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 Overflow Water (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.

Seawater rare earth element (REE) concentrations are increasingly applied to reconstruct water mass 18 histories by exploiting relative changes in the distinctive normalised patterns. However, the 19 mechanisms by which water masses gain their patterns are yet to be fully explained. To examine this, 20 we collected water samples along the Extended Ellett Line (EEL), an oceanographic transect between 21 Iceland and Scotland, and measured dissolved REE by offline automated chromatography (SeaFAST)  22 and ICP-MS. The proximity to two continental boundaries, the incipient spring bloom coincident with 23 the timing of the cruise, and the importance of deep water circulation in this climatically sensitive 24 gateway region make it an ideal location to investigate sources of REE to seawater and the effects of 25 vertical cycling and lateral advection on their distribution. concentration gradient from seafloor sediments to the overlying water column in the Rockall Trough,34 but not the Iceland Basin, highlights release of light and mid REE from resuspended sediments and 35 pore waters, possibly a seasonal effect associated with the timing of the spring bloom in each basin. 36 The EEL dissolved oxygen minimum at the permanent pycnocline corresponds to positive heavy REE 37 enrichment, indicating maximum rates of organic matter remineralisation and associated REE release. 38 We tentatively suggest a bacterial role to account for the observed heavy REE deviations. This study 39 highlights the need for fully constrained REE sources and sinks, including the temporary nature of 40 some sources, to achieve a balanced budget of seawater REE. 41

Introduction 43
The rare earth elements (REE) form a suite of 14 elements (i.e. the lanthanides) with chemical 44 properties that vary systematically across the group. The interpretation of relative changes in REE 45 concentrations makes them a powerful tool to investigate advection, cycling and inputs of trace metals 46 in seawater. When normalised to the Post Archaean Australian Shale (PAAS; Taylor and McLennan 47 1985), the balance of supply/removal processes that fractionate seawater REE away from their 48 lithogenic origins is highlighted (e.g. Elderfield andGreaves 1982, Bertram andElderfield 1993). This 49 fractionation is mainly attributed to the increasing strength of REE complexation to carbonate ions as 50 mass number increases (Byrne and Kim 1990), described by the lanthanide contraction effect (Zhang 51 and Nozaki 1996). While the heavy (H)REE are almost entirely bound by stable carbonate complexes, 52 the light (L)REE are present with a greater proportion of free metal ions that makes them more 53 susceptible to removal from solution through adsorption reactions (Cantrell and Byrne 1987, Byrne 54 and Kim 1990, Sholkovitz et al. 1994). This results in the characteristic PAAS-normalised seawater 55 REE pattern of HREE enrichment relative to LREE (e.g. Elderfield and Greaves 1982, Bertram and 56 Elderfield 1993, Alibo and Nozaki 1999). One exception to this is Ce, whose microbially mediated 57 redox chemistry results in substantially lower relative concentrations to neighbouring REE (Moffett 58 1990). 59 The relative changes in the distinctive pattern of dissolved seawater REE are increasingly applied to 60 reconstruct water mass histories, e.g. provenance, continental inputs, intensity of biogeochemical 61 cycling, and water mass isolation time (e.g. Zhang  Zheng et al. 2016) and the effects of biogeochemical cycling (particle sorption/desorption, 66 remineralisation) on vertical profiles of REE (e.g. Sholkovitz et al. 1994). Also important are the 67 processes operating at the continent-ocean interface, that dictate sources and sinks of REE to seawater 68 (e.g. Jeandel et al. 2011), and, gaining recognition, are the role of organics in altering the reactivity and 69 therefore the fractionation of the REE (Schijf et al. 2015). Demonstrating and ultimately quantifying 70 the impact of these mechanisms on seawater REE is essential for complete data interpretation. 71 Resolving seawater REE behaviour will also contribute to constraining the marine Nd budget (i.e. the 72 "Nd paradox"; Goldstein   EEL 2015 cruise for both upper water masses fall at the lower end or just below these ranges (Table  140 1), highlighting broader climate-induced changes in ocean circulation that influence nutrient 141 concentrations (Johnson et al. 2013 europium (i.e. >2%) and so are not considered to be significant. All data are presented in Table S1. 199 Over the course of this study, values ranged from 6 % to 16 % (2RSD) for the BATS and 7 % to 16 % 202 for the NASS-6 ( have relative differences similar to the external reproducibility, and on a few occasions were larger. 207 Total procedural blanks run through the preconcentration system were <1% of the average sample 208 signal, with the exceptions of Ce and Sm that represented 17 % and 10 % respectively of the smallest 209 sample signal. 210

