Search

Fjords are net carbon sinks with high organic carbon (OC) burial rates; however, the key drivers of OC burial in these systems are not well constrained. To study the role of water column redox condition and OC composition on OC preservation in fjord sediments, we determined OC accumulation rates (OCAR), OC source, and OC degradation in three Swedish fjords with variable redox conditions (long‐term oxic, seasonally hypoxic, and long‐term anoxic). Average OCARs were variable between and within the fjords studied (2–122 g OC m−2yr−1), but highest rates were at the mouth for each fjord. Based on a δ13C mixing model, Swedish fjords bury predominantly marine‐derived OC (∼83% of the total OC burial) likely because of relatively gentle slopes, low riverine discharge, and high marine inflow. Using a multi‐biomarker approach (lignin, photosynthetic pigments, and total hydrolyzable amino acids) we found, terrestrially‐ and marine‐derived OC were moderately degraded under the various redox conditions sampled, suggesting water column redox and OC source are not primary drivers of OC burial in these fjords. Rather, high sediment accumulation rates, common to fjords globally, lead to low oxygen exposure times, thus promoting efficient burial of OC regardless of its chemical composition. Fjords are net carbon sinks due in part to very high organic carbon burial rates. Despite being important in the regulation of earth's climate, the drivers of organic carbon burial in fjords are not well constrained. Here, we characterize the organic carbon buried in Swedish fjord sediments under different oxygen regimes (long‐term oxic, seasonally hypoxic, and long‐term anoxic) using bulk elemental and biomarker analyses. These fjords effectively bury organic carbon regardless of organic carbon source and water column oxygen conditions, likely because of high sediment accumulation rates. Fjords represent distinctive coastal systems with the capacity to retain reactive organic carbon, underscoring the importance of exploring these ecosystems in the context of global change. Swedish fjords bury primarily marine‐derived organic carbon under long‐term oxic, seasonally hypoxic, and long‐term anoxic water columnsOrganic carbon burial in Swedish fjords is largely controlled by sediment accumulation rate which can limit oxygen exposure timeMarine‐dominated fjords are uniquely efficient sinks of labile organic carbon Swedish fjords bury primarily marine‐derived organic carbon under long‐term oxic, seasonally hypoxic, and long‐term anoxic water columns Organic carbon burial in Swedish fjords is largely controlled by sediment accumulation rate which can limit oxygen exposure time Marine‐dominated fjords are uniquely efficient sinks of labile organic carbon

Modifications in land use and climate will result in shifts in the magnitude and composition of organic matter (OM) transported from wetlands to coastal waters, but differentiation between riverine and wetland OM sources in coastal areas remains a challenge. Here, we evaluate particulate and dissolved OM export dynamics in two representative estuary geomorphologies—Apalachicola Bay (AP) and Barataria Bay (BB), characterized primarily by blackwater river inputs and high particle abundance, respectively. The magnitude and composition of OM exported from each estuary was evaluated based on seasonal measurements of surface water dissolved organic carbon (DOC), particulate organic carbon (POC), particulate nitrogen, the stable isotopic composition of DOC and POC, dissolved and particulate lignin phenols, and carbon‐normalized dissolved lignin‐phenol yields. Data and discriminant analyses support the initial hypothesis; AP is dominated by a more terrestrial source of OM due to importance of fluvial dissolved OM inputs, while BB is a more particle‐rich and wetland carbon‐dominated system. Total lignin export (sum of mean dissolved and mean particulate) was higher in BB (5.73 ± 2.50 × 105kg/year) than in AP (4.21 ± 2.35 × 105kg/year). Particulate lignin export from BB was greater than the export of dissolved lignin at either BB or AP, suggesting coastal marsh erosion may be driving this comparatively large export of particulate lignin. These data have important implications for the stability of stored OM in coastal habitats, particularly since such habitats in this region are highly vulnerable due to relative sea level rise. Marsh organic matter sources are dominant in Barataria Bay due to a lack of direct river inputs and high relative sea level riseDissolved lignin export was 5 times greater than particulate in Apalachicola Bay, suggesting rivers are driving a large export of dissolved organic matterParticulate lignin export was 5 times greater than dissolved in Barataria Bay suggesting coastal marsh erosion is driving a large export of particulate organic matter

