INTRODUCTION. For more than 20 yr, magnetic susceptibility (MS) measurements of sedimentary sequences have been used in paleoclimatic studies. This in-.
Magnetosusceptibility event and cyclostratigraphy method applied to marine rocks: Detrital input versus carbonate productivity Brooks B. Ellwood Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, USA
Rex E. Crick Department of Geology, University of Texas, Arlington, Texas 76019, USA
Ahmed El Hassani Departement de Géologie, Institut Scientifique, B.P. 703 Rabat-Agdal, 10106 Rabat, Morocco
Stephen L. Benoist Richard H. Young Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, USA
ABSTRACT Magnetic susceptibility data from marine rocks can be used for global correlation due to synchronous variations in global erosion. We show here that the magnetic susceptibility signature, found in two forms, resides mainly in paramagnetic and other detrital constituents in most marine rocks. The first form is a short-term, low-magnitude, high-frequency cyclic climate signature that is often useful for regional correlation. The second form is a longer term, higher magnitude, lowfrequency signature resulting from transgressive and regressive events that can be used for global correlation. Fluctuations in detrital input, due to eustatic-based erosion, are the primary cause of events. These fluctuations are driven by large-scale processes such as global orogenic cycles. However, variations in carbonate productivity cannot be ruled out when explaining the lowmagnitude climate-driven cyclicity also observed in magnetic susceptibility data sets. Keywords: global correlation, climate cycles, magnetic susceptibility, Paleozoic, Devonian, Morocco. INTRODUCTION For more than 20 yr, magnetic susceptibility (MS) measurements of sedimentary sequences have been used in paleoclimatic studies. This includes work on loess sequences (Heller and Evans, 1995), lacustrine sediments (Allen et al., 1999), and archaeological sites (Ellwood et al., 1997). As a result, several different climate-driven mechanisms have been identified as a control on the MS variations. In part, this work has been successful because MS, independent of other measurements, has been shown to be very sensitive to small changes in total iron concentration of sediments (Banerjee, 1996) that are controlled by climate (Tite and Linington, 1975; Verosub et al., 1993; Maher, 1998). Some workers who have performed MS studies on rocks of late Paleozoic and Mesozoic age have argued that global correlations are possible (Hansen et al., 1993). However, these studies have suggested correlations between terrestrial and marine sequences that we believe are somewhat tenuous, and the authors have argued that the global MS trends observed are due to climatic variations. We agree that climate cyclicity (high-frequency fluctuation) does appear in MS data sets, and we agree that these cycles can be used in some instances for correlations. However, these cycles do not have global correlation power without excellent, high-resolution biostratigraphic or isotopic control, because high-frequency (short-term) cycles are too easily destroyed by erosion or nondeposition, thus disrupting the correlation power. On the other hand, long-term magnetosusceptibility event and cyclostratigraphy (MSEC; Crick et al., 1997; Ellwood et al., 1999) data, resulting from eustasy, do have global correlation power. The MSEC method involves measuring the MS of samples collected at close intervals throughout continuously exposed sections or from drill cores. Where section exposures are continuous, major unconformities can often be recognized. Even where stratigraphic gaps are not identified in outcrop, large abrupt changes in MS magnitude can be diagnostic indicators of major stratigraphic gaps. The MSEC method also works in relatively modern marine sediments, where it is used as a proxy for oxygen isotopic (δ18O ) variations and cliGeology; December 2000; v. 28; no. 12; p. 1135–1138; 4 figures; 1 table.
