Cumulated influence of natural and anthropogenic drivers on surface seawater barium: Evidence from a high-resolution coral record in the northern South China Sea

Barium (Ba) plays a crucial role as a tracer element in elucidating essential marine biogeochemical processes. However, the limited knowledge regarding Ba sources and variations impedes our comprehension of the diverse array of processes occurring in the marine environment. Although coral Ba/Ca ratios have demonstrated po-tential as a tracer of oceanic Ba, there remains a scarcity of long-term and high-resolution records to fully utilize this technique. Here, we presented a 32-year record of monthly coral Ba/Ca ratios and delta 18O from the Weizhou Island in the northern South China Sea to elucidate the sources and the influence factors on surface seawater Ba. The results indicated no significant correlation between coral Ba/Ca and sea surface temperature or growth rate, implying that coral Ba/Ca ratios could serve as a dependable proxy for surface seawater Ba concentrations. Significant increases and abrupt fluctuations in coral Ba/Ca ratios were observed during the period of oil drilling exploration and engineering construction, indicating that anthropogenic activities might lead to an elevation of surface seawater Ba levels, subsequently affecting coral Ba/Ca ratios. The winter coral Ba/Ca peaks on monthly timescales were confirmed to be caused by resuspended sediment driven by the winter monsoon. Extreme peaks of coral Ba/Ca occurring during the wet season demonstrated the potential of coral Ba/Ca to record tropical cyclones, which has not been found in low-resolution scale studies. The continuous, long-term, and high -resolution coral Ba/Ca time series provides compelling evidence for the combined influence of both natural and anthropogenic factors on seawater Ba concentrations. These findings significantly contribute to the comprehension of the intricate biogeochemical cycling of marine Ba.