THE SHURAM EXCURSION AS BURIAL OF BIOMINERALIZED CALCIUM CARBONATE

The Shuram excursion, a drop in carbonate δ¹³C to values as low as –12‰ (VPDB) in late Ediacaran shallow water carbonate sequences, has the lowest ¹³C/¹²C ratios of any globally synchronous negative δ¹³C_carb excursion on record. Since its initial discovery nearly thirty years ago, contention has surrounded the mechanism responsible for this anomaly in Earth's carbon cycle record. Models for the Shuram excursion invoke either atypical behavior of global carbon cycle reservoirs, or alteration effects that disproportionately affected Ediacaran sediments. Fossil records indicate the Shuram excursion coincided with a time of evolutionary advances in biomineralization and the development of new macroscopic organisms, in a lead-up to the Cambrian radiation of biomineralizing metazoans. Primitive biomineralization is well-established to have involved precipitation of minerals onto organic frameworks. Accordingly, I consider a carbon cycle reservoir model treating the Shuram excursion as recording deposition of a biomineralized shell phase combining marine-derived calcium carbonate with organic carbon. This organic carbon is taken to have decayed after burial to a secondary, ¹³C-depleted calcite phase reflecting the δ¹⁸O signature of altering meteoric porewater. Mass balance permits a Shuram excursion of any duration to be produced by this effect. The biomineralized material producing the excursion is estimated to have initially had an organic carbon fraction of ~0.6 relative to total carbon, in agreement with fossil evidence indicating a major fraction of organic carbon in many Ediacaran biomineralized structures. Because extensive recrystallization of marine-derived carbonates is not necessarily invoked, this hypothesis is consistent with geochemical evidence for limited diagenesis in some Shuram excursion carbonates. Elevated organic carbon productivity is predicted to have occurred during the Shuram excursion, with a peak globally averaged organic burial fraction of roughly 0.4. While it is commonly suggested that the development of Ediacaran multicellular organisms was stimulated by a rise in oxygen in the late Ediacaran, this model also appears to suggest that rising oxygen levels during this time resulted from sequestration of organic carbon through burial of biomineralized material.