Pb and Cs in sediments from Sagami Bay, Japan: sedimentation rates and inventories

Profiles of the radioisotopes ²¹⁰Pb and ¹³⁷Cs were determined in 15 sediment cores collected from Sagami Bay, Japan. The activities of ²¹⁰Pb_ex (unsupported) in core top sediments increased with water depth from 25 dpm g⁻¹ on the upper continental slope off the mouth of Tokyo Bay to an average of 283 dpm g⁻¹ at the deep-sea station SB. The high ²¹⁰Pb trapping efficiency of settling particles expected from the results of the sediment trap experiment near the SB site suggests that effective ²¹⁰Pb enrichment in surface sediments may occur during resuspension and lateral transportation of particles via the benthic nepheloid layer on the continental slope. In several cores, ¹³⁷Cs profiles showed an increase, a distinct peak, and then a decrease to an undetectable level downcore. These profiles can be compared with the temporal change of bomb-produced ¹³⁷Cs fallout.The mean sedimentation rates estimated by the ²¹⁰Pb_ex inventory method, rather than using ²¹⁰Pb_ex profiles, ranged from 0.06 g cm⁻² y⁻¹ to 0.14 g cm⁻² y⁻¹. The average value of the rates in SB cores was calculated to be 0.11 g cm⁻² y⁻¹, which was similar to that calculated under the assumption that the age of the ¹³⁷Cs peak corresponds to its maximum fallout year in 1963.Although ¹³⁷Cs inventories represented one tenth of the anthropogenic fallout of ¹³⁷Cs until 1997, they correlated with the increase in ²¹⁰Pb_ex inventory. This suggests that the scavenging of refractory ¹³⁷Cs as well as ²¹⁰Pb by settling particles in the water column can lead to the formation of a time marker layer even in deep-sea sediment core, such as at the SB site.     

Dispersion and accumulation of radionuclides in sediment of Urazoko Bay (I)

Contents of⁹⁰Sr,¹³⁷Cs and⁶⁰Co in surface sediments were investigated in and around Urazoko Bay, Fukui Prefecture, where a nuclear power plant has been operating since 1969 and the waste effluent has been released into the sea. The main source of⁹⁰Sr and¹³⁷Cs was recognized to be radioactive fallout, whereas⁶⁰Co was considered to originate from the nuclear power plant. Using the¹³⁷Cs concentration as an indicator of sorption capacity of the sediment, the behaviour of⁶⁰Co was investigated. No seasonal variation was observed in the distribution pattern of⁶⁰Co/¹³⁷Cs ratio and the retention of the heavy initial discharge in 1969 was considered to dominate the radionuclide level in the sediment. Correlation of⁶⁰Co/¹³⁷Cs ratio in the sediment and the distance from the discharge outlet was expressed by a simple exponential function of the distance. It was suggested that the contamination is spreading out gradually to the outer region of Urazoko Bay.     

Radiocaesium and 210Pb in Clyde sea loch sediments

Analysis of ²¹⁰Pb, ²²⁶Ra, ¹³⁷Cs and ¹³⁴Cs in short sediment cores provide first estimates of deposition rates in some Clyde sea lochs. The radio-caesium nuclides originate mainly in the liquid effluent released at distance from the Clyde by the Windscale nuclear fuel reprocessing plant and their concentrations in Clyde sediments provide information on (a) enrichment factors onto particulate matter, (b) surficial mixing coefficients and (c) sedimentation rates. A radiocaesium residence time in coastal waters of ca. 10³ years reflects the importance of scavenging by the high nearshore particulate flux. ²¹⁰Pb levels in sediments are controlled, in the unsupported fraction, by a major input sorbed on catchment particulates and, in the supported component, by ²²⁶Ra activities occasionally perturbed by unusually high surface values probably of planktonic origin. In one loch, detectable levels of ¹³⁴Cs and ⁶⁰Co are attributed to their discharge by nuclear submarines.     

The study of mixing rates and sedimentation rates in Meizhou Bay sediments

-The applicability of the doubie-layer model for 210Pb chronology in coastal marine environments was discussed,which is successfully used in the study of mixing rates and sedimentation rates in Meizhou Bay. Differences among sedimentation rates deduced from 210Pb, 210Po, and 137Cs were compared. Mixing rates in the sediment surface layer were determined by means of excess 234Th.     

