ALBERT

Description: Deep convection in the Subpolar Gyre (SPG) forms a link between the upper and lower limbs of the Atlantic Meridional Overturning Circulation (AMOC). The intensity of convection in ocean studies is usually estimated using mixed layer depth (MLD). Here MLD is derived using vertical profiles of potential density from the gridded ARMOR3D dataset and from in situ observations of the EN4 dataset. Given limited areas of convective chimneys, the robustness of the estimates from an available set of vertical profiles needs to be verified before accessing mechanisms of interannual variability of deep convection. For reaching this goal, we first outlined three convection domains in the SPG with a high frequency of deep convection events: the southwestern Labrador Sea (L-DC), the central Irminger Sea (I-DC), and the area south of Cape Farewell (F-DC). The minimum number of randomly scattered casts, required to be executed from January to April for a robust estimate of the maximum MLD, depends on the typical area of the convective regions within the domain and forms 50 casts for L-DC, 40 casts for I-DC and 10 casts for F-DC. For the investigated convection domains, a sufficient number of casts were collected for several standalone winters of the late 1990s, while continuous time series of the convection intensity can be obtained only since the mid-2000s.

Description: The impact of oxygen on the preservation of organic matter in marine surface sediments is still controversial. We revisited this long-standing debate by determining the burial efficiency of sedimentary organic matter in the Black Sea, the largest anoxic and euxinic basin in the modern ocean. Surface sediments were sampled in the Danube paleodelta on the northwestern margin of the Black Sea at 420–1550 m water depth. Steady-state modeling of solid species (particulate organic carbon and nitrogen) and solutes (ammonium, sulfate, and total alkalinity) in sediments was performed to quantify rates of mass accumulation, particulate organic matter (POM) degradation, and POM burial. We develop a novel analytical model to quantify these rates applying an inverse modelling approach to down core data accounting for molecular diffusion, sediment burial and compaction. Our model results indicate that 56.7 ± 6.6 % of the particulate organic matter deposited in the study area is not degraded in surface sediments but accumulates below 10 cm sediment depth. This burial efficiency is substantially higher than those previously derived for seafloor areas underlying oxygenated bottom waters. Hence, our study confirms previous studies showing that euxinic bottom water conditions promote the preservation of particulate organic matter in marine sediments.

Description: We present a detailed field and petrological study of charnockites and ultra-high temperature (UHT) granulites from the Gruf Complex, eastern Central Alps. Charnockites occur as up to 0.5 km wide and 8 km long, internally boudinaged, opx-bearing sheet-like bodies within the regionally dominant migmatitic biotite-orthogneisses. Granulites occur as garnet–orthopyroxene–biotite–alkali feldspar-bearing schlieren (± sapphirine, sillimanite, cordierite, corundum, spinel, plagioclase, and quartz) within charnockites and as residual enclaves both in the charnockites and the migmatitic orthogneisses. Thermobarometric calculations, P–T pseudosections and orthopyroxene Al content, show that both charnockites and granulites equilibrated at metamorphic peak conditions of T = 920–940 °C and P = 8.5–9.5 kbar. Peak assemblages were subsequently overprinted by intergrowth, symplectite and corona textures involving orthopyroxene, sapphirine, cordierite and spinel at T = 720–740 °C and P = 7–7.5 kbar. We suggest that granulites and charnockites are lower crustal relicts preserved in the migmatitic orthogneisses. Garnet diffusion modelling shows that metamorphic garnet–opx ± sapphirine ± sillimanite peak assemblages and post-peak reaction textures always involving cordierite developed during two separate metamorphic cycles. Peak assemblages reflect UHT metamorphism related to post-Varican Permian extension, but post-peak coronae and symplectites formed during the mid-Tertiary, upper amphibolite facies, Alpine regional metamorphism. Fluid-absent partial melting of pelitic and psammitic sediments during the Permian UHT event lead to the formation of charnockitic magmas and granulitic residues. Intense melt loss and thorough dehydration of the granulites (although retaining biotite) favoured the partial preservation of peak mineral assemblages during Alpine metamorphism.

