Behaviour of chromium and chromium isotopes during estuarine mixing in the Beaulieu Estuary, UK
Résumé
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 Cr 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 Cr 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 Cr values of dissolved inorganic Cr ranged from −0.59 to 1.68‰, Cr(VI) concentrations from 0.39 to 1.83 nmol kg−1 and Cr(III) concentrations from 0.11 to 3.21 nmol kg−1. Both Cr(VI) concentrations and Cr values increased linearly as a function of salinity, while Cr(III) concentrations decreased linearly with salinity. Thus Cr, Cr(III) and Cr(VI) all showed conservative behaviour in the estuarine mixing zone, and the Cr signature of Beaulieu River water was modified only by mixing between the river and seawater endmembers. The calculated average Cr 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 Cr 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 Cr values of estuarine and coastal waters on the local scale. The Cr 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 Cr records.