Marsh Sediments Research Articles

Enrichment and Fractionation of Rare Earth Elements in an Estuarine Marsh Soil Receiving Acid Discharges from Legacy Sulfide Mine Wastes

This paper provides new insights into the geochemical cycling of rare earth elements (REEs) in acid sulfate soils developed on salt marsh sediments of the Huelva estuary (Spain) as a result of sulfide mineral oxidation in abandoned ore stockpiles. The study was aimed at determining the REE abundance, fractionation pattern and mineralogical control of the dispersal and retention of REEs in the soil system. Forty-one samples were collected at 13 core sampling sites along two transects extending across the degraded marshland, and they were subjected to XRD, ESEM-EDS and ICP-MS analyses. Measurements revealed that the soil receiving acid discharges has relatively high concentrations of ΣREEs (174.77 ± 19.77 mg kg−1) compared to local baseline concentrations. Shale-normalized REE patterns are generally flat, but a slight middle REE (MREE) enrichment is consistently apparent in all soil samples, involving relatively low LaN/GdN ratios (0.83 ± 0.08) and GdN/LuN ratios up to 1.42. The convex-upward REE pattern supports the possibility that iron oxy-hydroxide minerals play an important role in MREE retention through adsorption and co-precipitation mechanisms. Efflorescent sulfate salts left on the topsoil by the evaporation of acid waters show a strong depletion of light REEs (LaN/GdN = 0.16 ± 0.10) and act as a temporary reservoir of labile MREEs and heavy REEs during dry periods.

Radiocarbon dating of historic mudflat sediments at Airth in the inner Forth estuary and the impact on the estuary of nineteenth century agricultural improvements

ABSTRACT The results of sediment-stratigraphic, diatom and pollen analyses, and AMS 14C dating of salt marsh sediments at Higgin’s Neuk, Airth in the inner Forth estuary in central Scotland are reported. Engineering borehole records of abundant peat within mudflat sediments encouraged work to establish rates of mudflat accretion over the last several centuries. However, six 14C assays on peat closely associated with buried archaeological structures of likely eighteenth century age record later prehistoric and early historic age-estimates. The assays are thought to be correct. The peat is not in situ. It probably originated from the well-documented, extensive late eighteenth and early nineteenth century clearance of raised mosses in the River Forth for agriculture. The sediment-stratigraphic evidence is important because it confirms the scale of the impacts of peat clearance on the inner estuary derived from contemporary documents. The peat may act as a marker horizon in mudflat and salt marsh sediments from which rates of post-eighteenth century mudflat accretion can be derived. At Higgin’s Neuk these were probably very high, given uncertainties in dating controls, around 0.8 cm/yr (1.2 yrs/cm) for the last ca. 150 years, a finding which accords with other studies around the North Sea.

Late Holocene stratigraphic evolution and sedimentary facies of an active to abandoned tide‐dominated distributary channel and its mouth bar

AbstractTo better understand the sedimentary facies of tide‐dominated deltas, a core dataset from the Ba Lai palaeochannel in the Mekong River Delta was obtained and studied. Nine sedimentary facies were identified and interpreted as representing the Late Holocene evolution of the Ba Lai palaeochannel, including its pre‐abandonment and post‐abandonment phases, as well as pre‐channel phases. The channel formed at 2.6 ka as a distributary channel connected to the deltaic network and was abruptly abandoned and rapidly infilled with sediment at 0.7 ka. The channel deposits are up to ca 11 m thick and overlie shelfal shell layers, which, in turn, overlie Mid‐Holocene and Pleistocene deposits. The active‐channel fill and mouth‐bar deposits consist of sand and mud with cyclical patterns, bidirectional lenses and abundant mud layers, suggesting deposition mainly driven by river and tidal processes. The abandoned‐channel fill consists entirely of organic‐rich mud, suggesting a predominance of tidal processes. Other sedimentary facies include tidal‐flat and marsh deposits; they mostly consist of mud and formed in shallow to subaerial areas near the channel margins or on barforms. Depending on the exact location of the core in this depositional setting, three possible stratigraphic successions and facies models are presented herein. Near channel margins, the deposits show an upward gradual change from heterolithic to mud with a well‐developed fining‐upward trend. Away from the channel margins, the deposits show an upward sharp change from heterolithic to mud due to the channel abandonment. The mouth‐bar‐area facies model shows an upward gradual change from heterolithic to heterolithic/sand to mud deposits with a coarsening‐upward to fining‐upward trend. Although differences among systems likely exist, the authors suggest that the sedimentary facies described in this study and the resulting facies models should be used to better understand tide‐dominated deltaic systems and to improve their interpretation in the geological record.

