Anna V. Domaniuk

Relevance. Gas flares or seeps consisting of bubbles continuously rising from the seabed have been recorded throughout the World Ocean at depths ranging from several meters to three kilometers or more. Measurements of the gas composition of the rising bubbles shown that they are dominated by methane (CH4). The East Siberian Arctic Shelf contains more than 30% of the world CH4 and carbon dioxide reserves, preserved in bottom sediments by underwater permafrost. In the shallow seas of the East Siberian Arctic Shelf, the main mechanism for transporting CH4 from bottom sediments into water is bubble transport. Therefore, it is extremely important to estimate the amount of CH4 transported by rising bubbles from bottom sediments into the water column and the atmosphere. Aim. Estimation of CH4 quantity transported by chains of rising bubbles from the seabed to the atmospheric surface layers. Methods. The manuscript presents a study of the mechanism of gas exchange between rising bubbles and a liquid column, carried out using a specially designed stand that allows simulating local upwelling. Results and conclusions. The paper shows that chains of bubbles coming from the seabed with intensities of ~40 ml∙min–1 and ~110 ml∙min–1, taking into account the hydrostatic pressure, deliver 206 mg∙min–1 and 616 mg∙min–1 of CH4 to the bottom layer of the water column, respectively. The results obtained during laboratory studies allowed us to specify the flux of CH4 from bottom sediments to the atmospheric surface layers. Taking into account the quantity of gas exchange and local upwelling, the amount of methane delivered to the atmospheric surface layers was 69 mg∙min–1 and 286 mg∙min–1. The paper presents an acoustic estimate of the amount of CH4 transported by the seep, which includes the considered chains of rising bubbles. According to calculations, the flux of CH4 into water from this area in 2012 was ~40 g∙min–1. At the same time, the amount of CH4 transported by this seep into the atmospheric surface layers, taking into account local upwelling and gas exchange occurring between the rising bubbles and the liquid column, is ~24.5 kg per day or ~9 tons per year.

The Russian sector of the arctic shelf is the longest in the world. Quite a lot of places of massive discharge of bubble methane from the seabed into the water column and further into the atmosphere were found there. This natural phenomenon requires an extensive complex of geological, biological, geophysical, and chemical studies. This article is devoted to aspects of the use of a complex of marine geophysical equipment applied in the Russian sector of the arctic shelf for the detection and study of areas of the water and sedimentary strata with increased saturation with natural gases, as well as a description of some of the results obtained. This complex contains a single-beam scientific high-frequency echo sounder and multibeam system, a sub-bottom profiler, ocean-bottom seismographs, and equipment for continuous seismoacoustic profiling and electrical exploration. The experience of using the above equipment and the examples of the results obtained in the Laptev Sea have shown that these marine geophysical methods are effective and of particular importance for solving most problems related to the detection, mapping, quantification, and monitoring of underwater gas release from the bottom sediments of the shelf zone of the arctic seas, as well as the study of upper and deeper geological roots of gas emission and their relationship with tectonic processes. Geophysical surveys have a significant performance advantage compared to any contact methods. The large-scale application of a wide range of marine geophysical methods is essential for a comprehensive study of the geohazards of vast shelf zones, which have significant potential for economic use.

Актуальность. В работе представлены результаты изучения морфологии и размеров аутигенного пирита, присутствующего в донных осадках как в рассеянном виде, так и в ассоциации с карбонатными стяжениями. С позиции изучения условий раннего диагенеза аутигенный пирит считается наиболее важным сульфидным минералом железа по причине его наибольшей диагенетической стабильности относительно других сульфидов железа. Ранее проведенные многочисленные исследования морфологии и размеров аутигенного пирита показывают возможность использования этой информации для оценки окислительно-восстановительных условий осадконакопления и раннего диагенеза как в современных, так и в древних осадочных бассейнах. Целью работы являлось изучения морфологических и размерных характеристик аутигенного пирита для оценки влияния сульфат-управляемого анаэробного окисления метана на условия раннего диагенеза. Методы: гидроакустические исследования (Kongsberg EA600), рентгенодифракционный анализ (Bruker D2 Phaser), сканирующая электронная микроскопия с локальным энергодисперсионным анализом (TESCAN VEGA 3 SBU). Основные результаты. В морфологическом отношении пирит представлен идиоморфными и гипидиоморфными кристаллами, фрамбоидами и их скоплениями, радиальными наростами, а также стержневидными агрегатами. Средний диаметр фрамбоидов, а также высокое значение стандартного отклонения в обеих рассматриваемых выборках свидетельствует о диагенетическом происхождении пирита. Сульфат-управляемое анаэробное окисление метана является доминирующим процессом, контролирующим формирование сульфидов железа, что подтверждается наличием достаточно крупных фрамбоидов диаметром до 49 мкм. Наблюдаемое разнообразие морфологических форм пирита может отражать изменение диагенетической среды с течением времени, вызванное вариативностью потока метан-содержащих флюидов.

The East Siberian Arctic Shelf (ESAS) is a key area of CH₄ venting in the Arctic Ocean. The ESAS region stores more than 80% of the world’s predicted subsea permafrost and associated permafrost-related gas hydrates. Gas emissions from subsea permafrost are controlled by its current thermal state, which, in turn, depends on environmental factors. The aim of the manuscript is to show that the thermal state of subsea permafrost and phase transitions of its pore moisture can be estimated remotely by echo soundings, which can resolve the structure of shallow bottom sediments. It has been found that the duration of the seabed acoustic response (echo duration, Δ) at frequencies of 50 and 200 kHz correlates with sediment temperatures and generally increases with cooling below 0.5 °C. This correlation, explained by assuming a layered structure of the bottom sediments, establishes the basis for high-frequency acoustic thermometry. The technique is an advantageous tool for many applications: fast contouring of low-temperature zones, remote measurements of seabed surface temperature, and estimation of the thickness of frozen sediments near the bottom. The latter estimates have implications for the distribution of subsea permafrost and the stability of gas hydrates on the Arctic shelf.