Nickel (Ni) is a bio-essential trace metal for marine phytoplankton, serving as a cofactor in key enzymes. Its nutrient-like distribution and the mirrored distribution of δ⁶⁰Ni in seawater suggest that δ⁶⁰Ni could serve as a tracer of biological Ni cycling. However, particulate Ni (pNi) remains understudied despite its important role. Here, we present three coupled dissolved and particulate δ⁶⁰Ni profiles from distinct biogeochemical regimes across the Subtropical Front in the Australian sector of the Southern Ocean.
In surface waters, pNi is isotopically lighter than co-located dissolved Ni, and particulate Ni/P in the mixed layer falls within the range observed for phytoplankton, reflecting an association with organic matter. These results support previous studies suggesting that phytoplankton preferentially remove isotopically light Ni. In the subtropics, higher surface productivity corresponds to elevated mixed layer pNi concentrations, below which pNi declines, associated with the regeneration of organic matter. However, particulate δ⁶⁰Ni shows limited variation, implying minimal isotope fractionation during this process. At subantarctic stations, pNi concentrations remain relatively constant, while particulate δ⁶⁰Ni decreases with depth in the upper 300 m. Moreover, δ⁶⁰Ni offsets between the dissolved and particulate phases are smaller than for the subtropical counterpart, likely associated with different phytoplankton assemblages. At greater depth, particulate Mn/P increases, and Ni/P ratios exceed those of phytoplankton. This suggests a shift in pNi association from organic-rich to Mn-rich phases, with minimal change in total particulate δ⁶⁰Ni. These findings suggest a more complex internal cycling of Ni in the oceans than one driven solely by biological processes.

Carbon dioxide fluxes associated with weathering and carbon export in river basins are highly spatially variable, with some locations acting as sinks whilst others represent CO₂ sources. Chemical weathering and the associated carbon fluxes are strongly modulated by bedrock geology, with orogenic systems constituting locations where metamorphic lithologies can be found juxtaposed with sedimentary rock mélanges. Hydrological processes also converge in mountain belt interiors, where water in the shallow critical zone can intersect with gas-rich fluids derived from deeper tectonic processes. The Engadin Valley in the Swiss Alps provides a natural geological setting to examine how these competing controls impact weathering and dissolved inorganic carbon (DIC) fluxes in alpine rivers. Using the radiocarbon and stable isotope composition of river DIC, alongside the dissolved products of weathering, we observed an increase in the fractional contribution of riverine DIC derived from metamorphic processes coincident with a distinct lithological transition from the upper to the lower Engadin Valley.
DIC export from the upper Engadin Valley is dominated by silicate weathering of igneous and metamorphic terrains, while Δ¹⁴C_DIC values < -100 ‰ suggest some of this DIC could originate from pre-aged organic sources from soils, permafrost thaw or glaciated catchments. Riverine DIC concentrations increase from headwaters to the most downstream station in the lower Engadin Valley, mirrored by a sharp drop in Δ¹⁴C_DIC values, while the magnitude of this decrease varies seasonally. This decline in Δ¹⁴C_DIC values, which is counter to the trend expected due to increasing exchange with atmospheric CO₂ downstream, is consistent with contributions from geogenic carbon sources, specifically DIC from weathering of carbonate minerals and petrogenic organic carbon (OC), spatially intertwined with inputs CO₂ from metamorphic processes. Overall, seasonal variations in hydrological processes and associated flow pathways appear to modulate DIC contributions from bedrock weathering, while metamorphic CO₂ inputs occur in a quasi-constant manner. This study demonstrates that geogenic CO₂ sources in the shallow critical zone contribute significantly to the riverine DIC pool, leading in the Engadin valley to a potential shift in CO₂ fluxes from uptake via silicate weathering upstream to potential CO₂ release downstream, highlighting both the importance and complexity of carbon cycle processes in mountainous river basins.

