Jean Louis Reyss

Resolving the ‘opal paradox’ in the Southern Ocean

In the Southern Ocean, high accumulation rates of opal—which forms by precipitation from silica-bearing solutions—have been found in the sediment in spite of low production rates of biogenic silica and carbon in the overlying surface waters. This so-called ‘opal paradox’ is generally attributed to a higher efficiency of opal preservation in the Southern Ocean than elsewhere. Here we report biogenic silica production rates, opal rain rates in the water column and opal sediment burial rates for the Indian Ocean sector of the Southern Ocean, which show that the assumed opal paradox is a result of underestimated opal production rates and overestimated opal accumulation rates. Our data thus demonstrate that the overall preservation efficiency of biogenic opal in this region is substantially lower than previously thought, and that it lies within a factor of two of the global mean. The comparison of our revised opal preservation efficiencies for the Southern Ocean with existing values from the equatorial Pacific Ocean and the North Atlantic Ocean shows that spatial differences in preservation efficiencies are not the primary reason for the differences in sedimentary opal accumulation. The reconciliation of surface production rates and sedimentary accumulation rates may enable the use of biogenic opal in the reconstruction of palaeoproductivity when the factors that affect the Si/C ratio are better understood.

Initial chronology of a recently discovered hydrothermal field at 14°45′N, Mid-Atlantic Ridge

Two expeditions of the ‘Sevmorgeologija’ association (1991–1994) led to the discovery of two new hydrothermal sites on the Mid Atlantic Ridge (MAR), south of the 15°20′ North Fracture Zone, one around 14°45′N and the other around 14°42′N. The northern one, between 14°45′ and 14°45.3′N has been studied in detail. About 12 mounds have been mapped and 3 of them have been sampled using a large hydraulic grab sampler. The largest one is about 200 m long and 200 m wide. When progressively moving up on the slope of an uplifted block of the rift valley floor, the sulphide samples have revealed ages ranging from about 10 ka to 60 ka. The ages were obtained using the 230Th/234U dating method used for chronological studies of diverse hydrothermal fields. The general picture of this lateral location of the samples of different ages provides evidence of a shift in the focus of hydrothermal activity with time. Moreover, there were rejuvenation stages of hydrothermal activity, including black and white smokers.

Growth of a manganese nodule from Peru Basin: A radiochemical anatomy

Abstract Attempts have been made to study the entire growth history of a manganese nodule from the northern part of Peru Basin in the Pacific using radiochemical profiles of 230 Th 232 Th , 227 Th 230 Th , and 10 Be 9 Be . Combined with the observations on Fe-Mn contents and textural variation, the radiochemical data indicate that the nodule grew more or less concentrically throughout most of its existence since it formed 1.5 my ago, receiving Mn from both bottom water and pore water. This condition appeared to have changed about 180 ky ago when the growth became asymmetric in that the top and bottom sides became fixed in their relative positions on the sea floor. Since then, the bottom side accreted with a fast rate of close to 200 mm/my, apparently fueled by the supply of diagenetically remobilized Mn in pore water from the sediment substrate. In the meantime, the top side accumulated at about 6 mm/my, a value which is in the normal range for deep-sea nodules having their Mn supplied from the hydrogenous source.

Actinide-series disequilibrium as a tool to establish the chronology of deep-sea hydrothermal activity

This paper describes the different radiochronological methods used to date geologically recent (i.e., <400,000 years) deep-sea hydrothermal deposits as well as the basic conditions necessary to obtain reliable dates. The limitations of the different techniques also are described. Using measurements of 210PbPb, 228Th228Ra, 230Th234U, 231Pa235U and 228Ra226Ra, the authors have undertaken an exhaustive chronological study of the hydrothermal deposits along the East Pacific Rise, the Mid-Atlantic Ridge, and in some back-arc basins. The objectives of this study were to obtain regional chronologies and to establish a general synthesis on the evolution of the hydrothermal processes at the scale of the mid-oceanic ridge system. Some results obtained by other authors are included in this synthesis. The dependence of the general trends of temporal development of the hydrothermal chimneys, edifices, and fields on their tectonic settings is discussed. This study demonstrates that hydrothermal activity does not represent a regular input of matter to the ocean, and that its pulsed character must be taken into account in all modeling attempts (chemical, biological and tectonic) affected by hydrothermal processes. In areas of rapid spreading, like the East Pacific Rise, recent and fossil deposits are spatially separated. By contrast, at the slow spreading Mid-Atlantic Ridge, fossil and present activity are found in the same location.

