rises in zone 1 as in the

previous cores, but is now accompanied

by a rise in L* (Fig. 15). Warm water

foraminifera are dominant in this zone

with 80 % and a decrease to 50% at 55

cm (Fig. 16).

Zone 2: in this zone Ca/Fe decreases

together with the L* where G. bulloides

first increases, but decreases at 75 cm

where 70 % is a warm water foraminifera

(Fig. 15&16). At 105 cm the tropical

species disappear and the polar species

and G. bulloides increase.

Zone 3: Only between 130 cm and 155

cm does the tropical species, G. ruber

white, returns. G. inflata has three peaks

where the abundance is 40%, whereas G.

truncatulinoides remains constant at 5 – 10% (Fig. 16). At 130 cm Ca/Fe has a sudden rise

and decrease at 190 cm. L* shows no overall decrease, but there is an negative excursion at

110 cm and 150 cm (Fig. 15).


The species are not displayed but the temperature range is.

0-6°C: N. pachyderma; 9-21°C: N. incompta;





23

Zone 4: Ca/Fe increases as seen in all other cores but is not accompanied by L* (Fig. 15). The

polar species increase to 20 % and G. bulloides to 60 %. During the high abundance of the

polar species G. tuncatulinoides disappears and G. ruber white increases (Fig. 16).

5.6 Time conversion

The correlation of the core depth with time is done with the δ18

O curve from (Cayre et al.,

1999b) based on the planktonic foraminifera G. bulloides. The dating of the δ18

O curve is

based on data taken from the same core taken by (Shackleton et al., 2000). On MD95-2042

core they preformed the δ18

O analyses on bentic foraminifera which provided an age model

that was corrected with GRIP in 2002 (Fig. 17A). The δ18

O curve of G. bulloides is

correlated with the Ca/Fe curve of MD08-3227 reference core. As the δ18

O value is based on

climate where high values of δ18

O indicate a warm period, high values of Ca also indicate

warm temperatures as the biological production is high in warmer periods (Croudace et al.,

2006; Rothwell, 2006). For this reason δ18

O and Ca/Fe can be compared to find some tie-

points for the conversion of depth to time in core MD08-3227. The tie-points that are used are

displayed in Table 4 (two left columns) where a depth is linked to a time. The visual

conversion is seen on Fig. 17A where the lines represent the used tie-points and the

correlation between MD08-3227 and MD95-2042. Between two tie-points a linear regression

is used to obtain a continuous time line. The correlation for the other cores is based on the

Ca/Fe data as this is the most presentable data of all (Fig. 17B). Again several tie-points are

used to link a certain time with depth (Table 4; 6 columns right) and linear regression between

the tie-points completed the time line. After 20,19 ka the data points were extrapolated till the

end of the sampled core. The reference core for these four cores is MD08-3227 with linear

regression and a visual correlation is presented in fig 19B. All chosen tie-points are based on

the assumption that the top cores, that starts at 0 m below sea level, starts at 0 ka.

Fig. 17: A: correlation of the MD08-3227 (upper graph) Ca/Fe data with the MD95-2042 core from (Cayre et al., 1999b)

where δ18O data of the G. bulloides is used; B: correlation of the five cores based on the ratio Ca/Fe XRF data.

A B

24

Table. 4: the used boundaries for the conversion of depth to time on all five cores

MD08-3227 conversion Conversion all cores (time to depth (cm))

Depth (cm) Time (ka

cal BP)

Time (ka cal

BP)

MD08-3227 MD04-2806 MD08-3213 MD08-3223 MD08-3228

116 6,1 2,07 40 24 16 8 16

306 11,7 6,22 120 80 35 48 56

454 15,6 9,05 216 160 80 72 104

503 17,7 13,97 392 256 128 120 152

793 35,3 17,82 505 328 152 152 176

943 56,3 20,19 557 344 160 184 200

1150 60,6

1282 72,8

1390 81,1

1541 97,9

1685 107,7

Plotting depth versus time the sedimentation rate can be found between two tie-points. The

slope between the tie-points (Table. 4) provides the actual sedimentation speed. To have an

continuous sedimentation rate, linear interpolation is used and the Linear Sedimentation Rate

(LSR) is calculated. From this value the Mass Accumulation Rate (MAR) can be calculated

by multiplying it by the dry bulk density (DBD) that is calculated with the volume of the

samples (10 cc) and the dry weight. It provides an amount in mass (g) per square centimetre

each thousand years (Rack et al., 1995). Both rates are plotted against time and compared to

each other (Fig. 18).

