Duofu Chen

A compositional kinetic model of hydrate crystallization and dissolution

Hydrates are crystallizing near and at the seafloor from gas vents on shelves where the sedimentation rate is high and hydrocarbons are being generated. When seafloor temperature, vent rate, or vent gas composition changes, these hydrates may become unstable and decompose. We have constructed a compositional kinetic model of hydrate crystallization and dissolution that can address these issues. The model crystallizes hydrate in compositional bins and allows each to dissolve at either a kinetically or compositionally controlled rate if vent gas composition or temperature causes it to become unstable. We empirically calibrate the model to venting at the Bush Hill hydrate mound in the offshore Louisiana Gulf of Mexico, show how variations in venting rate crystallize hydrate of diverse composition in the subsurface, and investigate how bottom water temperature variations similar to those measured could increase the rate of gas venting by destabilizing hydrates within a few meters of the seafloor. We show that increases in bottom water temperature can cause gas venting rates to increase similar to100%, as suggested by recent measurements, only if the dissolution kinetics are fast compared to the empirically calibrated crystallization kinetics and dissolution gases are removed rapidly enough that they do not thermodynamically inhibit the rate of dissolution. Model characteristics required to further investigate hydrate mound construction are identified.

A carbonate-based proxy for sulfate-driven anaerobic oxidation of methane

Sulfate-driven anaerobic oxidation of methane (SD-AOM) supports chemosynthesis-based communities and limits the release of methane from marine sediments. Formation of authigenic carbonates at active methane seeps is promoted by SD-AOM stoichiometry. While distinctively small δ 18 O/δ 34 S slopes of pore fluid sulfate have been shown to typify modern methane-rich environments, identification of such environments has been difficult for the geological past due to the lack of sedimentary pore fluids. However, if the isotopic composition of sulfate were archived in authigenic carbonate during early diagenesis, carbonate-associated sulfate (CAS) should display the characteristic δ 18 O-δ 34 S pattern. To test this hypothesis, we investigated the δ 18 O CAS , δ 34 S CAS , and 87 Sr/ 86 Sr signatures of authigenic carbonate minerals from three modern and two ancient methane-seep provinces. The data obtained demonstrate that all deposits regardless of age or location display consistently small δ 18 O CAS /δ 34 S CAS slopes (∼0.3) and CAS does not represent ambient seawater but pore-water sulfate. This finding confirms the utility of CAS as a recorder of SD-AOM in methane-rich environments. In addition, we report that aragonites bear higher CAS contents, 87 Sr/ 86 Sr ratios closer to that of contemporary seawater, and a larger δ 18 O CAS /δ 34 S CAS slope than calcites, reflecting the shallower formation depth of aragonite where pore-water has a composition close to that of seawater with high concentrations of sulfate. The new proxy can be used to constrain the record of SD-AOM through most of Earth history by measuring the δ 18 O and δ 34 S values of CAS of methane-derived diagenetic carbonates including but not limited to seep carbonates.

Evolving Yangtze River reconstructed by detrital zircon U‐Pb dating and petrographic analysis of Miocene marginal Sea sedimentary rocks of the Western Foothills and Hengchun Peninsula, Taiwan

The timing of the establishment of the Yangtze River, whether prior to the early Miocene (~24 Ma) or more recently (~2 Ma), has been a point of much debate. Here we applied detrital zircon U-Pb dating to Miocene sedimentary rocks from Taiwan and to estuary sands from modern rivers in SE China to trace sediment provenance and to further constrain the evolution of the Yangtze River. Detrital zircon U-Pb ages from Miocene sandstones of the Western Foothills show similar age spectra to Miocene and modern sediments in the Yangtze River drainage and some similarity to the Minjiang River sediments. However, they differ significantly from ages in some sandstones from the Hengchun Peninsula accretionary prism and from the estuary sands of the Jiulongjiang River. This information, together with petrographic and sedimentary facies analysis, argues that the Jiulongjiang and Minjiang Rivers were major sources to some Hengchun Peninsula turbidites (~12 Ma), while synchronous sedimentation in the Western Foothills was supplied from the Yangtze, Minjiang (or similar river), and possibly even the Yellow River. These sediments were transported southward/eastward via rivers or channels to the marginal sedimentary basins now inverted in the Western Foothills in Taiwan. The Yangtze River must have been established prior to the middle Miocene.

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.

Methane seepage in the Shenhu area of the northern South China Sea: constraints from carbonate chimneys

Two authigenic carbonate chimneys were recovered from the Shenhu area in the northern South China Sea at approximately 400 m water depth. The chimneys’ mineralogy, isotopic composition, and lipid biomarkers were studied to examine the biogeochemical process that induced the formation of the chimneys. The two chimneys are composed mostly of dolomite, whereas the internal conduits and semi-consolidated surrounding sediments are dominated by aragonite and calcite. The specific biomarker patterns (distribution of lipids and their depleted δ13C values) indicate the low occurrence of methanotrophic archaea ANME-1 responsible for the chimneys’ formation via anaerobic oxidation of methane. A significant input of bacteria/planktonic algae and cyanobacteria to the carbon pool during the precipitation of the carbonate chimneys is suggested by the high contributions of short-chain n-alkanes (69% of total hydrocarbons) and long-chain n-alcohols (on average 56% of total alcohols). The oxygen isotopic compositions of the carbonate mixtures vary from 3.1‰ to 4.4‰ in the dolomite-rich chimneys, and from 2.1‰ to 2.5‰ in the internal conduits, which indicates that they were precipitated from seawater-derived pore waters during a long period covering the last glacial and interglacial cycles. In addition, the mixture of methane and bottom seawater dissolved inorganic carbon could be the carbon sources of the carbonate chimneys.