Rare earth element distributions in salt marsh sediment cores reveal evidence of environmental lability during bioturbation and diagenetic processes

The distribution and accumulation of rare earth elements (REE) in the labile fraction of sediment cores collected from salt marshes in the Patos Lagoon estuary from southern Brazil were investigated. Sediment cores (ca. 40 cm) were obtained from three locations within the estuary to capture possible changes in REE content across the salinity gradient (i.e., where saline, brackish, and freshwater dominate). Salt marsh sediments from all three coring locations were enriched in the light REE (LREE) over the heavy REE (HREE) when normalized to the North American shale composite (NASC). Shale normalized values for the LREE of marsh sediments from sites M2 and M3 in the mid- and upper estuary commonly approximates unity indicating these sediments chiefly exhibit a terrigenous signature. In contrast, all 14 naturally occurring REE are depleted in the sediments from the M1 coring location in the lower estuary compared to shale. Sediments from the mid-estuary (M2 core location) where typical salinity values are ca. 10 practical salinity units, exhibit the greatest shale normalized LREE enrichments. The higher LREE contents of the M2 sediments likely reflect preferential removal of LREE from the water column owing to salt-induced coagulation of river-borne colloids that occurs during estuarine mixing processes. Sediments samples collected from the salt marsh in the lower estuary nearer the Atlantic Ocean (i.e., M1), have substantially lower REE contents than salt marsh sediments from sites M2 and M3 in the mid- and upper estuary, respectively. The more sand-rich lower estuary sediments are exposed to higher salinity water from the South Atlantic compared to the generally finer grained sediments from the mid- and upper estuary, which are dominated by brackish and freshwater conditions, respectively. Negative Ce-anomalies (i.e., Ce/Ce* < 1) in marsh sediments from depths generally greater than 27 cm support the occurrence of early diagenetic reactions consistent with suboxic to anoxic conditions whereby reductive dissolution of ferromanganese oxide/oxyhydroxide carrier phases, and/or Ce(IV) phases, release Ce(III) to the marsh pore waters. The negative Ce anomalies suggests that a suboxic or anoxic environment predominates to a depth of 9 cm, and then again below 27 cm depth in sediments from the M2 salt marsh core, and below a depth of 13 cm in the upper estuary marsh sediments (i.e., M3). In contrast, positive Ce anomalies (Ce/Ce* > 1) for much of the sediment from the M1 core, the mid depths of core M2 (i.e., 9 cm to 27 cm), and shallower than 13 cm in sediments from core M3 support oxic conditions where Ce is enriched in sediments from these depths by preferential scavenging from pore waters onto Fe/Mn oxides/oxyhydroxides. These trends in Ce/Ce* with depth in each sediment core are also consistent with known bioturbation that occurs within these marsh sediments. The relatively high REE contents of the sediments from location M3 are also related to the fact that these sediments are dominantly composed of clay-silt deposits that are enriched in organic matter. Finally, we find no conclusive evidence that the studied salt marsh sediments have been impacted by addition of REE from the local fertilizer manufacturing industry.

Effect of macropores on pore-water flow and soil conditions in salt marshes subject to evaporation and tides

Macropores such as crab burrows are commonly distributed in salt marsh sediments. They are well recognized to act as preferential flow paths, affecting physical and biogeochemical processes in salt marshes. While previous studies have examined the effect of macropores on pore-water flow in salt marshes, how and to what extent do macropores behave and interact with tide-induced flow and saturation-dependent soil evaporation remain unclear. This study conducted process-based and three-dimensional numerical simulations and examined effects of macropores on pore-water flow and soil conditions in salt marshes subject to evaporation and tides. The results demonstrate that in the marshes subject to inundating tides (tides flood marsh platform at high tides), macropores alter the pore-water flow and enhance the tide-induced water exchange between marsh soils and tidal water. Additionally, macropores increase the tidally averaged soil saturation in the shallow soil layer. As a result of higher soil saturation, soil evaporation is increased. In contrast, marshes subject to non-inundating tides (marsh platform is lower than high tidal level) do not show remarkable relationships with evaporation or soil saturation. Increases in the mean sea level increase soil saturation and reduce soil evaporation in marshes subject to inundating tides. These findings help to understand the chemical transport and soil conditions in salt marshes, which hold the key to protecting and restoring salt marshes.

