Detailed paleomagnetic and rock magnetic variability within three high-resolution study intervals from Site 1233

Abstract

We carried out a detailed rock magnetic and paleomagnetic study of deep-sea sediments from selected intervals of Site 1233 in order to assess whether they contain reproducible evidence for submillennial-scale environmental, climatic, and geomagnetic field variability. Three 1.5-m sediment intervals from oxygen isotope Stages 1, 2, and 3 were sampled using continuous U-channels; replicate sediment sequences were collected from two or three holes in each interval to test for reproducibility. Rock magnetic and paleomagnetic measurements were made at 1cm intervals. Rock magnetic results identify distinctive centennial (~150–300 yr)and millennial-scale variability that is probably continuous over the entire Site 1233 record. They also show evidence for intermittent multidecadal-scale rock magnetic variability. The rock magnetic variability allows us to modify the meters composite depth (mcd) correlation in each of the three high-resolution intervals to permit correlations between holes at ~5-cm resolution. Paleomagnetic results indicate that there is also a reproducible pattern of paleomagnetic secular variation at the centennial to millennial scale. The centennial-scale variability is dominated by ~5°–10° oscillations in both inclination and declination with an average interval of ~200–300 yr. Deconvolution studies and paleomagnetic secular variation (PSV) correlations between holes both suggest that the PSVs are real and not caused by similar-scale rock magnetic variability. The paleomagnetic results also document dis1Lund, S.P., Stoner, J., Channell, J., and Lamy, F., 2007. Detailed paleomagnetic and rock magnetic variability within three highresolution study intervals from Site 1233. In Tiedemann, R., Mix, A.C., Richter, C., and Ruddiman, W.F. (Eds.), Proc. ODP, Sci. Results, 202: College Station, TX (Ocean Drilling Program), 1–30. doi:10.2973/ odp.proc.sr.202.212.2007 2Department of Earth Sciences, University of Southern California, Los Angeles CA 90089-0740, USA. slund@usc.edu 3School of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis OR 97331-5503, USA. 4Department of Geological Sciences, University of Florida, Gainesville FL 32611, USA. 5GeoForschungsZentrum, Telegrafenberg, 14473, Potsdam, Germany. Initial receipt: 22 March 2005 Acceptance: 23 February 2006 Web publication: 18 January 2007 Ms 202SR-212 S.P. LUND ET AL. PALEOMAGNETIC AND ROCK MAGNETIC VARIABILITY 2 tinctive, larger-amplitude multicentennialand millennial-scale directional variability that is consistent with preliminary shipboard paleomagnetic studies. INTRODUCTION During Ocean Drilling Program (ODP) Leg 202, a variety of automated, quantitative shipboard measurements were made on the deepsea sediment cores recovered from each site as part of the routine initial scientific analysis (Mix, Tiedemann, Blum, et al., 2003). Some of these measurements were needed while coring progressed in order to develop a complete composite section (splice) among replicate holes from each site. This assured 100% recovery and sampling of all cored intervals. At Site 1233 (41.0°S, 74.4°W), shipboard measurements noted strong variability at the decimeter to meter scale throughout the sediment sequence, even though the sediments appeared to be generally uniform, hemipelagic, terrigenous-rich sediments. Such variability could result from oscillating regional, environmental, or climatic conditions. Initial shipboard estimates of sediment accumulation rates for Site 1233 were determined in three ways (Mix, Tiedemann, Blum, et al., 2003): (1) the Holocene sediments from site survey Core GeoB-3313-1 (41.0°S, 74.4°W) (Lamy et al., 2002), which has an accelerator mass spectrometry (AMS) radiocarbon chronology (AMS radiocarbon dates on carbonate microfossils), were correlated with the uppermost sediment sequence from Site 1233 using magnetic susceptibility variations; (2) a distinctive magnetic field excursion was identified at ~68 meters composite depth (mcd), which was correlated to the Laschamp Excursion (~41,000 cal. yr before present [BP]); and (3) an initial geomagnetic paleointensity record was developed using shipboard measurements of the sediment natural remanent magnetization (NRM) normalized by magnetic susceptibility; this record was then correlated with a number of other dated paleointensity records from around