12-14 September, 2011, Lucknow - Earth Science India

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National Conference on Science of Climate Change and Earth’s Sustainability: Issues and Challenges ‘A Scientist-People Partnership’ 12-14 September, 2011, Lucknow GLACIAL-INTERGLACIAL PRODUCTIVITY FLUCTUATIONS FROM INDIAN ANTARCTIC POLAR FRONT OF SOUTHERN OCEAN Sunil Kumar Shukla 1,3 , Xavier Crosta 1 , M. Sudhakar 2 , G.N. Nayak 3 and Olivier Ther 1 1 UMR-CNRS 5805 EPOC, Université Bordeaux 1, Avenue des facultés, 33405 Talence Cedex, France 2 Ministry of Earth Sciences, Govt. of India, CGO Complex, New Delhi – 110 003 3 Department of Marine Sciences, Goa University, Taleigao Plateau, Goa – 403 206 Southern Ocean (SO) plays a pivotal role in regulating glacial-interglacial variability of atmospheric CO 2 due to deep-water masses protrusion and exchanges of gases with the atmosphere. The circumpolar connection in the SO permits a global-scale overturning (thermohaline) circulation to exist. Diatoms are dominant primary producers in SO and play decisive role in global cycling of silicic acid and carbon. Their cell sizes determine carbon sequestration efficiency as large cells export disproportionately large amount of carbon to the ocean floor due to their faster sinking and slower dissolution. Diatom Fragilariopsis kerguelensis is endemic to SO and its abundance reaches up to 90% of the total assemblages in surface sediments. Due to its abundance and good preservation in the sediments, it can be used as a potential proxy to decipher the cycling of nutrients and productivity fluctuations through glacialinterglacial cycles in SO. Therefore, we present biometric investigations of F. kerguelensis from a well dated core SO136-111 from Indian Antarctic Polar Front (APF) of SO over the last 40,000 years. Apical length, trans-apical length and valve area of F. kerguelensis are compared to its absolute and relative abundances as a proxy for the species productivity, along with sea surface temperature and sea ice duration reconstructed through diatom-based transfer function by applying modern analogue technique. Downcore records demonstrate that F. kerguelensis valves were longer and more abundant during the Holocene than during the glacial-deglacial period which is opposite to the records of Atlantic sector of SO. The opposite records in the Indian and Atlantic sectors of the Southern Ocean may indicate that different factors regulated diatom biology in these basins. In the Atlantic sector, longer F. kerguelensis during the last glacial period may have resulted from greater iron availability from melting ice [Cortese and Gersonde, 2007]. In the Indian sector where little ice melt was registered during the glacial period, longer F. kerguelensis during the Holocene may have resulted from the prevalence of warmer oceanic conditions and greater upwelling, which is more favourable for sexually-induced production of long initial cells. Instead of being opposite records, both these studies argue against the high opal fluxes recorded during 10
National Conference on Science of Climate Change and Earth’s Sustainability: Issues and Challenges ‘A Scientist-People Partnership’ 12-14 September, 2011, Lucknow the 10,000-17,000 years BP period in the Southern Ocean and attributed to enhanced wind-driven upwelling [Anderson et al., 2009]. WAS MID-BRUNHES CLIMATE SHIFT (MBCS) A RESPONSE TO ECCENTRICITY MINIMUM REPEATED AT EVERY 400-KA IN THE LAST ~3 MILLION YEARS? S.M. Gupta, B. Nagender Nath and M.B.L. Mascarenhas-Pereira National Institute of Oceanography, (CSIR), Dona Paula, Goa, 403 004 email: smgupta@nio.org , nagender@nio.org, mariab@nio.org Mid-Brunhes Climate Shift (MBCS) at ~300 ka (kilo-years) was first recorded as a conspicuous change in the variation of eolian grain size data from northwest Pacific in the early eighties. Since then, it is recorded in several climate proxies, i.e., planktonic foraminifers, radiolarians, magnetic susceptibility and the calcium carbonate content in the world oceans. Recently, the MBCS are reported from the radiolarians/g data, and subsequently in planktonic foraminiferal variation from equatorial Indian Ocean. Here, we analyze radiolarians/g dry sediment, organic carbon, particle mean size data from a sediment record in the central Indian Ocean, and the differential summer solar insolation at 30 o N-30 o S, precession, and Earth’s orbital eccentricity by continuous wavelet (Morlet) transform (CWT) spectrum to study the variation in the strength of power through the time, which are associated with orbital forcing responsible for MBCS event. Results suggest that strength of the spectral power associated with insolation cycles due to its precession (23-ka) and eccentricity (126-ka) components weakened during the MBCE. Thus, the weakening of insolation might have resulted in climatic events similar to MBCS in the geological past. Analyses of our data, published data, orbital eccentricity and the solar insolation suggest that possibly, there were possibly seven climate shifts (S1-S7) similar to MBCS, at almost regular interval of ~400-ka cycles in the last 3 million years (Ma). 11
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