Benthic carbon mineralization in hadal trenches: Insights from in-situ determination of benthic oxygen consumption

Hadal trenches have been proposed as depocenters of organic material and hot spots for organic matter mineralization. In this study, we for the first time quantified the total benthic O-2 uptake in hadal trenches using in situ chamber incubations. Three trenches in the tropical Pacific were targeted and exhibited relatively high diagenetic activity given the great water depths, that is, the Mariana Trench (2.0x10(2)molO(2)m(-2)d(-1), 10,853m), the Mussau Trench (2.70.1x10(2)molO(2)m(-2)d(-1), 7,011m), and the New Britain Trench (6.00.1x10(2)molO(2)m(-2)d(-1), 8,225m). Combined with the analyses of total organic carbon and C-13 of total organic carbon in the sediments and previously published in situ O-2 microprofiles from hadal settings, we suggest that hadal benthic carbon mineralization partly is governed by the surface production and also is linked to the distance from land. Therefore, we highlight that terrestrial organic matter can be of importance in sustaining benthic communities in some hadal settings. Plain Language Summary Hadal trenches that refer to seafloor areas covered by a water column with depths >6,000m have been proposed as depocenters of organic material and hot spots for organic matter mineralization. We applied in situ benthic chamber incubation techniques within three trenches in the tropical Pacific Ocean (the Mariana Trench, the Mussau Trench, and the New Britain Trench) and thereby reported the first benthic total O-2 uptake rates measured in hadal settings. The benthic carbon mineralization rates generally show a positive correlation with the net primary production in respective provinces and the sedimentary total organic carbon (TOC) level. Analyses of TOC contents and C-13 of TOC indicated a downslope transport of sediment containing a large amount of terrestrial organic matter, possibly via mass-wasting events to the axis of New Britain Trench off the New Britain Island. Therefore, we speculate that both surface production regimes and the distance from land are closely connected with the benthic carbon mineralization rate at the trench axes. The elevated organic carbon turnover rate may in part result from preferential concentration of relatively labile organic matter in the surface sediments of trench axes or efficient utilization of refractory terrestrial material under extreme pressure

A model to predict the thermodynamic stability of abiotic methane-hydrogen binary hydrates in a marine serpentinization environment

Abiotic methane (CH4) and hydrogen (H2), which are produced during marine serpentinization, provide abundant gas source for hydrate formation on ocean floor. However, previous models of CH4–H2 hydrate formation have generally focused on pure water environments and have not considered the effects of salinity. In this study, the van der Waals–Platteeuw model, which considered the effects of salinity on the chemical potentials of CH4, H2, and H2O, was applied in a marine serpentinization environment. The model uses an empirical formula and the Peng–Robinson equation of state to calculate the Langmuir constants and fugacity values, respectively, of CH4 and H2, and it uses the Pitzer model to calculate the activity coefficients of H2O in the CH4–H2–seawater system. The three-phase equilibrium temperature and pressure predicted by the model for CH4–H2 hydrates in pure water demonstrated good agreement with experimental data. The model was then used to predict the three-phase equilibrium temperature and pressure for CH4–H2 hydrates in a NaCl solutions, for which relevant experimental data are lacking. Thus, this study provides a theoretical basis for gas hydrate research and investigation in areas with marine serpentinization

A quantitative assessment of methane cycling in Hikurangi Margin sediments (New Zealand) using geophysical imaging and biogeochemical modeling

Takahe seep, located on the Opouawe Bank, Hikurangi Margin, is characterized by a well-defined subsurface seismic chimney structure ca. 80,500 m2 in area. Sub-seafloor geophysical data based on acoustic anomaly layers indicated the presence of gas hydrate and free gas layers within the chimney structure. Reaction-transport modeling was applied to porewater data from 11 gravity cores to constrain methane turnover rates and benthic methane fluxes in the upper 10 m. Model results show that methane dynamics were highly variable due to transport and dissolution of ascending gas. The dissolution of gas (up to 3761 mmol m−2 yr−1) dwarfed the rate of methanogenesis within the simulated sediment column (2.6 mmol m−2 yr−1). Dissolved methane is mainly consumed by anaerobic oxidation of methane (AOM) at the base of the sulfate reduction zone and trapped by methane hydrate formation below it, with maximum rates in the central part of the chimney (946 and 2420 mmol m−2 yr−1, respectively). A seep-wide methane budget was constrained by combining the biogeochemical model results with geophysical data and led to estimates of AOM rates, gas hydrate formation and benthic dissolved methane fluxes of 3.68 × 104 mol yr−1, 73.85 × 104 mol yr−1and 1.19 × 104 mol yr−1, respectively. A much larger flux of methane probably escapes in gaseous form through focused bubble vents. The approach of linking geochemical model results with spatial geophysical data put forward here can be applied elsewhere to improve benthic methane turnover rates from limited single spot measurements to larger spatial scales

Temporal Variation in Natural Gas Seep Rate and Influence Factors in the Lingtou Promontory Seep Field of the Northern South China Sea

Natural hydrocarbon seeps in marine environment are important sources of methane and other greenhouse gases into the ocean and the atmosphere. This greenhouse gas seepage influences the global methane budget and global climate change. Hydrocarbon seeps on the shallow seabed produce a near-shore gas bubble zone along the western coast of Hainan Island, in the northern South China Sea. However, few studies on the quantitative value of the methane flux and on temporal variation and influence factors of hydrocarbon seeps have been conducted until now. This study describes the results of continuous gas vent measurements for 420 hours on the seabed of the Lingtou promontory shore. The amount of gas released from a single gas vent was 30.5 m3 during the measurement period. The gas flow rate ranged from 22 - 72 L h-1, with an average rate of 53.4 L h-1. The time series analyses of the 420-hour record clearly show three principal tidal components with periods of 5.4, 4.6, and 2.4 hours, which are the main factors controlling the gas flow rate. Low flow rates were associated with high tide and high flow rates associated with low tide. A 1-m increase in seawater height results in a decrease of 20 - 30 L h-1 or 35 - 56% of the hourly flow rate. Therefore, the changes in gas volume escape from the pore could be attributed to the hydrostatic pressure effect induced by water depth. This dominant mechanism controlled pore activation as well as the gas flow rate, suggesting that in the marine environment, especially the shallow-water shelf area, sea level changes may result in great variations in methane release into the ocean and atmosphere