Planktic foraminiferal assemblages and stable isotope records of the southern Japan Sea during the last 27,000 years

Abstrac

Indian Monsoonal Variations During the Past 80 Kyr Recorded in NGHP-02 Hole 19B, Western Bay of Bengal: Implications From Chemical and Mineral Properties

金沢大学理工研究域地球社会基盤学系Detailed reconstruction of Indian summer monsoons is necessary to better understand the late Quaternary climate history of the Bay of Bengal and Indian peninsula. We established a chronostratigraphy for a sediment core from Hole 19B in the western Bay of Bengal, extending to approximately 80 kyr BP and examined major and trace element compositions and clay mineral components of the sediments. Higher δ 18 O values, lower TiO 2 contents, and weaker weathering in the sediment source area during marine isotope stages (MIS) 2 and 4 compared to MIS 1, 3, and 5 are explained by increased Indian summer monsoonal precipitation and river discharge around the western Bay of Bengal. Clay mineral and chemical components indicate a felsic sediment source, suggesting the Precambrian gneissic complex of the eastern Indian peninsula as the dominant sediment source at this site since 80 kyr. Trace element ratios (Cr/Th, Th/Sc, Th/Co, La/Cr, and Eu/Eu*) indicate increased sediment contributions from mafic rocks during MIS 2 and 4. We interpret these results as reflecting the changing influences of the eastern and western branches of the Indian summer monsoon and a greater decrease in rainfall in the eastern and northeastern parts of the Indian peninsula than in the western part during MIS 2 and 4. © 2018. American Geophysical Union. All Rights Reserved

^<230>Th-normalized fluxes of biogenic components from the central and southernmost Chilean margin over the past 22,000 years

We report on ^Th-normalized fluxes of biogenic components—total organic carbon (TOC), total nitrogen (TN), calcium carbonate (CaCO_3), and biogenic opal (Si_)—from two sediment cores collected at 36°S off central Chile and covering the past 22 thousand years (kyr) (site PC-1), and at 52°S near the Pacific entrance of the Strait of Magellan and encompassing the past 13 kyr (site PC-3). During 13-8 calendar kyr before present (cal kyr BP), the ^Th-normalized TOC flux at the PC-1 site was relatively high, pointing to increased productivity, whereas a marked decrease in the flux characterized the periods around 22-14 and 8-5 cal kyr BP. In contrast, at the PC-3 site, the ^Th-normalized TOC flux was low during the last deglaciation until ~6 cal kyr BP, and then abruptly increased in the late Holocene. The ^Th-normalized fluxes suggest that, compared to other periods, the biological pump functioned less effectively during 22-14 cal kyr BP and the middle Holocene off central Chile, and during 13-6 cal kyr BP off southernmost Patagonia. The changes from glacial to interglacial in the ^Th-normalized biogenic components, which were controlled by changes in upwelling intensity at the PC-1 site and by the inflow of nutrients from the Pacific at the PC-3 site, can be explained by changes in wind direction and intensity associated with the latitudinal displacement of the southern westerly belt. In contrast, we found no obvious relationship during the deglacial and Holocene periods between El Niño activity and biogenic component fluxes at the PC-1 site, even though at present El Niño events are negatively correlated with primary productivity in the upwelling area off Chile

Deglacial-Holocene environmental changes at the Pacific entrance of the Strait of Magellan

We show environmental and nitrogen cycle changes between 13 and 2.5 kyr BP (from the latest deglaciation to the Holocene) based on biogeochemical records in a sediment core from the Pacific entrance of the Strait of Magellan. Organic carbon, total nitrogen, and C-37 alkenone contents were low during 13-9 kyr BP but increased rapidly at 8 kyr BP. The relative contribution of tetra-unsaturated C-37 alkenone, used as a salinity proxy, suggests low salinity during 13-9 kyr BP changing to high salinity at 8 kyr BP. Planktic foraminifer data showed that Globigerina bulloides was more abundant during the deglacial-early Holocene than in the mid-late Holocene, whereas abundances of Neogloboquadrina pachyderma showed the opposite pattern. This evidence plus biogenic opal data suggest that sea level rose, surface waters were relatively stratified, and in situ productivity was low except for organisms with opal tests from 13 to 9 kyr BP. Bulk delta N-15 values were high (9-11 parts per thousand) relative to present-day values during the deglacial-early Holocene, with consistently lower values (similar to 7 parts per thousand) after similar to 8 kyr BP. These relatively high delta N-15 values most likely reflect increased nutrient utilization in the Southern Ocean, which resulted in high-delta N-15 water being advected to the Patagonian fjords, although local denitrification caused by the delivery of copious terrestrial materials and organic matter to the ocean cannot be ruled out. By similar to 8 kyr BP continental influences and surface stratification were reduced and characteristic Holocene conditions were established, with increased influence of Southern Ocean water in the western Strait of Magellan. Sea surface temperatures showed millennial-scale changes during the deglaciation and early Holocene, with relatively warm temperatures (11-12 degrees C) corresponding to the Antarctic counterpart of the Younger Dryas cooling event of the Northern Hemisphere. The longer term pattern (similar to 10 kyr time scale) in alkenone-derived sea surface temperatures generally follows orbital changes in insolation during austral spring, suggesting a controlling influence of austral spring insolation on southern Patagonian climate