Spatially resolved correlative microscopy and microbial identification reveal dynamic depth- and mineral-dependent anabolic activity in salt marsh sediment.

SummaryCoastal salt marshes are key sites of biogeochemical cycling and ideal systems in which to investigate the community structure of complex microbial communities. Here, we clarify structural–functional relationships among microorganisms and their mineralogical environment, revealing previously undescribed metabolic activity patterns and precise spatial arrangements within salt marsh sediment. Following 3.7‐day in situ incubations with a non‐canonical amino acid that was incorporated into new biomass, samples were resin‐embedded and analysed by correlative fluorescence and electron microscopy to map the microscale arrangements of anabolically active and inactive organisms alongside mineral grains. Parallel sediment samples were examined by fluorescence‐activated cell sorting and 16S rRNA gene sequencing to link anabolic activity to taxonomic identity. Both approaches demonstrated a rapid decline in the proportion of anabolically active cells with depth into salt marsh sediment, from ~60% in the top centimetre to 9.4%–22.4% between 2 and 10 cm. From the top to the bottom, the most prominent active community members shifted from sulfur cycling phototrophic consortia, to putative sulfate‐reducing bacteria likely oxidizing organic compounds, to fermentative lineages. Correlative microscopy revealed more abundant (and more anabolically active) organisms around non‐quartz minerals including rutile, orthoclase and plagioclase. Microbe–mineral relationships appear to be dynamic and context‐dependent arbiters of biogeochemical cycling.

Long-Term Fertilization Alters Nitrous Oxide Cycling Dynamics in Salt Marsh Sediments.

Salt marsh sediments are known hotspots for nitrogen cycling, including the production and consumption of nitrous oxide (N2O), a potent greenhouse gas and ozone-depleting agent. Coastal eutrophication, particularly elevated nitrogen loading from the application of fertilizers, is accelerating nitrogen cycling processes in salt marsh sediments. Here, we examine the impact of long-term fertilization on nitrogen cycling processes with a focus on N2O dynamics in a New England salt marsh. By combining 15N-tracer experiments with numerical modeling, we found that both nitrification and denitrification contribute to net N2O production in fertilized sediments. Long-term fertilization increased the relative importance of nitrification to N2O production, likely a result of increased oxygen penetration from nutrient-induced increases in marsh elevation. Substrate utilization rates of key nitrogen cycling processes revealed links between functions and the corresponding microbial communities. Higher specific substrate utilization rates leading to N2O production from nitrification in fertilized sediments indicate a shift in the community composition of ammonia oxidizers, whereas the lack of change in specific substrate utilization of N2O production from denitrification under long-term fertilization suggests resilience of the denitrifying communities. Both are consistent with previous studies on the functional gene community composition in these experimental plots.

Lateral shoreline erosion and shore-proximal sediment deposition on a coastal marsh from seasonal, storm and decadal measurements

The persistence of coastal marsh is dependent on its ability to maintain elevation relative to sea level, particularly for marshes experiencing high rates of shoreline erosion due to wave-attack, storms, and sea level rise. Sediments eroded at the marsh edge are either delivered onto the marsh platform or into the estuary, the latter resulting in a net loss of marsh sediments and soil carbon. Knowledge on the timing, pattern, and quantity of sediment deposition versus shoreline erosion along the marsh-estuary interface is critical for evaluating the overall health and vulnerability of coastal marshes to future scenarios of sea level rise and for estimating sediment budgets. Here we examined marsh shoreline erosion and sediment deposition for marsh sites experiencing a range of shoreline erosion rates and different levels of wind-wave exposure within the Grand Bay National Estuarine Research Reserve and Wildlife Refuge in Mississippi. We developed a method for calculating an erosion-deposition sediment budget using marsh elevation profiles, shoreline erosion rate, and sediment deposition measurements. Sediment budgets were calculated at four sites with varying shoreline erosion rates. Much of the sediment eroded at the marsh edge can be accounted for as marsh platform deposition, except at the most erosive site, suggesting a possible erosion threshold where eroded sediment mass is greater than platform deposition. Consistent with other studies of marsh creeks, sediment delivery to the marsh platform appears to be largely driven by wave climate. These data suggest that for erosive bay-estuarine shorelines, sediment delivered into the marsh is largely concentrated near the marsh shoreline, although shoreline erosion does not always result in a net loss of sediments from the marsh system in either decadal or annual assessments.