Description: Highlights • Development of an autonomous DIC analyzer based on Conductometric technique using a cell with 4 hollow brass electrodes. • CO2 extraction from seawater using a gas diffusion cell with a “Tube In A Tube” configuration and a gas permeable membrane. • Formulation of mathematical temperature and salinity correction to determine accurate DIC concentration. • Demonstration of the analyzer performance in the southwest Baltic Sea. Abstract Background The increase in anthropogenic CO2 concentrations in the Earth's atmosphere since the industrial revolution has resulted in an increased uptake of CO2 by the oceans, leading to ocean acidification. Dissolved Inorganic Carbon (DIC) is one of the key variables to characterize the seawater carbonate system. High quality DIC observations at a high spatial-temporal resolution is required to improve our understanding of the marine carbonate system. To meet the requirements, autonomous DIC analyzers are needed which offer a high sampling frequency, are cost-effective and have a low reagent and power consumption. Results We present the development and validation of a novel analyzer for autonomous measurements of DIC in seawater using conductometric detection. The analyzer employs a gas diffusion sequential injection approach in a “Tube In A Tube” configuration that facilitates diffusion of gaseous CO2 from an acidified sample through a gas permeable membrane into a stream of an alkaline solution. The change in conductivity in the alkaline medium is proportional to the DIC concentration of the sample and is measured using a detection cell constructed of 4 hollow brass electrodes. Physical and chemical optimizations of the analyzer yielded a sampling frequency of 4 samples h−1 using sub mL reagent volumes for each measurement. Temperature and salinity effects on DIC measurements were mathematically corrected to increase accuracy. Analytical precision of ±4.9 μmol kg−1 and ±9.7 μmol kg−1 were achieved from measurements of a DIC reference material in the laboratory and during a field deployment in the southwest Baltic Sea, respectively. Significance This study describes a simple, cost-effective, autonomous, on-site benchtop DIC analyzer capable of measuring DIC in seawater at a high temporal resolution as a step towards an underwater DIC sensor. The analyzer is able to measure a wide range of DIC concentrations in both fresh and marine waters. The achieved accuracy and precision offer an excellent opportunity to employ the analyzer for ocean acidification studies and CO2 leakage detection in the context of Carbon Capture and Storage operations.

Description: The ocean region along the latitude of 40oS in the South Atlantic, characterized by enhanced primary productivity, forms a transition zone between the nutrient replete but iron depleted Southern Ocean, and the nitrate and iron depleted Subtropical Gyre. Here, we present distributions of nutrient-type dissolved and particulate trace metals (dTMs and pTMs) including cadmium (Cd), nickel (Ni), copper (Cu), and zinc (Zn) in the South Atlantic from the GEOTRACES GA10 cruises. Phytoplankton uptake, riverine and atmospheric inputs shaped dTM and pTM concentrations in surface waters (dCd 27.8±36.0 pmol kg-1, n=222; dCu 0.732±0.429 nmol kg-1, n=222; dNi 3.38±0.52 nmol kg-1, n=219; dZn 0.332±0.398 nmol kg-1, n=214). Subsurface nutrients and dTMs (dCd 563±184 pmol kg-1, n=335; dCu 1.819±0.773 nmol kg-1, n=334; dNi 6.19±1.06 nmol kg-1, n=330; dZn 3.71±2.10 nmol kg-1, n=333) were controlled by the mixing of Antarctic origin waters and North Atlantic Deep Waters (NADW) with negligible contributions from local remineralization. Dissolved and particulate TMs in the Argentine Basin showed elevated concentrations towards the seafloor because of benthic inputs. Direct hydrothermal inputs of dTMs and pTMs to deep waters were not observed along the transect. The Cd-Cu-Zn-phosphate stoichiometries of Antarctic origin waters were set by a combination of dynamic physical circulation and preferential uptake of Cd, Cu, and Zn relative to phosphate in surface waters because of a dominance by diatoms in the Southern Ocean. Water mass mixing subsequently produced convoluted dCu-P and dZn-P relationships and apparent linear dCd-P and dNi-P relationships in the South Atlantic. More importantly, endmember characteristics of Antarctic waters and NADW are largely fixed in their formation regions in high latitude oceans. Therefore, the highly dynamic high latitude oceans are key regions that supply nutrients and TMs at specific ratios to low latitude oceans via the thermohaline circulation. Changes to processes in the high latitude oceans may have consequences for marine primary productivity downstream, and hence the global carbon cycle.

