Carbon isotopic composition of waters in the South Atlantic, tracing water
masses and biological activity
Alex Piotrowski and Jo Clegg
University of Cambridge
Thanks to Alex Thomas, Malcolm Woodward, Mike Hall, & James Rolf.
Glacial:
Holocene:
Curry and Oppo, 2005
Seawater 13C from GEOSECSand WOCE
Benthic foraminiferal 13C from various studies
Ravelo and Hillaire-Marcel, 2007
Integrating Nd and C isotopes
Benthic 13C → deep ocean circulation + carbon cycling + air-sea gas exchangeNd isotopes → deep ocean circulation + REE cycling
Can Nd isotopes help to deconvolve the different signals contributing to the benthic 13C record?
Charles et al., 1996, Piotrowski et al., 2005
Site of those records…
Lower Nd and Higher 13C
Higher Nd and Lower 13C
Site of those records…
Mackensen 2012
Mackensen 2012
Methods
• Samples collected early to minimize gas exchange, glass containers overfilled to prevent air bubbles.
• Sealed poisoned (with mercuric chloride) seawater samples stored in refrigerator
• Subsamples run on Thermo MAT253 mass spectrometer with Gas Bench
d13C with depth for all stations (except 5)
0
1000
2000
3000
4000
5000
6000
0.40 0.90 1.40
d13C (per mil)
dep
th (
m)
station 8
station9
station 1
station 2
station 10
station 3
station 4
station 11
station 6
station 7
41S 18E depths 11 to 4395 Kroopnick 1980 has d13C -0.24 to 1.74‰ this is nearest to our station 7 which is (5 to 3531m) d13C -0.426-0.6‰ (so our d13C lower than this of Kroopnick’s; but similar sorts of values)
Cruise D357 Cape Basin
d13C with depth for all stations (except 5)
0
1000
2000
3000
4000
5000
6000
0.40 0.90 1.40
d13C (per mil)
dep
th (
m)
station 8
station9
station 1
station 2
station 10
station 3
station 4
station 11
station 6
station 7
41S 18E depths 11 to 4395 Kroopnick 1980 has d13C -0.24 to 1.74‰ this is nearest to our station 7 which is (5 to 3531m) d13C -0.426-0.6‰ (so our d13C lower than this of Kroopnick’s; but similar sorts of values)
Cruise D357 Cape Basin
nearcoast
test
12
3ss
4
56ss7
8 (0.5)
9 (1.5)
10 (2.5)
12 (3.5)
11ss (4.5)
13 (0.75)
First leg
Second leg
The mechanism for removing the biological contribution from the 13C is due to Broecker and Maier-Reimer 1992. They find the 1.1 slope with PO4 and use the -2.7 , an arbitrary constant, to bring deep water values from Pacific and Indian Ocean to 0 ‰.
13Cas = 13C + (1.1 x PO4) -2.7 (1)
Lynch-Stieglitz and Fairbanks (1994) adopted the notation 13Cas
– ‘the air/sea exchange signature’:
13Cas = 13C - (2.7 - 1.7 x PO4) (2)Lynch-Stieglitz at al (1995) for Antarctic
The numbers in this formula are not agreed. Other possibilities are:
13Cas = 13C - (1.92 - 0.7 x PO4) (3)Mackensen et al (1993) taking the regression computed by Kroopnick (1985).
13Cas = 13C - (2.4 – 0.93 x PO4) (4)Charles et al 1993
But in more recent papers, Mackensen uses the Broecker and Maier-Reimer equation (1).
Here we use: 13Cas = 13C + (1.1 x PO4) -2.7 (1)and PO4 in μmol/L rather than μmol/kg, to match most studies.
