North Atlantic Surface Waters Research Articles

Iodide, iodate & dissolved organic iodine in the temperate coastal ocean

The surface ocean is the main source of iodine to the atmosphere, where it plays a crucial role including in the catalytic removal of tropospheric ozone. The availability of surface oceanic iodine is governed by its biogeochemical cycling, the controls of which are poorly constrained. Here we show a near two-year time series of the primary iodine species, iodide, iodate and dissolved organic iodine (DOI) in inner shelf marine surface waters of the Western English Channel (UK). The median ± standard deviation concentrations between November 2019 and September 2021 (n=76) were: iodide 88 ± 17 nM (range 61-149 nM), iodate 293 ± 28 nM (198-382 nM), DOI 16 ± 16 nM (<0.12-75 nM) and total dissolved iodine (dIT) 399 ± 30 nM (314-477 nM). Though lower than inorganic iodine ion concentrations, DOI was a persistent and non-negligible component of dIT, which is consistent with previous studies in coastal waters. Over the time series, dIT was not conserved and the missing pool of iodine accounted for ~6% of the observed concentration suggesting complex mechanisms governing dIT removal and renewal. The contribution of excess iodine (I*) sourced from the coastal margin towards dIT was generally low (3 ± 29 nM) but exceptional events influenced dIT concentrations by up to ±100 nM. The seasonal variability in iodine speciation was asynchronous with the observed phytoplankton primary productivity. Nevertheless, iodate reduction began as light levels and then biomass increased in spring and iodide attained its peak concentration in mid to late autumn during post-bloom conditions. Dissolved organic iodine was present, but variable, throughout the year. During winter, iodate concentrations increased due to the advection of North Atlantic surface waters. The timing of changes in iodine speciation and the magnitude of I* subsumed by seawater processes supports the paradigm that transformations between iodine species are biologically mediated, though not directly linked.

Multidecadal Intensification of Atlantic Tropical Instability Waves

AbstractTropical Instability Waves (TIWs) are the dominant source of intraseasonal variability in the central equatorial Atlantic and play an important role in the redistribution of heat in the upper ocean. Here we use multidecadal records of sea surface temperature, sea level anomaly, sea surface salinity, and near‐surface currents constructed from in situ and satellite observations to reveal a long‐term intensification of the intraseasonal variability of these variables due to an increase of TIW activity. Enhanced barotropic energy conversion from increased covariance of horizontal current fluctuations, rather than low‐frequency changes of the mean zonal currents, drives the TIW intensification. As a consequence, boreal summer cooling of tropical North Atlantic surface waters through horizontal eddy temperature advection increased by 0.03°C month−1 decade−1 during 1993–2021, a change of 74% ± 53% relative to the long‐term mean. The presented multidecadal TIW trends are strongly modulated by interannual variations like the 2021 Atlantic Niño.

Comparison of discrete and underway CO2 measurements: Inferences on the temperature dependence of the fugacity of CO2 in seawater

The fugacity or partial pressure of CO2 in surface water (fCO2w) is a key parameter to determine air-sea CO2 fluxes and the evolution of ocean acidification. Despite its importance some key physical chemical characteristics are not fully resolved, notably its dependence on temperature. The fCO2w is mostly measured by autonomous underway systems near in situ sea surface temperature (SST). Subsurface measurements are commonly carried out on individual (discrete) samples at a fixed temperature, normally 20 °C. Here, the underway system observations are compared with co-located discrete observations to determine the consistency of these types of measurements. The co-located discrete fCO2w at 20 °C and underway fCO2w measurements at SST are used to infer the temperature dependence of CO2. In addition, calculated fCO2w from total alkalinity (TA) and total dissolved inorganic carbon (DIC) are compared with the underway and discrete fCO2w measurements. For 21 cruises spanning the major ocean basins from 1992 to 2020 a temperature dependence of 4.13 ± 0.01% °C−1 is determined in close agreement with a widely used previous empirical estimate of 4.23 ± 0.02% °C−1 for North Atlantic surface water. The temperature dependency of calculated fCO2w from TA and DIC using recommended constants is 4.10% °C−1 for 17 cruises where there are co-located measurements of fCO2w, TA and DIC.

