High resolution strontium isotope data from Nama Group, South Africa, constrain global stratigraphic relationships in the terminal Ediacaran

The Ediacaran was a pivotal period, during which the first metazoans emerged and then experienced several biotic turnover events. At the same time, the oceans were undergoing large-scale geochemical changes, including oscillating redox conditions and nutrient fluxes. Understanding global patterns of biotic change during this interval, and any environmental drivers, is limited by our ability to correlate geochemical and palaeontological records between sections. The strontium isotope ratio of seawater (87Sr/86Sr) represents a powerful chemostratigraphic tool, because it is globally homogeneous, evolves on - 1 myr timescales, and is well-preserved in a range of carbonate rocks. In addition, changes in 87Sr/86Sr track changes in continental weathering fluxes, which in turn can influence redox conditions and seawater chemistry. Recent age models for the Ediacaran have incorporated delta 13C and 87Sr/86Sr data, but there is a dearth of high-resolution, age-constrained 87Sr/86Sr data from the critical Ediacaran-Cambrian Boundary interval. To address this, we collected a high-resolution stratigraphic transect through an expanded section of terminal Ediacaran carbonate rocks, from the Nama Group, South Africa and Namibia, deposited -550-538 Ma. The samples are interbedded with a series of closely-spaced, dated ash beds. To ensure that the 87Sr/86Sr ratios were reflective of primary seawater signals, we tested a sequential digestion procedure, already shown to be effective in Phanerozoic samples, on Ediacaran bulk rock carbonate powders. The successful implementation of the sequential digestion technique was verified through careful examination of samples for diagenetic alteration using petrography, trace element ratios and delta 18O. This technique was then applied to 51 samples from a stratigraphic transect through the Kuibis Subgroup, collected at Zebra River, and the Schwarzrand Subgroup, collected at Orange River. The data through the Schwarzrand Subgroup are highly consistent, with an average of 0.708590 +/- 0.000076. The isotopic signature is consistent with other terminal Ediacaran age rocks, with typically low, positive delta 13C and relatively high 87Sr/86Sr ratios (-0.7085). Based on correlation with other Ediacaran sections, our data reveal an 87Sr/86Sr peak of -0.7086 pre-550 Ma before a drop-off to -0.7084 immediately post-550 Ma. These data can help improve age models and provide important context for a critical period in metazoan evolution.