ISOTOPIC STUDIES OF CARBONATES FROM CIROS-1 DRILLHOLE, WESTERN McMURDO SOUND, ANTARCTICA

• Main Authors: ワダ ヒデキ, オカダ ハクユウ, Hideki WADA, Hakuyu OKADA
• Format: Report
• Language: English
• Published: Institute of Geosciences, Faculty of Science, Shizuoka University 1990
• Subjects: McMurdo Sound; Antarc*; Antarctica; Ice Sheet
• Online Access: https://nipr.repo.nii.ac.jp/?action=repository_uri&item_id=2629; http://id.nii.ac.jp/1291/00002629/; https://nipr.repo.nii.ac.jp/?action=repository_action_common_download&item_id=2629&item_no=1&attribute_id=18&file_no=1

Carbonates from core samples from the CIROS-1 drillhole, western McMurdo Sound, Antarctica, were analyzed for isotopic, chemical and mineralogical compositions. Carbonates occur as both cement (Mg-calcite and aragonite) and vein filling (aragonite). Samples are generally depleted in ^<18>O to a level that is consistent with pore-water chemistry and downhole temperatures as high as 67℃. However, the samples from the upper part of the hole (<300m sub-bottom depth) contain extremely ^<18>O-depleted aragonite, suggesting precipitation from glacier- or ice-sheet-source meteoric water. Carbon isotopic rations of aragonite are enriched in ^<13>C relative to the neighboring Mg-calcite. Low δ^<13>C values of Mg-calcite may be ascribed to precipitation under the sulfate reduction layer where ^<13>C-depleted carbon dioxide might be released. It is plausible, however, that the high δ^<13>C values of aragonite are due to precipitation under the methane fermentation layer or CO_2 reduction layer, which underlay the sulfate reduction layer and produced ^<13>C-enriched carbon dioxide. Semi-micro scale isotopic analyses along two transects of aragonite veins revealed that δ^<13>C values vary from 3 to 5‰ within a vien. Near the bottom of the drillhole, δ^<18>O values in pore water remained constant during the aragonite precipitation. At shallow depths about 260m below the sea floor, δ^<18>O values in pore water were variable, probably owing to the different contribution of glacier or ice-melt water during the precipitation.