Preface
This study was accomplished with financial support from the Deutsche Forschungsgemeinschaft
(DFG). The work presented in this thesis was written in the frame
of the European Graduate College: Proxies in Earth History (EUROPROX)
at the University of Bremen. The subproject Phosphorus Cycle and
Phosphorite Formation in Marine Sediments of High Productivity
Areas that resulted in this thesis is dedicated to identify different
sedimentary phosphorus reservoirs and indication for diagenetic
impacts in surface sediments in various marine settings. This
work is submitted as a dissertation and has been proposed and
supervised by PD Dr. Matthias Zabel (MARUM . Center for Marine
Environmental Sciences, University of
Bremen, Germany) and by the project partner Prof. Dr. Gert J.
De Lange (Department of Earth Sciences - Geochemistry, Faculty
of Geosciences, University of Utrecht, The Netherlands). The work
was mostly conducted in the working group Geochemistry and Hydrogeology
headed by Prof. Horst D. Schulz (retired since October 2007) at
the Department of Geosciences, University of Bremen.
The presented work consists of a summary describing the discussed
topics (Abstract), a detailed introduction into the subject of
phosphorus cycling in the marine realm and the outline of the
thesis (Chapter 1). Three case studies in form of manuscripts
(Chapter 2-4) and two abstracts (Chapter 5) are included. Concluding remarks
and a brief outlook complete the scientific work, including a
summary of the main results, remaining open questions are addressed
and perspectives for future research on phosphorus cycling are
proposed (Chapter 6). Finally, the kind support
by many persons is acknowledged. Appendices for Chapter 2-4 are
given at the end of the thesis.
Table of contents
Abstract vii
Kurzfassung x
Chapter 1
Introduction …………………………………………………………………. 13
Chapter 2
Phosphorus cycling in marine sediments from the continental margin
off Namibia ………37
Chapter 3
Benthic phosphorus and iron budgets for three NW African slope
sediments: a balance approach ………69
Chapter 4
Governing factors of phosphorus speciation in surface sediments
of three highly productive shelf areas (NW Africa, Peru and Chile)
……93
Chapter 5 (Abstracts only)
Redox sensitivity of P cycling during marine black shale formation
- dynamics of sulfidic and anoxic, non-sulfidic bottom waters
………119
Phosphate geochemistry, mineralization processes, and Thioploca
distribution in shelf sediments off central Chile ………121
Chapter 6
Conclusions and Outlook …………………………………………………123
Danksagung ………………………………………………………………………127
Appendix
Chapter 2 ……………………………………………………………………III
Chapter 3 ……………………………………………………………………XI
Chapter 4 ……………………………………………………………………XIX
Marine biogeochemical cycling of the highly dynamic key nutrients
carbon, nitrogen and phosphorus starts with continuous production
of organic substances and remineralization in the oceanic water
column. Phosphorus is an essential nutrient required by all living
organisms and is suspected to control marine primary production.
The settling from surface waters to the sea floor is an important
pathway in transporting phosphorus-binding forms to the sediment.
The distribution of such components in marine sediments is therefore
strongly coupled to processes in the overlying oceanic waters.
Through burial in sediments bioessential phosphorus is removed
from the oceanic nutrient pool. Hence it is important to determine
the ability of sediments to regenerate and/or retain bioavailable
phosphate.
In the presented work the interactions and controls on sedimentary
phosphorus forms towards different geochemical boundary conditions
are shown. The major objective is a better understanding and quantification
of processes that control the benthic phosphorus cycle in selected
continental margin surface sediments.
The sequential extraction of sedimentary solid-phase phosphorus
yielded reservoir profiles of phosphorus at three sites of the
Namibian continental slope. Based on these results generally organic
and biogenic substances are the major carriers for phosphate to
the sediments. Another very reactive and dominant phase is phosphorus
associated with iron (oxyhydr)oxides, which is related to redox-dependent
pore
water-solid phase exchange. This observation indicates a strong
connection between phosphate and the ongoing benthic iron cycle.
Linked to that, for a correct understanding of these dynamic geochemical
processes, a quantification of phosphate transfer rates in the
investigated sediments is necessary.
By applying Fick’s First Law of diffusion and a diagenetic model
assuming steady state conditions phosphate production and removal
rates can be calculated. In particular, the release rate of phosphate
from reductive dissolution of iron (oxyhydr)oxides do not correlate
with the available amount of iron-bound phosphorus phases. The
contradiction stated for the balances between sediment composition
and transfer rates give indication for considering the process
of non-local transport in this setting. Such additional transport
process refills the iron (oxyhydr)oxide reservoir with material
from above that results in preferential phosphate release into
the pore water as reflected in concentration profiles.
