Ninth international conference on - Marum

Abstracts of oral presentations 39
of discharged fluid. Theoretical δ 18 O values for the dolomite were calculated according to the equation from
Fritz and Smith (1970) for bottom water with δ 18 OSMOW=0‰ and measured T=13 o C (Diaz-del-Rio et al., 2003).
Our calculations show that the oxygen isotopic composition of the dolomite precipitating in the present-day
environment is 3.7‰ (PDB), which is in the good agreement with the measured values. The differences between
the calculated and measured values one can explain by the variations in the temperature and isotopic
composition of the ambient water. An extremely light value of -3.39‰ (VPDB) measured in the top of the «Big»
chimney suggests either anomalous high temperature of carbonate formation or light isotopic composition of
original fluid.
Thus, the data obtained testify formation of studied chimneys at the water temperature over 13 o C during venting
of gas (perhaps thermogenic origin). The carbonate precipitation of the chimneys occurred from the canal
(around gas flux) to the outer side.
Methane seepage from the Arctic Shelf – Origin from permafrost, gas hydrate,
or river-borne organic matter?
T. D. Lorenson 1
1 U.S. Geological Survey 345 Middlefield Rd. MS-999 Menlo Park CA, 94025 USA
The Arctic shelf is currently undergoing warming related to Pleistocene and Holocene sea level rise starting 1.6
M.Y. ago. Recent global warming has dramatically increased the temperature of the Arctic region over the last
30 years. During the Holocene transgression warmer waters flooded the relatively cold permafrost of the Arctic
Shelf. The thermal pulse is still propagating down into the submerged permafrost resulting in thawing from both
above and below. Gas hydrates, if present in and beneath permafrost, may become thermally unstable, resulting
in methane emission into the water column and atmosphere.
A recent assessment of the Arctic region identified 1000 - 2000 Pg C of stored carbon that is vulnerable to
climate change over the next century. It appears that northern high latitude regions are a sink for atmospheric
carbon dioxide of approximately 0.5 Pg C per year and a source of atmospheric methane of approximately 50
Tg C per year. Gas hydrates and drowned permafrost were identified as having the greatest potential to release
methane.
Several studies conducted cooperation with the USGS during the past 20 years have attempted to find
geochemical evidence for gas hydrate dissociation on the Beaufort Sea shelf where a variety of overlapping
geologic scenarios exist: 1) drowned permafrost, 2) known petroleum systems of Prudhoe Bay and the
Mackenzie River, 3) submerged pingo-like features (PLF’s), 4) pockmark fields, and 5) several river deltas
entering the Arctic Ocean, the largest of which in the Mackenzie River. The results of these studies show that
methane source and location in the geosphere are variable.
On the Alaskan Beaufort Shelf, methane in seawater is elevated in water depths less than about 10 m, This
methane appears to be can be associated with drowned permafrost, and likely originates from microbial
degradation of organic matter deposited by rivers. Deeper on the shelf, north of the Prudhoe Bay oil field, some
exceptionally high bottom water methane concentrations were measured with carbon isotopic signatures very
similar (~ -46 to -48‰) to gas hydrate sampled from the Mt. Elbert 01 gas hydrate test well drilled in 2007. It is
clear that this methane is associated with the Prudhoe Bay petroleum system resulting in compelling but not yet
conclusive evidence that gas hydrate dissociation is occurring on the shelf.
Gas venting in and around the Mackenzie River delta is associated with offshore PLF’s and pockmarks. PLF’s
resemble onshore pingos, however their genesis is uncertain. The region is underlain by an active petroleum
system and drowned permafrost. Methane concentrations are elevated in cores from the PLF’s and remote
ocean vehicle (ROV) surveys revealed streams of microbial – sourced methane bubbles (-76 to -80‰)
emanating from at least two PLF’s. These PLF’s are associated with an exceptionally thick area of submerged
permafrost and are may be produced by upward-migrating, overpressured gas and water. It has been suggested
that the gas and water originate from decomposing, intra-permafrost gas hydrate, however migration of methane
from decomposition of deeply buried organic matter water in sequestered in permafrost cannot be ruled out.
Microbial methane (-72 to -99‰) from the Kugmallit pockmark field, near the mouth of the Mackenzie delta is
isotopically similar to methane emanating from sampled PLF’s. Methane in deeper gas hydrates of the
Mackenzie delta is thermogenic (mean value -42.7‰). Inland, gas seeps occur in shallow ponds and streams,
resulting in steep-sided pockmarks. This methane is thermogenic, has been venting vigorously for at least 45
years, and is very similar in composition to deeper gas hydrate and thermogenic gas in nearby gas fields.