Eastern Subtropical North Atlantic Research Articles

Single chamber Mg/Ca analyses of Globigerinoides ruber for paleo-proxy calibration using femtosecond LA-ICP-MS

Mg/Ca is an independent proxy in paleoceanography to reconstruct past seawater temperature. Femtosecond Laser Ablation Inductively Coupled Plasma Mass Spectrometry (fs-LA-ICP-MS) was employed to determine the Mg/Ca composition of tests (shells) of the planktic foraminifer species Globigerinoides ruber albus (white chromotype) and G. ruber ruber (red/pink chromotype) sampled alive from the temperate to subtropical eastern North Atlantic with the research sailing yacht Eugen Seibold. Mg/Ca data are compared to (i) the measured in-situ temperature of ambient seawater, (ii) average mixed layer temperature, and (iii) sea surface temperature (SST). The pooled mean chamber Mg/Ca from each plankton tow site exhibits a positive relationship with SST. Two chamber-specific calibrations are derived, which are consistent with previous calibration equations for comparable paleo-archives. The results confirm fs-LA-ICP-MS as reliable method for determining Mg/Ca in G. ruber, and both the penultimate and antepenultimate chambers of adult specimens may provide comprehensible Mg/Ca temperatures of the surface ocean.

Unravelling the transport of moisture into the Saharan Air Layer using passive tracers and isotopes

AbstractThe subtropical free troposphere plays a critical role in the radiative balance of the Earth. However, the complex interactions controlling moisture in this sensitive region and, in particular, the relative importance of long‐range transport compared to lower‐tropospheric mixing, remain unclear. This study uses the regional COSMO model equipped with stable water isotopes and passive water tracers to quantify the contributions of different evaporative sources to the moisture and its stable isotope signals in the eastern subtropical North Atlantic free troposphere. In summer, this region is characterized by two alternating large‐scale circulation regimes: (i) dry, isotopically depleted air from the upper‐level extratropics, and (ii) humid, enriched air advected from Northern Africa within the Saharan Air Layer (SAL) consisting of a mixture of moisture of diverse origin (tropical and extratropical North Atlantic, Africa, Europe, the Mediterranean). This diversity of moisture sources in regime (ii) arises from the convergent inflow at low levels of air from different neighbouring regions into the Saharan heat low (SHL), where it is mixed and injected by convective plumes into the large‐scale flow aloft, and thereafter expelled to the North Atlantic within the SAL. Remarkably, this regime is associated with a large contribution of moisture that evaporated from the North Atlantic, which makes a detour through the SHL and eventually reaches the 850–550 hPa layer above the Canaries. Moisture transport from Europe via the SHL to the same layer leads to the strongest enrichment in heavy isotopes (δ2H correlates most strongly with this tracer). The vertical profiles over the North Atlantic show increased humidity and δ2H and reduced static stability in the 850–550 hPa layer, and smaller cloud fraction in the boundary layer in regime (ii) compared to regime (i), highlighting the key role of moisture transport through the SHL in modulating the radiative balance in this region.

Multiproxy Reconstruction of Pliocene North Atlantic Sea Surface Temperatures and Implications for Rainfall in North Africa

AbstractWetter conditions in the African Sahel during the Pliocene (5.3–2.6 Ma) likely played an important role in hominid evolution and may be explained by warm North Atlantic sea‐surface temperatures (SSTs), similar to the modern warm phase of the Atlantic multidecadal oscillation (AMO). We reconstruct Pliocene North Atlantic (2°S to 60°N) SSTs through a multiproxy reduced‐dimension methodology that combines two new foraminiferal (Trilobatus sacculifer) Mg/Ca‐based SST records from Ocean Drilling Program sites in the northwestern Atlantic (Site 997) and Gulf of Mexico (Site 625) with previously published multi‐proxy SST records from seven additional Atlantic sites. Our Pliocene reconstructions indicate that the North Atlantic was overall warmer than the comparison period (0–0.5 Ma) with the most extreme warming occurring in the tropical and eastern subtropical North Atlantic and in the Labrador Sea. Furthermore, the mean state of North Atlantic warming at 5 Ma resembles that of the modern warm phase of the AMO with average SST anomalies of 3.8°C ± 0.5°C in the AMO region. With that reconstructed index, along with reconstructed El Niño‐like eastern Tropical Pacific SSTs from our previous research, we reconstruct rainfall over the Pliocene North Africa, covering the Sahara‐Sahel region and the Megalake Chad Basin and find up to twice the rainfall of that today. We surmise that this increased precipitation rate, along with other aiding land and sub‐surface processes, sustained Megalake Chad.

