1:45 PM - 2:00 PM
[MIS15-13] Response of Cape Darnley Bottom Water formation to sea ice and ice sheet variations during MIS 11
★Invited Papers
Keywords:Southern Ocean, Antarctic Bottom Water, Super Interglacial, grain size, paleocurrent, end-member modelling
The Southern Ocean acts as a net CO2 sink, regulated by physical and chemical processes (including wind speed, water temperature, and vertical mixing) and biological activity (primarily photosynthesis). Antarctic Bottom Water (AABW) formation is particularly crucial to the global carbon cycle, as it facilitates CO2 sequestration in deep ocean layers through vertical mixing and the transport of water masses with low carbonate ion concentrations. Despite AABW's critical role in climate regulation, its past dynamics remain poorly understood. Studies examining AABW variability during past warm periods are limited to a single report using sub-Antarctic sediment cores (Glasscock et al., 2020).
In this study, we reconstructed Cape Darnley Bottom Water (CDBW) formation over the past 450 thousand years (ka) and compared our findings with ocean model simulations to identify paleoenvironmental factors that contributed to weakened CDBW formation. We analyzed piston core WIC-6PC, collected from the eastern Wild Canyon during R/V Hakuho-maru KH-20-1 cruise. Multiple proxy analyses were conducted for paleoenvironmental reconstruction: X-ray CT scanning (Ice Rafted Debris; IRD counts), inorganic geochemical analysis (productivity and redox condition), grain size analysis (current speed), and diatom assemblages (sea ice). For the simulation of CDBW formation, we employed the eddy-resolving MITgcm (MIT General Circulation Model) (Marshall et al., 1997) with inclusion of dynamic/thermodynamic sea ice for the Cape Darnley Polynya region. We followed the model configuration and parameters from Mensah et al. (2021).
Paleoenvironmental records revealed a significant weakening of CDBW flow velocity during MIS 11. MIS 11 is a super-interglacial characterized by high sea levels and intense warmth. End-member mixing analysis of grain size distributions identified three distinct end-members in WIC-6PC, each representing different transport processes related to bottom current strength. End-member 3 (EM3), characterized by flow velocities >10-15 cm/s comparable to modern CDBW flow, showed high abundance during interglacial periods MIS 5-9 but decreased substantially during MIS 11, indicating reduced flow velocities (<5-10 cm/s). The weakening of CDBW formation during MIS 11 was supported by a shift toward sub-oxidative conditions, evidenced by decreased Mn/Fe ratios in seafloor sediments. Furthermore, the reduced relative abundance of summer sea-ice diatoms during MIS 11 suggested warmer conditions with extended open-water periods.
Model experiment results show that combined environmental changes of +2°C air temperature, +1°C surface water temperature, and -0.8 psu surface salinity induced attenuation of the CDBW velocity comparable to MIS 11 conditions. This weakening resulted from surface layer freshening (due to decreased summer sea-ice production and increased freshwater input) overwhelming the density increase from brine rejection. The similar phenomenon between paleoenvironmental records and model simulations indicates that reduced salinity was the primary driver of CDBW formation weakening. These findings highlight that the AABW formation could have fluctuated much more than previously thought, although this region is minimally impacted by ice sheet changes based on model results.
In this study, we reconstructed Cape Darnley Bottom Water (CDBW) formation over the past 450 thousand years (ka) and compared our findings with ocean model simulations to identify paleoenvironmental factors that contributed to weakened CDBW formation. We analyzed piston core WIC-6PC, collected from the eastern Wild Canyon during R/V Hakuho-maru KH-20-1 cruise. Multiple proxy analyses were conducted for paleoenvironmental reconstruction: X-ray CT scanning (Ice Rafted Debris; IRD counts), inorganic geochemical analysis (productivity and redox condition), grain size analysis (current speed), and diatom assemblages (sea ice). For the simulation of CDBW formation, we employed the eddy-resolving MITgcm (MIT General Circulation Model) (Marshall et al., 1997) with inclusion of dynamic/thermodynamic sea ice for the Cape Darnley Polynya region. We followed the model configuration and parameters from Mensah et al. (2021).
Paleoenvironmental records revealed a significant weakening of CDBW flow velocity during MIS 11. MIS 11 is a super-interglacial characterized by high sea levels and intense warmth. End-member mixing analysis of grain size distributions identified three distinct end-members in WIC-6PC, each representing different transport processes related to bottom current strength. End-member 3 (EM3), characterized by flow velocities >10-15 cm/s comparable to modern CDBW flow, showed high abundance during interglacial periods MIS 5-9 but decreased substantially during MIS 11, indicating reduced flow velocities (<5-10 cm/s). The weakening of CDBW formation during MIS 11 was supported by a shift toward sub-oxidative conditions, evidenced by decreased Mn/Fe ratios in seafloor sediments. Furthermore, the reduced relative abundance of summer sea-ice diatoms during MIS 11 suggested warmer conditions with extended open-water periods.
Model experiment results show that combined environmental changes of +2°C air temperature, +1°C surface water temperature, and -0.8 psu surface salinity induced attenuation of the CDBW velocity comparable to MIS 11 conditions. This weakening resulted from surface layer freshening (due to decreased summer sea-ice production and increased freshwater input) overwhelming the density increase from brine rejection. The similar phenomenon between paleoenvironmental records and model simulations indicates that reduced salinity was the primary driver of CDBW formation weakening. These findings highlight that the AABW formation could have fluctuated much more than previously thought, although this region is minimally impacted by ice sheet changes based on model results.