The Western Channel Observatory (Smyth et al., 2015) was established by the Natural Environmental Research Council (NERC), with Plymouth Marine Laboratory managing the two autonomous buoys that are located to the south of Plymouth in the English Channel (Stations L4 and E1). These two locations are now monitored continually and there is regular sampling of the water column and the sea floor at Station L4. At this location, despite it being located in water with a depth of 50 m, benthic foraminifera are regularly found in the surface water plankton samples. Some of these benthic foraminifera appear to contain algal symbionts, indicating that they may have been living at the time of capture. If benthic foraminifera can be entrained in the water column, while still living, then this provides a mechanism for dispersal or migration that is much more rapid and efficient than the rate at which protists could migrate within, or on, the sediment surface. Re-colonization by foraminifera, following disturbance, could well be facilitated by this mechanism which has only rarely been reported in the literature (Murray, 1965). It is clearly limited to depths impacted by fair weather (~30 m) or storm wave base (80–100 m).

Observations of vertical plankton tow (20 μm mesh) samples collected at Station L4 during the three winters (2015–2018) have shown that, following significant storms, the numbers of benthic foraminifera in the plankton tows are increased (Hart et al., 2017). Some of the specimens contain sediment, indicating that they have been picked up from the sediment surface and are in the process of being re-deposited. Such assemblage mixing has significant implications for the interpretation of both modern, and ancient, environments. Analysis of sea floor samples in the area has shown that the recorded species are from the area of Station L4 or Hillmars (in 50 m water depth) although some may have been transported from shallower-water settings by increased run-off during the storm events.

Clearly, re-distribution of foraminifera in the environment might make subsequent interpretations of ecology less accurate if the ‘living’ assemblage is not identified (by staining), but in the fossil record such changes could go completely un-detected and lead to inaccuracy in interpretations of palaeoecology.

Hart, M.B., Molina, G.S., Smart, C.W. and Widdicombe, C.E. 2017. The Western Channel Observatory: benthic foraminifera in the plankton following storms. Geoscience in South-West England, 14(1), 39–45. [for 2016]

Murray, J.W. 1965. Significance of benthic foraminiferids in plankton samples. Journal of Paleontology, 39, 156–157.

Smyth, T., Atkinson, A., Widdicombe, S., Frost, M., Allen, I., Fishwick, J., Queiros, A., Sims, D. and Barange, M. 2015. The Western Channel Observatory. Progress in Oceanography, 137, 335–341.

How to cite: Hart, M., Smart, C., Molina, G., and Widdicombe, C.: Benthic foraminifera in the plankton: how might this impact on palaeoecological interpretations?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-976, https://doi.org/10.5194/egusphere-egu23-976, 2023.

The magnesium/calcium ratio of seawater has oscillated throughout the geological time between aragonite seas (>2) and calcite seas (<2), which strongly influenced coccolithophores biocalcification, along with the variation of ocean temperature and pH. Here we proposed to analyze in a concomitant way the effect of temperature and pH together with Mg/Ca on coccolithophores. 

In this study, we cultured Emiliania huxleyi (RCC963) under a range of Mg/Ca ratios (Mg/Ca: 0.2, 1, 5), temperature (15℃, 21.5℃, 28℃) and pH (7.4, 7.8, 8.2) to evaluate the cell adaption and calcification sensitivity by measuring the growth rate(μ), Photosystem Ⅱ efficiency (Fv/Fm), calcification rate and coccolith morphology. We have found that in the calcite sea, E. huxleyi is more vulnerable to changes of temperature and pH than it is in the aragonite sea. The coccolith morphology also supports the different response to the Mg/Ca ratio variation where more malformed and incomplete coccoliths were observed with the decreasing of temperature and pH. Further exploration is needed to understand the multi-changes in environmental parameters in the distribution of Mg in coccoliths of E. huxleyi and morphogenesis. 

How to cite: Wang, H., Ruiz-Pino, D., Gardin, S., Probert, I., and Langer, G.: Calcification response of Emiliania huxleyi under calcite and aragonite sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10163, https://doi.org/10.5194/egusphere-egu23-10163, 2023.

