Early Silurian δ¹³Corg excursions in the foreland basin of Baltica, both familiar and surprising

The Sommerodde-1 core from Bornholm, Denmark, provides a nearly continuous sedimentary archive from the Upper Ordovician through to the Wenlock Series (lower Silurian), as constrained by graptolite biostratigraphy. The cored mudstones represent a deep marine depositional setting in the foreland basin fringing Baltica and we present high-resolution data on the isotopic composition of the section's organic carbon (δ 13 C org ). This chemostratigraphical record is correlated with previously recognized δ 13 C excursions in the Upper Ordovician–lower Silurian, including the Hirnantian positive isotope carbon excursion (HICE), the early Aeronian positive carbon isotope excursion (EACIE), and the early Sheinwoodian positive carbon isotope excursion (ESCIE). A new positive excursion of high magnitude (~4‰)is discovered in the Telychian Oktavites spiralis Biozone (lower Silurian)and we name it the Sommerodde Carbon Isotope Excursion (SOCIE). The SOCIE appears discernible in δ 13 C carb data from Latvian and Estonian cores but it is not yet widely recognized. However, the magnitude of the excursion within the deep, marine, depositional setting, represented by the Sommerodde-1 core, suggests that the SOCIE reflects a significant event. In addition, the chemostratigraphical record of the Sommerodde-1 core reveals the negative excursion at the transition from the Aeronian to Telychian stages (the ‘Rumba low’), and suggests that the commencement of the EACIE at the base of the Demirastrites triangulatus Biozone potentially is a useful chemostratigraphical marker for the base of the Aeronian Stage

Persistent global marine euxinia in the early Silurian

The second pulse of the Late Ordovician mass extinction occurred around the Hirnantian-Rhuddanian boundary (~444 Ma) and has been correlated with expanded marine anoxia lasting into the earliest Silurian. Characterization of the Hirnantian ocean anoxic event has focused on the onset of anoxia, with global reconstructions based on carbonate δ238U modeling. However, there have been limited attempts to quantify uncertainty in metal isotope mass balance approaches. Here, we probabilistically evaluate coupled metal isotopes and sedimentary archives to increase constraint. We present iron speciation, metal concentration, δ98Mo and δ238U measurements of Rhuddanian black shales from the Murzuq Basin, Libya. We evaluate these data (and published carbonate δ238U data) with a coupled stochastic mass balance model. Combined statistical analysis of metal isotopes and sedimentary sinks provides uncertainty-bounded constraints on the intensity of Hirnantian-Rhuddanian euxinia. This work extends the duration of anoxia to >3 Myrs – notably longer than well-studied Mesozoic ocean anoxic events

On the graptolites described by Baily (1871) from the Silurian of Northern Ireland and the genus Streptograptus Yin

Volume: 33Start Page: 937End Page: 94