Participants

V.Ramaswamy               Principal Investigator

Dr.D.V.Borole

Dr. P.D.Naidu

Objectives

The main objectives of this study are

1. To estimate present day Burial Fluxes of carbon (C.org and C.carb) in the

northern and western Bay of Bengal.

2. To study variation in temperature, salinity, ocean circulation, chemistry

and primary productivity changes during last 22000 years in response to

changes in monsoon intensity and freshwater influx.

3. To determine the accumulation rates of various elements in sediments

Background

The principal purpose of the process studies of JGOFS is to elucidate mechanisms controlling carbon cycle in different parts of the world oceans. It includes the need to define the nature, rates and controls of material transfer at the sediment-water interface (SCOR 1990). One of the main aims of the proposed study is to measure the burial fluxes of carbon in the Bay of Bengal (coastal sediments and in the Bengal Fan) and to relate it to sedimentation rate of particulate organic carbon in the water column and processes controlling its preservation at the sediment-water interface.

It has been suggested that changes in ocean productivity may partly account for atmospheric CO2 changes on glacial-interglacial time scales (Broecker and Peng, 1982; Dymond and Lyle, 1985; Mix, 1989). Burial of carbon, both as organic carbon and calcium carbonate, is an important process affecting the amount of carbon in the ocean -atmosphere system (Berner , 1982, 1991). There are a number of studies on glacial-interglacial changes in primary productivity in various regions of the ocean. These studies have shown that higher productivity occurred in many low and mid-latitude regions during the last glacial maximum. The decrease in Holocene accumulation rates of carbon has been attributed to diminished carbon export as a result of decrease in ocean upwelling as well as mixing of nutrient rich deep waters (Sarnthein, 1988; Mix, 1989). Similar studies at higher latitudes have shown different results. In the NE Pacific (Lyle et al., 1992) and high southern latitudes (Mortlock et al., 1991) productivity is comparatively lower during the glacials. Since primary productivity in the Bay of Bengal is to a large extent influenced by monsoon processes (Sirocko et al., 1991; 1992, Clemens et al., 1990), the productivity patterns in the Bay may be different compared to the other regions of the world oceans.

The Bengal Fan, the biggest submarine fan in the world is one of the major depo-center of carbon in the oceans (Ittekkot, 1985). The Bay of Bengal is one of the high productive areas of the world oceans and very high organic carbon fluxes of more than 3 g m-2 y-1 have been recorded in sediment trap experiments in the northern and central Bay of Bengal (Ramaswamy and Nair 1994). Primary productivity here is influenced by monsoon winds as well as freshwater influx from land (Ittekkot et al., 1991; Rao 1977). Organic carbon accumulation rates in the Bay of Bengal sediments should be very high because of relatively shallow depth as well as high as sedimentation rates. Moreover, high terrgienous matter influx can enhance organic carbon fluxes in the water column because of the "ballast effect" (Ittekkot, 1992). However, there is hardly any reliable information on the amount of carbon being buried in the Bengal Fan. Earlier studies have reported organic carbon percentages in the western Bay of Bengal but accumulation rates of organic carbon during the Holocene are few. It is therefore proposed to measure the present day burial fluxes of carbon in the JGOFS study area of the Bay of Bengal.

The burial of carbon in sediments should vary in response to variation in monsoon winds and rainfall. It is known that monsoon intensity has varied in response to orbital forcing, and that the Asian summer monsoon was weak during the LGM but stronger than present between 6000-12000 BP (COHMAP, 1988). However, the orbital changes of the Earth may not account the summer monsoon variability at centurion time scales (Naidu and Malmgren, 1995; 1996a). Freshwater influx from rivers should have been more during degalciation of Himalayan glaciers. Nutrient concentration in the column will effect the morphology and coiling direction of planktonic foraminifera in the northern Indian Ocean (Naidu and Malmgren, 1995b; 1996b). How has burial fluxes of carbon responded to these changes is the other main objective of this study. Increase in wind stress normally results in enhanced productivity in the ocean but it is not clear how productivity the Bay of Bengal will respond to changes in freshwater influx. Freshwater discharge from rivers bring in new nutrients, but at the same time it forms a strong stratified layer and prevents upwelling of nutrient rich waters from below.

Methods

Field Work

For studying present day burial fluxes of carbon it is proposed to collect short cores (0.5 to 1 m) with a Multicore or Spadecore along 4 zonal and 1 meridonal section in the western half of the Bay of Bengal (See Fig. 1) The cores will be collected from the continental shelf, continental slope and rise of the east coast of India as well as on the Bengal Fan proper. For paleo-fluxes and paleo-productivity studies 2 to 3 long cores (8 to 12 m) using gravity or piston corer will be collected on topographic highs. These cores will be sub-sampled and freeze dried onboard the ship. Present day and paleo-fluxes of carbon will be determined by the following laboratory studies.

Laboratory Studies

The short cores (0.5 to 1 m) will be sub-sampled and dated using Pb 210 chronology and amount of organic carbon and carbonate carbon (CaCO3) content in the sediments will be measured using elemental analyzer (CHN analysers).

The long cores collected from topographic highs will be dated using AMS C¹⁴ and O¹⁸ chronology. ¹⁴C dates on picked forams will be carried out to eliminate the risk of contamination with older detrital carbonates. ¹⁸O stratigraphy will be carried out (at the national AMS facility or on payment basis) to establish a glacial interglacial stratigraphy specifically to estimate stage 1/2 and 2/3.

First order components like percentages of calcium carbonate, organic carbon, opal and lithogenic material will be analyzed. Downcore variation in C.org/C carb and carb/opal rations will be measured. At present C.org/C carb ratios in sediment traps is 2 in the northern Bay of Bengal and 1 in the southern Bay of Bengal. The sediment trap samples have already been collected.

Down core variation in Ba, Mn, Cu, V which is an indicator of productivity and anoxicity and Al; Ti and Al/Ti ratios which is an indicator of lithogenic input will be studied. Trace element analysis will be done on ICP-AES at NIO. The present day accumulation rates of these elements will be measured by analysis of sediment trap samples collected in the northern, central and southern Bay of Bengal.

Planktonic and benthic foraminifera will be picked out for oxygen isotopes and to study variation in their taxonomy and morphology. The following parameters will be used as paleo-climatic indicators: Relative abundance of various species, morphological variation, coiling direction, size and shape variation and surface structure.

In addition the following sedimentological and mineralogical variation will be studied. Mass accumulation rates (biogenic and lithogenic), dry density, grain size, feldspar/quartz, quartz/mica/palygorskite, illite/smectite, chlorite/kaolinite, quartz abundance and accumulation rates.

Temporal and spatial comparison will be made between carbonate accumulation, planktonic foram taxonomy and morphology, organic carbon/ carbonate/ carbon and sedimentological, mineralogical and geochemical indices. Complimentary studies on d 13C , Cd/Ca Sr/Ca in CaCO3 etc. will be measured by PRL scientist. From this it will be possible to measure burial fluxes and paleo-fluxes of carbon and paleo-productivity and relate them to variation in regional paleo-climate and paleo-monsoon intensity.

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