CA ISOTOPE CONSTRAINTS ON THE ORIGIN OF EARTH'S DEEPEST δ13C EXCURSION

Understanding the extreme carbon isotope excursions found in carbonate rocks of the Ediacaran Period (635-541 Ma), where δ¹³C of marine carbonates (δ¹³C_carb) reach their minimum (-12‰) for Earth history, is one of the most vexing problems in Precambrian geology. Known colloquially as the ‘Shuram’ excursion, workers have long noted its tantalizing, broad concordance with the rise of abundant macro-scale fossils in the rock record, known as the 'Ediacaran Biota.' Thus, the event has been interpreted by many as connected to the rise of animal life, and a product of a profoundly different Ediacaran carbon cycle. More recently, diagenetic processes have been invoked, with the very negative δ¹³C values of Ediacaran carbonates explained via meteoric alteration, late-stage burial diagenesis or growth of authigenic carbonates in the sediment column, thus challenging models which rely upon a dramatically changing redox state of the Ediacaran oceans. Here we present δ^44/40Ca measurements, along with [Mg], [Mn] and [Sr] data, from carbonates of the Ediacaran-aged Wonoka Formation (Fm.) of South Australia to bring new isotope systems to bear on understanding the ‘Shuram’ excursion. Data from four measured sections spanning the basin reveal stratigraphically-coherent trends, with variability of ~1.2‰ in δ^44/40Ca. This Ca isotope variability dwarfs the 0.2-0.3‰ change seen coeval with the Permian-Triassic mass extinction, the largest recorded in the rock record, and is on par with putative changes in the δ^44/40Ca value of seawater seen over the Phanerozoic Eon. This variability is too large to explain with changes in the isotopic composition of Ca in global seawater given modern budgets and residence times, and thus must be products of alternative processes. Relationships between δ^44/40Ca and Mg and Sr abundance suggest a mineralogical control (i.e., aragonite vs. calcite, limestone vs. dolostone) on calcium isotope variability. The most pristine samples in the Wonoka dataset, preserving Sr concentrations and distinct values of δ^44/40Ca inherited from an originally aragonitic polymorph, have δ¹³C_carb of -8 to -7‰, thereby providing strong geochemical evidence that extremely negative δ¹³C_carb values are primary products of the Ediacaran surface environment.