Dispersion and accumulation of radionuclide in sediment of Urazoko Bay-II

Further studies on the transport in sea water and accumulation in marine sediment of the radionuclides, released from a nuclear power plant, on Urazoko Bay, Fukui Prefecture were reported.The partition of radionuclides among the particle size fractions of a sediment sample indicated that the accumulation of⁶⁰Co,¹³⁷Cs and⁵⁴Mn in the sediment were dominated substantially by the sorption on the particle surface in sea water, and the patterns of their sorption were considerably similar to each other.In regard to the⁶⁰Co contamination of marine sediments in an area of about 13 km² around the nuclear power plant, 60% of the⁶⁰Co contamination were retained in Urazoko Bay, which occupies only 7% of the bottom surface of the region.It is clear that a part of⁶⁰Co accumulated in the sediment of Urazoko Bay gradually moved to the outerward.     

Modern sediment characteristics and accumulation rates from the delta front to prodelta of the Yellow River (Huanghe)

Since 1976, the main channel of the Yellow River (Huanghe) has been on the east side of the delta complex, and the river has prograded a broad new delta lobe in Laizhou Bay of the Bohai Sea. In 2012, extensive bathymetric and high-resolution seismic profiles were conducted and sediment cores were collected off the new delta lobe. This study examined delta sedimentation and morphology along a profile across the modern subaqueous Yellow River delta and into Laizhou Bay, by analyzing sediment radionuclides (¹³⁷Cs, ²¹⁰Pb and ⁷Be), sedimentary structure, grain-size composition, organic carbon content, and morphological changes between 1976 and 2012. The change in the bathymetric profile, longitudinal to the river’s course, reveals subaqueous delta progradation during this period. The subbottom boundary between the new delta lobe sediment and the older seafloor sediment (before the 1976 course shift) was identified in terms of lithology and radionuclide distributions, and recognized as a downlap surface in the seismic record. The accumulation rate of the new delta lobe sediment is estimated to be 5–18.6 cm year^–1 on the delta front slope, 2 cm year^–1 at the toe of the slope, and 1–2 cm year^–1 in the shelf areas of Laizhou Bay. Sediment facies also change offshore, from alternations of gray and brown sediment in the proximal area to gray bioturbated fine sediment in the distal area. Based on ⁷Be distribution, the shorter-term deposition rate was at least 20 cm year^–1 in the delta front.     

Sedimentation and mixing rates of radionuclides in Barents Sea sediments off Novaya Zemlya

Radionuclide measurements have been conducted on sediment cores collected in 1992 in the south-eastern region of the Barents Sea, known as the Pechora Sea. Cesium-137 and ^239,24OPu activities in surface sediments are generally less than 30 Bq/kg, with the highest levels being measured in sediments off the southwestern coastline of the island of Novaya Zemlya. High correlations between both ¹³⁷Cs and ^239,24OPu and the concentration of fine (< 63 μm) particles in surface sediments indicate that much of the variance in radionuclide concentrations throughout the Pechora Sea can be explained by particle size fractionation. However, elevated activities of ¹³⁷Cs (138 Bq/kg), ⁶⁰Co (92 Bq/kg), ²⁴¹Am (433 Bq/kg), and especially ^239,24OPu (8.47 × 103 Bq/kg) were measured in one surface sediment sample from the fjord of Chernaya Bay on the southern coast of Novaya Zemlya. The source of radioactive contamination is two underwater nuclear tests conducted in Chernaya Bay in 1955 and 1957.The ²³⁸Pu/^239,240Vu activity ratio of 0.0245 in Chernaya Bay is equivalent to values measured in global fallout. The ²⁴⁰Pu/²³⁹Pu atom ratio (0.0304), measured by mass spectrometry, is much lower than values (0.18) typical of global fallout, but is consistent with ratios measured for fallout from the early (1951–1955) series of weapons tests at the Nevada Test Site. The timing of the Chernaya Bay source term, estimated from the ²⁴¹Am/²⁴¹Pu ratio, is consistent with the timing of the 1955 and 1957 underwater nuclear tests. Relatively low initial yields of ²⁴¹Pu (²⁴¹Pu/²³⁹Pu atom RATIO = 0.00 123) in these tests have resulted in relatively low ²⁴¹Am/^239,240Pu activity ratios (0.05) in recent sediments in Chernaya Bay.Radionuclide tracer profiles in cores from the Pechora Sea can be simulated using a two-layer biodiffusion model with rapid, near-homogeneous mixing in the surface mixed layer and reduced mixing in the deep layer. Lead-210 profiles are consistent with a wide range of sedimentation and mixing rates in the deep sediment layer. However, the ¹³7Cs and ^239,240Pu results further constrain the model parameters and indicate that the downward transport of radionuclides in the sediments is governed primarily by sediment mixing, with sediment burial playing a secondary role.     