Aerosol characterisation in the subtropical eastern North Atlantic region using long-term AERONET measurements

Abstract. A comprehensive characterisation of atmospheric aerosols in the subtropical eastern North Atlantic has been carried out using long-term ground-based Aerosol Robotic NETwork (AERONET) photometric observations over the period 2005–2020 from a unique network made up of four stations strategically located from sea level to 3555 m on the island of Tenerife. This site can be considered a sentinel for the passage of airmasses going to Europe from Africa, and therefore the aerosol characterisation performed here adds important information for analysing their evolution during their path toward Northern Europe. Two of these stations (Santa Cruz de Tenerife – SCO – at sea level and La Laguna – LLO – at 580 m a.s.l.) are located within the marine atmospheric boundary layer (MABL), and the other two (Izaña – IZO – at 2373 m a.s.l. and Teide Peak – TPO – at 3555 m a.s.l.) are high mountain stations within the free troposphere (FT). Monthly climatology of the aerosol optical depth (AOD), Ångström exponent (AE), aerosol concentration, size distribution and aerosol optical properties has been obtained for the MABL and FT. Measurements that are quite consistent across the four sites have been used to categorise the main atmospheric scenarios, and these measurements confirm an alternation between predominant background conditions and predominant dust-loaded Saharan air mass conditions caused by seasonal dust transport over the subtropical North Atlantic. Background conditions prevail in the MABL and FT for most of the year, while dust-laden conditions dominate in July and August. The MABL under background conditions appears as a well-mixed layer with a low aerosol concentration (the volume concentration, VolCon, ranges from 0.02 ± 0.01 to 0.04 ± 0.02 µm3 µm−2), a predominance of coarse-mode marine aerosols (the effective radius, Reff, changes from 1.60 ± 0.19 to 1.91 ± 0.34 µm), and a volume contribution of the fine-mode fraction Vf/Vt <0.35. The clean FT is characterised by remarkably low aerosol loading and a predominant impact of fine-mode aerosols throughout the year (Vf/Vt has a maximum value of 0.93 ± 0.13), with an average Reff of 0.16 ± 0.02 µm. However, under dust-laden conditions and mainly in summer, we observe a predominance of coarse-mode aerosols with maximum VolCon values of 0.26 ± 0.23 µm3 µm−2 for the MABL and 0.16 ± 0.12 (0.06 ± 0.05) µm3 µm−2 for IZO (TPO), and a similar and quite consistent fine-mode fraction of 0.12 ± 0.03 in the vertical within the MABL and FT. Similarities in micro-physical and optical intensive aerosol properties confirm that the Saharan Air Layer (SAL) is a well-mixed layer in terms of the particulate composition. An estimation of the difference in the aerosol loading in the 1 km layer between IZO and TPO (in terms of VolCon and AOD) is performed in this study, and this shows that aerosol loading at IZO is double that at TPO, but they have similar fine-mode fractions, effective radii and intensive optical properties. The long-term trend analysis at SCO shows a significant negative trend in the fine-mode AOD between 2005 and 2020 (−1.8 ± 0.5) × 10−5 yr−1, which might be linked to the large reduction in oil-refining SO2 emissions from the SCO refinery in 2012.

Disentangling different moisture transport pathways over the eastern subtropical North Atlantic using multi-platform isotope observations and high-resolution numerical modelling

Abstract. Due to its dryness, the subtropical free troposphere plays a critical role in the radiative balance of the Earth's climate system. But the complex interactions of the dynamical and physical processes controlling the variability in the moisture budget of this sensitive region of the subtropical atmosphere are still not fully understood. Stable water isotopes can provide important information about several of the latter processes, namely subsidence drying, turbulent mixing, and dry and moist convective moistening. In this study, we use high-resolution simulations of the isotope-enabled version of the regional weather and climate prediction model of the Consortium for Small-Scale Modelling (COSMOiso) to investigate predominant moisture transport pathways in the Canary Islands region in the eastern subtropical North Atlantic. Comparison of the simulated isotope signals with multi-platform isotope observations (aircraft, ground- and space-based remote sensing) from a field campaign in summer 2013 shows that COSMOiso can reproduce the observed variability of stable water vapour isotopes on timescales of hours to days, thus allowing us to study the mechanisms that control the subtropical free-tropospheric humidity. Changes in isotopic signals along backward trajectories from the Canary Islands region reveal the physical processes behind the synoptic-scale isotope variability. We identify four predominant moisture transport pathways of mid-tropospheric air, each with distinct isotopic signatures: air parcels originating from the convective boundary layer of the Saharan heat low (SHL) – these are characterised by a homogeneous isotopic composition with a particularly high δD (median mid-tropospheric δD=-122‰), which results from dry convective mixing of low-level moisture of diverse origin advected into the SHL; air parcels originating from the free troposphere above the SHL – although experiencing the largest changes in humidity and δD during their subsidence over West Africa, these air parcels typically have lower δD values (median δD=-148‰) than air parcels originating from the boundary layer of the SHL; air parcels originating from outside the SHL region, typically descending from tropical upper levels south of the SHL, which are often affected by moist convective injections from mesoscale convective systems in the Sahel – their isotopic composition is much less enriched in heavy isotopes (median δD=-175‰) than those from the SHL region; air parcels subsiding from the upper-level extratropical North Atlantic – this pathway leads to the driest and most depleted conditions (median δD=-255‰) in the middle troposphere near the Canary Islands. The alternation of these transport pathways explains the observed high variability in humidity and δD on synoptic timescales to a large degree. We further show that the four different transport pathways are related to specific large-scale flow conditions. In particular, distinct differences in the location of the North African mid-level anticyclone and of extratropical Rossby wave patterns occur between the four transport pathways. Overall, this study demonstrates that the adopted Lagrangian isotope perspective enhances our understanding of air mass transport and mixing and offers a sound interpretation of the free-tropospheric variability of specific humidity and isotope composition on timescales of hours to days in contrasting atmospheric conditions over the eastern subtropical North Atlantic.

