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Woods
Hole
Oceanographic Institution
Woods Hole, MA 02543 USA |
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Research Projects
Gas and Oil Seepage and Hydrothermal Venting
in the Ocean Bottom
— Detection by Fluorescence
Jean K. Whelan, Nathan Mah, and Greg Eischeid, Departement of Marine
Chemistry & Geochemistry, Woods Hole Oceanographic Institution, Woods
Hole MA 02543
Robert Chen and Xuchen Wang, Department of Environmental Sciences,
University of Massachusetts, Boston, MA 02125
Harry Roberts and Paul Aharon, Center for Coastal Studies &
Department of Geology & Geophysics, Louisiana State University, Baton
Rouge, LA 70803 |
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Introduction
An abundance of evidence suggests
continuous or episodic upward movement of fluids from deeper sediments
into surface sediments and ocean bottom waters
(Table 1). These seepages may have been volumetrically underestimated
in the past, both in oil and gas productive areas and in tectonic regimes
(e.g. hydrothermal vent areas) because they often occur through small
seemingly unimportant localized cracks in the seafloor . However, even if
these venting features are small, the volume of expelled fluid can be
important: even a small fracture can deliver orders of magnitude more
fluid than ordinary compaction and diffusion processes.
Here we present initial results on successful deployment of CTD
fluorescence for continuous detection of bottom venting fluids both in a
known oil and gas seep area (Green Canyon in the offshore Louisiana Gulf
of Mexico,
Figure 1a )
and in a hydrothermal vent area (Guaymas Basin, Gulf of California,
Figure 1c ).
Our initial results confirm the very localized and
possibly episodic nature of these venting features which would not have
been detected without a continuous localized detection technique.
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Figures and Tables
Figure 1: Locations of initial bottom CTD fluorescence measurements:
a) overview of location of Gulf
Coast Green Canyon area;
b) close-up: location of Green Canyon
manned submersible dive sites;
c) overview of location of Gulf of
California Guaymas Basin hydrothermal venting area;
d) close-up: Guaymas Basin,
location of Jason (unmanned submersible) cruise track
Figure 2: Green Canyon: CTD fluorescence data, Seatech DOC fluorometer.
Figure 3: Green Canyon: DOC
Figure 4: Green Canyon: laboratory quantitative fluorescence data
calibrated to Quinine standard
Figure 5: Green Canyon - excitation-emission fluorescence spectra:
a )
water and hexane extracts of unaltered oil;
b )
bottom water from Dive Site 2893;
c )
bottom water from Dive Site 2900;
d )
bottom water from Dive Site 2901;
e )
typical open ocean mid-water away from seepage site.
Figure 6: Green Canyon water, GCMS total ion chromatogram (TIC)
of methylene chloride extract in comparison to tetracosane recovery
standard.
Figure 7: Guaymas Basin: CTD fluorescence data, Seatech (DOC) and
Chelsea (aromatic hydrocarbon) fluorometers - overall.
Figure 8: Guaymas Basin comparative fluorometer measurements
(dive of 4/20/98).
Table 1: Summary observations suggesting seepage to bottom
sediments may have been underestimated in past
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Acknowledgements
Captain and crew of Edwin Link and Johnson Sea Link;
Dana Yoerger and the Deep Submergence Group for "piggybacking"
us on their Guaymas Jason
dive; Al Bradley and Ellen Druffel for helpful
discussions which got us to thinking about this work. Financial support
from the Vetlesen Foundation through a grant to the Woods Hole
Oceanographic Foundation and from the Department of Energy (grant no.
DE-FG02-86ER13466 to Jean Whelan) is gratefully acknowledged.
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