The shallow East Siberian Arctic Shelf (ESAS) shows high methane concentration in seawater; however, methane oxidation processes for the ESAS are poorly constrained. Here, we examined porewater geochemical profiles, microbial lipids, and their carbon isotope compositions in multiple sediment cores to explore molecular-isotopic diagnostics of microbial methane oxidation in the outer Laptev Sea. There was covariation between methane and sulfate profiles in only one core in the studied upper 20 cm. Across all cores, concentrations of microbial fatty acids (C₁₂–C₁₈) decreased sharply below the surface sediment with carbon isotope compositions (δ¹³C) ranging from −31 to −20‰. Notably, anaerobic methane oxidizing archaea (ANME)-derived lipid biomarkers, e.g., 2,6,10,15,19-pentamethylicosane, archaeal and hydroxyarchaeol, were absent or below detection limits, indicating that anaerobic methane oxidation (AOM) was not a dominant process in the studied sediment. However, the tetraether-based Methane Index increased from 0.03 to 0.8 in the core where sulfate and methane covaried, implying either a developing AOM microbial community with presently low activity or an episodic AOM event occurring within the past decades to a century. In contrast, hop-17(21)-ene was present throughout the cores, exhibiting strong ¹³C depletion, with values ranging from −66.1 to −44.0‰. The concentration and δ¹³C values of hop-17(21)-ene were distinct from the terrestrially-derived nC₃₁ alkane, which showed relatively constant δ¹³C values of −34.9 ± 1.1‰. This suggests that hop-17(21)-ene in the sediments is primarily derived from marine microbes. Given the oxic seawater in this system (< 60 m), the ¹³C-depleted hop-17(21)-ene likely reflects aerobic methane oxidation (AeOM) occurring in the water column and/or in surface sediments. AeOM may play a significant role in regulating methane in this subsea permafrost seepage system.

High-altitude terrain may intersect the upper atmospheric boundary layer and exhibit distinct environmental dynamics. We investigated the anthropogenic pollutants polychlorinated alkanes (PCAs, also known as chlorinated paraffins) in surface soils along a transect from the La Paz-El Alto metropolitan area in Bolivia (3200–4100 masl) to the upper slopes of Mount Chacaltaya (>5200 masl), around 16 km away. Concentrations of PCAs in urban soils (750–5,230 ng/g organic carbon [OC]) decreased exponentially with increasing distance from the urban boundary, declining to ∼150 ng/g OC at elevations below 4,700 masl. Beyond 4,700 masl concentrations increased again, reaching levels comparable to those in the urban area, 1,670–4,300 ng/g OC, above 5,000 masl. Given that pollutant concentrations typically decline with distance from their source, this altitudinal trend, together with a pronounced shift in PCA forensic fingerprints near 4,700 masl, strongly suggests contributions from sources beyond the local metropolitan area. Carbon and nitrogen isotope signatures in organic carbon further support long-range transport as a source, consistent with previous modeling and observations that the upper slopes of Mount Chacaltaya predominantly receive air masses and organic carbon from distant regions via transport in the free troposphere. Our observation that pollutant levels in high-altitude areas are comparable to those in the metropolis of 1.8-million inhabitants underscores the efficiency of long-range atmospheric transport.

Enrichment patterns of the redox- and bio-sensitive trace elements U, V, Cr, and Ni in margin sediments are essential for environmental reconstructions. However, their sedimentary accumulation pathways are not fully understood. While the accumulation of toxic trace elements like Cd, Hg, Pb, and Zn has been relatively well studied in fish scales, less is known about U, V, Cr, and Ni. To investigate the role of fish scales as trace elements sinks in marine sediments, we analyzed U, V, Cr, Ni, Ca, and P in fish scales from cores taken from the central and southern Namibian shelf, alongside the bulk sediment and lithogenic composition of the respective hinterlands. Fish scales from the central Namibian shelf showed significantly higher contents of U (up to 134 ppm), V (up to 196 ppm), and Cr (up to 97 ppm), 2–5 times higher than scales from the southern shelf. In contrast, Ni levels were low in all fish scales, and bulk sediment trace elements were similarly enriched in all cores. The central Namibian hinterland has a much higher lithogenic trace element background than the southern hinterland. Hence, it appears that local marine biology could provide a more direct reflection of terrestrial inputs than the sediment record. Our results indicate that fish uptake rather than diagenesis controls trace element accumulation in scales. Seasonal dust inputs, potentially amplified by mining, could make U, V, Cr, and Ni bioavailable, highlighting the importance of fish scales as sedimentary trace element sinks. This reveals a redox-independent enrichment pathway that should be considered in organic-rich shelf upwelling sediments.

