North East Atlantic Deep Water Research Articles

Impact of Water Mass Mixing on the Composition and Mineralization of Dissolved Organic Matter in the Deep Labrador Sea

AbstractThe Labrador Sea is a region of the North Atlantic known for its strong ocean currents and deep water formation, which contribute to the transport and mixing of water masses throughout the region. The absorbing and fluorescing properties of dissolved organic matter (DOM) were assessed to track the water masses and the in situ production in the Labrador Sea (>200 m). No significant differences in DOM composition were found in the mesopelagic waters (200–1,000 m). In the bathypelagic waters (1,000–4,000 m), the estimated dissolved organic carbon (DOC) and dissolved lignin concentrations, as well as humic‐like fluorescence intensities, allowed the discrimination of North East Atlantic Deep Water versus Denmark Strait overflow water. The humic‐like intensities were significantly different between upper Labrador Sea water (uLSW) and deep LSW (dLSW) suggesting their applications as tracers of deep winter mixing. The significant correlations with apparent oxygen utilization support the in situ production of humic‐like fluorescence and the net microbial consumption of DOC and lignin in the dark Labrador Sea. We also demonstrated that microbial activities play a role in the production of humic‐like compounds in the dLSW that experiences deep convection mixing.

Dissolved and suspended organic matter dynamics in the Cape Verde Frontal Zone (NW Africa)

The Cape Verde Frontal Zone (CVFZ) is a highly dynamic region located in the southern boundary of the Canary Current Eastern Boundary Upwelling Ecosystem. Due to the interaction of the Cape Verde Front with the Mauritanian coastal upwelling, the area features large vertical and horizontal export fluxes of organic matter. While the flux, composition and biogeochemical role of sinking organic matter have been thoroughly studied, much less attention has been paid to the dissolved (DOM) and suspended particulate (POM) organic matter fractions. Full-depth profiles of DOM and POM were recorded during an oceanographic cruise in the CVFZ, with four consecutive transects defining a box embracing the giant filament of Cape Blanc and the Cape Verde front. The distributions of DOM and POM and their C:N stoichiometric ratios in the epipelagic layer were strongly influenced by the position of the transects relative to the giant filament and the front. Geographical heterogeneity in POM and DOM distributions and elemental composition was also observed within each of the different water masses of contrasting origin present in the area (North and South Atlantic Central Water, Subpolar Mode Water, Mediterranean Water, Antarctic Intermediate Water, Labrador Sea Water and North East Atlantic Deep Water). These facts suggest that water masses properties are re-shaped by biogeochemical processes occurring within the CVFZ. Nevertheless, our analysis indicates that DOM and POM mineralisation represents only 8.1% of the inorganic carbon and 17.8% of the inorganic nitrogen produced by the local mineralisation of organic matter. Intense lateral export of POM and DOM out of the boundaries of the CVFZ is the likely reason behind these low contributions, which confirm the prominent role of sinking fluxes of organic matter for mineralisation processes in this region. The DOM distribution in the CVFZ interior is apparently affected by the dissolution of fast sinking particles.

Evidence for influx of Atlantic water masses to the Labrador Sea during the Last Glacial Maximum

The Last Glacial Maximum (LGM, 23–19,000 year BP) designates a period of extensive glacial extent and very cold conditions on the Northern Hemisphere. The strength of ocean circulation during this period has been highly debated. Based on investigations of two marine sediment cores from the Davis Strait (1033 m water depth) and the northern Labrador Sea (2381 m), we demonstrate a significant influx of Atlantic-sourced water at both subsurface and intermediate depths during the LGM. Although surface-water conditions were cold and sea-ice loaded, the lower strata of the (proto) West Greenland Current carried a significant Atlantic (Irminger Sea-derived) Water signal, while at the deeper site the sea floor was swept by a water mass comparable with present Northeast Atlantic Deep Water. The persistent influx of these Atlantic-sourced waters entrained by boundary currents off SW Greenland demonstrates an active Atlantic Meridional Overturning Circulation during the LGM. Immediately after the LGM, deglaciation was characterized by a prominent deep-water ventilation event and potentially Labrador Sea Water formation, presumably related to brine formation and/or hyperpycnal meltwater flows. This was followed by a major re-arrangement of deep-water masses most likely linked to increased overflow at the Greenland-Scotland Ridge after ca 15 kyr BP.