Ongoing global warming caused by a steady increase in carbon dioxide in the atmosphere urgently needs to be mitigated. This is possible if phytoplankton biomass is increased in the ocean, as this will remove additional atmospheric CO₂. In the Southern Ocean, Fe is a well-known growth-limiting element, but the role of the other micronutrients remains very unclear. Our aim is to describe the evolution of each nutrient in the Southern Ocean throughout the year and to identify nutrients that limit phytoplankton growth. Therefore, we created a model that calculates nutrient consumption rates and available nutrient pools, fueled by deep winter mixing and diapycnal supply. Annual consumed nutrient amounts are smaller than their labile dissolved pools, but this is not true for limiting elements Fe and Co (from the sub-Antarctic zone [SAZ] to the Antarctic zone [AZ]) and Zn and Si (only in the SAZ). Since we found several limiting elements, fertilization with multiple nutrients would be required to promote large-scale carbon capture.

Winter dissolved nickel (dNi) and particulate nickel (pNi) concentrations were measured in the Southern Ocean (GEOTRACES GIpr07 transect) to investigate biogeochemical cycling within the water column and over seasonal timescales. Concentrations of dNi ranged from 1.98 to 8.21 nmol kg⁻¹ with low surface concentrations and maxima in deepest sampled water masses. Combining our winter data with the GEOTRACES Intermediate Data Product (2021) shows insignificant seasonal dNi variation in surface waters north of the Antarctic Polar Front, indicating the dominance of year-round mixing processes. However, lower summer concentrations than winter in the Antarctic Zone (∆0.23 nmol kg⁻¹) suggest a role for biological processes at high latitudes. For pNi, concentrations ranged from 5 to 49 pmol kg⁻¹ with higher values in surface/near-surface water masses. Vertical attenuation factors (b values) for pNi (0.19 ± 0.06) and particulate phosphorus (pP; 0.43 ± 0.10) suggest a greater retention of Ni in particles than P, invoking scavenging processes or refractory Ni phases. Water mass analysis shows that remineralization of pNi contributes a maximum of 6% of the highest measured dNi. Instead, dNi distributions and macronutrient relationships were largely explained by phytoplankton uptake in surface waters, and mixing and advection of Atlantic and Antarctic origin water masses, each with different preformed nutrient compositions. Winter trace metal measurements provide new perspectives regarding the balance between biological and physical drivers in the Southern Ocean. For Ni, the biological component is small with respect to physical mixing processes and over the timescales in which water masses accumulate Ni during their transport.

Over the last decades, the chromium (Cr) stable isotope system (referred to as δ⁵³Cr) has emerged as a proxy to reconstruct past oxygenation changes in Earth’s atmosphere and oceans. Although Cr is a promising paleoproxy, uncertainties remain as to the modern marine Cr cycle, and limited data are yet available in large swaths of the ocean, including the Atlantic Ocean. Here we present dissolved seawater Cr concentrations ([Cr]) and δ⁵³Cr along a meridional transect from the North to the South Atlantic (AMT 29). Chromium concentrations range from of 2.51 to 3.96 nmol kg⁻¹ (n = 68) and δ⁵³Cr values range from +0.86 ± 0.04 ‰ (2SEM) to +1.20 ± 0.02 ‰ (2SEM) (n = 68). In contrast to data from other ocean basins [Cr] and δ⁵³Cr show only a weak correlation (δ⁵³Cr vs. Ln([Cr]) R² = 0.17), inconsistent with a closed-system Rayleigh distillation model. These results can mainly be explained by horizontal advection and water mass mixing, which our data demonstrate are the dominant processes controlling [Cr] and δ⁵³Cr distributions throughout much of the Atlantic, while the impact of in situ biogeochemical cycling is comparatively minor. There is, indeed no clear impact of biological productivity nor of dysoxic environments in the (sub)tropical Atlantic on the cycling of Cr along the transect. This is likely explained by insufficiently depleted oxygen concentrations and relatively low biological productivity, resulting in these processes being of secondary importance relative to water mass mixing in shaping the distribution of Cr in the low- to mid-latitude Atlantic Ocean.

