Nicolas Thouveny

Rock magnetic detection of distal ice-rafted debries: clue for the identification of Heinrich layers on the Portuguese margin

Abstract High resolution measurements of concentration and grain size dependent magnetic parameters along a sedimentary sequence deposited on the Portuguese margin (38°N) during the last 120 ka allowed the detection of multiple thin layers slightly enriched by a coarse lithogenic fraction. Sizes, mineralogy, shapes and mode of deposition identify the material as ice-rafted detritus (IRD). The four most recent events dated by calibrated radiocarbon ages are contemporaneous with Heinrich events HE1–HE4. Older events can be identified as HE5, HE6 or with minor IRD that occurred during stage 5. The resolution and sensitivity obtained with rock magnetic methods enable the detection of multiple secondary events showing that iceberg delivery was a frequent phenomenon directly linked to temperature variations over ice caps; Heinrich events representing the extreme cases.

Geomagnetic moment variation and paleomagnetic excursions since 400 kyr BP : A stacked record from sedimentary sequences of the Portuguese margin

Abstract A paleomagnetic study was performed in clayey-carbonate sedimentary sequences deposited during the last 400 kyr on the Portuguese margin (Northeast Atlantic Ocean). Declination and inclination of the stable remanent magnetization present recurrent deviations from the mean geomagnetic field direction. The normalized intensity documents a series of relative paleointensity (RPI) lows recognized in other reference records. Three directional anomalies occurring during RPI lows chronologically correspond to the Laschamp excursion (42 kyr BP), the Blake event (115–122 kyr BP) and the Icelandic basin excursion (190 kyr BP). A fourth directional anomaly recorded at 290 kyr BP during another RPI low defines the ‘Portuguese margin excursion’. Four non-excursional RPI lows are recorded at the ages of the Jamaica/Pringle Falls, Mamaku, Calabrian Ridge 1, and Levantine excursions. The RPI record is characterized by a periodicity of ∼100 kyr, paleointensity lows often coinciding with the end of interglacial stages. This record sets the basis of the construction of an authigenic 10Be/9Be record from the same sedimentary sequences [Carcaillet et al., this issue].

Early Pleistocene Presence of Acheulian Hominins in South India

Dates from a site in southeast India imply an early migration of Homo through Eurasia about 1.1 to 1.5 million years ago. South Asia is rich in Lower Paleolithic Acheulian sites. These have been attributed to the Middle Pleistocene on the basis of a small number of dates, with a few older but disputed age estimates. Here, we report new ages from the excavated site of Attirampakkam, where paleomagnetic measurements and direct 26Al/10Be burial dating of stone artifacts now position the earliest Acheulian levels as no younger than 1.07 million years ago (Ma), with a pooled average age of 1.51 ± 0.07 Ma. These results reveal that, during the Early Pleistocene, India was already occupied by hominins fully conversant with an Acheulian technology including handaxes and cleavers among other artifacts. This implies that a spread of bifacial technologies across Asia occurred earlier than previously accepted.

Revised magnetostratigraphy of the Plio-Pleistocene sedimentary sequence of the Olduvai Formation (Tanzania)

Abstract The magnetic stratigraphy of the Plio-Pleistocene sedimentary formation of the Olduvai Gorge (Tanzania), which contains a well known series of prehistoric pebble cultures and hominid remains, has been reinvestigated. Sixty one sites were sampled along 90 metres of stratigraphic section extending from the base of Bed I into the Masek Beds. Palaeo- and rock-magnetic analyses reveal that magnetizations are stable, carried by low coercivity magnetic minerals (titano-magnetites) and are likely to be of depositional origin. A revised magnetostratigraphic scale is proposed. The base of the major normal magnetozone, N1, interpreted as the Olduvai sub-chron, lies ca 6 m below Tuff IA, i.e. significantly below the previously defined position. This revision, associated to 40Ar/39Ar ages recently determined on the Tuffs of Bed I, supports the validity of the astronomical calibration of the geomagnetic polarity time scale for the Olduvai sub-chron. The correlation of the normal magnetozone N2 with Jaramillo (or Jaramillo + Cobb Mountain), is suggested by the discovery of a reverse magnetozone (R3) in the uppermost part of the Pleistocene sequence (Masek Beds), and implies that the fossiliferous sequence of the Olduvai Formation was entirely deposited before the Brunhes/Matuyama boundary.

