Application of Carbon and Nitrogen Isotopes to Past Productivity Determination

Shubham Tripathi*, Manish Tiwari, Siddesh Nagoji National Centre for Antarctic and Ocean Research

Headland Sada, Vasco-da-Gama, Goa 403 804, India

The isotopes of carbon and nitrogen can play an important role

in deciphering the past productivity changes and its sources.

The present study is carried out in the eastern Arabian Sea. This

region is important from the view point of the past Indian

summer (southwest) monsoon (ISM) reconstruction.

Upwelling is also reported during the ISM season related to its

strength, which also enhances productivity.

The present study is an attempt to decipher past productivity

changes using carbon and nitrogen content and their isotopes

from a marine sediment core from the eastern Arabian Sea off

Goa. This multi-proxy approach leads us to find four distinct

phases of enhanced and reduced productivity related to

monsoon strength.

Introduction Methods

Results and discussion

Agnihotri et al (2008), Variability of subsurface Denitrification

and surface productivity in the coastal eastern Arabian Sea

over the past seven centuries. The Holocene 18, 755–764

Meyers, P.A., (1997), Organic geochemical proxies of

paleoceanographic, paleolimnologic, and paleoclimatic

processes. Organic Geochemistry 27, 213–250.

R. Ramesh and Manish Tiwari, Significance of stable oxygen

and carbon isotopic compositions of individual foraminifera O.

universa d’Orbingy in a sediment core from the eastern

Arabian sea; In: Micropaleontology: Applications in

Stratigraphy and Paleoceanography (Ed. D.K. Sinha), , Narosa

Publishing House, New Delhi, pp. 331-346 .

References Conclusions

Paleoproductivity analysis carried out in the core

SK274/4G reveals that the organic matter at the core site

reflects both marine origin and terrestrial supply.

This can be related to the summer monsoon

strengthening, which increases marine productivity via

upwelling and also brings in terrestrial organic matter via

surface runoff.

Much higher resolution study needs to be carried out in

this core to fully comprehend the paleoproductivity

fluctuations that would illuminate the variability in the

past summer monsoon strength

Geological carbon cycle

Samples at an interval of 1cm is grinded manually

to fine powder to make it homogeneous.

Powdered sample were divided into two parts of

which one part is treated with 2N HCl to remove

inorganic carbon and by washing with Elix water we

get sediments having only organic carbon.

The carbon and nitrogen isotope values were

measured.

Isoprime Stable Isotope Ratio Mass Spectrometer

in continuous-flow mode coupled with an EA

(Isoprime, Vario Isotope Cube).

The external precisions on δ13C & δ15N are

±0.05‰ & ±0.05‰

The external precisions for C% & N% are ±0.27% &

±0.23% respectively (1σ standard deviation)

obtained by repeatedly running Sulfanilamide as

the standard (n=26). δ13C values are reported with

respect to V-PDB and δ15N values are reported with

respect to air N2.

The global Carbon Cycle

The total organic carbon (%) and %N contents vary from 1

to 3% and from 0.1 to 0.2% respectively.

Phase ‘A’ and Phase ‘C’ corresponding to 0-100 cmbsf

and 190-260 cmbsf respectively are the period when the

productivity declined indicating reduced monsoon

strength.

In contrast, Phase ‘B’ and Phase ‘D’ corresponding to

100-190 cmbsf and 260-356 cmbsf respectively show

clearly enhanced productivity indicating strengthened

monsoon.

δ15N varies from 3 to 8 ‰ with the average of 6.2%. Such

a high δ15N can be related to increased denitrification,

which in turn is controlled by the Indian Summer Monsoon

induced productivity increase.

The global nitrogen cycle

Based on the above facts during the 274th

expedition of Sager Kanya, a 359 cm long core

was recovered from off Goa (13 ̊ 59 ̍45 ̎ ̊ N; 72 ̊

0 ̍24 ̎ ̊ E) on which the present study is carried

out.

I thank Dr. Manish Tiwari for his support in the lab, I would also like to thank director NCAOR for his

encouragement and acknowledge BVS-2015 for giving an opportunity to present this poster.

*shubhamtripathi208a@gmail.com

Acknowledgement

• Down core variation in different productivity proxies

The C/N ratio can provide information about the

source of organic matter as the marine and

terrestrial organic matters have different ratios.

The marine organic matter is abundant in proteins

and lipids they have high content of nitrogen as

compared to terrestrial organic matter which is rich

in cellulose and lignin.

C/N values of algae generally range between 4 and

10, whereas, terrestrial C/N ratios are generally

higher than 20.

C/N ratio of the present study varies in the range

8-32% which is indicative of marine organic matter

(upwelling-induced) along with terrestrial influx via

surface run-off during the ISM season due to its

coastal location.

• Source characterization of organic matter using δ13Corg and C/N ratio

The geologic component of the carbon cycle is one of the most

important determinants of the amount of carbon in the

atmosphere, and thus of global temperatures.

Most of the earth's carbon is stored inertly in the earth's

lithosphere Of the carbon stored in the geosphere, about 80% is

limestone and its derivatives, which form from the sedimentation

of calcium carbonate stored in the shells of marine organisms

and the remaining 20% is stored as kerogens.

The global nitrogen cycle is required for understanding the

process leading to the nitrogen distribution in ocean and the

geological environment.

Nitrogen has two isotopes 14N and 15N with abundance of 99.64%

and 0.36% respectively. 99% of the known nitrogen on the earth

system is present as atmospheric N2 or is present in the ocean

as dissolved nitrogen.

As the nitrogen is found in several oxidation states so it is

suitable for understanding natural variation among different

phases. The variation of nitrogen isotope in biological system

was first reported by Schoenheimer and Rittenberg (1939).