Groundwater comprises 95% of the liquid fresh water on Earth and contains a diverse mix of dissolved organic matter (DOM) molecules which play a significant role in the global carbon cycle. ...Currently, the storage times and degradation pathways of groundwater DOM are unclear, preventing an accurate estimate of groundwater carbon sources and sinks for global carbon budgets. Here we reveal the transformations of DOM in aging groundwater using ultra-high resolution mass spectrometry combined with radiocarbon dating. Long-term anoxia and a lack of photodegradation leads to the removal of oxidised DOM and a build-up of both reduced photodegradable formulae and aerobically biolabile formulae with a strong microbial signal. This contrasts with the degradation pathway of DOM in oxic marine, river, and lake systems. Our findings suggest that processes such as groundwater extraction and subterranean groundwater discharge to oceans could result in up to 13 Tg of highly photolabile and aerobically biolabile groundwater dissolved organic carbon released to surface environments per year, where it can be rapidly degraded. These findings highlight the importance of considering groundwater DOM in global carbon budgets.
Dissolved organic matter (DOM) in groundwater is fundamentally important with respect to biogeochemical reactions, global carbon cycling, heavy metal transport, water treatability and potability. One ...source of DOM to groundwater is from the transport of organic matter from the vadose zone by rainfall recharge. Changes in precipitation patterns associated with natural climate variability and climate change are expected to alter the load and character of organic matter released from these areas, which ultimately impacts on groundwater quality and DOM treatability. In order to investigate potential changes in groundwater DOM character after rainfall recharge, we sampled shallow groundwater from a coastal peat-rich sand aquifer in New South Wales, Australia, during an extended period of low precipitation (average daily precipitation rate < 1.6 mm day−1 over the 8 months prior to sampling), and after two heavy precipitation events (84 mm day−1 and 98 mm day−1 respectively). We assess changes in DOM composition after correcting for dilution by a novel combination of two advanced analytical techniques: liquid chromatography organic carbon detection (LC-OCD) and negative-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). We also assess changes in water chemistry pre- and post-rainfall. Post-rainfall, we show that the dilution-corrected amount of highly aromatic DOM molecular formulae (i.e. those categorised into the groups polyphenolics and condensed aromatics) were 1.7 and 2.0 times higher respectively than in pre-rainfall samples. We attribute this to the flushing of peat-derived DOM from buried organic material into the groundwater. We also identify that periods of low precipitation can lead to low hydrophilic/HOC ratios in groundwater (median = 4.9, n = 14). Redundancy analysis (RDA) was used to compare the HOC fraction with FT-ICR MS compound groups. We show that HOC has a more aromatic character in pre-rainfall samples, and is less similar to the aromatic groups in post-rainfall samples. This suggests that the decline in water-borne hydrophobics observed post-rainfall could be associated with preferential adsorption of the hydrophobic aromatic DOM, making post-rainfall samples less treatable for potable water supply. Post-rainfall we also observe significant increases in arsenic (leading to concentrations greater than 3 times the World Health Organisation drinking water limit of 10 μg / L). Increases in coastal rainfall due to climate change may therefore alter the composition of groundwater DOM in coastal peatland areas in ways that may impact DOM bioavailability, and increase arsenic concentrations, reducing the ease of water treatment for human consumption. To the best of our knowledge, this is the first study to identify the chemical and molecular changes of shallow groundwater DOM pre-rainfall and post-rainfall in a sedimentary organic carbon rich environment through multiple analytical techniques.
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•We assess changes in groundwater DOM pre- and post-rainfall using LC-OCD and FT-ICR MS.•We identify the leaching of sedimentary organic carbon into groundwater post-rainfall.•Periods of low precipitation can lead to low hydrophilic/hydrophobic organic carbon ratios in groundwater.•We identify the preferential adsorption of aromatic hydrophobic organic matter after rainfall.•Increased coastal rainfall may make water treatment more difficult and expensive in the future.
