Autonomous hadal benthic lander for in situ tracer incubations and sediment recovery to study benthic community activities

Johannes Lemburg, Alfred Wegener Institute Helmholtz-Center for Polar and Marine Research Bremerhaven, Bremerhaven, Germany, Frank Wenzhofer, Alfred Wegener Institute Helmholtz-Center for Polar and Marine Research Bremerhaven, HGF-MPG Joint Research Group for Deep-Sea Ecology and Technology, Bremerhaven, Germany, Morten Larsen, University of Southern Denmark, Department of Biology, Odense, Denmark, Bo Thamdrup, University of Southern Denmark, Odense, Denmark and Ronnie N Glud, University of Southern Denmark, Department of Biology, Odense M, Denmark
Abstract:
Hadal trenches are the most remote parts of our global oceans, representing the most extreme and scantly explored habitats on Earth. However, recent investigations have shown that these ecosystems are biological hot spots compared to neighboring abyssal sites. A major challenge to investigate these habitats is the recovery of intact and undisturbed sediment samples. Hydrostatic pressure changes during sediment recovery will influence the activity of organisms and also the composition of the pore water (e.g. lysis of labile organic matter). Additionally, not many research vessels are capable to collect sediment samples from such depth.

Here we present an autonomous lander system for in situ sediment incubation and core recovery from hadal depth. Insertion of core liners and retraction of sediment cores is motor driven, providing mechanically undisturbed sediment samples and ensuring a safe instrument recovery. The lander is designed to collect 6 sediment cores and to vertically inject tracers in the sediment for in situ incubations. In each core we can apply 1 to 3 needles for injection over 10-20cm depending on the sediment penetration depth of the core liners. The tracers are injected in each core at a regular time interval of 2-6 hours (depending on bottom time) and a total volume of 100-250ml is injected along a vertical track at pre-programmed depth resolution. After injection the sediment cores are retracted into the bottom water for incubation until the end of the program. During a recent cruise to the Atacama trench system (RV SONNE cruise So268, 2018) we injected 15NO3-, 15NO2- and 15NH4+ to quantify key processes in the benthic nitrogen cycle. Additionally, we also injected Br- as an inert tracer for documenting the ability to evenly inject tracers in the sediment and explore to what extent lander recovery and core handling affected tracer distribution.

This study is part of the ERC project “Diagenesis and microbiology of Hadal trenches” (Grant 669947).

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