Calcareous foraminifera is a unicellular protist that possesses calcium carbonate shells and is commonly found in the oceans. Biomineralization of these organisms has been extensively studied, as the shells of these organisms hold valuable information about past environmental conditions. In particular, the evolution of foraminiferal assemblages in different environments has been well-documented throughout geologic time, and they are often used as facies fossils and reference fossils.

In this study, we aimed to estimate the calcification rate of the species Spirillina vivipara, which is known for its fast generational turnover and rapid shell growth rate. Foraminifera samples were collected from the Enoshima Aquarium in Fujisawa City, Japan, and kept in an incubator with a 12-hour light/dark cycle and a constant temperature of 20°C. The foraminifera was reared in filtered seawater with a salinity of 35 and fed twice a week with live Dunaliella sp.
To observe the calcification process, we recorded the elongation rate of testes by time-lapse observation using differential interference microscopy and calcein-containing seawater to mark the shells. We then used multiple techniques, including a focused ion beam (FIB)-SEM, to measure the thickness of the shells. In this presentation, we will present the results of our study on the shell growth rate of foraminifera and compare them with previous studies.

How to cite: Toyofuku, T. and Yukiko, N.: Estimation of calcification rate of Spirillina vivipara, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16323, https://doi.org/10.5194/egusphere-egu23-16323, 2023.

The chemical weathering of continental silicate rocks removes CO₂ from the atmosphere and exerts a fundamental control on the Earth’s climate over geological timescales. Characterizing silicate weathering in the past is therefore crucial for understanding the climate system. The lithium isotopic composition (δ⁷Li) of carbonates is considered to be a reliable archive of past seawater δ⁷Li values, which are useful as a tracer of silicate weathering. However, the Li isotopic fractionation during biogenic carbonate formation is complex, and local conditions such as carbonate system parameters could impact δ⁷Li values in marine calcifiers. For example, δ⁷Li values have been shown to be dependent on either pH or DIC in two studies using large benthic foraminifera. Those results are enigmatic, since both studies used similar species of the genus Amphistegina but reported differing controls on δ⁷Li values.

The aim of this study was to address the earlier contradictory results on the Li isotope behaviour in the hyaline species Amphistegina lessonii. We performed culture experiments under decoupled pH/DIC conditions, and analysed the δ⁷Li values and Li/Ca ratios in the foraminifera tests. Two different light treatments (light/dark and dark) were also implemented to investigate the potential role of the symbionts.

Contrary to the two previous studies, no links between either pH or DIC and δ⁷Li or Li/Ca values were observed for any of the treatments in our experiments. Additionally, growth rates also did not seem to influence the Li incorporation or isotopic fractionation. However, an effect of different light treatments was observed, probably due to different physiological processes of the symbionts occurring in dark conditions. Overall, these findings appear to support the use of Li isotopes in large benthic foraminifera to reconstruct past seawater chemistry and to infer changes in chemical weathering during carbon cycle perturbations over the last several hundred million years of Earth history.

How to cite: Charrieau, L., Rollion-Bard, C., Terbrueggen, A., Wilson, D., Pogge von Strandmann, P., Misra, S., and Bijma, J.: Lithium incorporation and isotopic fractionation in large benthic foraminifera under decoupled pH/DIC conditions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7935, https://doi.org/10.5194/egusphere-egu23-7935, 2023.

The Line Islands rank among the most isolated in the tropical Pacific, hold a biogeographic key position for the translocation of taxa across the Pacific, and constitute the ultimate stepping stone for the dispersal of species from east to west and vice versa. Because benthic foraminifera have limited dispersal capabilities, these isolated stepping stones are crucial for ecological and biogeographic studies. Mapping large-scale biogeographic patterns is vital for setting conservation priorities by revealing biogeographic variability and enabling the identification of species richness hot- and coldspots. Here we present the first results of a comprehensive survey of modern benthic foraminiferal assemblages from the Line Islands and compare the foraminiferal assemblages to other regions in the Pacific Ocean. We also document the diversity and composition foraminiferal biotas from inhabited and uninhabited islands to assess the impact of anthropogenic activities.