Oceanic cooling recorded in shells spanning the Medieval Climate Anomaly in the subtropical eastern North Atlantic Ocean

The Medieval Climate Anomaly (MCA; 900–1300 AD) was the most recent period of pre-industrial climatic warming in the northern hemisphere, and thus estimations of MCA signals can illuminate possible impacts of anthropogenic climate change. Current high-resolution MCA climate signals are restricted to mid- and high-latitude regions, which confounds inferences of how the MCA impacted some global/hemispheric climate mechanisms (e.g. North Atlantic Oscillation; NAO). To address this knowledge gap, we estimate seasonally-resolved sea surface temperatures (SSTs) from the oxygen isotope composition (δ18O) of serially sampled Phorcus atratus shells from archaeological sites spanning the MCA in the Canary Islands. Twelve archaeological and six modern P. atratus shells were analyzed, and archaeological shells were dated using carbonate-target radiocarbon dating. SSTs were estimated using the published aragonite-water equilibrium fractionation equation. Modern shells showed a mean SST of 20.0 ± 1.5 °C, with a seasonal amplitude of 5.3 ± 0.9 °C. Archaeological shells exhibited a mean SST of 18.2 ± 0.7 °C, with a mean seasonal amplitude of 5.5 ± 1.0 °C. Thus, shells that span the MCA in the Canary Islands recorded SSTs that were significantly cooler than the modern (P < .05), contrasting with warming estimates and model predictions elsewhere in the Northern Hemisphere. We propose that the observed cooling resulted from increased upwelling in NW Africa due to a strengthening of the prevailing westerlies and coastal winds along the African shoreline. The intensified upwelling scenario during the MCA is partially supported by in-situ carbon isotope data (δ13C) retrieved from the archaeological shells, which was compared to the δ13C values of modern shells and dissolved inorganic carbon in the ambient seawater. These results are consistent with other low-latitude temperature/precipitation anomalies associated with a positive NAO mode, suggesting a transition to a positive NAO index during the middle and late MCA that possibly extended later into the 13th century AD.

Oceanic cooling recorded in shells spanning the Medieval Climate Anomaly in the subtropical eastern North Atlantic Ocean

Oceanic cooling recorded in shells spanning the Medieval Climate Anomaly in the subtropical eastern North Atlantic Ocean

Response of Pelagic Calcifiers (Foraminifera, Thecosomata) to Ocean Acidification During Oligotrophic and Simulated Up-Welling Conditions in the Subtropical North Atlantic Off Gran Canaria

Planktonic Foraminifera and thecosome pteropods are major producers of calcite and aragonite in the ocean and play an important role for pelagic carbonate flux. The responses of planktonic foraminifers to ocean acidification (OA) are variable among the species tested and so far do not allow for reliable conclusion. Thecosome pteropods respond with reduced calcification and shell dissolution to OA and are considered at high risk especially at high latitudes. The present investigation was part of a large-scale in situ mesocosm experiment in the oligotrophic waters of the eastern subtropical North Atlantic. Over 62 days, we measured the abundance and vertical flux of pelagic foraminifers and thecosome pteropods as part of a natural plankton community over a range of OA scenarios. A bloom phase was initiated by the introduction of deep-water collected from approx. 650 m depth simulating a natural up-welling event. Foraminifers occurred throughout the entire experiment in both the water column and the sediment traps. Pteropods were present only in small numbers and disappeared after the first two weeks of the experiment. No significant CO2 related effects were observed for foraminifers, but cumulative sedimentary flux was reduced at the highest CO2 concentrations. This flux reduction was most likely accompanying an observed flux reduction of particulate organic matter (POM) so that less foraminifers were intercepted and transported downward.

