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Magnetostratigraphic Importance of Secondary Chemical Remanent Magnetizations

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Magnetostratigraphic Importance of Secondary Chemical Remanent Magnetizations Results of IODP Expedition 313: The History and Impact of Sea-Level Change Offshore New Jersey Themed Issue Magnetostratigraphic importance of secondary chemical remanent magnetizations carried by greigite (Fe3S4) in Miocene sediments, New Jersey shelf (IODP Expedition 313) Andreas Nilsson1, Youn Soo Lee2, Ian Snowball3,4, and Mimi Hill1 1The Geomagnetism Laboratory, School of Environmental Sciences, University of Liverpool, Oxford Street, Liverpool L69 7ZE, UK 2Department of Geology and Geoinformation, Korea Institute of Geoscience and Mineral Resources (KIGAM), 30 Gajeong-dong Yuseong-gu, Daejeon, 305-350, South Korea 3Geophysics, Department of Earth Sciences, Uppsala University, Villavägen 16, 752 36 Uppsala, Sweden 4Department of Geology, Lund University, Sölvegatan 12, 223 62 Lund, Sweden ABSTRACT et al., 1993; Mann et al., 1990; Vasiliev et al., shelf (Oda and Torii, 2004), some samples con- 2008). Kao et al. (2004) suggested that greigite tain evidence for gyroremanent magnetization Paleomagnetic and mineral magnetic preservation is favored by conditions with high (GRM) acquisition, which can be indicative of analyses were carried out on Miocene clays concentrations of reactive iron in fi ne-grained the presence of greigite (Snowball, 1997). from upper unit II at Sites M0027 and M0028 sediments with limited organic carbon supply. In this study, we attempted to identify the recovered during Integrated Ocean Drilling In such cases, the reactive iron exhausts supply main magnetic minerals in the Miocene clay- Program Expedition 313 on the New Jersey of dissolved sulfi de so effectively that the pyriti- rich horizons from Expedition 313. Through shallow shelf. A zone of mixed polarity in the zation process is arrested. detailed rock-magnetic analyses, we investi- lower section of Hole M0028A and dual over- It has been argued that the iron-sulfi de forma- gated the importance of magnetic iron-sulfi de lapping magnetization components in upper tion in continental shelf sediments is restricted diagenesis and assessed the fi delity of the mag- Hole M0027A indicate that the sediments to the upper anoxic ~1 m, where sulfate reduc- netic polarity stratigraphy at the studied site. may have been chemically remagnetized dur- tion occurs, and that chemical remanent magne- ing one or several events. Mineral magnetic tizations (CRMs) acquired by greigite will accu- SITE DESCRIPTION investigations reveal that the magnetization rately record the geomagnetic fi eld with only a is carried by the ferrimagnetic iron-sulfi de relatively small time lag (Roberts and Turner, The New Jersey shelf is a classic passive mar- greigite (Fe3S4), possibly with traces of titano- 1993; Tric et al., 1991). However, as numer- gin, with the main tectonics for the last ~165 m.y. magnetite. We fi nd that several changes in ous later studies have shown (e.g., Florindo dominated by simple thermal sub sidence, sedi- polarity coincide with variations in magnetic and Sagnotti, 1995; Horng et al., 1998; Roberts ment loading, and fl exure (Reynolds et al., 1991; mineral grain size and/or concentration. We and Weaver, 2005), iron-sulfi de diagenesis may Watts and Steckler, 1979). Holes M0027A, interpret these variations as different stages occur at different times after the sediments have M0028A, and M0029A were drilled in shallow of greigite growth, which were triggered been deposited, making it diffi cult to ascertain water depths (35–40 m) along a seaward tran- by changes in pore-water chemistry and/or the time at which the CRM was acquired. These sect (Fig. 1). The recovered sediments ranged upward migration of methane. studies highlight the need for detailed mineral from coarse sand to clay, which were depos- magnetic and/or petrographic analyses in order ited in cyclic nearshore to offshore successions INTRODUCTION to establish the timing of greigite formation from the Late Oligocene to present (Mountain and to evaluate the reliability of the paleomag- et al., 2010). The ferrimagnetic iron-sulfi de greigite (Fe3S4) netic signal. In this study, we concentrated on the clay- is frequently found in lacustrine (e.g., Dell, In 2009, coring associated with Integrated dominated upper part of unit II in Hole M0027A 1972; Frank et al., 2007; Roberts et al., 1996; Ocean Drilling Program (IODP) Expedition 313 (core 64–70; 192–208 m) and Hole M0028A Skinner et al., 1964; Snowball and Thomp- was conducted on the New Jersey shallow shelf (core 2–8; 224–244 m) directly overlying the son, 1988, 1990) and rapidly deposited marine to unravel the sedimentation history controlled m4.1 maximum fl ooding surface (Mountain sediments (e.g., Florindo and Sagnotti, 1995; by eustatic sea-level change. Initial paleomag- et al., 2010). The sequences are interpreted as Horng et al., 1998; Kao et al., 2004; Oda and netic results from Miocene clay-rich horizons being deposited during the middle Miocene in Torii, 2004; Roberts and Turner, 1993; Tric