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The distribution of subsurface microplastics in the ocean
No full textMarine plastic pollution is a global issue, with microplastics (1 µm–5 mm) dominating the measured plastic count1,2. Although microplastics can be found throughout the oceanic water column3,4, most studies collect microplastics from surface waters (less than about 50-cm depth) using net tows5. Consequently, our understanding of the microplastics distribution across ocean depths is more limited. Here we synthesize depth-profile data from 1,885 stations collected between 2014 and 2024 to provide insights into the distribution and potential transport mechanisms of subsurface (below about 50-cm depth, which is not usually sampled by traditional practices3,6) microplastics throughout the oceanic water column. We find that the abundances of microplastics range from 10−4 to 104 particles per cubic metre. Microplastic size affects their distribution; the abundance of small microplastics (1 µm to 100 µm) decreases gradually with depth, indicating a more even distribution and longer lifespan in the water column compared with larger microplastics (100 µm to 5,000 µm) that tend to concentrate at the stratified layers. Mid-gyre accumulation zones extend into the subsurface ocean but are concentrated in the top 100 m and predominantly consist of larger microplastics. Our analysis suggests that microplastics constitute a measurable fraction of the total particulate organic carbon, increasing from 0.1% at 30 m to 5% at 2,000 m. Although our study establishes a global benchmark, our findings underscore that the lack of standardization creates substantial uncertainties, making it challenging to advance our comprehension of the distribution of microplastics and its impact on the oceanic environment
Insights into silicon cycling from ice sheet to coastal ocean from isotope geochemistry
No full textThe polar regions are biologically productive and play a critical role in regional and global biogeochemical cycling. A key nutrient is dissolved silicon, required for the growth of siliceous phytoplankton, diatoms, which form an important component of polar ecosystems. Glacial weathering is thought to be an important dissolved silicon source to coastal waters, especially critical in regions experiencing seasonal silicon limitation of diatom growth. However, complex physical and biogeochemical interactions in fjords and coastal regions modulate the downstream supply of dissolved and particulate nutrients, including silicon. Here, we review the biogeochemical complexities of glaciated margins and the insights into this system that silicon isotope geochemistry offer. We show that stable and radioisotopic measurements and biogeochemical numerical modelling provide a quantitative mechanistic understanding of subglacial silica mobilisation and its cycling across the land-ocean continuum. Subglacial weathering produces isotopically light amorphous silica, which dissolves in seawater to release dissolved silicon. Our findings show that isotopically light, detrital silica, likely containing glacial material, reaches the ocean and there could support a substantial proportion of diatom productivity, especially in the Arctic. Outstanding questions about silicon cycling in these crucial environments will be addressed through novel and cross-discipline approaches that overcome traditionally viewed ecosystem boundaries
Characterizing the interannual variability of North Atlantic subpolar overturning
No full textVariability of the Atlantic Meridional Overturning Circulation (MOC) has drawn extensive attention due to its impact on the global redistribution of heat and freshwater. Here we present the latest time series (2014–2022) of the Overturning in the Subpolar North Atlantic Program and characterize MOC interannual variability. We find that any single boundary current captures ∼30% of subpolar MOC interannual variability. However, to fully resolve MOC variability, a wide swath across the eastern subpolar basin is needed; in the Labrador Sea both boundaries are needed. Through a volume budget analysis for the subpolar basins\u27 lower limbs, we estimate the magnitude of unresolved processes (e.g., diapycnal mixing) required to close the mean budget (∼2 Sv). We find that in the eastern subpolar basin surface-forced transformation variability is linked to lower limb volume variability, which translates to MOC changes within the same year. In contrast, this linkage is weak in the Labrador Sea.</span
A frontal ablation dataset for 49 tidewater glaciers in Greenland
No full textFrontal ablation at tidewater glaciers, which comprises iceberg calving and submarine and subaerial melting, is a key boundary condition for numerical ice sheet models but remains difficult to measure in-situ. Although previous studies have provided frontal ablation estimates over a range of spatiotemporal scales, most use ice discharge as an approximation, thereby neglecting the influence of terminus position change. Here, we present a dataset of frontal ablation estimates for 49 tidewater glaciers in Greenland that have reliable near-terminus bathymetry data. Near-terminus volume change over the period 1987–2020 is determined using previously published datasets of terminus positions (TermPicks) together with ice thicknesses estimated from ArcticDEM, AeroDEM, and Bedmachine v5 bed topography. Assuming a vertical terminus geometry and uniform ice density, we estimate frontal ablation as the residual between mass flux towards the terminus taken from a published dataset and mass change due to changes in terminus position. The frontal ablation dataset offers opportunities for developing new insights into ice dynamics, including helping to improve numerical model hindcasting and projections.</span
