Century-long record of Mo isotopic composition in sediments of a seasonally anoxic estuary (Chesapeake Bay)

Abstract A double-spike method was used to obtain Mo isotope data for sediments and waters of the seasonally anoxic Chesapeake Bay, and its primary tributary, the Susquehanna River. The dissolved Mo distribution in the estuary is non-conservative, reflecting minor Mo loss to the sediments, although removal of Mo to the sediments does not have a large influence on the isotopic composition of the water column. The δ 98 Mo of dissolved Mo in most of the estuary is dominated by seawater. Six samples with salinity > 15 have an average δ 98 Mo = + 2.17‰ (± 0.12), which agrees well with a δ 98 Mo value for the CASS-4 seawater standard of + 2.23‰. A single sample of Susquehanna River water has a δ 98 Mo of + 1.02‰, consistent with recent findings of positive δ 98 Mo in rivers worldwide. Susquehanna river sediments, in contrast, have δ 98 Mo ∼ − 0.1‰. The difference between the river water and sediment values implies that isotopic fractionation occurs within the river basin. The δ 98 Mo values for estuarine sediments are offset from values in the overlying water. Most samples deposited before 1925 have δ 98 Mo less than 0‰, similar to the Susquehanna sediments. Subsequently, there is an increase in the variability of δ 98 Mo, with values ranging up to + 0.8‰. The transition to increased variability coincides with the onset of authigenic Mo deposition, which was previously attributed to escalating summertime anoxia. Authigenic Mo concentrations correlate poorly with δ 98 Mo in core samples, suggesting that independent mechanisms influence the two parameters. Authigenic Mo concentrations may be controlled by shifting pore water H 2 S levels, while δ 98 Mo may be primarily affected by annual variations in Mn refluxing.