Water mass analysis along 22 °N in the subtropical North Atlantic for the JC150 cruise (GEOTRACES, GApr08)

This study presents a water mass analysis along the JC150 section in the subtropical North Atlantic, based on hydrographic and nutrient data, by combining an extended optimum multiparameter analysis (OMPA) with a Lagrangian particle tracking experiment (LPTE). This combination, which was proposed for the first time, aided in better constraining the OMPA end-member choice and providing information about their trajectories. It also enabled tracing the water mass origins in surface layers, which cannot be achieved with an OMPA. The surface layers were occupied by a shallow type of Eastern South Atlantic Central Water (ESACW) with traces of the Amazon plume in the west. Western North Atlantic Central Water dominates from 100 to 500 m, while the 13 °C-ESACW contribution occurs marginally deeper (500–900 m). At approximately 700 m, Antarctic Intermediate Water (AAIW) dominates the west of the Mid-Atlantic Ridge (MAR), while Mediterranean Water dominates the east with a small but non-negligible contribution down to 3500 m. Below AAIW, Upper Circumpolar Deep Water (UCDW) is observed throughout section (900–1250 m). Labrador Sea Water (LSW) is found centered at 1500 m, where the LPTE highlights an eastern LSW route from the eastern North Atlantic to the eastern subtropical Atlantic, which was not previously reported. North East Atlantic Deep Water (encompassing a contribution of Iceland-Scotland Overflow Water) is centered at ~2500 m, while North West Atlantic Bottom Water (NWABW, encompassing a contribution of Denmark Strait Overflow Water) is principally localized in the west of the MAR in the range of 3500–5000 m. NWABW is also present in significant proportions (>25%) in the east of the MAR, suggesting a crossing of the MAR possibly through the Kane fracture zone. This feature has not been investigated so far. Finally, Antarctic Bottom Water is present in deep waters throughout the section, mainly in the west of the MAR. Source waters have been characterized from GEOTRACES sections, which enables estimations of trace elements and isotope transport within water masses in the subtropical North Atlantic.

Oceanic mercury concentrations on both sides of the Strait of Gibraltar decreased between 1989 and 2012

Mercury (Hg) is a toxic metal that threatens the health of aquatic ecosystems and fish consumers. Its natural cycle has been deeply perturbed by Anthropogenic Hg emissions have deeply perturbed its natural cycle, especially since the start of the Industrial Revolution circa 1850 CE. Anthropogenic Hg emissions from North America and Europe have decreased by a factor of two in the last decades following the implementation of strict regulations. The response of North Atlantic Ocean and Mediterranean waters to this decrease remains poorly documented by field observations. A comparison of results obtained between 1989 and 2012 shows a significant decrease of Hg concentrations in waters on both sides of the Strait of Gibraltar. West of Gibraltar, the Hg decrease ranges from ∼35 % in the upper North East Atlantic Deep Water to ∼50 % in the North East Atlantic Central Water. East of Gibraltar, the observed decrease is ∼30 % in the Western Mediterranean Deep Water. No decrease is observed in the deep Atlantic Ocean layer that formed before the industrial era. These results strongly substantiate the effectiveness of global anti-pollution policies on Hg contamination in oceanic waters. A consequent decline of Hg bioaccumulation in Northeastern Atlantic and Western Mediterranean pelagic ecosystems still requires verification.

ISOW Spreading and Mixing as Revealed by Deep‐Argo Floats Launched in the Charlie‐Gibbs Fracture Zone

AbstractTo improve our understanding of deep circulation, we deployed five Deep‐Argo floats (0–4,000 m) in the Charlie‐Gibbs Fracture Zone (CGFZ), which channels the flow of Iceland‐Scotland Overflow Water (ISOW), a dense water mass of the North Atlantic Ocean. The floats were programed to drift at 2,750 dbar in the ISOW layer. The floats mainly moved westward in the CGFZ, although some of them followed different routes for few cycles depending on northward intrusions of the North Atlantic Current over the CGFZ. One float revealed a direct route for ISOW from CGFZ to the Deep Western Boundary Current at Flemish Cap. In the CGFZ, oxygen data acquired by the floats revealed that the ISOW layer, characterized by salinity higher than 34.94 and density greater than 27.8 kg/m, was mainly composed of the highly oxygenated ISOW and the less oxygenated North East Atlantic Deep Water (NEADW), a complex water mass from the East Atlantic. In the ISOW layer, the relative contribution of ISOW was generally larger in the northern valley than in the southern valley of CGFZ. Northward intrusions of the North Atlantic Current above the CGFZ increased the relative contribution of NEADW in the northern valley and favors mixing between ISOW and NEADW. The ISOW‐NEADW signal flowing westward from the CGFZ toward the Deep Western Boundary Current was progressively diluted by Labrador Sea Water and Denmark Strait Overflow Water. Oxygen measurements from Deep‐Argo floats are essential for a better understanding and characterization of the mixing and spreading of deep water masses.

