HIGH-RESOLUTION GEOCHEMICAL EVIDENCE FOR OXIC BOTTOM WATERS IN MULTIPLE CAMBRIAN BURGESS SHALE-TYPE DEPOSITS

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 interpretation strongly influences studies on the preservation of fossils in BST deposits and the paleoecology of Cambrian animals. 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, with V/Cr ratios less than 2.00 and V/(V+Ni) ratios less than 0.84. In addition, total organic carbon (TOC) and total sulfur (TS) levels in the Spence Shale are extremely low, with averages of 0.25 wt% TOC and 0.08 wt% TS. The average C/S ratio is 3.1, comparable to values for modern normal marine water. These low abundances of organic carbon and sulfur are consistent with the PI results and suggest that anoxia was not predominant during Spence Shale deposition. Bioturbation levels in all three shales were low (average ii =~1), with rare development of minimal to moderate horizontal bioturbation (ii2-3). While short-term oxygen fluctuations, or poikiloaerobic conditions, could still have sporadically existed in these settings, the results of this study show that these three BST deposits were deposited in a paleoredox setting dominated by oxic bottom waters. 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. These results have broad implications for interpreting the evolutionary paleoecology and preservation of animals during the Cambrian radiation.