Response of Subtropical Phytoplankton Communities to Ocean Acidification Under Oligotrophic Conditions and During Nutrient Fertilization

The subtropical oceans are home to the largest phytoplankton biome on the planet. Yet, little is known about potential impacts of ocean acidification (OA) on phytoplankton community composition in the vast oligotrophic ecosystems of the subtropical gyres. To address this question, we conducted an experiment with 9 in situ mesocosms (~35 m3) off the coast of Gran Canaria in the eastern subtropical North Atlantic over a period of 9 weeks. By establishing a gradient of pCO2 ranging from ~350 to 1025 µatm, we simulated carbonate chemistry conditions as projected until the end of the 21st century. Furthermore, we injected nutrient-rich deep water into the mesocosms halfway through the experiment to simulate a natural upwelling event, which regularly leads to patchy nutrient fertilization in the study region. The temporal developments of major taxonomic groups of phytoplankton were analyzed by flow cytometry, pigment composition and microscopy. We observed distinct shifts in phytoplankton community structure in response to high CO2, with markedly different patterns depending on nutrient status of the system. Phytoplankton biomass during the oligotrophic phase was dominated by picocyanobacteria (Synechococcus), which constituted 60-80% of biomass and displayed significantly higher cell abundances at elevated pCO2. The addition of deep water triggered a substantial bloom of large, chain-forming diatoms (mainly Guinardia striata and Leptocylindrus danicus) that dominated the phytoplankton community during the bloom phase (70-80% of biomass) and until the end of the experiment. A CO2 effect on bulk diatom biomass became apparent only in the highest CO2 treatments (>800 µatm), displaying elevated concentrations especially in the stationary phase after nutrient depletion. Notably, these responses were tightly linked to distinct interspecific shifts within the diatom assemblage, particularly favoring the largest species Guinardia striata. Other taxonomic groups contributed less to total phytoplankton biomass, but also displayed distinct responses to OA treatments. For instance, higher CO2 favored the occurrence of prymnesiophyceae (Phaeocystis globosa) and dictyochophyceae, whereas dinoflagellates were negatively affected by increasing CO2. Altogether, our findings revealed considerable shifts in species composition in response to elevated CO2 and indicated that phytoplankton communities in the subtropical oligotrophic oceans might be profoundly altered by ocean acidification.

Mesoscale contribution to the long-range offshore transport of organic carbon from the Canary Upwelling System to the open North Atlantic

Abstract. Several studies in upwelling regions have suggested that mesoscale structures, such as eddies and filaments, contribute substantially to the long-range transport of the organic carbon from the nearshore region of production to the offshore region of remineralization. Yet a comprehensive analysis of this mesoscale flux and of its impact across the Canary Upwelling System (CanUS) has not been provided. Here, we fill this gap using simulations with the Regional Oceanic Modeling System (ROMS) coupled to a Nutrient, Phytoplankton, Zooplankton and Detritus (NPZD) ecosystem model. We run climatological simulations on an Atlantic telescopic grid with an eddy-resolving resolution in the CanUS. Using both a Reynolds flux decomposition and structure-identification algorithms, we quantify and characterize the organic carbon fluxes driven by filaments and eddies within the upper 100 m and put them in relationship to the total offshore transport. Our analysis reveals that both coastal filaments and eddies enhance the offshore flux of organic carbon, but that their contribution is very different. Upwelling filaments, with their high speeds and high concentrations, transport the organic carbon offshore in a very intense, but coastally confined manner, contributing nearly 80 % to the total flux of organic carbon at 100 km offshore. The filament contribution tapers off quickly to near zero values at 1000 km off the coast, leading to a strong offshore flux divergence that is the main lateral source of organic carbon in the coastal waters up to 1000 km offshore. Some of this divergence is also due to the filaments inducing a substantial vertical subduction of the organic carbon below 100 m. Owing to the temporal persistence and spatial recurrence of filaments, the filament transport largely constitutes a time-mean flux, while the time-varying component, i.e., the turbulent flux, is comparatively small. At distances beyond 500 km from the coast, eddies dominate the mesoscale offshore transport. Although their contribution represents only 20 % of the total offshore flux and its divergence, eddies, especially cyclones, transport organic carbon offshore to distances as great as 2000 km from the coast. The eddy transport largely represents a turbulent flux, but striations in this transport highlight the existence of typical formation spots and recurrent offshore propagation pathways. While they propagate slowly, eddies are an important organic carbon reservoir for the open waters, as they contain, on average, a third of the organic carbon in this region, two thirds of which is found in cyclones. Our analysis confirms the importance of mesoscale processes for the offshore organic carbon transport and the fueling of the heterotrophic activity in the eastern subtropical North Atlantic, and highlights the need to consider the mesoscale flux in order to fully resolve the three-dimensionality of the marine organic carbon cycle.