mate fluctuations (Shackelton, 1999). Pleistocene δ18O variations are tied to ice volume and, therefore, to base-level (sea level) changes resulting in significant global, climate-controlled transgressive and regressive adjustments in coastal regions. These changes result in significant fluctuations in the transport of detrital material into the marine environment, thus producing the MSEC variations observed. In Paleozoic rocks (Crick et al., 1997, 2001; Ellwood et al., 1999), the MSEC method is proving to be a useful technique for regional and global correlation. Our work has been directed primarily toward Paleozoic rocks because other methods are generally not useful for solving global correlation problems during this time. We have argued that the MSEC signature observed (example shown in Fig. 1) is due to the influx of detrital grains into the marine environment as the result of (1) variations in weathering and erosion during transgressive and regressive sea-level changes (Crick et al., 1997; Ellwood et al., 1999), and (2) variable erosion driven by changes in climate (Crick et al., 2001). The cyclostratigraphic aspect of MSEC data sets is a climate-driven cyclicity superimposed on longer term events that result from erosiondriven base-level changes (Fig. 1). We relate the smaller amplitude and shorter duration cycles to climate change (Fig. 1B), the cyclostratigraphy part of the MSEC method (Crick et al., 2001). These cycles are superimposed on the longer term and greater magnitude events (Fig. 1A) that have global correlative power (Crick et al., 1997). There are also other elements in the data. For example, tempestites or storm deposits often give a very short term but high MS value (large-amplitude single data points in Fig. 1A). In addition, local or regional, relatively low magnitude and shortterm fluctuations can be distinguished in MSEC data because they appear in only one or two closely spaced sections. These result from regional or local tectonic processes (Ellwood et al., 1999). We are concentrating our MSEC work on global stratotypes to establish a direct link to the best available biostratigraphic data sets. MSEC has been empirically tested, and sampling intervals are extremely small (gener1135
Lower Devonian Jebel Issimour Section at Taalalt, Morocco
200.0
20 meter segment
185.0 184.0
190.0
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180.0 182.0
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Figure 1. A: Magnetic susceptibility results illustrating magnetosusceptibility event and cyclostratigraphy character typical for stratigraphic sections of marine limestones, shales, and marls. These data are from part of Jebel Issimour section, Morocco, including data from Pragian Stage of Devonian to upper Emsian. Ages are picked from Plodowski et al. (1999). Section heights here are slightly different from those reported by Plodowski et al. (1999) because we measured section independently. B: Expanded segment from overall section in A.
179.0
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ally 0.05–0.1 m) to clarify trends and for maximum time resolution. The MS signature of the Jebel Issimour section (Fig. 1) is typical. This section is ~270 m thick with a sample spacing of ~10 samples per meter, resulting in the collection and measurement of ~2700 samples from the section. Where the section is covered, we remove the cover to allow continuous sampling. We have argued (Crick et al., 1997; Ellwood et al., 1999) that the MSEC signature results from the influx of detrital grains, weathered and eroded from continents and delivered to the oceans, thus mixing in with the normal marine sediments. Therefore, the greater the influx of detrital material, the greater the MS. However, it might be argued that the observed MSEC signature is the result of dilution by CaCO3 as a result of variations in productivity. One problem with the latter argument is that it does not explain the MSEC signature in shales that have been shown to correlate over global distances with carbonate-rich sequences (Crick et al., 1997). MAGNETIC SUSCEPTIBILITY All materials are susceptible to becoming magnetized when placed in a magnetic field, and MS is an indicator of the strength of this magnetism within a sample. This characteristic is very different from remanent magnetism, the intrinsic permanent magnetization that accounts for the magnetic polarity of materials. MS can be quickly and easily measured on small samples and is largely a function of the concentration of the magnetizable material they contain. Magnetizable materials in marine sedimentary sequences include not only the ferrimagnetic minerals that may acquire a remanence (required for reversal magnetostratigraphy), but also any other mineral containing an odd number of electrons. These less magnetic, or paramagnetic, substances include clay minerals, ferromagnesian silicates such as biotite, iron sulfides such as pyrite, and other materials. Even though these paramagnetic constituents exhibit a very low MS, in relatively low abundance they can dominate the measured MS in limestones, marls, and shales. Magnetite is common in these rocks. However, very fine grained magnetite, such as that produced by magnetotactic bacteria or algae, is not 1136
GEOLOGY, December 2000
A
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B
Tourmaline
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Hornblende
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Biotite
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I-Type Granite
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Limestone
~0.2 mm
Percent Contaminant
15
35 µm
Figure 2. Paramagnetic minerals common in Devonian marine sedimentary rock (see text). A: Biotite grain. B: Tourmaline grain. Minerals were found in residual samples used in producing Figure 4.
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Aegirine-Augite
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Staurolite
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Epidote
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Figure 4. Modeled and natural effects of detrital minerals in typical marine limestones (Ls.), showing regression lines fit to different data sets illustrated in key in upper left margin. Synthetic samples were constructed from 0.05, 0.1, 0.5, and 1.0 g of each mineral used, which was then mixed with 5 g of CaCO3, percentage of mineral relative to total sample calculated, magnetic susceptibility measured, and results plotted.
Tourmaline
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Magnetic Susceptibility (m3/kg)
Goethite
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als is much greater than the MS of diamagnetic minerals, and therefore a small amount of a paramagnetic mineral can significantly outweigh the MS of volumetrically more abundant diamagnetic minerals.
A-Type Granite Pyrite
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Chert
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White Feldspar
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Pink Feldspar
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Magnetic Susceptibility (m 3/kg) Figure 3. Magnetic susceptibility (MS) mean values for typical detrital grains expected in marine deposits. Feldspar and chert samples, if pure, would be diamagnetic, but these MS values indicate fine-grained ferrimagnetic or paramagnetic contaminant.
easily magnetized in the very small inducing magnetic field (fields
