MSEC data sets record glacially driven cyclicity: Examples from the arrow canyon Mississippian–Pennsylvanian GSSP and associated sections

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Palaeogeography, Palaeoclimatology, Palaeoecology 255 (2007) 377 – 390 www.elsevier.com/locate/palaeo

MSEC data sets record glacially driven cyclicity: Examples from the arrow canyon Mississippian–Pennsylvanian GSSP and associated sections Brooks B. Ellwood a,⁎, Jonathan H. Tomkin b , Barry C. Richards c , Stephen L. Benoist a , Lance L. Lambert d a b

Louisiana State University, Department of Geology and Geophysics, E235 Howe-Russell Geoscience Complex, Baton Rouge, LA 70803 USA School of Earth, Society, and Environment, University of Illinois, 428 Natural History Building, 1301W. Green Street, Urbana, IL 61801 USA c Geological Survey of Canada, 3303 33rd St., N.W., Calgary, Alberta, Canada T2L-2A d University of Texas at San Antonio, Department of Earth & Environmental Science, One University Circle, San Antonio, TX 78249 USA Received 28 March 2007; received in revised form 12 July 2007; accepted 28 August 2007

Abstract Here we report magnetic susceptibility (MS) analyses from three, mainly marine sections, the Mississippian–Pennsylvanian (Mid-Carboniferous) Global Boundary Stratotype Section and Point (GSSP) in Arrow Canyon, SE Nevada, and two secondary sections nearby—one in Arrow Canyon (∼200 m distant) and the second in Battleship Wash, (∼ 2 km south of the GSSP). All three sections are easily correlated using the magnetosusceptibility event and cyclostratigraphy method (MSEC). Cyclicity is clearly apparent in all three sections, and Time-Series analysis independently verifies the MS zonation developed from the smoothed data set. The periods determined in the three sections support the argument that deposition of these Mid-Carboniferous rocks resulted from climate-controlled glacio-eustatic fluctuations that were driven by Gondwana glaciation. Given the published average thicknesses of T–R cycles (N 50 m) and the estimates for timing of glacial–interglacial cyclicity during the MidCarboniferous, we assign the strong FT peak observed for the three MS data sets a value of ∼400,000 years, corresponding to the Milankovitch E1 eccentricity band. We then calculate a Floating Point Time Scale (FPTS) for the sampled sequence. The FPTS results indicate that (1) sediment accumulation rates bracketing the Mid-Carboniferous boundary averaged ∼ 0.7 cm/1000 years, and (2) were relatively constant for these reference sections. © 2007 Elsevier B.V. All rights reserved. Keywords: Mid-Carboniferous glaciations; Magnetic susceptibility; Correlations; Floating Point Time Scale; Time-Series analysis

1. Introduction The Global Boundary Stratotype Section and Point (GSSP) system is part of a continuing effort to establish better biostratigraphic precision and correlations on an ⁎ Corresponding author. E-mail address: [email protected] (B.B. Ellwood). 0031-0182/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2007.08.006

international level. The International Commission on Stratigraphy (ICS) has been establishing reference standards for geological time units by defining a system of GSSPs for all geologic stage boundaries within the Phanerozoic. Once the GSSPs are chosen, these sections become well-studied stratigraphic successions that provide geologic standards to which other sections of equivalent age can be compared. GSSPs provide an age

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framework that can eventually link both marine and nonmarine successions all over the world. In addition, nontraditional stratigraphic methods can be tied directly to the critical biostratigraphic reference horizons. The Mid-Carboniferous GSSP is located in Arrow Canyon, SE Nevada (Fig. 1; Lane et al., 1999; Richards et al., 2002; Gradstein et al., 2004). It lies within a series of limestone beds alternating with various siliciclastics, including shale or paleosol and cross-bedded siltstone/ sandstone lithologies. The contacts between these units are abrupt and suggest that rapid paleoenvironmental changes occurred during times of transition. This change in lithologic character requires rapid and significant water depth fluctuations that have been attributed to glacial cycles in Gondwana (Lane et al., 1999; Richards et al., 2002). The most plausible causal mechanism for such changes is the glacial–interglacial fluctuations argued by a number of authors to have occurred during the latest

