Alkenone distribution in surface sediments of the southern Yellow Sea and implications for the U37K' thermometer

The U-37(K') index has been widely applied for sea surface temperature (SST) reconstruction in open ocean environments, but has inherently limited applications at smaller, regional scales including some marginal seas where both historical and reconstructed SST records are urgently needed for understanding regional climate evolution. We determined the spatial distribution of alkenone contents in surface sediments from the southern Yellow Sea to assess the regional U-37(K')-SST relationship for paleo-SST reconstructions. C-37:2 and C-37:3 alkenones were detected at all 36 sites covering most of the southern Yellow Sea. Alkenone content ranges from 17 to 1,063 ng/g, with high values (ca. 400 to 1,000 ng/g) at deep water sites and a decreasing trend shoreward. For six samples at shallower depths near the coast and further offshore, the values were too low for statistical evaluation. This spatial pattern of alkenone contents is consistent with existing knowledge on the spatial distribution and productivity of alkenone-producing coccolithophorid species in the region. There is a significant positive relationship (U-37(K') - 0.059SST - 0: 350, R = 0.912, n = 30) between the U-37(K') values and satellite-derived annual mean SSTs (0 m) for the last 27 years, providing support for the application of a region-specific U-37(K') index as paleothermometer in the southern Yellow Sea. However, the slope of the southern Yellow Sea calibration (0.059) is considerably larger than that of the well-known global core-top calibration (0.033). This implies that global SST trends may not adequately encompass regional SST patterns and/or that environmental factors other than temperature may gain importance in explaining coccolithophore dynamics in marginal seas.