『Abstract
In this study we investigate benthic phosphorus cycling in recent
continental margin sediments at three sites off the Namibian coastal
upwelling area. Examination of the sediments reveals that organic
and biogenic phosphorus are the major P-containing phases preserved.
High Corg/Porg ratios
just at the sediment surface suggest that the preferential regeneration
of phosphorus relative to that of organic carbon has either already
occurred on the suspension load or that the organic matter deposited
at these sites is already rather refractory. Release of phosphate
in the course of benthic microbial organic matter degradation
cannot be identified as the dominating process within the observed
internal benthic phosphorus cycle. Dissolved phosphate and iron
in the pore water are closely coupled, showing high concentrations
below the oxygenated surface layer of the sediments and low concentrations
at the sediment-water interface. The abundant presence of Fe(III)-bound
phosphorus in the sediments document the co-precipitation of both
constituents as P-containing iron (oxyhydr)oxides. However, highly
dissolved phosphate concentrations in pore waters cannot be explained,
neither by simple mass balance calculations nor by the application
of an established computer model. Under the assumption of steady
state conditions, phosphate release rates are too high as to be
balanced with a solid phase reservoir. This discrepancy points
to an apparent lack of solid phase phosphorus at sediment depth
were suboxic conditions prevail. We assume that the known, active,
fast and episodic particle mixing by burrowing macrobenthic organisms
could repeatedly provide the microbially catalyzed processes of
iron reduction with authigenic iron (oxyhydr)oxides from the oxic
surface sediments. Accordingly, a multiple internal cycling of
phosphate and iron would result before both elements are buried
below the iron reduction zone.
Keywords: benthic phosphorus cycle; benthic iron cycle; particle
mixing; Benguela Upwelling System 』
1. Introduction
2. Location and oceanic environment
3. Materials and methods
3.1. sampling
3.2. Pore water analyses
3.3. Solid phase analyses
3.4. Diagenetic model for P cycling in surface sediments
4. Results
4.1. Distribution of major diagenetic pore water compounds
in surface sediments
4.2. Distribution of sedimentary P and Fe in surface sediments
4.3. Carbon to phosphorus ratios in surface sediments
5. Discussion
5.1. Controls on sedimentary P cycling
5.2. Quantification of P transformation
6. Conclusions
Acknowledgements
References