The biogeochemistry of Cr and its cycling in Earth's surface environments is reviewed. A synthesis and critical evaluation of the major processes controlling Cr mobility and isotope composition (δ⁵³Cr) is presented, taking a source to sink view beginning with Cr mobilization from Earth's crust. Transport and cycling in inland waters and input to the oceans are discussed. Anthropogenic mobilization of Cr results in contributions to the atmosphere and inland waters that are of similar orders of magnitude as natural processes. The principal sources of Cr to the oceans are rivers and diffusive fluxes from marine sediments. Internal cycling of Cr in the ocean is largely controlled by reductive removal onto particles, particularly in O₂-depleted waters, and redistribution through ocean circulation. Chromium removal from the oceans occurs primarily in organic carbon-rich, O₂-poor shelf sediments. Despite theoretically poor mobility of Cr(III), reductive removal in anoxic waters is non-quantitative. As a result, isotope fractionation drives δ⁵³Cr offsets in removed Cr as well as residual dissolved Cr(III), which accumulates in anoxic water, compared to the corresponding source. The implications for δ⁵³Cr-based reconstructions are discussed, along with an outlook for future proxy applications based on the processes controlling Cr incorporation into sediments. The roles of different source and sink processes are quantified in an updated mass balance for the global ocean. Finally, priority topics for future research are suggested, which at present are the primary uncertainties of the modern Cr biogeochemical cycle and aspects of the Cr isotope mass balance.

The primary production of fjords across the Arctic and Subarctic is undergoing significant transformations due to the climatically driven retreat of glaciers and ice sheets. However, the implications of these changes for upper trophic levels remain largely unknown. In this study, we employ both bulk and compound-specific stable isotope analyses to investigate how shifts at the base of fjord food webs impact the carbon and energy sources of consumers. Focusing on two rapidly changing fjords in Southern Greenland, we used the migratory Arctic char as an indicator species, sampling populations along environmental gradients within the fjords, building upon the assumption that char populations feed primarily close to their natal stream, thereby integrating a dietary gradient. Our analysis of bulk stable isotopes in Arctic char tissue confirmed this premise, revealing a consistent change in resource use from the outer to the inner fjord, which nonetheless served as preferred feeding grounds. Essential amino acid analysis further indicated shifts in carbon and nitrogen sources, consistent with changes in nutrient use near glacier inputs characterized by low turbidity and high iron levels. Notably, these changes in the source of primary production were associated with shifts in trophic positions and the transfer of polyunsaturated fatty acids, with Arctic char in glacier-influenced inner fjords feeding at lower trophic level (size-corrected) and accumulating higher levels of high-quality docosahexaenoic acid (DHA). These findings highlight the usefulness of new analytical tools in revealing that glacial retreat can substantially alter food web dynamics, enhancing both carbon flow and the nutritional quality of fish in fjord ecosystems. The two Southern Greenland fjords studied could represent the future of other fjords, where retreating glaciers become land-terminating and glacial inputs decrease. Our study underscores the critical role of glacier dynamics in affecting high-level consumers, such as salmonids, with implications for fjords globally.

Inland waters play a crucial role in the global carbon cycle, with lakes integrating carbon from various sources within their catchment in addition to that fixed by local primary productivity. Isotopic measurements of carbon pools can differentiate contributions from these sources, with natural abundance radiocarbon (¹⁴C) a particularly powerful tool due to the large range in ¹⁴C characteristics among carbon sources. Here, we present ¹⁴C measurements of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) from monthly water column samplings over the course of a year in Lake Geneva, a large oligotrophic hard water perialpine lake in Western Europe. We find that DIC in the lake is significantly ¹⁴C-depleted relative to the atmosphere primarily due to the dissolution of carbonate rocks in the lake's catchment. Variability in DI¹⁴C is largely tied to the Rhône River inflow, where DI¹⁴C values were also found to vary seasonally. DOC has a ¹⁴C signature similar to that of DIC, reflecting the fact that much of the lake DOC pool is autochthonous. However, more ¹⁴C-depleted DOC was observed in July and tied to increased river discharge from snow and glacier melt within the upper Rhône River basin. These observations shed light on carbon sources and dynamics within Lake Geneva and its alpine catchment and highlight the importance of preaged dissolved carbon inputs to the largest natural lake in Western Europe.

