MULTIPROXY CHEMOSTRATIGRAPHIC CONSTRAINTS ON PALEOREDOX CONDITIONS DURING THE EARLY TO MIDDLE ORDOVICIAN INTERVAL IN THE BALTIC BASIN

The Early to Middle Ordovician represents an important period of Earth history as it captures a climate transition and the onset of one of the most prominent biodiversification events in the Phanerozoic (i.e., the Ordovician Radiation / Great Ordovician Biodiversification Event). While previous studies suggest atmospheric and marine oxygenation may have played a central role in driving the Ordovician Radiation, however, more recent studies suggest a nuanced paleoredox dynamics surrounding this event. In this study, we aim to refine our understanding of Ordovician marine paleoredox structures using a suite of redox-sensitive geochemical proxies such as iron speciation, pyrite sulfur isotope, and trace metal concentration data from an organic-rich shale succession from the paleocontinent of Baltica.

The focus of the study is the Lerhamn drillcore from Skåne, the southernmost province of Sweden, utilizing a multiproxy approach. This core consists of dark to light grey or greenish shale which forms part of the Tøyen Formation. Graptolite biostratigraphy suggests that the Tøyen Formation in this core spans the upper Tremadocian Stage (Lower Ordovician) up to the middle Darriwilian Stage (Middle Ordovician). Here we analyzed a total of 89 samples for trace metal concentrations and iron speciation. Ratios of various iron phases as well as manganese concentrations are used to track changes in local redox conditions, whereas variations in trace metal concentrations such as vanadium and molybdenum are likely to be reflective of changes in global extent of anoxia and euxinia. Pyrite sulfur isotopes, iron speciation and trace metal concentrations data primarily characterize locally reducing marine conditions in this part of the Baltic Basin. The geochemical data produced in this study, together with previously published data from the Baltic Basin (e.g., Grönhögen-2015, Röstånga-2 and Tosterup-2 cores) will provide a more comprehensive picture of the marine redox dynamics during the Early to Middle Ordovician.