During the first 7 ka the mass accumulation is low and then rises suddenly, only MD08-3223

shows an increase at 3 ka. The highest MAR is between 7 ka and 10 ka and decreases at 10

ka, but this is not seen in MD08-3227 and in MD08-3223 the decreases is at 15 ka.

Fig. 18: here is the comparison of the LSR with the MAR and sortable silt (SS) over time.

25

6. DISCUSSION

6.1 Chronostratigraphy

Fig. 19 shows the divisions in the

four previous discussed zones

through time. The high values of

Ca/Fe which correlated to the

increase in the biological cycle and

the increase in Ca (Croudace et al.,

2006; Rothwell, 2006) is during the

Holocene. In the Late-Holocene

there is a sudden decreases in Ca

production due to a decrease in

temperature, it indicates the

beginning of the Younger Dryas

(YD). This cooling of the climate is

the Pre-Boreal warming (Baas et

al., 1997; Schonfeld and Zahn,

2000). The YD is the onset of the

Pliocene Epoch and the MIS 2 at 9 – 10 ka cal BP. After the YD a warmer period is marked,

the Bølling-Allerød stage (Toucanne et al., 2007; Voelker et al., 2006), but it is only captured

in MD08-3223 (depression). The following 6 ka cal BP should cover Heinrich event 1(~15

ka) (Toucanne et al., 2007; Voelker et al., 2006), but this change is seen around 14 ka cal BP

in cores MD08-3227 and MD04-2806. However this is a climatic that impacts the entire Gulf

of Cadiz and should be seen in all cores, hence there is doubt whether the HE 1 is recorded.

At the boundary of zone 4 a sudden increase in Ca/Fe is located in all cores and indicates a

sudden warming (Croudace et al., 2006; Rothwell, 2006) called Termination I (LaViolette,

2005). After this warming the Last Glacial Maximum and the start of MIS 3 begins

(Toucanne et al., 2007; Voelker et al., 2006). The following discussions will cover these four

zones to discuss the temporal and lateral variability of bottom water strengths beginning at

zone 1 till zone 4. The second part will cover the effect of these variations on the existence of

cold-water corals.

6.2 Bottom current variability

Zone 1: the Late- to Middle-Holocene is characterized by a low MAR and high skewness in al

cores except MD08-3223 that increases at 3 ka cal BP (Fig. 20). The low MAR indicates that

the water column carries less load due to lesser input of material in the ocean and less lateral

transportation at the interface (Munoz et al., 2004). The Holocene is a warm period as seen in

the SST of the foraminifera and Ca/Fe. The warm and arid conditions weaken the rivers,

leading to less sediment load and less input in the Gulf of Cadiz (Barton et al., 1991). The

early increase of MAR in MD08-3223 is half as strong as the increase in MAR that follows in

all other cores. This indicates that the sediment input in the depression is not linked to climate

Fig. 19: division of the cores in five compartments based on XRF Ca/Fe.

YD: Younger Dryas, BA: Bølling-Allerød, HE 1: Heinrich Event 1

26

change but to regional variations in current strength. The high skewness (Fig. 20) is linked to

the distribution of the grain-size and indicates more coarse grained material on and around the

PDE. The source must be a change in water mass strength. A seasonal study of the NACW in

the Gulf of Cadiz by Machín et al. (2006) shows that there is an intensification of the NACW

during the summer and a more stratified and narrower NACW during the winter time. This

could be the same during glacial and interglacial times as the meridonial overturning

circulation is weak due to ice berg discharges and the hydrological cycle is strong in the Gulf

of Cadiz (Margari et al., 2010). It confirms that the NACW and the AAIW flow stronger

during interglacial periods and winnow the smaller grains from the PDE (Toucanne et al.,

2007) providing a higher skewness. Core MD08-3223 has a lower skewness (Fig. 20)

indicating that the bottom currents are weaker. This causes AAIW or NACW to flow over the

depression and because the skewness does not change during this event the source of the

sediment remains the same.

