Organic carbon isotopic study of Bay of Bengal sediments: Reconstruction of vegetational changes during last 30 ka

Abstract

Sedimentary organic carbon archived in the Bay of Bengal are mainly terrigenous in nature, and hence, serve as reliable proxy for reconstructing past changes in terrestrial vegetational patterns. Here, we have investigated total organic carbon (TOC) and stable organic carbon isotopic value of a sediment piston core (VM29-19PC; water depth ~3182 m; core length~ 7 m) from the western Bay of Bengal was investigated to assess climatic sensitivity of vegetation during last glacial and interglacial period. The TOC values of these sediments vary from 0.74 to 1.58% (mean: 1.17%), whereas the 13Corg values vary between -21.29 and -16.81 (average: -19 ± 1‰; n = 33). The 13Corg values exhibit significant temporal variation with enriched values being observed during the LGM. The temporal trends of 13Corg values point to relative expansion of C4 plants during the last glacial maxima (LGM) period, which are characterized by high 13Corg values than that of the C3 plants. This could have resulted as a combined effect of low atmospheric CO2 and arid conditions. The reduced SW monsoon during LGM should have caused arid conditions over the river basins to support the growth of C4 plants. Increase in the expansion of C4 plants can cause lower moisture content and increase the temperature-pressure difference over the land due to their lower transpiration and high-water use efficiency. This must have strengthened the ISM over the land, increasing the abundance of C3 plants and hence, caused the post-LGM decline in the C4 plants.