3
Results 211 The REE concentrations show relatively small increases with depth ( Figure 4,  Figure 9c (see discussion in Section 4.2). 238 In contrast to the coastal stations, the REE increase at depth in the Rockall Trough appears abruptly in 239 the deepest samples at each station and is associated with collection from water with a high particulate 240 load as determined from the beam transmission data (Supplementary Information Figure 1). To note, 241 IB4 has the strongest decrease in beam transmission but no sample was collected from within this layer. 242 These samples with high REE concentrations are discussed in Section 4.2. The HREE are reported in the literature as better tracers of water masses than LREE (e.g. Zheng et al. 273 2016) within ocean basins due to their longer residence times arising from their stronger aqueous 274 complexation and thus reduced particle reactivity compared to the LREE (Cantrell and Byrne 1987, 275 Byrne and Kim 1990). However, the limited distances and correspondingly short timescales for the 276 movement of these young water masses (ISOW and LSW) in the NE Atlantic in this study limits the 277 extent to which particle reactivity would influence the distribution of the REE (with the exception of 278 Ce). We therefore assume the REE behave conservatively and mainly reflect the lateral advection of 279 the water mass, with alteration of the REE signature chiefly attributable to mixing with other water 280 masses and extraneous inputs. In this section, we focus on the preformed nature of REE at depth in the 281 water column (below the permanent pycnocline). 282

Temporal record of ISOW 283
The ISOW data used for comparison to data in this study, expressed hereafter as pISOW, were collected 284 from the Faroe-Shetland Channel (Stn 23 in Figure 1 between Stn 23 and IB16 being relatively short at ~1000 km. At IB9, ISOW lies at greater depth and 291 the potential density gradient with the overlying LSW is shallower indicating more diffuse recirculation 292 and some mixing with LSW. This is observable in Figure 3c by the greater deviation of the deep waters 293 towards LSW compared to IB16. 294 To test the similarity between ISOW collected in 1999 and in 2015, we first normalise REE 295 concentrations at IB16 (1550 m), i.e. the sample with the strongest ISOW signal (i.e. σθ 27.85 kg/m 3 ), 296 by the pISOW REE concentrations. This is presented in Figure 6a and 6f (inverted light green triangles) 297 with a combined 2σ external error envelope of the IB16 (1550 m) and pISOW samples. The same data 298 are also normalised to DLSW in Figure 6g- Figure 1a, Figure 6f). Both this DLSW and the 363 LSW observed along the EEL have shared features relative to pISOW of ~15% lower LREE 364 concentrations (with the exception of La, which is similar in pISOW, DSLW and BATS 2000 m), fairly 365 prominent depletions of 50% in Ce and 30% in Eu, and steadily rising MREE to HREE concentrations 366 between Tb and Lu. 367 In Figure 6g Figure 7). This reveals a divergence in trends below ~1000 m depth, with low relative 422 increases with depth at the EEL stations and larger relative increases in the Southern Ocean. The 423 exceptions are the deepest samples in the Rockall Trough that show a steep concentration gradient and 424 an increase in Nd concentration relative to the surface waters that is similar to those observed in the 425 Southern Ocean at ~4500 m depth. 426 The most obvious reason for the elevated REE concentrations is sample collection from depths where 427 beam transmission is reduced ( Supplementary Information Figure 1)  shown) did not clarify the identification of the contributory phases. However, on the basis that pore 487 waters are derived from a combination of Fe-rich phases, dissolution of volcanic ash, and diagenesis 488 of organic matter, we attribute the excess REE to pore water inputs for the purposes of establishing 489 mixing proportions. 490

Mixing proportions 491
Considering pore waters as the source of excess REE in the deepest water column samples, the REE 492 composition (MREE/MREE* and HREE/LREE) can be combined with concentration data to 493 determine the proportional input of pore waters (Figure 10). Pore water concentrations can be highly 494 variable, but are generally at least one order of magnitude greater than seawater REE concentrations 495 (Elderfield and Sholkovitz 1987 little apparent contribution of pore water REE (e.g. ~≤2% HREE from pore waters; Figure 10). In this 503 instance a pore water contribution of the order of ~10% for both LREE and MREE is required, relative 504 to the BATS seawater, to account for the observed increase in the deep Rockall Trough (F, O, P) and 505 up to 25% at the coastal stations (IB22/23, 9G), with the caveat that actual pore water REE 506 concentrations from the sediments below the EEL may diverge from those of Abbott et al. (2015b). 507 The higher contributions to coastal station water columns are discussed below. 508 The four water column depths represented by the pore water data of Abbott et al. The reasons for REE compositional gradients in pore waters are likely associated with sediment 516 composition, reflecting the input of both different particle types and different amounts and reactivities 517 of organic matter to the seafloor to drive diagenetic reactions, as well as current action and benthic 518 activity that determines irrigation of the sediments and therefore contributes to the redox status of the 519 pore waters. 520 When estimating the sedimentary REE contribution to the water column, the effect of the spring bloom 521 on the seafloor needs to be considered. The samples in this study were collected in late May/early June 522 during the spring bloom. This represents a period of increased transfer of organic matter to the seafloor 523 and heightened benthic activity (e.g. Honjo  observed in this study may represent a temporary or seasonal shift. 535 A further consideration, as mentioned above, is the resuspension of sediments by currents, which 536 occurs along the slopes of the Rockall Trough (Lonsdale and Hollister 1979). Sediment resuspension This is a provisional file, not the final typeset article experiments noted significant increases in nutrient release, especially silica, attributable to pore waters, 538 desorption and potentially microbial activity on particle surfaces (Couceiro et al. 2013). These features 539 are notable in the silica concentrations in the deep Rockall Trough, and to a lesser extent in phosphate 540 ( Figure 11). They are possibly linked to the silica biogeochemical cycle dominated by remineralisation 541 of diatom frustules that are hypothesised to have high REE contents (Akagi 2013). Taken together, 542 diffusion, benthic activity and sediment resuspension may result in enhanced sedimentary REE fluxes 543 to the water column. The seasonal aspect of the sedimentary source of REE to seawater, as a response 544 to the spring bloom, cannot be evaluated in this study and requires further sampling either side of the 545 spring bloom, when the diffusive flux and sediment resuspension are likely to dominate. 546 The conspicuous differences between the water column profiles of REE concentrations at the two 547 coastal stations and the five open ocean stations can be partly attributed to effects related to water 548 depth, e.g. <200 m vs. ~1900 m, with the caveat for station 9G samples that were not filtered. The 549 drivers of diagenesis in the sediments are likely to be more intense on the shelf, for example the 550 reactivity and quantity of organic matter input, the intensity of benthic activity, which shows an inverse 551 relationship with water depth (Henderson et al. 1999), and sediment resuspension due to currents and 552 benthic activity. To establish a quantitative evaluation of the benthic flux (i.e. the cumulative effects 553 of diffusion, benthic activity, sediment resuspension), combined Nd isotope and REE concentration 554 measurements are required under different seasonal conditions. 555

