16:42 〜 17:00
[U05-05] A multi-dimensional reconstruction of East Asian monsoon-related changes in the South China Sea
★Invited Papers
キーワード：South China Sea, sea water temperatures, east Asian monsoon, Mg/Ca, alkenones, TEX86
The complex oceanography in the South China Sea (SCS) is influenced by the East Asian Monsoon (EAM) and the Western Pacific Warm Pool (WPWP). The relative importance of these two factors vary in space and time, with the WPWP exerting stronger control in the southern SCS while the winter monsoon strongly affects the hydrography in the northern SCS. One of the major characteristics that can be used to reconstruct how water column conditions changed in the SCS through time related to the EAM and WPWP is sea water temperature. Multiple proxies can be used to reconstruct sea water temperatures, but they do not always agree due to differences in the type and ecology of their respective proxy carrier.
In this study we are using the relationships between the proxies to disentangle the spatial pattern of the temperature history of the SCS since the Last Glacial Maximum (LGM) using differences in seasonality and habitat depth of the proxy carriers. We compare two sediment records from the northern (MD97-2146) and southern (MD01-2390; Mg/Ca by Steinke et al., 2006, 2008) SCS. At both sites we have generated paleo-temperature reconstructions using alkenones-based UK’37, archaeal tetraether-based TEX86, and planktonic foraminiferal-based Mg/Ca using both mixed-layer and thermocline-dwelling species.
Reconstructed temperatures in the southern SCS show different trends between the surface and the thermocline. Thermocline temperatures are quite constant, not showing a glacial-interglacial (G-IG) transition suggesting stable subsurface conditions. Sea surface temperatures based on Mg/Ca show a “traditional” G-IG transition with ~3°C warming. Depending on the selected calibration TEX86 temperatures vary up to 6°C but the trend is very similar to mixed-layer Mg/Ca-based temperatures suggesting TEX86 may represent mixed-layer temperatures rather than thermocline temperatures as was suggested previously. Alkenones show as expected the highest SSTs, although there is no clear G-IG transition.
Temperature history in the northern part of the SCS is slightly different. Thermocline temperatures decrease into the Holocene rather than increase indicating that colder subsurface water from outside the SCS was increasingly able to enter the SCS, but it may not have reached the southern SCS. For the mixed layer proxies a similar pattern is present as in the southern SCS, although the magnitude of change varies between Mg/Ca and UK’37.
Our results show that the mixed-layer throughout the SCS is mainly controlled by variations in the summer monsoon related to insolation. The summer EAM was weak during the LGM but increased in strength into the Holocene. The subsurface temperatures are mainly controlled by the presence or absence of the inflow of watermasses from outside the SCS related to variability in the winter monsoon. This supports previous studies that the winter EAM decreased in strength into the Holocene. Further work will synchronize the different proxy records in order to calculate gradients with regard to stratification, seasonality, and north-south transects.
In this study we are using the relationships between the proxies to disentangle the spatial pattern of the temperature history of the SCS since the Last Glacial Maximum (LGM) using differences in seasonality and habitat depth of the proxy carriers. We compare two sediment records from the northern (MD97-2146) and southern (MD01-2390; Mg/Ca by Steinke et al., 2006, 2008) SCS. At both sites we have generated paleo-temperature reconstructions using alkenones-based UK’37, archaeal tetraether-based TEX86, and planktonic foraminiferal-based Mg/Ca using both mixed-layer and thermocline-dwelling species.
Reconstructed temperatures in the southern SCS show different trends between the surface and the thermocline. Thermocline temperatures are quite constant, not showing a glacial-interglacial (G-IG) transition suggesting stable subsurface conditions. Sea surface temperatures based on Mg/Ca show a “traditional” G-IG transition with ~3°C warming. Depending on the selected calibration TEX86 temperatures vary up to 6°C but the trend is very similar to mixed-layer Mg/Ca-based temperatures suggesting TEX86 may represent mixed-layer temperatures rather than thermocline temperatures as was suggested previously. Alkenones show as expected the highest SSTs, although there is no clear G-IG transition.
Temperature history in the northern part of the SCS is slightly different. Thermocline temperatures decrease into the Holocene rather than increase indicating that colder subsurface water from outside the SCS was increasingly able to enter the SCS, but it may not have reached the southern SCS. For the mixed layer proxies a similar pattern is present as in the southern SCS, although the magnitude of change varies between Mg/Ca and UK’37.
Our results show that the mixed-layer throughout the SCS is mainly controlled by variations in the summer monsoon related to insolation. The summer EAM was weak during the LGM but increased in strength into the Holocene. The subsurface temperatures are mainly controlled by the presence or absence of the inflow of watermasses from outside the SCS related to variability in the winter monsoon. This supports previous studies that the winter EAM decreased in strength into the Holocene. Further work will synchronize the different proxy records in order to calculate gradients with regard to stratification, seasonality, and north-south transects.