High‐Resolution Coral Records of Cadmium in Surface Seawater: Biogeochemical Cycling and a Novel Proxy for Winter Monsoon

Abstract

Cadmium (Cd) is a toxic trace metal element, which can be transferred or accumulated through the food chain, and is one of the elements that must be measured in marine monitoring (Jurado-González et al., 2003). The behavior and distribution of Cd in the seawater are similar to that of macro-nutrient, being biologically depleted in surface seawater (Xu et al., 2008) and enriched at depth through dissolution of sinking organic matter (Boyle et al., 1976;Bruland et al., 1978), forming a scavenged depth profile of nutrient types (Grottoli et al., 2013). Cd is also considered as a micro-nutrient associated with a carbonic anhydrase that is ubiquitous in the bacteria, plants and animal kingdoms (Lionetto et al., 2005). Some researches suggested that Cd was an essential trace metal element in phytoplankton (Lane & Morel, 2000;Lane et al., 2005), but others suggested that Cd was non-specifically exploited by phytoplankton and stored intracellularly to avoid toxicity (Horner et al., 2013). Whichever mechanism may explain the nutrient-like behavior of Cd, its strong correlation with phosphate reflects its involvement in biological cycling processes (Delgadillo-Hinojosa et al., 2015). However, the geochemical Abstract Cadmium (Cd) geochemical cycle plays a significant role in the composition and function of the marine ecosystem. Skeletal cadmium-to-calcium (Cd/Ca) ratios in hermatypic corals have been applied to reconstruct the historical changes of oceanic and climatic processes, yet there was no systematic evaluation of this tracer's natural variability in high resolution over time. Here, we reported a coral skeletal Cd/Ca record in monthly resolution from 1999 to 2008 CE and reconstructed the history of Cd contents in surface seawater in the northern South China Sea. A significant seasonal variation (higher in the winter but lower in the summer) of Cd contents in surface seawater can be identified. We found that the seasonal variations in coral skeletal Cd/ Ca ratios exhibited significant trends coupled with the surface wind speeds, indicating that strong winds had likely driven the vertical seawater mixing process and then induced the process of sediment remobilization on the shelf, which significantly increased Cd contents in surface seawater. The reduction in Cd contents in surface seawater due to biological processes might be masked by the impacts of surface winds. Importantly, we also observed that coral skeletal Cd/Ca records in the winter showed significant correlations with the winter monsoon index, highlighting the possibility as a new proxy of winter monsoon in the non-upwelling shelf environments.Plain Language Summary Cadmium (Cd) is a nutrient element absorbed by phytoplankton in surface seawater, showing a distribution of low surface content and high bottom content. In order to determine the seasonal variations of Cd content in surface seawater in the northern South China Sea, we analyzed the coral skeletal Cd/Ca ratios in monthly resolution (1999 and finally concluded that the process of sediment resuspens...