d13C air-sea for all stations(station 3 no line through as two important data points missing)
0
1000
2000
3000
4000
5000
6000
-2 -1.5 -1 -0.5 0 0.5 1
depth
d1
3C
(a
ir-s
ea
)
station 8
station9
station 1
station 2
station 10
station 3
station 4
station 11
station 5
station 6
station 7
nearcoast
Southern Ocean surface has highest 13Cas, sub tropical gyre waters usually ~ -1 ‰
We then confirm the water mass identification using conservative propertiesSalinity and Potential temperature
Salinity(again leave out station 5 as so much data missing)
0
1000
2000
3000
4000
5000
6000
34 34.2 34.4 34.6 34.8 35 35.2 35.4 35.6 35.8
Salinity (psu)
Dept
h (m
)
Potential temperature (again leave out station 5 as so much data missing)
0
1000
2000
3000
4000
5000
6000
0.0000 5.0000 10.0000 15.0000 20.0000
Temp (deg C)
Dept
h (m
)
station 8
station 9
station 1
station 2
station 10
station 3
station 4
station 11
station 6
station 7
schneidl max
schneidl min
Max and Min for NADW: Pink lines Schmiedl at al 1997Grey lines Kroopnick 1980a
Max and Min for NADW: Dashed lines Schmiedl at al 1997
d13C air-sea for all stations(station 3 no line through as two important data points missing)
horizontal lines: blue NADW, pink AAIW, green LCDW
0
1000
2000
3000
4000
5000
6000
-2 -1.5 -1 -0.5 0 0.5 1
d13C (air-sea)
dep
th
AAIW
NADW
AABW
blue - east of MAR; red - west of MAR
0
1000
2000
3000
4000
5000
6000
0.00 0.50 1.00 1.50 2.00 2.50
d13C (per mil)d
ep
th (
m)
Comparison of eastern and western South Atlantic profiles
Red = west of mid-Atlantic Ridge Blue = east of mid-Atlantic Ridge
blue - east of MAR; red - west of MAR
0
1000
2000
3000
4000
5000
6000
0.00 0.50 1.00 1.50 2.00 2.50
d13C (per mil)d
ep
th (
m)
Comparison of eastern and western South Atlantic profiles
Red = west of mid-Atlantic Ridge Blue = east of mid-Atlantic Ridge
blue - east of MAR; red - west of MAR; triangles - silica
0
1000
2000
3000
4000
5000
6000
0.00 0.50 1.00 1.50 2.00 2.50
d13C
de
pth
(k
m)
0 50 100 150 200Silica
dissolved silica13C
blue - east of MAR; red - west of MAR PO4 triangles
0
1000
2000
3000
4000
5000
6000
0.00 0.50 1.00 1.50 2.00 2.50
d13C
de
pth
(m
)
0 0.5 1 1.5 2 2.5 3
PO4
13CPO4
Blue East of mid-Atlantic ridge
Yellow D357 cruise
RedWest of mid-Atlantic ridge
0
1000
2000
3000
4000
5000
6000
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0
d13C air sea (per mil)d
ep
th (
m)
aaiw
nadw
lcdw
weddell seadeep water?
0
1000
2000
3000
4000
5000
6000
-2.00 -1.00 0.00 1.00 2.00
d13C air-sea
de
pth
(m
)
WSDW from Provost et al, 1990 depth in Argentine Basin 4500-6000
Red = west of mid-Atlantic Ridge Blue = east of mid-Atlantic Ridge
aaiw
nadw
lcdw
0
1000
2000
3000
4000
5000
6000
34.0 34.5 35.0 35.5 36.0 36.5 37.0
salinity pss
dept
h
vertical lines potential temp values for cape basinaaiw max min green nadw violet lcdw pinkdepth ranges shown determined for D357
nadw cape basin
aaiw cape basin
lcdw cape basin
0
1000
2000
3000
4000
5000
6000
-5 0 5 10 15 20 25 30
potential temp
dept
h
Red = west of mid-Atlantic Ridge Blue = east of mid-Atlantic Ridge
Added Mackensen 1993 data as white squares.
