Examining (239+240)Pu, (210)Pb and historical events to determine carbon, nitrogen and phosphorus burial in mangrove sediments of Moreton Bay, Australia

Utilities of environmental radioactivity tracers in assessing sequestration potential of carbon in the coastal wetland ecosystems

Demand for accurate estimation of coastal blue carbon sequestration rates in a regular interval has recently surged due to the increasing awareness of nature-based climate solutions to alleviate adverse impacts stemming from the recent global warming. The robust estimation method is, however, far from well-established. The international community requires, moreover, to quantify its effect of "management." This article tries to provide the environmental isotope community with basic biophysical features of coastal blue carbon ecosystems to identify a suitable set of environmental isotopes for promoting coastal ocean-based climate solutions. This article reviews (i) the primary biophysical characteristics of coastal blue carbon ecosystems and hydrology, (ii) their consequential impact on the accumulation and preservation of organic carbon (OC) in the sediment column, (iii) suitable environmental isotopes to quantifying the sedimentary organic carbon accumulation, outwelling of the carbon-containing byproducts of decomposition of biogenic organic matter and acid neutralizing alkalinity produced in situ sediment to the offshore. Above-ground biomass is not cumulative over the years except for mangrove forests within coastal blue carbon systems. Non-gaseous carbon sequestration and loss occur mainly as a form of sediment organic carbon (SOC) and dissolved carbon in an intertidal and subtidal bottom sediment body in a slow, patchy, and dispersive way, on which this article focuses. Investigating environmental radionuclides is probably the most cost-effective effort to contribute to defining the offshore spatial extent of coastal blue carbon systems except for seagrass beds (e.g., Ra isotopes), to quantify millimeter per year scale carbon accretion and loss within the systems (e.g., 7Be, 210Pb) and a liter per meter of coastline per a day scale water movement from the systems (Ra isotopes). A millimeter-scale spatial and an annual (or less) time-scale resolution offered by the use of environmental isotopes would equip us with a novel tool to enhance the carbon storage capacity of the coastal blue carbon system.

Phosphate buffering in mangrove sediment pore water under eutrophication and deforestation influences

Phosphorus (P) behavior was evaluated in mangrove wetlands impacted by urban sewage, including a deforested site. Sediment cores were analyzed for grain size, organic carbon, total nitrogen, stable isotopes (δ13C and δ15N), P contents, and pore water PO43- concentrations and net consumption/production rates. Under stronger eutrophication influence, significantly higher P (1390 vs. <1000 μg/g), δ15N (8.9 vs. <6.7 ‰) and algal material contents (with lower C/N ratio and heavier δ13C) occurred. Depth-integrated PO43- consumption rates in eutrophicated sites were up to two orders of magnitude higher (at the deforested site) than in a moderately preserved mangrove. The whole core of the moderately preserved site presented no saturation of PO43- buffering capacity, while more eutrophicated sites developed buffering zones saturated at ∼18-26 cm depth. Contrasting to nearby subtidal environments, eutrophication did not cause larger pore water PO43- concentration, evidencing the role of PO43- buffering on P filtering by mangrove wetlands.

Developing a remote gamma-ray spectra collection system (RGSCS) by coupling a high purity Germanium (HPGe) detector with a cosmicguard background reduction device

Despite being widely used for high-resolution spectral analysis and quantifying low activity in natural samples, the operations and data analysis of High Purity Germanium (HPGe) gamma-ray detectors are seldom fully automated due to the excessive costs associated with commercially available automatic sample changing systems. This paper introduces the design and implementation of a cost-effective, customized remote gamma-ray spectra collection system centered around the HPGe detector coupled to a cosmic-ray veto background reduction device. The HPGe detector system, equipped with a Lynx DSA, is seamlessly integrated with an economically viable automatic sample changer. This sample vial changer is controlled by a high-torque NEMA 34 stepper servo motor from Vention. Web control of the rotary actuator is facilitated through a CAD-based programming tool. The remote-controlled sample pick-and-place procedure is executed using a robotic arm (Trossen Robotics, Viper X 250). The DYNAMIXEL servomotors of the robotic arm are programmed using Python software supported by the Robotic Operating System. Beyond its technical construction, this system is uniquely fashioned for academic research, providing invaluable hands-on experience in gamma spectrometry to both junior researchers and students.

