Metal induced chitinozoan malformations link the Mid-Ludfordian OAE to the venting of metalliferous hydrothermal basin brines into the Silurian Ocean
The mechanisms driving Silurian Ocean Anoxic Events (OAEs) and their impact on marine biota remain poorly understood. Previous studies linked chitinozoan (fossil zooplankton) malformations to heavy metal toxicity under expanding reducing oceanic conditions during the onset of some of these events. Importantly, these malformations represent in vivo reactions to changing paleoenvironmental stress and may serve as a sensitive indicator of marine metal loading and the geochemical processes driving Silurian extinction events. In the current study this hypothesis is tested for the prominent Mid- Ludfordian (upper Silurian) OAE. The onset of the event is examined using the thick and well preserved stratigraphic records in outcrop at Bodudd 1 (Gotland, Sweden). A bed-by-bed collection of 101 samples were analyzed for their whole rock geochemistry with chitinozoan malformations quantified 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, malformations appear in the lowermost sample in the section, below the carbon isotope excursion, 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 stratigraphic position of metal anomalies corresponds with an extraordinarily rapid rise in marine Sr isotope compositions. The pulsed nature of trace metal enrichments, the distinct elemental suite, and diagnostic spikes in radiogenic 87 Sr/ 86 Sr support the emerging model that episodic venting of metalliferous hydrothermal brines was the primary trigger for this and other Silurian OAEs.