Simple coprecipitation method combined with low-background γ-spectrometry: Determination of 7Be, 137Cs, 210Pb, and radium and thorium isotopes in small-volume coastal water samples

Application of a coprecipitation method combined with low-background γ-spectrometry allowed the isolation of ⁷Be, ¹³⁷Cs, ²¹⁰Pb, and radium and thorium isotopes together with BaSO₄, Fe(OH)₃, or AMP/Cs in 18 L of coastal water. This simple method showed high chemical yields of 77–91% (mean, 85%) for ²¹⁰Pb and radium isotopes, 87–92% (mean, 91%) for 7Be and thorium isotopes, and 98% for ¹³⁷Cs. The activity of the nuclides showed good reproducibility with standard deviations of 1–8%, while the deviation of activity in particle-reactive ²¹⁰Pb (32%) and ⁷Be (21%) in filtered water samples was exceptionally large.     

Radionuclides in Arctic sea ice: Tracers of sources,fates and ice transit time scales

Arctic sea ice can incorporate sediment and associated chemical species during its formation in shallow shelf environments and can also intercept atmospherically transported material during transit. Release of this material in ice ablation areas (e.g. the Fram Strait) enhances fluxes of both sediments and associated species in such areas. We have used a suite of natural (⁷Be, ²¹⁰Pb) and anthropogenic (¹³⁷Cs, ²³⁹Pu, ²⁴⁰Pu) radionuclides in sea ice, sea-ice sediments (SIS), sediment trap material and bottom sediments from the Fram Strait to estimate transit times of sea ice from source to ablation areas, calculate radionuclide fluxes to the Fram Strait and investigate the role of sea-ice entrained sediments in sedimentation processes. Sea ice intercepts and transports the atmospherically supplied radionuclides ⁷Be and ²¹⁰Pb, which are carried in the ice and are scavenged by any entrained SIS. All of the ⁷Be and most of the excess ²¹⁰Pb measured in SIS collected in the Fram Strait are added to the ice during transit through the Arctic Ocean, and we use these radionuclides as chronometers to calculate ice transit times for individual ice floes. Transit times estimated from the ²¹⁰Pb inventories in two ice cores are 1–3 years. Values estimated from the ⁷Be/²¹⁰Pb_excess activity ratio of SIS are about 3–5 years. Finally, equilibrium values of the activity ratio of ²¹⁰Pb to its granddaughter ²¹⁰Po in the ice cores indicate transit times of at least 2 years. These transit times are consistent with back-trajectory analyses of the ice floes. The latter, as well as the clay-mineral assemblage of the SIS (low smectite and high illite content), suggest that the sampled sea-ice floes originated from the eastern Siberian Arctic shelf seas such as the eastern Laptev Sea and the East Siberian Sea. This result is in agreement with the relatively low activities of ^239,240Pu and ¹³⁷Cs and the ²⁴⁰Pu/²³⁹Pu atom ratios (∼0.18, equivalent to that in global fallout) in SIS, indicating that prior global atmospheric fallout, rather than nuclear fuel reprocessing facilities, forms the main source of these anthropogenic radionuclides reaching the western Fram Strait at the time of sampling (1999). Transport of radionuclides by sea ice through the Arctic Ocean, either associated with entrained SIS or dissolved in the ice, accounts for a significant flux in ablation areas such as the Fram Strait, up to several times larger than the current atmospheric flux in the area. Calculated fluxes derived from sea-ice melting compare well to fluxes obtained from sediment traps deployed in the Fram Strait and are consistent with inventories in bottom sediments. ²⁴⁰Pu/²³⁹Pu atomic ratios lower than 0.18 in bottom sediments from the Fram Strait provide evidence that plutonium from a source other than atmospheric fallout has reached the area. Most likely sources of this Pu include tropospheric fallout from atomic weapons testing of the former Soviet Union prior to 1963 and Pu released from nuclear reprocessing facilities, intercepted and transported by sea ice to the ablation areas. Future work is envisaged to more thoroughly understand the actual mechanisms by which radionuclides are incorporated in sea ice, focusing on the quantification of the efficiency of scavenging by SIS and the effect of melting and refreezing processes over the course of several years during transit.     

底部边界层颗粒态物质迁移特征的放射性核素分析方法

利用颗粒态放射性核素携带的颗粒物历经过程信息,我们提出用泥沙扩散方程和颗粒态放射性核素扩散方程联解底部边界层颗粒态物质迁移参数的方法。分析实例的样品取自荷兰Waden Sea南部Balgzand潮滩(砂坪)和Mok湾潮滩(泥坪)的两个站位(BG1和Mok2)。示踪核素为~(234)Th,~(210)Pb和~(137)Cs,其放射性比度由r能谱测出。     

Environmental changes in the Amur Bay (Japan/East Sea) during the last 150 years revealed by examination of diatoms and silicoflagellates

Siliceous unicellular microalgae — diatoms and silicoflagellates from sediments in Amur Bay were analyzed with high temporal resolution to examine changes over the last 150 years. The age of sediments was estimated from unsupported ²¹⁰Pb controlled by ¹³⁷Cs. Siliceous microalgae examined in each cm of two sediment cores demonstrated significant changes in the ecological structure of the assemblages that reflected changes in sedimentation conditions. During the years 1860–1910 the sediments accumulated under the great influence of river runoff. For about the next 50 years the number of freshwater species and marine benthic diatoms in sediments sharply declined, which is probably connected with the weakening of the effects of river runoff due to deforestation. Since the early 1960s the sedimentation conditions in the Amur Bay changed significantly. Marine planktonic diatoms and silicoflagellates began to prevail in sediments and this reflects increasing microphytoplankton productivity. One consequence of this was the formation of seasonal bottom hypoxia in Amur Bay. The ecological structure of diatom and silicoflagellate assemblages indicates that the sea level began to rise since the early 1960s and this corresponds to the water and air temperature increase in the area for that period. The obtained data suggest that the environmental changes over the last 150 years in Armur Bay are associated with the weakening of river runoff due to deforestation, sea level rise caused by global warming, and the increase of siliceous microplankton productivity that resulted in the formation of seasonal bottom hypoxia.

     

Recent volumetric changes in salt marsh soils

Salt marsh sediment volume decreases from organic decomposition, compaction of solids, and de-watering, and each of these processes may change with age. Variability in the vertical accretion rate within the upper 2 m was determined by assembling results from concurrent application of the ¹³⁷Cs and ²¹⁰Pb dating techniques used to estimate sediment age since 1963/1964, and 0 to ca 100+ years before present (yBP), respectively. The relationship between ²¹⁰Pb and the ¹³⁷Cs dated accretion rates (Sed₂₁₀ and Sed₁₃₇, respectively) was linear for 45 salt marsh and mangrove environments. Sed₂₁₀ averaged 75% of Sed₁₃₇ suggesting that vertical accretion over the last 100+ years is driven by soil organic matter accumulation, as shown for the pre ¹³⁷Cs dated horizon. The ratio of Sed₂₁₀/Sed₁₃₇ declines with increasing mineral content. A linear multiple regression equation that includes bulk density and Sed₁₃₇ to predict Sed₂₁₀ described 97% of the variance in Sed₂₁₀. Sediments from Connecticut, Delaware and Louisiana coastal environments dated with ¹⁴C indicate a relatively constant sediment accretion rate of 0.13 cm year⁻¹ for 1000–7000 yBP, which occurs within 2 m of today's marsh surface and equals modern sea level rise rates. Soil subsidence is not shown to be distinctly different in these vastly different coastal settings. The major reason why the Sed₁₃₇ measurements indicate higher accretion rates than do the Sed₂₁₀ measurements is because the former apply to younger sediments where the effects of root growth and decomposition are greater than in the latter. The most intense rates of change in soil volume in organic-rich salt marshes sediments is, therefore, neither in deep or old sediments (>4 m; >1000 years), but within the first several hundreds of years after accumulation. The average changes in organic and inorganic constituents downcore are nearly equal for 58 dated sediment cores from the northern Gulf of Mexico. These parallel changes downcore are best described as resulting from compaction, rather than from organic matter decomposition. Thus most of the volumetric changes in these salt marsh sediments occurs in the upper 2 m, and declines quickly with depth. Extrapolation forwards or backwards, using results from the ²¹⁰Pb and the ¹³⁷Cs dating technique appear to be warranted for the types of samples from the environments described here.     

