Free-Vehicle Benthic Lander Technology for the Study of Biogeochemical Processes in Marine Sediments

Various sediment sampling and incubation techniques were used to characterize benthic biogeochemical processes important for an improved understanding of the oceanic carbon cycle. Recent flux data from the Arctic Siberian shelf are revealing differences (often by a factor of ten) between areas affected by the big Russian river outlets (high fluxes) and areas with low influence from land (low fluxes). When carrying out benthic flux studies in the deep-sea, pressure and temperature artifacts on sediment cores brought to the surface make it necessary to perform in-situ studies. The use of autonomous vehicles, benthic "landers", has proven to be the most efficient way, both practically and economically, to obtain high quality results from the sea-floor. Technical aspects on the construction, design and use of most existing benthic chamber and profiling landers are reviewed in detail (PAPER I). The BANYULS and G 6TEBORG landers, which are two recently developed modular landers mainly carrying chamber modules equipped with an innovative sediment sampling technique, are presented in PAPER II. Hydrodynamic aspects of descent, landing and ascent of the two landers were studied in detail resulting in formulas applicable to other free falling vehicles. An intercalibration of 14 different chamber designs was made to better understand several important hydrodynamic factors influencing flux measurements in chambers and to be able to compare results obtained with different chambers. Comparative flux incubations gave statistically significant differences in obtained fluxes of both oxygen and silicate between the chambers with up to a factor of two. A large part of this variation could be explained by differences in water column heights of the chambers, where higher overlying water column gave higher fluxes of both oxygen and silicate (PAPER III). By means of an experimental design evaluated with Partial Least Squares, the Clark type oxygen sensors used on the G 6TEBORG lander were calibrated with respect to variations in oxygen concentration, temperature, pH, salinity, total pressure, fugacity ([O2]water / [O2]sat water) and stirring rate. Fugacity and temperature explained a major part of the response, but also stirring-rate and the cross term (temperature) * (fugacity) were significant on the 95% level (PAPER IV). Recycling of carbon, nitrogen, phosphorus and silica in sediments were studied on a basin-wide scale in the Skagerrak using the G 6TEBORG lander on some of the stations. Nitrate fluxes (which dominated DIN fluxes) were in general reflecting sediment deposition patterns and were going into the sediment in the high accumulation areas. On average 62 % of the deposited POC was recycled to the overlying water and 38 % was buried in the sediment. An important component of the recycling flux was made up of DOC (DOC fluxes were on average 27 % of the total CO2 fluxes) showing the importance of including fluxes of both DOC and total CO2 when constructing benthic carbon mass balances. Similar observations were made for deep-sea sediments in the NE Atlantic Ocean, where the DOC fluxes were about 50 % of the total CO2 fluxes (PAPER V, VI and II)