doi:10.1016/j.pepi.2005.06.019
Copyright © 2006 Elsevier B.V. All rights reserved.
High-field magnetic susceptibility (χHF) as a proxy of biogenic sedimentation along the Antarctic Peninsula
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Stefanie A. Brachfeld
, a, 
aDepartment of Earth and Environmental Studies, Montclair State University, Montclair, NJ 07043, United States
Received 20 February 2005;
revised 29 April 2005;
accepted 24 June 2005.
Available online 13 March 2006.
Abstract
High-field mass-normalized magnetic susceptibility (χHF) is presented as a proxy for biogenic sedimentation in glacimarine sediment from the western Antarctic Peninsula. χHF is measured at field strengths that exceed the saturation field of ferrimagnetic minerals. These measurement conditions exclude the contributions of ferrimagnetic iron oxides that dominate low-field volume-normalized susceptibility (k) measurements. Therefore, χHF allows for closer examination of paramagnetic and diamagnetic minerals, the latter of which includes biogenic silica and biogenic calcite. In sedimentary sequences from the western Antarctic Peninsula, the main processes affecting the abundance of diamagnetic material in sediment is biological productivity and the subsequent flux of biogenic silica to the seafloor. χHF profiles were measured on two biosiliceous sediment cores from the western Antarctic Peninsula. Comparisons with quantitative biogenic silica measurements indicate that χHF tracks % opaline silica very well, and reveals the presence of century-scale cycles in sediment composition in intervals where k appears featureless. χHF is limited as a quantitative measure of biogenic sediment flux, since terrigenous paramagnetic and diamagnetic minerals also contribute to the measurement. The interpretation of χHF can also be complicated by the presence of unsaturated high-coercivity minerals such as hematite and goethite, or by the presence of ultra-fine superparamagnetic (SP) particles. However, in sediment sequences where the condition of saturation is met, χHF is very well suited for the rapid identification of temporal trends in biogenic sedimentation.
Keywords: Antarctic Peninsula; Magnetic susceptibility; High-field magnetic susceptibility; Palmer Deep; Gerlache Strait; Rock-magnetism
Fig. 1. Location map showing the Palmer Deep (PD, Ocean Drilling Program Site 1098), the Schollaert Drift within the Gerlache Strait (GS, JPC28), and Andvord Bay (AB). Stars denote sediment core locations. Bathymetry from Rebessco et al. (1998).
Fig. 2. Low-field volume-normalized magnetic susceptibility profiles (k) of sediment cores from the Palmer Deep (ODP Site 1098) and the Schollaert Drift (JPC28), measured with Bartington MS2C 80-mm and 125-mm diameter sensors, respectively. The late Holocene interval (0 to 
3.5 ka) is characterized by strong but variable k. The middle Holocene interval (9–3.5 ka) is characterized by uniformly weak k. Calibrated radiocarbon dates for ODP Site 1098 are from Domack et al. (2001). Uncorrected radiocarbon dates for JPC28 are from Domack et al. (2003).
Fig. 3. (A) The +M + H quadrant of a hysteresis loop. χHF is calculated as the slope of the M–H curve at applied fields that are higher than those required to saturate ferro- and ferrimagnetic minerals, i.e., field strengths above where the loop's upper and lower branches converge. (B). A schematic example of the selective dissolution method to determine % opaline silica. Aliquots of sample dissolving in NaOH are collected and measured every hour via spectrophotometry. A trend-line is fit to the data as the solution approaches Si saturation, and extrapolated back to time zero to determine the initial concentration of Si. Absorbance is converted to weight % silica using a series of standard solutions with known concentration of Si.
Fig. 4. Hysteresis loops from (A) the Palmer Deep late Holocene sediment, (B) the Palmer Deep middle Holocene sediment, (C) Schollaert Drift late Holocene sediment, and (D) Schollaert Drift middle Holocene sediment. All samples reach saturation below 0.7 T.
Fig. 5. (A) Room temperature M–H curves for SP particles in ferrofluid (Ferrofluidics MO1, solid line) with a mean grain-size of 10-nm (Carter-Stiglitz et al., 2001), and SP particles embedded in magnetic latex (ML) microspheres (Bangs Laboratories Inc., dashed line) with a mean grain size of 11-nm (Relle and Grant, 1998). Grain size was estimated from best-fit Langevin functions (Relle and Grant, 1998). M is plotted as M/Mmax, and the ML microsphere data has been multiplied by a factor of 0.8 for the purpose of distinguishing the two curves on the same plot. (B) Close-up of the high-field data for MO1. The saturation field is 800 mT.
Fig. 6. (A–D) Susceptibility as a function of temperature and applied field frequency [χ(fT)] for (A) the Palmer Deep late Holocene interval, (B) the Palmer Deep middle Holocene interval, (C) Schollaert Drift late Holocene interval, and (D) Schollaert Drift middle Holocene interval. There is no evidence of frequency dependence in the late Holocene samples. Schollaert Drift measurements were made on bulk samples, which may mask the frequency–temperature behavior of SP particles if they are present in very low concentrations. Magnetic extracts made from the Palmer Deep middle Holocene samples do show frequency dependence below 100 K (a property of titanomagnetites) and above 250 K. A detailed discussion of χ(fT) behavior observed in these samples in given in Brachfeld and Banerjee (2000).
Fig. 7. χHF as a function of % high-field slope used in the calculation. Values are normalized by the maximum value calculated. Twenty samples are displayed from each core, taken from 1, 2, up to 20 mcd. (A) Samples from Site 1098 show a consistent 2–4% variation, and there is no systematic pattern in the location of χHF(max). (B) Samples from the late Holocene interval of JPC28 display 2–14% variation as a function of % field, with χHF(max) typically occurring at the 70% calculation and χHF/χHF(max) decreasing at higher % field values. This may indicate the presence of unsaturated SP particles. Middle Holocene samples from JPC28 contain high-coercivity hematite and possibly goethite, and yet the samples show <2% variability in χHF. Longer averaging times and the absence of SP particles are likely responsible for the stability of χHF in this interval.
Fig. 8. (A) χHF and quantitative opaline silica from ODP Site 1098. χHF is presented as the average of the 70, 75, 80, 85, and 90% high-field slopes. Highs in % opaline silica correspond to lows in χHF, suggesting that diamagnetic silica is driving χHF values. The correlation coefficient between the two parameters is −0.638, which is lowered by the imperfections of the mcd correlation at the centimeter scale. (B) A comparison of k and 90% χHF in JPC28. Note the absence of a discontinuity at 10.5 mbsf, which suggests that hematite and goethite are not contaminating the χHF measurement.
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