In situ camera observations reveal major role of zooplankton in modulating marine snow formation during an upwelling-induced plankton bloom

Particle aggregation and the consequent formation of marine snow alter important properties of biogenic particles (size, sinking rate, degradability), thus playing a key role in controlling the vertical flux of organic matter to the deep ocean. However, there are still large uncertainties about rates and mechanisms of particle aggregation, as well as the role of plankton community structure in modifying biomass transfer from small particles to large fast-sinking aggregates.Here we present data from a high-resolution underwater camera system that we used to observe particle size distributions and formation of marine snow (aggregates >0.5 mm) over the course of a 9-week in situ mesocosm experiment in the Eastern Subtropical North Atlantic. After an oligotrophic phase of almost 4 weeks, addition of nutrient-rich deep water (650 m) initiated the development of a pronounced diatom bloom and the subsequent formation of large marine snow aggregates in all 8 mesocosms. We observed a substantial time lag between the peaks of chlorophyll a and marine snow biovolume of 9–12 days, which is much longer than previously reported and indicates a marked temporal decoupling of phytoplankton growth and marine snow formation during our study. Despite this time lag, our observations revealed substantial transfer of biomass from small particle sizes (single phytoplankton cells and chains) to marine snow aggregates of up to 2.5 mm diameter (ESD), with most of the biovolume being contained in the 0.5–1 mm size range. Notably, the abundance and community composition of mesozooplankton had a substantial influence on the temporal development of particle size spectra and formation of marine snow aggregates: While higher copepod abundances were related to reduced aggregate formation and biomass transfer towards larger particle sizes, the presence of appendicularia and doliolids enhanced formation of large marine snow.Furthermore, we combined in situ particle size distributions with measurements of particle sinking velocity to compute instantaneous (potential) vertical mass flux. However, somewhat surprisingly, we did not find a coherent relationship between our computed flux and measured vertical mass flux (collected by sediment traps in 15 m depth). Although the onset of measured vertical flux roughly coincided with the emergence of marine snow, we found substantial variability in mass flux among mesocosms that was not related to marine snow numbers, and was instead presumably driven by zooplankton-mediated alteration of sinking biomass and export of small particles (fecal pellets).Altogether, our findings highlight the role of zooplankton community composition and feeding interactions on particle size spectra and formation of marine snow aggregates, with important implications for our understanding of particle aggregation and vertical flux of organic matter in the ocean.

On the long-range offshore transport of organic carbon from the Canary Upwelling System to the open North Atlantic

Abstract. A compilation of measurements of net community production (NCP) in the upper waters of the eastern subtropical North Atlantic had suggested net heterotrophic conditions, purportedly supported by the lateral export of organic carbon from the adjacent, highly productive Canary Upwelling System (CanUS). Here, we quantify and assess this lateral export using the Regional Ocean Modeling System (ROMS) coupled to a nutrient, phytoplankton, zooplankton, and detritus (NPZD) ecosystem model. We employ a new Atlantic telescopic grid with a strong refinement towards the northwestern African shelf to combine an eddy-resolving resolution in the CanUS with a full Atlantic basin perspective. Our climatologically forced simulation reveals an intense offshore flux of organic carbon that transports about 19 Tg C yr−1 away from the nearshore 100 km over the whole CanUS, amounting to more than a third of the NCP in this region. The offshore transport extends beyond 1500 km into the subtropical North Atlantic, adding organic carbon along the way to the upper 100 m at rates of between 8 and 34 % of the alongshore average NCP as a function of offshore distance. Although the divergence of this lateral export of organic carbon enhances local respiration, the upper 100 m layer in our model remains net autotrophic in the entire eastern subtropical North Atlantic. However, the vertical export of this organic carbon and its subsequent remineralization at depth makes the vertically integrated NCP strongly negative throughout this region, with the exception of a narrow band along the northwestern African shelf. The magnitude and efficiency of the lateral export varies substantially between the different subregions. In particular, the central coast near Cape Blanc is particularly efficient in collecting organic carbon on the shelf and subsequently transporting it offshore. In this central subregion, the offshore transport adds as much organic carbon as nearly 60 % of the local NCP to the upper 100 m, giving rise to a sharp peak of offshore respiration that extends to the middle of the gyre. Our modeled offshore transport of organic carbon is likely a lower-bound estimate due to our lack of full consideration of the contribution of dissolved organic carbon and that of particulate organic carbon stemming from the resuspension of sediments. But even in the absence of these contributions, our results emphasize the fundamental role of the lateral redistribution of the organic carbon for the maintenance of the heterotrophic activity in the open sea.