Swiss geosciences are at the heart of major national and global challenges that our society is facing, including climate change and weather extremes, long-term environmental trends, natural hazards (e.g. landslides, floods, seismic and volcanic events), the energy transition towards a green and blue economy, sustainable management of natural resources (e.g. water, soils, ecosystems, critical metals, building materials), and the increasing interest towards extra-terrestrial life and resources. In order to remain at the forefront of geosciences research and provide the fertile environment necessary for groundbreaking findings addressing these societal challenges, the Swiss Geosciences Community proposes five large infrastructures, organised into four pillars built on a common datadriven research foundation. [...]

The highly biodiverse Lake Poso, located in Central Sulawesi, Indonesia, can be considered one of the least studied ancient lakes in the world. Here, we present a comprehensive analysis of Lake Poso's hydrology and hydrochemistry, shedding light on factors that may have contributed to the exceptional biodiversity. Riverine and lake water chemical compositions indicated a soft water lake and relative major cation and anion abundances of Ca²⁺ ≫ Mg²⁺ > Na⁺ > K⁺ and HCO₃^– ≫ SO₄²⁻ > Cl⁻, primarily a result of the high annual precipitation and chemical weathering of calcareous-siliceous metamorphic bedrock. Lake Poso’s nutrient concentrations were low (average DIN/TDP mass ratio of 6.2 and 50.9 for the lake surface water and its tributaries, respectively), indicating that most of the inlets were P-limited and that the lake was likely P-limited as well. Metal pollutants indicated a minor to moderate impact of anthropogenic land use (∼32 % of the catchment area). Water isotopic compositions of the different tributaries clearly delineated rivers draining higher elevation catchments with lower δ²H and δ¹⁸O from those draining lower elevation catchments with higher δ²H and δ¹⁸O. Surface lake water isotopic compositions indicated detectable evaporation from the lake leading toward more enriched isotope compositions than the integrated source signal. Overall, the findings suggested that Lake Poso remains relatively resilient to anthropogenic land use and related nutrient and pollutant inputs. However, ongoing alterations to its hydrological balance due to significant changes in land use may drive the lake towards higher trophic levels in the future.

L'état global de la qualité des lacs en Suisse a tendance à s'améliorer, mais de nombreux problèmes d'oxygénation dans le fond des lacs persistent. Ce phénomène, lié aux concentrations actuelles et passées de phosphore, demeure préoccupant. La modélisation au coeur de cette étude vise à guider le choix d'approches simples mais appropriées pour la gestion des lacs. Deux exemples d'applications pour les lacs de Schiffenen et de Baldegg sont illustrés.

Iron-rich, ferruginous waters were the dominant geochemical regime for most of Earth's history. Modern ferruginous waters are found in stratified, sulfur-poor lakes, and serve as crucial analogs for biogeochemical cycling throughout Earth's past. Here we present the first depth-resolved data of physical structure, nutrients and trace elements from Lake Poso (Indonesia), a deep oligotrophic ancient lake. Lake Poso is ferruginous, with anoxia below ~ 90 m depth, placing it among the world's largest ferruginous lakes. Physical stratification is weaker than other tropical anoxic lakes, indicating sensitivity for paleoclimate reconstructions. Trace elements and nutrients are predominantly shaped by the oxic–anoxic transition. Manganese– and Fe oxyhydroxide–driven biogeochemical cycling occurs at distinct depth horizons, with Co and Ni controlled by Mn and showing shallow release in anoxic waters, while V, Cr, P, and As are controlled by Fe, with release in surface sediments and diffusive transport. Chromium is nonquantitatively removed in anoxic waters, in contrast to widespread assumptions in Cr-based paleoreconstructions. Oxycline U and Se removal corresponds to a local N minimum, suggesting biological reduction and/or uptake. These first ferruginous water Se data also show removal in sediments, indicating sediment signals reflect multiple removal processes and informing Se-based paleoreconstructions, while the absence of sediment U removal contrasts other anoxic basins. A comparison with other ferruginous lakes demonstrates how local influences drive deviations from expectations in other systems, and highlight common, generalizable ferruginous basin features. Therefore, these data will guide research in ferruginous settings across space and time, and improve paleoreconstructions from ferruginous sediment records.