The resilience and sensitivity of Northeast Atlantic deep water εNd to overprinting by detrital fluxes over the past 30,000 years

The neodymium isotope proxy has become a valuable tool for the reconstruction of past ocean water mass provenance and mixing. For its accurate application, knowledge about the origin and preservation of Nd in sedimentary archives is crucial. Recently, concerns have emerged regarding the applicability of neodymium isotopes as a conservative palaeo water mass tracer, given potential Nd fluxes from sediments into bottom waters (Abbott et al., 2015a) and inferred relabelling of ocean waters by settling detrital material (Roberts and Piotrowski, 2015). Consequently, a decoupling of water mass provenance and proxy variations may arise. We investigate the mobility of Nd around extreme detrital sedimentation events such as glacial ice rafting pulses and turbidite deposition in the Northeast Atlantic. The constructed records from sediment leachates span extreme Nd isotope variations including volcanic (εNd ∼ 0) and Laurentian (εNd ∼ −27) sources. We find that Nd was released into pore waters from reactive detritus inside some detrital layers during early diagenesis, thereby overprinting any archived bottom water Nd signature and precluding the reconstruction of past water mass provenance during the affected time intervals. However, we do not observe any definite indication of diffusive vertical migration of Nd into adjacent layers. Furthermore, bottom water Nd isotope signatures were not modified to a measurable degree by any potential benthic flux of Nd during the deposition of these detrital sediment layers. Consequently, the Nd isotope composition of the pelagic glacial Northeast Atlantic water masses were resilient to such episodic large detrital fluxes.Apart from extreme local sedimentation events, we confirm the presence of detritally overprinted deep waters north of 47°N during the peak glacial from comparison of Northeast Atlantic depth transects. We furthermore suggest that the sensitivity of deep waters to this overprinting effect increased during periods of reduced Atlantic Meridional Overturning Circulation and elevated ice rafting.Overall, our study demonstrates that a thorough evaluation of the proportion of Nd originating from physical water mass advection versus in situ chemical inputs is crucial for the reliable application of Nd isotopes as a water mass tracer.

The <sup>226</sup>Ra–Ba relationship in the North Atlantic during GEOTRACES-GA01

Abstract. We report detailed sections of radium-226 (226Ra, T1∕2 = 1602 years) activities and barium (Ba) concentrations determined in the North Atlantic (Portugal–Greenland–Canada) in the framework of the international GEOTRACES program (GA01 section – GEOVIDE project, May–July 2014). Dissolved 226Ra and Ba are strongly correlated along the section, a pattern that may reflect their similar chemical behavior. Because 226Ra and Ba have been widely used as tracers of water masses and ocean mixing, we investigated their behavior more thoroughly in this crucial region for thermohaline circulation, taking advantage of the contrasting biogeochemical patterns existing along the GA01 section. We used an optimum multiparameter (OMP) analysis to distinguish the relative importance of physical transport (water mass mixing) from nonconservative processes (sedimentary, river or hydrothermal inputs, uptake by particles and dissolved–particulate dynamics) on the 226Ra and Ba distributions in the North Atlantic. Results show that the measured 226Ra and Ba concentrations can be explained by conservative mixing for 58 and 65 % of the samples, respectively, notably at intermediate depth, away from the ocean interfaces. 226Ra and Ba can thus be considered conservative tracers of water mass transport in the ocean interior on the space scales considered here, namely, on the order of a few thousand kilometers. However, regions in which 226Ra and Ba displayed nonconservative behavior and in some cases decoupled behaviors were also identified, mostly at the ocean boundaries (seafloor, continental margins and surface waters). Elevated 226Ra and Ba concentrations found in deepwater in the West European Basin suggest that lower Northeast Atlantic Deep Water (NEADWl) accumulates 226Ra and Ba from sediment diffusion and/or particle dissolution during transport. In the upper 1500 m of the West European Basin, deficiencies in 226Ra and Ba are likely explained by their incorporation in planktonic calcareous and siliceous shells, or in barite (BaSO4) by substitution or adsorption mechanisms. Finally, because Ba and 226Ra display different source terms (mostly deep-sea sediments for 226Ra and rivers for Ba), strong decoupling between 226Ra and Ba were observed at the land–ocean boundaries. This is especially true in the shallow stations near the coasts of Greenland and Newfoundland where high 226Ra ∕ Ba ratios at depth reflect the diffusion of 226Ra from sediment and low 226Ra ∕ Ba ratios in the upper water column reflect the input of Ba associated with meteoric waters.