Divergent Temporal Response of Abundant and Rare Bacterial Communities to Transient Escherichia coli O157:H7 Invasion.

The release of Escherichia coli (E. coli) O157:H7 has been widely found in various environments, but little is known about the probable influence of the transient E. coli O157:H7 invasion on the native microbial community. Here, we investigated the temporal response of two bacterial biospheres (abundant and rare) of two marsh sediments against E. coli O157:H7 during a 60-day incubation. The diversity of both biospheres showed no evident response to O157:H7 invasion. Temporal factor exhibited greater effects on bacterial variation than O157:H7 invasion. We found that O157:H7 invasion led to an increase in the niche breadth of the bacterial community while decreasing the efficiency of bacterial interaction of the abundant taxa. Moreover, the rare biosphere exhibited enhanced stability against O157:H7 invasion compared with the abundant biosphere, acting as the backbone in resisting external disturbance. Furthermore, each subcommunity assembly showed different randomness levels. The stochastic events were relatively more important in constraining the abundant taxa assembly after invasion. Collectively, E. coli O157:H7 exhibited diverse tangible impact on both biospheres, which unearthed differential responses of abundant and rare biosphere against transient microbial invasion.

EDXRF Detection of Trace Elements in Salt Marsh Sediment of Bangladesh and Probabilistic Ecological Risk Assessment

ABSTRACT The surface salt marsh sediments from marine ecosystems in the Bay of Bengal, Bangladesh was assessed for trace elements like iron (Fe), calcium (Ca), titanium (Ti), potassium (K), strontium (Sr), rubidium (Rb), zirconium (Zr), zinc (Zn), copper (Cu) and lead (Pb), their potential sources, and associated ecological risks. The range of trace elements in sediment (mg/kg) followed the decreasing order: Fe (25020–35176) > Ca (8679–16323) >Ti (2678–3340) > K (23680–16770) > Sr (138.32–209.64) > Rb (124–186.57) > Zr (96.21–161.35) > Zn (37.71–44.95) > Cu (36.51–51.66) > Pb (1.35–10.42). The concentrations of trace elements were under the average shale values of metals in sediment except for Fe, Cu and Rb, which can occasionally be associated with adverse biological effects. Contamination level and ecological risk were evaluated using contamination factor (CF), enrichment factor (EF), geo-accumulation index (I geo), pollution load index (PLI), and potential ecological risk index (PERI). Multivariate statistical technique was used to classify the metal sources. The results showed that Fe was significantly enriched in the sediments. Ecological risk factors illustrated a pollution-free condition of the salt marsh ecosystem in Bangladesh.

Lagoa do Quari (São Raimundo Nonato, Piauí): Palaeoenvironment and wetland archaeology in Northeastern Brazil

AbstractNortheastern Brazil has thousands of wetland fossiliferous deposits with megafauna and, in some cases, associated lithic artifacts. The timing of the arrival of humans in South America and the extinction of megafauna is still debated and these sites contribute both to this discussion and to the reconstruction of the Late Pleistocene and Holocene paleoenvironment. Lagoa do Quari, 40 km south of São Raimundo Nonato, Piauí state, is a marsh deposit in the present semiarid caatinga environment, excavated by an interdisciplinary team in 2002–2003. This paper presents the archaeological, palaeontological, geological, chronological, stratigraphical and palynological results and their implications for the Quaternary of northeastern Brazil. The site has a rich megafaunal bonebed at the bottom, dominated by Eremotherium giant sloth remains with poor biodiversity, and a sandy‐silty deposit at the top; two 14C dates constrain this latter layer to 9,944–9,557 and 6,308–6,177 cal. years BP and the associated rich lithic industry of quartz and chert shows a technical continuity throughout the Holocene in the region. Palynology describes an open landscape with periodic wet phases, which could explain the richness of the archaeological occurrences in Holocenic Brazilian prehistory. This study offers a model of interdisciplinary enquiry into the paleoenvironment and prehistory of lowland South America.

