Introduction

The northern North Atlantic and Arctic oceans are morphologically and geologically complex. The constructive axial plate margin of the northern North Atlantic is propagating through Fram Strait, forming a young oceanic crust in the Nansen Basin of the Eurasian end of the deep water Arctic Ocean (Fig. 1). A complex transform along the continental margin of the Laptev Sea is the present termination of this Atlantic-Arctic Ocean spreading center. The North American end of the Arctic Ocean is floored by older oceanic crust carrying a thick sediment prism in the western end of the Canada Basin. The Barents Sea, like the other wide shallow water margins of the Asian Arctic Ocean and narrower continental shelf elsewhere around the Arctic margin, is an epicontinental sea (Eldholm & Talwani, 1977).

The methane generating character of marine sediments is fundamental to the development of hydrate. Any area in which delivery of organic material is high and burial is rapid will lead to formation of biogenic methane. The Arctic and the North Atlantic Oceans are likely to be revealed as major hydrate provinces because the oceanographic conditions (around 0°C) are highly suitable for preservation and burial of organic material (>1.5%). Deeply buried sediments in the Arctic are rich in organic matter. The area comprises one of the largest contiguous sedimentary provinceswith significant amounts of organic carbon (Premuzic, 1980; Romankevich, 1984). The sedimentary framework has been similar in the North American end of the Arctic basin since mid-Mesozoic times and in the Eurasian end since not long after the formation of the plate margin in Magnetic Anomaly 23 times (~ 52 Ma bp) (Vogt, 1986; 1999; Eldholm et al., 1987).

Figure 1. Location and geographical names of generalized major basins, ridges and geological features in the Arctic and northern North Atlantic Oceans. From Max and Lowrie (1993). Polar equal area projection. Dot on Lomonosov Ridge is North rotational Pole. AR, Aegir Ridge; BAP, Barents Abyssal Plain; BB, Boreas Basin; Bl, Bennet Island; CC, Chukchi Cap; EI, Ellesmere Island; FAP, Fletcher Abyssal Plain; FJI, Franz Joseph Islands; FS, Fram Strait; HM, H&kon Mosby Mud Volcano; GB, Greenland Basin; GFZ, Greenland Fault Zone; GS, Greenland Sea; I, Iceland; JMF, Jan Mayen Fault Zone; KBR, Kolbensey Ridge; KNR, Knipovich Ridge; LB, Lofoten Basin; LS, Labrador Sea; MB, Malene Bukta; NB, Northwind Basin; NOB, Norway Basin; NR, Northwind Ridge; NS, Nares Strait; NOS, Norwegian Sea; NSL, North Slope; PB, Prudhoe Bay; PAP, Pole Abyssal Plain; SAP, Siberian Abyssal Plain; SS, Storegga Slide; SV, Svalbard; V, Vestnesa Ridge; VP, Voring Plateau; WAP, Wrangel Abyssal Plain; YFZ, Yermak Fault Zone; YP, Yermak plateau.

Figure 1. Location and geographical names of generalized major basins, ridges and geological features in the Arctic and northern North Atlantic Oceans. From Max and Lowrie (1993). Polar equal area projection. Dot on Lomonosov Ridge is North rotational Pole. AR, Aegir Ridge; BAP, Barents Abyssal Plain; BB, Boreas Basin; Bl, Bennet Island; CC, Chukchi Cap; EI, Ellesmere Island; FAP, Fletcher Abyssal Plain; FJI, Franz Joseph Islands; FS, Fram Strait; HM, H&kon Mosby Mud Volcano; GB, Greenland Basin; GFZ, Greenland Fault Zone; GS, Greenland Sea; I, Iceland; JMF, Jan Mayen Fault Zone; KBR, Kolbensey Ridge; KNR, Knipovich Ridge; LB, Lofoten Basin; LS, Labrador Sea; MB, Malene Bukta; NB, Northwind Basin; NOB, Norway Basin; NR, Northwind Ridge; NS, Nares Strait; NOS, Norwegian Sea; NSL, North Slope; PB, Prudhoe Bay; PAP, Pole Abyssal Plain; SAP, Siberian Abyssal Plain; SS, Storegga Slide; SV, Svalbard; V, Vestnesa Ridge; VP, Voring Plateau; WAP, Wrangel Abyssal Plain; YFZ, Yermak Fault Zone; YP, Yermak plateau.

