Carbon in thawing permafrost soils may have global impacts on climate change; however, the factors that control its processing and fate are poorly understood. The dominant fate of dissolved organic carbon (DOC) released from soils to inland waters is either complete oxidation to CO2 or partial oxidation and river export to oceans. Although both processes are most often attributed to bacterial respiration, we found that photochemical oxidation exceeds rates of respiration and accounts for 70 to 95% of total DOC processed in the water column of arctic lakes and rivers. At the basin scale, photochemical processing of DOC is about one-third of the total CO2 released from surface waters and is thus an important component of the arctic carbon budget.

Photochemical degradation of dissolved organic matter (DOM) to carbon dioxide (CO2) and partially oxidized compounds is an important component of the carbon cycle in the Arctic. Thawing permafrost soils will change the chemical composition of DOM exported to arctic surface waters, but the molecular controls on DOM photodegradation remain poorly understood, making it difficult to predict how inputs of thawing permafrost DOM may alter its photodegradation. To address this knowledge gap, we quantified the susceptibility of DOM draining the shallow organic mat and the deeper permafrost layer of arctic soils to complete and partial photo-oxidation and investigated changes in the chemical composition of each DOM source following sunlight exposure. Permafrost and organic mat DOM had similar lability to photomineralization despite substantial differences in initial chemical composition. Concurrent losses of carboxyl moieties and shifts in chemical composition during photodegradation indicated that photodecarboxylation could account for 40-90% of DOM photomineralized to CO2. Permafrost DOM had a higher susceptibility to partial photo-oxidation compared to organic mat DOM, potentially due to a lower abundance of phenolic moieties with antioxidant properties. These results suggest that photodegradation will likely continue to be an important control on DOM fate in arctic freshwaters as the climate warms and permafrost soils thaw.

In sunlit waters, photochemical alteration of dissolved organic carbon (DOC) impacts the microbial respiration of DOC to CO2. This coupled photochemical and biological degradation of DOC is especially critical for carbon budgets in the Arctic, where thawing permafrost soils increase opportunities for DOC oxidation to CO2 in surface waters, thereby reinforcing global warming. Here we show how and why sunlight exposure impacts microbial respiration of DOC draining permafrost soils. Sunlight significantly increases or decreases microbial respiration of DOC depending on whether photo-alteration produces or removes molecules that native microbial communities used prior to light exposure. Using high-resolution chemical and microbial approaches, we show that rates of DOC processing by microbes are likely governed by a combination of the abundance and lability of DOC exported from land to water and produced by photochemical processes, and the capacity and timescale that microbial communities have to adapt to metabolize photo-altered DOC.

Following the Deepwater Horizon (DWH) blowout in 2010, oil floated on the Gulf of Mexico for over 100 days. In the aftermath of the blowout, substantial accumulation of partially oxidized surface oil was reported, but the pathways that formed these oxidized residues are poorly constrained. Here we provide five quantitative lines of evidence demonstrating that oxidation by sunlight largely accounts for the partially oxidized surface oil. First, residence time on the sunlit sea surface, where photochemical reactions occur, was the strongest predictor of partial oxidation. Second, two-thirds of the partial oxidation from 2010 to 2016 occurred in less than 10 days on the sunlit sea surface, prior to coastal deposition. Third, multiple diagnostic biodegradation indices, including octadecane to phytane, suggest that partial oxidation of oil on the sunlit sea surface was largely driven by an abiotic process. Fourth, in the laboratory, the dominant photochemical oxidation pathway of DWH oil was partial oxidation to oxygenated residues rather than complete oxidation to CO. Fifth, estimates of partial photo-oxidation calculated with photochemical rate modeling overlap with observed oxidation. We suggest that photo-oxidation of surface oil has fundamental implications for the response approach, damage assessment, and ecosystem restoration in the aftermath of an oil spill, and that oil fate models for the DWH spill should be modified to accurately reflect the role of sunlight.

Please cite this article as: Ward, C.P., Cory, R.M., Chemical composition of dissolved organic matter draining permafrost soils, Geochimica et Cosmochimica Acta (2015), doi: http://dx. AbstractNorthern circumpolar permafrost soils contain roughly twice the amount of carbon stored in the atmosphere today, but the majority of this soil organic carbon is perennially frozen.Climate warming in the Arctic is thawing permafrost soils and mobilizing previously frozen dissolved organic matter (DOM) from deeper soil layers to nearby surface waters. Previous studies have reported that ancient DOM draining deeper layers of permafrost soils was more susceptible to degradation by aquatic bacteria compared to modern DOM draining the shallow active layer of permafrost soils, and have suggested that DOM chemical composition may be an important control for the lability of DOM to bacterial degradation. However, the compositional features that distinguish DOM drained from different depths in permafrost soils are poorly characterized. Thus, the objective of this study was to characterize the chemical composition of DOM drained from different depths in permafrost soils, and relate these compositional differences to its susceptibility to biological degradation. DOM was leached from the shallow organic mat and the deeper permafrost layer of soils within the Imnavait Creek watershed on the North Slope of Alaska. DOM draining both soil layers was characterized in triplicate by coupling ultra-high resolution mass spectrometry, 13 C solid-state NMR, and optical spectroscopy methods with multi-variate statistical analyses. Reproducibility of replicate mass spectra was high, and compositional differences resulting from interfering species or isolation effects were significantly smaller than differences between DOM drained from each soil layer. All analyses indicated that DOM leached from the shallower organic mat contained higher molecular weight, more oxidized, and more unsaturated aromatic species compared to DOM leached from the deeper permafrost layer. Bacterial production rates and bacterial efficiencies were significantly higher for permafrost compared to organic mat DOM; however, respiration rates were similar 3 between DOM sources. Increased release of permafrost DOM from arctic soils to surface waters will change the chemical composition of DOM and its lability to bacteria, but this study suggests that these shifts in DOM composition and lability may not increase the carbon dioxide produced by bacterial respiration of permafrost DOM exported to arctic surface waters compared to DOM currently draining the shallow active layer.4