The large number of species identified allow us to place the Line Islands into a biogeographic context and to assess the role of these remote islands as a stepping stone for the translocation of species across the Pacific Ocean. Our study provides novel insights into underlying mechanisms that promote species richness patterns in different shallow-water habitats.

The sediments analyzed includes material from the inhabited Tabuaeran Island, also known as Fanning and the uninhabited Palmyra Island. The environments of the island comprise a multitude of habitats including coral reef areas, lagoons, and algal covered shallow bays.

The foraminiferal associations from coral habitats are dominated large benthic foraminifera (LBF) such as Amphistegina, Sorites, and Peneroplis and low amounts of various miliolids. The preservation of nearshore assemblages revealed predominantly abraded and broken individuals. The assemblages from nearshore beaches and reef flats are composed of miliolids, numerous perforate-hyaline, and variable amounts of diverse LBF. The foraminiferal assemblages from algal covered beaches comprise a suite of diverse smaller miliolids and rare, mainly juvenile forms of Sorites. Our survey provides novel insight into the preservation, habitat-preferences, diversity and biogeography of benthic foraminifera across the tropical Pacific.

How to cite: Trubin, Y. and Langer, M.: Benthic foraminifera from extremely shallow-water habitats of Line Islands, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3402, https://doi.org/10.5194/egusphere-egu23-3402, 2023.

Tidal marshes are a threatened coastal ecosystem valued for their capacity as a carbon sink. Given that effective action on climate change includes the protection, enhancement and restoration of natural carbon sinks, it is crucial to accurately quantify and map the current soil carbon stocks in this ecosystem. We aim to produce the first globally consistent map of soil carbon storage in tidal marshes.

A globally distributed soil carbon core dataset was compiled by a systematic literature review (n = 2,127 locations), supplemented by data from the Coastal Carbon Research Coordination Network database (n = 1,798 locations). We then developed a list of global landscape-level environmental drivers that are important in determining variation in soil carbon, including vegetation indices, elevation, flow accumulation, water occurrence, tidal amplitude, and other climatic variables. 

Using the carbon cores as training data and the environmental driver data as covariate layers, we are developing a machine learning model to map global marsh carbon stocks and their uncertainties at 0-30 and 30-100 cm depths, at a high resolution, and applying these to a new globally consistent tidal marsh extent map. The model and the global map will be easily updatable when new information (i.e. additional soil cores) becomes available. This map will be valuable to support conservation efforts, compliment blue carbon studies, and Nationally Determined Contributions.

How to cite: Maxwell, T. L., Spalding, M., and Worthington, T. A. and the global marsh soil C team: Global map of soil carbon in tidal marshes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1332, https://doi.org/10.5194/egusphere-egu23-1332, 2023.

Mangrove forests provide a wide range of benefits for natural and human communities in tropical and subtropical coastlines. Mangrove restoration using seedlings is vital to counteract the loss of mangrove forests, currently estimated at 3.4% over the past 24 years, with mangroves disappearing at a rate twice as large as their gain. Restoration success depends on the successful establishment of seedlings in the new environment, which may be influenced by unexplored factors such as nutrient status and temperature in selected sites. In order to determine the role of these two factors on mangrove seedling growth and development once they have been outplanted, we set up a laboratory experiment consisting of two climate rooms at 25ºC and 30ºC mimicking a standard and a future global warming scenario. These two temperature levels were combined with two different nutrient levels representing a standard and an eutrophication scenario. We assessed the influence of increased nutrients and temperature on the development and photosynthetic performance of black mangrove seedlings over a period of 10 weeks, and studied their responses to the individual and combined effects of these drivers. Seedling growth was measured as leaf area, plant length and above/ below biomass development. In addition, we determined the photosynthetic performance of seedlings by applying the Pulse Amplitude Modulated (PAM) fluorometry technique. Our results showed reduced root growth and disproportionally long, bent stems, with lower net assimilation rates under the combined effects of high temperature and nutrient concentrations. These outcomes, specially the nearly-negligible root growth suggest a low stability as the seedlings fail to anchor themselves to the sediment, which may translate into a greater vulnerability to physical disturbance leading to their dislodgement. On this basis, we recommend that local hydrodynamic conditions and nutrient status are taken into consideration for improving future site selection and increasing restoration success under a warming climate. Our results also suggest that the impacts of global-scale processes such as global warming could be dampened by the implementation of local policies, such as preventing coastal eutrophication.

