Metal Induced Chitinozoan Malformations Link the Mid-Ludfordian OAE to the Venting of Metalliferous Hydrothermal Basin Brines into the Silurian Ocean

The primary processes driving Silurian Ocean Anoxic Events (OAEs) and their ecological consequences remain uncertain. Previous studies linked chitinozoan - fossil zooplankton - malformations to heavy metal toxicity under expanding reducing oceanic conditions during the onset of some of these events. Such deformities represent in vivo biological responses to changing paleoenvironmental pressure and may serve as a sensitive indicator of marine metal loading and the geochemical processes driving Silurian extinction events. Here we test this hypothesis for the prominent Mid- Ludfordian (upper Silurian) OAE. The island of Gotland (Sweden) in the western Baltic preserve some of the premier outcrop and subsurface records for study of the Mid-Ludfordian event. Here the pristine mixed carbonate-siliciclastic succession contains an +8‰ δ13Ccarb excursion and rapid shift in 87Sr/86Sr toward more radiogenic values. The onset of the event is examined using the thick and well-preserved stratigraphic records in outcrop at Bodudd 1 (Gotland, Sweden). Bed-by-bed collection of 101 samples was made for whole rock geochemistry and quantified chitinozoan malformations in 66 samples. A total of 587,241 specimens were counted with 133 confirmed malformations and 87 potentially malformed, occurring in two intervals that correlate with Mn enrichments. Notably, the lowermost sample in the section, below the carbon isotope excursion, already contains malformations, indicating that biotic stress preceded perturbations to the global carbon cycle. In addition to Mn, anomalies in Zn and Pb are present, alongside minor enrichments in Hg, Co, and Sb. The findings of this study demonstrate that the stratigraphic position of metal anomalies corresponds with the extraordinarily rapid rise in marine Sr isotope compositions. The episodic nature of trace metal enrichments, the distinct elemental suite, and diagnostic spike in radiogenic 87Sr/86Sr collectively support the emerging model that episodic venting of metalliferous hydrothermal brines acted as the primary trigger for this and other Silurian OAEs.