Sediments from the Senegal continental slope show very similar
geochemical patterns. Geochemical analyses of three surface sediments
document a close relationship between the benthic cycles of phosphorus
and iron. Although microbial mediated organic matter degradation
contribute to the pore water phosphate pool, most of the dissolved
phosphate must have been liberated from reductive dissolution
of
iron (oxyhydr)oxides.
Assuming steady state conditions, simple budget calculations
indicate that release and transformation rates of phosphate associated
to iron phases are not directly represented in the detected solid-phase
concentration profiles. Based on pore water gradients, release
rates of ferrous iron during reductive dissolution are much higher
than expected from the available amount of iron (oxyhydr)oxides.
Besides, in the surface layer above the area of reductive dissolution,
re-oxidation of ferrous iron along with subsequent re-adsorption
of phosphate do not match the content as detected from extraction
results. Only by downward transport of phosphorus associated to
iron (oxyhydr)oxides, most likely due to bioturbative replacements,
diffusive fluxes are maintained in this system.
Recording very different environmental conditions for geochemical
cycling of phosphorus, surface sediments investigated are derived
from cores drilled at sites off Senegal, Peru and Chile. The combination
of geochemical parameters offers an ideal opportunity to identify
depth distributions of phosphorus containing forms in highly productive
shelf areas.
In the open shelf mudbelt sediment off Senegal physical processes
in the shallow water column affect accumulation of phosphorus
phases derived from river discharge and organic matter production.
Thus, phosphorus bound to iron (oxyhydr)oxides contributes considerably
to the sedimentary phosphorus reservoir. In relation to the distribution
of major dissolved redox species, reductive iron (oxyhydr)oxide
dissolution is the major source for pore water phosphate. In addition,
bioavailable phosphate is efficiently retained in the sediment
in form of biogenic, precursor
apatite forms.
In the open shelf setting off Peru a strong connection exists
between primary production and the present deposition of biogenic
and organic phosphorus fractions. As the geochemical analyses
reveal, bottom waters are anoxic which directly controls phosphorus
recycling close to the sediment surface. Consequently, in such
an oxygen-depleted sediment re-oxidation of ferrous iron is not
quantitative. Thus, a return flux of dissolved phosphate to the
overlying waters is promoted, which is probably one of the factors
sustaining high productivity in this setting.
In contrast to both open shelf settings, the Chilean site is
located in a semienclosed shallow embayment. Here, phosphorus
cycling in the surface sediment is complex due to seasonal changes
in bottom water chemistry. The geochemical observation most probably
documents a temporary situation where conditions are in the transition
between a complete anoxic to a more oxic redox state.
This study reveals that the investigation of sedimentary phosphorus
and iron reservoirs in comparison to the pore water geochemistry
has a potential that takes step toward a better understanding
of site-specific sedimentary phosphorus budgets (sinks versus
sources). From the examples discussed in this study it can be
concluded that in particular the budget of phosphorus associated
with iron oxide phases is by far not balanced under the assumption
of simple release and transfer processes. In this context the
major question is addressed to a potentially active additional
transport process, which is emphasized as the most important in
understanding the coupling of the benthic phosphorus and iron
cycle.
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| 1 | 生物起源 | 25 ml 2M NH4Cl（塩化アンモニウム） (pH 7) | 残留孔隙水中のP、交換可能なP、生物起源アパタイト、アパタイト先駆鉱物、炭酸塩に伴うP |
| 1b | 交換可能 | 25 ml 0.35M NaCl | 交換可能で、ゆるく吸着したP |
| 2 | 酸化物結合 |
25 ml クエン酸亜ジチオン酸バッファー（pH 7.5）、 25 ml 2M NH4Cl, 25 ml dem. water（脱鉱物質水） |
吸着した還元性／反応性のFe3+結合P |
| 3 | 自生 |
25 ml 1M 酢酸ナトリウム（pH 4）、 25 ml 2M NH4Cl, 25 ml dem. water |
自生P |
| 4 | 砕屑性 |
25 ml 1M HCl、 25 ml dem. water |
砕屑性P |
| 5 | 有機 |
550℃での強熱（ignition）後に 25 ml 1M HCl |
有機P |