WATERMASS RECONSTRUCTION IN THE EARLY MISSISSIPPIAN APPALACHIAN SEAWAY BASED ON REDOX AND SALINITY PROXY VARIATION (Invited Presentation)

Redox state and salinity are fundamental properties of watermasses, and in modern environments, detailed spatial analysis of redox and salinity is possible through direct measurement. Watermass reconstruction is difficult in deep-time systems, however, because the sedimentary record of ancient watermasses is often incomplete or difficult to access on spatial scales large enough to permit basin-scale reconstruction. During the Devonian and Early Mississippian periods, the Appalachian Basin was characterized by a variably restricted watermass, leading to extensive deposition of black shale associated with bottom-water anoxia. Watermass reconstruction during deposition of these units has previously been attempted, but only on small spatial scales, with most studies focused on trace element proxies applied to individual study sections. Here, we focus on watermass reconstruction in the Lower Mississippian Sunbury Shale, and present a new geochemical dataset that spans five drill cores across a basin transect. Iron speciation and trace metal abundances reveal a strong paleoredox gradient that can be related to water depth and basin hydrography. In the northeastern-most core of Ohio, iron speciation, organic carbon, and trace metal contents reveal oxic conditions, which were likely related to proximity to the Catskill Delta that fed freshwater into the basin. In central and southern Ohio, however, paleoredox indicators reveal ferruginous conditions associated with rapid deepening into the central axis of the Appalachian Basin. Further to the south in Kentucky, our data reveal euxinic conditions, which developed as environments shoaled towards the basin-bounding sill. We also investigated paleosalinity across the Appalachian Basin using Sr/Ba ratios. Our data reveal a subtle paleosalinity gradient from low-brackish conditions close to the Catskill Delta to increasingly brackish conditions to the southwest, with this trend broken by large paleosalinity variation across a shallow mixing zone in the southwestern-most core. Ultimately, this study is among the first to present detailed watermass reconstruction in an ancient epeiric sea using both redox and salinity proxies. We suggest that a similar approach may be useful in other deep-time systems of geobiological or paleoceanographic significance.