Respiration in ocean margin sediments

Abstract

The aim of this thesis was the study of respiration in ocean margin sediments and the assessments of tools needed for this purpose. The first study was on the biological pump and global respiration patterns in the deep ocean using an empirical model based on sediment oxygen consumption data. In this thesis the depth dependence of respiration patterns was modelled using a compiled data set of sediment oxygen consumption rates. We showed that the depth relationship can best be described by a double exponential model. By assuming a uniform flux laterally across the global ocean the depth attenuation of POC could be derived. The results from this study imply a more efficient biological pump. The second study was on the short-term fate of phytodetritus, investigated across the Pakistan margin of the Arabian Sea. Stations ranged in water depths from 140 to 1850 m, encompassing the oxygen minimum zone. Phytodetritus sedimentation events were simulated by adding 13C-labelled algal material to surface sediments. The labelled carbon was subsequently traced into bacterial lipids as a proxy of bacterial biomass, foraminiferan and macrofaunal biomass as well as into dissolved organic and inorganic pools. The largest pool of processed carbon was found in dissolved inorganic carbon, attributed to respiration. Macrofaunal influence was most pronounced at the lower part of the oxygen minimum zone. The third study was on benthic respiration rates in the Gulf of Finland, Baltic Sea. Rates were based on in situ incubations using benthic chamber landers. Three contrasting stations with different sediment accumulation regimes were visited. The effect of changes in water masses on the benthic fluxes was investigated with a dynamic diagenetic model. Fluxes of dissolved inorganic carbon were highest at the station with accumulation bottom, intermediate at the station with transport bottom and lowest at the station with erosion sediments. The fourth study was primarily a theoretical investigation of how well a certain model parameter can be constrained based on a certain dataset. A model of bio-irrigation was used as a good example of this. The interpretation is based on fitting observed data with a model containing several parameters, where some parameters are a priori unknown. In this chapter, it was tested under what conditions the results obtained through this fitting are robust. The results from this study imply that using only the concentration change in the overlying water, it is not possible to constrain both the rate and the mechanism of bio-irrigation, thus, sampling the porewaters at the end of the incubation is a necessity. The fifth study was an an evaluation of the performance of an oxygen optode. The performance of the sensor was evaluated and compared with data obtained by other methods. The principal conclusion was that, owing to high accuracy, long-term stability (more than 20 months), lack of pressure hysteresis and limited cross-sensitivity, this method is overall more suitable for oxygen monitoring in the aquatic environment than other methods.