In a warming world, the input of glacier meltwater to inland water ecosystems is predicted to change, potentially affecting their productivity. Meta-ecosystem theory, which posits that the nutrient availability in the recipient ecosystem can determine the extent of cross-ecosystem boundary utilization, can be useful for studying landscape-scale influences of glacier meltwater on inland waters. Here, we investigate how the input of glacier meltwater in a river system in Southern Greenland influences the utilization of marine subsidies in freshwater fish. Our study system comprised four sites, with controls for glacial meltwater and marine subsidies, harboring a partially migrating population of arctic char, meaning that some individuals migrate to the ocean and others remain in freshwaters, and two fully resident populations as a freshwater reference. We assessed the incorporation of marine carbon in freshwater resident char using both bulk and amino acid stable isotope analysis of muscle tissue. In the population with partial migration, marine subsidies were a significant resource for resident char individuals, and estimates of trophic position suggest that egg cannibalism is an important mechanism underlying the assimilation of these marine subsidies. In proglacial streams, namely those with high glacial meltwater, the total dependence on marine subsidies increased and reached 83% because char become cannibals at smaller sizes. In the configuration of our focal meta-ecosystem, our results suggest that the importance of marine subsidies to freshwater fish strengthens within increasing meltwater flux from upstream glaciers.

Per- and polyfluoroalkyl substances (PFAS) pose significant environmental and health risks due to their ubiquitous presence and persistence in water systems. Herein, the efficacy of piezocatalysis using barium titanate nanoparticles under ultrasound irradiation for the degradation and defluorination of perfluorooctane sulfonate (PFOS) in water is investigated. The research demonstrates a substantial 90.5% degradation and 29% defluorination of PFOS after 6 h of treatment, highlighting the potential of piezocatalysis as a promising approach for PFAS degradation. Additionally, the quantification of degradation products elucidates the transformation pathways of PFOS, suggesting a stepwise chain-shortening mechanism. The findings underscore the importance of continued research in optimizing piezocatalytic processes and exploring synergistic approaches with other advanced oxidation methods to effectively address PFAS contamination challenges. These efforts are essential for advancing sustainable water treatment strategies and mitigating the environmental and health hazards associated with PFAS contamination.

Dissolved iron (dFe) availability limits the uptake of atmospheric CO₂ by the Southern Ocean (SO) biological pump. Hence, any change in bioavailable dFe in this region can directly influence climate. On the basis of Fe uptake experiments with Phaeocystis antarctica, we show that the range of dFe bioavailability in natural samples is wider (<1 to ~200% compared to free inorganic Fe′) than previously thought, with higher bioavailability found near glacial sources. The degree of bioavailability varied regardless of in situ dFe concentration and depth, challenging the consensus that sole dFe concentrations can be used to predict Fe uptake in modeling studies. Further, our data suggest a disproportionately major role of biologically mediated ligands and encourage revisiting the role of humic substances in influencing marine Fe biogeochemical cycling in the SO. Last, we describe a linkage between in situ dFe bioavailability and isotopic signatures that, we anticipate, will stimulate future research.

