Elevated methane concentrations in seawater have been reported over extensive areas of the East Siberian Arctic Seas, overlying thawing subsea permafrost. However, observed methane concentrations of the ephemeral seawater are highly variable across both space and time, compromised by both the timing of rare measurements and storm-driven exchanges to the atmosphere. Here, we applied time-integrated signals of the δ¹³C-composition of specific C₃₀ hopanoids (diploptene, hop-17(21)-ene, neohop-13(18)-ene and diplopterol) in surface sediments to trace aerobic methane oxidation as a proxy for enhanced methane cycling. Interpretations of hopanoids and possible sources were further assessed by 16S-rRNA analyses in the surface sediments. The consistently low δ¹³C-C₃₀ hopenes signals, ranging between −57.5 ‰ to −37.1 ‰ (n=23) across the Laptev Sea shelf indicated aerobic methane oxidation. This suggests ubiquitous methane cycling with the most pronounced intensities in the outer shelf region, broadly consistent with the observed methane concentrations. Notably, depleted δ¹³C-C₃₀ hopenes were also found in the mid-shelf region of the Laptev Sea, earlier thought to be an area of comparatively low methane cycling. High methane concentrations were also observed in the vicinity of the Lena River delta, yet the isotopically heavier δ¹³C-C₃₀ hopenes may here reflect a combination of lower aerobic methane oxidation, a greater relative abundance of type II methanotrophs (lower isotope fractionation during hopanoid production) and isotope dilution from non-methanotrophic sources. While this complicates the biomarker interpretation in the unique setting near the Lena River delta, the δ¹³C-C₃₀ hopenes were still much lower than δ¹³C-organic carbon, indicating aerobic methane oxidation and a clear methane cycling signal also in this regime. Taken together, the results unravel the wider cross-shelf patterns of enhanced methane cycling in the Laptev Sea through probing of methane fossilised in membrane lipids of aerobic methanotrophs, with the molecular-isotopic pattern being preserved in the sedimentary archive.

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.

Sulfate aerosols cool the climate and thus temporarily mask climate warming, but at a cost to air quality. Their short atmospheric lifetime leads to heterogeneous global coverage, with sulfate concentrations over South Asia being especially elevated and continuing to increase. It remains challenging to constrain the relative importance of different emission sources due to poor observational coverage and uncertainties in bottom-up technology-based emission estimates. The stable sulfur isotope composition (delta 34S-SO42-) quantitatively distinguishes natural and anthropogenic sources. This study aimed to constrain the sources of sulfate arriving at the Maldives Climate Observatory Hanimaadhoo (MCOH), which is ideally situated for intercepting the outflow from airsheds over the Indian subcontinent. The results show that anthropogenic sources of sulfate contributed 93 +/- 14 %, 87 +/- 10 %, and 66 +/- 12 % in winter (post-monsoon), spring (pre-monsoon), and summer (monsoon), respectively. There was also a moderate to strong correlation (r2= 0.75, p << 0.05, n=7) between continental anthropogenic (winter and spring) sulfate (delta 34S) and black carbon aerosols from fossil fuel combustion (pinpointed by Delta 14C). This study provides improved constraints on sulfate sources for South Asia - a key region for aerosol pollution and aerosol masking of climate warming.

Elevated methane concentrations are observed in the shallow water column above subsea permafrost on the East Siberian Arctic Shelf, including the inner Laptev Sea. The subsea source of this methane is poorly understood, yet crucial for predicting future methane emissions. Here, we combine analyses of dissolved methane concentrations, triple-isotopic fingerprinting, and Bayesian Markov Chain Monte Carlo statistics to constrain the source of high-concentration methane sampled during four expeditions, 2016-2020. In contrast to earlier findings of predominantly thermogenic methane release in the outer Laptev Sea, our results for the major methane release region of the inner Laptev Sea suggest that old microbial methane (radiocarbon age >48000 y BP) is being released from preformed methane pools stored within the subsea permafrost system. This observation reveals that several sources contribute to the elevated methane on the shelf and thus the necessity to consider a diversity of sources when estimating future methane release trajectories.

Black carbon (BC) aerosols perturb the climate and affect air quality/human health. In the highly populated and heavily polluted South Asian region, the wintertime modeled atmospheric abundance of BC has remained underestimated relative to surface observations. We hypothesize this is linked to underestimated (i) atmospheric lifetime (τBC) and/or (ii) regional emission fluxes of BC. To address this hypothesis, we developed a novel inversion framework combining multiwinter (2018-2020) hourly resolved BC and carbon monoxide (CO) measurements from a wide footprint site in the North Indian Ocean, intercepting wintertime South Asian outflow. The average ΔBC/ΔCO ratio in this continental outflow of 14 ± 5 ng m-3 ppb-1 was 2-3 times higher than in East Asian outflow and shows a profound regional wintertime presence of BC. The empirically derived τBC of 8 ± 0.5 days was higher than global-mean τBC of 5.5 days employed in climate models and suggests greater regional longevity of wintertime BC. The ΔBC/ΔCO inversion-estimated ‘top-down’ BC emission flux of ∼200 Gg/month was in fact higher by a factor of ∼1.5 than wintertime monthly BC emission flux from scaled ‘bottom-up’ emission inventory (∼125 Gg/month). Taken together, assimilating higher BC emissions with greater longevity seems promising to reconcile the model-observation offset of wintertime BC abundance for South Asia.