The Norwegian margin, including the northern North Sea and the Barents Sea (Vorren et al., 1993) along with the North American end of the Arctic Ocean and adjacent land areas (Chapter 5), are proven hydrocarbon provinces. Because the more deeply buried sediments have provided organic material that transformed into gas and oil, sediments in the upper 2 to 3 km of sediment which have not entered the oil window of hydrocarbon maturation (Max and Lowrie, 1993) are likely to provide a rich feedstock for methanogenic bacteria feeding hydrate formation (Chapter 8). Thus, the hydrocarbon-generating potential of marine sediments along this continental margin can be expected to have provided an excellent host for methane generation.

Hydrocarbon exploration in the Arctic has yet to move into many areas where hydrocarbons are likely to be concentrated. However, it is a great challenge because of the economic constraints associated with extreme cold in a remote area, shifting sea ice, environmental concerns, and very difficult logistics. Potentially methane-rich sediments almost certainly underlie large areas. It can be inferred that gas and oil, as well as hydrate deposits, will be more widely found than is currently proven.

Heat flow, which is critical to the thickness of the hydrate stability zone (HSZ) varies considerably in the Arctic and northern North Atlantic Oceans. Sea floor in the age range 100-200 my is 45-55 mW/m2, younger sea floor has higher heat flows and thinner sediment cover (references to heat flow in Max and Lowrie, 1993). On active ridge sites to 3-4 Ma off-ridge heat flows average about 300 mW/m2 with some heat flow measurements nearly 400 mW/m2. The oceanic crust of the northern Greenland-Norwegian Sea has a high heat flow of between 100 and 200 mW/m2 (Vogt and Sundvor, 1996).

The presence of natural submarine gas hydrates is commonly inferred from seismic reflection data (e.g. Hyndman and Spence, 1992). The base of the stability zone for gas hydrates (HSZ) is geophysically identified by the occurrence of a bottom simulating reflector (BSR) (Stoll et al., 1971). The BSR is a reflection at the boundary between a normal velocity layer or high-velocity gas hydrate cemented sediments and the underlying low-velocity gas-bearing sediments. Whereas compressional velocity values of 1700-2400 m/s are known to be typical for gas-hydrated sediments (Andreassen et al., 1990; Katzman et al., 1994; Lee et al., 1994; Minshull et al., 1994; Andreassen et al., 1995) values below the sound velocity of sea water (SVS) indicate free gas in the pore space. The BSR mimics the shape of the sea floor, often cuts the dominant stratigraphy and is characterized by a high, reversed polarity event (e.g. Lodolo et al., 1993; Katzman et al., 1994; Andreassen et al., 1995).

Oceanic hydrate (Chapter 6) has been geophysically recognized in deep water in a continuous zone along the North Slope of Alaska, in isolated localities in the Barents Sea (Laberg and Andreassen, 1996) and Norwegian continental margins (Mienert et al., 1998, Mienert and Posewang, 1999), and at least one locality in the eastern Labrador Sea (Fig. 1). In addition, hydrate formed independently from the presence of subsea permafrost (Chapter 5) and has been recognized in the Barents Sea (Lovo et al., 1990). Although considerable seismic data exist for the Barents Sea and Norwegian continental margin (Vorren et al., 1993), difficulty in carrying out seismic, bathymetric, and oceanographic surveys have yielded little data for the ice-covered Arctic basin as a whole.

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