Pierre Carbonel

Live benthic foraminiferal faunas from the Bay of Biscay: faunal density, composition, and microhabitats

In the meso-oligotrophic Bay of Biscay, a diminishing downward organic matter flux with depth is accompanied by an important decrease of the live foraminiferal density. Although bottom water oxygenation is not directly influenced by organic matter input, the oxygenation of interstitial waters and the primary redox fronts do change in response to variations of the organic matter flux. The occurrence of deep and intermediate infaunal taxa can be linked to fundamental redox fronts and putative associated bacterial consortia. Our data are in agreement with the TROX-model, which explains the benthic foraminiferal microhabitat as a function of organic flux and benthic ecosystem oxygenation. Both the depth of the principle redox fronts and the microhabitat of deep infaunal species show important increases with depth. At the deepest oligotrophic stations, deep infaunal faunas become relatively poor. Therefore, the exported flux of organic matter appears to be the main parameter controlling the composition and the vertical distribution of benthic foraminiferal faunas below the sediment-water interface. The oxygenation of pore waters plays only a minor role. A species-level adaptation of the TROX-model is presented for the Bay of Biscay.

Biological remains, geochemistry and stable isotopes for the reconstruction of environmental and hydrological changes in the holocene lakes from north Sahara

Abstract A detailed reconstruction of the Holocene palaeoenvironments and palaeohydrology is presented for wwamp and lacustrine calcareous sediments which lie in closed depressions along the northern margin of the Great Western Erg (Algeria). It is based on a multidisciplinary approach using numerous techniques. The chronological framework is established on the basis of 23 14 C dates (on inorganic calcite and mollusc shells) ranging from ca 9300 to 3000 yr B.P. (and on an isolated 14 C age of ca 10,900 yr B.P.). The significance of various environmental markers is discussed. Biological remains are abundant and diversified (about 150 taxa including diatoms, chrysophytes, charophytes, ostracods, molluscs and foraminifera). They provide information on the water chemistry (total salinity, ionic ratio, pH, and partial pressure of carbon dioxide), and on the habitats (water depth, development of aquatic plants, ephemerality, etc). Emphasis is given to the total salinity. Individual organisms and palaeobiological assemblages are ordered as a function of their salt requirements and tolerances. Calibration based on modern diatom samples allows a tentative estimate of the palaeosalinities which range from fresh to marine-like, and even hypersaline waters. Attention is drawn to the possible colonization of “marine” organisms in continental basins without any connection with the sea. Stable isotope analyses have been carried out on both inorganic and biogenic (mollusc and ostracod shells, charophyte gyronites) carbonates. The 18 O contents demonstrate wide changes in the hydrological balance. Fluctuations in 13 C contents inform on water mixing, residence time, and on the importance of biological effects on total dissolved inorganic carbon. Geochemical analyses were performed on inorganic calcite and on ostracod shells. High contents in Fe and Mn indicate reducing environments during stages of high salinity. Sr/Ca and Mg/Ca are used as salinity indicators. Zn/Mg ratios suggest that the major water supply was the aquifer of the Great Western Erg, today down to about — 50 m, rather than surface waters from the Atlas mountains. Salinities resulted from the balance between inputs from the water-table, evaporation rate, leaching of pre-existing salt crusts, and seepage through the lake bottom. A reconstruction of the evolution of the palaeosystems through time and space has been attempted, with special attention to the instability of the water and ionic balances for different time scales. Short term fluctuations occurring during a single hydrological cycle (annual or seasonal) are deduced from the apparent discrepancies between the various indicators found in a given sample. Long term (10 2 –10 3 years) changes confirm the existence of a climatic phase being wetter than today from at least ca 9300 (and may be from ca 10,900 yr B.P.) to 3000 yr B.P. For the first time, the salinity deduced from diatoms and the 18 O content of the water deduced from that of the carbonate (at 25°C) are used simultaneously to model the hydrological history of the basins and to estimate the relative humidity of the atmosphere during the Holocene. Major environmental trends recorded from ca 9300 to 3000 yr B.P. do not show a clear synchronims between the different profiles studied. Therefore, they cannot be interpreted in terms of palaeoclimate, but instead demonstrate the importance of local hydrology in the evolution of closed basins associated with groundwater, a common situation in arid and semi-arid zones.