Biological processes exert important controls on geomorphic evolution of karst landscapes because carbonate mineral dissolution can be augmented and spatially focused by production of CO2and biogenic acids from organic matter (OM) decomposition. In Big Cypress National Preserve in southwest Florida, depressional wetlands (called cypress domes) dissolved into surface‐exposed carbonate rocks and exhibit regular patterning (size, depth, and spacing) within the pine upland mosaic. To understand when wetland basins began to form and the role of spatially varying OM decomposition on bedrock weathering, we constructed age profiles of sediment accretion using compound‐specific radiocarbon analysis of long‐chain fatty acids and measured bulk OM properties and biomarker proxies (fatty acids and lignin phenols) in different zones (center vs. edge) of the wetlands. Based on compound‐specific radiocarbon analysis, landscape patterning likely began in the middle to late Holocene, with wetlands beginning to form earlier at higher elevations than at lower elevations within the regional landscape. Dominant vegetation appears to have shifted from graminoids to woody plants around 3,000 calendar years before the present, as reflected in downcore bulk carbon isotope data and lignin concentration, likely from increased precipitation and hydroperiods. OM is mostly accumulated in wetland centers, and wetland centers exhibit more carbonate dissolution due to inundation limiting atmospheric ventilation of CO2. Landscape development and patterning thus arise from interactions between hydrology, ecology, and ecological community evolution that control carbonate mineral dissolution. Regularly patterned depressional wetlands in southwest Florida were likely developed in the middle to late HoloceneWetland vegetation likely shifted from graminoids to woody plants as a result of precipitation change and concomitant hydroperiodsDecomposition of soil organic matter appears to be more efficient at bedrock weathering in wetland centers than edges

Wax Lake Delta, southern Louisiana, is a coastal delta that formed following the dredging of a river channel in 1941 and is a field model for investigating the geomorphology, ecology, carbon dynamics, and carbon storage capacity in young prograding deltas. However, it is unknown how the transition from subaqueous to subaerial sediments affects the sources and quality of the sequestered carbon. We investigated these variations within the sediments of Wax Lake Delta using amino acid, lignin, and stable carbon isotope compositions of the organic matter (OM). A principal component analysis of these proxies highlighted variability in organic carbon (OC) composition with changes in elevation. The transition from subaqueous to subaerial sediments at 0‐cm mean lower low water is an important component of the OM composition. In addition to the changes observed for OM source and quality, the OC loadings (OC/SA; mg C/m2) also increase as the delta aggrades and accumulates sediments with loadings typical of delta topsets and mobile mud banks (OC/SA < 0.4) to riverine sediments (0.5 < OC/SA < 1) and eventually to highly productive regions (OC/SA > 1). Linking this multiproxy approach with environmental variables such as elevation provides a path for incorporating OM dynamics into geomorphic models. A multibiomarker approach characterizes the sources and diagenetic state of organic matter in Wax Lake Delta sedimentsChanges in the source and diagenetic state of organic matter in Wax Lake Delta sediments are correlated with elevationThe OC/SA loadings increase as the delta aggrades, resulting in a semilog relationship between TOC and silt + clay content

To better understand the role of reactive Fe (FeR) in the preservation of sedimentary organic carbon (SOC) in estuarine sediments, we examined specific surface area, grain size composition, total OC (TOC), lignin phenols, FeR, FeR‐associated OC (Fe‐OC) and lignin phenols (Fe‐lignin), and δ13C of FeR‐associated OC (δ13CFe‐OC) in surface sediments of the Changjiang Estuary and adjacent shelf. An estimated 7.4 ± 3.5% of the OC was directly bound with FeRin the Changjiang Estuary and adjacent shelf. Unusually low TOC/specific surface area loadings and Fe‐OC/Fe ratios in mobile muds suggest that frequent physical reworking may reduce FeRbinding with OC, with selective loss of marine OC. More depleted 13CFe‐OCrelative to 13C of TOC (13Cbulk) in deltaic regions and mobile muds showed that FeRwas largely associated with terrestrial OC, derived from extensive riverine OC and Fe inputs. A higher proportion of hematite in the mobile muds compared to the offshore samples indicated that Fe oxides are likely subjected to selective sorting and/or become mature during long‐term sediment transport. When considering the percentage of Fe‐OC to SOC and SOC burial rates in different marine environments (e.g., nondeltaic shelf, anoxic basins, slope, and deep sea), our findings suggest that about 15.6 ± 6.5% of SOC is directly bound to FeRon a global scale, which is lower than the previous estimation (~21.5%). This work further supports the notion of a Rusty Sinkwhere, in this case, FeRplays an important role in the preservation and potential transport of terrestrial OC in the marine environment. Extensive sediment resuspension may reduce reactive iron binding with marine‐derived OCAbout 15.6% of sedimentary OC burial is directly associated with reactive iron on a global scaleThe reactive iron plays an important role in stabilization of terrestrial SOC