Description: The boundary between the American and European plates emerges in Iceland, an outstanding natural laboratory where it is possible to analyse ongoing rifting processes. In the North Volcanic Zone, we studied with unprecedented detail an active rift, known as the Theistareykir Fissure Swarm (ThFS). We surveyed an area of 85 km2 with 694 measurement sites along 1537 post-Late Glacial Maximum extension fractures. In the southern sector of the study area, fractures strike N30–40° with opening directions about N120°. Fractures in the central sector strike about N00° and opening directions are N90–100°. In the northern sector, fractures strike about N30° with opening directions about N125°. Through a comparison with older faults cropping out in the substrate at the shoulder of the ThFS, we are able to suggest that variations in fracture strike are the effect of substrate structural inheritance as well as the possible interaction with the Tjörnes Fracture Zone. With regard to kinematics, we highlight that most fractures show a small, but systematic, strike-slip component (a more frequent right-lateral component and a less common, left-lateral one). This cannot be explained as the result of fracture strike rotation relative to the regional, tectonic least principal stress. We conclude that the net opening directions can result from the combination of tectonic offsets and events caused by shallow magma chamber inflation and/or dyke intrusions. The latter can produce transcurrent components of displacement along new or already existing fractures.

Description: Highlights: • Acartia hudsonica shows strong seasonality in thermal tolerance. • The observed seasonal differences in are consistent with pheno-typic plasticity not adaptation. • Body size in A. hudsonica is negatively correlated to environmental and developmental temperature. Abstract: Seasonal changes in environmental conditions require substantial physiological responses for population persistence. Phenotypic plasticity is a common mechanism to tolerate these changes, but for organisms with short generation times rapid adaptation may also be a contributing factor. Here, we used a common garden design (11 °C and 18 °C) to disentangle the impacts of adaptation from phenotypic plasticity on thermal tolerance of the calanoid copepod Acartia hudsonica collected throughout spring and summer of a single year. Acartia hudsonica were collected from five time points across the season and thermal tolerance was determined using critical thermal maximum followed by additional measurements after one generation of common garden. As sea surface temperature increased through the season, field collected individuals showed corresponding increases in thermal tolerance but decreases in body size. Despite different thermal tolerances of wild collections, after one generation of common garden animals did not differ in within thermal treatments. Instead, there was evidence of phenotypic plasticity where higher temperatures were tolerated by the 18 °C versus the 11 °C treatment animals across all collections. Despite persisting differences between collections due to either adaptation or parental effects, acclimation also had significant effects on body size, with the warm treatment resulting in smaller individuals, consistent with the temperature size rule. Therefore, the differences in thermal tolerance and body size observed in field collected A. hudsonica were predominantly driven by plasticity rather than adaptation. However, the observed decrease in body size suggests that nutrient availability for higher trophic levels and ecosystem functioning could be impacted if temperatures consistently increase with no change in copepod abundance. This is the first record of A. hudsonica in the Baltic Sea known to the authors.

Description: Abundant mineral resources in the deep sea are prospected for mining for the global metal market. Seafloor massive sulphide (SMS) deposits along the Mid-Atlantic Ridge are one of the potential sources for these metals. The extraction of SMS deposits will expose adjacent marine ecosystems to suspended particle plumes charged with elevated concentrations of heavy metals and other potentially toxic compounds. Up to date there is no information about the impact of mining activities on deep-sea benthic ecosystems such as abundant deep-sea sponge grounds in the North Atlantic Ocean. Sponge grounds play a major role in benthic-pelagic coupling and represent an important habitat for a diversity of vertebrates, invertebrates and microorganisms. To simulate the effects of mining plumes on benthic life in the deep sea, we exposed Geodia barretti, a dominant sponge species in the North Atlantic Ocean, and an associated brittle star species from the genus Ophiura spp. to a field-relevant concentration of 30 mg L−1 suspended particles of crushed SMS deposits. Three weeks of exposure to suspended particles of crushed SMS resulted in a tenfold higher rate of tissue necrosis in sponges. All brittle stars in the experiment perished within ten days of exposure. SMS particles were evidently accumulated in the sponge's mesohyl and concentrations of iron and copper were 10 times elevated in SMS exposed individuals. Oxygen consumption and clearance rates were significantly retarded after the exposure to SMS particles, hampering the physiological performance of G. barretti. These adverse effects of crushed SMS deposits on G. barretti and its associated brittle star species potentially cascade in disruptions of benthic-pelagic coupling processes in the deep sea. More elaborate studies are advisable to identify threshold levels, management concepts and mitigation measures to minimize the impact of deep-sea mining plumes on benthic life.

