Ediacaran carbon cycling and Shuram excursion recorded in the Tarim Block, northwestern China

Abstract

The largest perturbation of the carbon cycle in geological history, named as the ‘Shuram Carbon Isotope Excursion (SCIE)’, has been widely reported in the Ediacaran Period in different continents over the world. In the Tarim Block, a complete Ediacaran carbonate succession was deposited; however, whether it recorded the SCIE or not still remains controversial, hindering establishment of a reliable stratigraphic framework and understanding of environmental perturbations and evolution during the Ediacaran Period. Here, we present two high-resolution carbon isotopic stratigraphy profiles from the upper Sugetbrak through the Qigebrak formations in the northwestern Tarim Block. Two negative excursions (TN1, TN2), and two positive excursions (TP1, TP2) are identified from both sections and supposed to be of the primary origin. Of these, the TN1 was recorded in a storm-dominated inner-to-middle ramp beginning with the marine transgression. It marks the largest negative excursion with a nadir down to ca. −12‰ which can be correlated to the SCIE of late Ediacaran in terms of its similar magnitude, variation pattern, time constraints and depositional characteristics. Other carbon isotope anomalies can also be correlated with coeval anomalies elsewhere around the world. The temporal coincidence of TN1 (or SCIE) with onset of transgressive, storm-dominated successions suggests that enhanced oceanic upwelling and storm agitations together may have greatly increased the depth of ventilated seafloor and extent/volume of oxygenated DOC pool in shallow marine realms, leading to the extremely δ13C excursion of carbonates. Our new carbon isotopic chemostratigraphic profiles of upper Ediacaran from the Tarim Block thus not only provide a more reliable time stratigraphic framework helpful for documenting depositional history and environment-life evolution, but also promote comparisons with other continental blocks globally.