Porewater Inputs Research Articles

A Carbon Source in a Carbon Sink: Carbon Dioxide and Methane Dynamics in Open‐Water Peatland Pools

AbstractPeatlands store organic carbon available for decomposition and transfer to neighboring water bodies, which can ultimately generate carbon dioxide (CO2) and methane (CH4) emissions. The objective of this study was to clarify the biogeochemical functioning of open‐water peatland pools and their influence on carbon budgets at the ecosystem and global scale. Continuously operated automated equipment and monthly manual measurements were used to describe the CO2 and CH4 dynamics in boreal ombrotrophic peatland pools and porewater (Québec, Canada) over the growing seasons 2019 and 2020. The peat porewater stable carbon isotope ratios (δ13C) for both CO2 (median δ13C‐CO2: −3.8‰) and CH4 (median δ13C‐CH4: −64.30‰) suggested that hydrogenotrophic methanogenesis was the predominant degradation pathway in peat. Open‐water pools were supersaturated in CO2 and CH4 and received most of these dissolved carbon greenhouse gases (C‐GHG) from peat porewater input. Throughout the growing season, higher CO2 concentrations and fluxes in pools were measured when the water table was low—suggesting a steady release of CO2 from deep peat porewater. Higher CH4 ebullition and diffusion occurred in August when bottom water and peat temperatures were the highest. While this study demonstrates that peatland pools are chimneys of CO2 and CH4 stored in peat, it also shows that the C‐GHG concentrations and flux rates in peat pools are comparable to other aquatic systems of the same size. Although peatlands are often considered uniform entities, our study highlights their biogeochemical heterogeneity, which, if considered, substantially influences their net carbon balance with the atmosphere.

Novel sulfur isotope analyses constrain sulfurized porewater fluxes as a minor component of marine dissolved organic matter

Marine dissolved organic matter (DOM) is a major reservoir that links global carbon, nitrogen, and phosphorus. DOM is also important for marine sulfur biogeochemistry as the largest water column reservoir of organic sulfur. Dissolved organic sulfur (DOS) can originate from phytoplankton-derived biomolecules in the surface ocean or from abiotically "sulfurized" organic matter diffusing from sulfidic sediments. These sources differ in 34S/32S isotope ratios (δ34S values), with phytoplankton-produced DOS tracking marine sulfate (21‰) and sulfurized DOS mirroring sedimentary porewater sulfide (∼0 to -10‰). We measured the δ34S values of solid-phase extracted (SPE) DOM from marine water columns and porewater from sulfidic sediments. Marine DOMSPE δ34S values ranged from 14.9‰ to 19.9‰ and C:S ratios from 153 to 303, with lower δ34S values corresponding to higher C:S ratios. Marine DOMSPE samples showed consistent trends with depth: δ34S values decreased, C:S ratios increased, and δ13C values were constant. Porewater DOMSPE was 34S-depleted (∼-0.6‰) and sulfur-rich (C:S ∼37) compared with water column samples. We interpret these trends as reflecting at most 20% (and on average ∼8%) contribution of abiotic sulfurized sources to marine DOSSPE and conclude that sulfurized porewater is not a main component of oceanic DOS and DOM. We hypothesize that heterogeneity in δ34S values and C:S ratios reflects the combination of sulfurized porewater inputs and preferential microbial scavenging of sulfur relative to carbon without isotope fractionation. Our findings strengthen links between oceanic sulfur and carbon cycling, supporting a realization that organic sulfur, not just sulfate, is important to marine biogeochemistry.

Mangroves as a Source of Greenhouse Gases to the Atmosphere and Alkalinity and Dissolved Carbon to the Coastal Ocean: A Case Study From the Everglades National Park, Florida