Bio-reef-chert suite and superlarge ore deposits

The bio-reef-chert suite is an important ore-bearing rock assemblage and one of the metallogenic rock suites of superlarge ore deposits. It is formed as a fixed and ordered suite in space and time, and composed of different rocks formed by different geological processes. It is the product of basin evolution at special stage in a special geological setting. It is also the comprehensive product of normal sedimentary process, biological process in basin, hydrothermal sedimentary process under basin base and magmatic process in the deep lithosphere.

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 hotspots for organic matter mineralization. In this study, we for the first time quantified the total benthic O2 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, i.e., the Mariana Trench (2.0×102 μmol O2 m-2 d-1, 10,853 m), the Mussau Trench (2.7±0.1×102 μmol O2 m-2 d-1, 7,011 m), and the New Britain Trench (6.0±0.1×102 μmol O2 m-2 d-1, 8,225 m). Combined with the analyses of total organic carbon (TOC) and δ13C of TOC in the sediments and previously published in-situ O2 microprofiles from hadal settings, we suggest that hadal benthic carbon mineralization partly is governed by the surface production but 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.

REDOX VARIATIONS AT COLD SEEPS RECORDED BY RARE EARTH ELEMENTS IN SEEP CARBONATES

Understanding the formation conditions of seep carbonate is crucial to better constrain the dynamic fluid flow and chemical fluxes associate with cold seeps on the seafloor. Rare earth element (REE) in seep carbonates collected from modern cold seeps of Gulf of Mexico, Black Sea, Congo Fan, ancient seeps of Beauvoisin (Oxfordian, J3, Southeastern France) and Marmorito (Miocene, Northern Italy) were studied. Our focus has been on 5% HNO3-treated solution (authigenic carbonate minerals) of carbonates. Several crystalline forms of carbonate minerals have been selected for analysis. Total REE (∑REE) contents in seep carbonates varies widely, from 0.068 to 43.655 ppm, but the common trend is that the ∑REE in microcrystalline phases is highest and lowest of in sparite, suggesting that the ∑REE of seep carbonates may be a function of diagenesis. The shale-normalized REE patterns of the seep carbonates show varied Ce anomalies across several seep sites and even within one site, suggesting that the formation condition of seep carbonate is variable and complex. Overall, our results show that apart from anoxic, oxic formation condition is also common at hydrocarbon seep environments.

Quantification of methane fluxes from hydrocarbon seeps to the ocean and atmosphere: Development of an in situ and online gas flux measuring system

Natural hydrocarbon seeps in the marine environment are important contributors to greenhouse gases in the atmosphere. Such gases include methane, which plays a significant role in global carbon cycling and climate change. To accurately quantify the methane flux from hydrocarbon seeps on the seafloor, a specialized in situ and online gas flux measuring (GFM) device was designed to obtain high-resolution time course gas fluxes using the process of equal volume exchange. The device consists of a 1.0-m diameter, 0.9-m tall, inverted conical tent and a GFM instrument that contains a solenoid valve, level transducer, and gas collection chamber. Rising gas bubbles from seeps were measured by laboratory-calibrated GFM instruments attached to the top of the tent. According to the experimental data, the optimal anti-shake time interval was 5 s. The measurement range of the device was 0–15 L min−1, and the relative error was ± 1.0%. The device was initially deployed at an active seep site in the Lingtou Promontory seep field in South China Sea. The amount of gas released from a single gas vent was 30.5 m3 during the measurement period, and the gas flow rate ranged from 22 to 72 L h−1, depending on tidal period, and was strongly negatively correlated with water depth. The measurement results strongly suggest that oceanic tides and swells had a significant forcing effect on gas flux. Low flow rates were associated with high tides and vice versa. The changes in gas volume escaping from the seafloor seeps could be attributed to the hydrostatic pressure induced by water depth. Our findings suggest that in the marine environment, especially in the shallow shelf area, sea level variation may play an important role in controlling methane release into the ocean. Such releases probably also affect atmospheric methane levels.

The distribution and variation in the gas composition of macro-seeps on the near-shore Lingtou Promontory in the South China Sea

Natural hydrocarbon seeps in a marine environment are one of the important contributors to greenhouse gases in the atmosphere, including methane, which is significant to the global carbon cycling and climate change. Four hydrocarbon seep areas, the Lingtou Promontory, the Yinggehai Rivulet mouth, the Yazhou Bay and the Nanshan Promontory, occurring in the Yinggehai Basin delineate a near-shore gas bubble zone. The gas composition and geochemistry of venting bubbles and the spatial distribution of hydrocarbon seeps are surveyed on the near-shore Lingtou Promontory. The gas composition of the venting bubbles is mainly composed of CO2, CH4, N2 and O2, with minor amounts of non-methane hydrocarbons. The difference in the bubbles’ composition is a possible consequence of gas exchange during bubble ascent. The seepage gases from the seafloor are characterized by a high CO2 content (67.35%) and relatively positive δ13CV-PDB values (–0.49×10–3–0.86×10–3), indicating that the CO2 is of inorganic origin. The relatively low CH4 content (23%) and their negative δ13CV-PDB values (–34.43×10–3—–37.53×10–3) and high ratios of C1 content to C1–5 one (0.98–0.99) as well point to thermogenic gases. The hydrocarbon seeps on the 3.5 Hz sub-bottom profile display a linear arrangement and are sub-parallel to the No. 1 fault, suggesting that the hydrocarbon seeps may be associated with fracture activity or weak zones and that the seepage gases migrate laterally from the central depression of the Yinggehai Basin.