Bivalve beds reveal rapid changes in ocean oxygenation in the Boreal Middle Triassic – a case study from Svalbard, Norway

A fossil-rich interval in the organic-rich marine black mudstone of the Middle Triassic Botneheia on eastern Svalbard was logged in high-resolution on an extremely well exposed section with on bivalve beds, taphonomic features, trace fossils and oxygenation proxies. The size distribution,, articulation and orientation of three bivalve beds of the epifaunal flat clam were analysed. The logged section was studied for the recurrent occurrences of trace fossils and beds, and size distribution of framboidal pyrite. Daonella is most common in the shaly interval, other taxa seem to outcompete Daonella in the siltier intervals. The comparison of fossil and data with geochemical proxies for oxygenation revealed that the bivalve beds formed dysoxic, conditions and due to low sedimentation rates and winnowing are not mass-mortality as previously suggested. The deposition on the sea floor was interrupted by anoxic intervals benthic life. Recurrent beds containing the trace fossil Thalassinoides, however, show that oxygen fluctuated. The combination of water currents and oxygen fluctuations is a key to understanding why black shales of the Botneheia Formation are so rich in benthic fossils.