Abrupt decline of microplankton species richness linked to coastal inflow in an Arctic fjord
No full textSpecies richness can be important for the functioning and resilience of marine ecosystems, with planktonic protists forming the base of the marine food web. Climate change-driven shifts in water mass distributions are projected to alter plankton diversity. This process is particularly relevant in Arctic fjords experiencing changes in coastal water inflow, affecting local hydrography, nutrients, and biodiversity. Yet, empirical data on the environmental effects on Arctic plankton diversity are scarce. In this study, we analyze a 15-year time series from Nuup Kangerlua, a West Greenland fjord, and report an abrupt decline in microplankton (> 20 μm) species richness following a high-richness period from 2009 to 2013. The high-richness period was also characterized by a period of substantial inflow of Atlantic type water (ATW). While ATW did not appear to directly introduce temperate species, it was associated with elevated surface temperatures and nitrate concentrations - conditions that may have supported elevated microplankton richness. Nitrate is typically the key limiting nutrient in Arctic fjords and may support higher microplankton species richness consistent with the Species Energy theory.</span
Occurrence of tetraester and mixed ether/ester-bound iso-diabolic acid membrane-spanning lipids in acidic, high-elevation mineral soils
No full textBranched glycerol dialkyl glycerol tetraethers (brGDGTs) are a suite of membrane lipids that are widely used as empirical proxies for past temperature and pH. Although the stereochemistry of their glycerol moiety suggests that they are produced by bacteria, the exact producers and the biosynthetic pathway of brGDGTs remain unclear. Here we report the occurrence of tetraester and mixed ester/ether membrane-spanning lipids with a backbone consisting of iso-diabolic acid (iso-DA) containing up to two additional methyl groups in high-elevation (>1900 masl), acidic (pH < 4.5) mineral soils from Nepal and Rwanda. These compounds are presumed intermediate products during brGDGT synthesis but had not been detected in cultures or the environment before. Interestingly, while acid hydrolysis of the GDGT fraction releases iso-DA in the soil from Nepal, monoalkyl glycerol ethers with iso-C15 and iso-C17 chains are released in the soil from Rwanda. Although speculative, these results could support both current hypotheses that brGDGT synthesis can occur via tail-to-tail condensation of two iso-C15 fatty acids to form iso-DA, as well as through the reduction of diesters to diethers and subsequent carbon–carbon linking as shown to occur during the synthesis of archaeal GDGTs.</span
Thermal stability of freshwater fern Azolla biomarkers as assessed by hydrous pyrolysis
No full textThe freshwater fern Azolla produces a unique series of ω20-hydroxy and ω9,ω10-dihydroxy lipids. These compounds were also detected in Eocene sediments (∼48.5 Ma) characterized by the occurrence of fossil remains of Azolla and are thought to be prime markers for the past occurrence of Azolla implying that they could serve as palaeo-environmental indicators of freshwater conditions. However, the suitability of these Azolla biomarkers for application to more mature sediments is unknown. In this study, Azolla caroliniana specimens were heated using hydrous pyrolysis at temperatures ranging from 220 to 365 °C for 72 h to examine the stability and degradation of the Azolla biomarkers during simulated diagenesis. Initially all ω20-hydroxy and ω9,ω10-dihydroxy lipids are converted to their ketone analogues. At 260 °C n-alkanes are formed. ω20-Hydroxy and ω9,ω10-dihydroxy lipids are degraded between 260 and 280 °C. At 300 °C, only ω20-keto-1-ols, ω20-keto fatty acids and ω20-ketones, along with their ω9 and ω10 analogues, remain to preserve their ω20, ω9 and ω10 functionalities, while at 320 °C and higher only the ω20-, ω9, and ω10-ketones were observed. At higher temperatures the proportions of n-alkanes became dominating. The high thermal stability of the ω20, ω9/ω10 functionality shows that it has a relatively high preservation potential. Hence, it may potentially be a robust source indicator of the depositional environment of Azolla-derived organic carbon reflecting freshwater or brackish surface waters.</span
Coarsening coasts: quantifying sensitivity of benthic communities to sandification
