Hydrographic restriction conditions in the Middle and Upper Yangtze region during the Early Silurian post-glacial transgression: Constraints from major, trace elemental geochemistry and Mo-TOC relationship

Abstract

Redox-sensitive trace metals have not only been widely used as the proxy for palaeoredox potential due to the strong enrichments under reducing conditions but also provide critical information into water-mass properties, such as the degree of basin restriction. In order to reveal the Early Silurian post-glacial transgression hydrographic restriction conditions throughout the Middle and Upper Yangtze region, trace metal concentrations and organic geochemical data of the lower Longmaxi Formation were analyzed by the Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) and Leco combustion techniques. We analyzed the hydrographic restriction conditions and redox conditions of different depocenters, including the Southern, Eastern, and Northern Sichuan Basin depocenters and the Western Hubei Region depocenter. The sedimentary Mo-TOC relationship, Mo_EF-U_EF covariation, and upwelling intensity suggest that the Southern, Eastern Sichuan Basin depocenters and the Western Hubei Region depocenter were deep water areas with moderate hydrographic restriction, whereas the restriction conditions of Northern Sichuan Basin depocenter was relatively weak. In addition, due to the combined effect of geographical location, connectivity with the open Qinling Ocean, and estuarine circulation, restriction degrees varied in the same depocenter. The redox proxy (C_org/P) indicated that the lower Longmaxi Formation was deposited in an anoxic water column. There are significant spatial differences in redox conditions in different depocenters and different locations within the same depocenter, which may be controlled by relative sea level rise, hydrographic restriction, and upwelling intensity. Anoxic conditions were more likely to occur in areas where water column was relatively deep and restricted. In addition, the anoxic conditions in the Northern Sichuan Basin depocenter may also associated with the strong upwelling intensity and the behavior of P cycling back to water column, which can promote the primary productivity and further maintain persistent anoxic conditions. The correlation between C_org/P and Mo/TOC indicates that, in relatively weak restriction basin, the trace elements enrichment in sediments was controlled by trace elemental concentrations in seawater. In moderate restriction basin, the trace elements enrichment was mainly controlled by redox conditions. Whereas under more reducing conditions, hydrographic restriction may tend to become the dominant factor. The application of sediment Mo-TOC relationship, Mo_EF-U_EF covariation, and upwelling can provide insights into hydrographic restriction conditions, but the Mo/TOC relationship is limited to anoxic facies, as little trace metal accumulation could also be the consequence of oxic conditions.