6700 yr sedimentary record of climatic and anthropogenic signals in Lake Aydat (French Massif Central)

A 19 m long sedimentary record retrieved in Lake Aydat (French Massif Central) covers the last 6700 yr at a high resolution. A multiproxy approach (density, magnetic susceptibility, XRF, Rock-Eval, pollen and non-pollen palynomorph analyses and a historical archives study) was used to characterise and propose a model of sedimentation. The high deposition rate results from the combined effects of the high suspension load of the river, autochthonous production and the narrow shape of the incised fluvial valley dammed by a lava flow c. 8550 years ago. Two contrasted periods (6700±200 to 3180±90 cal. BP, and 1770±60 cal. BP to present) were characterized. The lower unit (mid Holocene) displays a fine and regular lamination and holds a single, major, flood deposit. This unit is capped by an erosive mass-wasting deposit triggered c. 1770±60 cal. BP. The upper unit (late Holocene) is made of organic-rich and fine-grained faintly laminated sediment, with numerous interbedded flood deposits and diatom blooms. The sedimentation was principally controlled by climatic forcings until c. 1100 cal. BP, accompanied by detrital events linked to human activities around the lake. Then, a more detrital input attested by numerous and recurrent flood deposits can be linked to the intensification of a persistent anthropogenic impact on the catchment. Two phases of lake eutrophication are highlighted: 1200–1130 cal. BP, as a result of increased anthropogenic pressure, and the current phase that could have started c. 150 cal. BP.

TL dating of Upper Palaeolithic sites in the Coa Valley (Portugal)

Abstract In 1992, numerous animal engravings in styles associated with the Upper Palaeolithic (Gravetian to Magdalenian) were discovered along a long stretch of the schist cliffs bordering the Coa Valley (Northern Portugal). This suggested that not only caves and rock-shelters but also open-air Pleistocene sites may have been decorated in this manner more often than previously believed. Attempts to date the engravings themselves (radiocarbon of patination, micro-erosion analysis) were unsatisfactory, yielding contradictory and widely scattered dates. We are now reporting thermoluminescence ages of 15 burnt quartzite pebbles excavated in 1997 at three sites (Cardina, Olga Grande Sul, Quinta da Barca Sul) near the decorated cliffs. The ages of the Cardina and Olga Grande Sul pebbles indicated human presence at the sites between 30 and 27 ka ago, during the Gravetian period. The Quinta da Barca Sul pebbles were 13–10xa0ka old, indicating that humans were also present in the Valley during the Magdalenian period.

A rapid determination of oceanic sedimentation rates by non-destructive gamma-gamma coincidence spectrometry

Abstract A new technique based on gamma-gamma coincidence spectrometry was developed for the determination of oceanic sedimentation rates. The decrease of the ratio 230 Th/ 232 Th as a function of depth in a sediment is measured by the coincident events of gamma rays emitted from the descendants of the two thorium isotopes. The sedimentation rates of two Atlantic equatorial cores and of a North Atlantic core were determined to be 2.8 ± 0.3, 5.2 ± 0.6, and 2.4 ± 0.4 mm (10 3 yr) −1 , respectively, in excellent agreement with those derived from the microfauna. The present method is non-destructive and much more sensitive than single gamma counting. It permits rapid measurement of sedimentation rates for cores whose thorium content is diluted by a high carbonate content.

Nodules and associated sediments in the Madagascar Basin

Abstract Several buried nodule beds have been found in a sediment core sampled in the Madagascar Basin. 14C, 230Th, 231Pa and paleomagnetic measurements on sediments and nodules from several layers in the core show good agreement with data given by other investigators using microfaunal stratigraphic methods. The radiochemical data, on a carbonate-free basis, give low and regular sedimentation rates ( 1 mm 10 3 yr. ). The same measurements performed on nodules from three beds give identical accretion rates of ∼ 1 mm 10 6 yr . The implications of the results are briefly discussed in terms of nodule accretion rates: slow growth rates for which bioturbation is needed to allow nodules to escape from burial or rapid growth rates before burial in the sedimentary column.