Mississippian and into the Pennsylvanian (e.g. Veevers and Powell, 1987; Miller and Eriksson, 1999; Smith and Reed, 2000). These authors have argued that the observed fluctuations exhibit ∼400,000 year orbital-eccentricity cyclicity reflected in glacial–interglacial cycles in the Mid-Carboniferous. In support of this work, Rygel et al. (2006) have summarized published results showing that sea-level changes of over 50 m occurred through the latest Mississippian into the Pennsylvanian, and that these sealevel fluctuations are the result of glacio-eustatic cycles. Here we examine the magnetic susceptibility (MS) variability, one of many non-traditional stratigraphic methods currently being used in geology, for samples from three coeval Mid-Carboniferous sections: 1) the GSSP; 2) a second section in Arrow Canyon that we refer to as Arrow Canyon Two; and 3) a section in Battleship Wash (∼2 km to the south of the GSSP; Fig. 1). A TimeSeries analysis applied to the MS data from the Arrow

Fig. 1. Location of the three sites sampled in SE Nevada: GSSP in Arrow Canyon; 2, the secondary section in Arrow Canyon that we call Arrow Canyon Two in the text, and that we have chosen as the magnetostratotype; BT, a section located in Battleship Wash, ∼2 km south of the GSSP.

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Canyon Two and the Battleship Wash sections enables us to develop a Mid-Carboniferous boundary Floating Point Time Scale (FPTS). Evaluation of the timing and possible significance of the lithologic fluctuations within the studied sections is addressed relative to the temporal framework of the FPTS.

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particularly useful as an independent age control, because it can extract data from sections that are not amenable to other magnetostratigraphic techniques—such as the magnetostratigraphy polarity from analyses of remanent magnetization (RM) in sections (Berggren et al., 1995). 2.1. Magnetic susceptibility: general comments

2. Previous work It is now well-established that low-field magnetic susceptibility (MS) data sets in both unlithified and lithified marine Phanerozoic sediments commonly record Milankovitch cyclicity, and that the cyclostratigraphy of these successions can be used for astronomical calibration of geologic time scales (Mead et al., 1986; deMenocal et al., 1991; Hartl et al., 1995; Weedon et al., 1997; Shackleton et al., 1999a; Weedon et al., 1999). It has been argued that of those orbital cyclicities observed in geologic data sets, the ∼400,000 year eccentricity cycle has a robust, long-term paleoclimatic stratigraphic record. Shackleton et al. (1999b) state, “As Laskar (this issue) points out, despite the fact that a purely mathematical solution to the orbital calculations is intrinsically limited to a maximum extension into the past of ca. 30 Ma, some of the long-period frequencies that may be found in geological records are stable or calculable over much longer intervals. The 406 ka eccentricity cycle is particularly interesting in this respect, and indeed it seems realistic to propose the establishment of a stratigraphic scheme based on this cycle.” In support of this statement, a number of authors have reported the ∼400,000 year eccentricity cycle recorded in rocks of the Cenozoic and Mesozoic age (e.g. van Dobeneck and Schmieder, 1999; Shackleton et al., 1999a; Sprovieri et al., 2006; Pälike et al., 2006). In addition to its utility in paleoclimatic studies, magnetostratigraphy-susceptibility can be used for highresolution correlation among marine sedimentary rocks with broadly differing facies of regional and global extent (Crick et al., 1997, 2000; Ellwood et al., 1999, 2000, 2007). MSEC (magnetosusceptibility event and cyclostratigraphy) is a correlation method that provides a robust data set to independently evaluate and adjust stratigraphic position among geological successions. It requires reasonable biostratigraphic control to initially develop a chronostratigraphic framework where a distinctive MS zonation can be directly correlated with high precision among sections, even when biostratigraphic uncertainties or slight unconformities are known to exist within sections (Ellwood et al., 2007). Because the MSEC method does not require that the rock or sediment be consolidated or orientated for analysis, the method is

