Rock-Magnetic Properties of Pleistocene Passive Margin sediments: Environmental Change and Diagenesis offshore New Jersey

Abstract

Downhole changes in the concentration, grain size, and mineralogy of the magnetic components of Holocene through upper Pleistocene continental slope sediments (Ocean Drilling Program Holes 903A and 904A) were analyzed to judge the primary detrital vs. the diagenetic origin of the magnetic signal. Natural remanent magnetization (NRM), magnetic susceptibility (K), anhysteretic remanent magnetization (ARM), isothermal remanent magnetization (IRM), and additional hysteresis measurements were performed for densely sampled intervals (ca. every 15 cm). Single-domain through small pseudosingle-domain magnetite is identified as the dominant remanence carrying mineral. However, relative changes of the rock-magnetic properties on a sample-by-sample basis appear to be more instructive in detecting diagenetic overprints of the signal than are determinations of absolute grain sizes. Major downhole changes in the rock-magnetic properties are dominated by the original detrital signal and clearly record differences of glacial and interglacial modes of deposition. Postdepositional dissolution and the authigenic growth of magnetite modulate the detrital magnetic mineralogy dominantly in well-defined sections, which represent the contact of unconformably deposited sediment packages. This diagenetic alteration of the magnetic signal is restricted to the finer ferrimagnetic fraction and can be detected by analyzing the grain selectivity of the ARM acquisition process. The alteration of the magnetic mineralogy occurred repeatedly during sediment diagenesis.

The isolation of diagenetic groups in marine sediments using fuzzy c -means cluster analyses

“…total 'fuzziness'). Frequently used q values range between 1.5 and three; empirically, the best results are obtained using q = 1.5 (Vriend et al 1988;Dekkers et al 1994;Urbat et al 1997). …”

“…Conversely, the separation of different genetic components in the magnetic signal may yield additional information on the evolution of the sediments since deposition (e.g. Urbat 1995). Untangling the extent of post-depositional processes is hampered by the fact that rock magnetic values of diagenetic and detrital iron oxides, for example, are essentially the same.…”

“…In this paper a combination of these methods will be described, and their merits and limitations will be considered when applied to rock magnetic data. Marine sediment drilled at Ocean Drilling Program (ODP) Site 904, which has previously been successfully interpreted without the employment of cluster methods (Urbat 1995(Urbat , 1996, is used to provide an established background for comparison with the results of the multivariate statistical models using the same URBAT, M., DEKKERS, M. J. 8~ VRIEND, S. P. 1999.…”

“…dataset (Urbat et al 1997). In another example, the merits of cluster techniques are extended into other palaeomagnetic studies.…”

Site 1072

“…In Figure 19, NRM after 20-mT demagnetization, susceptibility of ARM (K arm , volume magnetic susceptibility (K), K arm /K, and median destructive field (MDF) of Hole 1072A are plotted vs. relative peak intensities of pyrite and siderite as determined by XRD analysis (see "Lithostratigraphy" section, this chapter). As with Site 1071 (see "Paleomagnetism" section, "Site 1071" chapter, this volume), the magnetic mineral is assumed to be magnetite, on the basis of rock magnetic results reported from Leg 150 (Urbat, 1996). From the top of the hole to 58 mbsf (Subunit IA/IB), rock magnetic parameters fluctuate.…”