Estuaries have been recognized as important sources of carbon dioxide (CO2) to the atmosphere; however, contributions of these systems to regional and global carbon budgets are not well constrained due to limited information on seasonal and spatial variability. In this study, we use satellite remote sensing to obtain seasonal pCO2distribution and air‐sea CO2fluxes in Apalachicola Bay, a national estuarine research reserve located in the northern Gulf of Mexico that receives seasonally varying dissolved organic matter‐rich waters from the Apalachicola River. A combination of time series (2005–2016) and seasonal field observations (2015–2016) of pH and biophysical variables were used to develop seasonal pH‐pCO2relationships for obtaining surface pCO2estimates and air‐sea CO2fluxes from Visible and Infrared Imaging Radiometer Suite (VIIRS) ocean color data. Monthly and seasonal maps of pCO2and air‐sea CO2fluxes showed a general trend of higher fluxes in winter and summer corresponding to high river flow and warm water temperatures. However, CO2fixation via photosynthesis and low water temperatures contributed to lower fluxes to the atmosphere in spring and fall, respectively. Throughout the study period, Apalachicola Bay was a net source of CO2with large seasonal and spatial variability and a mean annual CO2flux to the atmosphere of 3.4 ± 3.1 mol·m−2·year−1(9.4 ± 8.5 mmol·m−2·day−1), consistent with fluxes reported for other estuaries. This study demonstrates the critical role that satellite observations can play to improve the estuarine contributions to the global carbon flux estimates. Despite having high productivity, coastal water bodies are known to emit large amounts of carbon dioxide (CO2) to the atmosphere. Our study demonstrates the use of an ocean color satellite to estimate seasonal and annual rates of CO2release to the atmosphere in Apalachicola Bay, Florida (United States). Due to a lack of carbon dioxide measurements in Apalachicola Bay, we adopted a different approach with advanced statistical techniques to estimate CO2concentrations from water acidity (pH). Our study shows that Apalachicola Bay is a net source of CO2throughout the year; however, the rate of CO2emission varies among months and seasons, and even in different regions within the bay—likely due to the dominance of one or more controlling processes. The estimated rate of emission in Apalachicola Bay indicates that it is a weak source of CO2, which has an annual CO2emission rate ~95% lower than an average value for the subtropical estuaries around the world. This study also suggests that accurate CO2flux estimates for Apalachicola Bay and other estuaries in the Gulf of Mexico fill an important gap in current global carbon emission estimates. Seasonal and annual air‐sea CO2fluxes were estimated using VIIRS ocean color imageryApalachicola Bay (United States) is a net source of CO2to the atmosphere that also experiences distinct monthly and seasonal variations corresponding to various biophysical processesMean annual CO2flux for Apalachicola Bay is 3.4 ± 3.1 mol·m−2·year−1

We used seven 210Pb‐dated sediment cores from the Gaer Arm in the Doubtful Sound fjord complex, Fiordland, New Zealand to evaluate organic carbon (OC) dynamics in a temperate fjord‐head delta. The highly dynamic spatial features of this delta were clearly evident in the observed sediment properties such as mass accumulation rates that varied by a factor of 14, sediment grain size by a factor 5, and sedimentary OC content by a factor 6. Low lignin concentrations (e.g., 2.95 mg (100 mg OC)−1) and syringic/vanillic ratios of lignin phenols (S/V; e.g., 0.44) at the upper deltaic stations were representative of substantial autochthonous OC contributions to delta sediments. Significantly higher acid/aldehyde ratios of vanillic phenols [(Ad/Al)v] at the deltaic stations (0.45–0.82) than the surface grabs (0.26–0.30) indicated rapid degradation of OC within the delta. Despite being a “hot spot” for OC oxidation, the delta likely improves OC preservation in the adjacent fjord by filtering out coarse‐grained particles and exporting fine‐grained particles to fjord sediments. Our results showed that fjord‐head deltas can influence sedimentation and OC dynamics in select regions of fjords and thus warrant more examination of fjord‐head processes, particularly in areas where they are expanding. In particular, as Earth warms and glaciers retreat, the newly exposed fjord‐head platforms in high‐latitude environments may evolve into similar “hot spots” of OC oxidation, thereby altering the dynamics of OC burial in these systems. Variations in organic carbon content and sediment grain size are linked with hydrodynamic sorting of particlesLignin biomarker indices support local source inputs of OC on the fjord‐head deltaBiomarker and organic carbon accumulation rates suggest that fjord‐head deltas are likely hot spots for carbon oxidation

