François Guichard

Paleointensity of the geomagnetic field during the last 80,000 years

High-resolution records of the relative paleointensity of the geomagnetic field have been obtained from five marine cores. Three duplicate records were used to estimate the regional coherency of the data within a single area (Tyrrhrenian Sea) while the two others document the field variations in the eastern Mediterranean and the southern Indian Ocean. Careful investigations of distinct rock magnetic parameters have established the downcore uniformity of the sediments in terms of magnetic mineralogy and grain sizes. The time-depth control was provided by oxygen isotopes, and small-scale variations in the deposition rates were constrained by means of tephrachronology. The synthetic curve calculated from the Mediterranean records provides a continuous record of the intensity variations during the last 80,000 years (80 kyr), which correlates well with the sparse volcanic data available for the period 0–40 kyr. The fact that identical behavior is seen in both data sets and that they also compare quite well with results from a core collected in the Pacific Ocean establishes the truly dipolar character of these variations. The dipole field moment is characterized by large-scale changes as shown by the existence of pronounced drops (at 39 and 60 kyr) alternating with periods of higher intensity. The record suggests a periodic nature for these intensity variations; however, the period studied is not sufficiently long to state this conclusively. These results demonstrate the potential of sediments for such studies and constitute a first step towards obtaining a global paleointensity record over a long period of time.

Explosive activity of the South Italian volcanoes during the past 80,000 years as determined by marine tephrochronology

Abstract A chronological reconstruction of the explosive activity of the south Italian volcanoes over the past 80 ka is proposed from the geochemical study of marine tephra, interbedded in deep-sea sediment cores collected in the central Mediterranean Sea. Chronology is derived from detailed oxygen-isotope analyses, and from correlation of the marine tephra with their terrestrial equivalent, dated by the classical radiometric methods. The origin of the ash-layers was determined by major-element analyses. Products of Mount Etna, the island of Pantelleria, the Eolian islands and the Campanian area were studied. On the island of Ischia, two distinct major periods of activity were observed, related to the “Green Tuff Series” from 62 to 50 ka and to the “Citara-Ciglio-Serrara Series” from 42 to 25 ka. In the Campi Flegrei, the periods of major explosive activity occurred between about 40 and 25 ka, referred to as the “Campanian Ignimbrite Series”, and during the past 16 ka, known as the “Neapolitan Yellow tuff and Agnano Series”. Geochemical and chronological investigations of the marine tephra suggest that such larger terrestrial deposits, as the “Green Tuff” of Mount Epomeo on Ischia island, or the “Campanian Ignimbrite” in the Campi Flegrei result from several distinct eruptions. The “Green Tuff” of Mount Epomeo would have originated from at least 2 large eruptions, dated at 55.4 ± 2.2 and 60.3 ± 2.2 ka. The formation of the “Campanian Ignimbrite” would be the result of 5 distinct eruptions, the most important occurring at 36, 33.5 and 26.9 ka. Finally, the geographical shift in the activity, as is observed particularly by the decrease in the explosive activity in the past 15 ka on the island of Ischia, matching the increase in the Campi Flegrei, suggests modifications of the tectonic context of the Campanian area.

A Late Quaternary palaeoecological record from the Banda Sea, Indonesia: patterns of vegetation, climate and biomass burning in Indonesia and northern Australia

Abstract Palynological, charcoal, elemental carbon and elemental carbon stable isotope analyses on Banda Sea core SHI-9014 provide a detailed regional vegetation, fire and climate history for the Banda Sea area (eastern Indonesia and northern Australia) through the last 170–180,000 years. Reliable chronostratigraphic control is provided by a detailed oxygen isotope record and, in the younger part of the sequence, by radiocarbon dates. The results indicate that during the last two glacial periods (particularly stages 6, 4, and 2) drier climates prevailed in both eastern Indonesia and northern Australia and lower montane forests dominated by Fagaceae expanded, indicating cooler climatic condition in eastern Indonesia. High charcoal and elemental carbon values suggest increased burning during these periods. Expansion of tropical lowland rainforests, humid mid and upper montane forests, fern and woodland cover occurred in the interglacial periods (stage 5 and the Holocene), indicating warm and humid conditions. The Banda Sea record indicates that before 37,000 yr B.P. Dipterocarpaceae formed an important part of the tropical lowland vegetation of eastern Indonesia. Its subsequent demise coincides with an increase in disturbance pollen indicators, the replacement of Eucalyptus woodlands and open forests by open grassland vegetation and higher burning levels. It is likely that these changes relate to an increase in human impact on the landscape.

Tyrrhenian Sea tephrochronology of the oxygen isotope record for the past 60,000 years

Abstract A chronology of the marine oxygen isotope record of the past 60 ka is proposed by correlating eight marine ash-layers with terrestrial volcanic deposits, dated by the 14C and/or K Ar methods. Detailed oxygen isotope analyses were made on the planktonic foraminifera of three cores collected in the central Tyrrhenian Sea. The origin of the ash-layers was determined by major and trace elements analyses. Then the major pyroclastic terrestrial deposits were sampled, and analysed in order to provide land-sea correlations. Two ash-layers occurred during the warm event that followed the first step of the deglaciation, and which was dated at 12.6±0.5 ka B.P. Another ash-layer is present near the last glacial maximum that we have dated at 17.5±0.3 ka B.P. During isotopic stage 3, five ash-layers were recognized and dated, which allowed us to assign ages to the isotopically light oxygen peaks (warmer interstadials) at 31±1.5 ka and 33.4±1.6 ka (Denekamp), at 37.5±2 ka (Hengelo), and successively at 49±2 ka, 52.4±2.2 ka, 57±2.2 ka and 60.5±2.2 ka (Moershoofd complex).

