Organic-Walled Microfossils in Neoproterozoic Diamictites and the Implications for the Demise of Doushantuo-Pertatataka Acritarchs

Glaciations can have a calamitous effect on the biosphere, yet assessing their direct impact is often challenging. They are represented in the rock record by poorly sorted sediments (diamictites). Heterogeneous composition makes diamictites unsuitable for preserving evidence of ancient biosphere like the Precambrian organic-walled microfossils (OWM), but some diamictites may contain fine-grained matrix. During palynological maceration procedures, larger grains mechanically destroy microfossils that may have been preserved in the finer sediment. We employed a modified acid-maceration method for recovering microfossils from diamictites. The new approach recovered acritarch assemblages from fine-grained matrix of several late Neoproterozoic glaciogenic diamictites, and yielded more abundant and larger-sized microfossils compared to standard maceration, including long filaments and cellular-aggregates. A twofold increase was observed in the maximum number of taxa/level extracted using the new method. Using the novel approach and standard palynological maceration, we investigated the Cryogenian-Ediacaran succession in the Vestertana Group in northern Norway. OWM diversity changes therein demonstrate a turnover from large eukaryotic OWM to a microbial mat-builder community in the glacial intervals, to a low diversity post-glacial late Ediacaran assemblage during the rise of macroscopic life and the onset of bioturbation. We compared the Vestertana record to global DPA occurrences. Although one DPA assemblage zone postdates the Shuram carbon isotopic excursion, no DPA occur above Ediacaran glacial deposits where those are present. Considering this, and the community changes in the Vestertana succession, we suggest that DPA were hindered by the onset of an Ediacaran glaciation.