HR: 1340h
AN: GP43B-0859    [Abstracts]
TI: Authigenic Be-10/Be-9 Recorded in Western Equatorial Pacific Sediments over the Laschamp and Mono Lake Excursions. Implication for Cosmonuclide Production Rates.
AU: * LEDUC, G M
EM: leduc@cerege.fr
AF: CEREGE, EUROPOLE DE L'ARBOIS, BP 80, AIX EN PROVENCE, 13545 France
AB: The timing and the amplitude of geomagnetic moment variations and their direct impact on atmospheric cosmonuclides production rate during the late pleistocene are still debated. The effects of the Laschamp and Mono Lake excursions that occurred during Marine Isotope Stage 3 are particularly important for the radiocarbon chronology. Core MD97-2134 (9.9øS, 144.6øE, 760 m water depth) has been collected on the South New Guinean margin in the vicinity of the Fly river. The chronostratigraphy was established on 13 accelerator mass spectrometer (AMS) C-14 ages on planktonic foraminifera and on the age of the MIS4/3 boundary identified in the oxygen isotopes record. The average sedimentation rate of 25 cm/kyr allows a high resolution record of paleoclimate and paleoenvironmental variations through the 0-60 kyr BP time interval. Paleomagnetic measurements performed on Uchannels led to construct a quasi-continuous record of inclination and relative paleointensity (RPI) of the geomagnetic field. Therefore we have concentrated our sampling strategy for Be isotopes measurements around identified paleomagnetic features : one major directional deviation at 41 kyr BP and three major dipole lows at 32, 37 and 41 kyr BP. After extraction, authigenic Be-10 concentration was measured by AMS facility and normalized by the authigenic Be-9 concentration in order to reconstruct the authigenic Be-10/Be-9 ratio, proxy of the atmospheric Be-10 production rate during the studied interval. Three significant increases of the authigenic Be-10/Be-9 ratio coincide with the 3 RPI decreases between 30 and 45 kyr BP, including both the Laschamp and the Mono Lake events, usually dated at 41 and 32 kyr respectively. However, the strongest Be-10/Be-9 peak is centered at 37 kyr BP. The occurrence of enhanced deposits of atmospheric cosmonuclides between 30 and 40 kyr BP is consistent with the Be-10 flux reconstructed from the Vostok and Greenland (GRIP and GISP2) cores. Although these results may suggest that the VDM minimum associated with the Laschamp event is younger than previously admitted, our interpretation rather introduces a specific response of the cosmonuclide production rate during a significant, though until now undetected, dipole drop between the Laschamp and Mono excursions. If confirmed by other studies, these results should be considered in the reconstruction of cosmonuclides production rate variation, namely for C-14 dating.
DE: 1040 Isotopic composition/chemistry
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
DE: 1513 Geomagnetic excursions
DE: 1521 Paleointensity
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting