Потоки метана и газовые гидраты шельфов Карского моря и моря Лаптевых
ПОТОКИ МЕТАНА И ГАЗОВЫЕ ГИДРАТЫ ШЕЛЬФОВ КАРСКОГО МОРЯ И МОРЯ ЛАПТЕВЫХ
Худякова Екатерина Николаевна
Магистерская программа «Комплексное изучение окружающей среды полярных регионов» «КОРЕЛИС» / 05.04.06 «Экология и природопользование»
Научный руководитель: Алексей Алексеевич Крылов, к.г.-м.н., вед.н.с., ФГБУ “ВНИИОкеангеология”, доцент, СПбГУ
Научный консультант: Павел Серов, кандидат естественных наук, доктор, Арктический университет Норвегии
Метан является важным компонентом атмосферного воздуха и парниковым газом, вклад в парниковый эффект которого в 25 раз сильнее, чем от углекислого газа. В связи с тем, что содержание метана в воздухе непрерывно растет с прединдустриального периода, этот факт беспокоит исследователей всего мира. Если концентрация метана продолжит возрастать, эффект глобального потепления будет все более заметен.
Газовые гидраты являются одним из источников метана, которые при дестабилизации могут высвободить метан в атмосферу и, тем самым, содействовать изменению климата. Арктический регион обладает благоприятными условиями для формирования газовых гидратов из-за низких придонных температур. Повышение температуры воды, вследствие глобального потепления, может привести к высвобождению метана из газовых гидратов.
Цель магистерской диссертации – оценить потенциальные потоки метана в районе распространения газовых гидратов на шельфах Карского моря и моря Лаптевых. Для выполнения данной цели необходимо определить мощность зоны стабильности газовых гидратов по разрезам в настоящее время и в позднем Плейстоцене, а также рассчитать количество метана, которое может быть сгенерировано микробами на основании содержания органического углерода в донных осадках.
Для данных целей были выбраны 4 разреза в Карском море и море Лаптевых: 2 субмеридинальных и 2 субширотных, с 16 точками, расположенными на разных глубинах.
Расчеты мощности зоны стабильности газовых гидратов были сделаны с помощью программы Hydoff (Hydrate prediction program). Расчеты потоков метана были осуществлены на основании стехиометрии реакции образования метана из органического вещества.
В результате во всех точках разрезов были рассчитаны толщина, кровля и подошва зоны стабильности газовых гидратов (для четырех газовых смесей). На основании расчетов, мощность GHSZ в море Лаптевых меньше в среднем на 200 м, чем в Карском море, что говорит о ее большей степени уязвимости к полной диссоциации метана. 10 % примесь этана в составе газовых гидратов уменьшает глубину потенциального образования газовых гидратов в среднем на 100 м, а толщина зоны стабильности увеличивается в среднем на 150 м. Расчеты для периода позднего Плейстоцена показали, что мощность зоны стабильности в настоящее время больше, что вызвано повышением уровня океана и созданием дополнительного гидростатического давления.
Расчёты количества метана показали, что в выбранных точках нет дополнительного глубинного источника в виде газовых гидратов, что говорит нам о их стабильности в настоящий момент времени.
15-May-2020
Information about scientific supervisor, scientific consultant and reviver 2 Table of contents 3 Abstract in English 5 Abstract in Russian 6
1. Introduction 7 1.1. Background and importance of the research 7 1.2. Structure of the master thesis 8 1.3. The region of investigation 8 1.4. Goals and objectives 9
2. Theoretical background 10 2.1 Methane as greenhouse gas 10 2.2 Sources of methane 10 2.3 Physicochemical properties of methane 11 2.4 Impact on climate system 13 2.5 Gas hydrates origin 14 2.6 Gas hydrates distribution 16 2.7 Gas hydrates in the Arctic Region 17
3. Methodology 20
3.1 3.1.1 3.1.2 3.1.3 3.1.4 3.1.5
Gas hydrate stability zone 20 Сoncept of stability of gas hydrates 20 Hydoff (hydrate prediction program) 22
Bottom temperature
Bottom salinity Geothermal gradient
23
25
28
30
30
32
34
34
48
51
55
61
3.2 Methane fluxes
3.2.1 Organic carbon content
3.2.2 Methane content
4. Results
4.1 Gas hydrate stability zone
4.2 GHSZ during late Pleistocene 4.3 Methane fluxes
5. Discussion
6. Conclusion
3
Acknowledgements References Attachment 1 Attachment 2 Attachment 3
63 64 68 69 70
1.1 Background and importance of the research
Methane is one of the greenhouse gases, which has an important place among the atmospheric component of the air. Methane content in the atmosphere increased intensively since pre-industrial period, and that is why this fact is under huge interest among researches. Increasing of methane content can cause change of chemical processes in atmosphere and it can change environmental situation on Earth (Science- based proposals … 2015).
Since methane is greenhouse gas, which is 25 times more efficient than carbon dioxide, it contributes to the enhancement of the greenhouse effect. Thus, it has huge effect on global warming (Science-based proposals … 2015).
Methane can originate from natural and anthropogenic sources. Natural sources of methane include swamps, tundra, water bodies, insects (mainly termites), methane hydrates, and geochemical processes. Anthropogenic sources are rice fields, mines, livestock, and losses in the extraction of gas and oil, biomass burning, landfills (Science-based proposals … 2015).
