A paired sulfate–pyrite dS approach to understanding the evolution of the Ediacaran–Cambrian sulfur cycle

An anomalous enrichment in marine sulfate dSSO4 is preserved in globally-distributed latest Ediacaran–early Cambrian strata. The proximity of this anomaly to the Ediacaran–Cambrian boundary and the associated evolutionary radiation has invited speculation that the two are causally related. Here we present a high-resolution record of paired sulfate (dSSO4 ) and pyrite (dSpyr) from sediments spanning ca. 547–540 million years ago (Ma) from the Ara Group of the Huqf Supergroup, Sultanate of Oman. We observe an increase in dSSO4 from !20& to !42&, beginning at ca. 550 Ma and continuing at least through ca. 540 Ma. There is a concomitant increase in dSpyr over this interval from !"15& to 10&. This globally correlative enrichment, here termed the Ara anomaly, constitutes a major perturbation to the sulfur cycle. The absolute values of dSpyr reported here and in equivalent sections around the world, require the isotopic composition of material entering the ocean (dSin) to be significantly more enriched than modern (!3&) values, likely in excess of 12& during the late Ediacaran– early Cambrian. Against this background of elevated dSin, the Ara anomaly is explained not by increased fractionation between sulfate and pyrite (DdS), but by an increase in pyrite burial (fpyr), most likely driven by enhanced primary production and sequestration of organic carbon, consistent with earlier reports of elevated organic carbon burial and widespread phosphorite deposition. ! 2008 Elsevier Ltd. All rights reserved.