High-resolution sedimentological and geochemical records of three marshes in San Francisco Bay, California

The San Francisco Bay has the largest concentration of salt marshes in the state of California. In the last 170 years, the vast majority of the historic tidal wetlands in the Bay have been significantly altered or destroyed due to diking, filling and other processes. Many of the remaining marshes have been impacted by changing sedimentation regimes and related loadings of pollutants such as heavy metals (Sr, Al, Fe, Ti, Cu, Pb, Ni, Zn) over this period, making ecological trajectories and resilience to disturbance uncertain. Here we examine changing sediment accretion rates, coupled with the longest and highest resolution heavy metal accumulation data from three San Francisco Bay marshes, to examine European impacts in the context of earlier Holocene variability. The results are consistent with larger-scale analyses and indicate that European alterations of landscapes and shorelines had geographically variable impacts on marsh sediment accumulation. Despite differential impacts on net sediment accretion, initial European impacts appear to have decreased the proportion of organic material in the marsh sediments, likely due to the delivery of eroded inorganic sediment from landscapes and shorelines. The results confirm significant European impacts on the geochemistry of marshes in the San Francisco Bay over the last ~150–200 years. Post-European arrival levels of Pb are unprecedented throughout the earlier Holocene. However, these values have declined in recent years. Concentrations of Sr, Ti, Cu, Ni, and Zn also increased following European arrival. Our results show that post-European concentrations are not so far removed compared to pre-European maximum concentrations. As sedimentation regimes and emissions of Pb and delivery of other metals have decreased, and as organic productivity increased on the marshes, environmental trajectories are shifting back towards their immediate pre-European conditions.

Sea-level, storms, and sedimentation – Controls on the architecture of the Andros tidal flats (Great Bahama Bank)

Integrated time-separated remote sensing, geographic information systems (GIS), microbiology, and carbonate sedimentology of the tidal-flat portion of Andros Island (Great Bahama Bank) quantify the geomorphologic change of tidal-flat facies through time and illustrate how sea-level rise, among other controls, is recorded within these deposits. In the 75 years spanned by the remote-sensing datasets, sea level has risen by approximately 10 cm. In the face of this change: a) the seaward margin of the tidal flats has remained static, while b) the internal boundary, which separates the channelized and supratidal marsh deposits, retrograded up to 220 m, broadening the channelized zone, and c) in unison to the broadening of the channelized zone, the abundance of laminated (Scytonema) cyanobacterial fabrics decreased, d) accompanied by the lengthening and avulsion of the network of tidal channels that traverse this zone. Given the amount of sea level rise during the period of observation, such broad-scale restructuring of the Andros tidal-flat architecture is surprising. The muddy tidal flats did not prograde, but instead aggraded and locally retrograded. Whereas retrogradation was minimal at the coastline, the channelized zone retrograded substantially over the more landward supratidal inland marsh. These results question aspects of the autocyclic model for the development of peritidal carbonate cycles, which is underpinned by a dominant behavior of tidal-flat progradation. Whereas other controls cannot be explicitly excluded, sea-level oscillations seemingly exerted considerably more impact on the Andros tidal flat in the last decades than did major storm events.

Salt marsh sediments act as sinks for microplastics and reveal effects of current and historical land use changes

Microplastic particles are widespread in marine sediments and the abundance of the different types of particles vary widely. In this paper we demonstrate that salt marshes effectively capture microplastics in their sediments, and that microplastic accumulations increase with the level of urbanization of the land surrounding estuarine areas. We extracted microplastics from sediment cores in salt marshes of SE New England estuaries at different degrees of urbanization and land use intensity. Microplastics were present everywhere, but their abundances increased markedly with the degree of urbanization of the land. Microplastic fragment counts were linked to nearby urbanization and their abundances seemed to be linked to more local, within-watershed inputs. The number of fibers was similar across all sites suggesting that fiber accumulation in these sediments is likely influenced by effective long-distance transport from large-scale areas. The sedimentary record confirmed that microplastics have been accumulating in these estuaries since the early 1950s, and their abundances have increased greatly in more recent years in response to the progressive urbanization of the watersheds and intensification of land uses. Our results highlight the role of salt marsh sediments as sinks for microplastics in the marine environment.