All mineral grains are “susceptible” to becoming magnetized in the presence of a magnetic field, and MS is an indicator of the strength of this transient magnetism within a material sample (Nagata, 1961). MS is very different from remanent magnetism (RM), the intrinsic magnetization that accounts for the magnetic polarity of materials. In marine sediments, MS is generally considered to be an indicator of iron, ferromagnesian or clay mineral concentration (Ellwood et al., 2000, 2007), and can be quickly and easily measured on small samples. In the very low inducing magnetic fields that are generally applied, MS is largely a function of the concentration and composition of the magnetizable material in a sample. Mathematically, MS is a tensor of the second rank and therefore it has anisotropy (Nye, 1990). This means that the measured MS in a sample will be different in different directions, depending upon the mineral distributions and grain morphology. The anisotropy of magnetic susceptibility (AMS) of most rock types has been well-studied (Tarling and Hrouda, 1993). However, it is somewhat time consuming to measure and requires oriented samples, as do RM studies. Bulk (initial) measurements of MS are often performed without consideration of the AMS of samples, and for most samples the MS error due to the anisotropy is small. The AMS effect is further reduced when samples are crushed before the MS is measured, or where friable samples are broken up during sampling. MS is much less susceptible to remagnetization than is the RM in rocks and can be measured on small, irregular lithic fragments and on highly friable material that is difficult to sample for either AMS or RM measurement. Magnetizable materials in marine sediments include not only the ferrimagnetic minerals such as the iron oxide minerals (e.g. magnetite and maghemite), and iron sulfides (e.g. pyrrhotite), and iron sulfates (e.g. greigite) that can acquire an RM (required for reversal magnetostratigraphy), but also any other less magnetic, paramagnetic compounds. The important paramagnetic minerals in marine sediments include the clays, particularly illite, chlorite and smectite, ferromagnesian silicates such as biotite, tourmaline, pyroxene and amphiboles, iron sulfides including pyrite and marcasite, iron carbonates such as siderite and ankerite, and other iron and

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magnesium bearing minerals (Ellwood et al., 1989, 2000, 2007). Diamagnetic minerals like calcite and quartz, also present and usually dominant by volume in most marine rocks, have a negative MS, values so low that a very small amount of paramagnetic material will swamp the contribution to the MS of the diamagnetic material. This has been graphically illustrated and discussed by Ellwood et al. (2000). 2.2. Cyclic trends in MS data Cyclic trends in MS data sets have been shown to result from climate-controlled fluxes of detrital sediments into the marine environment (Weedon et al.,

1999; Ellwood et al., 2000; 2007), and this is supported by many others workers (Mead et al., 1986; Hartl et al., 1995; Weedon et al., 1997; Shackleton et al., 1999a,b; Weedon et al., 1999; Crick et al., 2001), as well as by identification of climate cycles in the MS data sets reported here. This is the result of enhanced erosion during portions of climate cycles, and may be due either to detrital or eolian components, or both being brought into the marine environment during that time and then being redistributed by ocean currents, as shown for recent sediments deposited in the South Atlantic Ocean and Gulf of Mexico (Sachs and Ellwood, 1988; Ellwood et al., 2006). Longer-term MS trends are due to factors such as eustasy.

Fig. 2. MS data for the Mid-Carboniferous (Mississippian–Pennsylvanian) boundary Global Boundary Stratotype Section and Point (GSSP) defining the beginning of the Bashkirian Stage and thus the end of the Serpukhovian Stage. Bar-logs (filled—high MS; open—low MS; methods discussed in text) developed from the MS data that were smoothed using splines (solid data curve; raw MS data are shown as dotted curve). MS data in m3/kg. MS chron numbers are: Ba1 to Ba4 in ascending order within the Bashkirian; SeZ and SeY in descending order within the Serpukhovian. Subchrons are identified by stippled zones within SeZ and SeY. Lithologies are labeled using unit labels (C to N) presented by Lane et al. (1999). Simplified lithologies are as follows: Units C, E, G, K and N are mainly limestones; Units D, F, I, H and J are mainly mudstone–siltstone units containing limestone nodules; Unit M is mainly a cross-bedded sandstone (see Lane et al., 1999 and Richards et al., 2002 for a detailed description of lithologies).