Description: Manganese (Mn) is an essential micro-nutrient that can limit or, along with iron (Fe), co-limit phytoplankton growth in the ocean. Glacier meltwater is thought to be a key source of trace metals to high latitude coastal systems, but little is known about the nature of Mn delivered to glacially-influenced fjords and adjacent coastal waters. In this work, we combine in-situ dissolved Mn (dMn) measurements of surface waters with Mn K-edge X-ray absorption spectroscopy (XAS) data of suspended particles in four fjords of West Greenland. Data were collected from transects of up to 100 km in fjords with different underlying bedrock geology from 64 to 70°N. We found that dMn concentrations generally decreased conservatively with increasing salinity (from 80-120 nM at salinity 〈8 to 〈40 nM at salinities 〉25). Dissolved Fe (dFe) trends in these fjords similarly declined with increasing distance from glacier outflows (declining from 〉20 nM to 〈8 nM). However, the dMn/dFe ratio increased rapidly likely due to the greater stability of dMn at intermediate salinities (i.e. 10 – 20) compared to rapid precipitation of dFe across the salinity gradient. The XAS data indicated a widespread presence of Mn(II)-rich suspended particles near fjord surfaces, with structures akin to Mn(II)-bearing phyllosilicates. However, a distinct increase in Mn oxidation state with depth and the predominance of birnessite-like Mn(IV) oxides was observed for suspended particles in a fjord with tertiary basalt geology. The similar dMn behaviour in fjords with different suspended particle Mn speciation (i.e., Mn(II)-bearing phyllosilicates and Mn(IV)-rich birnessite) is consistent with the decoupling of dissolved and particulate Mn and suggests that dMn concentrations on the scale of these fjords are controlled primarily by dilution of a freshwater dMn source rather than exchange between dissolved and particle phases. This work provides new insights into the Mn cycle in high latitude coastal waters, where small changes in the relative availabilities of dMn, dFe and macronutrients may affect the identity of the nutrient(s) proximally limiting primary production.

Description: The mesopelagic or ocean twilight zone (OTZ) in the ocean contains huge numbers of fish in a relatively pristine environment and may therefore attract interest as a commercial fishery. In this study we evaluate in economic terms, the likely trade-offs between the different services provided by the mesopelagic layer in the Bay of Biscay and the societal benefits of its commercial exploitation. Benefits arise mainly from the likely use of this group of species as raw material for producing fishmeal and fish oil. Costs are derived from the loss in climate regulating and cultural, services, but also from the loss in the provisioning service of other commercial species. To do so we compare the current non-exploited status with a situation in where mesopelagic fishes are harvested at levels capable of producing the Maximum Sustainable Yield. Results suggest that if mesopelagic fishes are harvested, a mean value of 1.2 million Euro loss in a year will be created in the Bay of Biscay, although in a range between 42 million Euro loss and 48 Euro million benefits. This uncertainty comes, mainly, from the limited existing knowledge of the mesopelagic fishes’ biomass but also from the uncertainty on the biomass of the rest of the species of the studied ecosystem. The large range indicates that a better understanding of the mesopelagic ecosystem is needed, however, results also show that ecosystem services under no exploitation provided by the OTZ could be more valuable than the fishmeal and fish oil that potentially could be obtained from the fishes harvested in this sea layer.