FE AND MN DISTRIBUTION IN PORE WATER: AN INDICATOR OF SUBMARINE GROUNDWATER DISCHARGE

Mixing of waters in subterranean estuaries causes variations in redox conditions, thereby controlling distributions of dissolved Fe and Mn. Under oxic conditions, these metals precipitate as oxyhydroxides, which sequester components such as PO4-3. Under anoxic conditions, the oxyhydroxides are used as electron acceptors to oxidize organic C . Metals are thus important for nutrient distributions and C budgets in estuarine sediments. Fe and Mn concentrations were measured in pore waters collected seasonally (11/04, 2/05, 5/05, 9/05) from Indian River Lagoon (IRL), Florida up to 230 cm below seafloor (cmbsf) at 8 stations in a 30 m shore-perpendicular transect. The samples cross a zone of mixed fresh water discharging from the Surficial aquifer and lagoon water circulated through the bottom sediments. Discharge of fresh water decreases linearly to 22 m offshore. All Fe+2 and Mn+2 concentration depth profiles display a single maximum, which decreases with depth and increases in concentration offshore. The Fe maximum increases to 360 µm at 22.5 m and the Mn+2 to 24 µm at 30 m offshore. Fresh water sampled from 210 cmbsf from a well onshore has Fe+2 and Mn+2 concentrations of 0 and 0.2 µm, respectively. DIC concentrations also increase offshore and show patterns similar to metals indicating a link between organic C and metal diagenesis. The Surficial aquifer is composed of quartz rich sandy sediments and sediment cores collected near the pore water samples are orange to yellow suggesting high concentrations of Fe and Mn oxyhydroxides. Our observed dissolved metal distributions contrast with the measurements of dissolved Fe and Mn in glacial outwash sediments of Waquiot Bay, MA where samples were collected across the boundary between freshwater and underlying salt water (Charette and Sholkovitz 2002). In Indian River Lagoon, metals appear to originate from estuarine sediments by microbial diagenesis unlike Waquiot Bay where Fe+2 and Mn+2 are carried to the estuarine system by terrestrial freshwater.