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3. Methods

3.2. Thermomagnetic susceptibility measurement

3.1. MS sampling and measurement

To characterize the ferrimagnetic and paramagnetic constituents in the sections sampled, thermomagnetic susceptibility measurements (TSM) were performed using the KLY-3S Kappa Bridge at LSU. Measurement using this instrument involves heating a sample from room temperature to 700 °C while measuring the MS as the temperature rises. At low measurement temperatures, up to ∼200 °C, paramagnetic minerals in natural samples show a parabola-shaped MS decay curve during this process because the MS in these samples is inversely proportional to the temperature of measurement (Hrouda,

Samples were collected at 5 cm intervals throughout each of three Mid-Carboniferous sections and measured using the susceptibility bridge at LSU. We report MS in terms of sample mass (in m3/kg) rather than volume, because it is much easier and faster to measure with high precision than in volume. Each sample is measured three times and the mean and standard deviation of these measurements is calculated. The mean of these measurements is reported here.

Fig. 3. MS data for the Arrow Canyon Two section (Fig. 1). MS and lithological data as in Fig. 2. Subchrons are identified by stippled zones within SeZ and SeY. T–R column represents the transgressive–regressive sea-level variations based on interpretation by Lane et al. (1999) and Richards et al. (2002). The Arrow Canyon Two section serves as the magnetostratotype for the Serpukhovian–Bashkirian boundary.

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1994). Ferrimagnetic minerals, on the other hand, usually show an increase in MS up to a point where the MS decays toward the Curie temperature of the minerals responsible for the MS. For magnetite this temperature is ∼580 °C; for maghemite this temperature varies somewhat, but usually is ∼610 °C; for hematite this temperature is ∼680 °C. 3.3. Presentation of MS data For presentation purposes and inter data-set comparisons, the bar-log format, similar to that previously established and routinely used since the 1960s for magnetic polarity data presentations, is used here. This technique is used because these data sets do not lend themselves to easy quantification and for simplicity. These bar-logs are accompanied by both raw and smoothed MS data sets so that the data used can be effectively evaluated

by the reader. Here, raw MS data (i.e. dashed curve in Fig. 2) are smoothed using splines (solid data curve in Fig. 2). The following bar-log plotting convention is used: if the MS cyclic trends increase or decrease by a factor of two or more, and if the change is represented by two or more data points, then this change is interpreted to be significant and the highs and lows associated with these cycles are differentiated by black (high MS values) or white (low MS values) bar-logs (shown in Fig. 2). This method is best employed when high-resolution data sets are being analyzed (large numbers of closely spaced samples). High-resolution data sets help resolve MS variations associated with anomalous samples. Such variations may be due to weathering effects, secondary alteration and metamorphism, longer-term trends due to factors such as eustasy (as opposed to shorter-term climate cycles), or events such as an impact (Ellwood et al., 2003), and to other factors. In addition, variations in detrital input

Fig. 4. MS data for the Battleship Wash section to the south of Arrow Canyon (Fig. 1). MS and lithological data as in Fig. 2. Subchrons are identified by stippled zones within SeZ and Ba2.

