Catherine Kissel

North Atlantic palaeointensity stack since 75ka (NAPIS–75) and the duration of the Laschamp event

Six relative palaeointensity records from the north Atlantic Ocean were stacked together to produce a new record for the last 75 kyr (NAPIS–75). Five of these records have been previously correlated at millennial scale and placed on the GISP2 age scale, the sixth record was tied to the others using magnetic susceptibility. From 75 ka the field strength exhibits some oscillations, with a first minimum ca.65 ka, followed by a progressive increase to a broad maximum centred at ca.48 ka. There is then a well–marked low at 40 ka, corresponding to the directional anomaly of the Laschamp event. Another intensity low, observed at ca.34 ka, corresponds in age to the Mono Lake event. After a high at 33 ka and two lows at 30 and 24 ka with a broad maximum between, the field strength seems to slowly increase to the upper limit of the studied interval. In the 10–20 kyr interval some differences exist between individual records, and fine–scale details are not always resolved. In the 20–75 kyr interval, on the other hand, well–resolved millennial–scale features are superimposed to the broader trends. The duration of the Laschamp event, which is recorded directionally in five cores, appears to be about 1500 years, consistent with a recent suggestion on the origin of geomagnetic excursions.

The Tertiary geodynamical evolution of the Aegean arc: a paleomagnetic reconstruction

Abstract The paleomagnetic results obtained in the last 5–6 years from Tertiary formations in the Aegean domain indicate that the Lower Miocene arc was almost rectilinear with an E-W trend and that its curvature has been acquired tectonically in two major phases. During the Middle Miocene a first phase of deformation is characterized by rotations occurring at the two terminations of the arc, clockwise in the west (Epirus) anticlockwise in the east (southeastern Anatolia). A second phase of rotation occurring in the last 5 Ma about a pole situated in the southern Adriatic Sea has affected only the northwestern part. The data also indicate that the Antalya basin has not been involved in the geodynamical evolution of the arc so that the Bey Daglari represent its easternmost unit. A more complex rotational pattern is observed in the internal zones: the large rotations measured in Evia (48°) and Skyros (26°) probably occurred in the last 5 Ma and result from rotation of fault bounded blocks in a shear zone connecting the North Aegean trough to the external arc. In the Izmir region, in the Karaburun peninsula of western Anatolia and in the island of Lesbos significant counterclockwise, clockwise or null rotations of coherent blocks have taken place during the neotectonic extensional regime. Furthermore, the values of the inclination obtained from both sedimentary and volcanic Oligo-Miocene formations are systematically shallower than expected on the basis of a geocentered dipole field. This suggests that the entire studied area has undergone a northward drift of over 1000 km since the Middle Miocene.

LOW-TEMPERATURE MAGNETIC BEHAVIOR OF TITANOMAGNETITES

Titanium substitution has a significant effect on the temperature dependence (5–300 K) of low-temperature saturation remanence and of initial susceptibility in synthetic and natural titanomagnetite (Fe3−xTixO4, 0≤x 0.4, and electronic and lattice relaxation phenomena over the entire titanomagnetite compositional range. These features produce diagnostic magnetic behavior useful for titanomagnetite identification in natural samples. Moreover, the apparent similarities between the thermal decay of low-temperature remanence of titanomagnetite with pyrrhotite, pure magnetite, and superparamagnetic phases can lead to complications in interpreting low-temperature remanence data when titanomagnetite is present.

RAPID CLIMATIC VARIATIONS DURING MARINE ISOTOPIC STAGE 3 : MAGNETIC ANALYSIS OF SEDIMENTS FROM NORDIC SEAS AND NORTH ATLANTIC

Abstract The bulk magnetic parameters of seven deep-sea cores distributed from the Nordic Seas (67°N) to the North Atlantic as far south as the Bermuda Rise (33°N) exhibit short-term variations which correlate with rapid climatic changes during marine isotopic stage 3 (MIS3). The magnetic mineralogy is uniformly dominated by well sorted low Ti-content magnetites indicating that these variations are due to variations in the relative amount of magnetic minerals. Because the magnetic minerals predominantly originate from one common source area (the Nordic basaltic province), these changes arise from changes in the efficiency of the transport of the magnetic particles by deep currents from the source to the site of deposition. These results therefore show that the fast climatic changes are related to coeval fast changes in the strength of the deep-sea circulation. The latter was active/reduced during the interstadials/stadials and Heinrich events transporting the magnetic particles from the Norwegian Sea into the North Atlantic ocean along a path similar to the present path of the NADW. It is tentatively suggested that the Faeroe-Shetland Channel and the Denmark Strait were the only two active paths for the overflow water during MIS3. The presence of magnetic oscillations in the Bermuda Rise core in phase with those from the North Atlantic indicates that the activity of the southern Newfoundland Basin gyre was linked to that of the NADW during MIS3.

