Processes that influence carbon isotope variations in salt marsh sediments (original) (raw)
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Carbon Sources in the Sediments of a Restoring vs. Historically Unaltered Salt Marsh
Estuaries and Coasts
Salt marshes provide the important ecosystem service of carbon storage in their sediments; however, little is known about the sources of such carbon and whether they differ between historically unaltered and restoring systems. In this study, stable isotope analysis was used to quantify carbon sources in a restoring, sparsely vegetated marsh (Restoring) and an adjacent, historically unaltered marsh (Reference) in the Nisqually River Delta (NRD) of Washington, USA. Three sediment cores were collected at "Inland" and "Seaward" locations at both marshes~6 years after restoration. Benthic diatoms, C3 plants, C4 plants, and particulate organic matter (POM) were collected throughout the NRD. δ 13 C and δ 15 N values of sources and sediments were used in a Bayesian stable isotope mixing model to determine the contribution of each carbon source to the sediments of both marshes. Autochthonous marsh C3 plants contributed 73 ± 10% (98 g C m −2 year −1) and 89 ± 11% (119 g C m −2 year −1) to Reference-Inland and Reference-Seaward sediment carbon sinks, respectively. In contrast, the sediment carbon sink at the Restoring Marsh received a broad assortment of predominantly allochthonous materials, which varied in relative contribution based on source distance and abundance. Marsh POM contributed the most to Restoring-Seaward (42 ± 34%) (69 g C m −2 year −1) followed by Riverine POM at Restoring-Inland (32 ± 41%) (52 g C m −2 year −1). Overall, this study demonstrates that largely unvegetated, restoring marshes can accumulate carbon by relying predominantly on allochthonous material, which comes mainly from the most abundant and closest estuarine sources.
Organic Geochemistry, 1999
Compound speci®c d 13 C analyses were used to determine the relative input of a C 4 temperate grass (Spartina anglica ) to primary biomass in a salt-marsh sediment. Lipid distributions revealed a C 32 n-alkanol homologue as a characteristically dominant component of Spartina anglica whilst the cohabiting C 3 species, Puccinellia maritima, exhibited a C 26 maximum. The C 32 n-alkanol component was used to create an isotopic mixing model, between organic matter derived from Spartina anglica and Puccinellia maritima, to estimate their relative contribution to the primary biomass input of salt-marsh sediments. The application of sedimentary lipid isotope data to the model gave values of Spartina anglica contributions ranging from 37 to 100%. This investigation represents the ®rst attempt to quantify inputs to sedimentary biomass based on compound speci®c stable carbon isotope techniques. #
Estuarine, Coastal and Shelf Science, 2010
Keywords: carbon isotopes saturated hydrocarbons salt marshes sea level carbon cycle a b s t r a c t Sources of sedimentary organic matter to a Morse River, Maine (USA) salt marsh over the last 3390 AE 60 RCYBP (Radiocarbon Years Before Present) are determined using distribution patterns of nalkanes, bulk carbon isotopic analysis, and compound-specific carbon isotopic analysis. Marsh foraminiferal counts suggest a ubiquitous presence of high marsh and higher-high marsh deposits (dominated by Trochammina macrescens forma macrescens, Trochammina comprimata, and Trochammina inflata), implying deposition from w0.2 m to 0.5 m above mean high water. Distributions of n-alkanes show a primary contribution from higher plants, confirmed by an average chain length value of 27.5 for the core sediments, and carbon preference index values all >3. Many sample depths are dominated by the C 25 alkane. Salicornia depressa and Ruppia maritima have similar n-alkane distributions to many of the salt marsh sediments, and we suggest that one or both of these plants is either an important source to the biomass of the marsh through time, or that another unidentified higher plant source is contributing heavily to the sediment pool. Bacterial degradation or algal inputs to the marsh sediments appear to be minor. Compound-specific carbon isotopic analyses of the C 27 alkane are on average 7.2& depleted relative to bulk values, but the two records are strongly correlated (R 2 ¼ 0.89), suggesting that marsh plants dominate the bulk carbon isotopic signal. Our study underscores the importance of using caution when applying mixing models of plant species to salt marsh sediments, especially when relatively few plants are included in the model.
Organic Carbon Isotope Systematics of Coastal Marshes
Estuarine Coastal and Shelf Science, 1997
Measurements of nitrogen, organic carbon and δ13C are presented forSpartina-dominated marsh sediments from a mineral marsh in SW Netherlands and from a peaty marsh in Massachusetts, U.S.A. δ13C of organic carbon in the peaty marsh sediments is similar to that ofSpartinamaterial, whereas that in mineral marshes is depleted by 9-12‰. It is argued that this depletion in δ13C of organic matter in marsh sediments is due to trapping of allochthonous organic matter which is depleted in13C. The isotopic composition and concentration of organic carbon are used in a simple mass balance to constrain the amount of plant material accumulating in marsh sediments, i.e. in terms of the so-called net ecosystem production. Net ecosystem production (∼2-100 g C m-2year-1) is a small fraction (1-5%) of plant production (∼2000 g C m-2year-1). This small amount of plant material being preserved is nevertheless sufficient to support marsh-accretion rates similar to the rate of sea-level rise.
