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Results for “JGR: Biogeosciences”

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27 records · Page 2Linked to original sources
USGS2025

Effects of total suspended solids on photomineralization of dissolved organic matter in the Peace-Athabasca Delta, Canada

Northern deltas receive chromophoric dissolved organic matter (CDOM) from their watersheds, which can be oxidized to carbon dioxide upon absorption of sunlight (i.e., photomineralized). These deltas also receive total suspended solids (TSS), which may shade sunlight absorption by CDOM, thus limiting photomineralization. To quantify this interaction for the first time, we measured photomineralization rates at 11sites in the Peace‐Athabasca Delta (PAD), Canada. We sampled waters during a July 2022 field campaign for TSS concentration, CDOM concentration ( α CDOM ,λ), total downwelling sunlight attenuation coefficients ( K d , tot , λ ), and light attenuation coefficients due to CDOM ( K d , CDOM,λ ). TSS ranged from <1 to 112 mg/L with an average of 19 ± 34 mg/L (mean ± one standard deviation), an order of magnitude lower than TSS reported in rivers entering the PAD earlier in the open water season. α CDOM ,λ at 305 nm ( α CDOM , 305 ) ranged from 23.3 to 65.2 m -1 , K d , CDOM,305 ranged from 26.3 to 74.1 m −1 , and K d , tot,305 ranged from 19.0 to 63.7 m −1 . The ratio of sunlight absorbed by CDOM relative to total sunlight attenuation K d , CDOM,λ/ K d ,tot,λ was inversely correlated with TSS concentration across all wavelengths measured (305–412 nm). TSS thus limited photomineralization rates by shading CDOM from ultraviolet A and visible wavelengths of sunlight, reducing photomineralization rates by up to 56% compared to rates in the absence of TSS or other non-CDOM particles that attenuate sunlight. Results suggest that shifts in delta hydrology that affect TSS concentration likely influence photomineralization rates within TSS-rich northern deltas.

Peace‐Athabasca Delta↗
USGS2025

Evidence of mineral alteration in a salt marsh subterranean estuary: Implications for carbon and trace element cycling

Subterranean estuaries (STE) in salt marshes are biogeochemically active zones where interactions between terrestrial groundwater and seawater drive complex cycling of carbon and trace elements, influenced by mineral dissolution. These systems, characterized by fine-grained organic-rich peat overlying permeable coastal aquifers, play a crucial role as a blue carbon sink, yet their geochemical dynamics remain poorly understood. We investigated dissolved trace elements, carbon, silica, and radium isotopes in a salt marsh STE (Sage Lot Pond, Waquoit Bay, MA) over seasonal and annual cycles. Our results reveal that groundwater and estuarine water circulation through marsh peat and aquifer sediments leads to enrichments of dissolved organic and inorganic carbon (DOC and DIC), Si, Ba, and Mn, with variable source/sink behavior of Fe and net removal of U. Submarine groundwater discharge dominated Ba fluxes, whereas pore water drainage from marsh peat acted as the main sink for U and source of Si. Fe cycling was variable, with terrestrial Fe largely removed as groundwater passed through the STE, consistent with Fe-sulfide and amorphous phase formation. Radium isotope ratios identified two distinct subsurface flow pathways, influenced by metal-oxide cycling and organic matter breakdown. Si production was decoupled from DIC, suggesting Si originates from mineral alteration, whereas DIC results from both mineral weathering and microbial respiration. Silicate mineral alteration, coupled with marsh pore water drainage, accounts for up to 16% of annual DIC exports (66 g C m −2 y −1 ), highlighting the importance of STEs in coastal carbon and trace element cycling, especially as marshes face environmental change.

Massachusetts↗
USGS2025

Exploring the use of non-invasive drone-based Ground Penetrating Radar (GPR) to characterize biogenic gas dynamics in subtropical peat soils

Peat soils are a critical component of the global carbon cycle as natural producers of biogenic greenhouse gases (e.g., methane and carbon dioxide) that accumulate within the soil matrix and are episodically released to the atmosphere. Previous studies have showed the unique ability of ground-based minimally-invasive geophysical methods like ground-penetrating radar (GPR) to characterize carbon dynamics in peat soils. However, the GPR methods are still limited by the scale of measurements and soil disturbance potentially altering biogenic gas releases during deployment. Here we explore the potential of drone-based GPR for identification of hot spots and hot moments of gas accumulation and release in subtropical peat soils. We collected drone-based GPR datasets across two grids (~ 17,500 m2) in the Florida Everglades during January, September, and November of 2023 to characterize peat thickness and seasonal variability of gas content. Results show that drone-based GPR is effective and efficient for: 1) capturing the temporal variation of in-situ biogenic gas content in peat soils with changes between 1-25% volumetric gas content over repeatable grids; 2) inferring peat thickness from discrete measurements ranging between 0.8-1.2 m; and 3) estimating methane flux releases of 63 and 135 mg CH4 m-2 day-1 for specific locations and time periods that are strikingly consistent with our coincident gas trap measurements. This work also indicates that (1) spatial distribution of gas content in the Everglades is strongly controlled by landscape morphology such as ridges and sloughs and (2) the temporal variation of gas content is seasonal with increased gas production during the wet season.