Special attention has been given to chromium (Cr) as a paleoproxy tracing redox cycling throughout Earth’s history, due to differences in the solubility of its primary redox species at Earth’s surface (Cr(III) and Cr(VI)) and isotope fractionation associated with their interconversion. In turn, chromium’s paleoproxy potential has motivated studies of the modern ocean to better understand which processes drive its cycling and to constrain their impact on the Cr isotope composition (δ⁵³Cr) of seawater. Here, we present total dissolved seawater Cr concentrations and δ⁵³Cr along the GEOTRACES GP13 section. This section is a zonal transect extending from Australia in the subtropical southwest Pacific Ocean. Surface signals of local biological Cr cycling are minimal, in agreement with distributions of dissolved major nutrients as well as biologically-controlled trace metals in this low productivity, oligotrophic environment. Depth profiles have Cr concentration minima in surface waters and maxima at depth, and are largely shaped by the advection of nutrient- and Cr-rich subsurface waters rather than vertically-driven processes. Samples close to the sediment-water interface indicate important benthic Cr fluxes across the section.
The GP13 transect crosses the hydrothermally-active Kermadec Arc. Hydrothermal fluids (consisting of <15% background seawater) were collected from three venting sites at the Brothers Volcano (along the Kermadec Arc). These fluids yielded near-crustal δ⁵³Cr values (-0.17 to +0.08‰) and elevated [Cr] (7.5-23 nmol kg^-1, hydrothermal endmember [Cr] ≈ 8-27 nmol kg^-1), indicating that the Kermadec Arc may be an isotopically light Cr source. Dissolved [Fe] enrichments have been reported previously in deep waters (∼1600-3000 m) along the GP13 transect, east of the Kermadec Arc. These same waters show elevated [Cr] compared to Circumpolar Deep Water ([Cr] = 3.88 ± 0.11, δ⁵³Cr = 0.89 ± 0.08, n = 32), with an average [Cr] accumulation of 0.71 ± 0.11 nmol kg^-1 (1 SD), and an estimated δ⁵³Cr of +0.46 ± 0.30‰ (2 SD, n = 9) for the accumulated Cr. Comparing high-temperature vent and neutrally buoyant plume data, hydrothermal-sourced Cr is likely negligable compared to Cr contributions from other processes (benthic fluxes, release from particles), and the advection of more Cr-rich Pacific Deep Water. It is unlikely that hydrothermal vents would be a major contributor within the regional or global biogeochemical Cr cycle, even if hydrothermal fluxes change by orders of magnitude, and therefore δ⁵³Cr trends in the paleorecord may be attributable, at least in part, to major changes in other controls on Cr (e.g. widespread anoxia).

Chromium stable isotope composition (δ⁵³Cr) is a promising tracer for redox conditions throughout Earth's history; however, the geochemical controls of δ⁵³Cr have not been assessed in modern redox-stratified basins. We present new chromium (Cr) concentration and δ⁵³Cr data in dissolved, sinking particulate, and sediment samples from the redox-stratified Lake Cadagno (Switzerland), a modern Proterozoic ocean analog. These data demonstrate isotope fractionation during incomplete (non-quantitative) reduction and removal of Cr above the chemocline, driving isotopically light Cr accumulation in euxinic deep waters. Sediment authigenic Cr is isotopically distinct from overlying waters but comparable to average continental crust. New and published data from other redox-stratified basins show analogous patterns. This challenges assumptions from δ⁵³Cr paleoredox applications that quantitative Cr reduction and removal limits isotope fractionation. Instead, fractionation from non-quantitative Cr removal leads to sedimentary records offset from overlying waters and not reflecting high δ⁵³Cr from oxidative continental weathering.

From winter 2013–14 to the end of 2015–16, a high pressure atmospheric system induced elevated sea surface temperatures in the offshore subarctic northeast Pacific, resulting in a marine heatwave. Increased stratification due to the heatwave resulted in shoaling of the winter mixed layer and a decrease in nutrient re-supply to the euphotic zone. Here, we investigate relationships between dissolved iron (dFe) and macronutrients, net community production (NCP), (micro)nutrient uptake ratios, and phytoplankton community composition in the winter and summer from 2012 to 2015 to gain insight into coupled biogeochemical responses to the heatwave. Our investigation highlights the importance of external dFe supply during marine heatwave events, as a more shallow mixed layer reduces the transport of essential (micro)macronutrients to the surface layer. We conclude that recycled dFe did not contribute to NCP in 2014, but rather the vertical displacement of dFe rich water unrelated to mixed layer deepening played a major role. In 2015, such transport was not detected, resulting in abnormally low dFe and shift toward higher biomass of pico- and nano-phytoplankton size-classes.