No full textSea level rise, increased storminess, and changes in sediment supply due to nourishments are all expected to drive coarsening (i.e., ‘sandification’) of muddy coastal sediments in the decades to come. Since the composition of soft-bottom benthic communities is associated with the sediment grain-size and mud content, this may result in habitats becoming less suitable for some species, leading to species shifts. Species-sediment relations can help to predict how this foreseen sandification may affect benthic fauna. We explore and quantify the sandification-sensitivity of benthic communities, with a tidal basin in the Dutch Wadden Sea as a model system. We identify the species\u27 sediment optima and tolerance ranges using non-linear quantile regression models, summarise preference and sensitivity at the community level, and determine the difference between optimal and realised sediment habitat. We find that sediment optima are taxon-specific and that most species in this area are sediment generalists. On community level, there is a difference between the preferred and realised sediment habitat. In many areas, the actual inhabited sediment is coarser and sandier than expected based on the preferences of the resident species. Future sandification of the area would further decrease sediment habitat suitability for benthic communities in these places. This detailed knowledge of area-specific sensitivity of benthos can be used to inform coastal management decisions
Maintaining shallow waters to maximize the potential of saltpans as foraging habitat for migratory shorebirds along an endangered flyway
No full textThe global expansion of human activities has led to a dramatic reduction in natural wetlands. While effectively managed artificial wetlands hold the potential to counteract this decline, there is often a lack of knowledge regarding how to enhance their value for wildlife. To bridge this gap, we studied one of the world’s largest saltworks in the Yellow Sea, China. This region has experienced significant natural wetland losses, resulting in marked declines in waterbird populations across the East Asian-Australasian Flyway (EAAF). Our research examined the impact of biotic factors such as prey abundance, and abiotic factors including water depth and management practices of saltpans, on the distribution of foraging shorebirds, as well as other waterbirds. Findings reveal that shorebirds, accounting for 95% of all waterbirds, heavily utilize saltpan evaporation ponds. Although various factors affect the distribution of shorebirds and other waterbirds, water depth and abandoned ponds (with low water depth) emerged as the primary factors. Notably, prey biomass (and abundance) did not exhibit a significant correlation with bird distribution across all shorebirds and other waterbirds, indicating that prey accessibility is more crucial than biomass in determining habitat suitability in these saltpans. Based on these insights, we propose tailored management strategies for the Yellow Sea saltpans that enhance conditions for shorebirds without impeding salt production, including lowering water levels through modifications to pond structures and reducing the volume of water evaporated during each stage of production. These straightforward, bird-friendly strategies provide practical solutions for adapting artificial wetlands to support migratory waterbirds in the EAAF and offer valuable insights for global waterbird conservation.</span
Seasonal and interannual variability in freshwater sources for Greenland\u27s fjords
No full textThe magnitude, source, release location, and timing of freshwater that ends up in the numerous Greenland fjords is of special interest for ice–ocean interactions and ecosystems. In this study, we investigate intra- and interannual variability in the various freshwater sources for Greenland\u27s fjords in seven climatologically distinct regions. For this, we use direct and statistically downscaled output from regional climate models for the mass fluxes, process-based estimates of basal melt, and observational data for solid ice discharge. For the period 1940/1958 through 2023, we separately quantify runoff from the Greenland ice sheet, peripheral ice caps and tundra regions, and precipitation directly falling in the fjords. From 2009 onwards, the available data allow us to resolve the full seasonal cycle of freshwater input. The results indicate a diverse range of relative contributions from freshwater sources between seasons and regions. Freshwater input in fjords in the wet South-East and North-West is dominated by solid ice discharge (55 % and 67 %, respectively) with a small contribution of tundra runoff, whereas in the relatively drier North, North-East, and South-West the contribution of tundra runoff is more important (20 %, 25 %, and 30 %, respectively). Precipitation in fjords and tundra runoff can represent a large fraction of the monthly total, i.e. up to 11 % and 35 %, respectively, for winter and spring. However, the relative contribution of tundra runoff has been decreasing with time as the result of rapid increases in ice sheet and ice cap runoff over the past decades following atmospheric and oceanic warming. We show that the regional glacier-integrated melt-over-accumulation (MoA) ratio is a good predictor for the relative contributions of solid ice discharge, tundra runoff, and ice sheet runoff. These findings have implications for the use of freshwater fluxes forcing in regional ocean models and fjord studies and enhance our understanding of their impact on ocean and fjord circulation and biogeochemistry.</span