Pierre Anschutz

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.

Interactions between metal oxides and species of nitrogen and iodine in bioturbated marine sediments

By using a gold amalgam (Au/Hg) voltammetric microelectrode, we have measured simulta- neously and with millimeter resolution the distributions of O 2, Mn(II), Fe(II), I(2I), and HS(2I) in bioturbated sediment cores from the Laurentian Trough. We also measured nitrate and ammonia in the pore water, total I and ascorbate- and HCl-extractable Fe and Mn in the solid-phase sediment, and fluxes of O 2, NO3 , and NH4 across the sediment-water interface. The concentrations of O2 and Mn(II) were below their respective detection limits of 3 and 5 mM between 4 and 12 mm depth, but a sharp iodide maximum occurred at the depth where upward diffusing Mn(II) was being removed. We propose that the iodide peak is maintained through the reduction of IO3 by Mn(II), reoxidation of I(2I) to IO3 in the oxic zone above the peak and oxidation to I2 below where it is ultimately trapped by reaction with organic matter. The iodide production rate is sufficient to account for the oxidation of all of the upward diffusing Mn(II) by IO3 . Nitrate plus nitrite (SNO3) decreased to a minimum within 10 mm of the sediment-water interface, in agreement with flux measurements which showed SNO3 uptake by the sediment. Below the minimum, SNO3 rebounded, and reached a maximum at 40- to 50-mm depth. This rebound is attributed to the anaerobic oxidation of ammonia by manganese oxides. Fe(II) was always first detected below the anoxic SNO3 maximum, and was accompanied by colloidal or complexed Fe(III). A sharp upward-directed ammonia gradient was recorded near the sediment-water interface, but no ammonia was released during the first 48 h of the incubations. If the ammonia removal were due to coupled bacterial nitrification- denitrification, more than one half of the total measured oxygen uptake (6.7 to 7.3 mmol/m 2 /d) would be required, and more organic carbon would be oxidized by nitrate than by oxygen. This scenario is not supported by nitrate flux calculations. Alternatively, the oxidation of ammonia to N2 by manganese oxides is a potential removal mechanism. It would require one quarter of the total oxygen flux. The high-resolution profiles of redox species support the conceptualization of bioturbated sediments as a spatially and temporally changing mosaic of redox reactions. They show evidence for a multitude of reactions whose relative importance will vary over time, and for reaction pathways complementing those usually considered in diagenetic studies. Copyright

The distribution of Mo, U, and Cd in relation to major redox species in muddy sediments of the Bay of Biscay

In order to understand the relationship between authigenic precipitation of U, Mo, Cd, and the redox properties of the marine environment, we have studied the vertical distribution of these metals and of the major redox species (oxygen, nitrate, manganese, reactive iron, sulfate, carbon, and sulfur) in modern sediment. We have sampled four sites at 150 to 2800 m depth in the Bay of Biscay. At the shallowest station, where sediment is highly bioturbated, organic carbon levels are above 2% and particulate sulfides are abundant. At the deepest stations, sediment is much less bioturbated, and organic carbon levels are lower. At all sites, early diagenesis follows a well-established depth sequence of redox reactions, based on the bacterially mediated oxidation of organic matter. Manganese-oxides and authigenic U and Mo were extracted by an ascorbate solution. Molybdenum is associated with these oxides in the oxic part of cores. In the anoxic layer of the sediment, Mo precipitates as a detectable authigenic phase only when sulfide minerals are present, i.e. when sulfate reduction becomes important. In anoxic sediments, Cd enrichment is a good indicator of sulfide production from sulfate reduction even if the production is weak. Uranium precipitates at the depth of reactive iron (III) reduction. No relationship was observed between U and S. The U concentration at the depth of precipitation agrees with values calculated using estimated accumulation rates and a downward diffusive flux of dissolved U from the bottom seawater into the sediment. However, authigenic U concentrations continue to increase in the anoxic part of the cores. Therefore, it is difficult to establish a direct relationship between the process of U accumulation and the flux of Corg to the sediment. D 2002 Elsevier Science B.V. All rights reserved.

