Decadal Changes in the Element Accumulation Rates Of the Red Sea Reef Sediments are Coupled with Pollution and Bleaching

The Red Sea has been recognized as a coral reef refugia, but it is vulnerable to warming and pollution. Here we investigated the spatial and temporal trends of 15 element concentrations (Na, Mg, P, S, Ca, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, Mo, and Cd) from 9 central Saudi Arabian Red Sea coral reef sediment cores (aged from the 1460s to the 1980s). We found several element concentrations were higher in the northern (Na, Ca, Cr, Fe, Co, Ni, and Sr) or southern (Mg, P, S, Mn, and Cd) Red Sea reef sediments, respectively (p < 0.05). In the central (i.e., Thuwal) to southern (i.e., Al Lith) Red Sea, the temporal trends of element concentrations were diverse, but the reef sedimentation rates (-36.4% and -80.5%, respectively) and all elemental accumulation rates (-49.4% for Cd to -12.2% for Zn, and -86.2% for Co to -61.4% for Cu, respectively) declined over time, potentially caused by warming-induced thermal bleaching. In the central to northern Red Sea (i.e., Yanbu), the severity of thermal bleaching is low, while the reef sedimentation rates (187%), element concentrations (6.7% for S to 764% for Co; except Na, Mg, Ca, Sr, and Cd), and all elemental accumulation rates (190% for Mg to 2697% for Co) exponentially increased from the 1970s, probably due the rapid industrialization in Yanbu. Our study also observed increased trace metal concentrations (e.g., Cu, Zn, and Ni) in the Thuwal and Al Lith coral reefs with severe bleaching histories, consistent with previous reports that trace metals might result in decreased resistance of corals to thermal stress under warming scenarios. Our study points to the urgent need to slow down global ocean warming and reduce the local discharge of trace metal pollutants to protect this biodiversity hotspot.