Implications for water mass identification 556
One last point to mention, based on inference from the REE concentrations, is alteration of other deep 557 water characteristics when located in the benthic nepheloid layer (or decreased beam transmission) 558 and/or during heightened benthic activity associated with the spring bloom. The REE concentrations 559 in those samples that lie within nepheloid layers demonstrate the influence of pore water release and/or 560 release from suspended particulates on elevated LREE and MREE concentrations in particular. What 561 of the other measured characteristics, e.g. nutrient and dissolved oxygen concentrations, that may also 562 be present in different concentrations in pore waters compared to seawater? The deep Rockall Trough 563 REE data demonstrate up to ~10 % contribution to the seawater REE load. This implies other chemical 564 characteristics of waters in the nepheloid layer may also be shifted to higher or lower values, depending 565 on their concentrations in pore waters, with no significant alteration in the defining properties of a 566 water mass (i.e. temperature, salinity, potential density). All measured nutrients are present in higher 567 concentrations in the very deepest parts of the eastern Rockall Trough, especially silica (Figure 2, 568 Figure 11). More detailed sampling of the lower water column and direct sampling and analysis of 569 sediment pore waters is needed to identify the influence of these on deep water characteristics. The Northeast Atlantic has an exceptionally productive annual spring bloom that results in Fe limitation 586 by the summer months (Nielsdottir et al. 2009). The impact of the spring bloom on the water column 587 can be observed in the distribution of dissolved oxygen concentrations, with a minimum at the 588 permanent pycnocline (Figure 2, Figure 12). This oxygen depletion zone (ODZ) is caused by particles 589 rich in organic matter from the surface ocean that linger and decay during their downward transit to 590 the seafloor. In the Rockall Trough, the ODZ is further enhanced by winter mixing that typically 591 reaches depths of 600 m, and therefore not as deep as the ODZ (i.e. ~800-1200 m in the Rockall 592 Trough), although it may reach ~1000 m in severe winters (Meincke 1986). Lateral advection at these 593 depths in the Rockall Trough is low (Holliday et al. 2000), implying a minimal inherited component 594 of dissolved REE but also a longer residence time of the water that equates to greater potential to 595 accumulate REE compared to elsewhere in the water column. This is not the case in the Iceland Basin, 596 where the ODZ is shallower and more diffuse, and hence more susceptible to obliteration by annual 597 winter mixing and by lateral advection. Also, at the time of sampling the spring bloom was not as well 598 developed in the Iceland Basin, with productivity at least ~4 times lower than in the Rockall Trough 599 (details below). It is noted that fish milt accomplishes a very similar effect by an almost identical mechanism to 646 bacteria, with preferential HREE complexation to external phosphate functional groups confirmed in 647 salmon milt (Takahashi et al. 2014 at the time of sampling, were experiencing the spring bloom, and also the relative increase in HREE at 726 the permanent pycnocline. We recognise that the conditions in the Rockall Trough allow for a 727 "distillation" effect to be preserved in the waters of the pycnocline, which may not be present 728 elsewhere. 729

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Conflict of Interest 730 The authors declare that the research was conducted in the absence of any commercial or financial 731 relationships that could be construed as a potential conflict of interest. 732

Author Contributions 733
All authors contributed to the design of the research and the preparation and revising of the manuscript. 734 All