Single foraminiferal test chemistry records the marine environment

We applied laser-ablation inductively coupled plasma‐mass spectrometry (LA-ICP-MS) as a new and precise technique for measuring trace elements in benthic foraminifera (Hoeglundina elegans). With this technique, trace element concentrations were accurately measured without the elaborate cleaning required in standard methods. Contaminated coatings are easily excluded during spatially resolved analysis. Application of this new technique allowed us to calibrate the trace elements incorporated in single tests of living (when sampled) benthic foraminifera to ambient seawater temperature (Mg and Sr) and redox conditions (Mn) for the first time. Incorporation of Ba showed a more complex pattern that cannot be explained by a direct correlation to water column concentration.

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.

A cadmium budget for the Lot-Garonne fluvial system (France)

Routine measurements of river discharge and total suspended sediment concentration (TSS) are combined with regular analyses for particulate and dissolved cadmium to produce a box model that allows us to propose a cadmium mass balance for the Lot-Garonne man-influenced river system (8400 km2). Nearly half the cadmium in the Garonne river is supplied by the tributary Lot river. Cadmium input onto the Lot river comprises wet deposition from the atmosphere, molecular diffusion at the sediment-water interface, surface-water runoff and discharge from the leaching of waste at a zinc refining plant. Approximately 85% of the cadmium in the Lot river is derived from anthropogenic origin. Cadmium in the industrial discharge is 80% dissolved and 20% in the particulate phase (4.2 and 1.1 t yr−1, respectively). Total inputs are estimated at 4.81 t yr−1 and 1.54 t yr−1 for the dissolved cadmium and for the particulate phase, respectively. Budgeting estimates an output onto the Garonne river of 0.54 t yr−1 for the dissolved cadmium (about 8%) and 6.13 t yr−1 for the particulate cadmium (about 92%) indicating that downstream sediment-associated cadmium fluxes are enhanced by the 4.27 t yr−1 removed from solution and the 0.32 t yr−1 remobilized by the erosion of sediment blanketing the Lot river bed. These figures are found to be comparable with those generated by a dilution model which suggests that 97% of dissolved cadmium is taken up by the particulate phase over 0.5 km downstream from the primary anthropogenic source.

The behaviour of arsenic in muddy sediments of the Bay of Biscay (France)

Abstract We have studied particulate and dissolved arsenic species in sediment and porewaters at sites in the Bay of Biscay, France, ranging in depths from 150 to 2,800 m. At all stations, major redox species (oxygen, nitrate, ammonia, total and reactive iron and manganese, sulphate and sulphur) reflect early diagenetic depth sequences of redox reactions comparable to other marine environments. Vertical distributions of dissolved and particulate As species and major redox species are related to changes in redox conditions and their major carrier phases, such as Fe and Mn-oxides. Arsenic diagenesis appears strongly dependent on Fe cycling. A subsurface maximum of dissolved As and surface enrichment of particulate As correspond to dissolution and precipitation of Fe (III) phases. Except for the shallowest and most bioturbated site, flux calculations show three different vertical diffusive As fluxes: two upwards and one downwards. Phase changes of recycled As result in local accumulations of reactive As at different redox fronts. Mass-balance calculations indicate that the upward As flux toward the oxidized layer can explain the enrichment of HCl extractable particulate As in this layer. A portion of the upward diffusing As can escape the sediment and may be fixed onto settling Fe-oxides by adsorption or co-precipitation and contribute to reactive particulate As input (i.e., As is recycled across the water sediment interface).

SEASONAL VARIABILITY OF BENTHIC FORAMINIFERAL FAUNAS AT 1000 M DEPTH IN THE BAY OF BISCAY

A 1000-meter-deep station in the Bay of Biscay (station A) was sampled 10 times between October 1997 and April 2001 for the purposes of studying the temporal variability of live foraminiferal faunas in the 63–150 mm and .150 mm size fractions. The results are compared with those obtained earlier for a 550-m-deep station nearby. The study area is marked by prolonged, two-month spring blooms and less clear autumn blooms that result in labile organic matter enrichment of the upper sediment layers. Episodic exportation of phytodetritus had a recognizable impact on early diagenetic processes only in April 2001. During the 2001 spring bloom, bottom-water oxygenation and the depth of the zero-oxygen boundary were minimum. Foraminiferal faunas respond to bloom events by increases in the abundance of opportunistic taxa. In the .150 mm size fraction, Uvigerina mediterranea and Uvigerina peregrina preferentially reproduced and thrived in shallow infaunal microhabitats that are seasonally enriched in phytodetritus. Although the seasonal changes in the 63–150 mm size fraction are less straightforward, Nuttallides pusillus and Uvigerina peregrina did show marked seasonal changes in abundance. The temporal changes in the foraminiferal faunas at the 1000m-deep station appear to be synchronous with those recorded at the 550-m-deep station.

