Water Column Biogeochemistry below the Euphotic Zone
The main focus of the International JGOFS research inititiatives was on the cycling of carbon and of associated elements within the surface layer, and their downward export from the upper ocean. Relatively few coordinated measurements and experiments were
- PDF / 650,243 Bytes
- 12 Pages / 547 x 765 pts Page_size
- 59 Downloads / 178 Views
Introduction
The main focus of the International JGOFS research inititiatives was on the cycling of carbon and of associated elements within the surface layer, and their downward export from the upper ocean. Relatively few coordinated measurements and experiments were made below the photic zone so our understanding and modeling of the biogeochemistry of the ocean’s interior is still in its infancy. However from the numerous data acquired in the 1990s during JGOFS and JGOFS-like process studies it is possible to extract sufficient information to make preliminary statements about the biogeochemistry of the water column below the euphotic zone. An important preliminary result of these studies is that we now are beginning to realize that the biogeochemistry of the surface ocean, of the ocean’s interior, and of the surface sediments appears to be more coupled than was thought fifteen years ago. Moreover, the rate of sedimentation of particulate biogenic carbon into the ocean’s interior can be very fast; particles can travel from the surface waters to the abysses in only a few days or weeks, and once the organic matter that is produced in the photic zone (Falkowski et al. 2003, this book) reaches the ocean’s interior it is subjected to intense processes of biodegradation and recycling that release nutrients and dissolved CO 2 into subsurface waters. The subsequent upward transport of nutrients and dissolved CO2 that have been produced and stored in the ocean’s interior occurs at a slow rate; the dissolved CO2 stored below the main ocean thermocline (the ‘ventilation depth’) will not return to the surface waters and to the atmosphere for centuries to millenia (Fig. 6.1). Whatever the exact time scale, it is clear that the biogeochemistry of the water column is largely driven by the biological pumping of CO2 (and of other associated essential nutrients), a major process for exporting matter and energy from the photic zone towards the deep layers, and ultimately towards the sediments (see Lochte et al, this book).
M. J. R. Fasham (ed.), Ocean Biogeochemistry © Springer-Verlag Berlin Heidelberg 2003
Fig. 6.1. A 1-D vision of the world ocean: the classical JGOFS scheme of the biological pump of CO2 in the ocean with time scaling in the different layers of the ocean and in the sediment. This scheme first shows that the focus of JGOFS was really on the surface layer, which makes sense because about 90% of the cycle of carbon occurs in the surface layer. It also shows the biological pump is responsible for the transfer of carbon (and of associated elements) to the ocean’s interior and ultimately to the sediments
146
6.2
Paul Tréguer · Louis Legendre · Richard T. Rivkin · Olivier Ragueneau · Nicolas Dittert
The Twilight Zone: Biology, Biogeochemical Processes and Fluxes
The ‘twilight’ zone, immediately below the photic zone between 100 and 1 000 m, is a key layer for remineralization/respiration and biotic processes resulting in an intensive oxygen consumption and in a dramatic decrease of the vertical distributions of t
Data Loading...
