4. RECONSTRUCTION OF OMZ PALEOCONDITIONS
4.1 Objectives of the paleoceanographic approach
The general objectives of paleoceanographic studies, within PRIMO, are to assess
changes in intensity and extension of the OMZ in relationship to different oceanographic and
climate conditions, at different time-scales, from interannual and decennal to millennial time
scales. If one considers that the system boundaries are probably met during the extreme
global situations of glacial and interglacial times, it may be justified to envisage also
comparisons of the paleoceanographic setting about 20 ky ago and the mid-Holocene (5 ky
ago).
Within PRIMO, the paleoceanographic study will be particularly focused upon
calibration measurements, diagenetic processes understanding and validation of proxies of
former OMZ structure and dimension. It will involve sedimentological, mineralogical,
geochemical and biological proxies recorded in the sediments of the eastern South Pacific
(ESP).
For a better reconstruction of the OMZ and its variability during the past, we need to
understand the whole series of diagenetical processes which may affect the composition and
structure of the sediments accumulated at the water-bottom interface. In this part of the
project, we shall not particularly be involved in the water column processes (dealt with by the
biogeochemistry team, see above). However, the paleo-group will be interested in the
sediment trap monitoring within the water column, and more specifically in the mineralogical
and sedimentary components, which should not be studied by the geochemistry team.
A major point of the study will be to calibrate the signals, of diverse nature, which are
included in the sedimentary record, and which refer to paleo-OMZ conditions. To achieve this
goal, we need to work in close association with the team which will analyse the benthic
realm. In particular, we should cooperate for the determination and identification of the
relevant proxies of the water-column conditions, within the interface sediments.
- Which are the reliable indicators of water column conditions susceptible to be
preserved in the sedimentary column?
- To which extent the bottom conditions may affect and alterate these signals and
proxies, once the sediment is deposited ?
- How can we detect diagenetic effects on some of the indicators ?
- Is it possible to infer variability of bottom conditions (from deep currents to biophysico-chemical conditions) from specific post-depositional alterations of the sedimentary
record? .
The validation of paleo-proxies will be based on comparison between sedimentary
archives and either experimental data or instrumental information for the past few decades.
This work will be done on relatively short cores that will be obtained from diverse sources as
well as within the framework of the PRIMO cruise. These short cores (multitubes, small
boxcores, short gravity cores) are, or will be, collected at different depths, and along a wide
latitudinal range, along onshore-offshore transects in Peru and Chile.
In a posterior stage, once calibration studies on the sedimentary proxies are
available, it will be possible to work upon long cores, encompassing thousands of years or
glacial/interglacial cycles. These studies will be performed on piston cores, or giant box cores
which will be collected through other projects (e.g. IMAGES for the GALOPER project).
Studies on long cores might be conducted at different resolutions: eiher seasonal to
interannual, during any time period (e.g. the last glacial maximum), or at lower resolution
(e.g. one sample every millennium) but during a longer period (e.g. the last 20 000 years).
In all the cases the purpose will be to characterize the OMZ structure and try to
determine its dynamics in different climatic scenarios. These studies on sediments
accumulated in the OMZ during quite different insolation situations, or global climate
extremes, or in different ocean-atmosphere interactive patterns, are essential for a full
understanding of the OMZ system and for modelling its complete range of variability.
4.2. Calibration and validation of sedimentary proxies
The calibration studies designed for the paleoecanographic approach will be performed
within the water column (in association with the biogeochemists) as well as on the sedimentwater interface and in the first centimetres of sediment.
Some specific components of the sediments (e.g. mineral detritus, or the oxygen and
Hydrogen indexes and petrographical analyses of the particulate organic matter) must be
studied within the water column, with sediment traps. The calibration work will thus begin
with the alteration of some original signals during the transit of these components to the sea
bottom.
Other studies will be focused upon the diagenetic changes of some proxies which are
produced at, or immediately below, the water-sediment interface. These aspects which will
involve multi-cores, aim at deciphering the different physico-chemical (or biological)
processes which affect the sedimentary record. It is expected that some of them are directly
linked to OMZ characteristics and/or circulation features.
