Beschreibung: General aspects of particle transport within the abyssal near-bottom water column [ up to 1000 meters above bottom (mab)] and within abyssal surface sediments of the Northeast Atlantic (Porcupine Abyssal Plain) and of the Arabian Sea have been investigated using natural particle-reactive radiotracers (234Th and 210Pb). lt is assumed that food supply controls the composition of the benthic community which, in turn, determines bioturbation intensity. Bioturbation intensity was investigated on different time scales using 234Th and 210Pb (234Th: 100 d; 210Pb: 100 yr). The intensity of biogenic particle mixing of surface sediments in the Arabian Sea was positively and negatively related to increasing food supply on the time-scales of 100 d and 100 yr, respectively. On a time-scale of 100 yr a negative relationship between macrofauna abundance and bioturbation intensity was observed. There was a positive relationship between the density of spoke traces and bioturbation intensity. Low food supply in the southeastern Arabian Sea seems to promote the selection of large infaunal benthic organisms with long particle-transport step lengths (spoke-trace producers) whereas high food supply in the northwestern Arabian Sea seems to promote the selection of infaunal macrofauna with short step lengths and presumably long rest periods. Thus, a lower abundance of large organisms may cause more rapid mixing than a higher abundance of small organisms. These results indicate that single components of the benthic community may dominate the distribution of bioturbation intensity in a given region. There is no a priori reason to assume that there is a general positive relationship between particulate-organic-carbon fluxes (food supply) and bioturbation intensity in all parts of the ocean. For the investigation of the near-bottom water column it was subdivided into the first meter above bottom, the benthic mixed layer (BML) and the layer above the benthic mixed layer up to the upper boundary of the bottom nepheloid layer (BNL). This subdivision was based on the distributions of total particulate matter, transmission, potential temperature and 234Th. The interplay of disaggregation and aggregation within the first meter above bottom and subsequent resuspension of rebound particles at the sediment-water interface are proposed to be the principal reasons for high particle-mass concentrations and increased freshness of particulate matter (PM) within the first meter above bottom. The BML, exhibiting thicknesses of ≈ 10 - 65 m, was characterized by more or less uniform distributions of transmission and/or potential temperature. In the BNL above the mixed layer the composition and distribution of PM is governed by lateral advection, settling, turbulent diffusion and presumably decomposition of PM (see below). A one-dimensional steady state box model was developed to investigate 234Th and particle cycling within the abyssal BNL and surface sediment. Mean particle residence times in the BML and in the resuspension zone of the surface sediment with respect to net transports out of these compartments suggest the BML and the adjoining compartments (upper BNL and resuspension zone of the surface sediment) to be a highly dynamic system with respect to particle cycling and sorptive reactions on time scales of several days up to a few weeks. Modelling results and comparison of the measured downward flux of settling particles with the calculated turbulent-diffusive upward flux of suspended PM indicate virtually simultaneous net upward and downward fluxes of 234Thpart and PM across the upper and lower boundaries of the BML, respectively. This finding requires PM to feed the BML presumably during sedimentation pulses which cause a transient non-steady state situation. An alternative source of PM is adsorption of dissolved organic matter (DOM) onto particles. Moreover, significant net decomposition / dissolution of PM in the upper BNL is implied. This result suggests that decomposition/dissolution of PM in the BNL should be considered in future studies of biogeochemical cycles in addition to particle decomposition/dissolution in the sediment. The BML is proposed to be a decisive filter for the exchange of matter and biogeochemical information between the ocean 's interior and the sediment. The roportion of the BML within the BNL and the thickness of the BNL are important controls of this exchange.