Floodplain Wetland Channel Planform, Cross‐Sectional Morphology, and Sediment Characteristics Along an Estuarine to Tidal River Gradient

AbstractThe formation of wetland channel networks on coastal river floodplains is affected by spatial transitions in tidal‐fluvial processes. This study evaluated sedimentation patterns in tidal marshes on the 147‐km tidal Columbia River and its 87‐km estuary and characterized the cross‐sectional geometry and planform morphometry of tidal wetlands along the estuarine to riverine gradient. Tidal marshes were predominantly depositional (median 0.7 cm yr−1, primarily silt), consistent with late‐Holocene rates. Marsh sediment accretion rate increased with annual maximum 7‐day average daily maximum water depth. Elevation was negatively correlated with accretion rate in the tidal river and positively correlated with total organic carbon (TOC%) in the tidal river and estuary. TOC% was greater in the estuary and, like percent fines, greater farther from the main‐channel Columbia River. For four single‐channel and six channel‐network planforms of tidal marsh, shrub‐dominated, and forested wetlands: (1) Spatial patterns of cross‐sectional morphology conformed to typical coastal wetland morphology in estuarine reaches up to the tidal river. (2) Reach‐based and common slope linear models for channel perimeter on wetland area were predictive in the estuary (R2 &gt; 0.81) and differed between island (n = 144) and mainland (n = 164) wetlands. No planform linear models were predictive for the entire study area or the tidal river. No models for channel outlets were acceptable. (3) Channel perimeter, surface area, and outlets were all highly variable within and among estuarine–riverine reaches. Analyses of sediment, cross‐section, and planform support the use of a “natural network paradigm” for wetland‐restoration design on tidal river floodplains.

The Decade-long Evolution and Toxicity of Oil Transformation Compounds in Louisiana Salt Marsh Sediments Revealed by FT-ICR Mass Spectrometry

The 2010 explosion of the Deepwater Horizon (DWH) drilling rig provides a unique opportunity to advance the understanding of the evolution of lingering oil in Louisiana salt marshes for nearly a decade after the initial spill. After an oil spill, petroleum hydrocarbons undergo physical and chemical processes that transform the native oil composition to a polyfunctional oxygenated hydrocarbon matrix that contains ketones, hydroxyl, and carboxylic acid functional groups. Preliminary analysis of saltmarsh sediments impacted by the DWH spill between 2010–2014 identifies highly polar oxygenated hydrocarbons with carboxylic acid functionalities. These highly polar, high molecular weight compounds are not detectable by conventional gas chromatography (GC) based techniques. Here, we employ advanced analytical technologies such as targeted fractionation and isolation methods with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to document and catalog oil transformation compounds detected in oiled saltmarsh sediments. The toxicities of these environmental transformation products are determined by commercial Microtox bioassay. Samples have been continuously collected from 21 previously established sampling stations located in northern Barataria Bay for the past ten years post-spill. The concentration of oil contamination from heavily and moderately oiled sites exceed non-oiled reference sites for years after the spill. Combining chromatographic fractionation, conventional GC MS, and ultrahigh-resolution FT-ICR Mass Spectrometry analyses, we identified tens-of-thousands of biotic and abiotic crude oil transformation products that remain persistent and keep evolving in the environment. Heavily oxygenated transformation products, which are mostly only accessible by FT-ICR MS, span a wide range of chemical functionalities. Toxicity of the non-GC amenable fraction of oil contaminants was measured to determine the health risk of weathered oil residues left in the salt marsh environment. The current study creates a decade-long assessment record, and a comprehensive understanding of petroleum compounds evolution of oil-impacted wetlands ecosystem.

