Abstract
Multibeam bathymetric mapping of the Santa Monica Basin in the eastern Pacific has revealed the existence of a number of elevated bathymetric features, or mounds, harboring cold seep communities. During 2013-2014, mounds at ~600 m water depth were observed for the first time and sampled by Monterey Bay Aquarium Research Institute’s ROV Doc Ricketts. Active cold seeps were found, but surprisingly one of these mounds was characterized by massive deposits composed of fossil serpulid worm tubes (Annelida: Serpulidae) exhibiting various states of mineralization by authigenic carbonate. No living serpulids with equivalent tube morphologies were found at the site; hence the mound was termed ‘Fossil Hill’. In the present study, the identity of the fossil serpulids and associated fossil community, the ages of fossils and authigenic carbonates, the formation of the fossil serpulid aggregation, and the geological structure of the mound are explored. Results indicate that the tubes were most likely made by a deep-sea serpulid lineage, with radiocarbon dating suggesting that they have a very recent origin during the Late Pleistocene, specifically to the Last Glacial Maximum ~20,000 years ago. Additional U-Th analyses of authigenic carbonates mostly corroborate the radiocarbon dates, and also indicate that seepage was occurring while the tubes were being formed. We also document similar, older deposits along the approximate trajectory of the San Pedro Basin Fault. We suggest that the serpulid tube facies formed in situ, and that the vast aggregation of these tubes at Fossil Hill is likely due to a combination of optimal physical environmental conditions and chemosynthetic production, which may have been particularly intense as a result of sea-level lowstand during the Last Glacial Maximum.
Highlights
Cold seeps are remarkable marine environments characterized by the effluence of near ambienttemperature reduced fluids, usually rich in methane or hydrogen sulfide, from the seafloor
Information from autonomous underwater vehicle (AUV) surveys has greatly aided the discovery of cold seeps in the deep sea, which are recognized by the presence of patches of irregular
Cold seeps often manifest as topographic mounds, pockmarks and outcrops of authigenic carbonates (Wagner et al, 2013), and are renowned for the often vast chemosynthesis-supported biological productivity that occurs at these sites compared to the otherwise generally food-limited deep sea
Summary
Cold seeps are remarkable marine environments characterized by the effluence of near ambienttemperature reduced fluids, usually rich in methane or hydrogen sulfide, from the seafloor. The elevated productivity of cold seeps provides habitats for a wealth of additional organisms that have not yet been found to possess such symbionts, such as fish, brachyuran crabs, polynoids, and serpulid tubeworms. The latter (Annelida: Serpulidae) are remarkable for their ability to form vast reef-like aggregations in shallow marine and brackish waters. We report on the discovery of, and investigations into, an unusually large accumulation of fossil serpulids within deep-sea waters that occurs in association with an active cold seep
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