How to cite: Pino Cobacho, S., Janssen, S., Brekelmans, M., van de Leemput, I., Holmgren, M., and Christianen, M.: Higher temperature and nutrient concentrations may decrease black mangrove seedling stability during coastline recovery   , EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1520, https://doi.org/10.5194/egusphere-egu23-1520, 2023.

Among the most important processes affecting the global biogeochemical cycles of carbon, nutrients and trace metals is the physicochemical transformation of terrestrial dissolved and colloidal material into suspended particles in estuaries. The gradient of increasing salinity and pH towards seawater induces the flocculation of riverine dissolved organic matter to the particulate form, providing a mechanism for the capture of dissolved metals to the newly formed particles. The role of pH is particularly relevant in the context of acid sulphate (AS) soils that can generate extremely low pH conditions and high soluble trace metal concentrations in river waters.

This study investigated organic matter flocculation, and trace metal (Al, Fe, Co, Mn and Cu) association with the newly formed organic particles, by the mixing of natural acidic river water and synthetic seawater in the laboratory. River water was collected from the Laihianjoki and Sulvanjoki rivers that are among the most AS-soil-impacted rivers in Finland and Europe. Continuous bucket experiments with an in situ LISST-100X particle size distribution analyser and YSI EXO2 multiparameter water quality sonde were run to simulate the development of suspended particle pool over the local salinity gradient from 0 to 6. Jar experiments with discrete salinity treatments were carried out to investigate metal behaviour with the collection of flocculated material on glassfibre filters. The filter samples were subjected to persulfate digestion of organic particles and metal oxyhydroxides (pH <2.3) and the digestion supernatants were analysed for metal concentrations by ICP-MS.

It was found that Al and Fe were strongly transferred to the particle pool and captured in large (>80 µm) organic flocs already in the lowest salinity range from 0 to 2, with a slightly lower transfer rate at higher salinities. The increasing concentration of small (median 11 µm) flocculi particles above pH ca. 5.5 (salinity 1 in Laihianjoki, salinity 2 in Sulvanjoki) indicate that the precipitation of Al and Fe oxyhydroxides contributed to the particle pool as a minor component. Co and Mn were weakly associated with organic complexes and relatively persistent in solution; however, the transfer of Co to the particle pool was roughly twice of that of Mn, indicating at least partial decoupling of Co from Mn oxyhydroxide precipitation. Cu to a significant proportion (<60 %) was bound to organic particles in the acidic and humic-rich rivers, and this proportion did not change significantly during mixing with seawater. The findings of this study demonstrate that salinity and pH in the estuarine mixing are important controls for the seaward transport and environmental impacts of metal loading from boreal AS-soil-impacted rivers.

How to cite: Virtasalo, J., Österholm, P., and Asmala, E.: Flocculation of trace metals from boreal acid sulphate soils over an estuarine gradient, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8734, https://doi.org/10.5194/egusphere-egu23-8734, 2023.

Shelf sea sediments are natural, long-term carbon sinks that may be managed for Blue Carbon (BC) storage to offset greenhouse gas emissions and contribute to nations ‘Net Zero’ ambitions. The Isle of Man in the Irish Sea has territorial waters equivalent to approximately 85% of its total jurisdiction, and significant potential for BC offsetting through seabed management. The Island’s Government is developing a comprehensive BC management plan to maximise carbon sequestration and restore seabed biodiversity and wider ecosystem services. We determined the effects of disturbance on sedimentary carbon stocks and accumulation rates in the western territorial sea, a region of mud-dominated sediment with elevated disturbance from Nephrops norvegicus bioturbation and bottom-contact fishing activities. Sediment cores were collected from approximately 5 to 12 nautical miles off the west coast of the Isle of Man, at depths from 60 m to 120 m. The cores were sectioned and analysed using: elemental analysis and isotope ratio mass spectrometry to quantify organic and inorganic carbon stocks; and gamma-ray spectrometry and alpha spectrometry to identify and quantify radionuclides of Cs-137, Pb-210, Am-241 and Po-210 to determine sedimentation rates. Complete sediment cores were scanned using X-ray to generate radiographs and laminographs, to determine internal core structure and identify N. norvegicus burrows. The results show that organic/inorganic carbon and sedimentation rates in offshore sediments vary over time and space. Analysis is ongoing to ascertain if these variations are related to disturbance of the sedimentary system, using indicators of fishing intensities and N. norvegicus bioturbation. Results of this analysis will be presented to elucidate the effects of disturbance on sedimentary carbon stocks and accumulation.