Increasing carbon and nutrient burial rates in mangroves coincided with coastal aquaculture development and water eutrophication in NE Hainan, China

Mangroves sequester and store large area-specific quantities of blue carbon (C) and essential nutrients such as nitrogen (N) and phosphorous (P). Quantifying C and nutrient burial rates in mangroves across a centennial time span and relating these rates to mangrove habitat is fundamental for elucidating the role of mangroves in carbon and nutrient budgets and their responses to environmental changes. However, relevant data are very limited in China. In this study, we used the radionuclides (210Pb and 137Cs) to determine chronologies and C, N and P burial rates in two mangrove forests located at different geomorphologic settings in NE Hainan Island, China. We found that the temporal patterns of C, N and P burial rates since 1900 fitted a quadratic function with a notable increase after 1960s in both mangroves, which coincided with the rapid development of coastal aquaculture since 1960s in NE Hainan and the subsequent coastal water eutrophication in this area. Sediment accretion rate (SAR) and mass accumulation rate (MAR) stayed relatively steady in the open-coastal mangroves, while they increased exponentially in the estuarine mangroves since 1900. The estuarine mangroves had significantly higher SAR and C, N and P burial rates than the open-coastal mangroves. C, N and P burial rates averaged at 141.52 g m-2 a-1, 6.27 g m-2 a-1 and 1.14 g m-2 a-1, respectively in the estuarine core, and these rates averaged at 61.71 g m-2 a-1, 3.71 g m-2 a-1 and 0.43 g m-2 a-1, respectively in the open-coastal core. The results suggest that estuarine mangroves may be more capable of surviving accelerating sea level rise under climate change and play a greater role in C accumulation and nutrient filtering under anthropogenic nutrient enrichment than marine-dominated mangroves. Blue C burial may be enhanced by coastal water eutrophication, but such a relationship needs to be tested in further studies.

Plutonium in sediments of the Eastern Guangdong coast-its sources and their contribution

The ^239+240Pu activities and ²⁴⁰Pu/²³⁹Pu atom ratios of surface sediments from the Eastern Guangdong coast (EGDC) were determined by sector field ICP-MS in order to examine the sources of plutonium (Pu) and quantify their contributions. The ^239+240Pu activities in the EGDC ranged from 0.113 to 0.451 Bq kg^-1, with an average of 0.225 ± 0.090 Bq kg^-1 (n = 17). Consistently high ²⁴⁰Pu/²³⁹Pu atom ratios, ranging from 0.218 to 0.274 (average = 0.254 ± 0.014, n = 17), indicate a non-global fallout Pu source in the EGDC. The horizontal distribution of the ²⁴⁰Pu/²³⁹Pu atom ratios in the EGDC sediment suggests the non-global fallout Pu is sourced from close-in fallout from the Pacific Proving Grounds (PPG). Using a simple two end-member mixing model, we calculated the relative proportions of Pu from the PPG and global fallout in the EGDC to be 57 ± 9 % and 43 ± 9 %, respectively. Moreover, from the well-defined relationship between ^239+240Pu activity and total organic carbon content in sediments and a two end-member mixing model using δ¹³C, we further calculated the Terr-global fallout (riverine input) and Mar-global fallout (direct atmospheric deposition) to be 11 ± 2 % and 32 ± 6 %, respectively. Finally, from the activity levels and atom ratios of Pu isotopes in the EGDC, we established a baseline for future use in environmental risk assessment related to nuclear power plant operations.

Vertical distribution, accumulation, and characteristics of microplastics in mangrove sediment in China

Mangroves in tropical and subtropical regions worldwide are recognized as important sinks for microplastics (MPs). However, recent studies have focused on surface sediments, and in China, the vertical distribution and characteristics of MPs in mangrove sediments remain poorly understood. In this study, sediment cores of 100 cm depth were collected from six representative mangroves in China to investigate MPs via chronological analysis. Futian had the highest abundance of MPs (0-3123 n/kg), followed by Dongfang, Yunxiao, Zhanjiang, Dongzhaigang and Fangchenggang. The earliest MPs occurring in mangroves were dated back to 1955, and their abundance increased exponentially from bottom to surface sediments. MPs were mainly white in color, fiber-shaped, 1000-5000 μm in size, and of polypropylene/polyethylene polymer types. Furthermore, the MPs in the urban mangrove also showed a higher diversity in color. The results showed that the MP stocks in the urbanized Futian mangrove reached 1828 mg/m³, an order of magnitude higher than in other areas (251 ± 180 mg/m³), contributing to 0.0057 % of the carbon storage of the sediment. The abundance of MPs in mangrove sediments is expected to increase by 2.38-9.54 times by 2030, and therefore deserve further attention.

