> Foraminiferal taxonomy and intra- and interspecific variability of the proxy signals

    > Impact of temporal variability of the ocean floor environment on the proxy records

    >  The impact of small scale variability (patchiness)

    >  The impact of ontogenetic variability

    >  The impact of diagenetic processes on the formation of the proxy signals

 

Research Strategy

 

Our research project aims to ameliorate all paleoceanographic proxies based on benthic foraminiferal carbonate by a better integration of our knowledge of deep-sea ecology and geochemistry. Although they are distinct, the two main objectives of the project can not be dissociated, because an integrated geochemical/ecological research strategy is needed for both of them.

The scientific questions which have been outlined before each need a specific strategy, which is outlined below:

 

A serious taxonomical study of the morphotypes which are routinely used for the geochemical studies is a prerequisite here. This study will be based on the abundant living material from the Bay of Biscay, on recently collected life material from the eastern Mediterranean, and on the ample material available in the laboratories of the main proposers (Angers, Gif-sur-Yvette).

 

a)                     The first step will be the production of a determination guide, combining correct taxonomical position with all available ecological and geochemical data for the various morphotypes.

b)                     Geochemical analyses will be performed on all of these morphotypes, in order to detect systematical isotopic offsets. As often as possible, measurements performed on faunas collected at different depths in the sediment will be compared.

 

The observation of seasonal and interannual variability asks for a major long-term research effort, whereas episodic events arriving at a decadal scale an only be correctly interpreted after prolonged periods of monitoring. This is out of the scope of the present project. Nevertheless, less time-intensive methods allow a first evaluation of the impact of temporal variability on the proxy records:

a)                                          seasonal samples are available for several stations (200-3000 m depth) in the Bay of Biscay, which have been sampled repeatedly between October 1997 and June 2001. In order to better cover the deeper NE Atlantic environments (with more abundant faunas of Fontbotia and Cibicidoides), new material will be sampled in Mai 2002 (water depth 1000-2000 m) during a sampling campaign in the NE Atlantic with the belgian research vessel "Belgica". Two sampling campaigns have been demanded for 2003. These sampling campaigns will allow the collection of life faunas in two contrasting seasons (spring/productive versus early winter/non productive).

b)                                          a comparative study of living and subfossil faunas at the same sites will allow the recognition of the taxa which preferentially form during the episodic high organic flux periods. For these taxa, a strong imprint on their geochemical composition is probable.

c)                                          For the same sites, single species analyses, either with highly sensitive mass spectrometers, or by UV laser ablation ICPMS (performed at Utrecht University, Netherlands), will give more insight into the geochemical variability due to this phenomenon.

 

For each site, a study of the microhabitat distribution of the dominant foraminiferal taxa, in combination with the chemical properties (oxygen, manganese, nitrate, sulfate, etc.) will be accompanied by d¹³C-measurements for living representatives of all microhabitat categories, as well as for the bottom and pore waters at various depths in the sediment. The obtained foraminiferal-based d¹³C-profile will be compared with the real bottom and interstitial water d¹³C-profile in order to detect eventual offsets. The execution of this type of study in a multitude of settings (in the NE Atlantic, Mediterranean and other areas), with different productivity and bottom water oxygenation regimes, will allow to describe the relations between bottom and pore water d¹³C-gradients, and surface water productivity patterns (obtained by satellite images). Since in open ocean systems empirical formula exist, which describe the export production as a function of overlying Primary Production and water depth (Herguera 1992), the d¹³C-gradients can then be compared with the downward organic flux.

Porewater d¹³C-profiles will be further used to derive estimates of sedimentary mineralization rates and associated fluxes. Organic matter respiration is the main driving force of interstitial d¹³C gradients (McCorkle et al., 1985; McCorkle and Emerson, 1988; Jahnke et al., 1997). Knowing the isotopic composition of organic C (-20 to –22‰ vs. PDB according to Wong and Sackett, 1978; Rau et al., 1982; Gearing et al., 1984, Meyers, 1994), porewater d¹³C can be used to evaluate rates of TCO₂ production from organic matter decay using a straight-forward modelling approach (Gehlen et al., 1999). The same approach, when applied to foraminiferal-generated d¹³C-profiles, will allow us to construct a powerful proxy tool.

 

In a first time, the extent of patchiness will be determined by a comparative ecological study of foraminiferal faunas from available replicate samples from the Bay of Biscaye and the eastern Mediterranean. In case of important small scale variability, the stable isotopes, and Mg/Ca ratios will be analyzed in the dominant Cibicidoides/Fontbotia and Uvigerina morphotypes for all replicate cores.

 

Two complementary strategies allow the recognition of the impact of ontogenetic variability on the geochemical proxy records:

a)       the analysis of narrowly defined size classes of benthic foraminifera, and

b)       the analysis of individual foraminifera. In the case of stable isotope measurements, a highly sensitive mass spectrometer will be used, in the case of Mg/Ca ratios, individual foraminifera will be analyzed by UV laser ablation ICPMS.

 

It is not easy to study the ongoing faunal as well as geochemical changes which take place during the transition of a living to a fossil fauna. The problem will be tackled by a high resolution (cm-scale) study of the Recent to Subrecent faunas collected in cores from areas with very high sedimentation rates. Progressive downcore changes in the faunal composition, due to losses of less resistant tests, will be compared with isotope measurements of several depth levels. Suitable material for this kind of analysis is available from the Bay of Biscay, deeper NE Atlantic and eastern Mediterranean.

Diagenetic processes affecting biogenic carbonates include dissolution and reprecipitation. Both are likely to affect trace element and isotopic composition. Evidence of a dissolution effect on benthic foraminiferal shell chemistry was reported by McCorkle et al. (1995). This author points to a possible dissolution-driven decrease of benthic foraminiferal d¹³C values. This important issue calls for a multi-proxy approach combining data on trace element composition (Cd/Ca, Ba/Ca, Sr/Ca, Mg/Ca) and stable carbon isotopes obtained on the same foraminiferal samples. We will compare our d¹³C records with benthic foraminiferal Cd/Ca-ratios. Cadmium is a phosphate proxy and a close correlation between foraminiferal d¹³C and Cd/Ca is expected as long as biological uptake and decomposition of organic matter control d¹³C and Cd distribution (Boyle, 1992). A recent study shows that Fontbotia wuellerstorfi is a recorder of bottom water Cd whereas Cibicides bradyi, U. peregrina and Melonis barleeanus record pore water Cd (Tachikawa and Elderfield, accepted). Thus, the combination of the four groups will allow us to reconstruct the pore water Cd gradient, which should vary with the organic flux.

 ates of organic carbon: Uvigerina peregrina group and Cibicidoides wuellerstorfi. Paleoceanography, 1, 27-42.