How to cite: Muir, H., Keenan, J., Henthorn, R., Strong, J., Reading, D. G., Duncan, P., Skov, M. W., Hiddink, J. G., Unsworth, R. K. F., Warwick, P. E., and Evans, C.: Influence of Disturbance on Sedimentary Carbon Stocks in a Temperate Seabed, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11942, https://doi.org/10.5194/egusphere-egu23-11942, 2023.

River-dominated continental margins receive large inputs of terrestrial organic matter (TerrOM). This TerrOM can potentially form a long-term sink for atmospheric CO₂ upon burial in continental margin sediments, thereby forming a key component of the global carbon cycle. However, only part of the TerrOM that is delivered to the coastal zone is preserved on the seafloor, as its sequestration efficiency depends on its properties and composition. The composition and quality of TerrOM on the seafloor is usually not determined, hampering estimates of its contribution to carbon sequestration on the continental margin. Moreover, TerrOM is thought to form associations with mineral surfaces that protect it from degradation, but it is not fully known to which extent these associations persist in the marine realm and if different types of TerrOM preferentially bind to certain minerals.  

Here, we investigated the TerrOM composition in different grain size fractions (>250, 250–125, 125–63, 63–30, 30–10 and <10 μm) of surface sediments along a land-sea transect (15–600 m water depth) in the northern Gulf of Mexico, using bulk properties (TOC, TN, δ¹³C_org) and lipid biomarkers for plant- (long-chain n-alkanes), fluvial- (long-chain C₃₂ 1,15-diol) and soil-microbial derived OM (branched glycerol dialkyl glycerol tetraethers; brGDGTs). In addition, we used mineral surface area analysis and X-ray diffraction (XRD) to assess whether different TerrOM types have an affinity for certain minerals.

We found that along the land-sea transect, the concentrations and mineral loading of n-alkanes remained constant, while the concentrations of both brGDGTs and the C₃₂ 1,15-diol rapidly decreased. Moreover, the concentrations of lipid biomarkers, in particular the n-alkanes, were highest in the smaller (<30 μm) size fractions along the transect. This suggests that n-alkanes likely form associations with clay minerals, in particular smectite, as shown by our XRD results.

However, n-alkanes might also be more resistant against degradation than the brGDGTs and diols,  further explaining their presence offshore. While brGDGT concentrations also increase towards smaller grain sizes, their molecular signature is constant among size fractions at each site, suggesting that they are not bound to a specific grain size fraction. Furthermore, an increase in the degree of cyclisation of the brGDGTs between 50 and 150 m water depth indicates that the initial soil-derived signal is strongly overprinted by an in situ marine contribution in this zone.

Our results show that in coastal waters, plant-derived OM is more likely to remain associated with mineral (smectite) surfaces than soil-microbial and fluvial OM, thereby facilitating its transport further offshore through hydraulic sorting and preferential burial on the shelf.

How to cite: Yedema, Y., Sangiorgi, F., Trabucho Alexandre, J., Vonk, J., and Peterse, F.: The influence of mineral associations on terrestrial particulate organic matter transfer and dispersal in the northern Gulf of Mexico, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12822, https://doi.org/10.5194/egusphere-egu23-12822, 2023.

The interaction between lava and seawater has received significant interest, since multiple reactions can take place which act as sources or sinks of elements, consequently changing the chemical properties of the surrounding seawater. However, most of these studies are based on underwater hydrothermal vent sites. On the 19th of September 2021, 6 km away from the coast, the Tajogaite volcano in La Palma, Canary Islands started its eruptive phase.

