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WP2 Sedimentary seafloor and sub-seafloor ecosystems: past, present & future links

WP leader: Tim Ferdelman, MPI, Germany
Judith McKenzie, ETH Zürich, Switzerland

Deep-sea sediments cover nearly two thirds of the Earth’s surface, and the microbial processes within them provide drive the nutrient regeneration and global biogeochemical cycles that are essential to sustain primary and secondary production in the oceans. The sediment-ocean interface forms a dynamic boundary that separates the mostly anoxic deep biosphere (WP3) and the ocean crust (WP1) ecosystems from the turbulent, oxidised deep ocean water column, which is the main conduit of energy to the seabed. A simple depth profile of biomass, principally as microbial cell counts, illustrates the importance of the sedimentary seafloor interface. In the water column 104 cells per cm3 are the typical densities of microbial cells in the water column. Volumetric abundances abruptly rise to 109 cells cm-3 within the surface sediments. Even in the most energy poor, ultra-oligotrophic region of the South Pacific Gyre, cell counts exceed 106 cells cm-3 at surface sediments. Below the surface meter of sediment, volumetric cell counts decline in a logarithmic fashion, reaching values of 104 cells cm-3 only after tens to hundreds of meters of sediment depth. Correspondingly, microbial activities, excluding hotspots or deeply buried high-energy interfaces, decline even more precipitously with increasing depth. Thus, the surface and near-surface seafloor represents a plate of high microbial abundance, high microbial diversity, high microbial activity separating the vast deep biosphere habitat from the deep ocean water masses that dictate climate. Furthermore, this zone is characterised by a close interaction between microbes, which make up around 90% of the seafloor benthic biomass, and the less abundant (macro-)fauna. The latter, however, have a strong impact on functioning and diversity of seafloor habitats.

Surface sedimentary ecosystems are the seafloor ecosystems most likely to be impacted by human activities. Activities such as deep-sea fishing (dredging), mineral prospecting (e.g. Mn nodules), offshore oil installation and exploration, waste storage, all may have dramatic and immediate consequences for the biogeochemical functioning of the deep surface seafloor. Less obvious, but still dramatic longer term changes in ocean chemistry (e.g. ocean acidification) and pollutants will also leave their mark on the structure and functioning of deep-sea ecosystems. This work package focuses on surface and near surface processes in deep-sea sedimentary environments. We broadly delineate the sedimentary surface and near-surface seafloor as those sediment depths extending downwards from the benthic-water boundary layer, through depths inhabited by traces of living fauna (to one meter) to those horizons where surface modulations in deep ocean water chemistry may be detected on decadal to millennial time-scales (up to 10 meters). Given the high activities, abundances, and diversities of macrofauna, meiofauna and microbial communities in and on surface marine sediments, these communities will have a tremendous impact on the sedimentary archive of past climate variations, as well as on seabed weathering. Deep-seated flows of material and energy link deep underlying geological and microbiological processes to distinct near-surface habitats. Deeper zones that are no longer in exchange with surface processes are also of relevance to this WP as archives for past seafloor habitats and their inhabitants. Therefore, from the perspective of surface and near-surface sediment ecosystems, two broad avenues of inquiry, one “top-down” and the second “bottom up” are proposed.

(1) How do processes and community structure in seafloor surface ecosystems affect the deep biosphere on millennial and greater time-scales? It has long been known that early diagenetic processes that alter minerals and organic matter deposited on the surface of the seafloor have an impact on the interpretation of the deep sedimentary paleo-record. However, the impact of paleo-biogeochemical processes in surface sediments on deep microbial activities has not been extensively explored. Buried ecosystems continue to influence deep sediment carbon, nitrogen, phosphorus and sulfur transformations; alteration of biomarkers and productivity proxies, and even mineral precipitation and weathering processes. Understanding these deep biosphere processes requires the fundamental understanding of the surface ecosystem linked with deep sediment records. Climate induced changes in surface conditions and associated pore water chemistry can be propagated through the dissolved chemical profiles at velocities at a far greater velocity than the burial of the original solid phase signal. Chemical fronts, induced from depositional changes above or changes in the chemistry of the deep basalt aquifer below may serve as drivers of deep biosphere microbial diversity and activity or zones of secondary mineral formation. Ultimately, does the deep sedimentary distribution of microbial diversity and activity somehow reflect initial surface conditions? Can we learn about changes in the biodiversity of communities and ecosystems – especially meiofauna, bacteria and archaea – from the so-called “paleoome”? This constitutes a rich, nearly unexplored treasure of potential paleo-proxies for ecosystem and climate-change studies.