From glacial times to late Holocene: Benthic foraminiferal assemblages from cold water coral habitats off northwest Scotland

Benthic foraminiferal assemblage data have been derived from a marine sediment core collected in a cold-water coral (CWC) site off northwest Scotland (near the Wyville Thomson Ridge - WTR). The record covers the last glacial period to the late Holocene. Our reconstructions enable to better constrain the paleoenvironmental changes that took place in the study area since the last glacial time and define the environmental conditions that allowed the onset, formation and decline of CWCs in the area. The sedimentary record was characterized by three distinct foraminiferal assemblages, dominant during the pre-coral time interval (i and ii) and the coral period (iii): (i) the “Glacial” assemblage (>13 cal ka BP) was characterized by the dominance of Cibicides refulgens and Discanomalina coronata. Low foraminiferal densities and diversities indicated unfavorable ecological conditions (i.e., low temperature and food availability, coarse substrate) that were most likely related to the extended British Ice Sheet; (ii) the “Deglacial-Early Holocene” assemblage (~13–9 cal ka BP) was characterized by the cassidulinids-Lobatula lobatula association, indicating a progressive increase of organic matter content and influx of warm and saline North Atlantic surface waters. We hypothesize that these were the main environmental factors that thereafter allowed the settlement of CWCs in the WTR area around 9 cal ka BP; (iii) the “Holocene” (<9 cal ka BP) assemblage was characterized by the cohabitation of diverse epifaunal species (e.g., Rosalina globularis) present when CWCs were highly abundant, and infaunal species (e.g., Trifarina angulosa) when corals were less frequent. The periods recording high presence of T. angulosa after 9 ka cal BP were interpreted as indicating a higher contribution of Eastern North Atlantic Waters during periods of retracted Subpolar Gyre (SPG), affecting probably the efficiency of food export to the seafloor.

Rapid microbial diversification of dissolved organic matter in oceanic surface waters leads to carbon sequestration

The pool of dissolved organic matter (DOM) in the deep ocean represents one of the largest carbon sinks on the planet. In recent years, studies have shown that most of this pool is recalcitrant, because individual compounds are present at low concentrations and because certain compounds seem resistant to microbial degradation. The formation of the diverse and recalcitrant deep ocean DOM pool has been attributed to repeated and successive processing of DOM by microorganisms over time scales of weeks to years. Little is known however, about the transformation and cycling that labile DOM undergoes in the first hours upon its release from phytoplankton. Here we provide direct experimental evidence showing that within hours of labile DOM release, its breakdown and recombination with ambient DOM leads to the formation of a diverse array of new molecules in oligotrophic North Atlantic surface waters. Furthermore, our results reveal a preferential breakdown of N and P containing molecules versus those containing only carbon. Hence, we show the preferential breakdown and molecular diversification are the crucial first steps in the eventual formation of carbon rich DOM that is resistant to microbial remineralization.

Export of bathyal benthos to the Atlantic through the Mediterranean outflow: Sponges from the mud volcanoes of the Gulf of Cadiz as a case study