Extension of the Lac du Bouchet palaeomagnetic record over the last 120,000 years

A high-resolution profile of the palaeomagnetic variations recorded in the upper 20 m of the Lac du Bouchet sedimentary sequence has been reconstructed from several parallel Mackereth and “Livingstone” type cores. Inter-core correlations were established by comparing individual core profiles of magnetic susceptibility. Individual records of AF cleaned directions were then “stretched” to a common depth scale and merged; the mean record was then filtered and smoothed in order to reduce the background noise. Accelerator radiocarbon determinations and pollen analyses show that the sequence covers the last interglacial/glacial cycle (back to ca. 120 kyr B.P.), and allow to perform a depth to time transformation. There is a noted absence of widely divergent directions back to 90 kyr B.P. It is estimated that the recorded signal (a PDRM signal) represents the geomagnetic input signal, attenuated by about 19%. It follows that the excursions reported for the Laschamp and Olby lavas, if they existed at all, must have been of rather short duration. Two durable excursions appear in narrow diameter sections at the base of three independent cores (B, C and D), while a reverse to normal transition is possibly documented in the lowermost meter (core D). This part of the sequence is dated from the recognition of major climatic events related to the 18 O isotopic stages defined in the oceanic record between 116 and 95 kyr. This introduces the possibility of new excursions at ca. 104 and 95 kyr, i.e, immediately after the recovery of normal polarity documenting the end of the Blake event.

A high resolution authigenic 10Be/9Be record of geomagnetic moment variations over the last 300 ka from sedimentary cores of the Portuguese margin.

A high resolution study of authigenic Be isotopes (10Be and 9Be) combined with continuous relative paleointensity records has been performed along the same marine sedimentary sequences from the Portuguese margin (N.E. Atlantic) covering the past 300 ka in order to assess relationships between geomagnetic moment variations and 10Be production rate variations. A carefull examination of the various ways of taking into account environmental disturbing effects on the authigenic 10Be concentration leads to the conclusion that the most reliable proxy of cosmonuclide production rates is presently the authigenic 10Be/9Be ratio. Eight intervals of significant authigenic 10Be/9Be enhancement evidence geomagnetic moment drops related to global paleomagnetic excursions, some being already admitted, others being proposed as new geomagnetic features. Since, contrarily to sedimentary magnetic remanence, the authigenic 10Be/9Be records dipole moment variations without significant acquisition delay, it provides better constraints on their timing. Comparison of 10Be/9Be and benthic δ18O records from the same cores suggests that dipole moment lows preferentially occured during or at the end of interglacial episodes, with a quasi-period of 100 ka.

Rapid ecosystem response to abrupt climate changes during the last glacial period in western Europe, 40-16 ka

We present a high-resolution and independently dated multiproxy lake sediment record from the paleolake at Les Echets in southeastern France that displays synchronous changes in independent limnic and terrestrial ecosystem proxies, in concert with millennial-scale climate oscillations during the last glacial period. Distinct lake-level fluctuations, low lake organic productivity, and open, treeless vegetation indicate cold and dry conditions in response to Heinrich events. Alternating phases of higher and low lake organic productivity, stratified surface waters and long-lasting lake ice cover, decreased or increased catchment erosion, and tree-dominated or herb-dominated vegetation resemble Dansgaard-Oeschger interstadialstadial variability. Transitions between different ecological states occurred in as little as 40–230 yr and seem to have been controlled by the position of the Polar Front. Ecosystem response after 30 ka suggests that local climate conditions became more important. Our results demonstrate that all parts of the terrestrial system responded to the abrupt and dramatic climatic changes associated with Dansgaard-Oeschger and Heinrich events, and that regional factors modulated ecosystem response.