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between localities or a change in detrital sediment source is resolved by comparing bar-logs between different localities. To make this comparison easier visually, we have labeled MS zones using the following convention: MS zones above the Mid-Carboniferous boundary are labeled consecutively upward from Ba1 (for the first zone in the Bashkirian); MS zones below the Mid-Carboniferous boundary are labeled consecutively downward from SeZ (for the last zone in the Serpukhovian; i.e., Fig. 2). Because we believe these MS zones represent ∼400,000 year cyclicity (see discussion below) and therefore should be robust (Shackleton et al., 1999a,b), we characterize these MS zones as magnetostratigraphic susceptibility chrons that should have correlation power, certainly regionally, and probably globally, although this still needs to be tested. 4. Results 4.1. Arrow Canyon GSSP The Mid-Carboniferous, Mississippian–Pennsylvanian Series GSSP, which also defines the base of the Bashkirian stage, was established in Arrow Canyon, SE Nevada (Lane et al., 1999; Fig. 1). We collected ∼ 6.7 m of the published sequence for MSEC (138 samples) and

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the results of measurement are reported (Fig. 2). Results include MS data (raw and smoothed) as well as MS zonation represented as bar-logs developed from the smoothed MS data set. In addition, the lithology log presented here is tied to the GSSP as described by Lane et al. (1999). Unit names (letters C through N) are from Lane et al. (1999) and represent that portion of the section that we sampled. (For greater lithologic detail see Lane et al., 1999; Richards et al., 2002). We extend their nomenclature to the Arrow Canyon Two and Battleship Wash sections as part of this study (below). 4.2. Secondary Arrow Canyon section—Arrow Canyon Two About 200 m from the GSSP section, on the south side of Arrow Canyon, is another exposure that is equivalent to the GSSP (Fig. 1). We call this Arrow Canyon Two, and the sequence was sampled. Arrow Canyon Two is less weathered than is the GSSP, and it is more easily accessible. Lithology and other details for this section are discussed in detail by Richards et al. (2002). MS samples covered ∼8.7 m and were collected at 5 cm intervals (175 samples). Results of MS measurements and general lithology for the section are reported here (Fig. 3).

Fig. 5. Fourier Transform (FT) of the raw MS data from the Arrow Canyon GSSP and Two and the Battleship Wash sections in Nevada. Frequency in cycles/m. The peak labeled in the diagram is interpreted to represent Milankovitch E-1 (∼ 400,000 years) eccentricity cyclicity (see text). The MS chron cyclicity is coincident with the E-1 eccentricity peak.

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4.3. Battleship Wash section A third section, located in Battleship Wash, ∼2 km to the south of Arrow Canyon (Fig. 1), was collected for MSEC (N = 200). The section (∼10 m) covers the same lithologic interval as do the two Arrow Canyon sections (Webster, 1969). Results of MS measurements and general lithologies are reported here (Fig. 4). 4.4. Time-Series analysis: Fourier Transform method For all three sections sampled we have performed a Time-Series analysis using the individual (raw) MS data (independent of smoothing). First, we assumed that the spacing of samples is linearly related with time, i.e., Δx ≈ Δt, so that a Fourier method could be used. The less this assumption is true, the more noise that will be produced in the spectral graph. The spectral power of the three MS data sets was obtained with the Fourier Transform (FT) method. The data were both detrended and subjected to a Welch window so as to reduce spectral leakage and increase the dynamic range (Jenkins and Watts, 1968). The results are reported in Fig. 5. Analysis of the spectral plot shows that there is one peak of special interest in all three data sets. For the Arrow Canyon Two and Battleship Wash sample sets, this peak is a narrow peak and centered at ∼0.3 cycles/m. The MS cycles from the bar-logs from all three data sets are essentially identical with this peak (shaded vertical bar in Fig. 5). The Arrow Canyon GSSP FT data set shows significant scatter and the main peak is offset slightly toward higher values. The SSA-MTM toolkit (Dettinger et al., 1995) was used to test for statistical significance of the spectral peaks highlighted in Fig. 5. The data was subject to an f-test (where the null hypothesis is that the MS data is the result of a red-noise distribution) by employing the multi-taper method (Ghil et al., 2002). For all three data sets (Arrow Canyon Two, Battleship Wash, and the GSSP), the initial peak highlighted in Fig. 5 is statistically significant, to at least the 95% confidence level. The unweathered Arrow Canyon Two and Battleship Wash sections are significant to the 99% level. The second, smaller peak of the Arrow Canyon Two data set (at about 0.7 cycles/m), is also significant to the 95% level. We interpret this as an artifact because its position suggests that it is the result of leakage from the much larger initial peak. No other peaks in the data are statistically significant.