Mercury distribution and transport in the North Atlantic Ocean along the GEOTRACES-GA01 transect

Abstract. We report here the results of total mercury (HgT) determinations along the 2014 Geotraces Geovide cruise (GA01 transect) in the North Atlantic Ocean (NA) from Lisbon (Portugal) to the coast of Labrador (Canada). HgT concentrations in unfiltered samples (HgTUNF) were log-normally distributed and ranged between 0.16 and 1.54 pmol L−1, with a geometric mean of 0.51 pmol L−1 for the 535 samples analysed. The dissolved fraction (< 0.45 µm) of HgT (HgTF), determined on 141 samples, averaged 78 % of the HgTUNF for the entire data set, 84 % for open seawaters (below 100 m) and 91 % if the Labrador Sea data are excluded, where the primary production was high (with a winter convection down to 1400 m). HgTUNF concentrations increased eastwards and with depth from Greenland to Europe and from subsurface to bottom waters. The HgTUNF concentrations were similarly low in the subpolar gyre waters ( ∼ 0.45 pmol L−1), whereas they exceeded 0.60 pmol L−1 in the subtropical gyre waters. The HgTUNF distribution mirrored that of dissolved oxygen concentration, with highest concentration levels associated with oxygen-depleted zones. The relationship between HgTF and the apparent oxygen utilization confirms the nutrient-like behaviour of Hg in the NA. An extended optimum multiparameter analysis allowed us to characterize HgTUNF concentrations in the different source water types (SWTs) present along the transect. The distribution pattern of HgTUNF, modelled by the mixing of SWTs, show Hg enrichment in Mediterranean waters and North East Atlantic Deep Water and low concentrations in young waters formed in the subpolar gyre and Nordic seas. The change in anthropogenic Hg concentrations in the Labrador Sea Water during its eastward journey suggests a continuous decrease in Hg content in this water mass over the last decades. Calculation of the water transport driven by the Atlantic Meridional Overturning Circulation across the Portugal–Greenland transect indicates northward Hg transport within the upper limb and southward Hg transport within the lower limb, with resulting net northward transport of about 97.2 kmol yr−1.

Water mass distributions and transports for the 2014 GEOVIDE cruise in the North Atlantic

Abstract. We present the distribution of water masses along the GEOTRACES-GA01 section during the GEOVIDE cruise, which crossed the subpolar North Atlantic Ocean and the Labrador Sea in the summer of 2014. The water mass structure resulting from an extended optimum multiparameter (eOMP) analysis provides the framework for interpreting the observed distributions of trace elements and their isotopes. Central Waters and Subpolar Mode Waters (SPMW) dominated the upper part of the GEOTRACES-GA01 section. At intermediate depths, the dominant water mass was Labrador Sea Water, while the deep parts of the section were filled by Iceland–Scotland Overflow Water (ISOW) and North-East Atlantic Deep Water. We also evaluate the water mass volume transports across the 2014 OVIDE line (Portugal to Greenland section) by combining the water mass fractions resulting from the eOMP analysis with the absolute geostrophic velocity field estimated through a box inverse model. This allowed us to assess the relative contribution of each water mass to the transport across the section. Finally, we discuss the changes in the distribution and transport of water masses between the 2014 OVIDE line and the 2002–2010 mean state. At the upper and intermediate water levels, colder end-members of the water masses replaced the warmer ones in 2014 with respect to 2002–2010, in agreement with the long-term cooling of the North Atlantic Subpolar Gyre that started in the mid-2000s. Below 2000 dbar, ISOW increased its contribution in 2014 with respect to 2002–2010, with the increase being consistent with other estimates of ISOW transports along 58–59° N. We also observed an increase in SPMW in the East Greenland Irminger Current in 2014 with respect to 2002–2010, which supports the recent deep convection events in the Irminger Sea. From the assessment of the relative water mass contribution to the Atlantic Meridional Overturning Circulation (AMOC) across the OVIDE line, we conclude that the larger AMOC intensity in 2014 compared to the 2002–2010 mean was related to both the increase in the northward transport of Central Waters in the AMOC upper limb and to the increase in the southward flow of Irminger Basin SPMW and ISOW in the AMOC lower limb.

