Sea-level changes and carbonate platform evolution of the Xisha Islands (South China Sea) since the Early Miocene

Abstract

Analyses of the biogeochemical branched and isoprenoid tetraether (BIT) index and elemental geochemistry from Well XK1 provide insight into the development of the Xisha Islands carbonate platform in the South China Sea (SCS) since the Early Miocene. BIT is the ratio of branched glycerol dialkyl glycerol tetraethers (bGDGTs) to isoprenoid glycerol dialkyl glycerol tetraethers (iGDGTs), which are derived from meteoric and marine environments respectively. BIT serves as a novel proxy for tracking sea-level changes.The BIT curve of Well XK1 is characterized by “low–high–low–high” alternating stages, indicating the superposition of sea-level changes on carbonate platform evolution in the SCS. Following the rapid expansion of the SCS in the early stages of the Early Miocene, biogenic reefs began to form, and BIT oscillated intensely. The SCS started to regress after its initial expansion during the Middle Miocene, at which time global sea level was also falling significantly. The combination of both tectonic activity and eustatic variation has resulted in the deposition of strata with reef-beach facies, which are characterized by high BIT values. Relative sea level reached its lowest point in the late stages of the Middle Miocene (~11.6Ma). Subsequent sea-level rise between the Late Miocene and Pliocene induced a negative shift in BIT, and lagoonal facies formed under optimal warm marine conditions. Periodic exposure caused the reefs of the Xisha Islands to suffer from erosion during the Pleistocene, which is consistent with a second elevation in the BIT index. However, modern trends show increasing regional water depths again.Major and trace element curves (Na, Si, P, B, Ga, Mo, Zn), as well as the associated ratios (Na2O/K2O, Na2O/SiO2, B/Ga, Ti/Sr, Zr/Sr, Al/Sr, Th/U), accurately record sea-level variations. Several significant discontinuities correlate with sea-level shifts, and provide important details about the geomorphology, sedimentation patterns, and diagenesis types on the SCS carbonate platform. The agreement between organic and inorganic geochemical data demonstrates that our reconstructions of SCS sea-level changes and modes of carbonate platform development are robust.Our findings also demonstrate that carbonate evolution in the Xisha Islands was shaped by relative sea-level changes, implying that sea level fluctuations in the SCS were controlled by global eustatic factors in addition to regional tectonic subsidence.