Salinity-redox covariation in Mississippian Bowland Shale (Craven Basin, UK) controlled by eustasy and climate

Abstract

Changes in watermass properties can be monitored in real time in modern depositional systems, but reconstruction in deep-time systems is difficult. Here, we analyze long-term changes in salinity (B/Ga, Sr/Ba, and S/TOC) and redox conditions (Fe_HR/Fe_T, Fe_py/Fe_HR, and C_org/P) in the upper unit of the Mississippian Bowland Shale Formation, Craven Basin, UK. Variations in paleosalinity and redox in the Upper Bowland Shale were tightly coupled to fourth-order glacio-eustatic cyclicity. At basinal sites (borehole MHD and outcrop HC), transgressive to highstand packages (‘marine bands’) were associated with weakly hypersaline to normal marine, euxinic bottom-water conditions. In contrast, falling-stage and lowstand packages were deposited under normal-marine salinity and fluctuating euxinic to ferruginous anoxic conditions punctuated by (sub)oxic events. HC, located within a bathymetric low, exhibits significantly higher C_org/P (∼100) than MHD (<50), located on an intrabasinal high, due to the more limited influence of oxygen-bearing hyperpycnal flows in the deep basin. A sharp shift to brackish conditions near the top of the HC section reflects the basinward progradation of the Pendle Paleodelta in response to long-term eustatic fall. In the relatively more proximal (landward) and stratigraphically younger COM borehole, the pattern of salinity-redox covariation in response to sea-level cyclicity is different: highstands were associated with normal-marine salinity and euxinic conditions, whereas lowstands exhibit shifts toward brackish, oxic bottom-water conditions, reflecting shoaling of this site into the ocean-surface layer and influence from the nearby Pendle deltaic system. The ultimate driver of late Mississippian sea-level fluctuations was growth and decay of Gondwanan icesheets during an early phase of the Late Paleozoic Ice Age, with interglacial stages potentially being marked by a shift toward stronger evaporation within the climatically sensitive zone occupied by the Craven Basin (∼10–15°S paleolatitude). Our findings provide new insights into the sea-level-redox-salinity dynamics of marginal-marine basins during a major ice age.