Paleoecology of limnic ostracodes: A review of some major topics

Abstract The paleoecology of limnic ostracodes is reviewd in the context of the evolution of their environment. In order to understand why there is a rich ostracode fossil record we examine data such as the quantitative and qualitative distribution of living ostracode species and ostracode assemblages (ways of describing and limitations). Importance of biological and physico-chemical factors for the local accumulation of high numbers of ostracodes and diversified assemblages is emphasized. We discuss the limits of extrapolations of information from ecology to paleocology. We stress how paleontological knowledge can be applied to neontological studies. The possibility of using Holocene ostracodes as indicators of morphometric changes of lakes, temperature, salinity, eutrophication and as a criterion of meromictic conditions is demonstrated. Relationships between ostracodes and the chemical environment and particularly the morphological response of the carapace are considered: e.g., diversity and qualitative composition of assemblages and chemical stability, types of carapace mineralization and ornamentation and chemistry of bottom waters. These ideas, developed from studies on Recent material, are tested for limnic Cenozoic faunas. In the Mesozoic, the main questions arising from the study of nonmarine ostracodes deal with the possible evaluation of paleosalinities in marginal lagoonal environments (Purbeck-Wealden) and with evaluation of depth and hydrochemical and paleoclimatological regime in truly lacustrine intracratonic basins. Some peculiarities of Paleozoic nonmarine ostracode assemblages are described. The paleoecological cause of changes in diversity of several ostracode groups in the Carboniferous and Permian is emphasized.

Early diagenetic processes in the muddy sediments of the Bay of Biscay

Abstract In order to understand the early diagenesis processes occurring in continental margin environment, modern sediments collected in six different sites from the Bay of Biscay have been studied. These sites can be separated into two groups. In the shallowest stations, where sediments are highly bioturbated, organic carbon levels are higher than 2%. In the deepest stations, sediments are much less bioturbated, and organic carbon levels are lower. In all sites, the vertical distribution of redox sensitive species can be explained by the well-established depth sequence of redox reactions, based on the bacterially mediated oxidation of organic matter. We have considered some alternative reaction pathways to explain the profiles of Fe, Mn, and N species. These reactions deal with the ammonia oxidation by manganese oxide, the aerobic denitrification and the oxidation of dissolved iron (II) by nitrate or Mn-oxides. Vertical flux calculations with a simple diffusion model indicate that these reactions could account for the reduction of all the Mn-oxides and the oxidation of all the upward diffusing Fe(II). They may also be responsible for a significant part of the total dinitrogen production. The relative importance of these pathways on early diagenetic processes and benthic fluxes has not been determined and must be examined with additional experimental works. Our study suggests, however, that the coupling between the benthic cycles of iron, manganese and nitrogen could strongly influence the carbon cycling at the ocean floor.

Holocene palaeohydrology of Lake Titicaca estimated from an ostracod-based transfer function

Lacustrine deposits of Holocene age (8000 14C yr BP–Present) were studied in Lake Titicaca, Bolivia. Ostracod assemblages were identified in six cores taken in the three basins which today form Lake Titicaca. Factorial correspondence and multiple linear regression analysis of the ostracods have permitted the reconstruction of (1) lake level, (2) water salinity and (3) water Mg/Ca ratio variations. Palaeolake levels indicate a complex evolution of the three basins, with some major phases. After a flooding at about 8000 yr 14C BP, the lake level fluctuated around a mean position until 3900 14C yr BP, when a second rise took place. It seems, however, that the present-day lake level was not reached until after 680 yr 14C BP. Some major dry events are also recorded at about 5300 14C yr BP and 2300 14C yr BP. The Lake Titicaca data are consistent with other records from South America, and also seem to reflect some world-wide events.

A quantitative method of palaeolake-level reconstruction using ostracod assemblages: an example from the Bolivian Altiplano

This paper provides quantitative information concerning the response of ostracods to environmental variability in order to reconstruct past environments. Ostracod faunas from modern sediments of Bolivian lakes and swamps were studied. Ostracod distribution is controlled by several ecological characteristics such as lake-level and water chemistry. Statistical results indicate that three transfer functions (on water depth, Total dissolved Salts and water in Mg/Ca ratio) can be developed, from ostracod species frequencies in lacustrine sediments, with some restrictions for the two last ones.

