65 Geochemistry II: Organic Geochemistry

Monday, 8 November 2004
8:00 AM - 12:00 PM

MOLECULAR CHARACTERIZATION OF MICROBIAL MATS FROM ALKALINE LAKES IN WARNER VALLEY, OREGON

Paper number: 65-13
Program: Technical Sessions
Time: 11:00 AM – 11:15 AM

Authors

Abstract

Alkaline brackish lakes and wetlands are the dominant environments in the northern part of Warner Valley which is closed-drainage half-graben within the tectonic province of the Oregon-Nevada Great Basin. Water, mat, and sediment samples were collected from a chain of alkaline lakes located adjacent to cliffs comprising layers of andesitic basalt that rise 500-1000 meters above the valley floor. Water chemistries in the lakes are dominated by Na-Cl-SO4. From early summer through fall, most of the lakes are covered by fibrous algal-cellulose mats overlying bacterial mats and anaerobic muddy sediments. Both algae and bacteria in these lakes are tolerant of arsenic. Concentrations of arsenic are highest (up to 700 μg/l) in seeps on the eastern margin of Anderson Lake where gelatinous microbial mats dominated by purple-sulfur bacterial blanket the marginal wetland. Algal and bacterial mats were characterized using solvent extraction of both untreated and pyrolyzed samples separation of the saturated hydrocarbons and gas chromatograph/mass spectrometry. Saturated hydrocarbons from untreated mat samples are dominated by n-C23, n-C25, n-C27, and n-C29, and have a Carbon Preference Index (CPI)=4.6. Saturated hydrocarbons from pyrolyzed mat samples are dominated by branched alkanes eluting in the range of n-C18 to n-C20, with a CPI=1.5. Hydrous pyrolysis yields 250 to 900 mg of extractable organic matter per gm of mat suggesting a high potential for preserving microbial biomarkers and for sourcing petroleum from alkaline lake deposits in extensional tectonic settings.