Benthic Foraminiferal Characteristics of Cenozoic Cold Seeps on the Northeastern Pacific Margin

Ruth Martin¹ and Elizabeth Nesbitt^2
1 University of Washington, Seattle, WA
² Burke Museum, University of Washington, Seattle, WA

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 δ¹³C 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 δ¹³C values of +8⁰/₀₀ 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.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005