The boundary between the Middle-Holocene and the Pre-Boreal warming is indicated by an

increase in sand fraction in MD08-3227 and MD04-2806 with a grain-size of 490 µm and in

MD08-3223 with a grain-size of 200 µm (Fig. 20). Because the grain-sizes are different

between the drift cores and the depression core the source of the sand fraction is different. As

the Ca/Fe of the MD08-3223 shows an increase from this period, indicating reworked

material, possibly due to mass-wasting (Rothwell, 2006). There is also an increase in MAR in

all cores but MD08-3223 as it decreases (Fig. 20). The increase in MAR is due to the cooling

of the climate and more rain (Barton et al., 1991) and more sediment is transported (Munoz et

al., 2004) to the Gulf of Cadiz from the continent. This increase in MAR is related to a

decrease in skewness indicating finer sediment and a weakening of the water masses (Margari

et al., 2010). The decrease in MAR of MD08-3223 (Fig. 20) is due to regional variations in

bottom water strength. The AAIW and NACW are strong during interglacial periods (Margari

et al., 2010) and thus cannot reach the bottom of the depression.

Zone 2: during the Pre-Boreal warming the skewness decreases as the MAR remains at a

maximum for all cores except MD08-3223, which has a regional variation in MAR and

skewness (Fig. 20). The decrease in skewness is related to the weakening of the NACW and

the AAIW (Margari et al., 2010) as more finer sediment remains on the seafloor (Foubert et

al., 2008). The high MAR can be related to the cooling of the climate during the Pre-Boreal,

which was also a wet period (Barton et al., 1991) increasing the sediment load in the rivers

and the input in the Gulf of Cadiz. The decrease in MD08-3223 in MAR could be related to

an local increase in bottom water current or change in lateral transport of sediment (Munoz et

al., 2004). At the end of the Pre-Boreal the MAR decreases rapidly in all core, whereas the

MAR in MD08-3223 increases (Fig. 20) to the value during the Middle-Holocene. This

decrease in MAR is related to the cooler climate that has become less arid (Barton et al.,

1991), hence less sediment is transported from the continent to the Gulf of Cadiz (Munoz et

al., 2004). The decreases is not seen in MD08-3227 as the AAIW has more influence in this

region. The AAIW might transport more sediment.

27

Zone 3: The skewness is overall lower than in zone 1 (Fig. 20) indicating that the water

masses are weaker during this zone (Foubert et al., 2008; Toucanne et al., 2007). The

weakening of the hydrography in the Gulf of Cadiz could be due to a stronger meridonial

overturning cycle (Margari et al., 2010). However in the depression the currents flow stronger

as the skewness is higher in zone 1. Because the AAIW and NACW are weak the currents can

reach the bottom of the depression to winnow the finer sediments (Foubert et al., 2008;

Toucanne et al., 2007). The MAR remains low during the entire zone, though it is higher than

during zone 1. This indicates that the input of sediment is a bit higher (Munoz et al., 2004)

during the glacial times than during the Holocene. However MAR remains high in MD08-

Fig. 20: this is the correlation between the grain size statistics, skewness and sand fraction, and the Mass Accumulation Rate

(MAR) of all the cores.

28

3227 (Fig. 20) indicating that the cooling of the climate and the reduction of sediment input

had no effect on the southern most core. As this core is located south and in the deeper

regions of the PDE the influence of AAIW is higher (Knoll et al., 2002; Machín et al., 2006).

It is not due to the winnowing of finer sediment by a channelized current (Munoz et al., 2004)

as the grain-size remains the same in all three drift cores (Fig. 20). As the influence of AAIW

is higher the input of sediment could be higher as this water mass has a different source then

the NACW (Criado-Aldeanueva et al., 2006b; Knoll et al., 2002; Machín et al., 2006). No

change in the skewness is observed within the MD08-3227 indicating that the source of the

sediment remains the same. At 19 ka cal BP a second decrease in MAR is observed in all

cores. In the drift cores the decrease is from 16 ka cal BP to 19 ka cal BP and in the

depression and plateau core the decreases happens between 20 ka and 21 ka in zone 4 and is

much faster. Cores MD08-3223 and MD08-3228 show an increase in MAR during 17 ka cal

BP– 18 ka cal BP (Fig. 20). This indicates that before Termination I the hydrodynamics

between the higher plateau region and the lower drift region are different. A possible

explanation is that the upper region was more influenced by the NACW and the drift region

more by AAIW as both water masses flow in the opposite direction and have different

sources. So between 17 ka cal BP and 19 ka cal BP the sediment load between NACW and

AAIW switched, as the NACW gained more sediment and AAIW less. This could be related

to a local change in climate at the source of the sediment input of the water masses. Between

the drift cores the decrease in MAR is fast in MD08-3227 and MD04-2806, but slow in

MD08-3213 and less. This indicates that the sediment load of AAIW could be lost before it

reaches the PDE.

