Global controls on phosphatization of fossils during the Toarcian Oceanic Anoxic Event

Sinjini Sinha,Günter Schweigert,A D Muscente,Rowan C Martindale,Matt Williams,James D Schiffbauer

doi:10.1038/s41598-021-03482-7

Abstract

Konservat-Lagerstätten—deposits with exceptionally preserved fossils—vary in abundance across geographic and stratigraphic space due to paleoenvironmental heterogeneity. While oceanic anoxic events (OAEs) may have promoted preservation of marine lagerstätten, the environmental controls on their taphonomy remain unclear. Here, we provide new data on the mineralization of fossils in three Lower Jurassic Lagerstätten—Strawberry Bank (UK), Ya Ha Tinda (Canada), and Posidonia Shale (Germany) —and test the hypothesis that they were preserved under similar conditions. Biostratigraphy indicates that all three Lagerstätten were deposited during the Toarcian OAE (TOAE), and scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) show that each deposit contains a variety of taxa preserved as phosphatized skeletons and tissues. Thus, despite their geographic and paleoenvironmental differences, all of these Lagerstätten were deposited in settings conducive to phosphatization, indicating that the TOAE fostered exceptional preservation in marine settings around the world. Phosphatization may have been fueled by phosphate delivery from climatically-driven sea level change and continental weathering, with anoxic basins acting as phosphorus traps.

Highlights

Konservat-Lagerstätten—deposits with exceptionally preserved fossils—vary in abundance across geographic and stratigraphic space due to paleoenvironmental heterogeneity
The Strawberry Bank (UK), Posidonia Shale (Germany), and Ya Ha Tinda (Canada) Lagerstätten (Fig. 1) all occur within the Dactylioceras tenuicostatum, Harpoceras serpentinum, Hildoceras bifrons, and equivalent (Dactylioceras kanense and Rarenodia planulata) biozones of the early Toarcian and exhibit signs of the negative isotope excursion that is a signature of the TOAE9,10,15 (Fig. 2)
We evaluate a model for exceptional fossilization during OAEs18, which relates the origin of Lagerstätten to redox-dependent processes

Summary

Introduction

Konservat-Lagerstätten—deposits with exceptionally preserved fossils—vary in abundance across geographic and stratigraphic space due to paleoenvironmental heterogeneity. The Strawberry Bank Lagerstätte (Beacon Limestone Fm.) is interpreted as a shallow marine lagoon d eposit[10], while both the Ya Ha Tinda (Fernie Fm.) and Posidonia Shale (Posidonienschiefer Fm.) Lagerstätten record deeper marine settings, which intermittently developed bottom water a noxia[29] between times of oxic and suboxic c onditions[9,16,30] These three Lagerstätten have similar fauna (Fig. 4) including ichthyosaurs, ray-finned fishes, crinoids, coleoids (belemnites and vampyropods), ammonites, bivalves, and crustaceans[9,10,16,31]. The Beacon Limestone Formation lies above the Marlstone Rock Formation and below the Bridport Sand Formation[35], and according to ammonite biostratigraphy, represents the lower to upper Toarcian interval, falciferum biozone (Fig. 2)

Methods

Results

Conclusion