Rare earth element distribution in the NE Atlantic: evidence for benthic sources, longevity of the seawater signal, and biogeochemical cycling

Seawater rare earth element (REE) concentrations are increasingly applied to reconstruct water mass 19 histories by exploiting relative changes in the distinctive normalised patterns. However, the 20 mechanisms by which water masses gain their patterns are yet to be fully explained. To examine this, 21 we collected water samples along the Extended Ellett Line (EEL), an oceanographic transect between 22 Iceland and Scotland, and measured dissolved REE by offline automated chromatography (SeaFAST) 23 and ICP-MS. The proximity to two continental boundaries, the incipient spring bloom coincident with 24 the timing of the cruise, and the importance of deep water circulation in this climatically sensitive 25 gateway region make it an ideal location to investigate sources of REE to seawater and the effects of 26 vertical cycling and lateral advection on their distribution. The deep waters have REE concentrations 27 closest to typical North Atlantic seawater and are dominated by lateral advection. Comparison to 28 published seawater REE concentrations of the same water masses in other locations provides a first 29 measure of the temporal and spatial stability of the seawater REE signal. We demonstrate the REE 30 pattern is replicated for Iceland-Scotland Overflow Water (ISOW) in the Iceland Basin from adjacent 31 stations sampled 16 years previously. A recently published Labrador Sea Water dissolved REE signal 32 is reproduced in the Rockall Trough but shows greater light and mid REE alteration in the Iceland 33 Basin, possibly due to the dominant effect of ISOW and/or continental inputs. An obvious 34 concentration gradient from seafloor sediments to the overlying water column in the Rockall Trough, 35 but not the Iceland Basin, highlights release of light and mid REE from resuspended sediments and 36 pore waters, possibly a seasonal effect associated with the timing of the spring bloom in each basin. 37 The EEL dissolved oxygen minimum at the permanent pycnocline corresponds to positive heavy REE 38 enrichment, indicating maximum rates of organic matter remineralisation and associated REE release. 39 We tentatively suggest a bacterial role to account for the observed heavy REE deviations. This study 40 highlights the need for fully constrained REE sources and sinks, including the temporary nature of 41 some sources, to achieve a balanced budget of seawater REE.