Methane hydrates are one of the forms of methane storage, which in solid ice form trapped in permafrost and under the seabed. Potentially, further rising air and sea water temperature can cause to release of methane from this gas hydrates, thus accelerate the effect of global warming (Bogoyavlensky et al. 2018).
Most of gas hydrates reservoirs are located on the continental slopes of the oceans and in the Arctic Region. The necessary thermobaric conditions for the formation of gas hydrates are well expressed in arctic conditions. Low (negative) bottom water temperature are favorable for such gas hydrates formation. Increase of sea water temperature may сause release of methane from hydrates and has significant impact on climate system (Bogoyavlensky et al. 2018). That is why it is important to determine the gas hydrates stability zones in the Arctic Region to predict potential methane fluxes from the areas, where gas hydrates can be influenced by degradation because of climate change.
Gas hydrates also are indicators of the prospecting for oil and gas deposits, and many researchers were concentrated on this direction. But as we stated above it is also important to study the influence of gas hydrates on climate system.
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1.2 Structure of the master thesis
This master thesis consists of six chapters.
The first chapter is introduction chapter, which determine importance and goals of the research.
The second chapter reviews background information about methane as greenhouse gas, its features, sources and impact on climate system, in particularly about gas hydrates and its distribution.
The third chapter represent methodological approach used in this master thesis, in particular it describes the concept of the gas hydrate stability zone and the program Hydoff (Hydrate prediction program), which was used to make calculations for defining of thickness, top and base of GHSZ. It also describes compounds, which are necessary for its determination: bottom temperature, bottom salinity and geothermal gradient, applied to the region of investigation. Finally, the chapter represent the method of methane amount calculation based on organic carbon content in bottom sediments.
The chapter four contains main results: calculated thickness, top and base of the gas hydrate stability zone in different points at the present time and late Pleistocene. Further, calculated methane fluxes are represented.
The chapter five contains discussion, where my results were accessed according to other works in this field of research.
Finally, in the chapter six – conclusion, I summarize key points of the research and briefly discuss main implications.
1.3 The region of investigation
The Kara and Laptev Sea shelves were chosen as region of investigation for the goals of this research.
Both seas are marginal seas of the Arctic Ocean with wide sea shelves where favorable conditions for formation of gas hydrates present.
The Laptev Sea occupies the shelf, captures the mainland slope and a small part of the ocean bed. The bottom relief of the shelf is relatively weakly crossed. Half of its total area relates to shelf depth until 50 m. In the northern part of the sea, continental slope abruptly breaks off and passes into the ocean bed, where the depths significantly increase with the maximum depth of of 3385 meters. The average depth of the Laptev Sea is 533 m (Dobrovol’skiy, Zalogin 1982).
The bottom relief of the Kara Sea compared to the Laptev Sea is very uneven.
Depths until 100 m predominate in the sea. The southern and eastern parts of the sea are
8
shallowest, the greatest depths are in the west and north-west of the Kara Sea (Dobrovol’skiy, Zalogin 1982). The maximum depth is 620 m in the northern part of St. Anna Trough. Narrow Novaya Zemlya Trough stretches along Novaya Zemlya. Additionally to St. Anna Trough, Voronin Trough and the accumulative elevations including the Central Kara Rise are distinguished (Kulakov et al. 2004).
Kara Sea and Laptev Sea have more harsh climatic conditions in comparison with Barents Sea, where the influence of the Golfstream is quite significant and where we can expect increased methane fluxes because of more intensive degradation of subaqual permafrost and gas hydrates. Nevertheless, it is important to evaluate potential methane fluxes from this region because of accelerating of global climate change in the whole Arctic Region.
1.4 Goals and objectives
The goal of the master thesis is to evaluate potential methane fluxes from gas hydrates distribution areas in the Kara and Laptev seas shelves.
Next objectives were defined:
To determine thickness, top and base of gas hydrates stability zones (GHSZ) in different point of profiles in the Kara and Laptev Seas
To determine thickness, top and base of GHSZ for Late Pleistocene and compare them with present time
To calculate amount of methane which can be generated in different points of profiles on the basis of organic carbon content in bottom sediments
In this research was decided to determine where in the Kara Sea and the Laptev Sea gas hydrates can lay directly near the bottom surface. It allows us to see zones where gas hydrates more vulnerable to dissociation and where then we potentially can expect higher methane fluxes, which can contribute to further climate change.
This Master Thesis is the result of the two year master program «Cold Region Environmental landscapes Integrated Science (CORELIS)» in the Institute of Earth Sciences in St. Petersburg State University.
I would like to say thanks to my scientific supervisor Dr. Alexey A. Krylov, Leading Reseacher in VNIIOkeangeologia and associate professor in St. Petersburg State University for his guidance and support during the writing of this Master Thesis. His feedback and comments were helpful and valuable to me.
I am grateful to the Department of the Geological mapping in FSBI
“VNIIOkeangeologia”, where I had my practice. There I collected some theoretical material for my research and got access to special program such as Hydoff (Hydrate prediction program), which helped me to make necessary calculations.
I also want to thank my scientific consultant Pavel Serov, PhD, VISTA postdoc at CAGE – Centre for Arctic Gas hydrate from The Arctic University of Norway for his valuable comments and feedback.
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