The influence of anthropogenic activities on heavy metal pollution of estuary sediment from the coastal East China Sea in the past nearly 50 years

Historical records of eight heavy metals (HMs: Cu, Zn, Cr, Ni, Pb, As, Hg and Cd) were analyzed in two dated sediment cores collected from the coastal East China Sea (Meishan Bay, MSB) to investigate the influence of anthropogenic activities on estuary sediment from 1972 to 2019. The sedimentary records of HMs in the two cores reflected the development of economy and change of energy consumption in China's east coastal areas. The contents of most HMs in sediments had no change or showed a downward trend before the 1980s. After the 1980s, the contents of HMs showed an increasing trend, mainly because of the rapid development of aquaculture, industry, and agriculture. The distribution of HMs outside the bay (OB) was mainly affected by industry, shipping, and agriculture (vegetable planting), while HMs in sediments inside the bay (IB) was affected by shipping, agriculture, and aquaculture. Principle component analysis (PCA) showed that Cd, Cr, and Ni could be attributed to industrial discharge, while As, Cu, Pb, and Zn were associated with shipping, agriculture, and aquaculture. Sediments were slightly polluted with Cd (I_geo:0.10-0.71, CF:1.90-7.74) and Ni (I_geo:0.08-0.92, CF:1.25-12.55), and seriously polluted with Hg (I_geo:0.95-1.76, CF:13.43-85.65). This study provides insights for the local governments to raise awareness of HM pollution in the coastal East China Sea and formulate corresponding pollution control measures.

Comparison of spatiotemporal carbon, nitrogen, and phosphorus burial in two plateau lacustrine sediments: implication for N and P control

The long-term accumulation, burial and release of nutrients, such as carbon (C), nitrogen (N), and phosphorus (P) in lacustrine sediments are responsible for the global lake eutrophication. Interpretation of the spatiotemporal sedimentary record of nutrients (C, N, and P) in contrasting trophic level of lakes is helpful for understanding the evolutionary process of water eutrophication. Based on the radiochronology of 210Pbex and 137Cs, a comparative study of spatial and temporal concentrations, burial of total organic carbon (TOC), total nitrogen (TN), and total phosphorus (TP), the sources of organic matter were conducted using sediment cores from two plateau lakes Dianchi (DC) and Fuxian (FX) of SW China. Results showed that concentrations and burial of C, N, and P in sediments of DC, a shallow hypertrophic lake with the maximum depth of 5.8 m, were both higher than those in FX, an oligotrophic deep lake with the maximum depth of 155.0 m. For both lakes the molar ratio of TOC/TN increased in the sediments moving from north to south. The values of TOC/TN molar ratios increased over time in DC and were higher than in FX. The extremely high values of TOC/TN appeared in the central and southern parts of FX, indicating the impacts of accumulation effect and sediment focusing in the deeper region and indirect supplement from the Lake Xingyun (XY), an adjoining lake connected with FX via the Gehe River. Time-integrated sources identification in DC indicated the contribution of allochthonous sources was dominant over the past few decades, which contributed to the increased trophic level of the lake. The comparison of relationships of carbon accumulation rates (CAR), nitrogen accumulation rates (NAR), and phosphorous accumulation rates (PAR), the ratios of N/P and the utilizations of N and P fertilizer between DC and FX implied that both of N and P inputs should be limited for reducing the trophic level, but N control was predominant in comparison with P for both lakes. The results indicated that caution is required in plateau lakes to limit transition from oligotrophic to eutrophic in these lakes.

