Salinity variation in Carboniferous cyclothemic successions

Late Paleozoic seas on the North American Craton were subject to large fluctuations in elevation and facies in response to Gondwanan glacio-eustasy, but quantification of salinity variation has proven difficult. Here, we reconstruct salinity variation using two proxies (B/Ga and “excess boron” [B_xs]) through ten cyclothems spanning the upper Desmoinesian through Missourian stages (∼308–303 Ma) in a single core from the Late Pennsylvanian Midcontinent Shelf. These proxies record cyclic variation between normal-marine (30–40 psu) and high-brackish conditions (15–30 psu), with occasional excursions to low-brackish conditions (1–15 psu). Most limestone formations accumulated under marine salinity conditions although a few (i.e., the argillaceous Exline and Raytown limestones) were brackish. Shales exhibit a wider range of salinity variation: organic-rich core shales are associated with fully marine conditions and outside shales mostly with reduced-salinity conditions. The lithofacies and salinity history of the study core provide insights into contemporaneous tectono-glacial events of the Middle to Late Pennsylvanian transition: the Pawnee-Altamont cyclothems record a major basinal subsidence event followed by large-scale influx of deltaic siliciclastics of the Nowata Shale linked to latest Desmoinesian uplift in the Ouachita-Marathon orogens. This North American tectonic event, coupled with other concurrent orogens globally, may have triggered a deep glacially driven sea-level fall around the Desmoinesian/Missourian stage boundary. This event heralded the onset of large-amplitude, high-frequency glacio-eustatic fluctuations of the Missourian Stage, signaling a fundamental shift toward increased Gondwanan icesheet instability.