Behaviour of chromium and chromium isotopes during estuarine mixing in the Beaulieu Estuary, UK

Abstract

Rivers are the principal source of chromium (Cr) to seawater and the Cr isotopic signatures of ancient marine sediments are widely considered to provide a record of the presence or absence of oxidative weathering processes on land. This assumes, however, that the δ53Cr value of river water is faithfully transferred to the oceans and is not modified in the estuarine mixing zone. To test this assumption we have determined the concentration and δ53Cr values of inorganic Cr (Cr(III) + Cr(VI)), and also Cr speciation for water samples collected within the estuarine mixing zone of the Beaulieu River, UK. The δ53Cr values of dissolved inorganic Cr ranged from −0.59 to 1.68‰, Cr(VI) concentrations from 0.39 to 1.83 nmolkg−1 and Cr(III) concentrations from 0.11 to 3.21 nmolkg−1. Both Cr(VI) concentrations and δ53Cr values increased linearly as a function of salinity, while Cr(III) concentrations decreased linearly with salinity. Thus δ53Cr, Cr(III) and Cr(VI) all showed conservative behaviour in the estuarine mixing zone, and the δ53Cr signature of Beaulieu River water was modified only by mixing between the river and seawater endmembers. The calculated average δ53Cr value of the river water endmember (−0.39 ± 0.08‰) was, however, lower than the range that has been observed in other rivers, which we attribute to input of organically-bound Cr(III) released by anoxic weathering processes. This is supported by the fact that Cr recovered by UV irradiation was found to have low δ53Cr values (−0.11 to −0.75‰). While input of Cr from anoxic weathering processes is unlikely to be an important source of Cr to the oceans today, this suggests that processes other than oxidative weathering may have an influence on the δ53Cr values of estuarine and coastal waters on the local scale. The δ53Cr value of the coastal seawater endmember (1.6 ± 0.4‰) was also higher than the range observed in the deep open ocean, due to in situ biogeochemical cycling of Cr. These factors need to be considered in the interpretation of marine sedimentary δ53Cr records.