The Impact of Benthic Processes on Rare Earth Element and Neodymium Isotope Distributions in the Oceans

Abstract

Neodymium (Nd) isotopes are considered a valuable tracer of modern and past ocean circulation. However, the promise of Nd isotope as a water mass tracer is hindered because there is not an entirely self-consistent model of the marine geochemical cycle of rare earth elements (REEs, of which Nd is one). That is, the prevailing mechanisms to describe the distributions of elemental and isotopic Nd are not completely reconciled. Here, we use published [Nd] and Nd isotope data to examine the prevailing model assumptions, and further compare these data to emergent alternative models that emphasize benthic processes in controlling the cycle of marine REEs and Nd isotopes. Our conclusion is that changing from a "top-down" driven model for REE cycling to one of a "bottom-up" benthic source model can provide consistent interpretations of these data for both elemental and isotopic Nd distributions. We discuss the implications such a benthic flux model carries for interpretation of Nd isotope data as a tracer for understanding modern and past changes in ocean circulation.

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

“…Our EDS elemental mapping suggests the primary iron host in the sediments studied here to be the clay mineral fraction (Figure 5). Indicators more commonly interpreted as evidence for oxyhydroxide coating recovery during reductive leaches, including trace metal ratios within the leachate (e.g., Gutjahr et al, 2007;Du et al, 2016), REE patterns (e.g., Gutjahr et al, 2007), and the agreement between the recovered neodymium with the overlying seawater (e.g., Elderfield and Greaves, 1982;Palmer and Elderfield, 1985;Rutberg et al, 2000;Roberts and Piotrowski, 2015) are also consistent with a benthic control model of REEs (Haley et al, 2017) that could be driven by clay minerals as we propose here. Further supporting this hypothesis is the correlation between finer average grain sizes, higher pore water REE concentrations (e.g., Abbott et al, 2016a), and higher sediment REE concentrations (e.g., Sa et al, 2018); consistent with higher reactivities of fine-grained, high surface clays than of coarser sediment component.…”

“…Indeed, the more distal sites with smaller grained sediments likely have larger fluxes (e.g., Holdren and Berner, 1979;Abbott et al, 2016a;Sa et al, 2018). A higher flux from distal sites could explain the isotopic agreement between modern core top leachates, pore water, and bottom water particularly if such a benthic flux is the dominant source of ocean Nd (e.g., Elderfield and Greaves, 1982;Abbott et al, 2015a;Haley et al, 2017). From this perspective, the absence of an anomalous isotopic signature in deep water regions is not indicative of a modest benthic flux, rather it is a predictable consequence of a long-term benthic flux from a major proportion of the seafloor.…”

Rare Earth Element Distribution in the NE Atlantic: Evidence for Benthic Sources, Longevity of the Seawater Signal, and Biogeochemical Cycling

“…The discussion in the literature on sources of REE to the deep marine water column describes vertical scavenging by particulate REE capture in surface waters and release at depth (e.g., Tachikawa et al, 1999;Siddall et al, 2008). A benthic flux from pore waters to the overlying water column may also be important for the overall marine REE budget (Elderfield and Sholkovitz, 1987;Haley et al, 2004Haley et al, , 2017Lacan and Jeandel, 2005b;Abbott et al, 2015a,b), where vertical scavenging alone cannot account for deep water REE concentrations (Elderfield and Greaves, 1982). Similar to the benthic flux are REE released into solution during sediment resuspension, through processes of fine particle dissolution, dissolution of labile phases adhered to the particles, and pore water release through disturbance of the sediment (Jeandel et al, 1998;Zhang and Nozaki, 1998;Lacan and Jeandel, 2005b;Arsouze et al, 2009;Pearce et al, 2013;Stichel et al, 2015;Grenier et al, 2018).…”

“…The emerging picture of REE cycling in the ocean is one of dominant removal (∼70%) of riverine REE in estuaries (Goldstein and Jacobsen, 1988;Sholkovitz, 1993), ∼20% (Nd) contribution from aeolian deposition (Tachikawa et al, 1999), negligible REE from hydrothermal venting (German et al, 1990), and variable contributions from sediments. This latter point includes diagenetic release of REE from pore waters (Abbott et al, 2015b;Haley et al, 2017), partial dissolution of particulates (Grenier et al, 2013;Pearce et al, 2013), and release from riverborne particulates in estuarine environments (Rousseau et al, 2015). These seawater-sediment interactions are described by "boundary exchange" and can result in release or scavenging of REE (Lacan and Jeandel, 2005b;Jeandel et al, 2007Jeandel et al, , 2011Jeandel and Oelkers, 2015;Jeandel, 2016).…”

Revealing past ocean circulation with neodymium isotopes

“…This can transfer Nd vertically through the water column and across watermass boundaries. At the ocean bottom, fluxes of Nd across the sediment-seawater interface have been observed both into and out of bottom waters(Haley et al 2017). In general, such benthic exchange depends on the reactivity of detrital material releasing Nd in the sediment.…”