2 Biogeochemistry at the benthic layer - Impact of biogenic fluxes at the benthic
boundary layer on OMZ
The physicochemical characteristics of the benthic interface (deep water and first sediment
decimeter?), in particular oxygen concentration, are critical drivers of the benthic micro- and
mega-fauna. Redox reactions modulate the chemical characteristics of the buried sediment in
one hand (Berner 1980), and diffusive fluxes on the other. These biogeochemical processes
take place while the sediment is subjected to vertical accumulation, dilution by terrigeneous
inputs and focusing. Off Peru, slope sediments are highly heterogeneous and lateral transport
and bioturbation are dominant processes over pelagic sedimentation (Levin et al., 2002).
Work carried out at the sediment surface will aim at deciphering the combined effect of both
types of processes on the construction of the sedimentary record and on the impact on water
column chemistry (oxygen availability). Analyses carried out on multicores and experimental
determination of critical rates will fuel a modeling integration.
Alike other compounds buried in the solid phase, all types of proxies ultimately recorded in
the sedimentary archive are influenced by a) the diagenesis at the sediment-water interface
and within the upper sedimentary layer driven by both redox chemistry and biological
activity, b) sediment transport dynamics.
a- Transport rates
The study area covers a wide range of sediment accumulation rates, as shown from 210Pb
measurements (0.05 to >2 cm yr-1on the continental shelf; 0.04 to 0.15 cm yr-1 on the
continental slope, data from Peru slope within this range, Levin et al., 2002). Sedimentation
rate is a critical estimate, and will be determined by Th230 (Schmidt/epoc, imarpe, please
implement). Detailed sedimentological studies will allow a comprehensive characterization of
the composition, structure/texture of the sedimentary sequence in the study area will be
derived from Rockeval analysis, Xray, palynofacies study (Abdel check and improve). This
approach applied at high resolution in the upper sedimentary column will permit to assess
how sedimentological transport processes may distort the record of proxy signals, and will
thus contribute in refining their interpretations in terms of paleoceanographic reconstructions.
b- redox conditions (Anschutz, Rabouille) and sediment composition
Exported carbon fluxes and physico-chemical properties of bottom waters exert primary
control on redox conditions in the benthic layer and upper sedimentary column and constrain
the vertical distribution of dissolved and particulate reactive chemical species.
Off central and Southern Peru, coastal and shelf sediments are intensely reduced and the
dominant microbial oxidation pathway is sulphate oxidation (Henrichs and Farrington, 1984;
Rowe and Howarth, 1985; Suits and Arthur, 2000). PRIMO studies will tackle redox
conditions and associated diagenetic reactions through a comprehensive assessment of pore
water chemistry focusing on a wide range of reactive dissolved species (O2, NO2, NO3-, NH4+,
PO4 3-, Mn 2+, Fe2+, HS-, SO42-, I-). In addition reactive compounds in solid phase will be
assessed, such as organic and inorganic carbon, elemental Fe, reactive Fe-oxides, total Feoxides, Fe-sulphide, Mn (III)- and Mn (IV)-oxides, Mn-carbonate, elemental Mn, various P
forms (associated to Fe-oxides, apatite, total P).
Generally, sediments under SEP OMZ are organic-rich environments. Chemical properties of
the sediments characterized during PRIMO will comprise bulk characteristics (CaCO3, Si,
OC) as well as trace elements.
Selected trace elements inform on one or several of the following processe: primary
productivity, vertical transport of particles, benthic activity, redox intensity, terrigeneous
inputs, denitrifiacation, sulfatoreduction,… etc. Quantification of associated fluxes in a well
constrain situation will provide a pertinent data set to better understand the mode of formation
and of burial of the information borne by these elements and by combining various elements.
Bearing together these information, those from benthic communities and those from redox
conditions will allow to constrain the impact of environmental conditions on the buried
sediment characteristics.
This approach is particularly relevant to the improvement of proxies. For instance the pattern
of accumulation of redox-sensitive metals (U, Mo, Re, Ag, some Rare Earth Elements?) in
marine sediments is thought to reflect oceanic productivity represented by a flux of organic
matter to the seafloor. The distribution of these metals in marine sediments can be used to
determine bottom water redox conditions at the time of deposition. Other selected elements
are biomarkers.
NB: at this location, I cannot introduce the species (nannofossils, diatoms, etc) because there
are in the paragraph b. However it would be logical to have them associated. Your opinion?
Abdel and others, do I say too much/ too little about proxies here?
Comprehensive understanding of early diagenetic redox processes is a requisite to implement
proxies understanding, since these processes transform proxies derived from the water column
or are at the origin and subsequent modification of authigenic proxy-signals.
Experimental work in controlled conditions will allow the determination of critical parameters
for modeling, for instance BSi dissolution rate respectively to organic carbon decay.
(Olivier, Christophe, Ioanna, Vincent: Should we detail more the experiments after your
meeting on the 14th of April? Please implement).
¿Measurements of co2 production, sulphite production, elements implied in the N cycle?
¿S cycle, S-biomarkers locean ? S cycle histo, laminated sediment, Xray microscopy epoc,
paleot?
c Benthic community structure; micro-, meio- and macro-benthos (Guiterrez Graco, imarpe
stora lmgem benthic foram’s Angers ?)
Benthic biota affects both sediment properties (e.g., biomixing) and distribution of chemical
species, and among them proxies, through the metabolism of the sedimentary fauna. Structure
and functioning of eukaryotic and prokaryotic benthic communities and their response and
feedback to changing oceanographic/biogeochemical conditions are key issues addressed
within PRIMO. Oxygen availability, flux of organic matter to the sediment, benthic
communities and bioturbation show complex interactions (Gutierrez et al., 2000, Lvin et
al.,2002). Living species that may become fossil proxies do not escape this rule (Jorissen et
al., 1996; Fontanier et al., 2002).
The inner shelf of the study area is oxygenated all year around. At the outer shelf-upper slope,
the OMZ impinges the sediments during ENSO years. Meiofauna, mainly calcareous
foraminifera and nematodes are the organisms most tolerant of nearly anoxic, organic-rich
conditions (Levin et al., 2002). Very low oxygen conditions see the development of giant
prokaryotes, principally sulfide oxidizer, but also nitrate reducers and chemosynthetic bacteria
(Jorgensen et Gallardo, 1999; Otte et al., 2000; Graco, 2002). Our present knowledge does not
allow to comprehend the role of benthic communities on fluxes of elements and on the
construction of the sediment.
PRIMO will address the ecological and biogeochemical roles of the prokaryotic and faunal
assemblages on anaerobic respirations by a process study, stressing denitrification processes.
Coupling between benthic processes studies and proxy measurements will allow
comprehending how proxies are altered through biotic and abiotic processes at the sedimentwater interface and during early burial (upper sedimentary layer).
Biomarkers will be used to infer the contribution of algal detritus fuelled by vertical transport
of particles, of their decay and transformation; biomarkers of the benthic microfauna will also
be determined.