Modelling the sedimentary source and diagenetic fractionation of Rare Earth Elements in the ocean

Rare Earth Elements (REE) are among the key tracers in modern chemical oceanography. Yet their budgetary imbalance in the modern ocean has limited our understanding of their marine biogeochemical cycling and their applications as paleo-proxies. The flux of REE across the sediment-water interface appears to be the dominant source of REE to the ocean. Most studies on the marine and sedimentary cycling of REE focus particularly on neodymium (Nd), which has a radiogenic isotope system that helps to constrain its ocean budget. Recently we developed a reactive transport model to study the diagenesis of Nd at a deep sea site on the Oregon margin, which successfully explained the distributions of Nd and its radiogenic isotope in pore water and authigenic sediment, and the diagenetic control of the benthic Nd flux to the ocean. Here we extend this model to include the whole REE series. We show that the transformation and fractionation of REE in sediment can be adequately modelled only via reactions between pore water, authigenic Fe/Mn oxides and phosphates. Using the model result we can scale the sedimentary sources of the rest of the REE elements to that of Nd, providing better constraints on the ocean budgets of all REE.