Florida↗
USGS2025

The δ13C signature of dissolved organic and inorganic carbon reveals complex carbon transformations within a salt marsh

Coastal wetlands have high rates of atmospheric CO 2 uptake, which is subsequently respired back to the atmosphere, stored as organic matter within flooded, anoxic soils, or exported to the coastal ocean. Transformation of fixed carbon occurs through a variety of subsurface aerobic and anaerobic microbial processes, and results in a large inventory of dissolved carbon. Carbon source and the roles of aerobic respiration, sulfate reduction, and methane cycling were evaluated within salt marsh peat and the underlying sandy subterranean estuary. There is a large increase in dissolved inorganic carbon (DIC, 7,350 ± 3,900 μmol L −1 ), dissolved organic carbon (DOC, 1,040 ± 1,480 μmol L −1 ) and CH 4 (14.5 ± 33.3 μmol L −1 ) within the marsh porewaters compared to creek waters. Alkalinity production (5,730 ± 2,170 μeq L −1 ) and sulfate removal (1,810 ± 1,970 μmol L −1 ) indicate anaerobic respiration, however, relative contributions from the various decomposition pathways cannot be identified due to overlapping geochemical signatures. The δ 13 C of the DOC (−29.0 ± 3.7‰) and DIC (−11.2 ± 1.1‰) produced within the marsh differed from the bulk soil organic matter δ 13 C (−14.5 ± 0.2‰). We explore a variety of mechanisms that could result in co-occurring depleted δ 13 C-DOC and enriched δ 13 C-DIC compared to the bulk soil organic carbon pool and salt marsh vegetation, including selective mineralization, production of δ 13 C-depleted bacterial biomass, and methane-derived DOC. While important questions remain about carbon cycling pathways, we found evidence of a cryptic methane cycle. Alteration of the δ 13 C of carbon species complicates source attribution in solid and dissolved phases and careful consideration should be used when carbon is partitioned between in situ salt marsh production and external marine and terrestrial sources.

Massachusetts↗
USGS2026

Biophysical controls on sediment erodibility in shallow estuarine embayments

The erodibility of cohesive sediment is known to vary both spatially and temporally but the factors governing its variation are not well understood. We conducted a field investigation of the influence of hydrodynamic forcing, sediment properties, and benthic infauna on erodibility in the muddy shallows of San Pablo and Grizzly Bays in northern San Francisco Bay in summer 2019 and winter 2020. An erosion rate parameter M c was determined from regressions between near-bed vertical turbulent sediment flux, as a proxy for erosion, and bed shear stress due to currents. During each 2-month study period, we measured benthic infauna abundance and dry bulk density, particle size distribution, percent organic carbon, chlorophyll a , pheophytin a , and carbohydrates carbon concentrations of surficial bed sediments five or six times. M c increased with bed shear stress due to waves in both embayments. In San Pablo Bay, erodibility was approximately 50% lower during the winter than the summer deployment, whereas in Grizzly Bay, there was no significant difference. The factor most strongly related to the decrease in M c in San Pablo Bay was increased abundance of the amphipod Ampelisca abdita . The observed reduction in erodibility may occur in many muddy estuaries because A. abdita is broadly distributed in the coastal waters of North America. Erodibility was also directly related to biomass of the invasive clam Potamocorbula amurensis . Erodibility did not depend on dry bulk density: bulk density did not vary seasonally in San Pablo Bay and was lower in winter than summer in Grizzly Bay.

California↗
USGS2025

Wetlands, groundwater and seasonality influence the spatial distribution of stream chemistry in a low-relief catchment

Evaluating stream water chemistry patterns provides insight into catchment ecosystem and hydrologic processes. Spatially distributed patterns and controls of stream solutes are well-established for high-relief catchments where solute flow paths align with surface topography. However, the controls on solute patterns are poorly constrained for low-relief catchments where hydrogeologic heterogeneities and river corridor features, like wetlands, may influence water and solute transport. Here, we provide a data set of solute patterns from 58 synoptic surveys across 28 sites and over 32 months in a low-relief wetland-rich catchment to determine the major surface and subsurface controls along with wetland influence across the catchment. In this low-relief catchment, the expected wetland storage, processing, and transport of solutes is only apparent in solute patterns of the smallest subcatchments. Meanwhile, downstream seasonal and wetland influence on observed chemistry can be masked by large groundwater contributions to the main stream channel. These findings highlight the importance of incorporating variable groundwater contributions into catchment-scale studies for low-relief catchments, and that understanding the overall influence of wetlands on stream chemistry requires sampling across various spatial and temporal scales. Therefore, in low-relief wetland-rich catchments, given the mosaic of above and below ground controls on stream solutes, modeling efforts may need to include both surface and subsurface hydrological data and processes.

