11:15 〜 11:30
[MIS12-18] Detritus supply changes to the eastern subantarctic Pacific and Patagonian Icesheet fluctuations
キーワード：パタゴニア氷床、海洋酸素同位体ステージ 5、帯磁率、X線回折分析、粒度分布
The northern and southern Patagonian ice fields (PIF), which are currently distributed on the higher parts of the Andes, have accelerated retreat during the last decades [1]. Understanding the current condition and predicting future PIF retreat are crucial issues, and the stability of glaciers under different past climatic conditions is essential information. The ice volume reached a maximum at 30 ka during the last glaciation, equivalent to ten times today's one based on Patagonian geomorphology and chronological data [2]. However, the glacial geomorphological records have been modified by subsequent glacier advances, and evidence for PIF fluctuations prior to the maximum advance is rare on land. To reconstruct PIF fluctuations over the last glacial cycle, we focused on marine sedimentary records retrieved from the Chilean margin. Here, we used two hemipelagic sediment cores with wide latitudinal coverage: IODP Site U1543 (54°S; 76°W; 3860 m water depth) and MR16-09 PC03 (46°S; 77°W; 3082 m water depth). We measured bulk CaCO3 content [3, 4], detrital mineral composition with X-ray diffraction (XRD), and detrital grain size distribution with laser-diffraction-scattering grain-size analyzer and analyzed magnetic susceptibility (MS) and Gamma-ray attenuation (GRA) density data. The detrital mineral compositions were similar to Patagonian fjord sediments, suggesting that detritus in the cores were derived from glacio-fluvial system. Variations in MS mainly reflect magnetic mineral concentration and can be diluted by non-magnetic minerals, such as CaCO3, so we corrected MS data with bulk CaCO3 content to remove the dilution effect. As a result, MS showed a notable positive anomaly around 90 ka (between Marine Isotope Stage 5c and 5b), the cold substage in the early last glacial. At the same time, the mode of detrital grain size (the most frequent bin of grain size) became coarser within the silt fraction. The amphibole concentration and anorthite/albite ratio also increased during the periods. Considering that two core sites are ~900 km apart, these detrital proxy changes could reflect simultaneous changes in terrigenous sediment supply at the regional scale. According to the petrological studies in the southern Andes, amphibole and anorthite are almost exclusively found in plutonic rocks of the Patagonian batholith, which is the major lithology of the coastal western side of the PIF. Therefore, the coarsening of detrital mode grain-size at ~90 ka suggests a change in detrital transport processes from the Patagonian batholith due to terrigenous environmental change, such as the onset of PIF advance at the early last glacial.
[1] Abdel Jaber et al. (2019), Cryosphere, 13, pp. 2511–2535
[2] Davies et al. (2020), Earth-Sci Rev, 204, pp. 103152
[3] Kasuya et al. (2023), Proc IODP 383, pp. 1–9
[4] Kasuya et al. (submitted)
[1] Abdel Jaber et al. (2019), Cryosphere, 13, pp. 2511–2535
[2] Davies et al. (2020), Earth-Sci Rev, 204, pp. 103152
[3] Kasuya et al. (2023), Proc IODP 383, pp. 1–9
[4] Kasuya et al. (submitted)