Temperature rise in the Arctic is causing deepening of active layers and resulting in the mobilization of deep permafrost dissolved organic matter (DOM). However, the mechanisms of DOM mobilization from Arctic soils, especially upper soil horizons which are drained most frequently through a year, are poorly understood. Here we conducted a short-term leaching experiment on surface and deep organic active layer soils, from the Yukon River basin, to examine the effects of DOM transport on bulk and molecular characteristics. Our data showed a net release of DOM from surface soils equal to an average of 5% of soil carbon. Conversely, deep soils percolated with surface leachates retained up to 27% of bulk DOM while releasing fluorescent components (up to 107%), indicating selective release of aromatic components (e.g., lignin and tannin), while retaining nonchromophoric components, as supported by spectrofluorometric and ultrahigh-resolution mass spectroscopic techniques. Our findings highlight the importance of the lateral flux of DOM on ecosystem carbon balance as well as processing of DOM transport through organic active layer soils en route to rivers and streams. This work also suggests the potential role of leachate export as an important mechanism of C losses from Arctic soils, in comparison with the more traditional pathway from soil to atmosphere in a warming Arctic. Significant amount of DOC export from surface soil by leachingNet retention of surface DOM leachates in deep soilsSelective molecular exchange of soil DOM leachate during percolation

Here we report on the effects of an experimental flood on the carbon cycling dynamics in the dry watercourse of the Colorado River in Mexico. We observed post‐flood differences in the degree of decay, age, and concentration of dissolved organic carbon (DOC), as well as dissolved CH4and CO2concentrations throughout the study site. Our results indicate that this flooded waterway was a limited source of CH4and CO2to the atmosphere during the event and that DOC age increased with time of flooding. Based on our findings, we suggest that the interplay between storage and mobilization of carbon and greenhouse gases in arid and semiarid regions is potentially sensitive to changing climate conditions, particularly hydrologic variability. Changes in the radiocarbon age of DOC throughout the flooding event suggest that organic matter (OM) that had been stored for long periods (e.g., millennial) was mobilized by the flooding event along with CO2. The OM residing in the dry riverbed that was mobilized into floodwaters had a signature reflective of degraded vascular plant OM and microbial biomass. Whether this microbial OM was living or dead, our findings support previous work in soils and natural waters showing that microbial OM can remain stable and stored in ecosystems for long time periods. As human appropriation of water resources continues to increase, the episodic drying and rewetting of once natural riverbeds and deltas may fundamentally alter the processing and storage of carbon in such systems. Experimental flow from dam to the dry watercourse of the lower Colorado RiverLinkages between controlled flooding event and dissolved greenhouse gas concentrationsChanges in composition and age of dissolved organic carbon over flooding period

Breithaupt et al. (2020, https://doi.org/10.1029/2019JG005349) investigated why rates of organic carbon (OC) burial in coastal wetlands appear to increase over the past ∼120 years. After comparing dating methods and applying biogeochemical analyses, we concluded that neither dating method nor carbon degradation contribute to the observed trend. Rather, we concluded that OC burial has increased in the past century. Parkinson's (2021) Comment disagrees with our conclusion, contending that: (1) use of a density correction to account for soil auto‐compaction is a flawed methodology that artificially shortens a core's length, (2) there is limited evidence for an acceleration in the regional sea‐level rise (SLR) rate, and (3) vertical accretion rates in previous papers by Breithaupt et al. (2014, https://doi.org/10.1002/2014JG002715; 2017, https://doi.org/10.1016/j.margeo.2017.07.002) are lower than the regional mean rate of SLR and are not to be believed as these wetlands should have converted to open water by now. We reject these contentions because: (1) no density correction was applied to the cores in this study, (2) local tide gauge records and analyses in the literature support an increase in SLR rates coinciding with the timeframe of our OC burial records, and (3) Parkinson's comparison of the 100‐yr mean rate of SLR neglects temporal variability and uncertainties in the long‐term sea‐level record, as well as biophysical feedbacks between wetland surface elevation and SLR. Here, we provide detailed responses to Parkinson's contentions and establish the importance of differentiating operational definitions of OC burial and accretion to clarify why an auto‐compaction correction is not applicable for OC burial measurements. Organic carbon burial rates are not altered by auto‐compaction and do not include a density correctionIncreasing rates of sea‐level rise are supported by regional data and analyses from the literature Organic carbon burial rates are not altered by auto‐compaction and do not include a density correction Increasing rates of sea‐level rise are supported by regional data and analyses from the literature

