Sediment Magnetic, Paleomagnetic, and Geochemical Data from Lacustrine Sediment in a Core from Grass Lake , Siskiyou County, California

By Patti J. Best¹, Richard L. Reynolds¹, Joseph G. Rosenbaum¹, John Drexler², David P. Adam³

1 U.S. Geological Survey, Denver, Colorado
2 University of Colorado, Boulder, Colorado
3 U.S. Geological Survey, Menlo Park, California

This report is preliminary and has not been reviewed for conformity with U. S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Introduction

Methods

Paleomagnetic Directions (Table 2): Natural Remanent Magnetization (NRM) was measured on oriented samples using a 90-Hz spinner magnetometer with a sensitivity better than 10^-5 A/m. Samples were demagnetized in steps through alternating-field (AF) demagnetization with peak fields of 5, 10, 15, 20, 30, 40, 60 and 80 milliTeslas (mT). Declination and inclination were calculated from a best fit line of the demagnetization data displayed as orthogonal vector diagrams as described in Kirschvink (1980).

Magnetic Susceptibility (Table 3): Volume susceptibility (MS) was measured using a susceptometer with a sensitivity better than 10^-5 volume SI. Samples were measured in a 0.1 mT induction at a low frequency of 600 Hz (MS_lf) and high frequency of 6000 Hz (MS_hf). For each sample, the MS value was determined as the mean of four measurements. Frequency dependent susceptibility was calculated as:

Laboratory induced magnetization (Table 3): After the paleomagnetic analysis, anhysteretic remanent magnetization (ARM) and isothermal remanent magnetization (IRM) experiments were conducted. Magnetizations were measured with a high speed spinner magnetometer. ARM was imparted in a decreasing AF from a peak induction of 100 mT and a DC bias of 0.1 mT. IRM magnetizations were generated at room temperature using an impulse magnetizer. First IRM was imparted in a 1.2T induction (IRM_1.2T). The samples were then magnetized in the opposite direction using an induction of 0.3T (IRM_-0.3T). Hard isothermal remanent magnetization (HIRM) and the S-parameter were calculated as follows (King and Channel, 1991):

Elemental Abundance (Tables 4 and 5): Elemental abundances were determined on selected samples using energy dispersive X-ray fluorescence analysis at the University of Colorado's Department of Geological Sciences. Contents of Cr, Cu, Fe, Mn, Mo, Nb, Ni, Rb, Sr, Ti, V, Y, Zn and Zr were measured. Organic carbon was determined on selected samples as the difference between total and inorganic carbon. Sample splits were combusted in oxygen for 5 minutes at 960°C. Total CO₂ was measured using a coulometer. Inorganic carbon was then determined through acidification with perchloric acid and measured with a coulometer (Engleman et al., 1985).

References

• Adam, D.P., Rieck, H.J., McGann, M., Schiller, K., Sarna- Wojcicki, A.M., 1994, Lithologic description of sediment cores from Grass Lake, Siskiyou County, California: U.S. Geological Survey Open- file Report 94-651, 36p.

• Best, P.J., 1996, Environmental controls on magnetic mineralogy and geochemistry of late Quaternary lacustrine sediments from Grass Lake and Tule Lake, northern California (M.S. thesis): Boulder, University of Colorado, 139 p.

• Engleman, E.E., Jackson, L.L., Norton, D.R. and Fischer, A. G., 1985, Determination of carbonate carbon in geological materials by coulometric titration: Chemical Geology, v. 53, p. 125-128.

• King, J.W., and Channel, J.E.T., 1991, Sedimentary magnetism, environmental magnetism, and magnetostratigraphy: Reviews of Geophysics, Supplement, p. 358-370.

• Kirschvink, J.L., 1980, The least-squares line and plane and the analysis of paleomagnetic data: Geophysical Journal of the Royal Astronomical Society, v. 62, p. 699-718.

Grass Lake Sample no.: A unique sample number assigned regardless of sample type.

