The sedimentary flux of dissolved rare earth elements to the ocean

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

“…One positive test for REE adsorption in the oceans may be seen in conservative Arctic Ocean REE profiles where particles are near absent (Yang and Haley, 2016). We do note, however, that a counter-argument can be made in that there does not appear to be an appreciable difference in the [Nd] profiles relating to marginal sites vs. distal sites, where particle loading may be very different (German and Elderfield, 1990;Grasse et al, 2012;Goswami et al, 2014;Haley et al, 2014;Abbott et al, 2015b;Stichel et al, 2015).…”

“…Other scenarios are obviously possible; for example, a 5 pmol/cm 2 /yr flux with ε Nd of −20 offers a similar 40 year exposure estimate, but predicts a final [Nd] of only 17 pM. The only existing pore waters from the Atlantic (Buzzard's Bay; Sholkovitz et al, 1989) show pore water [Nd] of >700 pM at ∼13 cm depth in the sediment, which compares to fluxes as large as any seen in the Pacific (up to 30 pmol/cm 2 /yr; Haley and Klinkhammer, 2003;Abbott et al, 2015b). Such a flux of 30 pmol/cm 2 /yr with an ε Nd of −25, as implied might be the case in this region , predicts only 4 years of exposure for ∼1 ε Nd unit change, which equates to 1.1 cm/s velocity, but an increase of [Nd] to just 16 pM.…”

“…Furthermore, eddy diffusivity is also an order of magnitude higher on the seafloor relative to the water column because of the turbulence created by topography (Waterhouse et al, 2014); therefore, a benthic flux will be dissipated with great efficiency into the water column. This dissipation is the rationale for the observation that there is an apparent jump in REE concentrations between pore water and overlying water (Abbott et al, 2015b(Abbott et al, , 2016.…”

“…There is little direct observational data regarding the benthic REE flux to the oceans Haley et al, 2004;Abbott et al, 2015b), and even less that tracks the ε Nd of this flux. However, comparison with radium offers some support for the hypothesis.…”

“…However, this supposition then begs the question of why reversible scavenging controls the modern deep [REE]/ε Nd ? On the other hand, in a benthic control scenario the ε Nd of bottom water may evolve through non-mixing processes: such as a change in the benthic flux or a change in the "exposure time" of the bottom water to this flux (Abbott et al, 2015b). As such, an ε Nd record interpreted through a benthic control model may reflect a change in benthic flux or exposure time, the latter of which may well be the result of changing circulation patterns or velocities.…”