Vegetation dynamics and climate variability over the past 2000 years inferred from Son Kul marsh in the western Tianshan Mountains

A pollen study was conducted on an alpine marsh sediment in the Son Kul Basin and was allowed to reconstruct changes in vegetation dynamics and climatic information in the western Tianshan Mountains during the past 2000 years. Pollen diagram reveals that regional vegetation is dominated by alpine meadow in the past 2000 years, being similar with modern vegetation components in the basin. The Artemisia/Chenopodiaceae-indicated moisture exhibits a warm-dry Roman Warm Period (RWP, ∼0–∼500 AD), a cold-dry Dark Ages Cold Period (DACP, ∼500–∼800 AD), a warm-wet Medieval Warm Period (MWP, ∼800–∼1350 AD), a cold-dry Little Ice Age (LIA, ∼1350–∼1850 AD) and a warm-dry Current Warm Period (CWP, since ∼1850 AD). Our pollen-based moisture reconstructions are supported by other nearby moisture records. Combined with other pollen data in the western Tianshan Mountains, we found that the vegetation was relatively stable before ∼1650–∼1750 AD and the anthropogenic activities obviously intensified afterwards (especially at the middle-elevation sites). Further work involving more and higher-resolution palaeovegetation records would contribute to fully understand the information on the complex links between environmental, climatic and anthropogenic changes in the western Tianshan Mountains.

Refining Estimates of Greenhouse Gas Emissions From Salt Marsh “Blue Carbon” Erosion and Decomposition

Coastal wetlands have sediments that contain organic matter preserved against decomposition for timespans that can range up to millennia. This “blue carbon” in wetland sediments has been proposed as a sink for atmospheric carbon dioxide and a potential source of greenhouse gases if coastal habitats are lost. A missing gap in the role of coastal habitats in the global carbon cycle is elucidating the fate of wetland sediment carbon following disturbance events, such as erosion, that can liberate organic matter to an oxygenated environment where decomposition can more readily occur. Here, we track the fate of previously stored salt marsh sediment by measuring the production of carbon dioxide (CO2) and methane (CH4) during an oxygenated incubation. Sediments from two depth horizons (5–10 cm and 20–25 cm) were incubated at two temperatures (20 and 30°C) for 161 days. Q10 of the decomposition process over the entire course of the experiment was 2.0 ± 0.1 and 2.2 ± 0.2 for shallow and deep horizons, respectively. Activation energy for the decomposition reaction (49.7 kJ ⋅ mol–1 and 58.8 kJ ⋅ mol–1 for shallow and deep sediment horizons, respectively) was used to calculate temperature-specific decomposition rates that could be applied to environmental data. Using high-frequency water temperature data, this strategy was applied to coastal states in the conterminous United States (CONUS) where we estimated annual in situ decomposition of eroded salt marsh organic matter as 7–24% loss per year. We estimate 62.90 ± 2.81 Gg C ⋅ yr–1 is emitted from eroded salt marsh sediment decomposition in the CONUS.

Toxicity evaluation of Louisiana nearshore marsh sediments following the Deepwater Horizon oil spill

A crucial step towards understanding potential impacts of the Deepwater Horizon oil spill to marsh ecosystems was to quantitatively determine the toxicity of oil remaining in the sediment. The objective of this study was to assess the potential injury to benthic species using standardized toxicity bioassays. Sediments were collected from locations with differing degrees of oiling based on previous assessments. Less than 13% of the 315 toxicity tests resulted in toxicity to amphipods, mysid shrimp, or sea urchins. There was no relationship among toxicity test results, oiling category or measured total polycyclic aromatic hydrocarbons (PAHs). Equilibrium partitioning sediment benchmarks and additive toxic units (ESBTUs) were applied as an additional line of evidence to evaluate the potential adverse effects based on PAH concentrations in field sediments. The ∑ESBTUs based on 34 PAHs at the 64 nearshore sampling locations were <1, indicating PAHs in sediments were unlikely to cause adverse impacts.

Onset of runaway fragmentation of salt marshes

Summary Salt marshes are valuable but vulnerable coastal ecosystems that adapt to relative sea level rise (RSLR) by accumulating organic matter and inorganic sediment. The natural limit of these processes defines a threshold rate of RSLR beyond which marshes drown, resulting in ponding and conversion to open waters. We develop a simplified formulation for sediment transport across marshes to show that pond formation leads to runaway marsh fragmentation, a process characterized by a self-similar hierarchy of pond sizes with power-law distributions. We find the threshold for marsh fragmentation scales primarily with tidal range and that sediment supply is only relevant where tides are sufficient to transport sediment to the marsh interior. Thus the RSLR threshold is controlled by organic accretion in microtidal marshes regardless of the suspended sediment concentration at the marsh edge. This explains the observed fragmentation of microtidal marshes and suggests a tipping point for widespread marsh loss.