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Air gun arrays are designed to generate strong energy pulses of less than 100 Hz for offshore oil and gas exploration. In shallow seas, due to the directivity of air gun arrays, the complex propagation conditions and the limitations of acquisition methods, it is difficult to accurately evaluate the spatial time and frequency transmission characteristics of airguns. To alleviate the existing lack of knowledge about the transmission characteristics of air gun pulses in the shallow waters of the Bohai Sea, we analysed three‐dimensional ocean bottom cable seismic data collected during a seismic survey in the Bohai Sea, China, in June 2017, which were used to evaluate the reception level, frequency domain characteristics and spatial distribution characteristics within a 5.55 × 5.575 km area. The results showed that in shallow water, multipath propagation causes downfrequency chirplike sounds and an increased sound field. Within 0.3 km, the transmission model follows a cylindrical diffusion, and the sound loss rate decreases as the transmission distance increases until reaching 8 km. Signal capture by the sound channel results in a high transmission loss frequency band, whereas multimode interference weakens the transmission loss. The gun array directivity and the transmission channel are the two main factors affecting the acoustic field. This research provides an improved understanding of the acoustic field of airguns in the Bohai Sea, which can benefit the acoustic modelling of air gun arrays, the design of future seismic exploration projects and the conservation of marine life. [ABSTRACT FROM AUTHOR]

Sediments on continental slopes, which are rich in hydrates and organic matters, may release methane into the overlying seawater and even into the atmosphere. To evaluate the impact of sediment emissions on oceanic methane cycling and greenhouse gas emissions, this study assessed the distribution of methane in surface sediments and water columns and calculated the sea-to-air methane flux in the Dongsha area of the northern South China Sea. Results show that the methane concentrations exhibited large spatio-temporal changes. In May 2019, the methane concentrations and sea-to-air methane flux (4.1 ± 3.7 μmol m−2 d−1) were close to the background values, indicating that in situ aerobic methane production was a weak atmospheric methane source. In contrast, in September 2020, abnormally high methane concentrations up to 26.6 nM were detected in the upper slope with water depths of 600–950 m and to the east of a seamount; the average sea-to-air methane flux increased to 11.3 ± 7.9 μmol m−2 d−1. The consistency of methane distribution throughout the water column and surface sediments, with a generally increasing trend with the increasing depth, indicates that in September 2020, sediment emissions were crucial methane sources in seawater and led to moderate emissions into the atmosphere. The heterogeneities of methane concentrations and sea-to-air methane fluxes were attributed to the uneven distribution of methane in the sediments. Besides, strong internal solitary waves in summer and autumn exacerbates methane release, especially around high terrain areas, such as in continental upper slopes and in the upstream side of seamounts. Our results demonstrate that methane released from the sediments, being perhaps affected by internal solitary waves induced sediments disturbance, substantially impacted the oceanic methane cycling in continental slopes. Thus, this study provides new insights into the methane budget of marginal basins. • Abnormally high methane concentrations were detected in the upper slope and to the east of a seamount. • Sediment emissions were crucial methane sources in seawater in the Dongsha area. • ISWs enhance methane released from sediments into atmosphere near high terrain areas in Autumn. [ABSTRACT FROM AUTHOR]

Successful bioremediation of oil pollution is based on a comprehensive understanding of the in situ physicochemical conditions and indigenous microbial communities as well as the interaction between microorganisms and geochemical variables. Nineteen oil-contaminated soil samples and five uncontaminated controls were taken from six major oilfields across different geoclimatic regions in China to investigate the spatial distribution of the microbial ecosystem. Microbial community analysis revealed remarkable variation in microbial diversity between oil-contaminated soils taken from different oilfields. Canonical correspondence analysis (CCA) further demonstrated that a suite of in situ geochemical parameters, including soil moisture and sulfate concentrations, were among the factors that influenced the overall microbial community structure and composition. Phylogenetic analysis indicated that the vast majority of sequences were related to the genera Arthrobacter, Dietzia, Pseudomonas, Rhodococcus, and Marinobacter, many of which contain known oil-degrading or oil-emulsifying species. Remarkably, a number of archaeal genera including Halalkalicoccus, Natronomonas, Haloterrigena, and Natrinema were found in relatively high abundance in some of the oil-contaminated soil samples, indicating that these Euryarchaeota may play an important ecological role in some oil-contaminated soils. This study offers a direct and reliable reference of the diversity of the microbial community in various oil-contaminated soils and may influence strategies for in situ bioremediation of oil pollution. [ABSTRACT FROM AUTHOR]

