Increased productivity as a primary driver of marine anoxia in the Lower Cambrian

The relationship between metazoan evolution and ocean-atmosphere oxygen levels has been extensively debated. Similarly, there is no consensus on the factors controlling the evolution of the marine redox landscape. The early Cambrian is a particularly critical time interval to examine, as there is a marked increase in metazoan body plan diversity and increased ecosystem complexity, but few constraints on marine redox conditions during this seminal interval. We present an assessment of early Cambrian marine redox conditions from the Luojiacun section in west Hubei Province (the upper Yangtze platform). There are low trace elements enrichments (U, Mo) in the Yanjiahe Formation followed by an abrupt increase in the overlying Shuijingtuo Formation. U-Mo systematics in the most metal-enriched samples suggest deposition under a weakly restricted offshore basin, which is consistent with previous studies. There is correlation between excess Ba (Ba-xs) and total organic carbon (TOC), suggesting a strong production control on organic matter concentration. Further, higher Ba excess in the Shuijingtuo Formation than the Yanjiahe Formation indicates dysoxic-oxic conditions in the Yanjiahe Formation is linked to lower productivity, and anoxic conditions in the Shuijingtuo Formation are linked to higher productivity. Although strong productivity controls on Proterozoic and Paleozoic marine redox conditions have been commonly invoked, we provide some of the first empirical evidence for this control on marine redox structure. More broadly, we find that excess barium coupled with redox proxies may provide useful constraints on the interplay between preservation and production controls on organic matter abundances in the Paleozoic successions.