Zone 4: Termination I is characterized by a sudden input of sand except in MD04-2806 (Fig.

20). All sand fractions are of the same size indicating a common source. Why MD04-2806

does not show an increased amount of sand is not known. The skewness in this zone is biased

due to the massive input of sand, which gives a bimodal grain-size distribution

(GRADISTAT). Only MD08-3223 and MD08-3228 show a decrease in MAR (Fig. 20)

indicating that the climate reduces the sediment inflow (Munoz et al., 2004) to the NACW

during the Termination I. The decrease levels the input of MAR to the same value as during

the Holocene. This indicates that the input of sediment is the same during the Holocene and

the Last Glacial Maximum. Even the skewness does not change, indicating the

hydrodynamical conditions were the same between the Holocene and LGM, but the

temperature.

To see if one of the sources is eolean dust from the main land (Larrasoana et al., 2008;

Yarincik et al., 2000) the Al/Ti curve is examined. Based on Fig. 22 there is no clear view of

eolean dust input from the continent. There is an overall higher value of Al/Ti after the

Termination I and it decreases in the middle of zone 3 in cores MD08-3227, MD08-3213,

MD08-3223 and MD08-3228 (black circles). Although the dust input is not seen in MD04-

2806, where also the input of sand was not visible. There is however an increased input of Al

during the Pre-Boreal warming (black circles and red arrows) in all cores which could

indicate the input of dust. This elevation goes together with an increase in sand fraction,

29

indicating that the input of sandy material could be due to eolean transport. Though in the Pre-

Boreal warming the input of sand was only seen in MD08-3227 and MD04-2806.

6.3 Effect on cold-water corals

As the bottom current changes from strong to weak and the variations of the climate it might

have an effect on the coral population. Previous studies have shown that there is a steady

decrease of coral population from the LGM (Wienberg et al., 2009) and that the CWC can

only live in a certain current strenght window (Dorschel et al., 2009). As indicated in this

study the MAR during cold periods is quite high, indicating that during glacial times the risk

for sediment coverage is high (Dorschel et al., 2009; Taviani et al., 2005) Based on this

parameter the cold periods are not suitable for coral growth. This is in contrast with Foubert et

al. (2008) but more with Wienberg et al. (2009) who stated that the CWC are not dependent

on climatic changes in the Gulf of Cadiz. The spatial difference between the drift cores and

the plateau region indicates that the plateau region accumulates less sediment due to a

different hydrodynamics, also the sediment accumulation near the PDE is low resulting in a

different hydrodynamics at the steep slope. Hence the PDE could have a suitable

environment, but the plateau region is submitted to mass-wasting (Van Rooij et al.,

submitted), which is negative for CWC growth.

The depression region is not a good place for CWC aside from the MAR is low CWC are

known to grow on hard elevated places so to maximize the input of nutrient from currents and

the removal of excess sediment (Foubert et al., 2008). The only suitable place for the CWC is

at the foot of the drift mound during glacial times as the MAR is weak and there is no

evidence of mass wasting (Van Rooij et al., submitted). This place is in line with the location

of CWC found by Foubert et al. (2008) as there were small mounds pressent at the foot of the

PDE. During warmer periods, from the LGM on, the conditions are more suitable for CWC to

grow, but because the climate is warm the temperature of the bottom water currents increases,

which is negative for the growth. This indicates there there is indeed a demise of CWC at the

PDE since the LGM (Wienberg et al., 2009) as the glacial nor interglacial periods are

perfectly suitable for CWC growth. The sudden demise of CWC at 12 ka (Wienberg et al.,

Fig. 21: this figure shows the correlation between the MS and the Al/Ti.