近30年辽河口盖州滩沉积环境研究

对采自辽河口盖州滩北部柱状样进行沉积物粒度分析和137Cs、210Pb年代测试,结合1988年、1994年、1999年和2013年4期遥感影像数据,对近30年盖州滩沉积环境进行了研究。沉积物样品主要为砂质粉砂和粉砂,分选较差,极正偏,峰态很窄至宽。近30年来盖州滩沉积速率为2.83 cm/a,沉积较为快速。1969年,盘山建闸后,虽然辽河入海径流量和输沙量减少,盖州滩沉积物粒度仍受到径流、潮流和波浪的共同影响,以径流影响为主;1999年,辽河截流后,径流影响较弱,潮流和波浪对沉积物改造作用增强。盖州滩发育过程受到径流、潮流和波浪以及人类活动的共同影响,但近30年来盖州滩主体形态无明显变化,仅北部沙洲不断淤高,与主沙体连为一起,南部受波浪改造呈鸟足状。     

Meiobenthic Organisms as Indicators of Mixing Processes in the Baltic Surface Sediments*

Abstract. Two sediment cores were collected in the southern Baltic Sea and sliced into 1.0cm-thick layers. Sediments of each layer were analysed for activities of ²¹⁰Pb, ¹³⁷Cs, ¹³⁴Cs and for the density of meiobenthic organisms (meiofauna). Zones with the rapid mixing occur in the uppermost layers (0–3 cm) of the cores based on ^I34Cs profiles. The extent and density of meiofauna confirmed the rapid mixing and revealed layers with slow mixing (4–8 cm). Sedimentation rates were derived from ²¹⁰Pb profiles below the mixing zones (1.21 and 1.72 mm a-^I) and were confumed by ¹³⁷Cs distribution. Of twenty-one major meiofauna taxa commonly found in the Baltic sediments, three were present in the cores.     

Processes affecting long-term changes in <Superscript>137</Superscript>Cs concentration in surface sediments off Fukushima

Temporal changes in cesium-137 (¹³⁷Cs) concentrations in the surface (0–10 cm) layer of seabed sediment were quantified from continuous observation data at 71 stations within a 150-km radius of the Fukushima Daiichi Nuclear Power Plant, and the primary processes affecting temporal changes were identified. From March 2011 to the end of 2015, about 80% of the initially deposited ¹³⁷Cs in the surface sediment in the coastal region (bottom depth ≤100 m) region has dissipated (radioactive decay is not included). Such a remarkable change in the ¹³⁷Cs concentration was not observed in the offshore (>100 m) region. This paper focuses on the following three processes that affected the decrease in the ¹³⁷Cs concentrations, and assesses their relative importance; (1) resuspension and transport of ¹³⁷Cs-bound sediment, (2) desorption of ¹³⁷Cs from the sediment, and (3) dilution of ¹³⁷Cs by vertical mixing of sediment. Consequently, it was estimated that the first two processes together have potentially contributed to reduce the ¹³⁷Cs inventory in the top 10 cm of the coastal region by at most 35%. Furthermore, by applying a pulse input sediment mixing model to the observed vertical distribution of sedimentary ¹³⁷Cs, it was also estimated that more than 43% of the ¹³⁷Cs in the surface sediment was transported to deeper sediment layers by vertical mixing of the sediment. This indicates that the decrease of ¹³⁷Cs concentrations in coastal sediments was mainly affected by mixing of ¹³⁷Cs-bound surface sediment with less contaminated sediment in the deeper layers.     

大亚湾近岸沉积物对人工放射性核素的吸附

用大亚湾近岸沉积物进行人工放射性核素的吸附研究。测得各放射性核素的吸附分配系数分别为：＾８５Ｓｒ，１６０；＾１３４Ｃｓ，３３０；＾６０Ｃｏ，６４００；＾１４１Ｃｅ，９５００；＾５９Ｆｅ，９４００；＾５４Ｍｎ，１５０００。并测得核素吸附分配系数随悬浮物浓度和示踪剂活度变化的规律。当悬浮物浓度增加时，＾８５Ｓｒ，＾５９Ｆｅ的Ｋｄ值显著降低，＾１４１Ｃｅ，＾５４Ｍｎ，＾６０Ｃｏ的Ｋｄ值稍有降低，而＾１３     