the world. All three chronostratigraphic estimates suggested that Site 1233 has an average sediment accumulation rate of nearly 150 cm/k.y. for the last ~70,000 yr. On that basis, the decimeterto meter-scale variability should convert to multidecadalto centennial-scale variability. Shipboard discussions focused on whether Site 1233 might reproducibly preserve such high-frequency environmental/climatic variability. It is extremely rare to find long marine records of submillennial-scale environmental/climatic variability, particularly in the Southern Hemisphere. A series of studies were formulated to evaluate this potential. Three intervals were chosen, one each in oxygen isotope Stages 1, 2, and 3. Each chosen interval would be 1.5 m in length, and a series of very high resolution (~1–2 cm) measurements would be made on the sediments from multiple holes in each interval to test for reproducibility. This paper summarizes the rock magnetic and paleomagnetic variability in the three high-resolution intervals. The focus of this study is to determine whether distinctive and reproducible evidence for submillennial-scale environmental/climatic or geomagnetic field variability can be recovered routinely at Site 1233. S.P. LUND ET AL. PALEOMAGNETIC AND ROCK MAGNETIC VARIABILITY 3 STAGE 1 (HOLOCENE) HIGH-RESOLUTION INTERVAL The Stage 1 high-resolution interval is located from 1.5 to 3.0 mcd. Four AMS radiocarbon dates from a neighboring piston core (Lamy et al., 2002; Mix, Tiedemann, Blum, et al. 2003), two of which are shown in Figures F1 and F2, bracket this interval. These dates estimate that the Stage 1 high-resolution interval extends from ~900 to 2000 cal. yr BP with an average sediment accumulation rate of 144 cm/k.y. Two core intervals from Holes 1233B and 1233D were sampled “off splice,” that is using sediments not part of the Site 1233 composite section (splice). We were also able to study the entire splice section for Site 1233 as part of our normal paleomagnetic studies. Thus, we also have paleomagnetic and rock magnetic data for Hole 1233C in this high-resolution interval. All of these intervals were sampled at the ODP Gulf Coast Core Repository (College Station, Texas) as continuous sediment columns using U-channels. U-channels are fairly rigid 2 cm × 2 cm square cross-section plastic tubes, which are as long as the individual core segments, usually 150 cm. The U-channels have one open side, which is pushed into the flat surface of a split sediment core and then carved out to recover a 2 cm × 2 cm × ~150 cm column of sediment. The U-channel open side is then capped to produce a long minicore of sediment that is protected by the U-channel and its plastic cap. All U-channels collected at the repository were measured using an automated 2G cryogenic magnetometer (cryomag) and automated magnetic susceptibility system at the University of Florida (USA). See the “Appendix,” p. 12, for a detailed breakdown of core segments and depths sampled for this study. Methods The NRM of all U-channels was initially measured and then sequentially demagnetized and remeasured in alternating fields (AF) from 25 to 100 mT. An artificial anhysteretic remanent magnetization (ARM) was then applied to each U-channel using a 0.05-mT steady field and 100-mT AF; the initial ARM was measured and then demagnetized/measured sequentially using the same sequence of steps as the NRM. Next, each U-channel was given a saturation isothermal remanent magnetization (SIRM) using a pulsed steady magnetic field of 950 mT; the initial SIRM was measured and then demagnetized/measured sequentially using the same sequence of steps. At some point in the measurement process, usually before initial NRM measurement, the magnetic susceptibility (χ) of each U-channel was also measured. In all cases, the measurements were made at a 1-cm interval, although the instrument response functions of the cryomag and susceptibility system are several centimeters wide. Rock Magnetic Results The overall intensities of the NRM, ARM, SIRM, and χ vary by less than a factor of two, indicating that the sediment magnetic characteristics are quite homogenous. This is corroborated by the homogenous visual character of the sediments (Mix, Tiedemann, Blum, et al., 2003). Even so, there is noticeable intensity variability in each of these parameters that is easily correlated between holes, as illustrated in Figures F1 F1. Stage 1 magnetic susceptibility, p. 13.