Institution: | (1) Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039-9644, USA;(2) University of Wyoming, Laramie, WY 82071, USA;(3) University of California Davis, Davis, CA 95616, USA;(4) University of Tromso, 9037 Tromso, Norway;(5) Department of Earth Science, University of Bergen, 5007 Bergen, Norway;(6) University of New Hampshire, Durham, NH 03824-3589, USA;(7) United States Geological Survey, Woods Hole, MA 02543, USA;(8) United States Geological Survey, Menlo Park, CA 94025, USA |
Abstract: | Seafloor pockmarks and subsurface chimney structures are common on the Norwegian continental margin north of the Storegga
Slide scar. Such features are generally inferred to be associated with fluid expulsion, and imply overpressures in the subsurface.
Six long gravity and piston cores taken from the interior of three pockmarks were compared with four other cores taken from
the same area but outside the pockmarks, in order to elucidate the origins and stratigraphy of these features and their possible
association with the Storegga Slide event. Sulfate gradients in cores from within pockmarks are less steep than those in cores
from outside the pockmarks, which indicates that the flux of methane to the seafloor is presently smaller within the pockmarks
than in the adjacent undisturbed sediments. This suggests that these subsurface chimneys are not fluid flow conduits lined
with gas hydrate. Methane-derived authigenic carbonates and Bathymodiolus shells obtained from a pockmark at >6.3 m below the seafloor indicate that methane was previously available to support a
chemosynthetic community within the pockmark. AMS 14C measurements of planktonic Foraminifera overlying and interlayered with the shell-bearing sediment indicate that methane
was present on the seafloor within the pockmark prior to 14 ka 14C years b.p., i.e., well before the last major Storegga Slide event (7.2 ka 14C years b.p., or 8.2 ka calendar years b.p.). These observations provide evidence that overpressured fluids existed within the continental margin sediments off Norway
during the last major advance of Pleistocene glaciation. |