Isotope (δD, δ18О) systematics in waters of the Russian Arctic seas

Oxygen and hydrogen isotope analysis was performed to study the processes of distribution of water masses and modification of their salinity in the Russian Arctic seas. A wealth of new isotopic data was obtained for freshwater (river runoff, Novaya Zemlya glaciers) and seawater samples collected along a set of extended 2D profiles in the Barents, Kara, and Laptev Seas. The study presents the first δD values measured for the Northeast Atlantic Deep Water NEADW dominated the water column of the Barents Sea (S = 34.90 ± 0.05, δD = +1.55 ± 0.4‰, δ18O = +0.26 ± 0.1‰, n = 44). This water mass is present in the Kara Sea and western Laptev Sea. The relationship between δD, δ18О, and salinity data was used to calculate the fractions of waters of different origin, including the fractions of continental runoff in waters of the Barents, Kara, and Laptev Seas. It was shown that the relationships between the isotopic parameters (δD, δ18О) and salinity in waters of the Kara and Laptev Seas is controlled by the intensity of continental runoff and sea ice processes. Sea ice formation is the main factor controlling the formation of the water column on the Laptev Sea shelf, whereas the surface waters of the middle Kara Sea are dominated by the contribution of river runoff. A very strong stratification in the Kara Sea is caused by the presence of a relatively fresh surface layer mostly contributed by estuarine water inputs from the Ob and Yenisei Rivers. The contribution of river waters reaches 40–60% in the surface layer in the central part of the sea and decreases to a few percent down 100 m water depth. Stratification in the western part of the Laptev Sea is controlled by the contribution of freshwater input from the Lena River and modification of salinity by sea ice formation.

Water mass circulation and weathering inputs in the Labrador Sea based on coupled Hf–Nd isotope compositions and rare earth element distributions

The Labrador Sea is one of the key areas for deep water formation driving the Atlantic thermohaline circulation and thus plays an important role in Northern Hemisphere climatic fluctuations. In order to better constrain the overturning processes and the origins of the distinct water masses, combined dissolved Hf–Nd isotopic compositions and rare earth element (REE) distribution patterns were obtained from four water depth profiles along a section across the Labrador Sea. These were complemented by one surface sample off the southern tip of Greenland, three shallow water samples off the coast of Newfoundland, and two deep water samples off Nova Scotia.Although light REEs are markedly enriched in the surface waters off the coast of Newfoundland compared to north Atlantic waters, the REE concentration profiles are essentially invariant throughout the water column across the Labrador Sea. The hafnium concentrations of surface waters exhibit a narrow range between 0.6 and 1pmol/kg but are not significantly higher than at depth.Neodymium isotope signatures (ɛNd) vary from unradiogenic values between −16.8 and −14.9 at the surface to more radiogenic values near −11.0 at the bottom of the Labrador Sea mainly reflecting the advection of the Denmark Strait Overflow Water and North East Atlantic Deep Water, the signatures of which are influenced by weathering contributions from Icelandic basalts. Unlike Nd, water column radiogenic Hf isotope signatures (ɛHf) are more variable representing diverse weathering inputs from the surrounding landmasses. The least radiogenic seawater ɛHf signatures (up to −11.7) are found in surface waters close to Greenland and near the Canadian margin. This reflects the influence of recirculating Irminger Current Waters, which are affected by highly unradiogenic inputs from Greenland. A three to four ɛHf unit difference is observed between Denmark Strait Overflow Water (ɛHf∼−4) and North East Atlantic Deep Water (ɛHf∼−0.1), although their source waters have essentially the same ɛNd signature. This most likely reflects different weathering signals of hafnium delivered to Denmark Strait Overflow Water and North East Atlantic Deep Water (incongruent weathering of old rocks from Greenland versus basaltic rocks from Iceland). In addition, the ɛHf data resolve two layers within the main body of Labrador Sea Water not visible in the ɛNd distribution, which are shallow Labrador Sea Water (ɛHf∼−2) and deep Labrador Sea Water (ɛHf∼−4.5). The latter layer was formed between the late 1980’s and mid 1990’s during the last cold state of the Labrador Sea and underwent substantial modification since its formation through the admixture of Irminger Water, Iceland Slope Water and North East Atlantic Deep Water, which is reflected in its less radiogenic ɛHf signature. The overall behavior of Hf in the water column suggests its higher sensitivity to local changes in weathering inputs on annual to decadal timescales. Although application of Hf isotopes as a tracer for global water mass mixing is complicated by their susceptibility to incongruent weathering inputs they are a promising tracer of local processes in restricted basins such as the Labrador Sea.