Influence of Ocean Acidification and Deep Water Upwelling on Oligotrophic Plankton Communities in the Subtropical North Atlantic: Insights from an In situ Mesocosm Study

Oceanic uptake of anthropogenic carbon dioxide (CO2) causes pronounced shifts in marine carbonate chemistry and a decrease in seawater pH. Increasing evidence indicates that these changes—summarized by the term ocean acidification (OA)—can significantly affect marine food webs and biogeochemical cycles. However, current scientific knowledge is largely based on laboratory experiments with single species and artificial boundary conditions, whereas studies of natural plankton communities are still relatively rare. Moreover, the few existing community-level studies were mostly conducted in rather eutrophic environments, while less attention has been paid to oligotrophic systems such as the subtropical ocean gyres. Here we report from a recent in situ mesocosm experiment off the coast of Gran Canaria in the eastern subtropical North Atlantic, where we investigated the influence of OA on the ecology and biogeochemistry of plankton communities in oligotrophic waters under close-to-natural conditions. This paper is the first in this Research Topic of Frontiers in Marine Biogeochemistry and provides (1) a detailed overview of the experimental design and important events during our mesocosm campaign, and (2) first insights into the ecological responses of plankton communities to simulated OA over the course of the 62-day experiment. One particular scientific objective of our mesocosm experiment was to investigate how OA impacts might differ between oligotrophic conditions and phases of high biological productivity, which regularly occur in response to upwelling of nutrient-rich deep water in the study region. Therefore, we specifically developed a deep water collection system that allowed us to obtain ~85 m3 of seawater from ~650 m depth. Thereby, we replaced ~20% of each mesocosm's volume with deep water and successfully simulated a deep water upwelling event that induced a pronounced plankton bloom. Our study revealed significant effects of OA on the entire food web, leading to a restructuring of plankton communities that emerged during the oligotrophic phase, and was further amplified during the bloom that developed in response to deep water addition. Such CO2-related shifts in plankton community composition could have consequences for ecosystem productivity, biomass transfer to higher trophic levels, and biogeochemical element cycling of oligotrophic ocean regions.

Optimality-basedTrichodesmiumdiazotrophy in the North Atlantic subtropical gyre

A recent optimality-based model for phytoplankton growth and diazotrophy was applied at two stations located in the oligotrophic western and the ultra-oligotrophic eastern subtropical North Atlantic. Contrary to the common view that diazotrophy is favoured by nitrogen (N) depletion relative to the Redfield equivalent of phosphorus (P), we find that optimality-based diazotrophy could explain N fixation in both regions in spite of relatively high N:P supply ratios. This is possible because the availability of an additional source of N for diazotrophs makes them strong competitors for P under oligotrophic conditions. The best reproduction of observations, especially of net primary production, is only achieved with preferential remineralization of P relative to N and atmospheric deposition. In line with observations, a higher rate of nitrogen fixation is predicted for the eastern site, owing to a larger niche for diazotrophs resulting from stronger oligotrophy and lower N:P supply ratios due to weaker atmospheric N deposition. Because the competitive advantage of diazotrophs under nutrient starvation diminishes with increasing supply N:P ratio, the predicted increase of atmospheric N deposition due to anthropogenic activity could negatively affect N2 fixation in the Atlantic Ocean.

Decadal variability in the oxygen inventory of North Atlantic subtropical underwater captured by sustained, long‐term oceanographic time series observations

AbstractHistorical observations of potential temperature (θ), salinity (S), and dissolved oxygen concentrations (O2) in the tropical and subtropical North Atlantic (0–500 m; 0–40°N, 10–90°W) were examined to understand decadal‐scale changes in O2 in subtropical underwater (STUW). STUW is observed at four of the longest, sustained ocean biogeochemical and ecological time series stations, namely, the CArbon Retention In A Colored Ocean (CARIACO) Ocean Time Series Program (10.5°N, 64.7°W), the Bermuda Atlantic Time‐series Study (BATS; 31.7°N, 64.2°W), Hydrostation “S” (32.1°N, 64.4°W), and the European Station for Time‐series in the Ocean, Canary Islands (ESTOC; 29.2°N, 15.5°W). Observations over similar time periods at CARIACO (1996–2013), BATS (1988–2011), and Hydrostation S (1980–2013) show that STUW O2 has decreased approximately 0.71, 0.28, and 0.37 µmol kg−1 yr−1, respectively. No apparent change in STUW O2 was observed at ESTOC over the course of the time series (1994–2013). Ship observation data for the tropical and subtropical North Atlantic archived at NOAA National Oceanographic Data Center show that between 1980 and 2013, STUW O2 (upper ~300 m) declined 0.58 µmol kg−1 yr−1 in the southeastern Caribbean Sea (10–15°N, 60–70°W) and 0.68 µmol kg−1 yr−1 in the western subtropical North Atlantic (30–35°N, 60–65°W). A declining O2 trend was not observed in the eastern subtropical North Atlantic (25–30°N, 15–20°W) over the same period. Most of the observed O2 loss seems to result from shifts in ventilation associated with decreased wind‐driven mixing and a slowing down of STUW formation rates, rather than changes in diffusive air‐sea O2 gas exchange or changes in the biological oceanography of the North Atlantic. Variability of STUW O2 showed a significant relationship with the wintertime (January–March) Atlantic Multidecadal Oscillation index (AMO, R2 = 0.32). During negative wintertime AMO years trade winds are typically stronger between 10°N and 30°N. These conditions stimulate the formation and ventilation of STUW. The decreasing trend in STUW O2 in the three decades spanning 1980 through 2013 reflects the shift from a strongly negative wintertime AMO between the mid‐1980s and mid‐1990s to a positive wintertime AMO observed between the mid‐1990s and 2013. These changes in STUW O2 were captured by the CARIACO, BATS, and Hydrostation S time series stations. Sustained positive AMO conditions could lead to further deoxygenation in tropical and subtropical North Atlantic upper waters.

