Geochemical characteristics and paleoenvironmental implications of beachrocks in northwestern Hainan Island

A systematic investigation was performed on the petrological and geochemical characteristics of Late Pleistocene beachrocks in the Paipu area, Danzhou City, northwestern Hainan Island. The results demonstrate that relative to the average composition of the Upper Continental Crust (UCC), the beachrocks are significantly enriched in MgO and CaO, slightly enriched in MnO, and depleted in all other elements, with the most pronounced depletion in Na 2 O. The Rare Earth Elements (REE) patterns of the six beachrock samples are generally characterized by a slight right-dipping trend and light REE enrichment, with weak fractionation between light and heavy REE, as well as weak negative Eu anomalies, weak negative Ce anomalies and weak positive La anomalies. From early to late stages, the decreasing Al 2 O 3 /MgO ratio and the gradual decline in the contents of terrigenous clastic components (including SiO 2 , Al 2 O 3 , Na 2 O, K 2 O and TiO 2 ) collectively indicate the weakening of terrigenous chemical weathering and an increasingly arid paleoclimate. Discrimination using paleosalinity Z values, Sr/Ba, Rb/K, V/Cr, V/(V + Ni) ratios and REE parameters reveals that the study area was a suboxic-oxic marine-terrestrial interactional environment, with a paleogeographic setting of a semi-closed marginal sea. The average paleo-seawater temperature of the Late Pleistocene calculated from δ 18 O is 28.2 °C, which is consistent with the paleotemperature data derived from the Mg/Ca ratio of planktonic foraminiferal tests in previous studies, confirming a tropical marine environment. The geochemical signatures of the beachrock cements are consistent with those of marine organism shells (e.g., foraminifera, corals and Tridacna ), recording the geochemical signals of paleo-seawater and providing important implications for reconstructing paleoclimatic and paleoenvironmental evolution.