432 / 2024-09-16 22:14:19

No detectable long-term tendency in the seasonal variability of sea surface carbon isotope ratios in the South China Sea during the late Quaternary

carbon isotope,individual foraminifera analyses

Carbon isotope (δ¹³C) analyses of individual-specimen planktonic foraminifera provide an important way to understand past changes in the δ¹³C seasonality of dissolved inorganic carbon in the upper ocean. We reconstructed the individual-specimen δ¹³C of Globigerinoides ruber since the Last Glacial Maximum, based on two sediment cores from the northern and the southern South China, respectively. The standard deviation of the individual-specimen δ¹³C shows no long-term tendency changes in both cores, suggesting limited changes in the seasonal variability of sea surface δ¹³C, despite the significant environmental changes that have occurred in the South China Sea since the Last Glacial Maximum. Modern investigations have revealed that the prevailing monsoon is the primary driver of seasonal δ¹³C variations. Summer monsoon winds tend to drive convergent motion of upper ocean water mass, causing the occurrence of downwelling, and the presence of relatively enriched δ¹³C values. In contrast, winter monsoon winds can induce the upwelling of nutrient-enriched subsurface water, corresponding to relatively depleted δ¹³C values. These processes might both function since the last glacial time and tend to cancel out each other, resulting in limited fluctuations of the standard deviation of individual-specimen δ¹³C.