Abstract
The rare earth elements (REEs) are an important tool for understanding biogeochemical cycling and sedimentary processes in the global ocean. However, ambiguities in the marine REE budgets, including questions around the dominant source of REEs to the ocean, hinder the application of this tool. A bottom-up model for REE release into the ocean has recently been proposed, driven by early diagenetic processes such as sediment dissolution, with potentially significant implications for the interpretation of marine REE and Nd isotope paleo-records. Here, our goal is to identify the phase or phases that interact with the pore fluids to drive such a benthic flux. We use new pore water REE, microbeam imaging and mineralogical data in combination with published pore water REE data to evaluate potential sedimentary REE host phases. Mineralogical and direct imaging observations suggest that authigenic Fe or Mn oxyhydroxides, which are widely considered a dominant REE host phase, are not sufficiently abundant sediment components to account for the high Nd concentrations recovered in reductive leaches, and are unlikely to be the primary source of pore water REEs. Pore water REE signatures similar to river sourced clays indicate a detrital clay dissolution source, while the spread in heavy to light REE enrichment in pore fluids and bottom waters relative to this clay source is best explained by fractionation during authigenic clay uptake of REEs. We therefore conclude that clay mineral dissolution and authigenesis are likely the primary influences on the REE cycling near the seafloor. We propose that the balance between dissolution and authigenesis controls the concentration, ratio of heavy and light REE abundances, and the isotopic composition of the pore waters. We discuss the implications of this hypothesis on an oceanic REE budget controlled by a benthic flux from a sedimentary REE source, and the use of authigenic neodymium isotopes as a paleoproxy for shifts in ocean circulation.
Highlights
IntroductionKnown as the lanthanides, the rare earth elements (REEs) are a series of 14 elements with largely coherent chemical properties (Elderfield and Greaves, 1982) used for applications ranging from reconstructing circulation and oxygen content to examining the influence of diagenesis
The rare earth elements (REEs) are widely used in paleoceanographic studies
We argue that a significant role of surface sediments in the REE cycle is consistent with observations that corals, Fe-Mn nodules, and co-located sediment phases have the same εNd as bottom waters, with sediment diagenesis providing the Nd to bottom waters as well as the nodules and corals (e.g., Elderfield and Greaves, 1982; Abbott et al, 2015a; Roberts and Piotrowski, 2015)
Summary
Known as the lanthanides, the REEs are a series of 14 elements with largely coherent chemical properties (Elderfield and Greaves, 1982) used for applications ranging from reconstructing circulation and oxygen content to examining the influence of diagenesis. Changes in global ocean circulation are inferred from the changes in the εNd of these authigenic records under the assumption that εNd is quasi-conservative in the global oceans (e.g., Vance and Burton, 1999; Frank, 2002; Haley et al, 2008; Böhm et al, 2015; Abbott et al, 2016b; Deaney et al, 2017)
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