The Mediterranean is a semi-enclosed sea, with a narrow natural connection —the Strait of Gibraltar— through its western basin to the North Atlantic. Many studies have investigated how the inflow of North Atlantic Surface water into the Mediterranean shapes the faunal composition and abundance of the shallow-water benthic communities of the Western Mediterranean. However, the reverse effect remains little explored, that is, at what level the relatively deep (>200 m deep) outflow of Mediterranean water (MOW) exports bathyal Mediterranean benthos into the North Atlantic and what is the fate of the exported fauna. In this study, we have investigated that process, using the bathyal sponge fauna known from a total of 9 biogeographical areas in the Northeastern Atlantic and 9 in the Western and Central Mediterranean, which accounted for a total of 456 spp. Prior to this general analysis, an exhaustive description of the bathyal sponge fauna (82 spp.) associated to 8 mud volcanoes located in the Gulf of Cadiz (Eastern North Atlantic) was conducted. This was necessary because the bathyal sponge fauna in the North Atlantic zone adjacent to the Strait of Gibraltar remained relatively poorly studied and that situation hindered relevant comparisons with the much better known bathyal fauna of the Western Mediterranean. The results of the clustering, ordination and regression analyses first revealed that the bathyal sponge fauna described from the mud volcanoes field in the Gulf of Cadiz was not essentially different from that previously described in pre-existing studies of other bathyal environments in the Gulf of Cadiz. The large scale subsequent assessment across the Atlantic-Mediterranean biogeographical gradient revealed that the sponge faunas of all Western Mediterranean areas form a relative cohesive group, except for the idiosyncratic nature of the Tyrrhenian Sea. More importantly, the deep-sea sponge fauna of the Gulf of Cadiz (in the easternmost Atlantic side of the Atlantic-Mediterranean gradient) showed more affinity with the fauna of the Western Mediterranean than with the fauna of the remaining Northeastern Atlantic areas considered in the study (i.e., Cape Verde, Canary Islands, Madeira, the Moroccan slope, Lusitanian Banks, Southern Azores Banks and Azores). The Mediterranean area with the highest faunal similarity to the Gulf of Cadiz was the Alboran Sea, followed by the Gulf of Lion, the Strait of Sicily and, the Gulf of Taranto, sharing collectively about 17% of their species. These patterns of faunal affinities clearly illustrate the importance of the MOW in transporting components of the Mediterranean deep-sea sponge communities towards the bathyal communities of the Gulf of Cadiz. The contrasting low faunal affinity between the deep-water sponge fauna of the Gulf of Cadiz and the remaining North Atlantic areas considered in the analyses also revealed that the Mediterranean faunal export is largely circumscribed to the Gulf of Cadiz. It is likely that the North-Atlantic trajectory of the MOW, turning north after the Strait of Gibraltar and staying attached to the slope of the Iberian margin, hinders subsequent colonization of the slopes of the Macaronesia region by the deep-water Mediterranean sponges exported to the Gulf of Cadiz. The results of this study, combined with previous literature on biogeographical sponge transport by marine currents, suggest that the sponge fauna provides a useful tool to reveal the future shifts in the biogeographic patterns predicted in our man-impacted and changing ocean.

Miocene Evolution of North Atlantic Sea Surface Temperature

AbstractWe reconstruct sea surface temperatures (SSTs) at Deep Sea Drilling Project Site 608 (42.836°N, 23.087°), north of the Azores Front, and Ocean Drilling Program Site 982 (57.516°N, 15.866°), under the North Atlantic Current, in order to track Miocene (23.1–5.3 Ma) development of North Atlantic surface waters. Mean annual SSTs from TEX86 and UK′37 proxy estimates at both sites were 10–15 °C higher than modern through the Miocene Climatic Optimum (17–14.5 Ma). During the global cooling of the Middle Miocene Climate Transition (~14.5–12.5 Ma), SSTs at midlatitude Site 608 cooled by ~6 °C, whereas high‐latitude Site 982 cooled by only ~2 °C, resulting in an ~4 Myr collapse of the SST gradient between the two sites. This regional pattern is inconsistent with an increased latitudinal surface temperature gradient, as generally associated with global cooling episodes linked to decreasing pCO2 levels. Instead, the pattern is best explained by enhanced ocean heat transport into the high‐latitude North Atlantic superimposed on the global cooling trend, probably due to enhanced Atlantic meridional overturning circulation and/or a stronger North Atlantic Current. During global late Miocene cooling (~8–7 Ma), surface waters cooled by ~6 °C at Site 982 while minimal change occurred at Site 608, reestablishing the North Atlantic SST gradient. The collapse and reemergence of the SST gradient between the middle‐ and high‐latitude North Atlantic suggests that interaction between changes in regional ocean circulation and the global response to changes in greenhouse gas concentration was important in Miocene climate evolution.