Lava flows reached the shoreline during three events on September 28th, November 10th, and November 22nd, strongly affecting the seawater properties as they interacted with the seawater. The evolution of surface and water column physicochemical properties (temperature, salinity, carbonate system variables, dissolved oxygen), iron concentrations (soluble (sFe), dissolved (dFe) and total dissolvable (TdFe)) concentrations, and iron ligands were characterized during 13 visits to the frontal zone of the newly formed deltas. A large volume of hot (50°C measured on November 12th) and high salinity seawater promoted pH values in the frontal zone of 7.0 with important decreases in alkalinity and total dissolved inorganic carbon. These waters were also characterised by high iron concentrations reaching 18 nM, 117 nM, and 2024 nM for sFe, dFe, and TdFe, respectively. The affected waters travelled in the top meter of the water column over 1 km away from the coast, naturally fertilising the surrounding photic layer.

How to cite: González Santana, D., González, A. G., Arnone, V., González-Dávila, M., and Santana-Casiano, J. M.: Subaerial lava as a source of coastal hydrothermal iron, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9006, https://doi.org/10.5194/egusphere-egu23-9006, 2023.

Burial of organic carbon (OC) in marine sediments controls atmospheric CO₂ and O₂ concentrations and is a key process in the global carbon cycle. Marine sediments bury ~160 Tg C yr-1 globally, of which ~90% in continental shelf sediments, which makes these sites a burial hotspot. It is generally assumed that OC is buried more efficiently in sediments underlying anoxic bottom waters. Recently, however, sediments of the Gotland Basin (Baltic Sea) have been shown to have unusually low OC burial efficiencies (~10%) considering their sediment accumulation rates, despite being overlaid by anoxic bottom waters.  

To investigate the reason for this lower-than-expected OC burial efficiency, we sampled five sites along a transect (including two sites for Fe-OM assessment) across the Western Gotland Basin. Sulphate reduction rate measurements showed that the OC reactivity was much higher than was expected for the age of the OC. Subsequent analysis of iron-organic matter associations and OC-to-surface area ratios showed that the potential for physical protection via either coprecipitation with iron minerals or mineral adsorption was very low (below the detection limit in one core and a maximum of 7% of OC linked to Fe in the other, which is low compared to the global average of ~20%). Our results suggest that in the Western Gotland Basin, the absence of physical protection seems to allow for unusually high mineralisation rates (low OC burial efficiency) despite anoxic conditions. Consequently, the link between oxygen exposure and OC mineralisation rate is likely not as universal as currently assumed.

How to cite: Placitu, S., van de Velde, S., Robertson, E. K., Hall, P. O. J., and Bonneville, S.: Unexpectedly low organic carbon burial efficiency in anoxic sediments is linked to the absence of physical protection: lessons from the Western Gotland Basin (Batic Sea), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14229, https://doi.org/10.5194/egusphere-egu23-14229, 2023.

The partial pressure of carbon dioxide (pCO₂) in surface waters is influenced by physical, chemical and biological variables. The interplay of these environmental variables is shown along a section in the central North Sea between Immingham (UK) and Halden (Norway) for the year 2013. Both, underway measurements by ships of opportunity and the results of a high resolution 3D – biogeochemical model show very high pCO₂ values near the UK coast and reduced values near the Norwegian coast. This pattern is due to high UK river loads at the western part and the outflow of Baltic water along the Norwegian coast at the eastern part of the section. Seasonal variations within the central North Sea off the coastal waters exhibit both the influence of temperature and biological activities. During the phytoplankton growth seasons, the pCO₂ is considerably lowered by the biological uptake of dissolved inorganic carbon and concurrently increased by higher sea surface temperatures.

The model is able to reproduce observed winter pCO₂ values using a variety of different bulk air-sea flux formulas. However, during spring and summer, the use of these formulas results in overestimated simulated pCO₂ values. Only the application of a formula which takes the presence of surfactants into consideration improved the simulated pCO₂ values. The latter formula exhibits lower piston velocities for the exchange of CO₂ between atmosphere and ocean than the common formulas.