Biospheric particulate organic carbon (POCbio) burial and rock petrogenic particulate organic carbon (POCpetro) oxidation are opposing long‐term controls on the global carbon cycle, sequestering and ...releasing carbon, respectively. Here, we examine how watershed glacierization impacts the POC source by assessing the concentration and isotopic composition (δ13C and Δ14C) of POC exported from four watersheds with 0%–49% glacier coverage across a melt season in Southeast Alaska. We used two mixing models (age‐weight percent and dual carbon isotope) to calculate concentrations of POCbio and POCpetro within the bulk POC pool. The fraction POCpetro contribution was highest in the heavily glacierized watershed (age‐weight percent: 0.39 ± 0.05; dual isotope: 0.42 (0.37–0.47)), demonstrating a glacial source of POCpetro to fjords. POCpetro was mobilized via glacier melt and subglacial flow, while POCbio was largely flushed from the non‐glacierized landscape by rain. Flow normalized POCbio concentrations exceeded POCpetro concentrations for all streams, but surprisingly were highest in the heavily glacierized watershed (mean: 0.70 mgL−1; range 0.16–1.41 mgL−1), suggesting that glacier rivers can contribute substantial POCbio to coastal waters. Further, the most heavily glacierized watershed had the highest sediment concentration (207 mgL−1; 7–708 mgL−1), and thus may facilitate long‐term POCbio protection via sediment burial in glacier‐dominated fjords. Our results suggest that continuing glacial retreat will decrease POC concentrations and increase POCbio:POCpetro exported from currently glacierized watersheds. Glacier retreat may thus decrease carbon storage in marine sediments and provide a positive feedback mechanism to climate change that is sensitive to future changes in POCpetro oxidation.
Plain Language Summary
Particulate organic carbon (POC) in rivers can come from bedrock and the biosphere. When POC produced by vegetation and soils gets buried in fjords, it can be stored for hundreds of thousands of years. In contrast, when bedrock erodes and rock organic carbon reacts with oxygen, carbon previously stored long term can enter the atmosphere as carbon dioxide. Thus, understanding the balance between these two sources of POC and their fates can help predict future carbon cycling. Here, we study POC in glacial rivers, where bedrock erosion produces rock derived POC and where soil and forests produce biospheric POC. We show how much POC comes from each source across the glacial melt season in four rivers in Southeast Alaska whose watershed range in percent glacier coverage from 0% to 49%. The watershed with the largest glacier coverage produces the most rock‐derived POC, providing evidence of the glacial source of rock‐derived carbon that can occur in Southeast Alaskan fjords. However, the most glacial watershed also carries the highest concentration of POC from the biosphere and the most sediment, suggesting that glacier rivers play an outsized role in carrying both rock‐ and plant‐derived carbon to fjords where its burial can help mitigate climate change.
Key Points
Carbon isotope data show a petrogenic source contributed to glacier river particulate organic carbon (POC) entering Southeast Alaska fjords
Glacier erosion and melt increased the river petrogenic POC:biospheric POC ratio, while non‐glacial runoff lowered the ratio
Heavily glacierized rivers can have high biospheric POC and sediment concentrations, implying high carbon burial potential in fjord sediments
Arctic rivers provide an integrated signature of the changing landscape and transmit signals of change to the ocean. Here, we use a decade of particulate organic matter (POM) compositional data to ...deconvolute multiple allochthonous and autochthonous pan-Arctic and watershed-specific sources. Constraints from carbon-to-nitrogen ratios (C:N), δ
C, and Δ
C signatures reveal a large, hitherto overlooked contribution from aquatic biomass. Separation in Δ
C age is enhanced by splitting soil sources into shallow and deep pools (mean ± SD: -228 ± 211 vs. -492 ± 173‰) rather than traditional active layer and permafrost pools (-300 ± 236 vs. -441 ± 215‰) that do not represent permafrost-free Arctic regions. We estimate that 39 to 60% (5 to 95% credible interval) of the annual pan-Arctic POM flux (averaging 4,391 Gg/y particulate organic carbon from 2012 to 2019) comes from aquatic biomass. The remainder is sourced from yedoma, deep soils, shallow soils, petrogenic inputs, and fresh terrestrial production. Climate change-induced warming and increasing CO
concentrations may enhance both soil destabilization and Arctic river aquatic biomass production, increasing fluxes of POM to the ocean. Younger, autochthonous, and older soil-derived POM likely have different destinies (preferential microbial uptake and processing vs. significant sediment burial, respectively). A small (~7%) increase in aquatic biomass POM flux with warming would be equivalent to a ~30% increase in deep soil POM flux. There is a clear need to better quantify how the balance of endmember fluxes may shift with different ramifications for different endmembers and how this will impact the Arctic system.
