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

Abstract

Seawater rare earth element (REE) concentrations are increasingly applied to reconstruct\n\nwater mass histories by exploiting relative changes in the distinctive normalised patterns.\n\nHowever, the mechanisms by which water masses gain their patterns are yet to be\n\nfully explained. To examine this, we collected water samples along the Extended Ellett\n\nLine (EEL), an oceanographic transect between Iceland and Scotland, and measured\n\ndissolved REE by offline automated chromatography (SeaFAST) and ICP-MS. The\n\nproximity to two continental boundaries, the incipient spring bloom coincident with the\n\ntiming of the cruise, and the importance of deep water circulation in this climatically\n\nsensitive gateway region make it an ideal location to investigate sources of REE to\n\nseawater and the effects of vertical cycling and lateral advection on their distribution. The\n\ndeep waters have REE concentrations closest to typical North Atlantic seawater and are\n\ndominated by lateral advection. Comparison to published seawater REE concentrations\n\nof the same water masses in other locations provides a first measure of the temporal\n\nand spatial stability of the seawater REE signal. We demonstrate the REE pattern is\n\nreplicated for Iceland-Scotland OverflowWater (ISOW) in the Iceland Basin from adjacent\n\nstations sampled 16 years previously. A recently published Labrador Sea Water (LSW)\n\ndissolved REE signal is reproduced in the Rockall Trough but shows greater light and\n\nmid REE alteration in the Iceland Basin, possibly due to the dominant effect of ISOW\n\nand/or continental inputs. An obvious concentration gradient from seafloor sediments to\n\nthe overlying water column in the Rockall Trough, but not the Iceland Basin, highlights\n\nrelease of light and mid REE from resuspended sediments and pore waters, possibly\n\na seasonal effect associated with the timing of the spring bloom in each basin. The\n\nEEL dissolved oxygen minimum at the permanent pycnocline corresponds to positive\n\nheavy REE enrichment, indicating maximum rates of organic matter remineralisation\n\nand associated REE release. We tentatively suggest a bacterial role to account for the\n\nobserved heavy REE deviations. This study highlights the need for fully constrained\n\nREE sources and sinks, including the temporary nature of some sources, to achieve\n\na balanced budget of seawater REE.