Quantification and regionalization of benthic reflux

Abstract

The observations made in the preceding chapters have demonstrated that the upper sediment layers usually contain the highest biochemical reaction rates within marine deposits. The decomposition of organic matter represents the major motive force of the benthic system and its processes which are mostly catalyzed by microbial activity. How important is the boundary between free water mass and the pore water system? In this chapter, we will be concerned with finding answers to this question. It is well established by a large number of long-term studies that most part of the primary organic substance is remineralized in the upper water zones after the organisms have died (cf. Chapter 4 and Section 5.5). Estimates on the marine carbon cycle, which were made for pelagic and hemipelagic domains of the ocean, demonstrate this process (Fig. 12.1). According to Berger et al. (1989), on average only about 1% of the organic matter reaches the sediment surface in the open ocean. Approximately 97% of this amount is decomposed in early diagenetic reactions so that only 0.03% of the organic carbon produced in the photic zone will ultimately become buried. Later on we will more thoroughly discuss the differences between pelagic and coastal regions of the ocean where the burial rates are estimated to be up to 30 times higher. Although the information contained in Figure 12.1 might vary depending on the applied method (e.g. de Baar and Suess 1993), in most cases relationships and dimensions usually remain quite consistent. This approximate balance may also apply to most nutrient cycles in the ocean. Regarding the water body simplistically as an indivisible entity, without concerning to its nume- 12 Quantification and Regionalization of Benthic Reflux MATTHIAS ZABEL AND CHRISTIAN HENSEN rous internal cycles, only the boundary conditions of the system will ultimately determine its potential changes in the course of time. Under such an assumption, the following boundary systems will have to be considered: 1) the atmosphere, 2) the continents and 3) the ocean floor (Fig. 12.2).