Discovery of an Extensive Deep-Sea Fossil Serpulid Reef Associated With a Cold Seep, Santa Monica Basin, California

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.

Methanotrophic bacterial symbionts fuel dense populations of deep-sea feather duster worms (Sabellida, Annelida) and extend the spatial influence of methane seepage

“…Thus, the occurrence of methanotrophic symbionts associated with these ubiquitous animals is likely to extend beyond the Costa Rica margin seeps and may even extend beyond species of Laminatubus and Bispira. Further, these results also explain the unusually high abundance of serpulids at ancient hydrocarbon seeps worldwide [up to ~50% of estimated total animal biomass; (17)].…”

“…Mass occurrences of serpulids in ancient methane seep deposits, known as "fossil serpulid seeps," have perplexed scientists for decades, and the recognition of paleoseep indicator species continues to be of value for geologists and geobiologists alike. For serpulids, which permanently inhabit calcareous tubes, seven taxa (including Laminatubus) have been reported from early Cretaceous to Miocene seep communities ~150 to 200 million years ago (15)(16)(17). They appear attached to, or in layers immediately adjacent to, known chemosynthetic vesicomyid clams, lucinid clams, or bathymodiolin mussels, prompting the suggestion that serpulids found in fossil seep deposits were actually regular members of the chemosynthetic community, as opposed to arriving only after seepage had ceased (15).…”

The dynamic influence of methane seepage on macrofauna inhabiting authigenic carbonates

“…was an exception; despite high abundance at transition sites, it did not thrive on the rocks transplanted to active sites. Notably, other species of serpulids have been reported at high densities at or near active seepage in Costa Rica (Levin et al 2012), Santa Monica Basin (Georgieva et al 2019), and in the Greenland Sea (Vinn et al 2013(Vinn et al , 2014.…”

Morphosedimentary, Structural and Benthic Characterization of Carbonate Mound Fields on the Upper Continental Slope of the Northern Alboran Sea (Western Mediterranean)

“…Carbonate mounds are formed by framework-building calcareous organisms distributed according to various transport, depositional and erosive processes [2,4,5]. Hence, they are firstly dependent on the biological activity of their reef-forming organisms, which include colonial suspension feeding invertebrates, such as cnidarians (e.g., scleractinians, stony hydrozoans and octocorals) [9,10], sponges [11], serpulids [12] and bryozoans [13]. These framework-building organisms sometimes have branching forms, increasing the complexity and diversity of substrates, food sources and associated sessile and mobile organisms, including those with calcareous skeletons (e.g., encrusting coralline algae, mollusks, cirripeds, etc.)…”