Ca and Mg isotope constraints on the origin of Earth’s deepest [formula omitted]C excursion

Abstract

Understanding the extreme carbon isotope excursions found in carbonate rocks of the Ediacaran Period (635–541Ma), where δ13C of marine carbonates (δ13Ccarb) reach their minimum (-12‰) for Earth history, is one of the most vexing problems in Precambrian geology. Known colloquially as the ‘Shuram’ excursion, the event has been interpreted by many as 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 257 δ44/40Ca and 131 δ26Mg 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 ∼1.5‰ in δ26Mg and ∼1.2‰ in δ44/40Ca. 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 δ44/40Ca value of seawater seen over the Phanerozoic Eon. Changes in both isotopic systems are too large to explain with changes in the isotopic composition of Ca and Mg in global seawater given modern budgets and residence times, and thus must be products of alternative processes. Relationships between δ44/40Ca and [Sr] and δ26Mg and [Mg] are consistent with mineralogical control (e.g., aragonite vs. calcite, limestone vs. dolostone) on calcium and magnesium isotope variability. The most pristine samples in the Wonoka dataset, preserving Sr concentrations (in the 1000s of ppm range) and δ44/40Ca values inherited from an originally aragonitic polymorph, have δ13Ccarb of -8‰ to -7‰, thereby providing strong geochemical evidence that extremely negative δ13Ccarb values are primary products of the Ediacaran surface environment.