Sample box no.: A unique sample number assigned to samples put into plastic cubes for magnetic mineral studies. The volume of each cube is 3.2 cubic centimeters.

Vial no.: A unique sample number assigned to sediment sample put into vials.

Drive no.: Identifies location in the core. The core was divided into drives numbered sequentially starting with 1 at the top. Some drives have been further divided into slugs indicated by letters, starting with A at the top of the drive.

Drive depth: The depth in meters of the top of the drive from the top of the core. Taken from Adam et al. (1994).

Depth interval of sample: The depth range of the sample within the drive.

Sample depth within drive (cm): Midpoint of the depth interval in centimeters.

Sample depth within drive (m): Midpoint of depth interval in meters.

Sample depth in core (m): Sample depth within drive (m) plus depth of drive from top of core.

Sample density (kg/m³): Density of sample calculated from the mass of the dried box samples used in magnetic mineral studies.

Sample Box no.: A unique sample number assigned to samples put into plastic cubes for magnetic mineral studies. The volume of each cube is 3.2 cubic centimeters.

Sample Depth: Depth of sample in meters from top of core.

Declination: Declination of the characteristic magnetization, clockwise from geographic north.

Inclination: The inclination of the characteristic magnetization.

NRM: Natural Remanent Magnetization in Am²/kg.

Demag. Interval: The demagnetization interval over which both Declination and Inclination were calculated.

Number of Points used in linear fit: Number of points used to create best fit line.

Subjective quality: A subjective grade (a is best, c the worst) given to the orthogonal vector diagrams from which the best fit line was calculated. NR indicates that the sample was not rated.

Comments:

• disturbed: Core was disturbed prior to sampling. The orientation of the sample was uncertain and therefore paleomagnetic directions were not calculated on these samples.
• poor fit: The vector diagram was extremely poor, and thus either paleomagnetic directions were not calculated or best-fit line used 2 points.
• not used: Applies to samples in which sediment was displaced during measurement using a magnetometer. A constant orientation could not be assured, and the sample was not used for paleomagnetic analysis.

TABLE 3. Sediment Magnetic Data

Sample Box no.: A unique sample number assigned to samples that are placed into plastic cubes for magnetic mineral studies. The volume of each cube is 3.2 cubic centimeters.

Sample depth.: Depth of sample in meters from top of core.

MSLF: Low-frequency magnetic susceptibility in m³/kg.

FDMS: Frequency-dependent magnetic susceptibility in percent.

ARM: Anhysteretic remanent magnetization in Am²/kg.

IRM (1.2T): Isothermal remanent magnetization from induction in a 1.2 tesla field at room temperature. Expressed in Am²/kg.

IRM (-.3T): Isothermal remanent magnetization from induction in a -0.3 tesla field at room temperature. Expressed in Am²/kg.

HIRM: Hard isothermal remanent magnetization: HIRM is calculated as:

[IRM(1.2T)+IRM(-0.3T)]/2 and expressed in Am²/kg.

S: (S Ratio) calculated as IRM(-0.3T)/IRM(1.2T).

TABLE 4. Elemental Abundance from X-ray Fluorescence

Vial no.: A unique sample number assigned to sediment samples placed in vials.

Sample depth: Depth of sample in meters from top of core.

Paired sample box no.: Sample box number which corresponds in depth to vial number.

Elements: The elements analyzed are listed below. The units are either weight percent (Wt%) or parts per million (ppm).

TABLE 5. Carbon Contents

Grass Lake Sample No.: A unique sample number assigned to samples regardless of type.

Vial No.: A unique sample number assigned to sediment put into vials.

Sample Depth: Depth of sample in meters from top of core.

Paired Sample Box no.: Sample box number which corresponds in depth to vial number.

Total Carbon: Percent of carbon (both organic and inorganic) in sample.

Carbonate Carbon: Percent of inorganic carbon in sample.

Organic Carbon: Percent of organic carbon in sample. Calculated as total carbon minus carbonate carbon.