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 histories by exploiting relative changes in the distinctive normalised patterns.

However, the mechanisms by which water masses gain their patterns are yet to be

fully explained. To examine this, we collected water samples along the Extended Ellett

Line (EEL), an oceanographic transect between Iceland and Scotland, and measured

dissolved REE by offline automated chromatography (SeaFAST) and ICP-MS. The

proximity to two continental boundaries, the incipient spring bloom coincident with the

timing of the cruise, and the importance of deep water circulation in this climatically

sensitive gateway region make it an ideal location to investigate sources of REE to

seawater and the effects of vertical cycling and lateral advection on their distribution. The

deep waters have REE concentrations closest to typical North Atlantic seawater and are

dominated by lateral advection. Comparison to published seawater REE concentrations

of the same water masses in other locations provides a first measure of the temporal

and spatial stability of the seawater REE signal. We demonstrate the REE pattern is

replicated for Iceland-Scotland OverflowWater (ISOW) in the Iceland Basin from adjacent

stations sampled 16 years previously. A recently published Labrador Sea Water (LSW)

dissolved REE signal is reproduced in the Rockall Trough but shows greater light and

mid REE alteration in the Iceland Basin, possibly due to the dominant effect of ISOW

and/or continental inputs. An obvious concentration gradient from seafloor sediments to

the overlying water column in the Rockall Trough, but not the Iceland Basin, highlights

release of light and mid REE from resuspended sediments and pore waters, possibly

a seasonal effect associated with the timing of the spring bloom in each basin. The

EEL dissolved oxygen minimum at the permanent pycnocline corresponds to positive

heavy REE enrichment, indicating maximum rates of organic matter remineralisation

and associated REE release. We tentatively suggest a bacterial role to account for the

observed heavy REE deviations. This study highlights the need for fully constrained

REE sources and sinks, including the temporary nature of some sources, to achieve

a balanced budget of seawater REE.