AbstractMost research evaluating the potential of mangroves as a sink for atmospheric carbon has focused on carbon burial in sediments. However, the few studies that have quantified lateral exchange of carbon and alkalinity indicate that the dissolved carbon and alkalinity export may be several‐fold more important than burial. This study aims to investigate rates and drivers of alkalinity, dissolved carbon, and greenhouse gas fluxes of the mangrove‐dominated Shark River estuary located in the Everglades National Park in Florida, USA. Spatial surveys and 29‐hr time series were conducted to assess total alkalinity (TAlk), organic alkalinity (OAlk), dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) dynamics. Dissolved carbon and greenhouse gas concentrations were coupled to porewater input, which was examined using radon‐222. Shark River was a source of CO2 (92 mmol/m2/day), CH4 (56 μmol/m2/day), and N2O (2 μmol/m2/day) to the atmosphere. Dissolved carbon export (DIC = 142 mmol/m2/day, DOC = 39 mmol/m2/day, normalized to mangrove area) was several‐fold higher than previously reported carbon burial rates in the study area (~28 mmol/m2/day). The majority of the DIC was exported as TAlk (97 mmol/m2/day), which remains dissolved in the ocean for millennia and, therefore, represents a long‐term sink for atmospheric carbon. By integrating our results with previous studies, we argue that alkalinity, dissolved carbon, and greenhouse gas fluxes should be considered in future blue carbon budgets.

The role of porewater exchange as a driver of CO2 flux to the atmosphere in a temperate estuary (Squamish, Canada)

Porewater exchange is an important yet poorly understood component of the coastal carbon cycle. Here, a high-resolution automated radon (222Rn, a natural porewater tracer) and CO2 time series was conducted in the Squamish Central Estuary (Canada) over eight consecutive tidal cycles to assess the relative importance of porewater exchange on estuarine carbon dynamics. Radon and CO2 observations revealed a clear tidal trend which is indicative of porewater exchange driven by tidal pumping. A radon mass balance indicated an average porewater exchange rate of 14.9 cm day−1 (4.3% of the tidal prism). The estuary was a net source of CO2 to the atmosphere (average 212 ± 19 mmol m−2 day−1). Porewater exchange accounted for 9%, 5% and 30% of net dissolved organic carbon (DOC), dissolved inorganic carbon (DIC) and CO2 exported out of the Squamish Central Estuary, respectively, while porewater inputs of free CO2 accounted for 38% of the atmospheric evasion. These flux estimates as well as strong correlations between pCO2 and 222Rn suggest that porewater exchange has a strong influence on CO2 concentrations in the estuary even though they are a small contributor to overall DIC fluxes.

Carbon dynamics and inconstant porewater input in a mangrove tidal creek over contrasting seasons and tidal amplitudes

Constraining the contribution of mangrove-derived carbon in tidal creeks is fundamental to understanding the fate of mangrove primary production and the role of mangroves as coastal carbon sinks. Porewater measurements and 24-h time series in a mangrove tidal creek were conducted during the dry and wet season, and over contrasting tidal ranges at the Can Gio Biosphere Reserve, Vietnam. Surface water carbon concentrations (dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), partial pressure of carbon dioxide (pCO2)) and their respective δ13C values were correlated with radon, suggesting that porewater input drives mangrove-derived carbon in the tidal creek. Based on three complementary mixing models, porewater input contributed to about 30% of the water volume and 46% to 100% of DOC and DIC pools in the tidal creek at low tide, with variabilities between seasons and tidal amplitudes. The creek carbon pool was 88% DIC, 6% DOC, and 6% particulate organic carbon (POC). The pCO2 values during the wet season (2973–16,495 μatm) were on average 5-fold higher than during the dry season (584–2946 μatm). This was explained by a potential greater mineralization attributed to higher organic matter availability and residual humidity that stimulate bacterial activity, and by a potential tidal dilution changing the pCO2/DIC ratio as suggested by the Revelle factor. Consequently, average CO2 evasion from the creek was estimated at 327–427 mmolC m−2 d−1 during the wet season and 92–213 mmolC m−2 d−1 during the dry season, using two independent approaches. Tidal amplitude seemed to influence porewater input and its carbon loads, with a higher contribution during frequent and high tidal amplitudes (symmetric). However, the highest input occurred in a tidal cycle which was preceded by tidal cycle of low amplitude (asymmetric). We explain this ambiguity by the influence of both, rapid water turnover intensifying porewater exchange, and long water residence time enhancing carbon load in porewater.