Vertical stratification of bacterial communities driven by multiple environmental factors in the waters (0–5000 m) off the Galician coast (NW Iberian margin)

The processes mediated by microbial planktonic communities occur along the entire water column, yet the microbial activity and composition have been studied mainly in surface waters. This research examined the vertical variation in bacterial abundance, activity and community composition and structure from surface down to 5000m depth following a longitudinal transect off the Galician coast (NW Iberian margin, from 43°N, 9°W to 43°N, 15°W). Community activity and composition changed with depth. The leucine incorporation rates decreased from the euphotic layer to the bathypelagic waters by three orders of magnitude, whereas prokaryotic abundance decreased only by one order of magnitude. The relative abundance of SAR11 and Alteromonas, determined by catalyzed reported deposition fluorescence in situ hybridization (CARD-FISH), decreased with depth. Meanwhile, the contribution of SAR 202 and SAR324 was significantly higher in the deeper layers (i.e. NEADW, North East Atlantic Deep Water and LDW, Lower Deep Water) than in the euphotic zone. Bacterial community structure, assessed by Automated Ribosomal Intergenic Spacer Analysis (ARISA), was depth-specific. A distance based linear model (DistLM) revealed that the variability found in bacterial community structure was mainly explained by temperature nitrate, phosphate, dissolved organic matter (DOM) fluorescence, prokaryotic abundance, leucine incorporation and to a lesser extent salinity, oxygen, CDOM absorbance and dissolved organic carbon concentration. Our results displayed a bacterial community structure shaped not only by depth-related physicochemical features but also by DOM quality, indicating that different prokaryotic taxa have the potential to metabolize particular DOM sources.

Neodymium isotopic composition and concentration in the western North Atlantic Ocean: Results from the GEOTRACES GA02 section

The neodymium (Nd) isotopic composition of seawater is commonly used as a proxy to study past changes in the thermohaline circulation. The modern database for such reconstructions is however poor and the understanding of the underlying processes is incomplete. Here we present new observational data for Nd isotopes and concentrations from twelve seawater depth profiles, which follow the flow path of North Atlantic Deep Water (NADW) from its formation region in the North Atlantic to the northern equatorial Atlantic. Samples were collected during two cruises constituting the northern part of the Dutch GEOTRACES transect GA02 in 2010. The results show that the different water masses in the subpolar North Atlantic Ocean, which ultimately constitute NADW, have the following Nd isotope characteristics: Upper Labrador Sea Water (ULSW), εNd=−14.2±0.3; Labrador Sea Water (LSW), εNd=−13.7±0.9; Northeast Atlantic Deep Water (NEADW), εNd=−12.5±0.6; Northwest Atlantic Bottom Water (NWABW), εNd=−11.8±1.4. In the subtropics, where these source water masses have mixed to form NADW, which is exported to the global ocean, upper-NADW is characterised by εNd values of −13.2±1.0 (2sd) and lower-NADW exhibits values of εNd=−12.4±0.4 (2sd). While both signatures overlap within error, the signature for lower-NADW is significantly more radiogenic than the traditionally used value for NADW (εNd=−13.5) due to the dominance of source waters from the Nordic Seas (NWABW and NEADW). Comparison between the concentration profiles and the corresponding Nd isotope profiles with other water mass properties such as salinity, silicate concentrations, neutral densities and chlorofluorocarbon (CFC) concentration provides novel insights into the geochemical cycle of Nd and reveals that different processes are necessary to account for the observed Nd characteristics in the subpolar and subtropical gyres and throughout the vertical water column. While our data set provides additional insights into the contribution of boundary exchange in areas of sediment resuspension, the results for open ocean seawater demonstrate, at an unprecedented level, the suitability of Nd isotopes to trace modern water masses in the strongly advecting western Atlantic Ocean.

