Jean-Louis Reyss

New age data for Mid‐Atlantic Ridge hydrothermal sites: TAG and Snakepit chronology revisited

The chronologies of TAG and Snakepit hydrothermal fields have been established using 210Pb/Pb, 230Th/234U and 14C dating. At the TAG field, a Mn-oxide record, indicative of low temperature events, began at least 125,000 years and possibly 140,000 years ago with maximum intensities at 15,000, 7000 and 4000 years before present. High temperature events, giving rise to sulfide deposits, began about 100,000 years ago and have been intermittent to the present day. A presently active site has experienced intermittent pulses of activity every 4000 to 6000 years over the past 20,000 years. Decrease in activity is often marked by low temperature aragonite precipitation in chimney conduits at 4000, 7000 and 9000 years ago. After a period of quiescence lasting about 4000 years this site was reactivated about 50 years ago. The Snakepit field is much younger and no sulfides older than 4000 years have been recovered. Relict sulfide deposits are dated between 2000 and 4000 years old indicating this site was active during a quiescent period at TAG. Reactivation of Snakepit. took place about 80 years ago, and is presently concurrent with that of TAG. Comparison with hydrothermal sites on the East Pacific Rise suggests that on slow spreading ridges the major fracture systems focussing the hydrothermal discharge can be reactivated at intervals and new deposits precipitated on top of older ones, while on faster spreading ridges each pulse of activity is separated in space and time resulting in discrete deposits.

Hydrothermal activity on a 105-year scale at a slow-spreading ridge, TAG hydrothermal field, Mid-Atlantic Ridge 26°N

New radiometric dates are presented for the hydrothermal deposits in the Alvin and Mir relict hydrothermal zones located on the lower east wall of the rift valley within the TAG hydrothermal field and placed in context of the evolution of the TAG field as a whole over the past 105 years. The field includes an active high-temperature sulfide mound situated near the base of the east wall and a low-temperature zone at middepth on the east wall all within a 5 by 5 kilometer area. The oldest hydrothermal deposits sampled in the Mir relict zone, the low-temperature zone, and an inactive sulfide mound in the Alvin relict zone are low-temperature manganese crusts dated at 140,000, 125,000 and 74,000 years, respectively. High-temperature activity began about 100,000 years ago in the Mir zone. A 50,000 year-high-temperature event took place at the active sulfide mound, in the Mir zone, and at the inactive sulfide mound in the Alvin zone. This event is followed by sporadic activity at the Mir zone and the active sulfide mound, until 10,000 and 5000 years ago, respectively. A reactivation of the system affected the Mir zone from about 2000 to 600 years ago. The presently active sulfide mound was reactivated only 60 years ago, after a quiescence of about 4000 years. The radiochronologic record reveals that the hydrothermal activity has been episodic with a major high-temperature event throughout the TAG field at 50,000 years and other more localized events.

Production of radionuclides by cosmic rays at mountain altitudes

Production rates of22Na (T1/2 = 2.6years) from aluminium by the action of cosmic rays are measured at the Mont Blanc (altitude 4600 m), the Aiguille du Midi (3840 m), and the Col du Lautaret (2070 m). They are2.3 ± 0.5,1.8 ± 0.3,and0.77 ± 0.18 atoms min−1 kg−1, respectively, in good agreement with the calculated production rates, 2.4, 1.7 and 0.6 atoms min−1 kg−1, respectively, at the three stations. Production rates of24Na (T1/2 = 15hours) from aluminium and magnesium are also measured at the Aiguille du Midi; the observed rates of3.4 ± 0.4and6.0 ± 1.7 atoms min−1 kg−1, respectively, agree well with the theoretically expected rates of 3.7 and 5.6 atoms min−1 kg−1. The production rates of3H,7Be,10Be,14C,22Na,26Al,36Cl,37Ar,39Ar,53Mn,54Mn, and55Fe in terrestrial rocks by the action of cosmic rays are calculated in order to show the possibility of applying the measurements of these cosmogenic radionuclides to the earth science.

