Publications

Export 3 results:
Sort by: Author Title Type [ Year  (Desc)]
2017
Hansman, RL, Thurber AR, Levin LA, Aluwihare LI.  2017.  Methane fates in the benthos and water column at cold seep sites along the continental margin of Central and North America. Deep-Sea Research Part I-Oceanographic Research Papers. 120:122-131.   10.1016/j.dsr.2016.12.016   AbstractWebsite

The potential influence of methane seeps on carbon cycling is a key question for global assessments, but the study of carbon cycling in surface sediments and the water column of cold seep environments is complicated by the high temporal and spatial variability of fluid and gas fluxes at these sites. In this study we directly examined carbon sources supporting benthic and planktonic food webs at venting methane seeps using isotopic and molecular approaches that integrate this variability. At four seep environments located along North and Central America, microorganisms from two size fractions were collected over several days from 2800 to 90501 of seawater to provide a time-integrated measure of key microbial groups and the carbon sources supporting the overall planktonic microbial community. In addition to water column measurements, the extent of seafloor methane release was estimated at two of the sites by examining the stable carbon isotopic signature (delta C-13) of benthic metazoan infauna. This signature reveals carbon sources fueling the base of the food chain and thus provides a metric that represents a time-integrated view of the dominant microbial processes within the sediment. The stable carbon isotopic composition of microbial DNA (delta C-13-DNA), which had values between -17.0 and -19.5%(0), indicated that bulk planktonic microbial production was not ultimately linked to methane or other C-13-depleted seep-derived carbon sources. Instead these data support the importance of organic carbon derived from either photo- or chemoautotrophic CO2 fixation to the planktonic food web. Results of qPCR of microbial DNA sequences coding for a subunit of the particulate methane monooxygenase gene (pmoA) showed that only a small percentage of the planktonic microbial community were potential methane oxidizers possessing pmoA (< 5% of 16S rRNA gene copies). There was an overall decrease of C-13-depleted carbon fueling the benthic metazoan community from 3 to 5 cm below the seafloor to the sediment surface, reflecting limited use of isotopically depleted carbon at the sediment surface. Rare methane emission as indicated by limited aerobic methane oxidation acts to corroborate our findings for the planktonic microbial community.

2015
Maloney, JM, Grupe BM, Pasulka AL, Dawson KS, Case DH, Frieder CA, Levin LA, Driscoll NW.  2015.  Transpressional segment boundaries in strike-slip fault systems offshore southern California: Implications for fluid expulsion and cold seep habitats. Geophysical Research Letters. 42:4080-4088.   10.1002/2015gl063778   AbstractWebsite

The importance of tectonics and fluid flow in controlling cold seep habitats has long been appreciated at convergent margins but remains poorly understood in strike-slip systems. Here we present geophysical, geochemical, and biological data from an active methane seep offshore from Del Mar, California, in the inner California borderlands (ICB). The location of this seep appears controlled by localized transpression associated with a step in the San Diego Trough fault zone and provides an opportunity to examine the interplay between fluid expulsion and restraining step overs along strike-slip fault systems. These segment boundaries may have important controls on seep locations in the ICB and other margins characterized by strike-slip faulting (e.g., Greece, Sea of Marmara, and Caribbean). The strike-slip fault systems offshore southern California appear to have a limited distribution of seep sites compared to a wider distribution at convergent plate boundaries, which may influence seep habitat diversity and connectivity.

2000
Rathburn, AE, Levin LA, Held Z, Lohmann KC.  2000.  Benthic foraminifera associated with cold methane seeps on the northern California margin: Ecology and stable isotopic composition. Marine Micropaleontology. 38:247-266.   10.1016/s0377-8398(00)00005-0   AbstractWebsite

Release of methane from large marine reservoirs has been linked to climate change, as a causal mechanism and a consequence of temperature changes, during the Quaternary and the Paleocene. These inferred linkages are based primarily on variations in benthic foraminiferal stable isotope signatures. Few modem analog data exist, however, to assess the influence of methane flux on the geochemistry or faunal characteristics of benthic foraminiferal assemblages. Here we present analyses of the ecology and stable isotopic compositions of living (Rose Bengal stained) and dead (fossil) foraminifera (>150 mu m) from cold methane seeps on the slope off of the Eel River, northern California (500-525 m), and discuss potential applications for reconstructions of methane release in the past and present. Calcareous foraminiferal assemblages associated with Calyptogena clam bed seeps were comprised of species that are also found in organic-rich environments. Cosmopolitan, paleoceanographically important taxa were abundant; these included Uvigerina, Bolivina, Chilostomella, Globobulimina, and Nonionella. We speculate that seep foraminifera are attracted to the availability of food at cold seeps, and require no adaptations beyond those needed for life in organic-rich, reducing environments. Oxygen isotopic values of the tests of living foraminiferal assemblages from seeps had a high range (up to 0.69 parts per thousand) as did carbon isotopic values (up to 1.02 parts per thousand). Many living foraminiferal isotope values were within the range exhibited by the same or similar species in non-seep environments. Carbon isotopic values of fossil foraminifera found deeper in the sediments (18-20 cm), however, were 4.10 parts per thousand (U. peregrina) and 3.60 parts per thousand (B. subargentea) more negative than living delta(13)C values. These results suggest that delta(13)C values of foraminiferal tests reflect methane seepage and species-specific differences in isotopic composition, and can indicate temporal variations in seep activity. A better understanding of foraminiferal ecology and stable isotopic composition will enhance paleo-seep recognition, and improve interpretations of climatic and paleoceanographic change. (C) 2000 Elsevier Science B.V. All rights reserved.