Coastal erosion mobilizes large quantities of organic matter (OM) to the Arctic Ocean where it may fuel greenhouse gas emissions and marine production. While the biodegradability of ...permafrost‐derived dissolved organic carbon (DOC) has been extensively studied in inland soils and freshwaters, few studies have examined dissolved OM (DOM) leached from eroding coastal permafrost in seawater. To address this knowledge gap, we sampled three horizons from bluff exposures near Drew Point, Alaska: seasonally thawed active layer soils, permafrost containing Holocene terrestrial and/or lacustrine OM, and permafrost containing late‐Pleistocene marine‐derived OM. Samples were leached in seawater to compare DOC yields, DOM composition (chromophoric DOM, Fourier transform ion cyclotron resonance mass spectrometry), and biodegradable DOC (BDOC). Holocene terrestrial permafrost leached the most DOC compared to active layer soils and Pleistocene marine permafrost. However, DOC from Pleistocene marine permafrost was the most biodegradable (33 ± 6% over 90 days), followed by DOC from active layer soils (23 ± 5%) and Holocene terrestrial permafrost (14 ± 3%). Permafrost leachates contained relatively more aliphatic and peptide‐like formulae, whereas active layer leachates contained relatively more aromatic formulae. BDOC was positively correlated with nitrogen‐containing and aliphatic formulae, and negatively correlated with polyphenolic and condensed aromatic formulae. Using estimates of eroding OM, we scale our results to estimate DOC and BDOC inputs to the Alaska Beaufort Sea. While DOC inputs from coastal erosion are relatively small compared to rivers, our results suggest that erosion may be an important source of BDOC to the Beaufort Sea when river inputs are low.
Plain Language Summary
Arctic coastlines are rapidly eroding into the ocean. Soils along these coastlines contain large quantities of organic matter (OM) that dissolves in seawater and may be consumed by microbes. If this dissolved organic matter (DOM) is biodegradable, it can be an important energy source to coastal food webs and/or quickly decomposed to greenhouse gases. We used laboratory experiments to examine the chemical composition of the DOM that is released from eroding soils into seawater and measured its biodegradability. We compared results for three different layers of soil at Drew Point, Alaska, including near‐surface soils that thaw seasonally and deeper soils that are perennially frozen (permafrost). Our results show that different layers, which contain OM of different sources and ages, have distinct chemical characteristics that impact biodegradability. While rivers supply more OM to the Alaska Beaufort Sea than coastal erosion, our results show that DOM released from all soil layers is highly biodegradable, and that DOM from deep permafrost is the most biodegradable. We deomonstrate that coastal erosion can be an important source of OM to Arctic coastal ecosystems, particularly in locations and seasons (e.g., late summer) that receive fewer river inputs.