A conceptual model of nitrogen dynamics for the Great Barrier Reef catchments

Nitrogen (N) from anthropogenic sources has been identified as a major pollutant of the Great Barrier Reef (GBR), Australia. We developed a conceptual framework to synthesise and visualise the fate and transport of N from the catchments to the sea from a literature review. The framework was created to fit managers and policymakers' requirements to reduce N in the GBR catchments. We used this framework to determine the N stocks and transformations (input, sources, and outputs) for ecosystems commonly found in the GBR: rainforests, palustrine wetlands, lakes, rivers (in-stream), mangroves and seagrasses. We included transformations of N such as nitrogen fixation, nitrification, denitrification, mineralisation, anammox, sedimentation, plant uptake, and food web transfers. This model can be applied to other ecosystems to understand the transport and fate of N within and between catchments. Importantly, this approach can guide management actions that attenuate N at different scales and locations within the GBR ecosystems. Finally, when combined with local hydrological modelling, this framework can be used to predict outcomes of management activities.

A record of fallout 239Pu and 240Pu at World Heritage Bathurst Harbour, Tasmania, Australia

This study presents the first measurements of anthropogenic plutonium (²³⁹Pu and ²⁴⁰Pu) concentrations and atom ratios (²⁴⁰Pu/²³⁹Pu) for Tasmania, in sediment collected from Bathurst Harbour, in the Tasmanian Wilderness World Heritage Area, Australia. The weighted mean ²⁴⁰Pu/²³⁹Pu atom ratio measured at this site was 0.172 ± 0.007 which is consistent with published data from mainland Australia and global and Southern Hemisphere averages. The ²⁴⁰Pu/²³⁹Pu atom ratios ranged between 0.11 and 0.21 with the earliest recorded ²⁴⁰Pu/²³⁹Pu atom ratios being the lowest, suggesting an influence of low atom ratio fallout from nuclear testing in Australia. Post-moratorium fallout ²⁴⁰Pu/²³⁹Pu atom ratios were consistent with other records. Lead-210 (²¹⁰Pb) sediment chronologies indicate sediment accumulation rates have increased since the early part of the 19th century at this location.

A review of current and emerging approaches for Quaternary marine sediment dating

Dating methodologies for Quaternary marine sediments play increasingly important roles in the reconstruction of paleoenvironments and paleoclimate in (paleo)oceanography. Previous reviews or studies have focused mainly on one or two methodologies, and their applications in one specific environment. With the continuing technological and methodological advances in different methods over the past few decades, an up-to-date comparison of the pros and cons of each dating methodology is needed to clearly understand their applications in marine geoscience research. In this review, we first briefly summarized the common methods of absolute dating and relative dating. These are (1) radioisotope dating with different half-lives using natural nuclides of ²³⁴Th, ²¹⁰Pb, ²³⁰Th, and ²²⁶Ra, cosmogenic nuclides of ⁷Be, ¹⁴C, ¹⁰Be, ³²Si, ²⁶Al, ³⁶Cl and ²¹Ne, and the artificial radionuclides of ¹³⁷Cs, ^{239, 240}Pu, ²⁴¹Am and ¹²⁹I that have been induced by atmospheric nuclear tests, accidents in nuclear plants, and discharges of radioactive wastes; (2) radiation exposure dating of luminescence and electron paramagnetic resonance (ESR) dating; and (3) stratigraphic dating of δ¹⁸O and paleomagnetic sequence. Applications and limitations from the marine terraces, estuaries, to hadal trenches have been summarized to each technique in the study of Quaternary marine geoscience extending from the Anthropocene through the Pleistocene. Finally, we introduced some emerging event dating methods, namely the arrivals of microplastics, mercury isotopes, and organic pollutant deposition that all appeared after the industrial resolution in our now changing ocean influenced by acidification, global warming, and anthropogenic activities. We ended by discussing future perspectives for reliable and high-resolution chronology by interdisciplinary methods including computer programming to better understand the natural geological evolution and predict the future changes in earth science.

Macro- and Micronutrient Cycling and Crucial Linkages to Geochemical Processes in Mangrove Ecosystems

High mangrove productivity is sustained by rapid utilization, high retention efficiency and maximum storage of nutrients in leaves, roots, and soils. Rapid microbial transformations and high mineralization efficiencies in tandem with physiological mechanisms conserve scarce nutrients. Macronutrient cycling is interlinked with micronutrient cycling; all nutrient cycles are linked closely to geochemical transformation processes. Mangroves can be N-, P-, Fe-, and Cu-limited; additions of Zn and Mo stimulate early growth until levels above pristine porewater concentrations induce toxicity. Limited nutrient availability is caused by sorption and retention onto iron oxides, clays, and sulfide minerals. Little N is exported as immobilization is the largest transformation process. Mn and S affect N metabolism and photosynthesis via early diagenesis and P availability is coupled to Fe-S redox oscillations. Fe is involved in nitrification, denitrification and anammox, and Mo is involved in NO3− reduction and N2-fixation. Soil Mg, K, Mn, Zn and Ni pool sizes decrease as mangrove primary productivity increases, suggesting increasing uptake and more rapid turnover than in less productive forests. Mangroves may be major contributors to oceanic Mn and Mo cycles, delivering 7.4–12.1 Gmol Mn a−1 to the ocean, which is greater than global riverine input. The global Mo import rate by mangroves corresponds to 15–120% of Mo supply to the oceanic Mo budget.

