Threshold Behavior of a Marine‐based Sector of the East Antarctic Ice Sheet in Response to Early Pliocene Ocean Warming

We investigate the stability of the East Antarctic Ice Sheet (EAIS) on the Wilkes Land continental margin, Antarctica, utilizing a high‐resolution record of ice‐rafted debris (IRD) mass accumulation rates (MAR) from Integrated Ocean Drilling Program Site U1359. The relationship between orbital variations in the IRD record and climate drivers was evaluated to capture changes in the dynamics of a marine‐based ice sheet in response to early Pliocene warming. Three IRD MAR excursions were observed and confirmed via scanning electron microscope microtextural analysis of sand grains. Time series analysis of the IRD MAR reveals obliquity‐paced expansions of the ice sheet to the outer shelf prior to ~4.6 Ma. A decline in the obliquity and a transition into a dominant precession response of IRD MAR occur at ~4.6 Ma along with a decline in the amplitude of IRD MAR maxima to low background levels between ~4.0 and ~3.5 Ma. We speculate that as sea surface temperatures began to peak above 3°C during the early Pliocene climatic optimum, the ice shelves thinned, leading to a greater susceptibility to precession‐forced summer insolation and the onset of persistent retreat of a marine‐based portion of the EAIS

Orbital Variations of Biogenic CaCO3 and Opal Abundance in the Western and Central Equatorial Pacific Ocean During the Late Quaternary

Biogenic CaCO3 and opal abundances were measured in two piston cores (PC313 and PC5101) collected, respectively, along the equator in the western and central Pacific Ocean. The age model for core PC313, which extends to about 750 ka, was developed by comparing the oxygen isotope stratigraphy of planktonic foraminifera (N. dutetrei) to the SPECMAP stack. The age model for core PC5101, which extends to about 600 ka, was developed by stratigraphic correlation of CaCO3 contents to those in the well-dated core RC11-210 (Chuey et al. 1987). Both cores distinctly exhibited a series of CaCO3 and opal variations, which are mainly controlled by the orbital eccentricity cycle of about 100-kyr. The orbital-scale cyclic variations of CaCO3 and opal contents appear to be contrasting in both cores such that high CaCO3 and low opal contents occurred during the glacial periods. In contrast, during the interglacial periods, low CaCO3 and high opal contents occurred. Mostly remarkable is the distinct occurrence of a mid-Bruhnes event (MBE) at around 350 ka. The CaCO3 content was higher in core PC5101 than in core PC313 before the MBE, whereas biogenic opal abundance became higher in core PC5101 after the MBE. Such a characteristic discrepancy of biogenic (CaCO3 and opal) production, i.e., a succession of primary producers from coccolithophore to diatom, between cores PC313 and PC5101 may be attributed to the prevailing dominant hydrographic conditions (i.e., the South Equatorial Current), in the path of which both cores are located. The intensity of westward propagation might have been an important factor in contrasting biogenic production centering around the MBE

Biogenic CaCO3 and Opal Depositions and Their Latitudinal Comparison During the Past 600 ka in the Central Equatorial Pacific

The orbital-scale variations in biogenic CaCO3 and opal abundance in two piston cores collected in the central equatorial Pacific (core PC5101 from a southern site at _ and core PC5103 from a northern site at _ were compared to assess latitudinal differences. The correlation between the oxygen isotope stratigraphy of planktonic foraminifera (Globigerinoides sacculifer) of PC5103 with the LR04 stacks provides the age of PC5103 to be approximately 950 ka. The age of PC5103 was further refined by correlating the CaCO3 content with the well-dated core RC11-210. The age of PC5101 was also constrained by the same CaCO3 chronostratigraphic correlation with RC11-210, resulting in an age of approximately 650 ka. Distinct orbital-scale series of CaCO3 and opal variations appear to be parallel between the two cores during the past 600 ka, which are controlled mainly by eccentricity with an approximate periodicity of 100 ka. It is worth noting that the biogenic CaCO3 and opal deposition patterns in the two cores differ between interglacial and glacial periods. During interglacial periods the biogenic opal content is higher in the southern core than in the northern core, which corresponds with the present-day condition. In contrast the CaCO3 content is higher in the northern core, which is contradictory to the present-day northward decreasing CaCO3 deposition pattern from the Equator. The collection site of PC5101 is approximately 350 m deeper than that of PC5103, which significantly promotes CaCO3 dissolution and causes unexpectedly high CaCO3 content at the northern site in contrast to the biogenic opal content

Northward shift of oceanic front systems in the Southern Ocean during the last glacial maximum

第2回極域科学シンポジウム/第31回極域地学シンポジウム 11月16日（水） 国立国語研究

Sediment deposition in response to the glacial-interglacial changes on the continental slope of eastern Pennell-Iselin Bank in the Ross Sea, Antarctica