Michigan↗
USGS2026

Season and antecedent conditions impact concentration-discharge relationships for dissolved organic carbon and alkalinity in southeast Alaskan watershed

Fluvial export of dissolved carbon plays an important role in watershed-scale biogeochemistry. Predicted changes in climate are expected to impact watershed hydrologic regimes, and in turn, the sources and export of dissolved carbon from watersheds. Here, we utilize high resolution measurements of discharge and dissolved carbon concentration to examine how concentration-discharge (CQ) relationships vary seasonally and during high flow events over the main runoff season (May–October) in a temperate forested watershed in Southeast Alaska. Concentration-discharge relationships for dissolved organic carbon (DOC) and alkalinity demonstrated strong seasonal patterns, with more linear relationships in May and June versus other months. Changing power law model slopes ( b values; the exponent in a power law regression between runoff and carbon yields) indicated potentially shifting watershed sources (biogenic vs. geologic) and contrasting dominant flowpaths (shallow vs. deeper groundwater) for DOC and alkalinity over the sampling period. During the largest storm event of the study, DOC and alkalinity b values shifted from an overall pattern of transport (mean b = 1.58 values >1.0 indicate transport limitation) and source limitation (mean b = 0.48, values <1.0 indicate source limitation) to chemostatic (DOC, b = 0.99; alkalinity, b = 1.019). In June through August, patterns in hysteresis index suggest that CQ relationships were altered when storms followed in close succession to each other. Together, these findings indicate that seasonal and antecedent flow conditions play a role in dissolved carbon export from forested watersheds. Understanding these dynamics, particularly during winter months, will become increasingly important as changes to hydroclimate impact riverine carbon export.

Alaska↗
USGS2026

Mercury cycling across a U.S. semi-arid mountain ecosystem elevation gradient

Mountains comprise ∼30% of the Earth's surface, but mercury (Hg) cycling in these regions remains understudied, particularly in the semi-arid western U.S. where strong climatic and ecological gradients in mountainous landscapes influence Hg deposition, retention, and bioaccumulation. In this study, we quantified growing season inputs, storage, and bioaccumulation of Hg along a ∼2,000 m elevation gradient in the Colorado Rocky Mountains, spanning the plains to the alpine. We measured Hg in atmospheric deposition, vegetation, soil, and 12-day-old chickadees. Accounting for percent canopy cover, open precipitation was the largest component of atmospheric deposition at all elevations, followed by throughfall and litterfall fluxes. Atmospheric Hg fluxes peaked at mid-elevations, likely due to cloud-cap dynamics and denser canopy cover. Total gaseous Hg and precipitation fluxes were highest at low elevations, likely reflecting local emissions and meteorological pooling. Surface soil Hg storage was more strongly predicted by organic matter content ( R 2 = 0.49; p < 0.01) and water retention ( R 2 = 0.45; p < 0.01) than by elevation ( R 2 = 0.21; p < 0.05). Alpine soils (66.3 ± 25.3 ng g −1 ) had significantly higher total Hg concentrations than lower elevations (<41.0 ± 12.7 ng g −1 ; p < 0.01), likely reflecting slower organic matter turnover. Soils on north-facing slopes also retained significantly higher pools of Hg in surface soils compared with south- and east-facing slopes. Vegetation Hg pools were greatest in the alpine region, likely due to long-lived plant species. Methylmercury (MeHg) concentrations in chickadee feathers peaked at mid-elevations (205 ± 155 ng g −1 ), corresponding to higher ecosystem Hg inputs via throughfall. Our results show that deposition, canopy cover, and meteorological conditions—not elevation alone—predict Hg retention and bioaccumulation.

Colorado↗
USGS2026

Effects of wildfire on soil hydraulic properties in the western Oregon Cascades

Wildfires can substantially impact the hydrology of forested watersheds, increasing the risk of hydrologic hazards such as flash floods and debris flows. Soil hydraulic properties related to infiltration are a key control in determining the timing and magnitude of these hydrogeomorphic events. In our study, we collected 445 soil cores from burned (216 cores) and unburned (229 cores) reference catchments and analyzed them for soil hydraulic properties 10 months after the 2022 Cedar Creek Fire in Oregon, USA. We observed significantly greater field-saturated hydraulic conductivity ( K fs ), sorptivity ( S ), and wetting front potential ( Ψ f ) in burned soils relative to unburned soils, with median ratios of 5.7, 4.4, and 5.0, respectively. Among low-, moderate-, and high burn severity groups, soil hydraulic properties were not statistically different. Reductions in median soil bulk density with increasing burn severity suggested an expansion of pore sizes, which may have been partially responsible for increasing K fs and S . Additionally, in some burned soil samples, the increase in soil hydraulic properties may have been partially related to a concurrent reduction in “natural background” water repellency that is characteristic of dry, unburned soils in the Western Cascades. We observed no evidence of spatial autocorrelation in K fs using semivariogram analysis. Principal component analysis paired with a k- means cluster analysis suggested that soil physical properties explained variations in soil hydraulic properties better than landscape attributes. Although there is a lack of regional results for comparison, our results trend in the opposite direction from drier, lower net primary productivity regions that are typically studied for post-wildfire soil hydraulic properties.

Oregon↗