• Ajai K Rai

      Articles written in Journal of Earth System Science

    • Pleistocene oceanographie changes indicated by deep sea benthic foraminifera in the northern Indian Ocean

      Ajai K Rai M S Srinivasan

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      An attempt has been made to understand the Pleistocene bottom water history in response to the paleoclimatic changes in the northern Indian Ocean employing quantitative analyses of deep sea benthic foraminifera at the DSDP sites 219 and 238. Among the 150 benthic foraminifera recorded a few species show dominance with changing percent frequencies during most of the sequence. The dominant benthic foraminiferal assemblages suggest that most of the Pleistocene bottom waters at site 219 and Early Pleistocene bottom waters at site 238 are of North Indian Deep Water (NIDW) origin. However, Late Pleistocene assemblage at site 238 appears to be closely associated with a water mass intermediate between North Indian Deep Water (NIDW) and Antarctic Bottom Water (AABW).

      Uvigerina proboscidea is the most dominant benthic foraminiferal species present during the Pleistocene at both the sites. A marked increase in the relative abundance ofU. proboscidea along with less diverse and equitable fauna during Early Pleistocene suggests a relative cooling, an intensified oceanic circulation and upwelling of nutrient rich bottom waters resulting in high surface productivity. At the same time, low sediment accumulation rate during Early Pleistocene reveals increased winnowing of the sediments possibly due to more corrosive and cold bottom waters. The Late Pleistocene in general, is marked by relatively warm and stable bottom waters as reflected by low abundance ofU. proboscidea and more diverse and equitable benthic fauna.

      The lower depth range for the occurrence ofBulimina aculeate in the Indian Ocean is around 2300 m, similar to that of many other areas.B. aculeata also shows marked increase in its abundance near the Pliocene/Pleistocene boundary while a sudden decrease in the relative abundance ofStilostomella lepidula occurs close to the Early/Late Pleistocene boundary.

    • Late Quaternary changes in surface productivity and oxygen minimum zone (OMZ) in the northwestern Arabian Sea: Micropaleontologic and sedimentary record at ODP site 728A

      Ajai K Rai S S Das

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      Changes in the abundance of selected planktic foraminiferal species and some sedimentological parameters at ODP site 728A were examined to understand the fluctuations in the surface productivity and deep sea oxygenation in the NW Arabian Sea during last ∼540 kyr. The increased relative abundances of high fertility taxa, i.e., Globigerinita glutinata and Globigerina bulloides mainly during interglacial intervals indicate intense upwelling. Strong SW summer monsoon probably increased the upwelling in the western Arabian Sea during interglacial intervals and caused high surface productivities due to the lateral transport of eutrophic waters. Most of the glacial periods (i.e., MIS 2, 4, 6, 8 and 12) are characterized by higher relative abundances of Neogloboquadrina pachyderma and Neogloboquadrina dutertrei associated with Globigerinoides ruber. The more stratified condition and deep mixed layer due to increased NE winter monsoon are mainly responsible for the higher relative abundances of N. pachyderma during glacial periods. Some of the glacial intervals (i.e., MIS 6 and 8) are also characterized by pteropod spikes reflecting deepening of aragonite compensation depth (ACD) and relatively less intense oxygen minimum zone (OMZ) in this region due to deep sea mixing and thermocline ventilation, and relatively less intense surface productivity during winter monsoon. The interglacial periods are largely devoid of pteropod shells indicating more aragonite dissolution due to increased intensity of OMZ in the northwestern Arabian Sea.

      In general, the interglacial periods are characterized by low sediment accumulation rates than the glacial intervals. On an average, the total biogenic carbonate percentages were higher during interglacial and during periods of higher surface productivity. Most terrigenous material was trapped on shelf during intervals of high sea level stands of interglacial, whereas more erosion of shelf increased the sedimentation rates during glacial periods. In addition, the fragmentation record may be the result of changes in intensity and vertical distribution of the OMZ with time. During glacial intervals, the lower boundary of the OMZ probably was in a shallower position than during interglacial periods, when dissolution increased as a result of higher organic production. The higher rates of sinking organic matter result into a stronger OMZ as oxygen is used to disintegrate the organic matter. This process lowers the $p^H$ of water which results into increased dissolution of calcium carbonate.

    • Paleoenvironmental significance of clay mineral assemblages in the southeastern Arabian Sea during last 30 kyr

      Siddhartha Sankar Das Ajai K Rai Vaseem Akaram Dhananjai Verma A C Pandey Koushik Dutta G V Ravi Prasad

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      A gravity core SK-221 recovered from the southeastern Arabian Sea near Laccadive–Chagos Ridge was examined to identify the sources of detrital clay minerals and to decipher paleoenvironmental changes for the last 30 kyr. The clay mineral assemblages predominantly consist of illite, kaolinite and chlorite with small amounts of smectite. Quartz, feldspar and occasionally gibbsite are the clay-sized non-clay minerals present in the examined section. The detrital clay minerals primarily originated from the hinterland and were supplied to the present site by the numerous small rivers draining western India during preglacial and Holocene periods, and partly by the strong reworking of Indian continental shelf during glacial period. The low values of humidity proxies (kaolinite content, kaolinite to illite and smectite to illite ratios) and better illite crystallinity indicate relatively weak summer monsoon condition that resulted in reduced chemical weathering during glacial period, which was interrupted by a discrete event of winter monsoon intensification at ∼20–17 ka. The increased kaolinite content, higher values of humidity indices and poorer illite crystallinity reflect high humidity that resulted in strong hydrolysis activity during the preglacial and Holocene periods. The increased CaCO3 during above periods also indicates less terrigenous dilution and intensified southwest monsoon-led upwelling which result in higher surface biogenic productivity. The characteristic clay mineral associations broadly suggest dry to semi-drier conditions during Heinrich Events H1, H2, and H3 and also during Younger Dryas. The low values of biogenic carbonate and organic carbon also indicate low productivity associated with weak summer monsoons during Heinrich Events. Abrupt increased humidity was recorded at 15–12.7 ka (Bølling/Allerød Event) sandwiched between two lows of Heinrich Events. Cycles of millennial timescale variations 2300, 1800, 1300 and 1000 yr have been observed from the clay mineralogical data. All the cycles observed in the monsoonal climate appear to be part of global oscillations.

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