In order to understand the growth and retreat of glaciers in response to the glacial-interglacial changes, subglacial marine sedimentary sequences have been studied extensively in the continental shelf areas of the Ross Sea. The purpose is to comprehend the glaciomarine sedimentation change on the continental slope of eastern Pennell-Iselin Bank in the Ross Sea, using three gravity cores (C1, C2, C3) and three box cores (BC1, BC2, BC3) collected from sites (RS14-1, 2, 3), respectively, across the continental slope to the eastern side of the Pennell-Iselin Bank during XXIX° (2014) PNRA expedition (Rosslope Ⅱ project). Several sedimentological (grain size, magnetic susceptibility), elemental (XRF), geochemical (biogenic opal, total organic carbon, total nitrogen, C/N ratios, CaCO3), and isotopic (δ13C and δ15N of organic matter) parameters were measured along sediment cores with AMS 14C dating of bulk sediments. Core-sediments consist mostly of hemipelagic sandy clay or silty clay with scattered IRDs (Ice-Rafted Debris). A comparison of sediment properties between box cores and the top of gravity cores reveals that the loss of sediment during sampling is trivial. Sediment colors of gravity cores alternate between brown and gray downward. Based on the variation patterns of sediment properties, sediment lithology was divided into different units (A and B), and subunits (B1 and B2). AMS 14C dates and sediment properties assign Unit A, Unit B1, and Unit B2 to interglacial, deglacial, and glacial conditions, respectively. Unit A represents the Holocene and interglacial sediments deposited mainly by the suspension settling of biogenic particles with IRDs in the open marine condition. Unit B1 reflects the deglacial sediments with an increase in IRDs showing the transition of sediment properties from Unit B2 to Unit A by the retreat of subglacial ices. Unit B2 is characterized by different sediment properties, mainly supplied by the continuously lateral melt-water plume or distal part of debris flow originating from the front of grounding floes in the subglacial continental shelf under the ice shelf during the glacial period. Thus, Unit B contains mostly reworked and eroded sediments from the continental shelf with scattered IRDs. The influence of subglacial continental shelf sedimentation in terms of melt-water transport and/or distal stage of debris flow was limited as far as to the middle slope areas (Site 2) during the deglacial and glacial periods. The deeper Site 1 remains in seasonally open marine conditions during the glacial period, due to the peaks of biogenic opal and TOC contents. Keywords: sediment property, subglacial activity, continental slope, Ross Se

Glacial-interglacial variations in paleoproductivity in the Indian sector of the Southern Ocean

第2回極域科学シンポジウム/第31回極域地学シンポジウム 11月16日（水） 国立国語研究所　2階フロ

Antarctic Cryosphere Evolution Project (AnCEP): New IODP proposal for transect drilling in the Southern Ocean

第3回極域科学シンポジウム　横断セッション「海・陸・氷床から探る後期新生代の南極寒冷圏環境変動」11月26日（月） 国立国語研究所 ２階講

Preparation for deep sea drilling proposal in the Indian Sector of the Southern Ocean: Conrad Rise and off Enderby Land

第2回極域科学シンポジウム/第31回極域地学シンポジウム 11月16日（水） 国立国語研究所　2階フロ

Postglacial marine environmental changes in Maxwell Bay, King George Island, West Antarctica

Sediment textural properties and total organic carbon (TOC) contents of three sediment cores from Maxwell Bay, King George Island, West Antarctica, record changes in Holocene glaciomarine sedimentary environments. The lower sedimentary unit is mostly composed of TOC-poor diamictons, indicating advanced coastal glacier margins and rapid iceberg discharge in proximal glaciomarine settings with limited productivity and meltwater supply. Fine-grained, TOC-rich sediments in the upper lithologic unit suggest more open water and warm conditions, leading to enhanced biological productivity due to increased nutrient-rich meltwater supply into the bay. The relationship between TOC and total sulfur (TS) indicates that the additional sulfur within the sediment has not originated from in situ pyrite formation under the reducing condition, but rather may be attributed to the detrital supply of sand-sized pyrite from the hydrothermal-origin, quartz-pyrite rocks widely distributed in King George Island. The evolution of bottom-water hydrography after deglaciation was recorded in the benthic foraminiferal stable-isotopic composition, corroborated by the TOC and lithologic changes. The Ø18O values indicate that bottom-water in Maxwell Bay was probably mixed gradually with intruding 18O-rich seawater from Bransfield Strait. In addition, the Ø13C values reflect a spatial variability in the carbon isotope distribution in Maxwell Bay, depending on marine productivity as well as terrestrial carbon fluxes by meltwater discharge. The distinct lithologic transition, dated to approximately 8000 yr BP (uncorrected) and characterized by textural and geochemical contrasts, highlights the postglacial environmental change by a major coastal glacier retreat in Maxwell Bay