The relationships between Late Ordovician sea-level changes and faunal turnover in western Baltica: Geochemical evidence of oxic and dysoxic bottom-water conditions

Abstract

The late Sandbian to the early Katian (Late Ordovician) in southeastern Norway is dominated by marine mudstones that contain an exceptional spectrum of macrofauna and trace element geochemistry, recording both abrupt and gradual faunal changes during major sea-level and environmental shifts. This study investigates the relationships between sea-level changes and their influence on the source of clastic material, oxygen levels in the bottom water and faunal changes, with special emphasis on the brachiopod fauna. Trace element ratios indicate that nearly stable upper dysoxic bottom-water conditions prevailed in the northwestern part of the Baltoscandian Sea. The exception is during the major shallowing in the earliest Katian, when there was an abrupt shift to oxic conditions as the sea bottom came within a normal storm wave base. Nonetheless, the pre-shallowing epibenthic fauna (though not the shelly endobenthic) in this area was rich and diverse and two major immigration phases of new brachiopod taxa are seen well before the shallowing. This indicates that the immigration of new taxa was not the result of an increase in oxygen content in the bottom water. More brachiopod genera stayed during the abrupt shallowing and increased oxic level than did during the following gradual return to deeper, dysoxic environments. The major brachiopod immigration phases took place markedly earlier in this northwestern part of the Baltoscandian Sea than in the central part (Eastern Baltic) and possibly also the comparable faunal turnover in Laurentia. The following disappearance of taxa during an early Katian transgressive event coincided with the faunal turnover in the shallow-water environments of the East Baltic. The depositional history was consistent within the investigated mudstone-dominated offshore facies of the Oslo Region, which are comparable to the Central Baltoscandian Confacies Belt of Baltoscandia. The composition of the siliciclastic material which was deposited in the basin was nearly constant through the sequence, though the land area expanded to include ophiolites, expressed by Cr enrichment, coinciding with the major sea-level drop. The Cr enrichment indicates that, by that time at least, the ophiolite complexes north of the Oslo Region became subaerially exposed. This enrichment thus forms a potential geochemical marker horizon representing the basal Katian Stage.