Strong interplays between eustacy, nitrogen dynamics, and biodiversification between the Ediacaran and Silurian interval

The Ediacaran to Silurian geological interval of the Podolia Basin, southwestern Ukraine, is associated with significant environmental change and biodiversification of morphologically complex biota. Using a multiproxy approach, including total Nitrogen (TN), nitrogen and carbon isotopes, total organic matter (TOC), and whole-rock geochemistry, we show that localised and transient nitrate scarcity may have persisted in otherwise oxygenated and nitrate-replete oceans during the period of extensive animal diversification. Bulk δ 15 N bulk data suggest stable partial denitrification between the middle Ediacaran and Silurian, indicating the existence of a large seawater nitrate reservoir in redox stratified waters that sustained the complex biota. Importantly, the switch to climate warming combined with the nitrate-rich waters in the early Paleozoic triggered a rapid phytoplankton and zooplankton bloom. This transition likely supported efficient nutrient flow to higher trophic levels, resulting in an increase in animal body size, biodiversification, and complexity during the Great Ordovician Biodiversification Event. Nonetheless, the upper Ediacaran and upper Silurian strata suggest a progressive near-quantitative denitrification with diazotrophic primary production as the primary nitrogen source. The generated nitrate scarcity was likely transient with no significant biological incidence since the early Paleozoic emergence and diversification of animals in the Podolia Basin are well recorded. The shift to dominant lighter marine sedimentary δ 15 N bulk composition perhaps resulted from the expansion of anoxic conditions in open marine environments during the Silurian sea-level rises and falls.