Transpressional segment boundaries in strike-slip fault systems offshore southern California: Implications for fluid expulsion and cold seep habitats
Jillian M. Maloney, Benjamin M. Grupe, Alexis L. Pasulka, Katherine S. Dawson, David H. Case, Christina A. Frieder, Lisa A. Levin, Neal W. Driscoll
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.
Methane seep ecosystem functions and services from a recently discovered southern California seep
Benjamin M. Grupe1,*, Monika L. Krach1,†, Alexis L. Pasulka1,‡, Jillian M. Maloney2,§, Lisa A. Levin1 and Christina A. Frieder1,¶
1 Center for Marine Biodiversity and Conservation, Integrative Oceanography Division, Scripps Institute of Oceanography, University of California San Diego, La Jolla, CA, USA
† Farallones Marine Sanctuary Association, San Francisco, CA, USA
‡ Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
§ Department of Geological Sciences, San Diego State University, San Diego, CA, USA
¶ Department of Biological Sciences, University of Southern California, Los Angeles, CA, USA
The recent discovery of a methane seep with diverse microhabitats and abun- dant groundfish in the San Diego Trough (1020 m) off the coast of Del Mar, California raised questions about the role of seep ecosystem functions and ser- vices in relation to continental margins. We used multicorer and ROV grab samples and an ROV survey to characterize macrofaunal structure, diversity, and trophic patterns in soft sediments and authigenic carbonates; seep micro- habitats and taxa observed; and the abundance and spatial patterns of fishery- relevant species. Biogenic microhabitats near the Del Mar Seep included micro- bially precipitated carbonate boulders, bacterial mats, vesicomyid clam beds, frenulate and ampharetid beds, vestimentiferan tubeworm clumps, and fields of Bathysiphon filiformis tubes. Macrofaunal abundance increased and mean faunal d13C signatures decreased in multicorer samples nearer the seep, suggesting that chemosynthetic production enhanced animal densities outside the seep center. Polychaetes dominated sediments, and ampharetids became especially abundant near microbial mats, while gastropods, hydroids, and sponges domi- nated carbonate rocks. A wide range of stable isotopic signatures reflected the diversity of microhabitats, and methane-derived carbon was the most prevalent source of nutrition for several taxa, especially those associated with carbonates. Megafaunal species living near the seep included longspine thornyhead (Sebas- tolobus altivelis), Pacific dover sole (Microstomus pacificus), and lithodid crabs (Paralomis verrilli), which represent targets for demersal fisheries. Sebastolobus altivelis was especially abundant (6.5–8.2 fish 100 m 2) and appeared to aggre- gate near the most active seep microhabitats. The Del Mar Methane Seep, like many others along the world’s continental margins, exhibits diverse ecosystem functions and enhances regional diversity. Seeps such as this one may also con- tribute ecosystem services if they provide habitat for fishery species, export production to support margin food webs, and serve as sinks for methane- derived carbon.