The CDE hydrothermal field was first discovered during a Chinese cruise to the East Lau Basin Spreading Centre in 2007. Apart from significant amounts of loose Fe-Si-Mn (oxyhydr) oxide (referred to as oxide below) precipitates, a small Si-rich oxide chimney was also recovered on this cruise. In this study, we report on the mineralogical and geochemical analyses of this chimney and a model for its growth that has been developed. Based on the mineralogy and O isotope results, the chimney walls can be divided into four growth generations (layers) from the inner to the outer layers: amorphous opal and barite layer (precipitation temperature 68.5°C based on oxygen isotope determinations), a rod-like amorphous layer (precipitation temperature 39.6°C), a filamentous Fe-Si oxide layer, and an outer Fe-Mn oxide layer. Investigations based on SEM and EDS showed that neutrophilic Fe-oxidizing bacteria play an important role in the formation of this chimney, particularly in the outer two generations. In the first stage, the metabolic activity of the microbes results in the pervasive precipitation of the filamentous Fe-rich oxides inside a ring formed by some amorphous opal and barite; therefore, a loose porous layer forms. In the second stage, amorphous opal then precipitates inside this wall as a result of conductive cooling and gradually controls the mixing between the hydrothermal fluids and ambient seawaters. In the third stage, barite and some amorphous opal form from the higher temperature fluids at the summit of the chimney growth history. In the last stage, the chimney wall becomes thicker and denser and the exchange of hydrothermal fluids and seawater ceases. As a result, a Fe-Mn oxide layer precipitates onto the outer surface of the chimney wall as neutrophilic Fe-oxidizing bacteria reoccupy the surface of the chimney. This mineral sequence and the resultant growth generations are confirmed by the chemical characteristics of the chimney wall. Sr isotopes extracted from the Fe oxides of the four-generation wall generally show a decreasing trend of the Sr/Sr ratios from the second layer to the inner layer (from 0.707008 to 0.705877) except for the outer layer (0.706502). The Sr isotope and chondrite normalized REE patterns of the corresponding bulk samples from the chimney wall also display a similar trend. Our study shows that the biogenic filament network plays a key role in the formation of the chimney in contrast to previous growth models of higher temperature chimneys, which often ignore the influence of biogenic factors. [ABSTRACT FROM AUTHOR]

The molecular and carbon and hydrogen isotope compositions associated with the aerobic consumption of methane (C l), ethane (C 2) and propane (C 3) were determined from incubations of marine sediments collected from the seafloor surface of the gas- and oil-bearing Bohai Bay in eastern China. The fresh sediment samples were incubated aerobically at 28 °C and treated with three types of hydrocarbon gas mixtures as follows: mixture I (C 1 :C 2 :C 3 = 100:0:0), II (C 1 :C 2 :C 3 = 99.00:0.70:0.30) and III (C 1 :C 2 :C 3 = 79.96:10.04:10.00), corresponding to incubations i, ii and iii, respectively. All the hydrocarbons are of thermogenic origin, except C 1 (microbial) in incubation ii. The oxidation of C 1 -C 3 was observed in all three treatments, with variations in carbon isotope fractionation factors (ε C) of C 1 and average values of −10.5 ± 0.1, −5.7 ± 1.6, and −10.9 ± 2.9, respectively. The hydrogen isotope fractionation factors (ε H) of C 1 in the three incubations also display variation, with average values of −118.4 ± 3.0, −68.3 ± 15.0 and −56.3 ± 8.4, respectively. Experiments with thermogenic C 1 display greater carbon fractionation than those with microbial C 1. The ε C and ε H values for C 2 and C 3 were obtained only in incubation iii. Compared to C 1 , C 2 and C 3 exhibit weaker carbon isotope fractionation but stronger hydrogen isotope fractionation. The fractionation factors presented in this study can be applied to estimate the extent of C 1 -C 3 oxidation in natural environments, especially at seafloor and terrestrial seeps where aerobic microbes are commonly present. Our study has important implications in oil and gas exploration, with an alert for exercising caution when using the molecular and isotopic characteristics of hydrocarbon gases to evaluate the gas sources because these are susceptible to aerobic microbial oxidation. • Experimental evaluation of aerobic oxidation of methane (C l), ethane (C 2), propane (C 3). • C 1 -C 3 display different hydrogen and carbon isotope fractionation patterns. • Aerobic microbial oxidation can erase genetic information of hydrocarbon gas. [ABSTRACT FROM AUTHOR]

In an effort to assess the potential contamination and determine the environmental risks associated with heavy metals, 51 surface sediment samples (0-4 cm) were collected in the Zhoushan Islands coastal sea of the East China Sea (ECS). The heavy metal concentrations, grain size and total organic carbon (TOC) contents of these surface sediments were comprehensively analyzed. The results showed that the order of the average concentrations of heavy metals is Zn > Cr > Cu > Pb > As > Cd > Hg. Zn, Cr, Cd and Hg exhibit similar distribution and which derived from a common source, however, Cu, Pb and As present different distribution. The metal enrichment factor (EF) and geoaccumulation index (I geo ) values both suggest that Cu and Cd likely pose environmental risks. The results of this study could provide scientific data to authorities in charge of sustainable marine management., (Copyright © 2021 Elsevier Ltd. All rights reserved.)