marine sediments (Ellwood et al., 2000; Ellwood et al., 2007). For example, recent work by Ellwood et al. (2007), using thermomagnetic and XRD measurements of Ordovician sediments from the North American Midcontinent compared to standards, demonstrated that the minerals illite and chlorite dominated the MS in these rocks. This was due in part to the unique thermal breakdown signature in illite-containing samples, where a strong conversion-peak is produced as maghemite forms from illite (the source of the iron) at high temperatures during sample measurement. In addition, at low temperatures clay standards showed the parabolic decay curves known for paramagnetic minerals. In the case of thermomagnetic measurements for Mid-Carboniferous samples examined here, there appears to be very little illite in these samples. A combination of ferrimagnetic and paramagnetic minerals, assumed to be detrital, dominates the MS in many samples (the relatively straight line or slight convex upward decay in limestone and shale samples shown in Fig. 6). Exceptions are those samples like the siltstone sample in Fig. 6 that show a strong convex downward decay during heating, indicating paramagnetic components strongly dominating the MS at low temperatures. No large illite conversion-peak is observed, although there are some minimal alteration effects observed. In many cases there is also a ferrimagnetic component (probably maghemite) that appears to have formed from weathering. At high temperatures

4.5. Thermomagnetic measurements It has been argued and experimentally shown that paramagnetic minerals often dominate the MS in lithified

Fig. 6. Thermomagnetic susceptibility (TSM‑MS vs T) results for measurements of typical limestone, shale and siltstone samples from Arrow Canyon Two.

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this component is destroyed by conversion to very lowMS hematite. 5. Discussion The Mid-Carboniferous GSSP (Fig. 2) is an important boundary that falls within an interglacial period during a time of glacial‑interglacial cyclicity. This is reflected in the alternating lithologic character of the GSSP, where limestone beds alternate with shale/paleosol/siltstone/ sandstone horizons resulting from relatively large sealevel fluctuations. The MS data from the GSSP section (Fig. 2) reflect these transgressive–regressive (T–R) cycles by exhibiting high MS during regressions and low MS during transgressions, and this essentially follows the lithologic changes. However, there are some interesting but subtle differences between the MS and lithologic

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data sets within the GSSP. For example, although the lowest MS values are associated with limestone Units C and N, some of the highest values are also associated with a limestone, the base of Unit G. The transition to higher or lower MS values commonly occurs well before the lithologic change, which indicates the general independence of MS and lithology at finer levels of cyclicity. There appears to be more MS scatter within the GSSP data set (Fig. 2) than for the MS data from the Arrow Canyon Two section (Fig. 3). Upon careful examination, much of the scatter in the GSSP is associated with local weathering (i.e. samples defining MS chron SeZ and into the base of Ba1). Weathering at the base of the boundary limestone, Unit G, is associated with some modern root penetration and chemical alteration. As a consequence, this has resulted in the local leaching of iron (causing reduced MS) and the formation of maghemite (increased

Fig. 7. Graphic comparison of the MS chron boundaries and lithologic boundaries for the Mid-Carboniferous intervals in Arrow Canyon; the Arrow Canyon GSSP and the Arrow Canyon Two sequences. The Serpukhovian–Bashkirian stage boundary level is shown, as is the MS chron zonation and lithologies for these sections. Filled circles represent the intersection of corresponding MS chron tops and bases between sections, while patterned squares represent the intersection of corresponding lithologic unit tops and bases between sections. Open circles represent the Mississippian– Pennsylvanian Series boundary level. A line-of-correlation (LOC) is fit to these data.