(2) Just as processes occurring within surface sediments can impact down-sediment microbiology, geochemistry, interpretation of paleo-proxies and mineralogy, deep-seated sedimentary processes can directly impact the surface seafloor and deep-sea. Processes that impact the surface world can range from diffusive fluxes from deep microbial activities to advective flows of energy yielding compounds (such as methane or hydrogen sulfide; WP5) to chaotic, catastrophic events such as eruptions and slides (WP4). A major question is the quantitative impact of these deep processes on the surface world and their effect on climate response.

To explore these questions, we turn to key deep-sea sedimentary seafloor ecosystems, which may include: - Deep low energy ecosystems – Characterised by low temperature, high pressure, low sedimentation rates and extremely low energy flows, these environments are the most extensive, contiguous environments on the planet. Due to their large areal extent they important for regulating global flows of mass and energy (links to WP3 and WP1). - Turbidite slump systems/deep sea fans, canyon systems – The importance of these as mass transfer agents between the continental margins and the deep sea are only beginning to be appreciated. The biogeochemistry and microbial activities and diversities within these structures are even more poorly constrained (WP 4). - Anoxic/low oxygen environments – Anoxic and hypoxic basins are not well represented in the modern ocean, with the Black Sea being the exception. However, ocean-anoxic events are important paleo-events in the rock record. Furthermore, low-oxygen environments are predicted to expand during the Anthropocene. - Ice-covered oceans (the Arctic) – These low-temperature, high productivity, high latitude ecosystems are at the forefront of current research in oceanographic global change research, yet are probably the most poorly studied sedimentary ecosystems. Here, European scientists are at the leading edge (e.g., IODP-MSP Expedition 302 Arctic Coring Expedition). - Cold-water coral ecosystems – One of the more unique and important marine ecosystems whose broad extent around the European margins has only recently been appreciated. These ecosystems are characterised by high nutrient throughput, high diversity and productivity, impact on carbonate budgets, and for their relevance as paleo-signals. The underlying sediments can be significantly impacted by sub-surface hydrocarbon flow (link to WP6) - Seeps – Deep-sea ecosystems characterised by focused fluid flow and a connection to subsurface gas and petroleum reservoirs, including methane clathrates, may be small in number and area, but have enormous impact on ocean chemistry and biology. They represent windows to the deep-biosphere and may be relevant for the paleo-record with regard to the distribution of productivity, hydrate stability, carbonate formation and slope stability (link to WP 5 on geofluids).

List of tentative WP2 participants (Level 3)

Judy McKenzie - ETH, Zürich, Switzerland; Tim Ferdelman - MPI, Bremen, Germany; Rachel James - NOC, Southhampton, UK; Filip Meysman - NIOO, Yserke, Netherlands; Karline Soetaert - NIOO, Yserke, Netherlands; Ronnie N. Glud - SDU, Odense, Denmark; Caroline Slomp - U. Utrecht, Netherlands; Giovanni Aloisi - CNRS/LOCEAN Paris, France; Daniel Conley - U. Lund, Sweden; Francisca Martina Ruiz - Granada, Spain; Martine Buatier - Besançon, France; Tina Treude - IfM GeoMar, Kiel, Germany; Anneleen Fourbert - U. Leuwen, Belgium; Roberto Danovaro - CONISMA, Italy; Gian Marco Luna - CONISMA, Italy