Impact of Sahara dust on solar radiation at Cape Verde Islands derived from MODIS and surface measurements

Based on radiometer measurements of solar irradiance (direct and diffuse light) and Aeronet-based aerosol optical depth (AOD) obtained at the Cape Verde atmospheric observatory during a major cloud-free dust outbreak event on February 7, 2012, the relationship between Saharan mineral dust outbreaks and a reduction of solar irradiance is quantified. The investigation is representative of the eastern subtropical North Atlantic region where the wind mobilization of mineral desert dust from the Sahara results in aerosol signals that are large enough to outweigh those from other aerosol types such as anthropogenic and marine aerosols. Ground-based estimates of AOD show frequency dependence as is expected from Mie theory. Our AOD signals agree well with satellite-based MODIS products and reveal AOD values exceeding 2.5 during the investigated dust storm event. We also demonstrate the use of satellite imagery with an atmospheric trajectory model to simulate time series of measurements at a given location. Using this approach, variations in AOD observed during February 7, 2012 can be rationalized as spatial inhomogeneities in the atmospheric dust load being advected laterally over the observing site. Our measurements suggest a dust forcing efficiency of around −90W/m2/AOD at a wavelength of 380nm, which is about 10–15% greater than reported in the literature indicative of a possible non-linear behavior at high AODs.

Northern source for Deglacial and Holocene deepwater composition changes in the Eastern North Atlantic Basin

Over the last decades extensive research has been carried out on changes in the Atlantic Meridional Overturning Circulation and its role during the last deglacial period. At present, only a few high-resolution data sets from the deep eastern North Atlantic exist for this period. It therefore remains uncertain whether deepwater changes in the Eastern North Atlantic Basin were governed by alternating contributions of northern and southern deepwater or whether the Eastern North Atlantic Basin reflects changes in the initial composition and source of North Atlantic deepwater. Furthermore, it is still debated whether such changes are triggered by Northern or Southern Hemisphere climatic changes.In this centennial to decadal scale resolution study we investigate deepwater composition changes in the Eastern North Atlantic Basin over the last 15 ka BP. We used sediment cores GEOFAR KF16 and MD08-3180 (37.984° N; 31.118° W, wd 3050 m/37.999° N; 31.134° W, wd 3064 m), obtained from a small basin at the eastern flank of the Mid Atlantic Ridge south of the Azores Islands. Under modern conditions the coring site is situated at the boundary between southern Lower Deep Water and northern Eastern North Atlantic Deep Water consisting of Iceland Scotland Overflow Water and Labrador Sea Water. Distinct differences between the three water masses in terms of ventilation state, temperature and salinity signatures are ideal for tracking changes in the deglacial North Atlantic deepwater distribution using paired benthic foraminiferal stable isotopes (δ13C, δ18O) and Mg/Ca bottom water temperature reconstructions.The results show a close coupling of low bottom water temperature (1.5 °C) and δ13C(0–0.5‰) values during cold Heinrich event 1, the Younger Dryas and the Preboreal, that were coeval with a major δ18O depletion of deepwater. The strong similarities between subtropical Eastern North Atlantic Deep Water and subpolar North Atlantic surface and deepwater changes indicate that deglacial changes in Eastern North Atlantic Deep Water distribution were triggered in the North Atlantic. Consequently, this would imply that changes in the North Atlantic also contributed to deglacial changes in Antarctic bottom water composition. During the early Holocene stepwise increasing δ13C values suggest increasing Eastern North Atlantic Deep Water production and/or increasing Eastern North Atlantic Deep Water ventilation with minor but distinct interruptions (at 10.8, 10.6, 9.1, 8.4, 8.1 and 7.2 ka BP), which were most probably also triggered in the subpolar North Atlantic.