Elemental mercury in the marine boundary layer of North America: Temporal and spatial patterns

Gaseous elemental mercury (Hg0) comprises the majority of atmospheric mercury (Hg) and results in long-range transport of Hg. Secondary emissions from the marine surface mixed layer result in spatial and temporal Hg0 variability, which subsequently determine marine pools of divalent mercury (HgII) available for methylation. We measured atmospheric Hg0 concentrations in the marine boundary layer (MBL) around North America in regions of the Atlantic, Pacific, and Arctic Oceans (6.7°–77.3° N, 52.3°–150.0° W). Observed spatial Hg0 distributions across these regions match patterns revealed from current published biogeochemical models. These models have supported marine processes as the major factors controlling MBL Hg0. We observe differences in variability between ocean basins. Asian emissions appear to be well-mixed in the Eastern Pacific MBL, with stable measurements across a 3200 km transect in the North Pacific. Hg0 concentrations decrease but remain relatively stable in tropical regions of the Pacific and Atlantic. In contrast, we measured several 100–200 km regions of elevated Hg0 above North Atlantic surface waters. Repeated offshore sampling of Arctic Hg0 reveals strong seasonal trends that are similar to those collected at coastal long-term Arctic monitoring stations. These similarities suggest marine processes control seasonal Arctic trends, which are linked to sea ice formation and melt. Together our observations are consistent with marine controls on MBL Hg0 concentrations although further study is needed to fully distinguish between marine and atmospheric controls.

Community structural differences shape microbial responses to high molecular weight organic matter.

The extent to which differences in microbial community structure result in variations in organic matter (OM) degradation is not well understood. Here, we tested the hypothesis that distinct marine microbial communities from North Atlantic surface and bottom waters would exhibit varying compositional succession and functional shifts in response to the same pool of complex high molecular weight (HMW-OM). We also hypothesized that microbial communities would produce a broader spectrum of enzymes upon exposure to HMW-OM, indicating a greater potential to degrade these compounds than reflected by initial enzymatic activities. Our results show that community succession in amended mesocosms was congruent with cell growth, increased bacterial production and most notably, with substantial shifts in enzymatic activities. In all amended mesocosms, closely related taxa that were initially rare became dominant at time frames during which a broader spectrum of active enzymes were detected compared to initial timepoints, indicating a similar response among different communities. However, succession on the whole-community level, and the rates, spectra and progression of enzymatic activities, reveal robust differences among distinct communities from discrete water masses. These results underscore the crucial role of rare bacterial taxa in ocean carbon cycling and the importance of bacterial community structure for HMW-OM degradation.

Reservoir Ages in the Western Tropical North Atlantic from One Coral off Martinique Island (Lesser Antilles)

AbstractSea surface reservoir ages (R) are reported from radiocarbon (14C) measurements of the annual growth bands of coral Siderastrea siderea collected on the Atlantic coast off Martinique Island, in the Lesser Antilles volcanic arc. Mean values of R are similar between 1835 and 1845 during pre-anthropogenic times at 385±30 yr and between 1895 and 1905 at 382±20 yr when there was a huge eruption from the Montagne Pelée volcano in 1902–1903. Limited 14C aging of sea surface (~40 yr) may be due to enhanced volcanic activity. Variability of R is slightly greater during 1835–1845 than during 1895–1905. It is linked to a moderate increase of ∆14C of 5‰, strengthened by a clear increase of δ18O of 0.4‰. This is attributed to a decrease of the northward advection of the South Atlantic Waters into the western tropical North Atlantic and Caribbean Sea and relative enhanced westward flux of the tropical North Atlantic surface waters, the southern waters having lower values of 14C and δ18O than the North Atlantic ones. From 1835 to 1845, the fraction of the South Atlantic Waters transported up to Martinique Island was reduced from 25% to 15%.

