Distributions of dissolved titanium in porewaters of estuarine and coastal marine sediments

Sediment porewater and water column samples were collected in Chesapeake Bay and the Mid-Atlantic Bight (MAB) and were analyzed for dissolved (<0.45 μm) Ti as well as dissolved Al, Fe, Si, and NH4+. Dissolved Ti concentrations in the uppermost porewater (PW) intervals were from 8 to >50 times greater than bottom water (BW) Ti concentrations, with the steepest gradients occurring in muddy sediments of the mid-bay site (0.4–0.5 nM BW vs. 29 nM PW) and shallower gradients in siltier sediments of the lower bay and MAB (0.3–0.5 nM BW vs. 2.7–5.5 nM PW). Porewater Ti concentrations at all sites increased from the sediment–water interface to 6–9 cm depth, below which concentrations were relatively constant or decreased slightly to the bottom of the profiles (10–15 cm). The shapes of the profiles suggest the release of Ti from mineral phases or biological detritus in shallow sediments, and adsorption onto or incorporation within mineral phases or organic matter in deeper sediments. Calculated benthic diffusive fluxes of dissolved Ti range from 11–68 nmol m−2 day−1 in fine-grained mid-bay sediments to 1.5–4.2 nmol m−2 day−1 in sediments of the lower bay and MAB. A tentative estimate of the annual benthic flux of Ti in Chesapeake Bay (3.3×104 mol year−1) is comparable to the riverine input of 3.5×104 mol year−1. Experiments using both mud and sand indicated that sediment resuspension only slightly increased the Ti concentrations in overlying waters. These results suggest that benthic fluxes may be a significant source of Ti to estuaries and the ocean.