Hurricanes and climate in the Caribbean during the past 3700 years BP

A multiproxy analysis of lacustrine sediments cored in Grand-Case Pond at Saint-Martin, north of the Lesser Antilles archipelago, reveals three distinct climatic periods for the last 3700 years. From 3700 to ~2500 yr cal. BP and from 1150 yr cal. BP to the present, carbonate mud deposition occurred in connection with pond lowstands. These periods were also punctuated by severe drought events, marked by gypsum laminae, and hurricane landfalls, leading to marine sand inputs into the pond. The intermediate time interval, from 2500 to 1150 yr cal. BP, is typified by black organic mud deposition, suggesting that hypoxic to anoxic conditions prevailed at the pond bottom. These were probably linked with a perennial pond highstand and reflect more uniform and wetter climatic conditions than today. The carbon isotopic composition of the ostracod Perissocytheridea bisulcata shows that the lowest δ13C values are recorded during the hypoxic periods, as a consequence of bacterial recycling of isotopically depleted organic matter. Such a climatic history agrees closely with that documented from other records in the Caribbean area, such as the Cariaco Basin, central coast of Belize or Barbados. By constrast, discrepancies seem to emerge from the comparison between hurricane activity recorded at Saint-Martin on the one hand and Vieques (Puerto Rico) on the other hand. We explain this apparent contradiction by a balance between two distinct storm paths in response to latitudinal shifts of the Intertropical Convergence Zone (ITCZ). Stronger storm activity over the Gulf coast and the inner Caribbean Sea is favoured by a southern position of the ITCZ in connection with dry climatic conditions. Plausible links with the North Atlantic Oscillation (NAO) are also suggested.

Freshwater ostracods as environmental tracers

This paper revises the response of freshwater ostracods to different environmental conditions and anthropogenic impacts, with a worldwide overview of the potential use of these microcrustaceans as bioindicators and several examples of applications in different scenarios. The development of either a single species or an ostracod assemblage is influenced by physical–chemical properties of waters (salinity, temperature, pH, dissolved oxygen), hydraulic conditions, bottom grain sizes or sedimentation rates. In addition to population and community changes, morphological and geochemical changes can also be detected in the ostracod carapace, which serves as a tracer of the water quality. All these features permit to delimit the spatial effects of urban sewages, mining effluents, agricultural wastes, watershed deforestation or road building. These data are the basis for the palaeoenvironmental reconstruction of cores, with an interesting application to archaeology. In addition, favourable results of recently developed bioassays, coupled with an important variability of local assemblages under changing conditions in both waters and sediments, suggest that these microcrustaceans may included between the most promising sentinels groups in freshwater areas. These microcrustaceans show high sensitivity to pesticides, herbicides, heavy metal pollution and oil inputs.

How a low tissue O2 strategy could be conserved in early crustaceans: the example of the podocopid ostracods

SUMMARY An adaptation strategy whereby O2 partial pressure, PO2, in the tissues is maintained within a low, narrow range of 1–3 kPa, largely independent of the inspired PO2, has been reported in water- and air-breathing poikilotherms and in homeotherms. Based on the postulate that this basic cellular mechanism has been established since the early stages of evolution, it has been hypothesized that it could be the consequence of an early adaptation strategy to maintain cellular oxygenation within the same low and primitive range. To test this hypothesis we studied the basic mechanisms of oxygen regulation in podocopid ostracods, minute crustaceans that have existed on earth for at least 500 million years. Podocopids lack any regulatory mechanism for adapting their ventilation to cope with changes in water oxygenation, and instead adjust their tissue oxygenation status by migrating through the O2 gradient to sediment layers where the PO2 of the water is 3–5 kPa. Experimental manipulation of the O2 profile induced their vertical migration to follow this precise water PO2 and demonstrates the existence of a regulation strategy. This strategy must be associated with the lower PO2 values within the animals carapace valves, showing that podocopids can actively regulate their tissue PO2 at constant but even lower values than the water. In conclusion, the low tissue PO2 strategy could have existed in early crustaceans and, by extension, in early animals.

Geoscience rediscovers Phoenicia's buried harbors

After centuries of archaeological debate, the harbors of Phoenicia9s two most important city states, Tyre and Sidon, have been rediscovered, and including new geoarcheological results reveal how, where, and when they evolved after their Bronze Age foundations. The early ports lie beneath their present urban centers, and we have indentified four harbor phases. (1) During the Bronze Age, Tyre and Sidon were characterized by semi-open marine coves that served as protoharbors. (2) Biostratigraphic and lithostratigraphic data indicate the presence of early artificial basins after the first millennium B.C. (3) The harbors reached their apogees during the Greco-Roman and Byzantine periods. (4) Silting up and coastal progradation led to burial of the medieval basins, lost until now.