Four years (2011–2015) of total gaseous mercury measurements from the Cape Verde Atmospheric Observatory

Abstract. Mercury is a chemical with widespread anthropogenic emissions that is known to be highly toxic to humans, ecosystems and wildlife. Global anthropogenic emissions are around 20 % higher than natural emissions and the amount of mercury released into the atmosphere has increased since the industrial revolution. In 2005 the European Union and the United States adopted measures to reduce mercury use, in part to offset the impacts of increasing emissions in industrialising countries. The changing regional emissions of mercury have impacts on a range of spatial scales. Here we report 4 years (December 2011–December 2015) of total gaseous mercury (TGM) measurements at the Cape Verde Observatory (CVO), a global WMO-GAW station located in the subtropical remote marine boundary layer. Observed total gaseous mercury concentrations were between 1.03 and 1.33 ng m−3 (10th, 90th percentiles), close to expectations based on previous interhemispheric gradient measurements. We observe a decreasing trend in TGM (−0.05 ± 0.04 ng m−3 yr−1, −4.2 % ± 3.3 % yr−1) over the 4 years consistent with the reported decrease of mercury concentrations in North Atlantic surface waters and reductions in anthropogenic emissions. The decrease was more visible in the summer (July–September) than in the winter (December–February), when measurements were impacted by air from the African continent and Sahara/Sahel regions. African air masses were also associated with the highest and most variable TGM concentrations. We suggest that the less pronounced downward trend inclination in African air may be attributed to poorly controlled anthropogenic sources such as artisanal and small-scale gold mining (ASGM) in West Africa.

Modern palynological record in the Bay of Brest (NW France): Signal calibration for palaeo-reconstructions

Modern palynological investigations (pollen and non-pollen palynomorphs) have been carried out for the first time in the entire Bay of Brest (BB; NW France), a semi-enclosed oceanic basin flooded during the Holocene that today is exposed to both marine (North Atlantic surface waters) and fluvial (Aulne and Elorn rivers) influences. Palynological analyses were conducted on 41 surface sediments collected in several mudded bays where low-energy conditions prevail, allowing for fine particle decantation, and also on 12 moss samples considered to be pollen rain collectors taken on the periphery of the BB. Furthermore, the BB allows a direct comparison between landscapes and fossilized pollen assemblages in sediments thanks to relatively small watersheds and well-studied vegetation cover on land. Our data display a well-known distortion between pollen taxa percentages calculated from palynological slides and current vegetation cover on land, with a particular overestimation of arboreal taxa as well as an underestimation of herbaceous ones, especially of cereal crops that represent a major component of the regional continental vegetation. Various factors may be involved, including pollen transport issues, taphonomic processes, and different pollination rates according to trees/herbaceous taxa. Under close examination, we find that while all surface sediments show the same major species, palynological maps established in the BB regarding some specific pollen taxa exhibit differences and upstream-downstream gradients. Also, the comparison between the pollen content in BB surface sediments and in mosses supports the hydrodynamic influence acting on the pollen distribution in sediments. More specifically, we suggest that some arboreal taxa reflect a watershed effect, especially Alnus, which seems to be highly correlated to river influences and could therefore be considered as a potential proxy for fluvial Holocene palaeo-discharges within the BB.

