HR: 14:40h
AN: PP13B-05    [Abstracts]
TI: Oxygen Isotopic Stratigraphy and Geomagnetic Field Intensity
AU: Meynadier, L
EM: meynad@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4, Place Jussieu, Paris, 75252 France
AU: * Valet, J
EM: valet@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4, Place Jussieu, Paris, 75252 France
AB: Nick Shackleton intellectual leadership in isotope stratigraphy had profound implications for paleoclimatology and paleoceanography. What is not so well enough known is that Nick contributed also to significant advances in studies of variations of the Earth's magnetic field. The first link between the two disciplines was certainly the paper that he produced with Neil Opdyke in 1973. About twenty years later Nick Shackleton obtained a very detailed isotope stratigraphy after analyzing the oxygen isotopes of bulk sediment of two cores from the Somali basin characterized by high resolution records of relative paleointensity for the past 140 kyr. This stratigraphy allowed us to correlate these two records with other independent data from the Mediterranean sea and to propose that the signal which was recorded at the two locations was global and thus of geomagnetic origin. The fast growing database made possible a stacking of the results for the past 200 ka and then for the past 800 kyr. The resulting curve was constructed from 33 paleointensity records and Shackleton's isotopic records were essential in many cases. Indeed without high resolution stratigraphy much information could not be retrieved due to uncertainties in correlating different records. The results revealed the very variable character of the field with large 20 to 60 kyr oscillations and changes in amplitude that can exceed a factor of five, but no apparent periodicity. Short periods of very low intensity occur at more or less regular intervals (roughly every 100 kyr) and correspond with geomagnetic excursions. The next step was to obtain a much longer record that would document the field changes across reversals and during entire polarity intervals. The opportunity was met during ODP Leg 138 with the recovery of beautifully magnetized sequences that covered at least 4 Myr of geomagnetic history. Nick was responsible for correlating sedimentary columns taken at each site. He performed a tremendeous task by orbitally tuning the density variations for thousands of meters of sediment. This allowed us to correlate the paleointensity signals from several holes and to produce the first long dataset with a very accurate time-depth control. Very recently the accumulation of data made it possible to stack records from different oceans for the past 2 Myr and to extract features of field intensity which add significant constraints to the modelling of the geodynamo. Alternatively this curve can be used as a stratigraphic reference, similarly to isotopes records.
DE: 4870 Stable isotopes
DE: 1521 Paleointensity
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting