High-resolution geochemical evidence for oxic bottom waters in three Cambrian Burgess Shale-type deposits

Abstract

The Cambrian radiation of complex animals is a fundamental event in the history of life on Earth. Much of our understanding of this event is made possible through the study of exceptionally preserved fossils in Burgess Shale-type (BST) deposits. Based on bioturbation levels in BST deposits, they are often interpreted as representing restricted oxygen settings. This study tests the low-oxygen interpretation through analysis of geochemical paleoredox proxies and bioturbation levels in three BST deposits: the early Cambrian Maotianshan Shale (China), the middle Cambrian Wheeler Shale (USA), and the middle Cambrian Spence Shale (USA). Results from 96 samples show fine-scale geochemical evidence for oxic bottom water conditions during the deposition of these three BST deposits. Trace element paleoredox indices (PI) give consistent oxic signals in all three shales. In addition, total organic carbon (TOC) and total sulfur (TS) levels in the Spence Shale are extremely low. The average C/S ratio is 3.1, comparable to values for modern oxic marine water. Bioturbation levels in all three shales are low, with rare development of minimal to moderate horizontal bioturbation. While short-term oxygen fluctuations, or poikiloaerobic conditions, could still have sporadically existed in these settings, this study shows that these BST deposits were deposited in a dominantly oxic paleoredox setting. Evidence for low bioturbation levels coupled with paleoredox geochemistry indicative of oxic bottom waters suggests that pervasive exaerobic conditions, with an oxic water column positioned above oxygen-depleted sediment pore waters, may have existed in many Cambrian BST deposit paleoenvironments.