Benthic Processes Sub-Project

Objectives

To establish, assess and quantify transport, settling accumulation and burial fluxes of particles along the European ocean margins.

To evaluate the role of benthic biota in the respiration and preservation of organic matter in the sediments.

To study the diagenetic processes occurring in the recent sedimentary column and to quantify the related fluxes of elements associated with the carbon cycle.

Transport, Accumulation and Burial

Particle transport in the shelf environment is the result of a complex interaction of a number of processes. Particles originate from the shelf and through biological activity in the surface waters over the shelf break. They may also be resuspended into the water column under certain physical conditions into mid-water depths from sites on the slope. The concentration of particles in water column was measured by the deployment of transmissometers and nephelometers, either as part of an instrument package lowered from a research vessel or on instrument moorings. These measurements showed the expected high particle concentrations in the surface layers and, through resuspension, near the sea bed. However, in addition a zone of enhanced particle concentration, termed an intermediate nepheloid layer, was frequently observed during OMEX I at a depth of about 1000 m. This is believed to result from sediments resuspended from the shelf break floor being advected into deeper waters. Numeric models were used within OMEX I to investigate this phenomenon. Sediment samples from the sea floor were collected along the Goban Spur transect. From these, a picture of sediment accumulation and organic carbon burial rates across the shelf break was built up.

Benthic Biota

During OMEX I a quantitative assessment of the benthic community, from minute nematodes to large holuthurians, was made in the vicinity of the Goban Spur. The carbon remineralisation rates by this community were assessed through the determination of oxygen demand on sediment samples and through in-situ measurements obtained by the deployment of benthic landers on the sea bed. Biogeochemical activity levels within the sediment were assessed through quantification of adenylates and bacterial biomass was determined through the measurement of DNA, proteins and lipids.

Diagenetic Processes

Benthic carbon mineralisation rates along Goban Spur were derived from steady-state modelling of the pore water profiles of oxygen, nitrate, ammonium, dissolved manganese and dissolved iron in combination with fluxes of these solutes across the sediment water interface. Pore water profiles of oxygen were obtained in-situ with a benthic lander and on-deck in sediment cores retrieved by multi-coring. With water depths increasing from 200 to 1500 m benthic carbon oxidation rates decreased, while the interfacial organic carbon concentrations increased. On the lower slope (>1500 m), neither benthic carbon oxidation nor interfacial organic carbon concentrations showed further trends with depth.

Oxic mineralisation accounted for more than 70% of the depth-integrated carbon oxidation rate at all stations. Suboxic and anoxic mineralisation processes were only identified on the upper slope, whereas on the lower slope, more than 90% of the organic matter mineralisation was performed by oxic respiration.

Organic matter deposited on Goban Spur sediments was efficiently mineralised rather than buried. Carbon burial rates, derived from biostratigraphically determined sedimentation rates and the organic carbon concentration at depth in the sediment, represented less than 2.5% of the input of organic carbon to the sediment. Thus, the absence of a confined area characterised by elevated carbon mineralisation rates, a pronounced maximum in sedimentary organic carbon concentrations and enhanced burial rates indicated that no so-called "Carbon Depocenter" could be evidenced along the Goban Spur transect.

Benthic oxygen fluxes showed no direct response to pulses of organic material settling on the sea floor, as appearing in sediment traps (see OMEX sub-project Biological Processes), suggesting that the organic material deposited is dominated by refractory compounds. This finding was supported by steady-state modelling of pore water oxygen profiles which showed that the organic matter being mineralised at stations deeper than 200 m had very low degradation rate constants (<1 yr^-1).

Comparison of the measured oxygen and nutrient fluxes with the diffusive fluxes calculated from pore water profiles indicated that within the experimental errors there was no significant contribution of bio-irrigating organisms to the sediment-water exchange fluxes.