87Sr/86Sr of coral reef carbonate strata as an indicator of global sea level fall: Evidence from a 928.75-m-long core in the South China Sea

Reconstructing the precise timing of sea level fall is important for understanding earth system during the geologic time. However, previous studies are limited by the absence of effective indicators for sea level fall. To overcome this limitation, we investigated the Sr-87/Sr-86 and petrologic features of carbonate strata in a 928.75-m-long coral reef core (CK2) from Xisha Islands, northern South China Sea and their relationship to sea level fluctuations. Ten exposed surfaces were identified by their petrologic features. The Mn and Sr content, Mn/Sr, Sr/Ca and delta O-18 suggest that most of the coral reef carbonate in the CK2 core preserved the original seawater Sr-87/Sr-86 except at intervals of 672-616, 596-575, 521-491, 423-414, and 192.5-161 m, which suffered alteration of meteoric water and showed higher Sr-87/Sr-86 during low sea level period. The 160 carbonate Sr-87/Sr-86 showed a continuously increasing trend, which is consistent with that of seawater over the same period. The continuously increasing Sr-87/Sr-86 curve of CK2 was divided into seven units by six Sr-87/Sr-86 hiatuses. Inconsistency between the sedimentation rate of CK2 and tectonic subsidence rate in the Xisha area suggest that the continuously increasing Sr-87/Sr-86 was controlled by the coupled effect of tectonic subsidence and sea level change, and the hiatus and higher Sr-87/Sr-86 interval in the Sr-87/Sr-86 curve were controlled by sea level fall. The higher Sr-87/Sr-86 usually produced during low sea level (< 5 m), Sr-87/Sr-86 hiatus usually produced when sea level changed from deeper to shallower. The depth of hiatuses and higher intervals of Sr-87/Sr-86 correspond well with the exposed surface in CK2 core, verifying that the hiatuses and higher intervals of Sr-87/Sr-86 in the coral reef carbonate succession provide an effective indicator of sea level fall. Using this indicator and the CK2 Sr-87/Sr-86 profile, we identified eleven large-scale sea level falls and three dramatic regressions during 19.6-0.2 Ma and constrained the age based on strontium isotope stratigraphy. These interpreted sea level falls are mostly consistent with other records of global sea level changes, confirming the reliability of our records. The Sr-87/Sr-86 profile of a coral reef carbonate is an efficient indicator of past sea level falls, including their timing.

Automated summary

Reconstructing the timing of sea level fall is important for understanding the earth system during geological time. This study uses Sr-87/Sr-86 ratios and petrologic features of carbonate strata to investigate sea level fluctuations in the Xisha Islands. The results show that the Sr-87/Sr-86 ratio in the coral reef carbonate is an effective indicator of sea level fall, allowing for the identification of past sea level falls and their timing.