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.
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Assessing the Role of Photochemistry in Driving the Composition of Dissolved Organic Matter in Glacier Runoff
Dissolved organic matter (DOM) in glacier runoff is aliphatic‐rich, yet studies have proposed that DOM originates mainly from allochthonous, aromatic, and often aged material. Allochthonous organic matter (OM) is exposed to ultraviolet radiation both in atmospheric transport and post‐deposition on the glacier surface. Thus, we evaluate photochemistry as a mechanism to account for the compositional disconnect between allochthonous OM sources and glacier runoff DOM composition. Six endmember OM sources (including soils and diesel particulate matter) were leached and photo‐irradiated for 28 days in a solar simulator, until >90% of initial chromophoric DOM was removed. Ultrahigh‐resolution mass spectrometry was used to compare the molecular composition of endmember leachates pre‐ and post‐irradiation to DOM in supraglacial and bulk runoff from the Greenland Ice Sheet and Juneau Icefield (Alaska), respectively. Photo‐irradiation drove molecular level convergence between the initially aromatic‐rich leachates and aromatic‐poor glacial samples, selectively removing aromatic compounds (−80 ± 19% relative abundance) and producing aliphatics (+75 ± 35% relative abundance). Molecular level glacier runoff DOM composition was statistically indistinguishable to post‐irradiation leachates. Bray‐Curtis analysis showed substantial similarity in the molecular formulae present between glacier samples and post‐irradiation leachates. Post‐irradiation leachates contained 84 ± 7.4% of the molecular formulae, including 72 ± 17% of the aliphatic formulae, detected in glacier samples. Our findings suggest that photodegradation, either in transit to or on glacier surfaces, could provide a mechanistic pathway to account for the disconnect between proposed aromatic, aged sources of OM and the aliphatic‐rich fingerprint of glacial DOM.
more » « less- Award ID(s):
- 1757348
- NSF-PAR ID:
- 10448394
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Biogeosciences
- Volume:
- 126
- Issue:
- 12
- ISSN:
- 2169-8953
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract 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.
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Abstract Glaciers and ice sheets cover over 10 % of Earth's land surface area and store a globally significant amount of dissolved organic matter (DOM), which is highly bioavailable when exported to proglacial environments. Recent rapid glacier mass loss is hypothesized to have increased fluxes of DOM from these environments, yet the molecular composition of glacially derived DOM has only been studied for a handful of glaciers. We determine DOM composition using ultrahigh resolution mass spectrometry from a diverse suite of Arctic glacial environments, including time series sampling from an ice sheet catchment in Greenland (Russell Glacier) and outflow from valley glaciers in catchments with varying degrees of glacial cover in Svalbard. Samples from the Greenland outflow time series exhibited a higher degree of similarity than glacier outflow between glaciers in Svalbard; however, supraglacial meltwater samples from Greenland and Svalbard were more similar to each other than corresponding glacial outflow. Outflow from Russell Glacier was enriched in polyphenolic formulae, potentially reflecting upstream inputs from plants and soils, or inputs from paleosols overridden by the ice sheet, whereas Svalbard rivers exhibited a high level of molecular richness and dissimilarity between sites. When comparing DOM compositional analyses from other aquatic systems, aliphatic, and peptide‐like formulae appear particularly abundant in supraglacial meltwater, suggesting the DOM quickly metabolized in previous incubations of glacial water originates from energy‐rich supraglacial sources. Therefore, as glaciers lose mass across the region, higher‐quality fuel for microbial degradation will increase heterotrophy in coastal systems with ramifications for carbon cycling.
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Abstract River organic matter transformations impact the cycling of energy, carbon, and nutrients. The delivery of distinct dissolved organic matter (DOM) sources can alter aquatic DOM cycling and associated biogeochemical processes. Yet DOM source and reactivity are not well‐defined for many river systems, including in western Canada. Here, we explore DOM cycling in the mainstem of the Oldman River (stream order 6–7), a heavily regulated river network in southern Alberta (Canada). We compared seasonal river DOM content, composition, and bioavailability with nine endmember leachates from the river valley using optical properties and incubations to estimate biodegradable dissolved organic carbon (BDOC). River DOM composition was most similar to terrestrial soil leachates, followed by autochthonous DOM leachates. River DOM bioavailability was low (BDOC = 0%–16.6%, mean of 7.1%). Endmember leachate bioavailability increased from soils (BDOC = 23.9%–53.7%), to autochthonous sources (fish excretion, macrophytes, biofilm; BDOC = 49.9%–80.0%), to terrestrial vegetation (leaves, shrubs, grass; BDOC > 80%), scaling positively with protein‐like DOM content and amount of leachable dissolved organic carbon (DOC), and negatively with aromaticity. Seasonally, DOC concentrations changed little despite >15‐fold increases in discharge during spring. River DOM composition shifted modestly toward soil‐like endmembers in spring and more bioavailable autochthonous end members in autumn and winter. Low DOM bioavailability in the river mainstem and low DOC yields shown in previous work point to limited internal processing of DOM and limited bioavailable DOM delivery to downstream habitats, possibly due to upstream flow regulation. Our observations provide important insights into the functioning of western Canadian aquatic networks.
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Abstract Most terrestrial allochthonous organic matter enters river networks through headwater streams during high flow events. In headwaters, allochthonous inputs are substantial and variable, but become less important in streams and rivers with larger watersheds. As allochthonous dissolved organic matter (DOM) moves downstream, the proportion of less aromatic organic matter with autochthonous characteristics increases. How environmental factors converge to control this transformation of DOM at a continental scale is less certain. We hypothesized that the amount of time water has spent travelling through surface waters of inland systems (streams, rivers, lakes, and reservoirs) is correlated to DOM composition. To test this hypothesis, we used established river network scaling relationships to predict relative river network flow‐weighted travel time (FWTT) of water for 60 stream and river sites across the contiguous United States (3090 discrete samples over 10 water years). We estimated lentic contribution to travel times with upstream in‐network lake and reservoir volume. DOM composition was quantified using ultraviolet and visible absorption and fluorescence spectroscopy. A combination of FWTT and lake and reservoir volume was the best overall predictor of DOM composition among models that also incorporated discharge, specific discharge, watershed area, and upstream channel length. DOM spectral slope ratio (R2 = 0.77) and Freshness Index (R2 = 0.78) increased and specific ultraviolet absorbance at 254 nm (R2 = 0.68) and Humification Index (R2 = 0.44) decreased across sites as a function of FWTT and upstream lake volume. This indicates autochthonous‐like DOM becomes continually more dominant in waters with greater FWTT. We assert that river FWTT can be used as a metric of the continuum of DOM composition from headwaters to rivers. The nature of the changes to DOM composition detected suggest this continuum is driven by a combination of photo‐oxidation, biological processes, hydrologically varying terrestrial subsidies, and aged groundwater inputs.