Oxygen isotope measurements of seawater (18O/16O): A comparison of cavity ring‐down spectroscopy (CRDS) and isotope ratio mass spectrometry (IRMS)

AbstractLaser‐based spectroscopic techniques, such as cavity ring‐down spectroscopy (CRDS), provide a new, cost effective and more widely available approach to measure the oxygen isotope ratio in water molecules, 18O/ 16O (δ18O), and are used increasingly to measure δ18O in the world's oceans. Here, we present results from an interlaboratory comparison designed to evaluate the quality of CRDS‐derived measurements, and their consistency with values measured by isotope ratio mass spectrometry (IRMS). We also discuss the influence of salt on instrument performance and sample throughput for the analysis of seawater samples. This study compared measurements of δ18O from natural samples with a wide range of salinities (0, 29.4, and 34.6) performed by four independent labs: two using CRDS and two using IRMS. We also compared δ18O measurements of Northeast Atlantic Deep Water collected in 2013, 2012, 2009, and 1995 from the AR7W repeat hydrography transect across the Labrador Sea. The within‐lab precision of ocean‐based CRDS measurements is seen to approach 0.03‰, which is better than the manufacturer's typically stated analytical precision (around +/− 0.05‰), and comparable to that achievable with IRMS. The interlaboratory differences of measurements (highest‐lowest) reported by the four labs is taken as an indicator of overall accuracy, and is estimated conservatively as being < 0.1‰, with the potential to approach 0.05‰. Overall, these results show that CRDS based 18O measurements of seawater can be equivalent to high‐quality measurements by IRMS.

Oceanographic regimes in the northwest Labrador Sea since Marine Isotope Stage 3 based on dinocyst and stable isotope proxy records

Sea surface temperature (SST), salinity and density gradients in the upper water column of the northwest Labrador Sea have been reconstructed based on high resolution analysis of a core (HU2008-029-004PC) spanning the last ∼36 ka, raised off Hudson Strait. The modern analogue technique was applied to dinocyst assemblages and combined with stable isotope data from Neogloboquadrina pachyderma left-coiled (Npl) for this purpose. Three oceanographic regimes were identified, broadly corresponding to the “glacial”, “deglacial” and “postglacial” intervals. The site remained under the direct influence of the Laurentide Ice Sheet (LIS) margin until the postglacial and did not record the Bølling-Allerød warming and weakly recorded the Younger Dryas event. The “glacial” regime lasted until ∼12.2 cal ka BP. It was characterized by generally low concentrations of dinocysts within an assemblage indicative of quasi-perennial sea ice. The “deglacial” regime (ca 12.2–8.3 cal ka BP) was marked by increased biogenic fluxes and more diversified dinocyst assemblages and possibly an enhanced subsurface inflow of North East Atlantic Deep Water. Warm summer (∼11 °C) but low winter (∼0 °C) sea surface temperatures, sea ice cover during about 3 months per year, and low summer salinity (∼28) suggest strong stratification in the upper water layer in relation to meltwater supply from the LIS. Following the final drainage of glacial Lake Agassiz through Hudson Strait, which is dated here at ∼8.3 cal ka BP, and the subsequent LIS collapse, increased summer salinity (up to ∼35) was accompanied by a reduced seasonal gradient of sea surface temperature from winter (∼3.8 °C) to summer (∼8.6 °C) suggesting enhanced penetration of North Atlantic Water. Weakened stratification of the surface water layer then allowed for winter convection and Labrador Sea Water formation, which is consistent with increased Npl-δ13C values in response to higher ventilation of the subsurface water layer.

Deepwater circulation in the northeastern Iceland Basin in the Late Pleistocene

Based on changes in planktonic foraminifer assemblages and carbonate content of deposits in the sedimentation column from the northeastern Iceland Basin, nine isotope stages are recognized, covering 300 ka. The study of benthic foraminifer assemblages in the section has shown that, most of this time, it contained “old” water, isolated for a long time from the surface, with a low content of oxygen. At transitions from glacial conditions to interglacials, it was displaced by North East Atlantic Deep Water.