Episodic speleothem deposition tracks the terrestrial impact of millennial-scale last glacial climate variability in SW Ireland

Abstract Eighty four new U-Th ages are presented for twenty randomly selected broken, displaced and reworked calcite speleothems retrieved from clastic sedimentary fill and from isolated bedding-plane shelves in Crag cave (SW Ireland). The dated pre-Holocene samples span much of the last glacial, ranging in age from 85.15 ± 0.60 to 23.45 ± 0.17 ka. Speleothem deposition requires the presence of liquid water, and because Crag cave is a shallow system, deposition is considered likely only when mean annual air temperatures (MAAT) exceed the freezing point of water. Deposition at this mid-latitude ocean-marginal site occurred episodically during MIS5a through to MIS2, synchronously within dating uncertainties, with the timing of Greenland Interstadials (GI). In the latter part of Marine Isotope Stage 3 (MIS3), deposition was particularly intense, consistent with regional scale climate amelioration inferred previously from radiocarbon ages for sparse MIS3 organic and freshwater surficial deposits in N. Ireland. A brief episode of speleothem deposition at c.23.40 ± 0.22 ka coincides with GI-2, demonstrating the sensitivity of the site to brief climate amelioration episodes in Greenland during MIS2. Conditions favourable for speleothem deposition occurred periodically during the last glacial, indicating temperature changes of at least 10 °C between stadials and interstadials at this mid-latitude site. Deposition ceased during Greenland Stadials (GS), including during periods of ice-rafting in the adjacent N. Atlantic Ocean (Heinrich events). Oxygen and carbon isotope ratios of the last glacial speleothems are generally elevated, reflecting non-equilibrium isotope fractionation effects. However, establishment of low δ13C values often occurred within a few decades of climate amelioration, indicating that biogenic CO2 production resumed rapidly at this site, particularly during MIS3. Speleothem δ18O variability was driven largely by long-term changes in the δ18O value of the adjacent North Atlantic surface water, in turn largely reflecting changes in global ice volume. In common with published speleothem datasets, warming episodes in Ireland associated with GI events typically pre-date their timing in the GICC05 and GICC05modelext time scales, but lie comfortably within the maximum counting uncertainties of these ice core age models.

Organochlorine pesticides and polychlorinated biphenyls along an east-to-west gradient in subtropical North Atlantic surface water

Despite the fact that most persistent toxic substances have hardly been primarily emitted for several decades, their concentrations are only slowly decreasing in the global oceans. Surface seawater samples were collected along a 38°–24° N/28°–67° W transect in the subtropical North Atlantic Ocean. While the concentration levels of hexachlorobenzene (2.1–6.1 pg L−1), dichlorodiphenyltrichloroethane (DDT, up to 2.1 pg L−1) and polychlorinated biphenyls (PCB, 10.8–24.9 pg L−1) were in the same range as observed earlier in the North Atlantic, hexachlorocyclohexane (HCH, 90–627 pg L−1) was found elevated, partly also relative to previous measurements in the same sea region. Hereby, the ratio α-HCH/γ-HCH was very low, 0.09–0.13. Chlordane and endosulfan were found in the range <3.0–11.1 and <5.8–8.8 pg L−1 respectively. DDT metabolites, endrin and related pesticides were found below quantification limits. Spatial pollution patterns in surface seawaters seem to be determined by atmospheric and oceanic transport patterns, rather than by mixing and air-sea equilibrium. The comparison with global multicompartment chemistry-transport model predictions of surface seawater levels indicate underestimated degradation of PCBs and overestimated emissions of endosulfan.

Deep water circulation within the eastern Mediterranean Sea over the last 95 kyr: New insights from stable isotopes and benthic foraminiferal assemblages