Tidal elevation is the key factor modulating burial rates and composition of organic matter in a coastal wetland with multiple habitats

This study examines long-term burial rates of organic carbon (OC), organic nitrogen (ON), and total sulphur (TS) in a tidal-dominated coastal wetland with a high spatial heterogeneity and habitat diversity, and long history of human impacts, Cádiz Bay (SW Spain). Using replicate sediment cores, we quantified fluxes of these elements over a transect, extending from the lower saltmarsh (Spartina maritima, ~0.3 m mean sea level, MSL) to the lower intertidal region (Zostera noltei, ~ - 0.7 m MSL). Potential organic matter (OM) sources to the sediment were examined using an extensive dataset on carbon and nitrogen stable isotopes, and C:N molar ratios of primary producers in the region. OC burial rates decreased from the sites below MSL (~80 gC·m^-2·y^-1) to the lower saltmarsh (~50 gC·m^-2·y^-1), whereas ON burial rates showed an opposite pattern (~3 gN·m^-2·y^-1 and ~4 gN·m^-2·y^-1 observed below and above MSL, respectively). TS burial rates (0.5-46 gS·m^-2·y^-1) did not show any trend along the sea-land gradient. Hence, (tidal) elevation appeared to be an important determinant of sediment biogeochemical properties, and predictor of OM burial rates. The Bayesian mixing model suggested a well-mixed combination of subtidal and terrestrial/high-marsh OM sources to the surface sediments, with no clear indication of an increased contribution from the particular vegetation species inhabiting the sediments. The indication that there is substantial transport, remineralization and cycling of OM between habitats, suggests diversity may play an important role in maintaining this function, reinforcing the idea that a holistic, catchment-scale view is appropriate for understanding and preserving the long-term burial of OM in coastal wetlands.

Significant nitrate attenuation in a mangrove-fringed estuary during a flood-chase experiment

Intertidal wetlands can sequester pollutants along estuarine conduits. Here we test the effectiveness of a mangrove-dominated estuary in removing dissolved nitrogen during a rain event. We intensively and simultaneously sampled surface water nutrients upstream and downstream of an estuary before, during and after a 63 mm rain event in Coffs Creek (Australia). NO_x was the main form of dissolved nitrogen upstream of the estuary (∼60%), while dissolved organic nitrogen (DON) was an important form at the downstream station (∼46%) during observations. High NO_x attenuation (71%) occurred during the rain event when the loads reached 31 μmol m^-2 catchment area day^-1. In contrast, the estuary was found to be a source of NH₄⁺ (∼5 μmol m^-2 catchment area day^-1). This implies a moderate conversion of upstream NO_x into NH₄⁺ and DON along the transport pathway, likely due to tidally-driven pore water exchange within the anoxic estuarine mangrove sediments. Overall, the mangrove-lined estuary attenuated upstream total dissolved nitrogen loads, maintaining water quality and minimizing exports to the coastal ocean even during high flow conditions.

Methane emissions partially offset "blue carbon" burial in mangroves

Organic matter burial in mangrove forests results in the removal and long-term storage of atmospheric CO₂, so-called "blue carbon." However, some of this organic matter is metabolized and returned to the atmosphere as CH₄. Because CH₄ has a higher global warming potential than the CO₂ fixed in the organic matter, it can offset the CO₂ removed via carbon burial. We provide the first estimate of the global magnitude of this offset. Our results show that high CH₄ evasion rates have the potential to partially offset blue carbon burial rates in mangrove sediments on average by 20% (sensitivity analysis offset range, 18 to 22%) using the 20-year global warming potential. Hence, mangrove sediment and water CH₄ emissions should be accounted for in future blue carbon assessments.

