Reconstructing paleoceanographic conditions during the middle Ediacaran: Evidence from giant ooids in South China

Abstract

The Ediacaran was a period of intense environmental change in Earth history. Transient pulses of oxygenation and shifts in ocean chemistry have been interpreted from a range of geochemical proxies, including rare earth elements and yttrium (REY), but it is still a challenge to reconstruct paleoceanographic conditions due to the scarcity of reliable recorder. In this study, we explore the possibility that a newly-discovered deposit of giant ooids in the Nanshanping outcrop in South China contains a relatively intact record of the REE composition of Ediacaran seawater, providing an important window into the redox state of the surface ocean at this time. These ooids are relatively large in size (~1–6 mm), and exhibit densely banded laminae. A regional chemostratigraphic correlation shows that the giant-ooid-bearing succession in South China is in the upper part of the second Ediacaran positive carbon isotopic shift (EP2), and thus its occurrence was not associated with the Shuram Excursion. Additionally, the occurrences of giant-ooid-bearing oolites in different blocks were not synchronous during the middle Ediacaran. In-situ laser ablation analysis of giant-ooid cortices reveals that the shale-normalized REY patterns are similar to those of modern seawater, including depleted light REEs, superchondritic Y/Ho ratios, and positive La anomalies. Yet, no Ce anomalies (1.0 ± 0.15) in numerous ooid cortices seems to indicate a suboxidized condition of surface ocean in South China at that time. The petrographic and geochemical evidence support the weak diagenetic alteration of the pure giant ooids during burial. They formed in open-ocean settings with energetic hydrodynamic environments and were affected by early dolomitization that might preserve mostly original petrographic fabrics and REY signatures from further diagenetic overprinting. The characteristics of Ce anomalies and REY compositions acquired from the giant ooids indicate that the surface ocean was suboxidized in South China during the late stage of EP2, implying the very low atmospheric O2 level at that time. Our results suggest that giant ooids may be a reliable analytical target that provide a snapshot into palaeoceanographic conditions during this crucial period of dynamic changes in early Earth history.