The response of eastern Mediterranean Sea (EMS) circulation to climate forcings over the last 95kyr BP was studied using core MD04-2722, collected at 1780m water depth within the Levantine Sea. Foraminiferal stable isotopes and benthic assemblages were combined in order to reconstruct deep water ventilation and oxygenation in relation to surface water freshening. Over the last deglaciation, benthic foraminiferal δ13C values and oxygen index decreased, while the δ18O gradient between benthic and planktonic foraminifera increased. The results, respectively, indicate slower ventilation, bottom water oxygen depletion, and stronger stratification prior to S1 sapropel deposition. A combination of deglacial sea level rise and fresher north Atlantic surface water contributions were determined to be a precondition for S1 formation within the Levantine Sea. Local Nile River freshwater supplied during the African Humid Period further strengthened water column stratification. As for S3 deposit, the central role of monsoonal precipitation was estimated. For the last glacial period, three events at around 53, 46 and 37ka BP were marked by a deep water circulation reduction at the core location. Considering the influence of north Atlantic surface water salinity on Mediterranean Sea circulation, we propose that the 46 and 37ka BP events are responses to Heinrich Events 4 and 5 that supplied fresher surface water to the Mediterranean Sea. Since the ‘53ka event’ was also characterized by the appearance of low oxygen benthic indicators as observed within the S1 and S3 layers, we tentatively attributed it to ‘missing’ sapropel S2. Our results indicate that intense stagnation within the EMS could occur when both the local freshwater supply and fresher north Atlantic surface water are contributors. The influence of north Atlantic conditions was significant to EMS circulation under warm and cold climate conditions.

Climate versus geological controls on glacial meltwater micronutrient production in southern Greenland

Low concentrations of micronutrients in subarctic North Atlantic surface waters limit phytoplankton growth. Iron, phosphorous, and silicon are all potentially bio-limiting nutrients; iron is the most well documented in the subarctic North Atlantic. Manganese, nickel, copper and zinc are also essential trace metals for phytoplankton cell function. However, the spatial and temporal variability in the flux of these elements to the subarctic North Atlantic is undercharacterized. Here we show new data from the meltseason peak in 2013 indicating that glacial meltwater from the southern tip of Greenland has elevated dissolved major and trace metal concentrations compared to glacial meltwater draining shorter melt season glacial catchments to the north. Fe concentrations range from 0.13 to 6.97 μM, Zn from 4 to 95 μM, and Si from 4 to 36 μM, all higher than the depleted surface waters of the subarctic North Atlantic. Measured hydrochemical data modeled by PHREEQC indicates meltwater is undersaturated in pyrite and silicate phases but supersaturated with respect to oxyhydroxides, hematite and goethite, all phases that precipitate Fe as colloids, of which the nanoparticle phases should remain biologically available. The variability in geologic units between the sites indicates that subglacial lithology is a minor but not the dominant control on meltwater chemistry. The disparity in concentrations is directly correlated with climate, and an extended melt season, suggesting that future warming in Greenland will lead to increased trace element, and potential micronutrient, flux to the subarctic North Atlantic surface waters.

Rapid shifts in environmental conditions inferred from geochemical analyses of Lake Stiucii lacustrine record, Transylvanian Lowlands, Romania

Extensive proxy data and modeling suggest that the environmental response to climate forcing varied both spatially and temporarily even during the current interglacial (Davis et al., 2003; Davis and Brewer, 2009). The drivers of these short-term shifts in climate are of multiple origins, ranging from solar irradiance, periodic shifts in the regional expression of atmospheric circulation (e.g., NAO phases) to the episodic cold-water forcing of North Atlantic surface waters. Recently Magny et al. (2013) provided a comprehensive overview of regional climate change during the Holocene and its reflection in local hydrological data and water levels. Although these studies suggest a degree of regionalism in the climate signal, which might be expected, the most intriguing hypothesis is the so-called seesaw gradient, expressed as latitudinal and longitudinal differences in climate over relatively short distances (Davis et al., 2003; Davis and Brewer, 2009). In central-eastern Europe it is likely that the distribution of mountain ranges contributed even further to this climate fragmentation, therefore a regional scale perspective of the temporal development of climatic conditions and phase-relationships requires an increasing spatial resolution in proxy record (Dragusin et al., 2014)

