PSBenthic
Foraminiferal Characteristics of Cenozoic Cold Seeps on the Northeastern Pacific
Margin*
By
Ruth Martin1 and Elizabeth Nesbitt2
Search and Discovery Article #50015 (2005)
Posted August 14, 2005
*Poster presentation at AAPG Annual Convention, with SEPM, Calgary, Alberta, June 19-22, 2005.
Click
to view poster in PDF format
1University of Washington, Seattle, WA ([email protected])
2Burke Museum, University of Washington, Seattle, WA
Abstract
Hydrocarbon seeps within Cenozoic formations of western
Washington and Oregon display different morphological characteristics. Some,
such as the Eocene-Oligocene Lincoln Creek Formation are characterized by
carbonate mounds, lenses and chimneys with distinctive macrofauna adapted to low
oxygen organic-rich environments. Others, such as seeps in the Pliocene Quinault
Formation contain diffuse seep signatures including small, irregular carbonate
nodules, abundant glendonite crystals, and occasional seep-specific bivalves.
The purpose of this investigation was to begin with the Quinault Formation to
characterize fossil methane seeps using benthic
foraminifera.
Foraminifera were extracted from strata showing obvious
seep and non-seep signatures. Analytical methods included (1) determining
species diversity and population densities, and (2) comparing
d13C values of seep and
non-seep foraminiferal tests and authigenic carbonates. Optical and scanning
electron microscopy were used to establish that foraminifera were free of
authigenic and diagenetic carbonate. Foraminiferal assemblages of the different
samples are distinctive. Non-seep samples contain more foraminifera, averaging
85 individuals/gram of sediment, and are dominated by epifaunal species. Samples
from the most intense seepage zones contain fewer individuals, averaging 43
individuals/gram of sediment, and are dominated by infaunal taxa, particularly
species of Globobulima, which is tolerant of low oxygen environments in Recent
sediments. Inorganic carbonates yielded d
13C values of +8 0/00 to -38.6 0/00.
These values indicate an environment rich in thermogenic methane. Seep
foraminifera are significantly depleted in 13C, also indicating the
influence of hydrocarbon-rich pore waters. Thus, benthic
foraminifera can be
used to characterize fossil methane seeps, and may be particularly useful in
areas of diffuse seepage.
Two localities of Tertiary foraminifera-bearing cold seep deposits.