44-15 : Paleosalinity Analysis of the Late Devonian Marine-to-Terrestrial Transition, Appalachian Basin, North America: Implications for Eustatic Cycles and Vertebrate Habitats

Abstract:

Late Devonian mass extinctions, together constituting a ‘Big Five’ mass extinction of the Phanerozoic, were composed of multiple globally correlative biotic crises that occurred through the Frasnian and Famennian stages. In the latest Famennian, the Hangenberg Crisis strongly affected fauna that inhabited shallow epeiric seas and the marine-to-terrestrial transition zone, with a proposed driver of this extinction being the intrusion of toxic euxinic water into shallow seas during marine transgression. However, it can be difficult to reconstruct the exact timing and spatial extent of marine transgression relative to extinction pulses to determine potential cause-and-effect relationships. A proxy that can potentially facilitate reconstructing sequences of sea-level rise and fall relative to extinction horizons is the ratio of boron to gallium (B/Ga), a paleosalinity proxy applied to clay-rich siliciclastic rocks. Because clay minerals naturally absorb B in proportion to the salt content of the water they were formed in, higher ratios (>6) indicate marine conditions, intermediate ratios (>3&<6) indicate brackish conditions, and lower ratios (<3) indicate freshwater. Here, we present a paleosalinity analysis of field and core samples covering a transect from more distal (relative to the ancient Acadian Highlands) Famennian environments of Ohio (Cleveland Shale Member) to proximal localities in north-central Pennsylvania (Catskill, Lock Haven, and Oswayo formations). Preliminary data from northeast Pennsylvania field samples from an environment interpreted as west-facing rivers, estuaries, and deltas draining the ancient Acadian Mountains exhibit freshwater to brackish B/Ga paleosalinity values ranging from 2.2 (freshwater) to 4.4 (brackish), with these values implying lower salinity conditions than the brackish to marine gradient recorded in Ohio's Cleveland Shale. Paleosalinity interpretations are supported by primary physical observations such as sedimentary structures (e.g., symmetrical ripples vs desiccation cracks) and fossils (e.g., brachiopods vs root traces) that indicate deposition in distal fluvial channel-floodplain and tidal flat-channel environments. Our integrated paleosalinity, sedimentological, and paleontological data reveal previously undocumented relative sea-level fluctuations that may have influenced paleobiodiversity. Stratigraphic profiles will be developed for B/Ga paleosalinity in correlated Pennsylvania and Ohio Famennian sections of core and outcrop. Further analysis will include comparisons of B/Ga values with isotopic analysis of bioapatite from fish bone samples (independent paleosalinity proxy) to infer the salinity habitats of fish taxa and develop a robust multi-proxy reconstruction of changing sea-level during the latest Famennian. 

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