Water masses and 129I distribution in the Nordic Seas

The application of the radioactive isotope iodine-129 as a tracer of water circulation in the oceans has provided interesting information with respect to sources and mixing of different water masses. We here present results of 129I distribution in water profiles located in the Nordic Seas and use the isotope to fingerprint water masses in the region. The samples were collected by the US research vessel Knorr in May–June 2002. 129I signatures along the Norwegian Sea reflect a mixing of 129I-rich surface water along the Scandinavian continental slope and 129I-poor North Atlantic surface water. These two water masses become less segregated along the Fram Strait where apparent 129I enrichment penetrates the return Arctic flow into the East Greenland Current. The 129I data further suggest existence of a water mass that is not entirely labeled with respect to origin at the Denmark Strait bottom water. This water parcel probably originates from the Iceland Sea. 129I data also shed light on the major deep water outflow from the Nordic Seas located at the Faeroe Bank Channel.

Detecting anthropogenic carbon dioxide uptake and ocean acidification in the North Atlantic Ocean

Abstract. Fossil fuel use, cement manufacture and land-use changes are the primary sources of anthropogenic carbon dioxide (CO2) to the atmosphere, with the ocean absorbing approximately 30% (Sabine et al., 2004). Ocean uptake and chemical equilibration of anthropogenic CO2 with seawater results in a gradual reduction in seawater pH and saturation states (Ω) for calcium carbonate (CaCO3) minerals in a process termed ocean acidification. Assessing the present and future impact of ocean acidification on marine ecosystems requires detection of the multi-decadal rate of change across ocean basins and at ocean time-series sites. Here, we show the longest continuous record of ocean CO2 changes and ocean acidification in the North Atlantic subtropical gyre near Bermuda from 1983–2011. Dissolved inorganic carbon (DIC) and partial pressure of CO2 (pCO2) increased in surface seawater by ~40 μmol kg−1 and ~50 μatm (~20%), respectively. Increasing Revelle factor (β) values imply that the capacity of North Atlantic surface waters to absorb CO2 has also diminished. As indicators of ocean acidification, seawater pH decreased by ~0.05 (0.0017 yr−1) and ω values by ~7–8%. Such data provide critically needed multi-decadal information for assessing the North Atlantic Ocean CO2 sink and the pH changes that determine marine ecosystem responses to ocean acidification.

Persistent influence of ice sheet melting on high northern latitude climate during the early Last Interglacial

Abstract. Although the Last Interglacial (LIG) is often considered as a possible analogue for future climate in high latitudes, its precise climate evolution and associated causes remain uncertain. Here we compile high-resolution marine sediment records from the North Atlantic, Labrador Sea, Norwegian Sea and the Southern Ocean. We document a delay in the establishment of peak interglacial conditions in the North Atlantic, Labrador and Norwegian Seas as compared to the Southern Ocean. In particular, we observe a persistent iceberg melting at high northern latitudes at the beginning of the LIG. It is associated with (1) colder and fresher surface-water conditions in the North Atlantic, Labrador and Norwegian Seas, and (2) a weaker ventilation of North Atlantic deep waters during the early LIG (129–125 ka) compared to the late LIG. Results from an ocean-atmosphere coupled model with insolation as a sole forcing for three key periods of the LIG show warmer North Atlantic surface waters and stronger Atlantic overturning during the early LIG (126 ka) than the late LIG (122 ka). Hence, insolation variations alone do not explain the delay in peak interglacial conditions observed at high northern latitudes. Additionally, we consider an idealized meltwater scenario at 126 ka where the freshwater input is interactively computed in response to the high boreal summer insolation. The model simulates colder, fresher North Atlantic surface waters and weaker Atlantic overturning during the early LIG (126 ka) compared to the late LIG (122 ka). This result suggests that both insolation and ice sheet melting have to be considered to reproduce the climatic pattern that we identify during the early LIG. Our model-data comparison also reveals a number of limitations and reinforces the need for further detailed investigations using coupled climate-ice sheet models and transient simulations.