Recent turbidite deposition in the eastern Atlantic: Early diagenesis and biotic recovery

An interface core taken in Capbreton canyon shows a succession of sedimentary facies interpreted as classical Bouma turbiditic sequences. Activities of 234Th and 210Pb suggest that the deposition of the most recent turbidite was triggered by the violent storm that affected the Atlantic coast of southern France on the 27th of December 1999, about four months before the sampling of the core. This turbidite allows us to study the ongoing diagenesis of the new sediment layer and of the previous sediment-water interface, which has been buried and only slightly eroded. A study of benthic foraminiferal populations informs us about the rate of benthic ecosystem recovery after such a major ecosystem disturbance event. The composition of the benthic foraminiferal fauna suggests that the benthic ecosystem in Capbreton canyon remains in an early stage of colonization. The rare agglutinant taxon Technitella melo appears to be the first colonizing species. It is suggested that Technitella melo is advantaged by the food-impoverished conditions in the days following turbidite deposition. Almost all of the turbidite layer and the previous oxic sediment-water interface contain reduced dissolved metal species and were anoxic. The buried interface contains Fe- and Mn-oxides inherited from its recent oxic past. The reduction of manganese oxides was in progress at the time of core collection. The reduced Mn remained trapped in the sediment as Mn-containing carbonates. Iron-oxides did not undergo significant reductive dissolution. The top of the newly deposited turbidite formed an oxic layer, which was rapidly enriched in metal-oxides. The enrichment of manganese oxides was mostly due to the oxidation of dissolved Mn2+, which diffused from below. The enrichment of iron oxides is explained both by the oxidation of the upward flux of dissolved Fe2+, and by the input of detrital iron oxide after, or as a result of the turbidite deposition.

Heat and salt fluxes in the Atlantis II Deep (Red Sea)

The Atlantis II Deep is located on the Red Sea axial rift. It is a topographic depression enclosing a volume of about 17 km3 between 1,900 m and 2,200 m depth that contains layered brines of temperatures up to 66°C and salinities up to 270‰. Previous geochemical investigations showed that the hot brines result from discharge of hydrothermal solutions that have exchanged heat and chemical components with the basaltic substratum. The last investigation of the Deep in 1992 showed that the brines occurred in four well mixed layers with the shallowest at a depth of 2,000 m. The temperature and salinity profiles describe a transition zone from 2,000 m to 1,900 m, above which Red Sea water occurs. The distribution of temperature and salt in this transition zone appears to be controlled by the topography of the Deep. The hydrographic structure of the brine column has been documented in the literature for seven time intervals between 1966 and 1992. Examination of these data shows that the system changed with time. The evolution of the entire package of brines that fill the Deep shows the following changes: (1) the temperature of the brines increased; (2) the salinity of the solutions increased; (3) the two brine layers described in 1966 still existed in 1992, but new layers appeared above them; (4) for 26 years, almost all additional heat and salt supplied in the Deep were confined to the depression and were not dispersed into the overlying seawater. The fourth point indicates that a heat and salt balance for the Deep can be calculated. The calculation of the heat that entered into the system was divided into two components: the temperature increase of the brines and the heat loss at the wall-rock; the latter component was negligible. The rate of heat input to the Deep was constant during the period considered, and amounted to 0.54 × 109 W. The salt input was also constant, and equalled 250–350 kg/s. During the period 1966–1992, heat and salt were most probably supplied by a hydrothermal solution with an average range of flow rate, temperature, and salinity of 670–1000 kg/s, 195–310°C, and 270–370‰, respectively.