High-frequency surface water changes in the Tagus prodelta off Lisbon, eastern North Atlantic, during the last two millennia

Abstract A high-resolution sedimentary sequence recovered from the Tagus prodelta has been studied with the objective to reconstruct multi-decadal to centennial-scale climate variability on the western Iberian Margin and to discuss the observations in a wider oceanographic and climatic context. Between ca. 100 BC and AD 400 the foraminiferal fauna and high abundance of Globorotalia inflata indicate advection of subtropical waters via the Azores Current and the winter-time warm Portugal Coastal Current. Between ca. AD 400 and 1350, encompassing the Medieval Climate Anomaly (MCA), enhanced upwelling is indicated by the planktonic foraminiferal fauna, in particular by the high abundance of upwelling indicator species Globigerina bulloides. Relatively light δ18O values and high sea surface temperature (SST) (reconstructed from foraminiferal assemblages) point to upwelling of subtropical Eastern North Atlantic Central Water. Between ca. AD 1350 and 1750, i.e. most of the Little Ice Age, relatively heavy δ18O values and low reconstructed SST, as well as high abundances of Neogloboquadrina incompta, indicate the advection of cold subpolar waters to the area and a southward deflection of the subpolar front in the North Atlantic, as well as changes in the mode of the North Atlantic Oscillation. In addition, the assemblage composition together with the other proxy data reveals less upwelling and stronger river input than during the MCA. Stronger Azores Current influence on the Iberian Margin and strong anthropogenic effect on the climate after AD 1750 is indicated by the foraminiferal fauna. The foraminiferal assemblage shows a significant change in surface water conditions at ca. AD 1900, including enhanced river runoff, a rapid increase in temperature and increased influence of the Azores Current. The Tagus record displays a high degree of similarity to other North Atlantic records, indicating that the site is influenced by atmospheric–oceanic processes operating throughout the North Atlantic, as well as by local changes.

Interannual Variability of the Mixed Layer Winter Convection and Spice Injection in the Eastern Subtropical North Atlantic

AbstractThe Argo dataset is used to study the winter upper-ocean conditions in the northeastern subtropical (NEA) Atlantic during 2006–12. During late winter 2010, the mixed layer depth is abnormally shallow and a negative anomaly of density-compensated salinity, the so-called spiciness, is generated in the permanent pycnocline. This is primarily explained by unusual weak air–sea buoyancy flux during the late winter 2010, in contrast with the five other studied winters. Particularly deep mixed layers and strong spiciness anomalies are observed during late winter 2012. The 2010 winter conditions appear to be related to historically low North Atlantic Oscillation (NAO) and high tropical North Atlantic index (TNA). Interannual variability of the eastern subtropical mixed layer is further investigated using a simple 1D bulk model of mean temperature and salinity linear profiles, based on turbulent kinetic energy conservation in the upper-ocean layer, and forced only with seasonal air–sea buoyancy forcing corresponding to fall–winter 2006–12. It suggests that year-to-year variability of the winter convective mixing driven by atmospheric buoyancy flux is able to generate interannual variability of both late winter mixed layer depth and spiciness in a strongly compensated layer at the base of the mixed layer and in the permanent pycnocline.

The effect of nutrients on carbon and nitrogen fixation by the UCYN-A-haptophyte symbiosis.

Symbiotic relationships between phytoplankton and N2-fixing microorganisms play a crucial role in marine ecosystems. The abundant and widespread unicellular cyanobacteria group A (UCYN-A) has recently been found to live symbiotically with a haptophyte. Here, we investigated the effect of nitrogen (N), phosphorus (P), iron (Fe) and Saharan dust additions on nitrogen (N2) fixation and primary production by the UCYN-A-haptophyte association in the subtropical eastern North Atlantic Ocean using nifH expression analysis and stable isotope incubations combined with single-cell measurements. N2 fixation by UCYN-A was stimulated by the addition of Fe and Saharan dust, although this was not reflected in the nifH expression. CO2 fixation by the haptophyte was stimulated by the addition of ammonium nitrate as well as Fe and Saharan dust. Intriguingly, the single-cell analysis using nanometer scale secondary ion mass spectrometry indicates that the increased CO2 fixation by the haptophyte in treatments without added fixed N is likely an indirect result of the positive effect of Fe and/or P on UCYN-A N2 fixation and the transfer of N2-derived N to the haptophyte. Our results reveal a direct linkage between the marine carbon and nitrogen cycles that is fuelled by the atmospheric deposition of dust. The comparison of single-cell rates suggests a tight coupling of nitrogen and carbon transfer that stays balanced even under changing nutrient regimes. However, it appears that the transfer of carbon from the haptophyte to UCYN-A requires a transfer of nitrogen from UCYN-A. This tight coupling indicates an obligate symbiosis of this globally important diazotrophic association.