Late-Holocene variation in vertical thermal gradient from the coastal East China Sea: Links to monsoon climate and coastal upwelling

Abstract

The evolution history of coastal upwelling systems in the East China Sea over the late-Holocene are important yet difficult to be characterized. Here, we present ~3000-year temperature records based on both alkenones (U^K’₃₇) and glycerol dialkyl glycerol tetraethers (TEX^H₈₆) from a sedimentary core in the coastal upwelling site of the southern East China Sea. By examining the similarity and difference between the down-core TEX^H₈₆-derived temperature and records of sea surface temperature, summer monsoon, and winter monsoon, we suggest that the TEX^H₈₆ mainly reflects temperature in the subsurface water in the studied site. In this sense, paired temperature records from both surface (U^K’₃₇) and subsurface (TEX^H₈₆) water would yield an estimation on the vertical thermal gradient, which provides insight into the upwelling strength in the summer season. Weak upwelling periods occurred at 2600–2800 yr BP, 2000–2400 yr BP, 1200–1550 yr BP, 350–750 yr BP, and the recent ~100 years. Positive correlations among the coastal upwelling, the Asian summer monsoon, and the solar irradiance in centennial scales at ~100–3000 yr BP indicate that solar-induced summer monsoonal wind might be responsible for the coastal upwelling in the studied area. However, weak upwelling intensity and weak monsoonal strength under the background of relatively warm global temperature and strong solar irradiance since the end of the ‘Little Ice Age’ (LIA) might suggest a different climatic response to the natural irradiance, asking for further work on the underlying mechanisms of this phenomenon in a widespread area.