Impact of water mass mixing on the biogeochemistry and microbiology of the Northeast Atlantic Deep Water

The extent to which water mass mixing contributes to the biological activity of the dark ocean is essentially unknown. Using a multiparameter water mass analysis, we examined the impact of water mass mixing on the nutrient distribution and microbial activity of the Northeast Atlantic Deep Water (NEADW) along an 8000 km long transect extending from 62°N to 5°S. Mixing of four water types (WT) and basin scale mineralization from the site where the WT where defined to the study area explained up to 95% of the variability in the distribution of inorganic nutrients and apparent oxygen utilization. Mixing-corrected average O2:N:P mineralization ratios of 127(±11):13.0(±0.7):1 in the core of the NEADW suggested preferential utilization of phosphorus compounds while dissolved organic carbon mineralization contributed a maximum of 20% to the oxygen demand of the NEADW. In conjunction with the calculated average mineralization ratios, our results indicate a major contribution of particulate organic matter to the biological activity in the NEADW. The variability in prokaryotic abundance, high nucleic acid containing cells, and prokaryotic heterotrophic production in the NEADW was explained by large scale (64-79%) and local mineralization processes (21-36%), consistent with the idea that deep-water prokaryotic communities are controlled by substrate supply. Overall, our results suggest a major impact of mixing on the distribution of inorganic nutrients and a weaker influence on the dissolved organic matter pool supporting prokaryotic activity in the NEADW.

Dynamics of North Atlantic Deep Water masses during the Holocene

High resolution flow speed reconstructions of two core sites located on Gardar Drift in the northeast Atlantic Basin and Orphan Knoll in the northwest Atlantic Basin reveal a long‐term decrease in flow speed of Northeast Atlantic Deep Water (NEADW) after 6,500 years. Benthic foraminiferal oxygen isotopes of sites currently bathed in NEADW show a 0.2‰ depletion after 6,500 years, shortly after the start of the development of a carbon isotope gradient between NEADW and Norwegian Sea Deep Water. We consider these changes in near‐bottom flow vigor and benthic foraminiferal isotope records to mark a significant reorganization of the Holocene deep ocean circulation, and attribute the changes to a weakening of NEADW flow during the mid to late Holocene that allowed the shoaling of Lower Deep Water and deeper eastward advection of Labrador Sea Water into the northeast Atlantic Basin.

Contribution of Crenarchaeota and Bacteria to autotrophy in the North Atlantic interior

Marine Crenarchaeota are among the most abundant groups of prokaryotes in the ocean and recent reports suggest that they oxidize ammonia as an energy source and inorganic carbon as carbon source, while other studies indicate that Crenarchaeota use organic carbon and hence, live heterotrophically. We used catalysed reporter deposition fluorescence in situ hybridization (CARD-FISH) to determine the crenarchaeal and bacterial contribution to total prokaryotic abundance in the (sub)tropical Atlantic. Bacteria contributed ~ 50% to total prokaryotes throughout the water column. Marine Crenarchaeota Group I (MCGI) accounted for ~ 5% of the prokaryotes in subsurface waters (100 m depth) and between 10 and 20% in the oxygen minimum layer (250-500 m depth) and deep waters (North East Atlantic Deep Water). The fraction of both MCGI and Bacteria fixing inorganic carbon, determined by combining microautoradiography with CARD-FISH (MICRO-CARD-FISH), decreased with depth, ranging from ~ 30% in the oxygen minimum zone to < 10% in the intermediate waters (Mediterranean Sea Outflow Water, Antarctic Intermediate Water). In the deeper water masses, however, MCGI were not taking up inorganic carbon. Using quantitative MICRO-CARD-FISH to determine autotrophy activity on a single cell level revealed that MCGI are incorporating inorganic carbon (0.002-0.1 fmol C cell⁻¹ day⁻¹) at a significantly lower rate than Bacteria (0.01-0.6 fmol C cell⁻¹ day⁻¹). Hence, it appears that MCGI contribute substantially less to autotrophy than Bacteria. Taking the stoichiometry of nitrification together with our findings suggests that MCGI might not dominate the ammonia oxidation step in the mesopelagic waters of the ocean to that extent as the reported dominance of archaeal over bacterial amoA would suggest.