Ni Yugen 1, 2; Xia Zhen 1, 2; Ma Shengzhong 1, 2 (1. Guangzhou Marine Geological Survey, Guangzhou 510760; 2. Key Laboratory of Seabed Mineral Resources, Ministry of Land and Resources, Guangzhou 510760) Fund project: Key Laboratory of Seabed Science of the State Oceanic Administration is open
Fund (KLSG0905).
Brief introduction of the first author: Ni Yugen (1984—), male, master's degree, mainly engaged in marine geology and natural gas hydrate research.
Email:niyugen@163.com.
Abstract In geological history, submarine landslides caused by the decomposition of natural gas hydrates have been widely distributed in the world's oceans. The famous ones include the Storegga landslide off the coast of Norway, the Beaufort Sea landslide in northern Alaska, the Cape Fear landslide on the South Carolina Continental Rise on the east coast of the United States,
The Amazon fan on the continental margin of northeastern Brazil, and the giant turbidite layer in the Balearic Basin of the Western Mediterranean; abrupt climate events caused by the decomposition of natural gas hydrates have also occurred many times, the famous one being the Jurassic Early Thorian oceanic gap.
Oxygen event (Early Toarcian OAE), Cretaceous Aptian ocean anoxic event (Aptian OAE), late Paleocene thermal extreme event (LPTM), and Quaternary interglacial global warming, etc.
Whether due to the rapid reduction of hydrostatic pressure during the cold period of geological history, or due to the warming of bottom water during the warm period of geological history, natural gas hydrates may become unstable and decompose, thereby inducing submarine landslides (slumps) and releasing huge amounts of gas hydrates.
Methane entering the atmosphere causes drastic changes in global climate.
Seafloor landslides and climate change events caused by the decomposition of natural gas hydrates can occur not only in the past but also in the future, and their effects may be catastrophic.
Therefore, while we are exploring and developing natural gas hydrates, we should also conduct in-depth research on its environmental effects, evaluate and weigh the pros and cons of human development of natural gas hydrates, in order to grasp the balance between natural gas hydrate resource benefits and environmental effects.
Keywords: natural gas hydrate, submarine landslide, climate change 1 Introduction Natural gas hydrate is a solid, non-fixed ratio clathrate compound composed of certain specific gas molecules (mainly methane) and water molecules under high pressure and low temperature conditions.
As a new type of clean energy, natural gas hydrate has broad development prospects, especially in the context of today's energy shortage.
Conservative estimates suggest that the energy contained in natural gas hydrates is twice that of all other fossil fuels combined [1].
Natural gas hydrate resources mainly exist in the marine environment, and the methane (including natural gas hydrate and free gas) stored in the global continental margins is as high as 10 to 20 trillion tons [2-4].
Countries such as the United States, Japan, Canada, Germany, India and China have invested heavily in the exploration and development of natural gas hydrate resources and have made major breakthroughs.
Several countries have set timetables for the commercial extraction of gas hydrates.
However, while natural gas hydrates have huge resource benefits, once they decompose, they can cause catastrophic submarine landslides and climate mutations.
2 Submarine landslides caused by the decomposition of natural gas hydrates Submarine landslides (slides) caused by the decomposition of natural gas hydrates are widely distributed around the world.
The most studied are the Storegga landslide off the coast of Norway formed during the last glacial period, the Beaufort Sea landslide in northern Alaska, the Cape Fear landslide on the South Carolina continental rise on the east coast of the United States, the Amazon fan on the continental margin in northeastern Brazil, and the Balearic landslide in the western Mediterranean.
Megaturbidity accumulation in the basin, etc.
The Storegga ("Great Edge") landslide system [5] off the coast of Norway is one of the best-studied submarine landslides. Its valley head steep wall is located on the edge of the continental shelf 100km offshore and is 290km long.
The landslide system extends from the continental slope to the 3600m deep sea basin, a distance of more than 800km. The debris deposits caused by the landslide are up to 450m thick, with a total volume of about 5600km3.
This landslide system has three phases of activity. The first phase is the largest (about 3880km3) and may have occurred between 30,000 and 50,000 years ago. The other two phases occurred between 6,000 and 8,000 years ago.
The second phase of landslides traced back 6 to 8 km compared to the first phase of landslides, destroying 450 km3 of the continental shelf edge. In this landslide, two 150 to 200 m thick and 10 × 30 km wide soil layers were formed along the continental slope (average slope 0.3°
) moved downward about 200km.
The third-stage landslide was limited to the remnants of the second-stage landslide, and may be the final activity of the second-stage landslide.
In the deepest part of the Norway Basin, more than 700km away from the landslide valley head, a fine-grained turbidite body more than 6m thick was deposited, which may be related to the second phase of the landslide.
The slip surface of the Storegga landslide is at the same depth as the bottom boundary of gas hydrates (BSR).
Bugge et al. [5] believed that earthquakes and natural gas hydrate decomposition caused sediment liquefaction, which triggered the Storegga landslide.
The first phase of the landslide may have resulted in the release of 5×1015 g or more methane [6].