DSpace Collection:

A paleoproductivity shift in the northwestern Bay of Bengal (IODP Site U1445) across the Mid-Pleistocene transition in response to weakening of the Indian summer monsoon

2020-12-01T00:00:00Z

Title: A paleoproductivity shift in the northwestern Bay of Bengal (IODP Site U1445) across the Mid-Pleistocene transition in response to weakening of the Indian summer monsoon Authors: Lee, Jongmin; Kim, Sunghan; Khim, Boo-Keun Abstract: The long-term variability of the Indian monsoon in the Bay of Bengal remains inconclusive due to the lack of proximal sedimentary records. To further elucidate the long-term variability of the Indian monsoon, we analyzed the paleoproductivity regime over the last 2.3 Myr at the International Ocean Discovery Program (IODP) Expedition 353 Site U1445 located near the Mahanadi Basin in the northwestern Bay of Bengal. We measured the downcore concentrations and mass accumulation rates (MARs) of biogenic opal, CaCO3, total organic carbon (TOC), and total nitrogen over the Mid-Pleistocene Transition (MPT) to identify the links between surface water marine biogenic production and the Indian summer monsoon. TOC MARs were found to reflect both surface water marine productivity and terrestrial organic matter through the measurements of sediment C/N ratios and δ13CSOM values. Nonetheless, we identified a shift in the paleoproductivity regime from a dominance of biogenic opal deposition prior to the MPT to the dominance of CaCO3 deposition following the MPT. The shift in biogenic marine productivity across the MPT was closely related to riverine discharge, which was primarily controlled by the intensity of the Indian monsoon. Our results, therefore, infer a decrease in riverine discharge to the Bay of Bengal across the MPT in response to a weakened Indian summer monsoon (and/or strengthened Indian winter monsoon). In addition, changes in the intensity of the Indian monsoon across the MPT were more closely linked to the global climate cooling rather than the gradual uplift of the Himalaya and Tibetan Plateau.

Paleoceanographic changes in the Southern Ocean off Elephant Island since the last glacial period: Links between surface water productivity, nutrient utilization, bottom water currents, and ice-rafted debris

2020-12-01T00:00:00Z

Title: Paleoceanographic changes in the Southern Ocean off Elephant Island since the last glacial period: Links between surface water productivity, nutrient utilization, bottom water currents, and ice-rafted debris Authors: Kim, Sunghan; Yoo, Kyu-Cheul; Lee, Jae Il; Roh, Youn Ho; Bak, Young-Suk; Um, In-Kwon; Lee, Min Kyung; Yoon, Ho Il Abstract: To understand past changes in ocean-cryosphere interactions in the Southern Ocean off the Antarctic Peninsula, multi-proxy analyses of three sediment cores located off Elephant Island were used to reconstruct changes in paleoproductivity, nutrient utilization, bottom current intensity, and iceberg calving since the last glacial period. The glacial period was characterized by low surface water produc-tivity with high nutrient utilization, indicating surface water stratification. During the deglaciation, surface water productivity increased with decreasing nutrient utilization, implying that the increase is associated with increased nutrient supply from the subsurface water by enhancing Antarctic Circumpolar Current (ACC) influence as fronts migrate southward with warming. Abundant occurrence of grains >1 mm during the deglacial period indicates rapid ice sheet retreat with large-scale melting and calving. During the glacial period, however, coarse silt-fine sand-sized fraction represented ice-rafted debris (IRD). The different IRD grain size characteristics are thought to be related to the IRD source material characteristics. Regardless of IRD input, the running downcore correlation (5 to 9-point) between sortable silt mean grain size and percentage showed that sediments are well sorted by bottom current. However, the cross plot of them showed different temporal relationships. Sediments were sorted by the ACC and southwestward flowing bottom current. Along with southward migration of fronts and the ACC, southwestward flowing bottom current influence diminished, whereas the ACC influence increased particularly from 7 ka. Our results indicate that the sedimentary processes in the Scotia Sea largely depend on the regional interactions between the ocean and the cryosphere. (C) 2020 Elsevier Ltd. All rights reserved.

