1. "Amino acid dating and biomarkers of carbonate fossils in relict sediments of the continental shelf off Visakhapatnam, East coast of India"

Principal Investigator: Dr. Nittala S. Sarma, Professor of Marine Chemistry & Director (Retired), School of Chemistry, Andhra University, Visakhapatnam - 530 003

Co-Investigator: Dr. K. Mohana Rao, Scientist, NIO Regional Centre, Visakhapatnam

Time schedule: Three years

Project status: Completed (Final Technical Report submitted to MoES)

MoES Administrative Order No. & Date: DOD/11-MRDF/4/2/UNI/97-MRC dated 18.3.1999

MoES sanctioned project cost: Rs.32,93,598/-

Grants released by MoES: Rs.32,93,260/-

Project commencement date: 16.9.1999

Project completion date: March 2005

Objectives:

• To establish amino acid dating method as a routine tool for the dating of geological samples of biological origin and develop the facility as an alternative / preliminary to the AMS method,
• To establish the suite of biomarkers characteristic of marine sediments by a study of organic geochemistry,
• To investigate paleotemperature record using alkenone unsaturation index as a tool, and
• To extend the above studies on longer cores going back in age to the late Pleistocene to be collected from deeper bottoms of the Bay of Bengal and Arabian Sea.

Summary:
The principal objective of the project was to develop indigenous methodology for dating of marine sediments by the amino acid racemization procedure and to identify long chain alkenones in marine sediments for paleo SST estimation. The output is expected to be of extreme utility to a large section scientists working on paleoceanography. World over, only a few laboratories notably of the USA have the needed expertise in this frontier area of research. As experimental samples, it was originally visualised that offshore sediments of Visakhapatnam would be worked. However, befitting the dimension of the expected outcome, on suggestion from the Management Board, samples from all over the Indian marginal seafloor, with a wider age coverage and depositional conditions were investigated. The project work was carried out on 6 deep-sea cores (of the 13 collected), 2 each from the eastern Arabian Sea (cores 1 and 2), the western Bay of Bengal (cores 3 and 4) and the Indian Ocean (cores 5 and 6).
A new protocol was developed for the preparation of amino acid volatile esters suitable for gas chromatography. According to the reported sequence of reactions (adopted for the study), the first step is one pot conversion of the amino acids to carboxylate derivatives. This step is now divided into two sub-steps: (i) preparation of (amino) acid chloride initially under mild conditions, and (ii) the condensation of acid chloride with alcohol (isopropanol), also under mild conditions when carboxyltes are formed. We adopted the second step of the synthetic procedure as reported in literature. The reaction was standardized to produce reproducible results, and a large sample throughput. By this procedure, we were able to address the problem of artificial racemization also.
Using the method developed, we calibrated our ratios of amino acid enantiomers (D/L ratios) against radiocarbon ages in three cores. Racemization rate constants in core 1 are 0.85*10-5 for aspartic acid and 4.08*10-6 for valine. In core 2, the constant for phenyl alanine is 0.85*10-5. For aspartic acid, it is calculated as 1.4*10-5 in Holocene sediments, and 0.19*10-5 in the earlier sediments of this core. In core 4 (off Tuticorin), the amino acid racemization constant for aspartic acid is 4.44*10-5 yr-1  in the surface layer, 0.52*10-5 yr-1 in the mild region of the core (5-29 k yrs BP) and 0.11*10-5 yr-1 in the region below. For alanine, obtained for the first time in literature, the value is nearly uniform at 3.98*10-6 yr-1. With inflections absent in racemization rate curves, alanine seems to be a reasonably good candidate for amino acid dating. The racemization constants of valine (core 1) and phenyl alanine that are measured for core 2 are analysed for the first time in literature.
For applying amino acid racemization methodology for marine sediments beyond the radiocarbon's age capability, the amino acids of a number of the core 6 sectional sediments were derivatised. We also made a programme to subject monospecific forams for optimising the capability of the method and in the interpretation of depositional (calcareous/sileceous) characteristics, thermal (cf., the volcanogenic inputs), conditions, role of bacteria etc. But, since the crucial NPD (nitrogen-phosphorus detector) of the Gas chromatograph requiring replacement, these studies could not be extended.
Organic extracts of a large number of sectional sediments from 3 cores viz., cores 1, 4 and 6 were prepared. We targeted the long chain alkenones (LCAs), which are paleo SST proxies, brassicasterol, epibrassicasterol and dinosterol which are productivity proxies and pristane and phytane which are proxies of red-ox status of the depositional environment. By extensive GC-MS studies, we identified C37:2Me and C37:3Me as also C37:2Et of the LCAs and other biomarkers in our gas chromatograms.
In core 1, the profile of the alkenone unsaturation index Uk’37 and its derived paleo SST (based on reported calibration equation) and the Pr/Ph ratios are exact mirror images. Conditions representing high surface productivity (low pr/ph ratio) are also the conditions under which the index is lower (i.e., cooler surface water). In core 4, the unsaturation index ranged between 0.85 and 0.98, corresponding temperature range being 24.0-27.6°C. The derived SSTs are in agreement with the stable oxygen isotope data on Globigerinoides ruber, which themselves are in agreement with SPECMAP data. The results of Uk’37 and the derived paleo SSTs are also in agreement with reported alkenone-derived SSTs of nearby regions of the Indian Ocean. In core 6 (of the EIO region), the unsaturation index (0.77-0.85) and pr/ph ratio are also co-varying. This core dates back to ~300 kyr in age. The profile of Uk’37 matches with biostratigraphic profiles. Since no dates beyond the radiocarbon dates of surface sediments are available, alkenone index (Uk’37) turns out to be a good stratigraphic tool for this core.
We also performed supporting investigations to meet the exigency of non-availability of age-controls. The data needed for primary calibration of our results ensued when the project tenure was nearly at its end for radiocarbon dates and much after the tenure of the project for the stable oxygen stratigraphy. Our studies in this context have not only been helpful for meeting this eventuality but in deed, some new insights could be gained regarding the paleoceanographic conditions of the region.
The additional investigations we performed were: (i) bulk geochemistry (notably CaCO3, OC, Ba, Sr), (ii) trace metal (notably Cd, Ba, Sr, Mn, Nd and V) geochemistry of the planktonic foraminifer Globigerinoides sacculifer and (iii) biostratigraphy (profiles of 10 ecologically significant major planktonic foraminifera). Foram geochemistry is currently a HR tool investigated globally by paleoceaographers. Our studies on core 4 have revealed that during the glacial phase, the primary productivity was more intense (than the Holocene) due to convective currents that gave rise to a thicker, and cooler (i.e., nutrient richer) SML off the south east coast of India.
Papers published:  

1. Aminostratigraphy of sediments of the SW Bay of Bengal. Current Science, v. 85, pp. 435-436, 2003.
2. Geochemistry of biogenic elements in the eastern Arabian Sea sediments. Proc. AP Akad. Sci., v. 8, pp. 183-190, 2004.
Presented five papers at the symposia.