Shallow coastal marine habitats are hotspots for carbon dioxide (CO2) and methane (CH4) exchange with the atmosphere, yet these fluxes remain poorly quantified, limiting their integration into global and regional carbon budgets. Using floating chambers, this study quantified seasonal and annual CO2 and CH4 fluxes in common Baltic Sea habitats, including macroalgae-covered coarse sediments, sparsely to densely vegetated sands, submerged plant-covered mixed substrates, and reed-dominated muds. Monthly average CO2 fluxes ranged from −937 ± 161 to 3512 ± 704 mg CO2 m−2 d−1, with macroalgae and reed habitats exhibiting distinct flux ranges. Apart from macroalgae, all habitats exhibited a net annual CO2 efflux. Diffusive CH4 fluxes varied seasonally, from 0.1 ± 0.01 to 26 ± 1.5 mg CH4 m−2 d−1, with peak emissions in summer. Ebullition occurred from March to October, reaching up to 232 mg CH4 m−2 d−1 and contributed substantially to annual carbon-based greenhouse gas fluxes in the sand, mixed-substrate, and reed habitats. Contrary to previous findings that ebullition is confined to muddy, organic-rich sediments, this study found the highest CH4 ebullition in vegetated sand habitats, indicating a broader spatial extent of intense CH4 release than previously assumed. Upscaling to the shallow-water (< 6 m) zone of the Stockholm archipelago yielded total CO2-equivalent fluxes of between −0.01 and 0.2 Tg CO2eq yr−1 (100-year timescale). For comparison, Stockholm’s energy- and transport sectors emit ∼ 1.2 Tg CO2-eq yr−1, suggesting the shallow coastal zone could be a small, but non-negligible regional source for carbon-based greenhouse gases.

Thermospores, the dormant resting stages of thermophilic bacteria, have been shown to be frequent but enigmatic components of cold marine sediments around the world. Multiple hypotheses have been proposed to explain their distribution, emphasizing their potential as model organisms for studying microbial dispersal via ocean currents. In the Arctic Ocean, the abundance and diversity of thermospores have previously been assumed to be low. However, this assessment has been based on data mainly from the western fjords of Svalbard, thus leaving most of the Arctic unexplored. Here, we expand the knowledge about the distribution of thermospores in the Arctic Ocean by investigating the abundance and diversity of thermospores in heated shelf sediments from three sites in the outer Laptev Sea. Two of the sites are located in an area with methane-emitting cold seeps with a thermogenic source signature suggestive of an origin in a deep hydrocarbon reservoir, while the third site is a reference site not known to be impacted by seepage. We found that activity of viable thermospore populations was more prominent at one of the investigated seep sites. This finding is supported by both radiotracer growth experiments showing thermophilic, sulfate-reducing activity triggered by heating, as well as 16S gene sequence analyses showing significantly enriched ASVs affiliated to the phylum Firmicutes following high-temperature incubations. An enrichment of the sulfate-reducing, endospore-forming class Desulfotomaculia in heated samples compared to unheated samples was also observed. Furthermore, several ASVs identified at the seep site are closely related to thermospore-producing bacteria associated with the deep biosphere, including hydrocarbon and hydrothermal systems. Based on the combined information from induced activity, estimated abundance, and phylogenetic composition using 16S rRNA gene sequencing, we propose likely source environments and dispersal vectors for thermospores in the Arctic Ocean.