Particle-size dependent magnetic properties of Scotia Sea sediments since the Last Glacial Maximum: Glacial ice-sheet discharge controlling magnetic proxies

2020-11-01T00:00:00Z

Title: Particle-size dependent magnetic properties of Scotia Sea sediments since the Last Glacial Maximum: Glacial ice-sheet discharge controlling magnetic proxies Authors: Shin, Ji Young; Kim, Sunghan; Zhao, Xiang; Yoo, Kyu-Cheul; Yu, Yongjae; Lee, Jae Il; Lee, Min Kyung; Yoon, Ho Il Abstract: The strong glacial-interglacial similarity between the magnetic susceptibility (MS) of Southern Ocean sediments and Antarctic ice core dust records has often been used to reconstruct Southern Hemisphere atmospheric variability. Although evaluation of various magnetic properties is essential for identifying the magnetic carriers linked to sedimentological variation, detailed magnetic studies are not sufficient in the Scotia Sea. Here we investigate the bulk and particle-size dependent magnetic properties of Scotia Sea sediments over the past similar to 22 kyr, to determine the main sediment transport mechanism driving bulk magnetic proxies including MS. In bulk sediments, MS is highest during the last glacial period and is accompanied by an increase in the concentration and grain size of ferrimagnetic and antiferromagnetic minerals. For magnetic mineral assemblages, coarse detrital magnetite is dominant. Of three particle-size fractions ( > 63, 16-63, and < 16 mu m), the coarse silt fraction (16-63 mu m) is responsible for the magnetic properties of bulk glacial sediments. Such dominant contribution of coarse silts rules out a major input of dust, which is expected as finer silt and clay. The silt fraction exhibits a co-varying magnetic mineral concentration with that of the sand fraction ( > 63 mu m) throughout the last deglaciation, indicating a close linkage between their input mechanisms. Thus, the sediment particles ranging from sand to coarse silt, which control the bulk glacial magnetic proxies, are most plausibly transported by iceberg-rafted debris (IRD). As hematite is relatively concentrated in the sand fraction, the hematite contribution in the bulk sediment can highlight IRD-related magnetic signals rather than magnetite. The bulk hematite contribution simultaneously varies with the deglacial influx of coarse IRD particles ( > 1 mm) in Scotia Sea sediments, although their glacial inconsistency possibly suggests a different IRD input mechanism during the advancement and retreat of the ice sheet. Consequently, the glacial increase in the bulk magnetic concentration indicates vigorous iceberg calving activity in the Scotia Sea and further suggests the coupled cryosphere-atmosphere system.

Post-LGM dynamic deglaciation along the Victoria Land coast, Antarctica

2020-11-01T00:00:00Z

Title: Post-LGM dynamic deglaciation along the Victoria Land coast, Antarctica Authors: Rhee, Hyun Hee; Lee, Min Kyung; Seong, Yeong Bae; Lee, Jae Il; Yoo, Kyu-Cheul; Yu, Byung Yong Abstract: The post-Last Glacial Maximum (LGM) deglaciation of Antarctica holds important clues for understanding past environmental changes and predicting future changes in the Antarctic Ice Sheet. Cosmogenic nuclide exposure dating of glacial erratics documents the spatial and temporal glacier changes during the most recent deglaciation. We collected 55 erratic cobbles from the eight glaciated benches on Inexpressible Island, which is at the terminal area of Priestley Glacier, Terra Nova Bay, Victoria Land, to elucidate its post-LGM deglaciation pattern. Analyses of the 10Be ages and 26Al/10Be ratios suggest that Priestley Glacier underwent ~254 m of lowering during the mid-Holocene, between 8.9 and 5.9 ka. This lowering rate (~0.09 m a 1) is faster than those observed along other Victoria Land outlet glaciers to the north (Tucker and Aviator), but slower than the one to the south (Mackay). The post-LGM lowering of these outlet glaciers was triggered by marine ice sheet instability, with their asynchronous onsets of deglaciation derived from their diachronous response times to the southwestward migration of the grounding-line retreat until their synchronous termination of deglaciation at ~6 ka. A post-LGM deglaciation model with the southwestward migration of the western Ross Ice Shelf grounding line provides the best match to the terrestrial exposure dating results of the thinning patterns of the outlet glaciers along the Victoria Land coast.