Avoiding timescale bias in assessments of coastal wetland vertical change

There is concern that accelerating sea-level rise will exceed the vertical growth capacity of coastal-wetland substrates in many regions by the end of this century. Vertical vulnerability estimates rely on measurements of accretion and/or surface-elevation-change derived from soil cores and/or surface elevation tables (SETs). To date there has not been a broad examination of whether the multiple timescales represented by the processes of accretion and elevation change are equally well-suited for quantifying the trajectories of wetland vertical change in coming decades and centuries. To examine the potential for timescale bias in assessments of vertical change, we compared rates of accretion and surface elevation change using data derived from a review of the literature. In the first approach, average rates of elevation change were compared with timescale-averaged accretion rates from six regions around the world where sub-decadal, decadal, centennial, and millennial timescales were represented. Second, to isolate spatial variability, temporal comparisons were made for regionally unique environmental categories within each region. Last, comparisons were made of records from sites where SET-MH stations and radiometric measurements were co-located in close proximity. We find that rates vary significantly as a function of measurement timescale and that the pattern and magnitude of variation between timescales are location-specific. Failure to identify and account for temporal variability in rates will produce biased assessments of the vertical change capacity of coastal wetlands. Robust vulnerability assessments should combine accretion rates from multiple timescales with the longest available SET record to provide long-term context for ongoing monitoring observations and projections.

Carbon accumulation and storage capacity in mangrove sediments three decades after deforestation within a eutrophic bay

A dated sediment core from an eutrophic mangrove area presented non-significant differences in carbon accumulation rates before (55.7±10.2gm^-2yr^-1) and after three decades of deforestation (59.7±7.2gm^-2yr^-1). Although eutrophication effects appear to compensate the loss of mangrove organic matter input, the results in this work show a threefold lower carbon accumulation than the global averages estimated for mangrove sediments. The effects of increasing eutrophication and enhanced sediment dry bulk density observed after deforestation (~30% higher) did not result in higher carbon stocks. Moreover, the lower TOC:OP (<400) and C:N (~20) molar ratios, as well as increased nutrient accumulation, reflect the dominance of phytoplankton-derived organic matter after deforestation, resulting in less-efficient sedimentary carbon sinks. These results indicate that the organic material deposited from eutrophication may not compensate mangrove deforestation losses on carbon accumulation in mangrove ecosystems.

Temporal variation of sedimentation rates and potential factors influencing those rates over the last 100years in Bohai Bay, China

Fourteen sediment cores were collected from Bohai Bay, China, which is close to the fast-developing megacities such as Beijing and Tianjin, and dated using excess Pb-210 and Cs-137. Using the constant rate of supply model (CRS), the temporal variation of sedimentation rates over the last 100years in Bohai Bay was determined, and its main factors influencing sedimentation rates were discussed. The sedimentation rates before 1980 were relatively stable, ranging from 0.26±0.04g/(cm²·y) in 1920 to 0.39±0.08g/(cm²·y) in 1980. A gradual increase in sedimentation rate was found from 0.39±0.08g/(cm²·y) in 1980 to 0.84±0.13g/(cm²·y) in 2010. Riverine input was the main factors influencing sedimentation rates in Bohai Bay before 1980. After 1980, the accelerated increase in sedimentation rate may be attributed to the large-scale reclamation along the coastline. It is worth noting that eutrophication caused by an increase in sewage discharge and overuse of chemical fertilizer, also may influence recent acceleration in sedimentation rate.

Mercury dilution by autochthonous organic matter in a fertilized mangrove wetland

A dated sediment core from a highly-fertilized mangrove wetland located in Cubatão (SE Brazil) presented a negative correlation between mercury (Hg) and organic carbon contents. This is an unusual result for a metal with well-known affinity to organic matter. A dilution of Hg concentrations by autochthonous organic matter explained this observation, as revealed by carbon stable isotopes signatures (δ(13)C). Mercury dilution by the predominant mangrove-derived organic matter counterbalanced the positive influences of algal-derived organic matter and clay contents on Hg levels, suggesting that deleterious effects of Hg may be attenuated. Considering the current paradigm on the positive effect of organic matter on Hg concentrations in coastal sediments and the expected increase in mangrove organic matter burial due to natural and anthropogenic stimulations of primary production, predictions on the influences of organic matter on Hg accumulation in mangrove wetlands deserve caution.