To assess and validate paleo-proxies of hydrological, biogeochemical and climatic
conditions bio-geochemical and mineralogical analyses on sedimentary sequences will be
confronted to available instrumental records for the last decades.
4.3. Experiments and analyses
The sedimentological analyses consist in determinations of: grain-size, bulk density,
porosity (water content) of each subsample. Combined with the previously indicated
descriptions and (X-ray, SCOPIX, thin-section) analyses, the sedimentological study must
lead to a comprehensive characterization of the physical parameters of the whole sedimentary
sequence at a sub-centimetre scale. In a general way, it is highly recommendable to proceed to
geochemical, micropaleontological and geochronological studies on sub-samples which are
precisely located in their stratigraphic context and which are precisely described, or
adequately isolated from the matrix or the surrounding material.
The study of sedimentary cores necessarily involves a series of steps which can be
resumed in the following way:
- X-ray radiographic analyses and numerical photography to detect the general structure of
the sedimentary sequence, and eventual stratigraphic anomalies like unconformities,
slump features, and bioturbation impacts;
- SCOPIX measurements on vertical slices of the sediment cores, which provide a first
semi-quantitative evaluation of grey level variations downcore.
- Detailed lithologic description, including colour variations, sedimentary structure and
texture of the individual layers, as well as identification of all the visible elements (shells,
fish scales and bones, etc.);
- Preparation of giant thin sections along the core, for more detailed and thorough
observation of the structural features and composition of the sediment;
- 210Pb measurements in the upper part of the cores provide information on the bioturbation
effects and recent accumulation rates, and may also be used to determine an age model
for the last century or so of sedimentation accumulation. Thus these radiometric
measurements may help to constrain, at least for the core tops (which only suffered limited
diagenetic effects), age models otherwise determined with radiocarbon data.
4.4. Studied parameters
The proxies that will be specifically addressed from the water-column to the sedimentary
archive are:
Geochemical parameters
* Redox-sensitive metals (U, Cd, V, Mo, Etc.) used to evaluate the intensity and
displacement of OMZ;
* Rock-eval analysis (hydrogen and oxygen index) in order to evaluate the
oxidation/preservation of organic matter in relation with the OMZ;
* Elemental composition (C, N, S, etc…) of the organic matter in order to evaluate its
origin, amount and mineralization;
* Petrographical analyses of the organic matter as an indicator of its source and
preservation;
* Biomarkers (lipids) as indicators of organic matter sources and modification;
Alkenones-Uk37 as a proxy of sea-surface temperature;
* Nitrogen isotopes in order to evaluate nutrient (nitrate) dynamics (denitrification, nutrient
utilization and cycling);
* Taxonomical investigation of shell-producing benthic species groups (ostracoda,
benthic foraminifera) as diagnostic species of oxygen content and organic matter supply.
* Carbon isotopes (within shells of benthic and planktonic foraminiferas) as an indicator of
water masses (ventilation) and productivity;
* Biogenic Si (opal), organic carbon and calcium carbonate contents as a proxy for
productivity;
* The identification and quantification of the mineral fraction of the sediments by X-ray
diffraction and Fourier-Transform Infra-Red spectrometry (FTIR) allow the determination or
quantification of: 1. Allochthonous mineral fluxes, related to erosion processes, sediment
transport from the continent, and depositional parameters; 2. autochthonous biogenic and
mineral fraction which are related to the intensity of the productivity and the physico-chemical
conditions, respectively. These combined informations may be used to characterize oceanic
circulation patterns.
* Proxies sediments properties, as well as short-lived and mefdium-lived radioisotopes as
234
Th, 137Cs and 210Pb.
Biological parameters –
* Taxonomy of shell-producing species: taxonomical investigation of shell-producing
species groups involved into the primary and secondary production in the surface mixed
layer (dinoflagellates, silicoflagellates, diatoms, planktic and benthic foraminifera,
coccolithophores) as diagnostic species of upwelling dynamics, water-mass distribution and
associated productivity.