查看更多>>摘要：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.
查看更多>>摘要：Wave and/or current supported gravity-driven sediment flows (GDSFs) cause substantial sediment movement across the continental shelf, contributing to morphological evolution in many regions worldwide. However, they appear to occur episodically and ephemerally and, therefore, it remains a challenge to document them in detail with in situ measurements. Here we present solid evidence of frequent generation of such flows over the shallow sea floor of a muddy open coast. They were triggered by wave resuspension and/or sediment settling from the overlying water column, maintained by wave-and/or current-induced bed stress, and terminated due to upward spreading of bottom sediment within the high concentration layer. Randomly selected GDSF events were analyzed to realize parameterization with a buoyancy-friction model; the resultant bed drag coefficient for two of the cases is higher than the value of 0.003, which is attributed to the additional drag at the interface between the overlying flow and the moving GDSFs. Although short-lived, the observed GDSF events in the three different seasons indicate that they occur more frequently than previously thought.
NSTL
Elsevier