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MS) as a weathering by-product. While this has produced scatter in the MS, it has not destroyed the MS character in the GSSP. Similar effects are generally reduced in the Arrow Canyon Two section, in part because rooting and other weathering is less intense away from the crest of the hill where the GSSP is located. For this reason we have chosen the Arrow Canyon Two section as the preferred reference section for the Mid-Carboniferous magnetostratigraphy susceptibility stratotype (Fig. 3). We have used the MS data from this section for our Time-Series analysis and as the MS reference section (MS RS) for inter-site comparisons. T–R cycles reported by Lane et al. (1999) are shown in Fig. 3, and are visually compared with the MS and lithologic variations exhibited in the Arrow Canyon Two section. While there is a generally good correspondence between the T–R cyclicity, as defined on both the lithologic and MS data sets, there are some differences. For example, while limestone Unit E generally represents

a regression (Fig. 3), there is a short but significant MS low (defining an MS subchron, dark stippled in MS chron SeZ, Fig. 3) that indicates a short transgressive event near the top of limestone Unit E. Such shorter-term cyclicities may reflect other orbitally controlled climatic trends that do not persist throughout the data set, or may reflect a period of reduced sediment accumulation rates. A second subchron lies within MS chron SeY (light stippled zone, Fig. 3). 5.1. Graphic comparison of Mid-Carboniferous sections Simple graphic comparisons between the tops and bottoms of lithologic units (stippled squares) and MS chrons (filled circles) are used in Fig. 7 to correlate the two Arrow Canyon Mid-Carboniferous sections. A lineof-correlation (LOC) was then fit through the data distribution; the less accurate the correlation, the greater the departure of data points from the LOC. As expected

Fig. 8. Graphic comparison of the MS chron boundaries and lithologic boundaries for the Mid-Carboniferous intervals, the Arrow Canyon Two and Battleship Wash sequences. Symbols as in Fig. 7.

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for the GSSP and Arrow Canyon Two sections, which are only ∼ 200 m apart, the correlation is good (Fig. 7). A similar comparison between the GSSP and the Battleship Wash data sets (Fig. 8) shows a greater scatter of data points for both the MS and lithologic data sets than is seen in Fig. 7, with the greater scatter found in the lithologic comparison. With the exception of one data point (base of MS chron Ba2), the MS data fit the LOC very well. The best fit is found in Fig. 9, where the Arrow Canyon Two and the Battleship Wash sequences are compared. Here, even though the two sections are ∼ 2 km apart, the relatively low level of weathering and well-defined lithologic units show good correlation for both the lithologic and MS chron data sets. An interesting variation emerges when all three LOCs are compared. The Battleship Wash-GSSP comparison shows that sediment accumulation rates at the two

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sites are essentially the same (LOC ∼ 45°, Fig. 8). However, this angle is ∼35° for comparisons between the Arrow Canyon Two section and both of the other sections (Figs. 8 and 9), indicating that the Arrow Canyon Two, even though it is only ∼200 m from the GSSP, is a slightly expanded section. This is another reason why we chose this section as the magnetosusceptibility stratotype. 5.2. Timing of glacio-eustatic fluctuations Berger, Shackleton and others (Berger, 1978; personal communication; Shackleton et al., 1999b) have argued that frequencies with ∼400,000 year periods are expected in geological data sets. This frequency, the Milankovitch E1 eccentricity band, is confirmed by a number of studies on Mesozoic and Cenozoic sedimentary

Fig. 9. Graphic comparison of the MS chron boundaries and lithologic boundaries for the Mid-Carboniferous intervals, the Arrow Canyon GSSP and Battleship Wash sequences. Symbols as in Fig. 7.

