Rare earth elements as indicators of post-Marinoan (∼635 Ma) paleoceanographic changes from the Amazon Craton

Abstract

The extensive deglaciation linked to the Snowball Earth climate that elapsed in the terminal Neoproterozoic caused dramatic changes in ocean chemistry, exceptionally recorded in globally distributed cap carbonate successions. One of the most spectacular examples is the Puga cap carbonate (∼635 Ma), which is exposed in the Amazon Craton and associated with extensive sea level changes related to glacial-isostatic adjustment and local ice gravity, resulting in continuously mixed waters. This study meticulously evaluates complex post-Marinoan dynamics using a comprehensive multiproxy approach. The rare earth element + yttrium (REE+Y) patterns in the Puga cap carbonate do not accurately reflect the global ocean water composition; instead, they primarily fractionate in response to local expression of the post-Snowball Earth event, including alkalinity levels and freshwater mixing in the aftermath of the Marinoan glaciation. The flattened REE+Y pattern, accompanied by a positive Eu anomaly, may suggest the influence of continental weathering. Specifically, low Y/Ho ratios in the cap dolostone are consistent with seawater dilution due to meltwater influx (Y/Ho ∼ 29–32). Conversely, superchondritic Y/Ho ratios up to 71 in the basal cap dolostones suggest upwelling of hypersaline seawater in coastal areas. The shallow-water recurrence influenced by ice gravity resulted in continuous coastal uplift, forming isolated shelves and the deformation in diamicton, resulting in irregular substrate relief morphologies. This post-glaciation scenario was succeeded by significant landward shoreline migration concomitant with rapid recovery of primary productivity, with large microbial communities flourishing, inducing dolomite precipitation under restricted paleoenvironmental conditions on dolomitic platforms. The rapid rise in sea level led to the dilution of evaporative fluids, ultimately halting dolomicrite precipitation. Following the pos-tglacial transgression, the stratified waters gradually became more mixed, resulting in the termination of dolomitic platform deposition and coinciding with an increase in detrital components, as indicated by the increase of insoluble elements (e.g., REE, Zr, and Th), followed by abrupt replacement by CaCO₃-oversaturated seas during the post-glacial transgression. These observations elucidate the interplay between post-glaciation and paleoceanographic dynamics during the Cryogenian-Ediacaran boundary.