Key Points
Eroding soils and permafrost along the Alaska Beaufort Sea coast leach biodegradable dissolved organic carbon into seawater
Biodegradability was higher in leachates from Late‐Pleistocene relict marine permafrost than Holocene terrestrial soils and permafrost
Permafrost leached aliphatic and peptide‐like molecular formulae that were not present or less abundant in active layer soils
Stream water carbon concentrations can be highly dynamic on the time scales of both individual storm events and seasonal hydroclimatic shifts. We collected stream water daily over a 6‐day storm from ...three headwater subcatchments of varying landcover (poor fen, forested wetland, and upland forest) and the catchment outlet to evaluate how precipitation events impact the concentration and speciation of carbon (organic vs. inorganic) as well as the composition of dissolved organic matter (DOM) exported laterally from coastal temperate rainforest catchments. Dissolved and particulate organic carbon concentrations increased during the storm at all sites, while dissolved inorganic carbon concentrations were diluted during peak flows. These results highlight the importance of quantifying all forms of lateral carbon export when evaluating the role of storms in catchment‐scale carbon cycling. Isotopic hydrograph separation using stream water δ18O showed that percent new water was significantly related to carbon concentration and form providing a clear link between stream water sources (i.e., recent event water) and soil carbon source areas that become connected to surface water during storms. Furthermore, ultrahigh‐resolution mass spectrometry showed that stream water DOM exported from the upland forest contained the greatest molecular diversity of the three landscape types and had the largest changes in composition over the storm suggesting that the wetland‐dominated subcatchments were less compositionally diverse with regard to soil DOM pools active during the storm. Overall, this study provides insight into hydro‐biogeochemical drivers that control lateral carbon export from forested catchments in a region where an increasing fraction of precipitation is falling as rain.
Plain Language Summary
Streams transport large amounts of terrestrially derived carbon to the ocean, especially during large rainstorms. We collected water samples daily over a 6‐day storm from small drainage areas of varying landcover to see how the concentration and type of carbon changed over the course of a storm. Our results show that the amount and type of carbon in the stream changed dramatically during the storm and originated from different areas of the landscape. The flow of water through the soil also changed during the storm and was related to the type and amount of carbon entering the stream. Storm events not only impact carbon entering the stream but also may impact its transfer to coastal marine ecosystems. Climate in the study region is projected to become warmer and wetter in coming decades. These shifts in climate could lead to more carbon export during storms, especially during winter because of more precipitation falling as rain rather than snow.
Key Points
During storms, the carbon species entering the stream change reflecting shifts in the terrestrial flowpaths contributing to streamflow
Percent new water was related to carbon concentration and form providing a clear link between stream sources and soil carbon source pools
Ultrahigh‐resolution mass spectrometry showed stream DOM molecular properties draining the upland forest converging on those of the wetlands
Groundwater organic matter is processed within aquifers through transformations such as the adsorption of dissolved organic matter (DOM) to minerals and biodegradation. The molecular character of DOM ...varies according to its source and this can impact its bioavailability and reactivity. Whilst the character of DOM in riverine and oceanic environments is increasingly well understood, the sources, character and ultimately the fate of groundwater DOM remains unclear. Here we examine groundwater DOM from contrasting hydrogeological settings in New South Wales, Australia. For the first time, we identify the distinct molecular composition of three groundwater DOM end-members including a modern terrestrial input, an aged sedimentary peat source, and an aged stable by-product pool. We also identify and characterise the processing pathway of DOM in semi-arid, low sedimentary organic carbon (OC) environments. Based on size exclusion chromatography, ultrahigh-resolution Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS), isotopic analyses (13C, 14C and 3H) and principle component analysis (PCA), we show that in higher rainfall temperate coastal peatland environments, large amounts of aged sedimentary organic carbon can leach into groundwater resulting in higher molecular weight (500 g mol−1 < molecular weight > 1000 g mol−1) and highly aromatic groundwater DOM with high O/C ratios and low H/C ratios. We show that in semi-arid environments with low rainfall rates and high groundwater residence times, groundwater dissolved organic carbon (DOC) is processed into increasingly low molecular weight (<350 g mol−1), low aromaticity DOM with low O/C ratios and high H/C ratios by subsurface processing mechanisms such as biodegradation and adsorption. We provide the first comprehensive study of groundwater DOM characterisation based on multiple analytical techniques, and highlight the impact of source inputs and processing on groundwater DOM composition at a molecular level.