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sequences (e.g. van Dobeneck and Schmieder, 1999; Shackleton et al., 1999a; Sprovieri et al., 2006; Pälike et al., 2006). In addition, several authors have argued that Mid-Carboniferous glacial–interglacial cyclicity was ∼400,000 (e.g. Veevers and Powell, 1987; Miller and Eriksson, 1999; Smith and Reed, 2000). Finally, Lane et al. (1999) and Richards et al. (2002) have argued that glacial–interglacial T–R cyclicity was responsible for the lithologic changes that occurred in the Mid-Carboniferous GSSP. In turn, our FT data indicate one significant peak corresponding with the FT analysis of the raw MS data from the Mid-Carboniferous sections sampled. This peak is coincident with the smoothed MS bar-log data sets (Fig. 5). These are graphically shown to be equivalent to the lithologic variations (Figs. 7, 8 and 9). Therefore, we have assigned the main, long-wavelength FT peak identified in Fig. 5 to the E1 eccentricity band. We conclude from these data that the MS chron and lithologic cycles observed here for the Mid-

Carboniferous GSSP and associated sections, represents ∼400,000 year eccentricity-controlled, glacial–interglacial fluctuations. 5.3. A Floating Point Time Scale Graphic comparisons of bar-logs for sections used in this study show that all three sections are highly correlated, and this is supported by lithologies and biostratigraphy. Because MS chrons appear to represent Milankovitch climate cyclicity in the ∼400,000 year eccentricity band, the zonation presented in Fig. 10 can also be considered as a Floating Point Time Scale (FPTS), with each MS chron representing ∼200,000 years of time. Therefore, assuming that the ∼400,000 climate cyclicity must be fairly uniform in time, we have assigned the FPTS segments uniform time, and these are graphically displayed in Fig. 10. We have then graphically compared the Arrow Canyon Two magnetostratotype section to the

Fig. 10. A Floating Point Time Scale for an E1 cyclicity (Fig. 5) and comparison with the Arrow Canyon Two section, chosen as the magnetostratotype. Symbols as in Fig. 7.

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FPTS to ascertain rock accumulation rates at Arrow Canyon Two for the Mid-Carboniferous. The result is that for most of the section, bar-log character is highly correlated to the FPTS, indicating relatively uniform sediment accumulation rates for the Arrow Canyon Two section. Only in the upper part of the section is there a change, and here it appears that the section is somewhat condensed. Finally, the FPTS pattern can be interpreted to indicate that a bit more than 1.2 Ma is represented by the Arrow Canyon Two sedimentary sequence. This in turn allows calculation of an average sediment accumulation rate for the Arrow Canyon Two section, resulting in an average rate of ∼0.7 cm/Kyr, typical for marine sediment accumulation rates. 6. Conclusions Here we develop the magnetosusceptibility stratigraphy MS chron zonation for the Mid-Carboniferous, Serpukhovian–Bashkirian stage (Mississippian–Pennsylvanian Series) Global Boundary Stratotype Section and Point (GSSP) at Arrow Canyon, Nevada, and compare it among two additional sections, Arrow Canyon Two, ∼ 200 m from the GSSP, and a section in Battleship Wash, ∼ 2 km to the south (Fig. 1). Due to MS scatter within the GSSP resulting from weathering, we have established Arrow Canyon Two, exhibiting less weathering and being slightly expanded, as the magnetosusceptibility stratotype for the Mid-Carboniferous boundary. Our results indicate: 1. The MS chron and lithologic cyclicity for the sections examined exhibit well-defined T–R cyclicity that is consistent with the glacio-eustatic interpretation. 2. Graphic comparison shows an excellent correlation among sections, using either the MS chron zonation, or the lithologic unit previously defined (Lane et al., 1999). 3. Time-Series analysis of raw, non-smoothed MS data from the three sections sampled, using the Fourier Transform method, results in well-defined peaks for each section that, when compared to other MidCarboniferous studies, indicates a ∼ 400,000 year Milankovitch eccentricity (E1) glacio-eustatic cyclicity. Smoothing produces MS bar-logs that exhibit the same cyclicity. 4. Assuming that E1 cyclicities for these data are correct, then Floating Point Time Scale assignment to the Arrow Canyon Two data set indicates that the sampled sequence was deposited in ∼1.2 Myr with a relatively steady sediment accumulation rate of ∼ 0.7 cm/ 1000 years.

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