PRELIMINARY INVESTIGATION OF ENVIRONMENTAL CONTROLS ON TRACE ELEMENT INCORPORATION INTO POTOMAC RIVER AND CHESAPEAKE BAY BENTHIC FORAMINIFERA

The Chesapeake Bay and its tributaries represent a dynamic estuarine ecosystem that, due to anthropogenic activities and climate change, has experienced increased eutrophication, summertime hypoxia, and changes in salinity gradients across the estuary over the last century. In this study, we use this dynamic environment as a natural laboratory to investigate environmental controls on trace element incorporation in the shells of two benthic foraminiferal taxa (Ammonia spp. and Elphidium spp.) as a means of developing geochemical paleoproxies of bottom water dissolved oxygen and salinity. Here we present preliminary results from samples collected across a salinity gradient in the Potomac River, a major tributary to the Chesapeake Bay, with samples collected during three different seasons, capturing varying dissolved oxygen, salinity, temperature, and pH conditions. Trace element geochemistry (i.e., Mg, Na, Sr, Ba, Mn, Zn) of foraminifera shells was measured using laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Preliminary results show foraminiferal Ba/Ca and Mg/Ca covary with salinity, where Ba/Ca negatively covaries and Mg/Ca positively covaries with salinity. Additionally, Zn/Ca shows a negative covariance with dissolved oxygen, suggesting its potential usage as a paleoredox proxy. Lastly, geochemical-environmental covariance tends to be stronger when examining sites along the salinity gradient during a single season when compared to data collected over all seasons and sites; this result shows the complex nature of trace element incorporation into foraminifera shells and suggests multiple environmental factors influence the incorporation of several of the examined elements. These preliminary results represent important building blocks in the use of benthic foraminiferal trace element geochemistry for reconstructing past environmental conditions in the Chesapeake Bay and similar estuarine environments.