Are Clay Minerals the Primary Control on the Oceanic Rare Earth Element Budget?

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 waters 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 waters 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.

Fractionation Trends and Variability of Rare Earth Elements and Selected Critical Metals in Pelagic Sediment from Abyssal Basin of NE Pacific (Clarion-Clipperton Fracture Zone)

“…These observations support conclusions that during oxic diagenesis, REE which initially had adsorbed into Fe-Mn minerals (mostly vernadite, birnessite and todorokite) were later released into pore waters, and resorbed by apatite [22,29]. Additionally, REE sources may be related to Fe-rich authigenic (diagenetic) clay minerals and admixtures of allogenic components from continental areas [14,19,30]. The REE accumulation in pelagic sediments may also be related to external influx of Fe/Mn-microparticles [31].…”

Small-scale heterogeneity of trace metals including rare earth elements and yttrium in deep-sea sediments and porewaters of the Peru Basin, southeastern equatorial Pacific

“…The sedimentary REY SN patterns with La/Yb 1; negative Ce SN anomaly; and positive La SN , Gd SN , and Y SN anomalies are similar to REY SN patterns reported for nontronites (Fig. 9, Murnane and Clague, 1983;Alt, 1988;Mascarenhas-Pereira and Nath, 2010), which are expected to occur in these sediments because of the observed tan-green color change and the high Fe/Al ratio. The published nontronite REY SN patterns, however, refer exclusively to hydrothermally produced nontronites and the nontronite in cores from this study are not hydrothermally affected but rather derived from altered clay minerals or Fe (oxyhydr)oxides (e.g., Cole, 1985).…”

“…The carrier phase for the REY could therefore be a Fe-rich clay such as nontronite. Clay minerals have been postulated by others as the primary phase controlling porewater and solid-phase REY cycling (Zhang et al, 2016;Abbott et al, 2019). The REY also correlate with Al at Small Crater and at DEA West until approx.…”

“…The analyses, however, were based on one core per work area only, separated by hundreds of kilometers. Similarly, many studies in the past collected cores for porewater and solid-phase geochemical analyses based on sparse sampling distribution and spread over large areas (e.g., Froelich et al, 1979;Klinkhammer, 1980;Toyoda and Masuda, 1991;Drodt et al, 1997;König et al, 1997König et al, , 1999Haley et al, 2004;Schacht et al, 2010;Kim et al, 2012;Soyol-Erdene and Huh, 2013;Kon et al, 2014;Deng et al, 2017;Volz et al, 2018;Abbott et al, 2019). Processes might, however, vary even on small spatial…”

“…Clay minerals such as illite and kaolinite show flat REY patterns when normalized to post-Archean Australian shale (PAAS), European shale (EUS), or any other analogue of average upper continental crust material, due to their detrital origin (Cullers et al, 1975;Prudêncio et al, 1989;Tostevin et al, 2016). Nontronite of hydrothermal origin displays seawater-like REY SN patterns, except for a less pronounced Ce SN anomaly (Murnane and Clague, 1983;Alt, 1988) and sometimes a Eu SN anomaly (Mascarenhas-Pereira and Nath, 2010). To the best of our knowledge, no REY data from non-hydrothermal nontronite have been published yet.…”