5:15 PM - 6:45 PM
[MIS10-P09] Weakening in Cape Darnley Bottom Water during MIS11 and its Factor Estimation
Keywords:Antarctic Bottom Water, Super Interglacial, MITgcm model
Antarctic Bottom Water (AABW) is a major component of the global overturning circulation (Talley, 2013), with the potential reservoir of heat and carbon. Moreover, AABW supplies oxygen to the global ocean abyss, thus affecting marine ecosystems and the biogeochemical cycle (Henley et al., 2020). Despite AABW’s key role in regulating Earth’s climate and in recording Southern Ocean conditions on long-time scales, its past and present dynamic behavior remains poorly understood. Studies focusing specifically on the past warm period are limited to reports suggesting a weakening of Weddell Sea Bottom Water due to freshening by Antarctic ice sheet melting or a change in the position and/or strength of the Southern Hemisphere westerlies (Hayes et al., 2014, Glasscock et al., 2020). In this study, we reconstructed Cape Darnley Bottom Water (CDBW) over the past 500 ka, and compared it with the ocean model to understand the paleoenvironmental factors that caused the weakening of CDBW formation.
We used a piston core WIC-6PC, which was collected from the east side of the Wild Canyon during the R/V Hakuho-maru KH-20-1 cruise. For paleoenvironmental analysis, X-ray CT (Ice Rafted Debris; IRD counts), inorganic geochemical composition (redox condition), grain size (current speed), stable carbon isotope of fatty acids (productivity in sea ice edge), and diatom assemblages (paleoenvironment) were conducted. For the simulation of CDBW formation, we employed the eddy-resolving MITgcm (MIT General Circulation Model) (Marshall et al., 1997) for the Cape Darnley Polynya region with hydrostatic approximation and dynamic/thermodynamic sea ice (Losch et al., 2010). We followed the model configuration and parameters from Mensah et al. (2021).
The results show a significant weakening (< 5–10 cm/s) of long-term CDBW formation in MIS 11, unlike other interglacial periods. At the same time, sea ice-related diatoms showed a decreasing trend, and there were no significant peaks in IRD. Ocean observations suggest that surface water warming leads to reduced sea ice and increased basal melting of ice shelves, which could reduce CDBW formation (Aoki et al., 2022). Basal melting released IRD under the ice shelves (Williams et al., 2002), so it is possible that the more pelagic study areas were not IRD supplied. For these reasons, the weakening of CDBW in MIS 11 may be related to the freshening and decreasing sea ice influence. In addition, the model experiments showed a delay in the onset of autumn sea ice production and a reduction of more than half of CDBW velocity associated with the combined decrease of salinity (-0.1), and increase of air temperature (+2°C) and sea surface temperature (+0.7°C). The CDBW slightly weakened when the ice tongue on the east side of the Cape Darnley Polynya was removed. Therefore, these modeling results are consistent with paleoenvironmental records and suggest that the weakening of CDBW formation during MIS11 is associated with reduced sea ice influence and low salinity in the bottom water-forming region.
We used a piston core WIC-6PC, which was collected from the east side of the Wild Canyon during the R/V Hakuho-maru KH-20-1 cruise. For paleoenvironmental analysis, X-ray CT (Ice Rafted Debris; IRD counts), inorganic geochemical composition (redox condition), grain size (current speed), stable carbon isotope of fatty acids (productivity in sea ice edge), and diatom assemblages (paleoenvironment) were conducted. For the simulation of CDBW formation, we employed the eddy-resolving MITgcm (MIT General Circulation Model) (Marshall et al., 1997) for the Cape Darnley Polynya region with hydrostatic approximation and dynamic/thermodynamic sea ice (Losch et al., 2010). We followed the model configuration and parameters from Mensah et al. (2021).
The results show a significant weakening (< 5–10 cm/s) of long-term CDBW formation in MIS 11, unlike other interglacial periods. At the same time, sea ice-related diatoms showed a decreasing trend, and there were no significant peaks in IRD. Ocean observations suggest that surface water warming leads to reduced sea ice and increased basal melting of ice shelves, which could reduce CDBW formation (Aoki et al., 2022). Basal melting released IRD under the ice shelves (Williams et al., 2002), so it is possible that the more pelagic study areas were not IRD supplied. For these reasons, the weakening of CDBW in MIS 11 may be related to the freshening and decreasing sea ice influence. In addition, the model experiments showed a delay in the onset of autumn sea ice production and a reduction of more than half of CDBW velocity associated with the combined decrease of salinity (-0.1), and increase of air temperature (+2°C) and sea surface temperature (+0.7°C). The CDBW slightly weakened when the ice tongue on the east side of the Cape Darnley Polynya was removed. Therefore, these modeling results are consistent with paleoenvironmental records and suggest that the weakening of CDBW formation during MIS11 is associated with reduced sea ice influence and low salinity in the bottom water-forming region.