Pollen-based vegetation changes in southern Tanzania during the last 4200 years: climate change and/or human impact

The age-constrained pollen data of a sedimentary sequence from the crater Lake Masoko, southern Tanzania (9‡20PS, 33‡45PE, 770 m), display a continuous record of vegetation for the past 4200 years. This record provides evidence that wetter Zambezian woodlands always occupied this area during the late Holocene, reaching a maximum extent between 2800 and 1650 cal yr BP related to increase in summer monsoon intensity. However, three main episodes of decline have been detected, between 3450 and 2800 cal yr BP, between 1650 and 1450 cal yr BP and from 1200 to 500 cal yr BP, for which a climatic interpretation, decrease in the summer monsoon strength, was preferentially advanced. The first is synchronous with lowstand of many tropical African lakes and, so, mainly induced by increased aridity. In contrast, the abrupt change in the pollen record at 1650^1550 cal yr BP is marked by a large extension of grasslands at the expense of arboreal cover, further by an increase in Ricinus communis and an intensification of burning. It could thus indicate local clearance of vegetation by man. However, at the same time, the decline of montane forest suggests the impact of a more regional change. During the last episode, between 1200 and 500 cal yr BP, dry climatic conditions are inferred from a combination of pollen, diatom and magnetic proxies, although the occurrence of Late Iron Age settlements in the region means that local human interference cannot be excluded. This study illustrates the difficulties in deciphering ecological and anthropological changes from pollen data in African tropical regions. A 2003 Elsevier B.V. All rights reserved.

Climatic and oceanographic changes in the Northeast Atlantic reflected by magnetic properties of sediments deposited on the Portuguese Margin during the last 340 ka

Abstract Rock magnetic parameters measured along two giant piston cores MD95-2040 (40°34′N, 9°51′W) and MD95-2042 (37°47′N, 10°09′W) collected off the Portuguese Margin, related to other proxy-climatic data, have been used to reconstruct magnetic mineralogical changes of, in relation to environmental and climatic conditions over the North Atlantic, Western Europe and Northwest Africa during the last three climatic cycles (since isotope stage 10). Thin discrete layers containing coarse grains of titano-magnetite are associated with events of iceberg discharge during Heinrich events 1–6 [Heinrich, Quat. Res. 29 (1988) 142] that have equivalent events in isotope stages 5–8. Concentrations of fine-grained (Ti-) magnetite and hematite/goethite, varying in phase opposition, are directly linked with alternations of cold and warm climatic periods. Spectral analyses of the rock magnetic signals reveal Milankovitch periods at 100 and 41 ka, confirming the relationship with long-term climatic changes at high latitudes. The nature (Ti-magnetite) and size range of the finest ferrimagnetic fraction as well as its variation, suggest a control by deep currents carrying a colloidal/clayey fraction from remote sources (Iceland, Faeroes, mid-Atlantic Ridge). Variation of hematite/goethite contents is linked with transport by rivers and winds from the neighbouring continent. A tight correlation with the D–O cycles in Greenland ice records confirms that North Atlantic oceanic regimes and continental wind regimes were strongly influenced by millennial scale climatic changes throughout the last 350 ka.

Formation and preservation of greigite (Fe3S4) in sediments from the Santa Barbara Basin: Implications for paleoenvironmental changes during the past 35 ka

Diagenetic processes are known to modify sedimentary records, but they can also reveal important paleoenvironmental changes. Here we investigate variations in sedimentary iron diagenesis and depositional environments for the last 35 ka by analyzing the rock magnetic and geochemical properties of two sediment cores collected in the Santa Barbara Basin (California). In organic-rich sediments, early diagenesis often leads to partial dissolution of detrital iron oxides and stepwise formation of authigenic pyrite (FeS2). The pyritization process takes place following several geochemical pathways, sometimes involving intermediate iron sulfide species such as greigite (Fe3S4). Sedimentary conditions in the basin appear to have recurrently favored preservation of greigite (identified by its magnetic properties) and inhibited its complete transformation into pyrite. The Holocene interval contains a series of centimeter-thick greigite-bearing layers that are associated with large flood deposits, which are known in the basin as ‘‘gray layers.’’ We propose that greigite preservation was enabled in these sediments by their relative enrichment in reactive iron over organic matter and/or hydrogen sulfide (because of their high ratio of terrigenous over organic material), which limited pyritization reactions. Within the glacial deposits, formation and preservation of meter-thick greigite layers occurred in terrigenous-rich and organic-poor sedimentary layers and is proposed to result from a similar diagenetic process to that in the Holocene greigite-bearing layers (dominance of reactive iron over organic matter and/or HS�). The terrigenous enrichments in the glacial greigite-bearing layers are probably related to climatic or sea level changes because they occur at times of massive iceberg releases in the North Atlantic, the so-called Heinrich events.