How to cite: Paetsch, J., Macovei, V. A., Voynova, Y. G., Petersen, W., and Möller, L.: Observed and simulated variations of marine partial pressure of CO2 within the central North Sea: The choice of the bulk air-sea flux formula matters, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3898, https://doi.org/10.5194/egusphere-egu23-3898, 2023.

Climate warming is increasing glacial melt on the Antarctic continent, in turn shifting hydrostatic pressure on subglacial and surface lakes. This has a direct impact on the rates ofgroundwater discharge, with feedbacks to local and regional ocean and climate dynamics, as well as ice sheet retreat rates. The magnitude and diversity of Antarctic subglacial flux and its interaction with the marine realm is currently unknown, yet it may be critical to projecting hydrologic dynamics and the fate of coastal ecosystems in the Ross Sea under future climate change scenarios. Here, we report the first holistic quantification of the magnitude, composition, and impact of subglacial fluxes documented in the Antarctic region. With extensive water column, sediment, and ecological sampling we have explored the magnitude, biogeochemical composition, and temporal variability of subglacial water and dissolved constituent fluxes in the Ross Sea. We will highlight initial findings from this research, including the magnitude and spatial variability of subglacial fluxes from the Victoria Land coast, and the sphere of influence of these fluids on the coastal marine ecosystem in the Ross Sea.  We will place discoveries to date in the context of this emerging field, including incorporation into ice sheet and oceanographic models of the region. With this, we provide the first direct evidence of land-sea connectivity in the Ross Sea coastal environment through submarine groundwater discharge (SGD) and establish a critical baseline for measuring and predicting future change.

How to cite: Seabrook, S., Law, C., Druce, M., Cummings, V., Deppeler, S., Cox, S., Westerhoff, R., Hillman, J., Paytan, A., Mikucki, J., Ladroit, Y., Maurice, A., Spain, E., and Safi, K.: Antarctic subglacial flux: a driver of climate sensitivity on coastal margins, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2517, https://doi.org/10.5194/egusphere-egu23-2517, 2023.

Rising CO₂ concentrations in the atmosphere have a multidimensional influence on marine ecosystems. Through amplifying the greenhouse effect they lead to seawater temperature increase, which initiates cascade changes in the environment. In addition to this climatic pathway, increasing atmospheric CO₂ concentrations cause also an overall increase of CO₂ concentrations in surface seawater and, consequently, a pH decrease – a mechanism called Ocean Acidification (OA). OA is already fairly well understood and traceable in the open ocean waters, where large-scale projects and actions supply an enormous amount of observations and experimental data and where the magnitude of OA is to a large extent thermodynamically consistent with the increase in atmospheric pCO₂. In the coastal and shelf seas, OA is still a considerably understudied phenomenon despite their high socio-economic importance and potentially great vulnerability of these regions to acidification due to often lower salinity and corresponding lower buffer capacity of waters as compared to open ocean.

In the present study, we underline the importance of total alkalinity (TA) as the key factor shaping the OA dynamics and pH fields in the coastal and shelf seas. The extensive research performed in 2018-2022 extended from the brackish Baltic Sea through the open waters of the eastern Fram Strait to the Spitsbergen fjords affected by the high inflow of meltwaters. It revealed extremely high variability in the marine CO₂ system structure and significantly different influences of freshwater input in the investigated regions. The observed overall spatial and temporal (seasonal) variability in TA extended in the broad range between <350 and 4,320 µmol kg^-1. This makes TA a fundamental variable for studying the large-scale pH and pCO₂ changes and forecasting the development of OA in the coastal zone in the future high-CO₂ world. This large-scale study can be considered as the reference for future OA research in the investigated regions and simultaneously calls for action to include TA in the routinely observed parameters in the coastal waters.

How to cite: Kulinski, K., Aguado Gonzalo, F., Bromboszcz, L., Diak, M., Koziorowska-Makuch, K., Makuch, P., Palka, I., Prusinski, P., Saghravani, S. R., Szymczycha, B., and Winogradow, A.: Climate-related changes in total alkalinity as a key to understanding ocean acidification in the coastal zone, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16078, https://doi.org/10.5194/egusphere-egu23-16078, 2023.