Particle mixing rates in sediments of the northeast tropical Atlantic: Evidence from 210Pbxs, 137Cs, 228Thxs and 234Thxs downcore distributions

Particle mixing rates were estimated from distribution of 137Cs and excess activities of 210Pb, 228Th and 234Th measured in nine sediment cores taken from the northeast tropical Atlantic, i.e. at the three sites of the French EUMELI (EUtrophic, MEsotrophic and oLIgotrophic) programme. At the oligotrophic site, activities of 137Cs and excess 210Pb decrease smoothly with depth. On the other hand, at the mesotrophic and eutrophic sites, 210Pbxs and 137Cs profiles suggest a mixing process with three different regimes, with subsurface sediments being mixed more rapidly than those at the surface. Bioturbation rates were found to be slowest at the oligotrophic site, intermediate at the mesotrophic site, and fastest at the eutrophic site. These mixing intensities appear closely linked to benthic fauna abundance, organic carbon flux to the sediments, and primary production in overlying surface waters. The use of a multitracer approach for studying sediment particle reworking has allowed bioturbation rates to be better constrained and an assessment of whether radionuclide distributions in surface sediments are in steady state. For only one core from the mesotrophic site were surface sediments not in steady state, perhaps because of the occurrence of a benthic animal (i.e., a xenophyophore) at the surface of this core.

226Ra in barite: Absolute dating of Holocene Southern Ocean sediments and reconstruction of sea-surface reservoir ages

Absolute dating of marine sediments is necessary to achieve reliable correlations of paleoclimate records. The 2 2 6 Ra decay in barite separated from a deep-sea core of the Southern Ocean was used to determine absolute ages for the Holocene Epoch. Comparison with 1 4 C ages measured on planktic foraminifers provides the first past sea-surface reservoir ages in the Antarctic zone of the Southern Ocean. Throughout the middle to late Holocene, our results indicate a reservoir age of ca. 1100 yr, comparable to modern estimates. A significantly higher reservoir age (ca. 1900 yr) is obtained for the early Holocene, which suggests a major change in the Southern Ocean circulation at that time.

Radium as internal tracer of Mediterranean Outflow Water

Radium-226 and 228Ra data are presented for the Western Mediterranean Sea and the NE Atlantic. In the NW Mediterranean Sea, the near-shore surface waters of the Ligurian Current contain much higher 228Ra concentrations than surface waters collected in the open Ligurian seawater. The gradient of 228Ra along a Villefranche - Calvi transect has been used to calculate values of the horizontal eddy diffusion coefficient perpendicular to the flow direction of the Ligurian Current of about 106 cm2 s−1. The observed decay of 228Ra, as the Mediterranean Deep Water flows toward the Gibraltar Straits, is in good agreement with the assumption of a rapid turnover of the Mediterranean basin. Concentration profiles of 226Ra and 228Ra were measured in the Mediterranean Outflow Water (MOW) off the Gibraltar Straits in the NE Atlantic basin at two stations, 36°30′N - 15°35′W and 36°27′N - 10°35′W. These sites showed the hydrographic characteristics of Meddies (MOW which propagates as eddies in the NE Atlantic) at intermediate depth. At the center of the Meddies, around 1000 m depth, the activity of the short half-life 228Ra increases up to 0.9 × 10−2 disintegrations per minute per liter, higher than usually found in North Atlantic Deep Water. This “young” 228Ra results from the contact, a few months earlier, of the MOW with the Spanish continental shelf in the vicinity of the Gibraltar Straits. The decay of 228Ra, used as an internal tracer of water mass motion, leads to a transit time of the Meddy Yseult of less than 1 year since its formation.

10Be in a deep-sea core: implications regarding10Be production changes over the past 420 ka

Abstract The influx of 10 Be into a globigerinid ooze core (CH72-02) from the eastern North Atlantic has been studied. This core contains a depositional record of the first 11 δ 18 O stages covering the last 423 ka. It is shown that the marine deposition of 10 Be is strongly influenced by the sedimentation of clays. Clay particles appear 10 times more efficient than the carbonate component as a carrier in bringing 10 Be to the bottom sediments. In core CH72-02, the deposition rates of 10 Be averaged over each oxygen-isotope stage for the past 11 stages show a scatter of ±40% about the mean value of 6.6 × 10 8 atoms cm −2 ka −1 . However, after correction for changes in lithology, the data show that the production rate of 10 Be over the same period has varied no more than ±25%, and the variations are not systematic in that high or low 10 Be production appear to be associated with either cold or warm climates. On the time scale of this investigation (intervals of ca. 50 ka over the last 420 ka, with resolutions as fine as 10 ka for portions of the record), it is unlikely that the shielding effect of the solar wind has deviated by more than ±25% or the geomagnetic field intensity has deviated by more than a factor of 1.6 from their long-term averages.