Porewater inputs drive Fe redox cycling in the water column of a temperate mangrove wetland

Iron is a vital micronutrient within coastal marine ecosystems, playing an integral role in the scale and dynamics of primary production and carbon cycling in the world's oceans. We investigated the relative importance of in situ Fe(II) production from photochemical, microbial and thermal Fe reduction in the surface water column as well as advective porewater inputs in a temperate saline wetland in Australia containing mangrove and saltmarsh vegetation. The diel average concentration of Fe(II) (0.63 ± 0.21 μM, accounting for >70% of the total dissolved Fe present in surface water) was much higher than commonly reported in oxygenated marine waters despite high dissolved oxygen concentrations (81–112% saturation), pH (7.7–7.8) and salinity (33–36) that favor Fe oxidation. In situ production of Fe(II) in the surface water column was primarily driven by microbial processes rather than photochemical and thermal reduction, with a maximum production rate of 4.9 × 10−3 nM s−1. Advective porewater Fe(II) inputs to the wetland averaged over a diel cycle (3.0 × 10−1 nM s−1) were an order of magnitude greater than the combined Fe(II) production rate from autochthonous water column processes (1.0 × 10−2 nM s−1). A bottom up model based on the estimated individual fluxes was used to explain the high Fe(II) concentrations measured during a 24 h time series experiment. Combined, different lines of evidence suggest that advective porewater exchange provides significant quantities of Fe(II) to the estuarine wetland.

Using the radium quartet to quantify submarine groundwater discharge and porewater exchange

The specific ingrowth rates of different radium isotopes make them valuable tracers to distinguish processes occurring at different temporal scales. Here we demonstrate the use of the radium quartet (223Ra, 224Ra, 226Ra and 228Ra) to differentiate flows of submarine groundwater discharge and porewater exchange to a coastal embayment (Alfacs Bay, NW Mediterranean Sea), based on the assumption that these processes occur on different time scales. In order to evaluate the seasonal dynamics of groundwater and porewater inputs to the bay, we conducted three seasonal samplings at Alfacs Bay, during which samples for Ra isotopes were collected from bay waters, groundwater springs, porewaters and irrigation channels. Activities of short-lived Ra isotopes in the bay showed a strong seasonality, (e.g. average 224Ra activities in summer (∼32dpm 100L−1) up to 4 times higher than in winter (∼8dpm 100L−1)). In contrast, the activities of the long-lived Ra isotopes were fairly constant throughout the year (e.g. activities of 226Ra were ∼16 and ∼14dpm 100L−1 in summer and winter, respectively). The relatively short exposure to sediments of recirculation fluxes resulted in porewaters significantly enriched in short-lived Ra isotopes relative to the long-lived ones (e.g. 224Ra=1100–1300dpm100L−1; 226Ra=17–99dpm100L−1), whereas coastal groundwaters were enriched in all the Ra isotopes (e.g. 224Ra=120–150dpm100L−1; 226Ra=200–400dpm100L−1). The distinct signatures of different sources allowed us to construct seasonal Ra mass balances to estimate both groundwater discharge, which ranges from (40±60)·103m3·d−1 in summer to (310±200)·103m3·d−1 in winter, and porewater exchange fluxes, ranging from (1200±120)·103m3·d−1 in summer to (270±40)·103m3·d−1 in winter. Whereas the seasonal variability of groundwater inputs is likely governed by the terrestrial hydraulic gradient, a qualitative evaluation of the drivers of porewater exchange suggested that the strong seasonality of the seawater recirculation inputs is likely mediated by seasonal cycles on the activity of benthic infauna. Ra isotopes are thus valuable tracers to differentiate fluxes of both submarine groundwater discharge and porewater exchange, allowing a more accurate evaluation of the fluxes of freshwater and solutes to coastal ecosystems, as well as their implications for coastal biogeochemical cycles.

Using Optical Properties to Quantify Fringe Mangrove Inputs to the Dissolved Organic Matter (DOM) Pool in a Subtropical Estuary

Dissolved organic carbon (DOC) concentration and dissolved organic matter (DOM) optical properties were analyzed along two estuarine river transects during the wet and dry seasons to better understand DOM dynamics and quantify mangrove inputs. A tidal study was performed to assess the impacts of tidal pumping on DOM transport. DOM in the estuaries showed non-conservative mixing indicative of mangrove-derived inputs. Similarly, fluorescence data suggest that some terrestrial humic-like components showed non-conservative behavior. An Everglades freshwater-derived fluorescent component, which is associated with soil inputs from the Northern Everglades, behaved conservatively. During the dry season, a protein-like component behaved conservatively until the mid-salinity range when non-conservative behavior due to degradation and/or loss was observed. The tidal study data suggests mangrove porewater inputs to the rivers following low tide. The differences in quantity of DOM exported by the Shark and Harney Rivers imply that geomorphology and tidal hydrology may be a dominant factor controlling the amount of DOM exported from the mangrove ecotone, where up to 21 % of the DOC is mangrove-derived. Additionally, nutrient concentrations and other temporal factors may control DOM export from the mangroves, particularly for the microbially derived fluorescent components, contributing to the seasonal differences. The wet and dry season fluxes of mangrove DOM from the Shark River is estimated as 0.27 × 109 mg C d−1 and 0.075 × 109 mg C d−1, respectively, and the Harney River is estimated as 1.9 × 109 mg C d−1 and 0.20 × 109 mg C d−1.