The Elbe Estuary is strongly impacted by anthropogenic activities such as dredging and eutrophication. Together, these cause oxygen minimum zones (OMZ) regularly during summer in the Hamburg Port area, within the tidal freshwater region of the estuary. Over the last years, this OMZ has expanded spatially and temporally. We present an analysis of an extraordinary oxygen minimum event in June 2022, when an all-time lowest oxygen concentration was observed upstream of the Hamburg Port.

We combine data from six transect cruises (early May -late June, 2022) and monitoring stations (2016-2022), to show the decrease of oxygen, and the increasing number of oxygen minimum events. In June 2022, the OMZ moved upstream due to the collapse of a phytoplankton bloom upstream of the tidal weir. This was accompanied by particularly warm temperatures and low river discharge, providing a glimpse into the potential future changes of central European estuaries under climate change.

How to cite: Sanders, T., Heineke, M., Götz, F., Russnak, V., Husmann, E., Dähnke, K., Schöl, A., Große, F., and Voynova, Y. G.: Increasing of oxygen minimum events in a temperate estuary caused by warming and reduced discharge, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13978, https://doi.org/10.5194/egusphere-egu23-13978, 2023.

The land-ocean continuum is a key component of the biogeochemical cycling of carbon and nutrients. At the same time, the rapidly changing conditions at the river-sea interface make the nearshore regions highly dynamic and variable. For better understanding of the drivers of this variability, high temporal and spatial frequency observations are needed. Here, we use ship-of-opportunity measurements in the outer estuaries of the Humber (UK) and Elbe (Germany) rivers to show that essential ocean variables in these regions vary with a periodicity that matches the spring-neap tidal cycles. The tidal disturbance was observed up to 10 km offshore from the river mouth. While at neap tide, the conditions at this location were typical of the open North Sea, during spring tide, the salinity dropped below 15, the turbidity was high (>50 FTU), dissolved oxygen dropped to under-saturated values and the partial pressure of carbon dioxide (pCO₂) in seawater rose above 700 µatm. Since neap tide seawater pCO₂ values were below 400 µatm, lower than atmospheric levels, the near-shore area shifts between being a sink and a source of carbon dioxide to the atmosphere on roughly one-week timescales. This behaviour is currently not resolved in some biogeochemical models. Correctly integrating the drivers of this variability into regional assessments is essential for the accurate evaluation of shelf sea carbon budgets.

How to cite: Macovei, V., Rewrie, L., and Voynova, Y.: Spring-neap tidal cycles are major drivers for biogeochemical variability in two large outer estuaries, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11848, https://doi.org/10.5194/egusphere-egu23-11848, 2023.

Significant progress has been made regarding the spatial distribution of sources and sinks of atmospheric CO₂ within the global coastal ocean. The physical and biochemical drivers that govern these CO₂ sources and sinks as well as their variability on seasonal timescales are, however, poorly understood. In this study we aim at filling this knowledge gap and assess to what extend the CO₂ dynamics in the coastal ocean is controlled by the adjacent open ocean or conversely is dominated by continental influence. We use the global ocean biophysical model MOM6-COBALT, carefully evaluated against a well-established coastal data-product (Laruelle et al., 2017), to quantify the individual influences of the oceanic transport, thermal changes, freshwater influence, and biological activity on the annual mean spatial distribution of CO₂ sources and sinks as well as their seasonal variability for the coastal ocean worldwide. Our analysis reveals complex spatial and temporal dynamics, depending on the coastal region and/or the time scale investigated (seasonal or annual) resulting from interactions and compensations between the different processes. We identify five typical behaviors of coastal systems (i.e., coastal regions under biological drawdown, vertical transport, imprint of land, dominated by intracoastal alongshore currents, and weak CO₂ sources and sinks coastal regions) and propose a processed-based typology of the coastal ocean based on the processes that control the spatial and seasonal air-sea CO₂ dynamics. 

How to cite: Resplandy, L., Roobaert, A., Laruelle, G. G., Liao, E., and Regnier, P.: Process-based typology of the global coastal ocean based on physical and biogeochemical controls of the sea surface CO2 dynamics, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16578, https://doi.org/10.5194/egusphere-egu23-16578, 2023.