Blue holes are unique geomorphological features with steep biogeochemical gradients and distinctive microbial communities. Carbon cycling in blue holes, however, remains poorly understood. Here we describe potential mechanisms of dissolved carbon cycling in the world's deepest blue hole, the Yongle Blue Hole (YBH), which was recently discovered in the South China Sea. In the YBH, we found some of the lowest concentrations (e.g., 22 μM) and oldest ages (e.g., 6,810 years before present) of dissolved organic carbon, as well as the highest concentrations (e.g., 3,090 μM) and the oldest ages (e.g., 8,270 years before present) of dissolved inorganic carbon observed in oceanic waters. Sharp gradients of dissolved oxygen, H2S, and CH4and changes in bacterially mediated sulfur cycling with depth indicated that sulfur‐ and/or methane‐based metabolisms are closely linked to carbon cycling in the YBH. Our results showed that the YBH is a unique and easily accessible natural laboratory for examining carbon cycling in anoxic systems, which has potential for understanding carbon dynamics in both paleo and modern oceans—particularly in the context of global change. Here, we report for the first time concentrations and isotopic composition of dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) along with geomorphological and geochemical parameters in the Yongle Blue Hole (YBH), South China Sea, the world's deepest blue hole. We found some of the lowest concentrations of DOC and the highest concentrations of DIC observed in coastal waters, that were both very old (their 14C ages are 6,810 and 8,270 years, respectively). Such low DOC concentrations have not been observed in relatively shallow oceanic environments. DIC in surface waters of the YBH is mainly controlled by air‐sea exchange, which only impacts the upper 80 m. The decomposition of organic carbon and the sulfidic acid dissolution of carbonates in the YBH enhance the production of DIC in deep waters of the YBH. The profiles of physical, chemical, and microbial parameters in the YBH provide a natural laboratory for examining carbon cycling and other redox‐sensitive elements. Understanding the key mechanisms of biogeochemical pathways in anoxic systems is critical in our understanding of carbon cycling in both paleo and modern oceans. Concentrations and isotopic composition of dissolved carbon in the world's deepest blue hole were examined for the first timeLowest concentrations of DOC and highest concentrations of DIC in oceanic waters were observed, and both were radiocarbon depletedCarbon cycling in the Yongle blue hole is unique from those of the open ocean

Rates of organic carbon (OC) burial in some coastal wetlands appear to be greater in recent years than they were in the past. Possible explanations include ongoing mineralization of older OC or the influence of an unaccounted‐for artifact of the methods used to measure burial rates. Alternatively, the trend may represent real acceleration in OC burial. We quantified OC burial rates of mangrove and coastal freshwater marshes in southwest Florida through a comparison of rates derived from 210Pb, 137Cs, and surface marker horizons. Age/depth profiles of lignin: OC were used to assess whether down‐core remineralization had depleted the OC pool relative to lignin, and lignin phenols were used to quantify the variability of lignin degradation. Over the past 120 years, OC burial rates at seven sites increased by factors ranging from 1.4 to 6.2. We propose that these increases represent net acceleration. Change in relative sea‐level rise is the most likely large‐scale driver of acceleration, and sediment deposition from large storms can contribute to periodic increases. Mangrove sites had higher OC and lignin burial rates than marsh sites, indicating inherent differences in OC burial factors between the two habitat types. The higher OC burial rates in mangrove soils mean that their encroachment into coastal freshwater marshes has the potential to increase burial rates in those locations even more than might be expected from the acceleration trends. Regionally, these findings suggest that burial represents a substantially growing proportion of the coastal wetland carbon budget. Coastal mangroves and marshes use photosynthesis to change carbon dioxide into plant material like leaves, wood, and roots. As these materials die over time, they do not decompose the same way they might in a dry soil environment. Instead, daily ocean tides flood the soil, removing the oxygen and allowing dead plant materials to be buried and preserved for hundreds to thousands of years or more. This “carbon burial” process is a global benefit because it means that carbon dioxide, a greenhouse gas, is stored belowground where it can no longer warm the atmosphere. Mangroves and marshes bury carbon more quickly than most other ecosystems, but little is known about how burial rates change over time. In this study, we quantify how rates of carbon burial have changed in southwest Florida coastal wetlands in the past century. We find that carbon burial is 2.5–4.5 times greater today than it was a century ago in mangroves and 1.9–2.3 times greater in marshes. We propose sea‐level rise as the most likely cause of increased carbon burial. Rising seas provide the means and opportunity for wetlands to bury more carbon, but if sea level rises too quickly, these wetlands may drown. Organic carbon burial rates increased by factors of 2.5 to 6.2 for mangroves and 1.4 to 2.4 for coastal freshwater marshes in the past ~120 yearsIncreasing rates represent acceleration rather than a record influenced by an artefact of the dating method or postdepositional degradationAcceleration has increased soil carbon stocks by 5.7–8.0 kg m−2for mangroves and 4.2–4.5 kg m−2for coastal freshwater marshes