Refractory organic matter in coastal salt marshes-effect on C sequestration calculations
• We studied how refractory carbon inputs affect marsh age and C sequestra-tion budgets. • We measured Δ 14 C and δ 13 C of total organic carbon (TOC) and refractory carbon (C RF). • TOC was dominated by autochthonous inputs, C RF was dominated by allochtho-nous C. • Allochthonous C delivery was controlled by the size and slope of each watershed. • Steep-gradient rivers delivered Δ 14 C-depleted C RF to their estuarine marshes. Editor: Mae Sexauer Gustin The age and ability of salt marshes to accumulate and sequester carbon is often assessed using the carbon isotopic signatures (Δ 14 C and δ 13 C) of sedimentary organic matter. However, transfers of allochthonous refractory carbon (C RF) from the watershed to marshes would not represent new C sequestration. To better understand how refrac-tory carbon (C RF) inputs affect assessments of marsh age and C sequestration, Δ 14 C and δ 13 C of both total organic carbon (TOC), C RF , and non-C RF organic matter fractions were measured in salt marshes from four contrasting systems on the North Atlantic coast. To our knowledge, no salt marsh sediment study has considered refractory or allochthonous carbon in carbon budget calculations or the impact on chronologies. Stable and radiogenic isotope data suggest that while TOC was dominated by autochthonous plant inputs, C RF was dominated by locally recycled or allochthonous C, the delivery of which was controlled by the size and slope of each watershed. Steep-gradient rivers analyzed delivered Δ 14 C-depleted C RF to their estuarine marshes, while the site located in the low-gradient river was associated with larger C RF content. Finally, the marsh isolated from riverine input contained the least fraction of TOC as C RF. Laterally transported C RF caused only a small offset in Δ 14 C in relation to TOC in low-gradient systems (average Δ 14 C offset was −44.4 and −24.2‰ at each location). However, the presence of allochthonous Δ 14 C-depleted C RF in sediments of steep-gradient rivers led to large overestimates of the time of organic matter deposition (i.e. apparent age was older than the 'true' time of deposition) (Δ 14 C offset ranged from −170.6 to −528.9‰). Further, reliance on TOC or loss on ignition analyses to calculate C
Carbon flow through oxygen and sulfate reduction pathways in salt marsh sediments
Limnology and Oceanography, 1984
We measured surface 0, uptake, 35S0.,2-reduction, and total sediment metabolism (CO, production) in sediments (O-30 cm) supporting stands of short Spartina alterniflora in a New England salt marsh. Sediment CO, production was highest at the surface where 0, was present and declined rapidly with depth. In deeper sediments (below 5 cm) CO, production was equal to 35S0,2-reduction as determined by the Cr2+ reduction technique. Time-course experiments using CO, production and 35S04?-reduction (by Cr2+ reduction and aqua regia digest) indicate that the aqua regia technique is not reliable for measuring SOd2-reduction and that the rate ofSOd2-reduction is much less than previously reported for this marsh.
Quantifying C and N contents and isotope signatures of SOM pools in the HJ Andrews DIRT plots
2004
The mechanisms governing short-and long-term belowground carbon dynamics need to be understood. As part of a larger project developed to assess the effect of quantity and quality of litter inputs on the rate of soil organic matter (SOM) formation, I examined SOM in the H. J. Andrews Detritus Input and Removal Treatments (DIRT) plots. This study was designed to: (1) determine how five years of treatment had changed the SOM in the reduced input plots and the added input plots relative to the control plots; (2) determine if the more labile (light) fraction of the soil had changed more from the manipulations than the more recalcitrant (heavy) fraction of the SOM; (3) document how the light and heavy fractions changed with depth in this coniferous forest relative to the published trends in other forest types; and, (4) determine if density fractionation conserved the C and N of the sample, or if the method resulted in any losses or transformations that might yield the method untrustworthy. To accomplish these objectives, I measured carbon and nitrogen concentrations and isotope values of SOM at different depths in the soil profile and by density fraction because previous work has shown that these parameters are good indicators of soil age! recalcitrance. I separated soil into labile (light) and more recalcitrant (heavy, mineral-bound) fractions from three depths in each of 18 treatment plots at the H. J. Andrews Forest, Cascades, OR using sodium polytungstate. Soil light fraction averaged 5.
Carbon Storage in Tagus Salt Marsh Sediments
Water, Air, & Soil Pollution: Focus, 2000
Seasonal variation of above ground and belowground biomass of Spartina maritima and Halimione portulacoides, decomposition rates of belowground detritus in litterbags, and carbon partitioning in plant components and sediments were determined in two Tagus estuary marshes with different environmental conditions. Total biomass was higher in the saltier marsh from 7,190 to 6,593 g m −2 dw and belowground component contributed to more than 90%. Litterbag experiment showed that 30 to 50% of carbon is decomposed within a month (decomposition rate from 0.024 to 0.060 d −1 ). Slower decomposition in subsequent periods agrees with accumulation of carbon concentration in sediment. Atmospheric carbon annually transferred to the plant belowground biomass is stored more efficiently in sediments of Corroios than Pancas.
Isotopic variations of dissolved inorganic carbon
Chemical Geology, 1966
The isotopic composition of the dissolved inorganic carbon in near shore waters varies systematically w~h chlorinity. Extrapolation to 0-chlorinity shows a513C range of-5 to-ll/oo relative to the Chicago PDB 1 standard for the Hudson and Potomac Rivers and the bayous feeding into Mississippi Sound. The composition of a few samples of open ocean water shows a range of -1 to +2~oo. The data suggest that the amount and isotopic composition of the total CO2 and chlorinity of estuary and coastal waters may be used to characterize and to study the mixing of various water masses, and that it is necessary to re-evaluate paleoenvironmental methods of sediment classification based on the isotopic composition of sedimentary organic and inorganic carbon.
Estuaries, 2000
Surface soil and sediment samples collected along a forest-brackish marsh-salt marsh transect in a southeastern U.S. estuary were separated into three different fractions (sand, macro-organic matter, and humus) based on size and density. Elemental, stable carbon isotope, and lignin analyses of these samples reveal important contrasts in the quantity, composition, and sources of organic matter between forest and marsh sites. Elevated nitrogen contents in humus samples suggest nitrogen incorporation during humification is most extensive in forest soils relative to the marsh sites.