The central Arctic Ocean (CAO) plays an important role in the global carbon cycle, but the current and future exchange of the climate-forcing trace gases methane (CH4) and carbon dioxide (CO2) between the CAO and the atmosphere is highly uncertain. In particular, there are very few observations of near-surface gas concentrations or direct air-sea CO2 flux estimates and no previously reported direct air-sea CH4 flux estimates from the CAO. Furthermore, the effect of sea ice on the exchange is not well understood. We present direct measurements of the air-sea flux of CH4 and CO2, as well as air-snow fluxes of CO2 in the summertime CAO north of 82.5 N from the Synoptic Arctic Survey (SAS) expedition carried out on the Swedish icebreaker Oden in 2021. Measurements of air-sea CH4 and CO2 flux were made using floating chambers deployed in leads accessed from sea ice and from the side of Oden, and air-snow fluxes were determined from chambers deployed on sea ice. Gas transfer velocities determined from fluxes and surface-water-dissolved gas concentrations exhibited a weaker wind speed dependence than existing parameterisations, with a median sea-ice lead gas transfer rate of 2.5cmh-1 applicable over the observed 10m wind speed range (1-11ms-1). The average observed air-sea CO2 flux was -7.6mmolm-2d-1, and the average air-snow CO2 flux was -1.1mmolm-2d-1. Extrapolating these fluxes and the corresponding sea-ice concentrations gives an August and September flux for the CAO of -1.75mmolm-2d-1, within the range of previous indirect estimates. The average observed air-sea CH4 flux of 3.5μmolm-2d-1, accounting for sea-ice concentration, equates to an August and September CAO flux of 0.35μmolm-2d-1, lower than previous estimates and implying that the CAO is a very small (‰ 1%) contributor to the Arctic flux of CH4 to the atmosphere.

In highly seasonal systems, the emergence of planktonic resting stages from the sediment is a key driver for bloom timing and plankton community composition. The termination of the resting phase is often linked to environmental cues, but the extent to which recruitment of resting stages is affected by climate change remains largely unknown for coastal environments. Here we investigate phyto- and zooplankton recruitment from oxic sediments in the Baltic Sea in a controlled experiment under proposed temperature and light increase during the spring and summer. We find that emergence of resting stage differs between seasons and the abiotic environment. Phytoplankton recruitment from resting stages were high in spring with significantly higher emergence rates at increased temperature and light levels for dinoflagellate and cyanobacteria than for diatoms, which had highest emergence under cold and dark conditions. In comparison, hatching of copepod nauplii was not affected by increased temperature and light levels. These results show that activation of plankton resting stages are affected to different degrees by increasing temperature and light levels, indicating that climate change affects plankton dynamics through processes related to resting stage termination with potential consequences for bloom timing, community composition and trophic mismatch.

The expansion of transient and permanent coastal benthic anoxia is one of the most severe problems for the coastal ocean globally. We report frequent, hidden hypoxia in the bottom 5 cm of the water column of a coastal site in the central Baltic Sea by continuous high-resolution profiling of oxygen (O₂) directly above the sediment surface. This hypoxia stood in stark contrast to 30-yr O₂ monitoring records at this site that suggest apparent continuous well-oxygenated conditions. In situ measurements showed highly dynamic conditions in the bottom 30 cm recording frequent gradual and abrupt changes between normoxic (> 63 μmol L⁻¹) and hypoxic (< 63 μmol L⁻¹) conditions that would remain undetectable by conventional bottom water O₂ monitoring. The temporal variability of these “hidden” hypoxia is tied to the dynamic current field and to changes in O₂ consumption following resuspension events. Our observations suggest that transient benthic hypoxia is much more common than routine monitoring data indicate.

Background In anoxic coastal and marine sediments, degradation of methylated compounds is the major route to the production of methane, a powerful greenhouse gas. Dimethylsulphide (DMS) is the most abundant biogenic organic sulphur compound in the environment and an abundant methylated compound leading to methane production in anoxic sediments. However, understanding of the microbial diversity driving DMS-dependent methanogenesis is limited, and the metabolic pathways underlying this process in the environment remain unexplored. To address this, we used anoxic incubations, amplicon sequencing, genome-centric metagenomics and metatranscriptomics of brackish sediments collected along the depth profile of the Baltic Sea with varying sulphate concentrations.Results We identified Methanolobus as the dominant methylotrophic methanogens in all our DMS-amended sediment incubations (61-99%) regardless of their sulphate concentrations. We also showed that the mtt and mta genes (trimethylamine- and methanol-methyltransferases) from Methanolobus were highly expressed when the sediment samples were incubated with DMS. Furthermore, we did not find mtsA and mtsB (methylsulphide-methyltransferases) in metatranscriptomes, metagenomes or in the Methanolobus MAGs, whilst mtsD and mtsF were found 2-3 orders of magnitude lower in selected samples.Conclusions Our study demonstrated that the Methanolobus genus is likely the key player in anaerobic DMS degradation in brackish Baltic Sea sediments. This is also the first study analysing the metabolic pathways of anaerobic DMS degradation in the environment and showing that methylotrophic methane production from DMS may not require a substrate-specific methyltransferase as was previously accepted. This highlights the versatility of the key enzymes in methane production in anoxic sediments, which would have significant implications for the global greenhouse gas budget and the methane cycle.

Sediments of the continental shelf anoxic zones of the Benguela upwelling system (BUS) and the Peruvian upwelling system are present-day hotspots of trace element accumulation. However, contribution of the lithogenic trace element fraction and early diagenetic transformation processes are poorly constrained. The identification of source and accumulation mechanisms is necessary for the validation of trace elements as proxies for productivity and redox cycling, notably in highly dynamic upwelling systems such as the BUS. Here, we analyzed redox- and biosensitive elements (U, Ba, Ni), lithogenic tracers (Al, Ti, Zr), total organic carbon (TOC) and P in eleven short sediment cores from the Namibian shelf (ca. 22–25°S) as well as grain size fractionated hinterland samples from the Tsauchab Valley at Sossusvlei (∼25°S). These samples help to constrain the lithogenic trace element contributions, a prerequisite for a realistic interpretation of marine authigenic trace element data. Our findings corroborate previous findings that Zr is a sensitive tracer of the coarse fraction in marine sediment for dust input and/or sediment reworking. Hence, the combined analysis of the coarse fraction (Zr), organic matter, and P enrichments (TOC/P) differentiates between a calm, organic matter-rich central shelf environment and a more energetic phosphorite-rich southern shelf environment. A significant correlation of U with P in cores from the southern shelf corroborates previous findings of U incorporation into apatite, the initial formation of which requires oscillating oxic to sulfidic redox conditions. Non-apatite associated U dominates in the cores from the central shelf, where it is only enriched in the buried, anoxic parts of the sediment. This also supports oscillating oxic to sulfidic redox conditions in surface sediment, where U may be recycled. Hence, U accumulates either (i) in apatite under oscillating oxic to sulfidic conditions or (ii) in buried anoxic sediment sections. This strongly questions the use of U as indicator of suboxic conditions but rather anoxic conditions. While lithogenic fractions of U were negligible, the lithogenic fractions of Ba and Ni were found to be elevated. The calculation of authigenic Ba when accounting for lithogenic Ba from the Sossusvlei fine fractions gives more realistic authigenic Ba values. The high Ba enrichment in the BUS is interpreted as uptake of Ba by diatoms and/or nucleation of barite in P-rich diatom remains, as suggested previously. Given the shallow water depth of the cores, barite formation must already take place in subsurface waters (<50 m water depth) as opposed to previous suggesting greater water depths. The lithogenic background of Ni strongly varies from 23°S to 25°S based on Sossusvlei data and the literature. When authigenic Ni contents are calculated accordingly and plotted versus TOC, more systematic trends are seen (compared to total Ni versus TOC). Essentially all cores show an increase in the authigenic Ni/TOC ratio with sediment depth suggesting better preservation and retention of Ni as compared to TOC consistent with previous observations in sediments from the Peruvian upwelling system. This trend corroborates the use of Ni as a productivity indicator for upwelling sediments.