Atmospheric methane concentration (AMC) surged by ~50% during the last deglaciation, with northern (>30°N) sources accounting for ~40% of the rise. However, hypothesized sources including expanding lakes, peatlands, and destabilized permafrost or hydrates fail to explain this rapid increase. We use biomarkers, isotopes, and radiocarbon data to reconstruct temperature change, methane cycling, and permafrost thaw from a Tibetan thermokarst lake. Radiocarbon evidence and ultradepleted δ13C values (−80.3 per mil) of methane-diagnostic lipids indicate intense cycling of ancient (~2500-year-old) methane during the Younger Dryas–Preboreal transition, coeval with the AMC surge and the most rapid warming. By contrast, methane cycling was weak during the Holocene Climatic Optimum despite peak temperatures. These findings imply that anomalously high rates of warming, rather than absolute temperature alone, may play an important role in triggering enhanced paleo-methane cycling. Rapid warming likely intensified emissions from existing northern lakes, fueling the elusive yet clearly amplified northern methane source that contributed to the deglacial abrupt rise in AMC.

Atmospheric aerosols strongly influence the global climate through their light absorption properties (e.g., black carbon (BC) and brown carbon (BrC)) and scattering properties (e.g., sulfate). This study presents simultaneous measurements of ambient-aerosol light absorption properties and chemical composition obtained at three large-footprint southern Asian receptor sites during the South Asian Pollution Experiment (SAPOEX) from December 2017 to March 2018. The BC mass absorption cross section (BC-MAC678) values increased from 3.5 ± 1.3 at the Bangladesh Climate Observatory at Bhola (BCOB), located at the exit outflow of the Indo-Gangetic Plain, to 6.4 ± 1.3 at two regional receptor observatories, the Maldives Climate Observatory at Hanimaadhoo (MCOH) and the Maldives Climate Observatory at Gan (MCOG), representing an increase of 80 %. This likely reflects a scavenging fractionation, resulting in a population of finer BC with higher MAC678 that has greater longevity. At the same time, BrC-MAC365 decreased by a factor of 3 from the Indo-Gangetic Plain (IGP) exit to the equatorial Indian Ocean, likely due to photochemical bleaching of organic chromophores. The high chlorine-to-sodium ratio at the BCOB, located near the source region, suggests a significant contribution of chorine from anthropogenic activities. Particulate Cl− has the potential to be converted into Cl radicals, which can affect the oxidation capacity of polluted air. Moreover, Cl− is shown to be nearly fully consumed during long-range transport. The results of this synoptic study, conducted on a large southern Asian scale, provide rare observational constraints on the optical properties of ambient BC (and BrC) aerosols over regional scales, away from emission sources. They also contribute significantly to understanding the aging effect of the optical and chemical properties of aerosols as pollution from the Indo-Gangetic Plain disperses over the tropical ocean.

A key challenge in climate change research is apportioning the greenhouse gas methane (CH4) between various natural and anthropogenic sources. Isotopic source fingerprinting of CH4 releases, particularly with radiocarbon analysis, is a promising approach. Here, we establish an analytical protocol for preparing CH4 from seawater and other aqueous matrices for high-precision natural abundance radiocarbon measurement. Methane is stripped from water in the optionally field-operated system (STRIPS), followed by shore-based purification and conversion to carbon dioxide (CO2) in the CH4 Isotope Preparation System (CHIPS) to allow Accelerator Mass Spectrometry analysis. The blank (±1σ) of the combined STRIPS and CHIPS is low (0.67 ± 0.12 μg C), allowing natural sample sizes down to 10 μg C-CH4 (i.e., 30 L samples of 40 nM CH4). The full-system yield is >90% for both CH4-spiked seawater and ambient samples from CH4 hotspots in the Baltic Sea and the Arctic Ocean. Furthermore, the radiocarbon isotope signal of CH4 remains constant through the multistage processing in the STRIPS and the CHIPS. The developed method thus allows for in-field sampling and sample size reduction followed by precise and CH4-specific radiocarbon analysis. This enables powerful source apportionment of CH4 emitted from aquatic systems from the tropics to the polar regions.

Effects of aerosols such as black carbon (BC) on climate and buildup of the monsoon over the Indian Ocean are insufficiently quantified. Uncertain contributions from various natural and anthropogenic sources impede our understanding. Here, we use observations over 5 y of BC and its isotopes at a remote island observatory in northern Indian Ocean to constrain loadings and sources during little-studied monsoon season. Carbon-14 data show a highly variable yet largely fossil (65 ± 15%) source mixture. Combining carbon-14 with carbon-13 reveals the impact of African savanna burning, which occasionally approach 50% (48 ± 9%) of the total BC loadings. The BC mass-absorption cross-section for this regime is 7.6 ± 2.6 m²/g, with higher values during savanna fire input. Taken together, the combustion sources, longevity, and optical properties of BC aerosols over summertime Indian Ocean are different than the more-studied winter aerosol, with implications for chemical transport and climate model simulations of the Indian monsoon.

Light-absorbing Brown Carbon (BrC) aerosols partially offset the overall climate-cooling of aerosols. However, the evolution of BrC light-absorption during atmospheric transport is poorly constrained. Here, we utilize optical properties, ageing-diagnostic delta C-13-BrC and transport time to deduce that the mass absorption cross-section (MACWS-BrC) is decreasing by similar to 50% during long-range oversea transport, resulting in a first-order bleaching rate of 0.24 day(-1) during the 3-day transit from continental East Asia to a south-east Yellow Sea receptor. A modern C-14 signal points to a strong inverse correlation between BrC light-absorption and age of the source material. Combining this with results for South Asia reveals a striking agreement between these two major-emission regions of rapid photobleaching of BrC with a higher intrinsic absorptivity for BrC stemming from biomass burning. The consistency of bleaching parameters constrained independently for the outflows of both East and South Asia indicates that the weakening of BrC light absorption, thus primarily related to photochemical processes rather than sources, is likely a ubiquitous phenomenon.