Climate change is decreasing watershed glacial coverage throughout Alaska, impacting the biogeochemistry of downstream ecosystems. We collected streamwater fortnightly over the glacial runoff period ...from three streams of varying watershed glacier coverage (0–49%) and a subglacial outflow to assess how glacier recession impacts the relative contributions of glacier and terrestrial plant derived dissolved organic matter (DOM) inputs to streams. We show an increase in the fraction of old dissolved organic carbon (up to ∼ 3200 yr old radiocarbon age) with increasing glacial meltwater contribution to streamflow. We use a dual isotopic mixing model (δ13C and Δ14C) to quantify the relative contribution of terrestrial and glacial sources to streamwater DOM. The endmember contributions were further compared to DOM molecular compositional data from Fourier‐transform ion cyclotron resonance mass spectrometry to assess whether DOM composition can be linked to streamwater DOM source in watersheds with varying contributions of glacial runoff. This approach revealed the glacial fraction was positively correlated with percent relative abundance of heteroatom‐containing DOM molecular formulae, aliphatics, and peptide‐like formulae, while the terrestrial fraction was positively correlated with condensed aromatics and polyphenolics. These results provide information about how the retreat of mountain glaciers will impact the composition and thus biogeochemical role of DOM delivered to downstream ecosystems. Our findings highlight that combining an isotopic mixing model and ultrahigh resolution mass spectrometry data can provide novel insights into how changes in watershed landcover impact the source and chemical properties of streamwater DOM.
Climate change is decreasing watershed glacial coverage throughout Alaska, impacting the biogeochemistry of downstream ecosystems. We collected streamwater fortnightly over the glacial runoff period ...from three streams of varying watershed glacier coverage (0-49%) and a subglacial outflow to assess how glacier recession impacts the relative contributions of glacier and terrestrial plant derived dissolved organic matter (DOM) inputs to streams. We show a decrease in the fraction of modern dissolved organic carbon (up to ~3,200 years old radiocarbon age) with increasing glacial meltwater contribution to streamflow. We use a dual isotopic mixing model (δ13C and Δ14C) to quantify the relative contribution of terrestrial and glacial sources to streamwater DOM. The endmember contributions were further compared to DOM molecular compositional data from Fourier-transform ion cyclotron resonance mass spectrometry to assess whether DOM composition can be linked to streamwater DOM source in watersheds with varying contributions of glacial runoff. This approach revealed the glacial fraction was positively correlated with percent relative abundance of heteroatom containing DOM molecular formulae, aliphatics, and peptide like formulae, while the terrestrial fraction was positively correlated with condensed aromatics and polyphenolics. These results provide information about how the retreat of mountain glaciers will impact the composition and thus biogeochemical role of DOM delivered to downstream ecosystems. Our findings highlight that combining a traditional isotopic mixing model and ultrahigh resolution mass spectrometry data can provide novel insights into how changes in watershed landcover impact the source and chemical properties of streamwater DOM.
Black applicants are 13% less likely than white applicants to receive research funding from the National Institutes of Health (NIH) 22. ...the benefits of our research are not distributed equally; ...since the large majority of research subjects and tissue donors are white (even for many diseases that disproportionately affect BIPOC), biomedical research often has reduced relevance for BIPOC 29,30. When healthcare professionals are taught to identify medical conditions by the presence of rashes, skin becoming pale, or lips turning blue, BIPOC patients may be overlooked in initial screenings; their quality of care is lower even before treatment because their symptoms are less likely to be recognized 33. Racial bias in medicine not only exacerbates distrust of biomedical research, but also entrenches systemic healthcare disparities between racial and ethnic groups 34,35.
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