Erosional history of the Himalayan and Burman ranges during the last two glacial–interglacial cycles

Abstract The results of a clay mineralogy study combined with major element geochemistry, strontium, neodymium and oxygen isotopes, and 14C AMS stratigraphy are reported for deep-sea gravity cores located in the Bay of Bengal (MD77-180) and the Andaman Sea (MD77-169). eNd(0) and 87Sr/86Sr from Holocene and last glacial maximum (LGM) sediments of fifteen other cores have also been investigated to identify sediment sources and to estimate oceanic sedimentary transport. The data show the contribution of three sources: (1) Ganges/Brahmaputra rivers; (2) Irrawaddy River; and (3) sediment derived from the western part of the Indo–Burman ranges. The dispersion of the detrital material issuing from these sources has been constrained by the geographic distribution of Nd and Sr isotopic ratios. The LGM sediments are characterized by a significant increase of 87Sr/86Sr, small changes in eNd(0), and a general decrease of smectite/(illite + chlorite), which together imply a decrease in weathering intensity. The increased 87Sr/86Sr are attributed to a decrease in chemical weathering, which should release preferentially 87Sr during weathering processes. In the Andaman Sea sediments, smectite/(illite + chlorite) and kaolinite/quartz ratios combined with a chemical index of alteration (CIA*) indicate that the weathering intensity of the Irrawaddy River basin is mainly controlled by the summer monsoon rainfall intensity. The wet summer monsoons increase vegetation cover in the plains and favor soil development by the production of pedogenic clays (smectite and kaolinite).

High-resolution record of the Upper Olduvai transition from Po Valley (Italy) sediments: support for dipolar transition geometry?

Abstract A detailed record of the Upper Olduvai polarity transition, composed of > 100 transitional directions, has been obtained from the Crostolo section in northern Italy. A careful examination of the rock magnetic properties of the sediments using standard paleomagnetic techniques, thermomagnetic, scanning electron microscopy, microprobe, X-ray diffraction and Mossbauer analysis, shows that greigite is the main magnetic carrier of the remanence. The correlation of a shift in the magnetic record to a small-scale sedimentary feature observed in the section indicates that the magnetization was acquired at deposition, or very shortly after. The virtual geomagnetic pole (VGP) path is largely confined along a great circle over North and South America ∼ 90° west of the site and consists of three stages: first, the VGP moves to southern mid-latitudes, then, after a period of standstill, it comes back to almost true north, and, finally, the south pole is reached in a third step. Although different from a record from the southern Indian Ocean, this path is virtually identical to those obtained for the same transition from North Pacific deep-sea cores, and partly coincides with a North Atlantic record, which suggests that a dipolar component may be present in the transitional field during the Upper Olduvai reversal. A review of the recently obtained records of various transitions shows that in more than two-thirds of the cases the VGP paths are similarly confined along a meridian over the Americas or antipodal to them, irrespective of the sampling site and of the sense of the transition. Although not deterministic, this tendency suggests that a similar dipolar component might be present in the transitional fields of other reversals of different ages.

