Cr isotope systematics in the Connecticut River estuary

Abstract

Stable chromium (Cr) isotopes (expressed in δ53Cr as the deviation of 53Cr/52Cr in samples from the international standard NIST SRM 979) are an emerging paleoredox proxy that has been used to track Earth's redox evolution over a range of timescales. There have been several major advances in our understanding of the Cr isotope system, but we are still developing a basic knowledge of the global Cr isotope mass balance and there are key Cr fluxes with poorly constrained isotopic compositions. The Cr isotope behavior in estuaries is one such poorly constrained aspect of the modern Cr cycle. Therefore, we collected water and suspended particulate samples from the estuary of the Connecticut River, and present the first Cr isotope dataset from a salinity gradient. As pH and salinity increases from brackish to salt water, dissolved Cr increases from 118.1 ± 25.7 ng/kg (1σ, n = 6) to 225.1 ± 26.0 ng/kg (1σ, n = 8), while particulate Cr decreases from 341.3 ± 319.8 ng/kg (1σ, n = 6) to 208.4 ± 364.5 ng/kg (1σ, n = 8); dissolved δ53Cr decreases from 1.30 ± 0.27‰ (1σ, n = 6) to 0.64 ± 0.16‰ (1σ, n = 8), whereas particulate δ53Cr remained stable at 0.11 ± 0.07‰ (1σ, n = 9). These data suggest that Cr is lost from particles to solution when transitioning from brackish to salt water in the Connecticut River estuary. If these observations can be confirmed in other estuaries, estuary systems can potentially enhance riverine dissolved Cr fluxes and thus lower the Cr oceanic residence time, which has important implications for the Cr isotope system as a paleoceanographic redox proxy. There is a clear inverse correlation between dissolved Cr and δ53Cr in the Connecticut River estuary, which yields a Cr isotope fractionation factor of ~0.89‰ that is indistinguishable from previous estimates based on open seawater samples. Large δ53Cr variation in oxygenated open and estuary seawater suggests that marine δ53Cr variations need not necessarily be exclusively tied to Cr(VI) reduction in low oxygen zones. Therefore, interpreting δ53Cr in the sedimentary record requires taking multiple processes into consideration.