Late Ordovician Ocean Chemistry and Biological Evolution

Late Ordovician was a time of dramatic changes of environment and biosphere. It was estimated that atmospheric CO2 was probably 10-16 times of the present atmospheric level. However, under such high pCO2 condition, a major glaciation was developed at high-latitude areas. Coupled with climate changes, the Late Ordovician fossil records indicate that global diversity of marine animals decreased following the Middle Ordovician biodiversification. Subsequently, the diversity of marine animals rebounded into early Ashgillian before the mass extinction at the end of the Ordovician.

To investigate influence of ocean chemistry change on the Late Ordovician biological evolution, we performed detailed S-C systematic study on the Late Ordovician sedimentary rocks from the Lysogory Block in the south-eastern Poland. The siliciclastic succession of the Late Ordovician in the Lysogory Block is composed monotonously of mudstones. We collected ~200 samples in high-resolution from two drill cores in the northern Holy Cross Mountains spanning early Caradoc to late Ashgill Stage. The early-middle Caradocian mudstones are characterized by high pyrite contents with most of Spy between 0.5 to 2.0%. The pyrite contents decrease into middle-late Caradoc with an average of 0.5% for Spy. In contrast, the Ashgillian mudstones contain much less pyrite with most of Spy<0.1%. The decreases of pyrite content in time may reflect oceanic redox chemistry changes. S-isotopic values of pyrites are consistent with this model. A wide spread S-isotopic values of pyrites (-10 to +15 per mil) in the early-middle Caradocian mudstones is observed. However, the S-isotopic compositions of pyrite become progressively heavier into the upper Caradoc that may have resulted from restricted input of sulfate. The Ashgillian mudstones show wider spread S-isotopic values. In context of paleogeographic reconstruction and sedimentology, we will discuss implications of oceanic redox chemistry change for decreased biodiversity and the following rebound in the Late Ordovician.