Numerous international governmental agencies that steer policy assume that polystyrene persists in the environment for millennia. Here, we show that polystyrene is completely photochemically oxidized to carbon dioxide and partially photochemically oxidized to dissolved organic carbon. Lifetimes of complete and partial photochemical oxidation are estimated to occur on centennial and decadal time scales, respectively. These lifetimes are orders of magnitude faster than biological respiration of polystyrene and thus challenge the prevailing assumption that polystyrene persists in the environment for millennia. Additives disproportionately altered the relative susceptibility to complete and partial photochemical oxidation of polystyrene and accelerated breakdown by shifting light absorbance and reactivity to longer wavelengths. Polystyrene photochemical oxidation increased approximately 25% with a 10 °C increase in temperature, indicating that temperature is unlikely to be a primary driver of photochemical oxidation rates. Collectively, sunlight exposure appears to be a governing control of the environmental persistence of polystyrene, and thus, photochemical loss terms need to be included in mass balance studies on the environmental fate of polystyrene. The experimental framework presented herein should be applied to a diverse array of polymers and formulations to establish how general these results are for other plastics in the environment.

Increasing wildfire activity in the Alaskan Arctic may result in new sources of black carbon (BC) to arctic watersheds. Black carbon, primarily comprised of condensed aromatics, is one of the most chemically recalcitrant fractions of organic carbon. However, lateral transfer of particulate and dissolved BC from soils to sunlit surface waters is increasingly suggested to result in the photochemical mineralization of BC to CO₂. While sunlight can also partially photooxidize aromatic compounds in surface waters, producing compounds with a higher O/C than the parent compound, this degradation pathway has not yet been identified for either particulate or dissolved BC. To address knowledge gaps on the photochemical degradation of particulate and dissolved BC, we quantified the complete and partial photooxidation of particulate and dissolved BC derived from arctic biomass as photochemical CO₂ production and O₂ consumption relative to dark controls. Concurrently, we investigated shifts in the chemical composition of dissolved BC following exposure to sunlight using UV-visible absorbance, fluorescence spectroscopy, and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The chemical and physical properties of BC produced from charring arctic biomass were similar to BC produced by wildfires in terrestrial ecosystems based on elemental analysis and FT-ICR MS. Based on the concentration of light-absorbing carbon in each fraction, dissolved BC was disproportionately more susceptible to complete and partial photooxidation compared to particulate BC. Upon exposure to sunlight, the predominant fate of dissolved BC was partial photooxidation, while a smaller fraction of dissolved BC was photomineralized to CO₂. Shifts in both the optical and mass spectrometry spectra suggested that condensed aromatics likely comprised the fraction of dissolved BC that was completely and partially photooxidized. To further refine the meaning of sunlight as a sink for aquatic BC, the reactivity of partially oxidized photoproducts of BC in the aquatic organic carbon pool must be determined.

Chemical dispersants are one of many tools used to mitigate the overall environmental impact of oil spills. In principle, dispersants break up floating oil into small droplets that disperse into the water column where they are subject to multiple fate and transport processes. The effectiveness of dispersants typically decreases as oil weathers in the environment. This decrease in effectiveness is often attributed to evaporation and emulsification, with the contribution of photochemical weathering assumed to be negligible. Here, we aim to test this assumption using Macondo well oil released during the Deepwater Horizon spill as a case study. Our results indicate that the effects of photochemical weathering on Deepwater Horizon oil properties and dispersant effectiveness can greatly outweigh the effects of evaporative weathering. The decrease in dispersant effectiveness after light exposure was principally driven by the decreased solubility of photo-oxidized crude oil residues in the solvent system that comprises COREXIT EC9500A. Kinetic modeling combined with geospatial analysis demonstrated that a considerable fraction of aerial applications targeting Deepwater Horizon surface oil had low dispersant effectiveness. Collectively, the results of this study challenge the paradigm that photochemical weathering has a negligible impact on the effectiveness of oil spill response and provide critical insights into the "window of opportunity" to apply chemical dispersants in response to oil spills in sunlit waters.