[M-IS06] Evolution and variability of the Tropical Monsoon and Indo-Pacific climate during the Cenozoic Era

[MIS06-P07] Attenuation of U^K'₃₇-based temperature variability in the northern South China Sea: Implications for East Asian Winter Monsoon reconstruction

Chia-Yu Chung¹、*Sze Ling Ho¹ (1.Institute of Oceanography, National Taiwan University)

キーワード：East Asian Winter Monsoon, Alkenones, UK'37, South China Sea

The East Asian Monsoon plays a key role in driving seasonal changes in surface circulation and sea surface temperature (SST) in the South China Sea (SCS). SST in the northern SCS decreases substantially in winter, partly due to strong winter monsoonal winds. By analogy, many studies have attributed reconstructed low SSTs during glacial periods to the increased intensity of the East Asian Winter Monsoon (EAWM). One widely used proxy for SST reconstruction is the alkenone-based U^K’₃₇ index. Due to the higher abundance of alkenone producers during the winter months in the SCS, the U^K’₃₇ index has been interpreted as reflecting changes in the EAWM. However, the sedimentary U^K’₃₇ signal shows a stronger relationship with annual mean SST than with winter SST. The application of the U^K’₃₇ index in reconstructing past SSTs further relies on the assumption that sedimentary alkenones originate from the mixed layer of the overlying water column; this assumption may not hold true in the northern SCS, given its complex hydrodynamics. To evaluate these assumptions commonly adopted in U^K’₃₇-based EAWM reconstructions, we generated 2-year-long alkenone flux and U^K’₃₇ time series using sediment trap samples collected at a weekly interval from the northern SCS. The alkenone flux increased during the winter months, following the general seasonal pattern of marine productivity without an apparent temporal offset, indicating the rapid settling of alkenones. However, the U^K’₃₇ temperature time series show an approximately 4 degC attenuation compared to satellite SSTs and exhibit a temporal shift relative to satellite SSTs. Interestingly, U^K’₃₇ temperature records align better with the mean SST records averaged over 20 to 30 weeks. These results suggest that the alkenones intercepted by the traps were likely produced months or even years earlier, indicating a long residence time in the water column. The aged alkenones may originate from the overlying water column or represent resuspended alkenones produced on the continental shelves, which were transported laterally to the sediment trap site. Homogeneous U^K’₃₇ values in the surface sediments of the northern SCS basin further suggest that the processes governing the temporal pattern of our U^K’₃₇ data are probably regional in extent. Together, these factors may explain why the U^K’₃₇ signal in the northern SCS sediments is not biased toward the winter months, despite a clear increase in alkenone production during winter monsoons. In conclusion, our findings suggest that U^K’₃₇ index may not be a sensitive recorder of the EAWM, but may instead reflect the annual mean and possibly regional mean SST in the northern SCS.