Paleoenvironmental changes during the last 8400 years in the southern Yellow Sea: Benthic foraminiferal and stable isotopic evidence

We used benthic foraminiferal, stable isotopic and grain-size data from an AMS14C dated core from the southern Yellow Sea to decipher Holocene paleoenvironmental changes. Large-amplitude changes in δ18O, δ13C and benthic foraminiferal assemblage composition indicate that major environmental changes occurred during the last 8400 years in the southern Yellow Sea. We recognize 3 stages of environmental development: 1. A low salinity estuarine environment stage, at 8.4–6.9 cal. kyr BP, 2. A low salinity shallow sea environment stage at 6.9 to about 6–5 cal. kyr BP and 3. A modern marine shelf environment stage after about 6–5 cal. kyr BP The δ18O and δ13C values of Ammonia aomoriensis (Asano), combined with benthic foraminiferal assemblage data indicate a low bottom-water salinity (< 15‰) during stage 1, suggesting a large amount of river runoff. Abrupt increases in δ18O and δ13C around 6.9 cal. kyr BP were accompanied by a decrease in mean grain-size and in abundance of benthic foraminifera, possibly caused by a North-ward shift of the Yellow River's outlet. During subsequent stage 2 the southern Yellow Sea was still characterized by a low salinity (< 28‰) even though sea-level was high, possibly due to the continued strong influence of river input, and/or limited exchange between the southern Yellow Sea and the open ocean. At about 6–5 cal. kyr BP, an A. aomoriensis dominated benthic foraminiferal assemblage was replaced by an Ammonia ketienziensis (Ishizaki) dominated assemblage, indicating an increase in bottom-water salinity. Grain-size and stable isotope data, suggest that the modern circulation was probably established at that time. We speculate that the combined impacts of reduced river runoff, enhanced East Asia Winter Monsoon, and increased influence of southern, warm ocean currents caused this transition.