Summary Growing evidence indicates that tree‐stem methane (CH4) emissions may be an important and unaccounted‐for component of local, regional and global carbon (C) budgets. Studies to date have focused on upland and freshwater swamp‐forests; however, no data on tree‐stem fluxes from estuarine species currently exist. Here we provide the first‐ever mangrove tree‐stem CH4 flux measurements from  >50 trees (n = 230 measurements), in both standing dead and living forest, from a region suffering a recent large‐scale climate‐driven dieback event (Gulf of Carpentaria, Australia). Average CH4 emissions from standing dead mangrove tree‐stems was 249.2 ± 41.0 μmol m−2 d−1 and was eight‐fold higher than from living mangrove tree‐stems (37.5 ± 5.8 μmol m−2 d−1). The average CH4 flux from tree‐stem bases (c. 10 cm aboveground) was 1071.1 ± 210.4 and 96.8 ± 27.7 μmol m−2 d−1 from dead and living stands respectively. Sediment CH4 fluxes and redox potentials did not differ significantly between living and dead stands. Our results suggest both dead and living tree‐stems act as CH4 conduits to the atmosphere, bypassing potential sedimentary oxidation processes. Although large uncertainties exist when upscaling data from small‐scale temporal measurements, we estimated that dead mangrove tree‐stem emissions may account for c. 26% of the net ecosystem CH4 flux.

Molybdenum precipitates preferentially under reducing conditions; therefore, its occurrence in sediment records is used as an indicator of paleoredox conditions. Although thiomolybdates (MoO4-xSx(2-) with x = 1-4) supposedly are necessary intermediates in the process of molybdenum precipitation under anoxic conditions, there is no information about their abundance in natural environments, because of a lack of element-specific methods with sufficiently low detection limits. Here, we optimized ion-pair chromatographic separation for coupling to an inductively coupled plasma-mass spectrometry detector (IPC-ICP-MS). 2-Propanol (10%-25% gradient) replaced the previously used acetonitrile (25%-75%) as the solvent, to reduce the carbon load into the plasma. In synthetic solutions, formation of thiomolybdates was found to occur spontaneously in the presence of excess sulfide and the degree of thiolation was highest at pH 7. Excess hydroxyl led to a transformation of thiomolybdates to molybdate. Under acidic to neutral conditions, precipitation of molybdenum and hydrolysis of tetrathiomolybdate were observed. Flash-freezing was found to be suitable to stabilize tetrathiomolybdate, with <4% transformation over more than two months. High ionic strengths matrices (>2 mM) negatively affected the detection of molybdate, which eluted mainly in the dead volume, but had no negative effect on higher thiolated molybdates. Detection limits were ∼10 nM. With the newly developed IPC-ICP-MS method, thiomolybdates were found to form spontaneously in euxinic marine waters after adding a molybdate spike and occur naturally in sulfidic geothermal waters.

Most research evaluating the potential of mangroves as a sink for atmospheric carbon has focused on carbon burial in sediments. However, the few studies that have quantified lateral exchange of carbon and alkalinity indicate that the dissolved carbon and alkalinity export may be several-fold more important than burial. This study aims to investigate rates and drivers of alkalinity, dissolved carbon, and greenhouse gas fluxes of the mangrove-dominated Shark River estuary located in the Everglades National Park in Florida, USA. Spatial surveys and 29-hr time series were conducted to assess total alkalinity (TAlk), organic alkalinity (OAlk), dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), carbon dioxide (CO 2), methane (CH 4), and nitrous oxide (N 2 O) dynamics. Dissolved carbon and greenhouse gas concentrations were coupled to porewater input, which was examined using radon-222. Shark River was a source of CO 2 (92 mmol/m 2 /day), CH 4 (56 μmol/m 2 /day), and N 2 O (2 μmol/m 2 /day) to the atmosphere. Dissolved carbon export (DIC = 142 mmol/m 2 /day, DOC = 39 mmol/m 2 /day, normalized to mangrove area) was several-fold higher than previously reported carbon burial rates in the study area (~28 mmol/m 2 /day). The majority of the DIC was exported as TAlk (97 mmol/m 2 /day), which remains dissolved in the ocean for millennia and, therefore, represents a long-term sink for atmospheric carbon. By integrating our results with previous studies, we argue that alkalinity, dissolved carbon, and greenhouse gas fluxes should be considered in future blue carbon budgets. Plain Language Summary Protecting mangroves can help us deal with one of the biggest challenges of our time: Climate change. Mangroves remove carbon dioxidethe gas that is making the world hotterfrom the air around us and store it in their surrounding soils. Many scientists have studied how much carbon is trapped in mangrove soils, and the newest studies say that some of that trapped carbon is flushed out to the coastal ocean. We wanted to find out for ourselves so we went to Everglades National Park, which protects the biggest mangrove forest in North America. We cruised along the Shark River estuary and spent day and night on the boat to uncover this carbon mystery. We found that the estuary loses greenhouse gasses to the atmosphere, but much less compared to what it loses to the Gulf of Mexico. We could also confirm that the amount of carbon lost was much greater than what is trapped in soils. We encourage other researchers to investigate dissolved carbon export in other mangroves in order to find out how much mangroves can alleviate the impacts of climate change.

Mangroves, saltmarshes, and seagrasses mitigate climate change by sequestering and storing atmospheric carbon. Carbon sequestration and storage per unit area are considerably higher in these coastal ecosystems than in terrestrial ecosystems (Mcleod et al., 2011;Nellemann & Corcoran, 2009) and have been referred to as "blue carbon." Previous blue carbon research focused mainly on sedimentary and biomass carbon stocks

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