Comparison with data of Lynch-Stieglitz et al 1995
-0.50
0.00
0.50
1.00
1.50
1.00 1.50 2.00 2.50 3.00
13Cas
PO4 (mol/kg)
WOCE A10 track25S to 25S
Data from JC068 stations 8-21
-1
4
9
14
19
24
0 0.5 1 1.5 2 2.5 3
PO4 (uM/kg)
Mackensen (2012)Bottom water values
lat/long water d13Cdepths only 1000-2000 (AAIW in Cape Basin)
-80
-70
-60
-50
-40
-30
-20
240 290 340 390
lat (recall 0=360)
lon
g (
S)
-ve
0-0.2
0.2-0.4
0-4-0.6
0.6-0.8
0.8-1
ours 0.4--0.6
ours 0.6--0.8
lat/long water d13C air-seadepths only 1000-2000 (AAIW in Cape Basin)
-80
-70
-60
-50
-40
-30
-20
240 290 340 390
lat (recall 0=360)
lon
g (
S)
-ve
0-0.2
0.2-0.4
0-4-0.6
ours 0.2-0.4
ours 0.4-0.6
d13C
d13Cas
Triangles: our valuesSquares: Mackensen 2012
AAIW
20W 30E70W120W
20W 30E70W120W
Our coretop valueson benthic foramsare 1.24 ‰ while overlying seawater is 0.73 ‰
South America
South America
SouthAfrica
SouthAfrica
d13C
lat/long water d13Cdepths only 2000-3500 (NADW in Cape Basin)
-80
-70
-60
-50
-40
-30
-20
240 260 280 300 320 340 360 380 400 420
lat (recall 0=360)
lon
g (
S)
-ve
0-0.2
0.2-0.4
0-4-0.6
0.6-0.8
0.8-1
0.2-0.4 ours
0.6-0.8 ours
0.8-1 ours
lat/long water d13Cair-seadepths only 2000-3500 (NADW in Cape Basin)
-80
-70
-60
-50
-40
-30
-20
240 260 280 300 320 340 360 380 400 420
lat (recall 0=360)
lon
g (
S)
0-0.2
0.2-0.4
0-4-0.6
0--0.2 ours
0.2-0.4 ours
NADW
d13Cas
Triangles: our valuesSquares: Mackensen 2012
20W 20E60W120W 40E 60E40W80W100W 0
20W 20E60W120W 40E 60E40W80W100W 0
South America
South America
SouthAfrica
SouthAfrica
Our coretop valueson benthic foramsare 0.85 ‰ while overlying seawater is 0.90 ‰
lat/long water d13Cdepths only below 3500 (LCDW in Cape Basin)
-80
-70
-60
-50
-40
-30
-20
240 260 280 300 320 340 360 380 400 420
lat (recall 0=360)
lon
g (
S)
-ve
0-0.2
0.2-0.4
0-4-0.6
0.6-0.8
0.8-1
ours 0.2--0.4
ours 0.4--0.6
ours 0.6--0.8
ours 0.8--1
lat/long water d13C air-seadepths only below 3500 (LCDW in Cape Basin)
-80
-70
-60
-50
-40
-30
-20
240 260 280 300 320 340 360 380 400 420
lat (recall 0=360)
lon
g (
S)
-ve
0-0.2
0.2-0.4
0-4-0.6
0.6-0.8
0.8-1
ours 0--0.2
ours 0.2--0.4
ours 0.4--0.6
ours 0.6--0.8
d13C
d13Cas
LCDW
Triangles: our valuesSquares: Mackensen 2012
20W 20E120W 40E40W80W100W 060W
20W 20E120W 40E 60E40W80W100W 060W
RC11-83 benthic d13Chas a “coretop” (~5ka)value of 0 ‰
Cape Basin bottomwater is clearly more positive
South America
South America
SouthAfrica
SouthAfrica
Conclusions
• Seawater 13C clearly shows water mass structure, both surface hydrography and deep water.
• Air-sea exchange component of strong 13C gradient in surface ocean near Agulhas likely has subtropical (Indian) source.
• The 13C offset at NADW/AABW boundary in west is shallower than in east
• Nutrient signal? May not appear in 13C air-sea exchange component. Need to check potential density.
• Coretop calibration to benthic foraminifera 13C remains questionable.
