Constraining causes of fish mass mortality using ultra-high-resolution biomarker measurement

•Sub-annual, high resolution biomarker data obtained for a mass mortality horizon (MMH)•Biomarker parameters suggest incursions of marine water varied over time.•For the studied MMH they had become infrequent.•Fish killed by hypoxic conditions subsequent to an incursion of marine water

Lamina by lamina measurement of biomarkers at a sub-millimetre resolution within the Achanarras Limestone Member has helped to resolve the changing environmental conditions associated with a fish mass mortality horizon. An anomalous proportion of C30 sterane (24-n-propylcholestane) marks the beginning of the horizon and likely corresponds to an influx of marine water. This appears to have been short lived and was likely analogous to a modern day storm tide. The subsequent laminae record an increased incidence of water column stratification and hypoxic bottom waters in the form of an elevated gammacerane index. The mass mortality horizon studied was from an upper interval of the Achanarras Limestone Member with a fossil fish assemblage comprising mostly Dipterus, an early Dipnoan (lungfish). However, lower intervals of the Achanarras Limestone Member have greater assemblage diversity, including species associated with marine conditions such as Coccosteus, and evidence higher proportions of C30 sterane indicating better connection to the marine environment. Therefore, it appears that ingressing seawater in and of itself was not responsible for creating a stressed environment. Rather, disconnection of the lake from marine waters stranded fish in a lake, that when perturbed by storm tides, killed en masse by exposing fish to hypoxic conditions in a similar way to modern water bodies affected by storm tides generated during hurricanes.