Geomagnetic paleointensity and environmental record from Labrador Sea core MD95-2024: global marine sediment and ice core chronostratigraphy for the last 110 kyr

Piston core MD95-2024 from the Labrador Rise provides a continuous record of rapidly deposited detrital layers denoting Laurentide ice sheet (LIS) instability. The core also provides a high-resolution record of geomagnetic paleointensity, that is consistent with, but at higher temporal resolution than previous Labrador Sea records. Correlation to the Greenland Summit ice cores (GRIP/GISP2) is achieved by assuming that Labrador Sea detrital layers correspond to cold stadials in the ice cores. This allows a GISP2 official chronology to be placed on MD95-2024 which is consistent with the inverse correlation between paleointensity and the flux of cosmogenic isotopes ( 10 Be and 36 Cl) in Greenland ice cores. Synchronous millennial scale variability observed from the MD95-2024 paleointensity

Changes in the carbon cycle during the last deglaciation as indicated by the comparison of 10Be and 14C records

The variations in atmospheric radiocarbon (C-14) concentration during the last 50 000 years can be attributed to changes in the C-14 production rate (due to changes in solar activity, the geomagnetic field and/or interstellar galactic cosmic ray flux) and to changes in the global carbon cycle. The relative contributions of these processes is the subject of current debate. Although the discrepancies between the various reconstructions of the past atmospheric radiocarbon concentration increase with age, the relatively good agreement over the last 25 000 years allows a quantitative discussion of the causes of the observed C-14 variations for this period. Using Be-10 measurements from Greenland Summit ice cores, we show that, in addition to solar and geomagnetically induced production rate changes, significant changes in the carbon cycle have to be considered to explain the measured C-14 concentrations. There is evidence that these changes are connected to: (1) global deglaciation and (2) climate changes in the North Atlantic region on centennial to millennial time scales related to changes in the ocean circulation. Differences between Be-10 and geomagnetic field records, however, suggest that uncertainties of about 20% still exist in determinations of past changes in the C-14 production rate

Chlorine-36 evidence for the Mono Lake event in the Summit GRIP ice core

Abstract A distinct peak has been discovered in the 36 Cl data from the GRIP ice core between the Dansgaard Oeschger (D–O) events 6 and 7 at approximately 32 kyr BP. This peak can be attributed to a minimum of the geomagnetic dipole field associated with the Mono Lake event. Since the 36 Cl peak reflects a higher production rate of all cosmogenic radionuclides, it has an impact on the 14 C dating of the last ice age. Furthermore, it provides an additional time marker similar to a peak found earlier corresponding to the Laschamp event at approximately 39 kyr BP.

South Atlantic and North Atlantic geomagnetic paleointensity stacks (0–80 ka): implications for inter-hemispheric correlation

Recent coring of high-deposition-rate Atlantic sediments has led to the development of North Atlantic (NAPIS) and South Atlantic (SAPIS) geomagnetic paleointensity stacks. The SAPIS stack comprises five records from the sub-Antarctic South Atlantic (41–471S, 6–101E). Four are from piston cores collected during the Ocean Drilling Program (ODP) Leg 177 site survey cruise (Earth Planet. Sci. Lett. 175 (2000) 145). The other is ODP Site 1089 (Leg 177) (J. Geophys. Res., 2001, submitted for publication). All but one (4-PC03) of the SAPIS cores have oxygen isotope records which are used in conjunction with lithologic and geomagnetic variability to derive an optimized correlation to Site 1089. The Site 1089 age model is derived by mapping the benthic and planktic isotope data to those from nearby core RC11-83 which has 14 calibrated radiocarbon ages in the 11–41 ka interval. Below this level, the chronology is derived by matching the Site 1089 benthic oxygen isotope data to the SPECMAP stack. The NAPIS stack comprises six records from a wide area of the North Atlantic (Philos. Trans. R. Soc. Ser. A 358 (2000) 1009). Correlation between cores was based on marine isotopic stage boundaries, augmented by millennial-scale features (Heinrich events and fluctuations in concentration-dependent magnetic parameters). NAPIS was placed on the GISP2 chronology, using the marine to ice-core oxygen isotope correlation proposed by Voelker et al. (Radiocarbon 40 (1998) 517) for one of the NAPIS cores. In the resulting age model, the LaschampEvent, recorded in five of six cores, falls in a very narrow ( o1 kyr) age range and coincides with the 10 Be and 36 Cl peak measured in the ice cores. Comparison of the two stacks, placed on their own independent age models, indicates that common millennial scale paleointensity features are preserved. Although more work is needed to define the ‘‘true’’ global content of the paleointensity record as well as the precise age of many features, it is readily apparent that paleointensity can provide a global correlation tool at a resolution unattainable from isotope data alone. r 2002 Published by Elsevier Science Ltd.