identifi ed several reversal boundaries with an offshore environment at water depths ranging et al., 1991). Greigite can form authigenically in partly stable polarity in between, which suggests from 50 to 100 m (Mountain et al., 2010). anoxic sediments as a precursor to pyrite (FeS2), that magnetostratigraphic interpretations could Mountain et al. (2010) described the upper associated with bacterial degradation of organic potentially be used to develop age models for unit II sediments in M0027A as dark- and pale- matter (Berner, 1984), or through biomineral- the sediments (Mountain et al., 2010). However, gray clay with occasional color banding and ization by magnetotactic bacteria (Bazylizinki similar to a previous study from the New Jersey silty laminae. There is a general increase in bio- Geosphere; June 2013; v. 9; no. 3; p. 510–520; doi:10.1130/GES00854.1; 10 fi gures. Received 4 August 2012 ♦ Revision received 31 January 2013 ♦ Accepted 4 February 2013 ♦ Published online 4 April 2013 510 For permission to copy, contact [email protected] © 2013 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/9/3/510/3343994/510.pdf by guest on 28 September 2021 Magnetostratigraphic importance of secondary chemical remanent magnetizations to Bremen, the remaining samples were only This study Offshore ODP Long Island demagnetized up to 60 mT. Onshore ODP In August 2010, the remaining samples were further demagnetized at 10 mT increments to a maximum fi eld of 100 mT. Anhysteretic rema- helf ntal s nent magnetizations (ARMs) were induced in t Contine a peak AF of 100 mT and a DC bias fi eld of Piedmon Sea Girt 50 µT. This is also expressed as susceptibility ic outcrop 40° Cretaceous outcropzo Hole M0027A of ARM (KARM), after normalizing by the 50-µT N Ceno Hole M0028A DC bias fi eld. Isothermal remanent magneti- Island Hole M0029A Beach zations (IRMs) were induced in a DC fi eld of Ancora Site Bass River 1071 700 mT. After each treatment, the samples were New Jersey Site Site AF demagnetized using the same steps as for Atlantic City 1072 1073 the NRM. Millville Ocean View Cape May Mineral Magnetic Analyses Zoo Site 906 39° All mineral magnetic analyses were carried Cape Site 903 Site 902 May out after the paleomagnetic measurements were 0 Site 904 10 0 00 1 00 completed using facilities in both the Paleomag- 20 Site 905 netic and Mineral Magnetic Laboratory at the 75° 74° 73° 72° Department of Geology, Lund University, and Figure 1. Map of the Expedition 313 drill sites and locations of the Geomagnetism Laboratory at the School of previous onshore and offshore Integrated Ocean Drilling Program Environmental Sciences, University of Liver- (IODP) drill sites. pool. However, for comparison, we also show κ magnetic susceptibility ( initial) measured imme- diately after the cores were split in 2009 using a turbation from bottom to top, culminating in a this study, 277 paleomagnetic samples were Geotek multisensor core logger (MSCL) at the darker-gray–colored, relatively homogeneous collected from the working halves of the cores. IODP Bremen Core Repository, Bremen. clay sequence between 199 and 195 m. Above Oriented plastic sample boxes (6.2 cm3) were The IRM induced in a fi eld of 700 mT 207 m, the bedding is contorted at a range of carefully pushed directly into the sediments. was assumed to represent the saturation IRM scales, which indicates postdepositional disrup- For harder sediments, a custom-made stain- (SIRM). A few samples exhibited SIRMs in tion of the successions. The lower section of the less steel “square shaped” tube was fi rst used excess of the dynamic range of the cryogenic clay-dominated sediments in M0028A (between to extract the sediments, which were then trans- magnetometer, and therefore an SIRM was also 244 and 239 m) consists of similar dark- to pale- ferred into the sample boxes. Sediments with induced on selected samples using a Redcliffe gray clays as in M0027A, with contorted bed- evidence of postdepositional deformation, 700 BSM pulse magnetizer and measured with a ding and high degrees of bioturbation. The over- e.g., soft-sediment folding structures, were Molspin Minispin magnetometer. Volume-spe- lying sediments consist of gray-yellow-brown avoided. The samples were stored in refriger- cifi c magnetic susceptibility (κ) was measured color-banded clay with chondrite trace fossils ated conditions. using a Geofyzica Brno Kappabridge (KLY-2) and pyritic silty laminae. magnetic susceptibility meter. Paleomagnetic Analyses The anisotropy of magnetic susceptibility METHODS (AMS) was measured on 67 samples using an All paleomagnetic measurements were car- AGICO Multi-Function Kappabridge (MFK1- Coring and Initial Sampling ried out using a 2G Enterprises pass-through FA) equipped with an automated rotation sys- direct-current (DC) cryogenic magnetometer tem. Using the same equipment, fi nely ground All coring was carried out between 30 April (model 755 R, horizontal orientation) equipped ~300 mg extracts of six samples were selected and 17 July 2009 during the offshore phase of the with an automated alternating fi eld (AF) demag- for measurements of low- and high-tempera- expedition.
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