30

2009) could be due to the sudden increase in bottom water temperature, because the amount

of MAR drops, which is positive for CWC (Dorschel et al., 2009; Taviani et al., 2005). There

is evidence that during the LGM the MAR was low and the temperature of the bottom

currents is low. This environment is beter suited for CWC and supports the statement of

Wienberg et al (2009) that from the LGM the CWC growth in the Gulf of Cadiz decreases.

The effect of meddies is not visible in this data as no Nd values have been measured or

bottom water temperatures, however the MOW during the warm periods will shed less

meddies as the water column is more stratified and stable (Ambar et al., 2008; Foubert et al.,

2008) and in colder periods the meddy numbers are high and could reach the PDE and interact

with the NACW to supply fresh and nutrient rich water (Foubert et al., 2008).

7. CONCLUSION

The Pen Duick Escarpment (PDE) is one of the four ridges in the El Arriachi mud volcano

field. It is topped with mounds build up by dormant cold-water corals (CWC) and at the foot

of PDE a drift mount is present. These CWC were dependent on the hydrodynamics around

the PDE, now these conditions are unsuitable. There were times when CWC flourished, but

the hydrodynamics are poorly known. This study revealed temporal and spatial variations in

the water masses North Atlantic Central Water (NACW) and Antarctic Intermediate Water

(AAIW) from the Last Glacial Maximum (LGM) till now. During the cold periods the NACW

and AAIW were weak (Margari et al., 2010), which lead to the strengthening of internal tides

that are formed in the region (McCave and Hall, 2002). These internal tides provide high

amounts of organic material and nutrients to the CWC on the PDE (Mienis et al., 2007).

During cold periods this supply is weakened and all particles in the water column deposit, but

the formation of meddies is increased during the cold periods (Ambar et al., 2008), which

counteracts the weakening of the internal tides as Mediterranean Outflow Water (MOW)

water can reach the study area bringing more nutrients and sediment (Cacho et al., 2000).

Also during cold periods the MAR is high due to more sediment input from the continent or

lateral transport (Munoz et al., 2004) which can cover up the CWC. This does not support the

statement of Foubert et al. (2008) that during cold periods the conditions for coral growth

were suitable in the Gulf of Cadiz but it supports the statement of Wienberg et al. (2009) that

the CWC are not bound to the glacial-interglacial cycles.

The spatial variability indicates that the least suitable place for the coral growth is in the

depression and near the drift mound as the sedimentation is minimal and not free from

obstacles. The highest sediment deposition is in the southern region, far from the elevated

PDE, which is also a negative aspect as this can cover the corals due to the high sedimentation

rate and slow grow speed (Dorschel et al., 2009). The best place for the CWC in the colder

periods, based on the information of the five cores, is on the plateau after the PDE. Here the

sedimentation is average, the currents are strong enough, it is elevated and rather free from

surrounding obstacles however there is some evidence of a mass wasting. During the cold

periods this plateau would have been suitable for CWC from the LGM on. The data shows

stronger currents during the LGM as the amount of finer grain-sizes increases together with a

high Mass Accumulation Rate during the Holocene. The change to weaker currents in the

31

glacial periods and stronger currents in the interglacial period occurred after Termination I,

which could have an effect on the hydrodynamics and sediment supply to and around the PDE

and confirms the statement of Dorschel et al. (2009) that the CWC only live in a certain

limitation of current strength. More research is needed between these periods to see if there is

a change in hydrodynamics or source of the sediments. More research is also needed to see if

it is correct that the characteristics of MOW (meddies) are present during colder periods and

study the changes of the internal tides due to a weakening of water masses and their effect on

the suspension and deposition rate on sediment.

8. ACKNOWLEDGEMENTS

This thesis would not have been possible without the work of all the people on board the

cruises R/V Marion Dufresne 169, R/V Pelagia 64PE253 and R/V Belgica GENESIS “Pen

Duick”. By providing the cores and initial analysis of the petrophysical properties like the MS

and the XRF analysis on some of the cores. Special thanks goes to Rineke Gieles from the

Royal NIOZ at Texel (The Netherlands) who instructed and helped me with for the analysis

with the XRF AVAATECH. Many thanks to the given guidance from Lies De Mol for writing

the paper and lab work and to Danielle Schram for the introduction to the workings of the

laboratory at UGent. For the writing of the thesis I specially thank my promoter, Dr. David

Van Rooij, for the guidance and comments. I am grateful for all the help and company during

the lab work from Koen De Rycker

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