The coupling between benthic and pelagic processes plays a fundamental role in marine biogeochemistry, since the oceans’ nutrient and carbon inventories are regulated by fluxes across the sediment-seawater interface. The modes and mechanisms of this benthic-pelagic coupling are still poorly understood for many ecosystems. Moreover, the ongoing anthropogenic perturbation of the natural system affects not only the surface ocean but also the seabed and the biogeochemical fluxes between these two environments. The main research questions of WP3 a

• What are key biogeochemical processes in benthic-pelagic coupling and what is their sensitivity to natural and anthropogenic change?
• How do geomicrobiological processes interact with the marine geosphere and vice versa?
• What biogeochemical archives and proxies have to be tested and applied to reconstruct, interpret and quantify past biogeochemical cycles and the chemical history of seawater?

Contents and Goals

WP3 will investigate material fluxes, turnover and proxy archives at the sediment-water interface affected by natural and anthropogenic induced warming, acidification and deoxygenation. With respect to this overarching goal, different themes have been identified:

• Characterize processes that control sub-seafloor fluid formation, geochemical interactions and element cycling at hydrocarbon seep environments,
• Investigate areas of potential temperature-controlled gas hydrate destabilization and improve models for the estimation of methane releases from melting gas hydrates,
• Budget element fluxes across the sediment-water interface and identify key processes in benthic element cycling of oxygen minimum zones,
• Identify mechanisms of biomineralization in marine calcifiers and evaluate the relevance of these processes for marine biogeochemistry, and
• Study the Phanerozoic history of the most abundant alkaline earth elements and their isotopes to reconstruct past oceans´ biogeochemical cycles.

Key processes and tasks to be addressed in this WP:

Modern and past fluid seepage processes: Both methane and petroleum charged seeps are sites of enhanced material turnover and volatile emissions. WP3 provides information about the interaction between processes in the sedimentary deep and the sediment-seawater interface. Furthermore, the capacity of sediments to recycle or retain climate-relevant substances such as methane and CO₂ and to decompose natural petroleum compounds will be studied. Research on this theme will be realized using sophisticated lander-based systems including in-situ sampling and measuring of sediments, solutes and volatiles at the benthic boundary layer. On the basis of carbonate proxy archives, as well as, geochemical and biological field data, the dynamic change of benthic fluxes will be quantified using transport-reaction models. The microbial alteration and degradation of petroleum compounds in marine sediments will be investigated in laboratory systems, which enable the simulation of natural petroleum seepage and oil spills in intact sediments cores. Since the Gulf of Mexico oil spill catastrophe in 2010, the capability of the marine ecosystem to degrade petroleum compounds has been under much debate. Here, WP3 can make a valuable contribution to elucidate some of the benthic microbial mechanisms involved in petroleum degradation. [link with Topic 4-WP2]

Processes connected to gas hydrate destabilization: Global warming eventually triggers the destabilization of shallow gas hydrate reservoirs in sensitive regions such as the Arctic Ocean. WP3 will investigate the behaviour of natural gas hydrates with model-based studies to predict potential methane emissions and correlated biogeochemical responses in surface sediments. In addition, field-based studies will be conducted to assess methane seepage in sensitive regions with respect to potential correlations to recent gas hydrate destabilization. Such correlations will be identified by analysing the geophysical, geochemical and biological characteristics of suspect sites, which provide information about the onset and origin of methane releases. In addition, a methane blow out accidently created during oil drilling in the northern North Sea 20 years ago, will serve as a natural laboratory to directly study the response time of the marine environment to instantaneous methane releases. [links with Topic 1-WP2+3 and Topic 4-WP1+2]

Trace element and isotope pathways during biomineralization: Marine calcifying organisms play a major role in the earth´s CO₂ budget. However, the physiology of calcification and its relevance for the marine biogeochemistry is poorly constrained. In WP3, uni- and multi-cellular pelagic and benthic calcifying organisms will be studied to unveil pathways and processes of trace element and isotope transport from seawater to the site of calcification. In this regard, field and culturing studies under controlled laboratory conditions will be performed. Specifically, culturing studies with heterotrophic marine microbes, e.g., sulphate-reducing bacteria are ideal to decipher microbial mechanisms that are involved in initial crystal nucleation and progressive formation of minerals such as aragonite, calcite or dolomite. The role of microbes in the formation of marine geological archives is still poorly understood but potentially not insubstantial. Understanding how microbes generate or alter geological archives is vital for the interpretation of past climates and biogeochemical cycles. The biological calcifying studies proposed above will be extended and completed by inorganic precipitation experiments in order to constrain the thermodynamic background of these processes. [link with Topic 3-WP2]

Reconstructing the chemical history of seawater: Continental weathering controls the current chemical composition of seawater, which is subject to short and long term change. WP3 will study the Phanerozoic history of the most abundant alkaline earth elements (Ca, Mg, Sr) and their isotopes (δ^44/40Ca, δ^26/24Mg, δ^88/86Sr) to reconstruct the dynamic interaction of the oceans´ biogeochemical cycles with continental weathering products and solutions originating from the hydrothermal vents at mid-ocean ridges. With this knowledge, the background of modern ocean element and isotope composition can be well constrained. [link with Topic 1-WP3]

WP3 research highlight: A special research theme, which unites all members of WP3 and has strong cross-links to pelagic processes in WP2, is the investigation of benthic element fluxes in oxygen-deficient regions. Modern oxygen minimum zones (OMZ) indicate a lateral extension possibly linked to ocean temperature rise. WP3 aims to provide a greater understanding of the role of benthic processes in moderating water column elemental budgets in OMZs, with an emphasis on phosphorus, nitrogen and iron cycling. Core analyses of this project are facilitated through autonomous deep-sea lander systems, developed at GEOMAR. A close collaboration with oceanographic colleagues is intended, in order to understand how physical conditions such as currents and turbulent fluxes in the water column directly impact exchange processes at the sediment-water interface. [link with Topic 1, WP1]

Budgets of solutes will be constrained by combining field data, in-situ determined nutrient fluxes and numerical reaction-transport models. Results will be used to derive vertically-integrated benthic models, which can be coupled to the pelagic ocean and general circulation models. Further goals will be the reconstruction of past bottom water oxygen levels and their major control mechanisms in the Peruvian upwelling cell over the past 20,000 years by using novel trace elemental and morphological proxies [link with Topic 1-WP3 & Topic 3-WP2].