Rare earth element cycling and reaction path modeling across the chemocline of the Pettaquamscutt River estuary, Rhode Island

Abstract

Rare earth elements (REEs) were measured in the water column of the highly stratified and euxinic Upper Basin of the Pettaquamscutt River estuary (Rhode Island) to investigate their cycling and evolution across the chemocline. The euxinic bottom waters of the estuary exhibit total dissolved sulfide (i.e., ΣS-II) concentrations up to 4 mmol kg−1, which exceeds values of the Black Sea by nearly 10-fold and makes this system ideal for study of redox-sensitive elements. The REE concentrations decrease with depth in the oxic surface waters, before substantially increasing across the chemocline, corresponding to redox cycling of manganese (Mn) and iron (Fe). For example, the mean ± 1σ neodymium (Nd) concentration in the oxic surface waters is 511 ± 184 pmol kg−1, but increases by approximately 5-fold across the chemocline reaching 2476 pmol kg−1 at 7.4 m depth. Concurrently, Mn and Fe concentrations increase by 5- and 11-fold, respectively, within the chemocline relative to their oxic surface water values. The pore water within the Upper Basin sediments exhibits REE concentrations that are on average 130-fold greater than their values in the overlying waters, driving an upward diffusive REE flux into the euxinic bottom waters. An outstanding feature of the REEs in this study is the preservation of negative cerium (Ce) anomalies across the chemocline into the highly euxinic bottom waters. Likewise, our reaction path model shows that the mass balance of the local water sources is the major driver of the REE compositions of the water column. Primarily, the negative Ce anomaly in the euxinic bottom waters highlights the influence of the Ce depleted sediment pore water on the REE composition and fractionation patterns of the euxinic bottom waters. This study underscores the importance of local sources and indicates that negative Ce anomalies can persist in highly euxinic waters that evolve from Ce depleted sources, further demonstrating that negative Ce anomalies are not necessarily a good proxy of the redox conditions of aqueous environments.