Determination and comparison of uranium and radium isotopes activities and activity ratios in samples from some natural water sources in Morocco.

Radiochemical results (238U, 226Ra and 228Ra activities; 234U/238U, 228Ra/226Ra and 226Ra/238U activity ratios) are reported for 42 natural water samples collected from wells, hot mineral springs, rivers, tap water, lakes and irrigation water in 15 Moroccan locations. Results show that 238U activity varies between 4.5 and about 309 mBq l(-1) in wells, 0.6 and 8.5 mBq l(-1) in hot springs, 9.7 and 28 mBq l(-1) in rivers, 2.5 and 16 mBq l(-1) in tap waters and between 6 and 24 mBq l(-1) in lakes. The 234U/238U activity ratio varies in the range 0.87-3.35 in all analyzed water samples except for hot springs where it reaches values higher than 7. Unlike well water, mineral water samples present low 238U activities and high 234U/238U activity ratios and 226Ra activities. The highest activity of radium in mineral water is 150 times higher than the highest activity of 226 Ra found in well water. 226Ra/238U activity ratios are in the ranges 0.07-1.14 in wells, 0.04-0.38 in rivers, 0.04-2.48 in lakes, and 1.79-2115 in springs. The calculated equivalent doses to all the measured activities are inferior to the maximum contaminant levels recommended by the International Commission of Radioprotection and they do not present any risk for public health in Morocco.

Production cross sections of26Al,22Na,7Be from argon and of10Be,7Be from nitrogen: Implications for production rates of26Al and10Be in the atmosphere

Abstract The cross sections for production of7Be(1.2 ± 0.3; 2.6 ± 0.7and5.2 ± 1.4mb),22Na(1.5 ± 0.3; 2.8 ± 0.6and2.8 ± 0.6mb),and26Al(1.9 ± 0.4; 3.0 ± 0.6and3.2 ± 0.7mb) from the spallation of argon at, respectively, 480, 880 and 2980 MeV are measured. In the course of the same irradiation as that of argon, the cross sections for production of7Be(9.0 ± 2.1and9.3 ± 2.1mb)and10Be(1.5 ± 0.4and2.6 ± 0.6mb) from nitrogen at, respectively, 450 and 2950 MeV are also measured. Experimental results are compared with the values calculated from the semiempirical formula of Silberberg and Tsao. The production rates of26Al and10Be in the atmosphere by galactic cosmic rays calculated from the measured cross sections are:(1.1 ± 0.3) × 10−4and(2.1 ± 0.5) × 10−2 atoms cm−2 s−1, respectively. These rates are compared with experimental results. The potential use of26Al/10Be ratio for dating marine sediments and evaluating cosmic dust influx is discussed in the light of these cross section values.

SEDIMENTARY IMPRINT OF MICROCYSTIS AERUGINOSA (CYANOBACTERIA) BLOOMS IN GRANGENT RESERVOIR (LOIRE, FRANCE)1

Analysis of a sediment core taken from the Grangent reservoir in 2004 showed the presence of high concentrations of Microcystis aeruginosa Kütz. colonies at the sediment surface (250 colonies · mL sediment−1) and also at depths of 25–35 cm (2300 colonies·mL sediment−1) and 70 cm (600 colonies · mL sediment−1). Measurements of radioactive isotopes (7Be, 137Cs, and 241Am) along with photographic analysis of the core were used to date the deep layers: the layer located at −30 cm dates from summer 2003, and that located at −70 cm from 1990 to 1991. The physiological and morphological conditions of those benthic colonies were compared with those of planktonic colonies using several techniques (environmental scanning electron microscopy [ESEM], TEM, DNA markers, cellular esterases, and toxins). The ESEM observations showed that, as these colonies age, peripheral cells disappear, with no cells remaining in the mucilage of the deepest colonies (70 cm), an indication of the survival thresholds of these organisms. In the benthic phase, the physiological conditions (enzyme activity, cell division, and intracellular toxins) and ultrastructure (particularly the gas vesicles) of the cells surviving in the heart of the colony are comparable to those of the planktonic form, with all the potential needed for growth. Maintaining cellular integrity requires a process that can provide sufficient energy and is expressed in the reduced, but still existing, enzymatic activity that we measured, which is equivalent to a quiescent state.