239,240Pu and137Cs in the East China and the Yellow Seas

The distribution and inventory of artificial radionuclides,^239,240Pu and¹³⁷Cs were determined in the East China and the Yellow Seas in 1987. Almost all of^239,240Pu and 50 to 80% of¹³⁷Cs in the continental shelf area are retained in the sediment column.^239,240Pu sediment inventory in the sea area is larger than the fallout input and tends to increase southwardly. This excess^239,240Pu and the lateral distribution are attributable to the supply of^239,240Pu by the Yangtze River discharge. On the contrary,¹³⁷Cs sediment inventory shows a decrease to the south, and the fact can be accounted for by the southward dispersion of fine silt particles discharged from the Yellow River. Total¹³⁷Cs inventory is smaller than the estimated fallout input, and the fact seems to indicate dispersion of¹³⁷Cs out of the shelf region. Vertical profiles of^239,240Pu and¹³⁷Cs contents in sediments differ from that of natural²¹⁰Pb, implying the effect of varied accumulation rates of the artificial radionuclides over the sediment particle mixing by benthic organisms. Apparent maximum sediment particle mixing coefficient (D _B ) calculated from the excess²¹⁰Pb profiles in stations located between the inner and outer shelves ranged from 1.4 to 8.3 cm²y^–1. ThisD _B value is higher than that in the Okinawa Trough (1.0 cm²y^–1), but lower than previously estimatedD _B value (26 cm²y^–1) in the outer shelf mud.     

渤海湾沉积物中钚的测定

1958年,Thomas等人首次测定了海洋生物中的鈈(Pu),从而开始了海洋环境中超铀元素的研究。七十年代,超铀元素的研究已成了海洋同位素化学的重要內容之一,甚至超过了对~(90)Sr,~(187)Cs等核素的研究。因为超铀元     

A sediment budget for the Choptank River estuary in Maryland,U.S.A.

A sediment budget for the Choptank River, one of the three largest estuaries on the eastern shore of Chesapeake Bay, was developed from measurements of sediment carried in upland runoff, shore erosion, sedimentation, and levels of suspended sediments in estuarine waters. Shore erosion was the major source of sediment (340 × 10⁶ kg y^?1), contributing seven times more sediment than upland runoff. Low relief, the rural character of the Coastal Plain drainage basin, and the susceptibility of poorly consolidated shoreline materials to erosion contributed to the dominance of shore erosion over runoff as a sediment source. Box modelling indicated a net annual flux (14–44 × 10⁶ kg y^?1) of sediment from the Choptank River to Chesapeake Bay. A mass balance estimate of sedimentation, calculated as the difference between total inputs and loss at the mouth of the estuary, (350 × 10⁶ kg y^?1) agreed well with an estimate based on ²¹⁰Pb profiles (340 × 10⁶ kg y^?1) measured along the longitudinal axis of the estuary. Lead-210 sedimentation rates correspond to accumulation rates of 1·5–7·9 mm y^?1.     

Impact of dynamic feedbacks between sedimentation,sea-level rise,and biomass production on near-surface marsh stratigraphy and carbon accumulation

Salt marshes accrete both organic and inorganic sediments. Here we present analytical and numerical models of salt marsh sedimentation that, in addition to capturing inorganic processes, explicitly account for above- and belowground organic processes including root growth and decay of organic carbon. The analytical model is used to examine the bias introduced by organic processes into proxy records of sedimentation, namely ¹³⁷Cs and ²¹⁰Pb. We find that accretion rates estimated using ²¹⁰Pb will be less than accretion rates estimated using the ¹³⁷Cs peak in steadily accreting marshes if (1) carbon decay is significant and (2) data for ²¹⁰Pb extend below the ¹³⁷Cs peak. The numerical model expands upon the analytical model by including belowground processes such as compaction and root growth, and by explicitly tracking the evolution of aboveground biomass and its effect on sedimentation rates. Using the numerical model we explore how marsh stratigraphy responds to sediment supply and the rate of sea-level rise. It is calibrated and tested using an extensive data set of both marsh stratigraphy and measurements of vegetation dynamics in a Spartina alterniflora marsh in South Carolina, USA. We find that carbon accumulation in marshes is nonlinearly related to both the supply of inorganic sediment and the rate of sea-level rise; carbon accumulation increases with sea-level rise until sea-level rise reaches a critical rate that drowns the marsh vegetation and halts carbon accumulation. The model predicts that changes in carbon storage resulting from changing sediment supply or sea-level rise are strongly dependent on the background sediment supply: if inorganic sediment supply is reduced in an already sediment poor marsh the storage of organic carbon will increase to a far greater extent than in a sediment-rich marsh, provided that the rate of sea-level rise does not exceed a threshold. These results imply that altering sediment supply to estuaries (e.g., by damming upstream rivers or altering littoral sediment transport) could lead to significant changes in the carbon budgets of coastal salt marshes.