Development and compartmentalization of chalky carbonate reservoirs: The Urgonian Jura-Bas Dauphiné platform model (Génissiat, southeastern France)

The chalky limestones of the upper Barremian-middle Aptian Urgonian Limestone Formation of the Jura-Bas Dauphiné platform (southeastern France) are characterised by a total porosity between 17.9 and 24.5%, a threshold radius between 1.04 and 2.13 μm, a trapped porosity between 51.4 and 68.3% and a gas permeability between 161 and 410 mD. They were formed in response to the diagenetic evolution of dominantly micritic sediments deposited during a “calcite sea” interval. Their present-day reservoir qualities and transport properties are controlled by the intercrystalline microporosity of microcrystalline areas (peloids or matrix). This intercrystalline microporosity is the product of a meteoric porewater input (installation of a shallow-burial phreatic environment characterised by a short residence time of porewaters) which promoted an aggrading neomorphism (Ostwald ripening process). The impact of the burial diagenesis on the reservoir qualities and the transport properties of these chalky limestones is limited. The chalky units are presently compartmentalized by horizontal layers of compact limestones characterised by a total porosity between 2.9 and 4.5%, a threshold radius between 0.04 and 0.09 μm, a trapped porosity of 92.4–92.8% and a gas permeability of 2 mD. Like the chalky limestones, the present-day reservoir qualities and transport properties of compact limestones are controlled by the intercrystalline microporosity of microcrystalline areas. The petrophysical differences between the chalky and compact limestones result from differences in the pore network of microcrystalline areas due to an increase in the average size of particles and a modification of crystal–crystal contacts. These petrographical differences reflect an evolution of micritic areas of original sediments during submarine lithifications (precipitation of microcrystalline cements within the pore space and/or an increase in solid–solid contacts by crystal growth) which preceded the aggrading neomorphism induced by the meteoric porewater input.

Driving Forces Behind Nutrient and Organic Matter Dynamics in a Mangrove Tidal Creek in North Brazil

In the mangrove of the Caeté Estuary (North Brazil) a detailed sampling was carried out in order to gain insight into the dynamics of inorganic nutrients and organic matter in one of the world's largest mangroves and to identify the driving forces behind these processes. Throughout 36 tidal cycles in the course of one year, concentrations of dissolved and particulate organic carbon and nitrogen (DOC, POC, DON, PON) and dissolved inorganic nutrients (N, P, Si compounds) were determined in a mangrove tidal creek. Annual average concentrations (in μM) were DOC 360, POC 240, DON 20, PON 29, dissolved inorganic nitrogen (DIN) 11, silicate 170 and phosphate 2·4. Ammonium and nitrite were about 80 and 17% of DIN, respectively. Nutrient dynamics in the creek were significantly influenced by porewater input from the upper forest sediment layer. This led to characteristic tidal signatures of nutrient and organic matter concentrations with maximum values during low tide. Annual phosphate and DOC oscillations were caused mainly by tidally dependent variations of porewater input. Their concentrations in the creek were directly proportional to the hydraulic gradient between creek and sediment water table. Due to autotrophic activity, dissolved oxygen, pH and DOC were higher and DIN was lower during daytime than at night. Annual oscillations of DIN and DON could also be attributed to varying phytoplanktonic activity. Silicate and phosphate showed only weak response to aquatic photosynthesis with slightly lower concentrations during the day. Assuming equilibrated fluxes, nitrogen fixation in the forest could be estimated to be 2·3mmolNm−2d−1. Based on these findings, tidal range and porewater concentrations were identified as driving forces behind coastal outwelling of nutrients and organic matter from mangroves. Outwelling probably occurs only from mangroves where the nutrient concentration in porewater exceeds the demands of the benthic community and trees, caused by positive sedimentation balances and high nitrogen fixation rates, and only in macrotidal regions where porewater can flow in considerable amounts to the tidal creeks and the ocean.