Carbon and sulfur cycling during the mid-Ludfordian anomaly and the linkage with the late Silurian Lau/Kozlowskii Bioevent

The Ludfordian (late Silurian) records one of the largest perturbations in the Phanerozoic marine carbon cycling, the mid-Ludfordian positive delta C-13(carb) excursion (MLCIEcarb), which occurred in the aftermath of the Lau/Kozlowskii Bioevent (LKB). There are still controversies regarding origin of the MLCIE and its possible causal link (s) with respect to the preceding LKB. Surprisingly, the isotope fractionation of carbon between inorganic and organic carbon pools during the MLCIE was not investigated in detail. To constrain better this parameter of marine C cycle, its linkage to the LKB, and local sulfur cycle, we analysed high-resolution delta C-13(carb), delta C-13(org), and delta S-34(py) records in the Prague Basin (peri-Gondwana) representing one of the most complete records of the MLCIEcarb in the world. Evaluation of these high-resolution delta C-13(org) and delta C-13(carb) records reveals their tight coupling, which in turn indicates the photosynthetic origin of the MLCIEorg via links to primary production and changing rates of organic carbon burial. On the other hand, our data also reveal a short-term decoupling of the delta C-13(org) and delta C-13(carb) records preceding the MLCIE which may have been related to a purported increase in atmospheric pCO(2) levels or anoxic event. The peak of the Delta C-13 anomaly exactly coincides with the LKB and, therefore, for the first time suggests a tight link between the change in marine carbon cycling and the bioevent. Subsequent rapid drop of the Delta C-13 values (and by inference also atmospheric pCO(2)) fits well with a rapid cooling and globally documented regression reported for this time interval. Finally, the documented strong coupling of delta C-13(carb) and delta S-34(py) records during the MLCIE points to an intensive microbial sulfate reduction which should increase carbonate alkalinity input, thus contributing to formation of authigenic carbonates and an increase in delta C-13 values of DIC (and anticipated seawater carbonate saturation). The coupling of delta C-13(org) and delta S-34(py) values suggests a global enhancement of organic carbon and pyrite burial likely driven by an expansion of euxinic (anoxic and sulfidic water column) conditions in the oceans. Present study of C and S isotope variations during the MLCIE reveals for the first time a tight link among carbon and sulfur cycling, global cooling, and reorganization of faunal communities and fits well with purported seawater carbonate hypersaturation at the onset and during MLCIE.

Automated summary

The Ludfordian period saw a significant perturbation in marine carbon cycling, with the MLCIEcarb being one of the largest events. Controversies exist regarding its origin and connection to the LKB, but studies suggest a link to organic carbon storage and primary productivity fluctuations.