6. Data Report: High-resolution Study of Magnetic Properties

High-resolution magnetic measurements were performed on sediments recovered from Sites 966, 967, and 969 during Leg 160 of the Ocean Drilling Program to the eastern Mediterranean Sea. Measurements of u-channel samples were made at 1-cm intervals from nearly complete composite stratigraphic sequences at each site. The measurements are integrated over stratigraphic intervals of 4.5 cm, which allows high spatial resolution of sediment magnetic properties, including detailed characterization of variations in magnetic mineralogy, concentration, and grain size, as well as paleomagnetic determinations. We present u-channel magnetic data for more than 100 m of core from three eastern Mediterranean sites. Cyclic variations are observed over short and long stratigraphic intervals, which reflect variations in magnetic concentration and mineralogy of the sediments. The high-frequency variability is related to variations in nonsteady state diagenesis that results from cyclic deposition of organic-rich sapropels. This suggests that magnetic parameters may be an excellent proxy for diagenesis-related cyclicity in these sediments. Longer term changes may reflect significant variations in the climatic evolution of the eastern Mediterranean Sea. INTRODUCTION Extreme changes in environmental conditions in the eastern Mediterranean Sea are marked by organic carbon–rich sediment layers that are intercalated within lower Pliocene to Holocene hemipelagic marine sediments. These so-called sapropels have been used, along with carbonate cycles, to develop time series that have been tied to variations in the precessional index of the Earth’s orbit, which has allowed development of a detailed Pliocene–Pleistocene time scale for marine sequences that now crop out above sea level in the Mediterranean region (Hilgen, 1991a, 1991b; Lourens et al., 1996). Sediments recovered during Ocean Drilling Program (ODP) Leg 160 preserve significantly more sapropels than are found in the land sections. In addition, other oxidized and/or bioturbated sapropels are often discernible only as “ghosts” in Leg 160 sediments (Emeis, Robertson, Richter, et al., 1996). Because of the significant differences in the number of sapropels that are preserved in different records and the difficulty in developing magnetic stratigraphies from Mediterranean marine sediments (Richter et al., Chap. 5, this volume), correlation between marine and on-shore sequences has had limited success. A major finding of the shipboard paleomagnetic research during Leg 160 was that the sapropels are strongly magnetized relative to the surrounding sediments. Diagenetically reduced gray sediments that underlie the sapropels are typically extremely weakly magnetized, as would be expected if detrital magnetic grains in these sediments had been subjected to dissolution (Roberts et al., 1996, in press). Because magnetic properties may allow an increased understanding of sapropel diagenesis (Roberts et al., in press) and because they may provide a means of identifying oxidized sapropels that may not be easily distinguished using other means (van Santvoort et al., in press), a high-resolution study of u-channel samples was undertaken to doc1Robertson, A.H.F., Emeis, K.-C., Richter, C., and Camerlenghi, A. (Eds.), 1998. Proc. ODP, Sci. Results, 160: College Station, TX (Ocean Drilling Program). 2Department of Geology, University of Florida, 1112 Turlington Hall, Gainesville, FL 32611, U.S.A. [email protected] 3Ocean Drilling Program, 1000 Discovery Drive, Texas A&M Research Park, College Station, TX 77845-9547, U.S.A. 4Department of Oceanography, University of Southampton, Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, United Kingdom. ument and identify the magnetic signature of sapropeland nonsapropel-bearing intervals. Initial results of this study are reported here. METHODS Data were acquired from u-channel samples (Tauxe et al., 1983) that were measured on a narrow-access 2G Enterprises cryogenic magnetometer (Weeks et al., 1993; Nagy and Valet, 1993) at the University of California, Davis. U-channel samples are collected by pushing rigid, u-shaped plastic liners (2 × 2 cm cross section, up to 1.5 m in length) into the archive halves of core sections. The sample is freed from the core by guiding a nylon fishing line under the uchannel. A tightly fitting cap is then placed on the u-channel sample and the ends sealed to minimize dehydration of the sediment. Magnetic measurements on u-channel samples were made at 1-cm intervals; however, the half-width of the response function of the magnetometer pick-up coils (4.5 cm; Weeks et al., 1993) is such that adjacent measurements are not independent, and the data are smoothed over a 4.5-cm measurement window. This smoothing is more than would occur in a continuous row of discrete samples; however, this disadvantage is offset by the decreased sediment deformation in uchannel samples and the significantly greater speed with which the measurements can be made (u-channel measurements are at least 20 times faster). U-channel samples were taken from the archive halves of core sections from throughout the composite sequences of Sites 964, 966, and 967 and from Hole 969B. Because of time limitations for data processing, only data from Sites 966, 967, and 969 are reported here (see Tables 1–3, CD-ROM, back pocket, this volume). Before uchannel sampling, archive halves of core sections were measured aboard the JOIDES Resolution using a 2G Enterprises pass-through cryogenic magnetometer at 10-cm intervals, after alternating field (AF) demagnetization at up to 25 mT. This wide-access magnetometer has a 15-cm smoothing window (Roberts et al., 1996). Because the archive halves had already been partially demagnetized, the natural remanent magnetization (NRM) was measured for all u-channel samples after AF demagnetization at 30, 40, 50, 60, and 70 mT (Tables 1–3, CD-ROM, back pocket, this volume). An anhysteretic re-