Relative geomagnetic intensity during the last 4 m.y. from the equatorial Pacific

A complete sequence of relative geomagnetic field intensity was obtained for the last 4 m.y. from multiple holes drilled at Sites 848, 851, and 852 (Leg 138) in the east equatorial Pacific Ocean. The time vs. depth correlation has been derived by tuning the gamma-ray attenuation porosity evaluator (GRAPE) density record (and, in some cases, the susceptibility) to the insolation curve, yielding one calibration point every 25,000 yr. Relative paleointensities were determined after normalizing the remanent magnetization of U-channels, single samples, and long core measurements by anhysteretic remanent magnetization (ARM) and low-field susceptibility (K). Identical results are obtained with every technique. The composite records obtained at the three sites are very similar within their temporal resolution, which varies as a function of the deposition rates and the different sampling methods. The most noticeable features are large decreases in field intensity accompanying the field reversals. The process is clearly asymmetrical with a long-term decrease before a transition and a rapid field recovery immediately after, thereby suggesting two different processes. Once the field has reached a maximum, it starts relaxing until the occurrence of the next reversal. Thus, the duration of the polarity intervals appears to be controlled by the field regeneration prevailing during very short episodes after major geomagnetic events, which appear themselves to result from a progressive degradation of the field. Shorter intensity minima observed during the last 2 m.y. are correlated to geomagnetic excursions and/or short events already noticed in other data sets. This record provides a framework for a stratigraphy based on the intensity variations of the geomagnetic field. INTRODUCTION Most studies related to geomagnetic field variations in the past have focused on the directional changes that prevailed during reversals, excursions, and paleosecular variation. Therefore, much more is known about shortand long-term variations in field direction than in paleointensity. This is somewhat paradoxical as variations in field intensity tell us more than any other parameter about the evolution of processes governing the geodynamo. The main reason is that determinations of paleointensity are difficult and often yield unreliable results. Until recently, studies in this field were mostly obtained from archeomagnetic and volcanic materials. Archeological objects obviously are restricted in time and space. Lava flows give accurate, but discontinuous, readings for young as well as old rocks. So far, we have no other information about long-term changes in field intensity than a few scarce volcanic records. New hope emerged recently with the publication of detailed and continuous records of relative paleointensity from sedimentary sequences (Constable, 1985; Constable and Tauxe, 1987; Tauxe and Valet, 1989; Tauxe and Wu, 1990; Trie et al., 1992). Indeed, several studies (Tauxe and Wu, 1990; Meynadier et al., 1992) showed that a good agreement existed among records from distinct sites, as well as an overall coherence among relative paleointensity, archeomagnetic, and volcanic data for the period covering the last hundred thousand years. Four records spanning a part or the entire Brunhes Chron have also been published (Kent and Opdyke, 1977; Tauxe and Wu, 1990). However, the length of these records does not allow examination of field intensity behavior across the last reversals. Dominant frequencies have been observed within the last 350 thousand years (k.y.), but there is no clear indication of whether they are truly connected to underlying processes that drive the geodynamo. Pisias, N.G. , Mayer, L.A. , Janecek, T.R. , Palmer-Julson, A. , and van Andel, T.H. (Eds.), 1995. Proc. ODP, Sci. Results, 138: College Station, TX (Ocean Drilling Program). 2 Institut de Physique du Globe de Paris, 4 Place Jussieu, 75252 Paris Cedex 05, France. 3 Godwin Laboratory for Quaternary Research, Cambridge University, Free School Lane, Cambridge CB2 3RS, United Kingdom. Among other questions that remain to be answered, is whether orbital parameters of the Earth have consequences on the fluid motions in the fluid core driving the geodynamo. For these reasons, we attempted to obtain a long paleointensity record that spanned the last 3 or 4 Ma. Except for zones having very low deposition rates, which are useless for the present study, conventional piston cores do not provide sequences of sediment long enough to document the field variations during these periods. The Ocean Drilling Program offers the only opportunity for retrieving long sequences of sediment. The sites drilled during Leg 138 were appropriate because they mostly are located close to the equator, with mean deposition rates between 1.5 and 3 cm/k.y. (i.e., large enough to record variations with characteristic times on the order of a few thousand years). Finally, the distribution of the sites, along two parallel latitudinal transects, separated by relatively large distances, was appropriate to average out most of the effects of any possible nondipole components. SAMPLING, CORRELATION, AND DATING Sites 848, 851, and 852 (Fig. 1) were selected for this study on the basis of the magnetostratigraphic results obtained from the shipboard measurements (see Leg 138 Initial Reports volume). The results from Site 848 show the succession of all polarity intervals back to the Gilbert Epoch. Site 851 is characterized by the highest deposition rates with an average value of 1.8 cm/k.y. for the last 4 Ma (Shackleton et al , 1992). Site 852 provided one of the longest records, with excellent magnetostratigraphy, but with resolution lower than that at the other sites by at least a factor of two. Because more than three distinct holes had been drilled at Sites 848 and 851, we were able to sample Cores 1H, 2H, and 3H from Hole 848D and Cores 1H and 2H from Hole 85 ID with U-channels (Tauxe et al, 1983). A total of 657 complementary standard paleomagnetic cubic samples were taken from Cores 138-848C-2Hto3H, 138-848D-4Hto5H, 138-851C-1H, 138-851E2H to 8H, 852D-1H to 7H (1) to fill the gaps between successive cores, (2) to provide independent checks about the spatial resolution of the U-channels, and/or (3) to check the magnetic properties of the sediment (Fig. 2).