NEW INSIGHTS INTO EARLY VASCULAR LAND PLANTS AS GEOBIOLOGICAL AGENTS OF PHOSPHORUS WEATHERING

The emergence of vascular land plants with deep rooting systems during the late Silurian and Early Devonian periods is suggested to be one of the most profound changes to the Earth system. Previous work has suggested that the rise of vascular land plants ushered in prominent changes to marine biogeochemical cycling. This view arises from the fact that land plants with roots can increase both physical and chemical weathering. As weathering on land dictates nutrient and trace element transport to the oceans, vascular land plants may have modified ocean chemistry by impacting weathering. Because the nutrient phosphorus (P) is ultimately derived from the continents, the radiation of vascular land plants may have played an important role in altering P delivery to the ocean. However, P can enter the ocean as either unreactive detrital apatite, or as bioavailable dissolved P. Apatite dissolution can be enhanced by physical weathering from plant roots that increases mineral surface area, as well as by organic acids secreted from plant roots that lower soil pH. This study tests the hypothesis that early vascular plants increased bioavailable P delivery to the oceans through increased continental apatite weathering during the Silurian-Devonian transition.

Here we present new P geochemistry data from a >700-meter succession of lower–middle Paleozoic marine shales from Bathurst Island, Nunavut, Canada. We highlight stratigraphic patterns contemporaneous with a shift from a pre-vascular plant baseline in the mid-upper Silurian to the Lower Devonian when vascular plants first emerged and radiated. We report P/Al and P/Ti data recording detrital apatite fluxes, as well as chemical index of alteration data to constrain trends in chemical weathering intensity through this critical interval. This dataset brings new insights to the longstanding question of the extent to which the radiation of vascular land plants impacted continental nutrient fluxes or marine P cycling.