These are all Mackensen 2012 data for d13C of forams incoretops vs d13C in bottom water. And now we can add our two more – the big stars!They don’t seem to match much but then neither do Mackensens.
lat/long water d13Cwith forams superimposed
-80
-70
-60
-50
-40
-30
-20
240 260 280 300 320 340 360 380 400 420
lat (recall 0=360)
lon
g (
S)
-ve
0-0.2
0.2-0.4
0-4-0.6
0.6-0.8
0.8-1
forams -ve
forams 0-0.2
forams 0.2-0.4
forams 0.4-0.6
forams 0.6-0.8
forams 0.8-1
forams >1
ours 0.8-1
ours >1
And the big circles are our bottom water samples just above the forams.(Note very few of Mackensens data have water taken just above the forams)
lat/long water d13Cwith forams superimposed
-80
-70
-60
-50
-40
-30
-20
240 260 280 300 320 340 360 380 400 420
lat (recall 0=360)
lon
g (
S)
-ve
0-0.2
0.2-0.4
0-4-0.6
0.6-0.8
0.8-1
forams -ve
forams 0-0.2
forams 0.2-0.4
forams 0.4-0.6
forams 0.6-0.8
forams 0.8-1
forams >1
ours 0.8-1
ours >1
our w ater 0.6-0.8
our w ater 0.8-1
Worksheets provided
• ‘jo's current look at results-xtra DIC’
• ‘jo's current look at results’
• ‘just before Oxford jc068 geotraces’
• jc068 geotraces
• jc068 geotraces TDD
• and try for final
Comparisons with other data:Working out d13Cas using PO4 and d13C of Mackensen 1993 at 44S 10.3E AAIW 0.52 to 0.59‰NADW 0.003 to 0.053‰LCDW -0.11 to 0.022‰The discrepancy in values is disappointing as I worked them out. I have checked the units and they seem to be the same – umol/L.
Lynch-Stieglitz and Fairbanks 1994 had d13Cair-sea for Atlantic all negative (their values were from Cd not PO4) and I can’t see where they get their modern ocean values from. We could perhaps say our values will be higher, being pulled up by mixing with positive Antarctic. But surely not this much.
( Mackensen 2012 says NADW characterised by negative d13Cair-sea, despite the values above 0.003 to 0.053‰)
So we find d13Cair-seaAAIW -0.55 to 0.1‰NADW 0.21 to 0.42‰LCDW 0.19 to 0.77‰
AAIW 500-2000NADW 2000-3500LCDW below 3500
For surface waters our d13C and PO4 values are similar to Broecker and Maier-Reimer S Atlantic; despite possibly having more anthropogenic effects that are not accounted for. (We don’t have their very high phosphate level ones.)
This suggests our values of d13Cas are different to other work because I have calculatedthe equation wrong. But I’ve checked several times!
Note the values of d13Cas are similar to those of mackensen 2012 –slides at end
If we decide to use this slide it needs redoing and to be put somewhere else as it
uses JC068 cruise data and up to now all we have discussed is D357
surface (highest value we have)d13C vs PO4
0
0.5
1
1.5
2
2.5
0 0.2 0.4 0.6 0.8 1 1.2
PO4
d13
C
d13C vs DIC concentration for all stations grey shallow, blue deep
-2
-1.5
-1
-0.5
0
0.5
1
2000 2100 2200 2300 2400 2500
DIC
d13
C a
ir s
ea
station 8station 1 deepstation 1 shallowstation 9 deepstation 9 shallowstation 2 deepstation 2 shallowstation 10 deepstation 10 shallowstation 3 deepstation 3 shallowstation 4 deepstation 4 shallowstation 5 deepstation 5 shallowstation 7 deepstation 7 shallowstation 6 deepstation 6 shallow
As we have not got DIC for second cruise perhaps delete this slide?
NADWAAIW
LCDW
Green max min of AAIW; pink of NADW;blue of LCDW from Schmiedl at al 1997 for Cape Basin
)Figures below right: Provost et al 1990 for Argentine BasinLeft: Memery 2000 for southwest Atlantic
-2
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
33.000 34.000 35.000 36.000 37.000
salinity pss
po
ten
tial
tem
p
AAIWNADW
LCDW
WOCE A11 track30S to 45S
WOCE A10 track25S to 25S
WOCE A10 track25S to 25S
Data from JC068 stations 8-21
-1
4
9
14
19
24
0 0.5 1 1.5 2 2.5 3
PO4 (uM/kg)
So the PO4 and potential temp data agree quite well with WOCE.But we have problems that it does not lay on Lynch Steiglitz fig 6 – next slide