Pleistocene forced regressions and tidal sand ridges in the East China Sea

Abstract Tidal sand ridges are common features on modern shelves but only few examples of such preserved sand bodies are described in Pleistocene deposits. In the stratigraphic record, some sand bodies encased in shales, previously interpreted as sand ridges, have been reinterpreted as shoreface deposits. More than 5000 km of high-resolution seismic data from the East China Sea, correlated to geotechnical boreholes and shallow cores, demonstrate the potential of sand ridge preservation and allow reconstruction of the depositional history of Pleistocene fourth order (100-kyr) depositional sequences. A high subsidence rate of about 300 m/Myr allows the preservation of three elementary sedimentary facies, constitutive of a ‘motif’ which was repeated during glacio-eustatic cycles. They consist of (1) regressive marine prodeltaic prograding wedges, (2) estuarine and continental (deltaic) facies, and (3) transgressive sand ridges, similar in shape and orientation to modern sand ridges. Major discontinuities, traceable over the entire outer continental shelf along distances of hundreds of kilometers, are transgressive and regressive surfaces of marine erosion, whereas sequence boundaries formed by fluvial erosion are difficult to identify on this low-gradient shelf. Because of the asymmetry of the Pleistocene glacio-eustatic cycles, most of the preserved sedimentary record (with the exception of sand ridges) corresponds to forced regressive deposits (deposits that formed during a seaward shift of the shoreline due to relative sea-level lowering).

The Blake geomagnetic event: transition geometry, dynamical characteristics and geomagnetic significance.

Abstract Two records of the Blake geomagnetic event have been obtained from marine cores in the Mediterranean. The upper bound for duration of the event, calculated from accurate oxygen isotope stratigraphy and/or tephrochronology, is ∼ 4000 years. One record is very detailed with 70 transitional directions and allows some characteristics of the event to be precisely retrieved. The directional changes do not occur at constant speed, rather a stop-and-go behaviour is observed as in detailed records of reversals. The virtual geomagnetic pole (VGP) path, confined to two bands of longitude over the Americas and antipodal to them, is virtually identical to the only other path of this event obtained from lacustrine sediments in the western USA, suggesting that a dipolar component may be present in the transitional field during the Blake event. The two bands of longitude coincide with those of many recent reversals, suggesting that the physical processes leading to reversals or events are basically the same and that the same mechanism is statistically observed over long periods of time.

The Blake magnetic polarity episode in cores from the Mediterranean Sea

Abstract Palaeomagnetic and oxygen isotope studies on samples taken from five cores in the Mediterranean area are described. Directions of the stable remanent magnetisation show the presence of an anomalous (reversed) zone during the transition between the oxygen isotope substages 5e and 5d. This position corresponds to the age of about 117 kyr B.P. and thus to the Blake polarity episode. The duration of this episode is estimated to be about 6000 years. The study of the anisotropy of the magnetic susceptibility shows no indication of sedimentological disturbances in recorded sections. The Blake episode therefore represents a stratigraphic marker which may be used to link the continental and marine palaeoclimatic and sedimentary records.

Rare earths in barites: distribution and effects on aqueous partitioning

Abstract A large variety of barites collected from marine and continental environments was analyzed by neutron activation for the rare-earth elements (REE) La, Ce, Sm, Eu and Dy. Relative to chondrites, all barites show a decrease of the lighter REE from La toward Eu. The abundance and distribution of rare earths in barites show a distinction of barite types. Deep-sea barites have large REE concentrations as do other authigenic deep-sea minerals and display the chondrite normalized Eu minimum, but not the negative Ce anomaly, of sea water. Other barites, mostly on land, some hydrothermal, and others of shallow marine origin, display lower total Ree concentrations. Chondrite normalized positive Eu anomalies are displayed by those varieties of reducing sedimentary and metamorphic origin. Distribution of REE in barite can be attributed both to crystallographic constraints of substitution, and to solution complexing of REE in the precipitating medium. Plots of rare earth partitioning versus effective ion size suggest that the decreasing enrichment toward Eu for all barite types is caused by crystallographic constraints due to contraction of the substituting REE ion sizes relative to the size of the host Ba ion. Solution effects on REE substitution in barite can be evaluated by writing solid solution distribution equations based on mass action of REE and Ba sulfates and the lanthanide (Ln) solution species Ln (CO3)−54), LnSO+4, LnCl+2 and LnF+2. Under normal sea water conditions, solution complexing plays a minor role. However, increased alkalinities of reducing sediments and increased brine chlorinities could cause significant complexing and deplete REE heavier than Eu. Besides Dy in barites, this could be true for aqueous precipitation of REE in general.

Low-latitude hydrological cycle and rapid climate changes during the last deglaciation

Sea surface temperature and oxygen isotopic records from two well-dated Indian Ocean cores covering the last deglaciation show the occurrence of two periods of increased salinity along the route of warm surface water transport from the Indian to the Atlantic Ocean, one between 18 and 14.5 ka and the other during the Younger Dryas. Our results imply that during these periods, salt accumulated in the tropical Atlantic, creating favorable conditions for an abrupt resumption of the thermohaline circulation and abrupt northern hemisphere warming. Furthermore, we suggest that the observed pattern of millennial climate variability during the last glacial and deglaciation resulted from the interaction between the relatively slow rhythm of expansion and decay of the northern hemisphere ice sheets, and El Nino–Southern Oscillation variability, through changes in the position of the Intertropical Convergence Zone. This interaction generated an oscillator with millennial time response that operated at times of sufficient northern hemisphere ice sheets extent.

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.