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U.S. Geological Survey Open-File Report 00-306: Chapter 11

Physical Properties of Marion-Dufresne Cores MD-99 2204 - 2209 and Magnetic Secular Variation Studies of MD-99 2207

by John W. King and Clifford W. Heil
Graduate School of Oceanography, University of Rhode Island
South Ferry Road, Narragansett, Rhode Island 02882

Abstract

Variations in the physical properties from cores MD99-2204 through -2209 are used to identify different lithologies in the sedimentary record of the Chesapeake Bay. When calibrated to other paleoenvironmental proxies, the downcore variation in lithologies found in these cores may provide another means of establishing a paleoenvironmental record for this area.

Using a radiocarbon based age model (Colman and others, this volume) and the location of the Tsuga (hemlock) decline (Willard and Korejwo, this volume) from core MD99-2207, secular variation data from the Chesapeake Bay could be correlated to a radiocarbon dated regional secular variation curve that also had a well-defined hemlock decline. By visually matching similar troughs in these two curves and correlating the pollen datum, it was estimated that the radiocarbon ages for core MD99-2207 are ~1000 14C years too old for the late Holocene (1000-4500 14C years), and ~2000 14C years too old for the mid to early Holocene (4500-9000 14C years). Based on visual correlation, the basal age of core MD99-2207 is ~9100 B.P. which is ~3000 14C years younger than the basal age of Colman and others (this volume).

Introduction

GEOTEK® logging and magnetic secular variation studies of split cores were done at the Graduate School of Oceanography at the University of Rhode Island under the supervision of Dr. John W. King. The goals of these studies were to characterize stratigraphic variations in lithology and to derive preliminary age models from suitable cores.

Methods

The MD99 sediment cores were split in order to measure several bulk property parameters including: gamma density, magnetic susceptibility, P-wave velocity, and paleomagnetics. The split cores were also X-rayed and digitally imaged for RGB color analysis. Each core section was split into an archive and a working half along the same longitudinal axis in order to maintain continuity of paleomagnetic directional measurements. The working halves were sub-sampled and the archive halves were kept for the non-destructive bulk property measurements and color analysis.

The gamma density, P-wave velocity, and magnetic susceptibility were all measured simultaneously using a split-core GEOTEK® Multi-Sensor Core Logging System (MST). Using the MST, the split cores were measured in a continuous order from top to bottom at 1-cm intervals. Digital imaging and RGB color analysis was performed using the MST as well, however, this required a second pass because of the data acquisition software used. The results are presented in Figures 11.1, 11.2, 11.3, 11.4, 11.5, 11.6, 11.7, 11.8, 11.9, 11.10, 11.11, and 11.12.

Gamma Density

Gamma density (GRAPE) is the measurement of the wet bulk density of the sediment. It is measured using a Cs-137 source and detector. A 5 mm gamma beam passes through the sediment and is attenuated by Compton scattering. This attenuation is directly proportional to the number of electrons blocking the gamma beam. Therefore, by measuring the number of unscattered photons that pass through the sediment unattenuated, the density of the core material can be determined.

P-wave Velocity

Split core P-wave velocity is measured using two transducers; a pad-type transducer for the sediment surface and an oil-filled, roller-bearing transducer for the underside of the core. A short P-wave is transmitted through the sediment and is detected by a receiver. The travel time is measured and divided by the sediment thickness in order to determine the velocity. In order to measure the P-wave velocity, it is essential that there is a good acoustic coupling between the transducers and the core liner or sediment and core liner. The P-wave velocity is affected by gaps in the core, gaseous sediment, poor conductivity, and dewatering of coarse-grained sediment during the core splitting process.

Magnetic Susceptibility

Magnetic susceptibility of the split core sections was measured using a Bartington Instruments® point sensor. The magnetic susceptibility is measured by an oscillator circuit that produces a low intensity, non-saturating, alternating magnetic field. If the sensor is placed near anything that is susceptible to magnetization, the oscillator frequency will change. This change in frequency is then converted to magnetic susceptibility. The point sensor is very sensitive to temperature fluctuations, so the split core sections were allowed to equilibrate to the same temperature as the sensor before logging.

Digital Imaging and RGB Color Analysis

A digital image is taken of the split core section using a 300 dpi digital camera and GEOTEK® acquisition software. The image is stored in 20-cm increments which are then pieced together to create a composite image of the split core section by the GEOTEK® software. The composite image is then used to generate downcore RGB data at 1-cm intervals.

X-radiography

The split core sections were X-rayed using standard medical X-radiography. After removing the end caps, the core sections were placed in a sand trough to provide even exposure across the width of the core. The X-radiography was performed at 40-cm intervals (with 10-cm overlap) downcore.

Paleomagnetics

The split core sections were run on a pass through 2-G Enterprises® cryogenic magnetometer system with a 12.5- cm access in order to measure the paleomagnetic parameters. The natural remnant magnetization (NRM) was measured at 2-cm intervals and then each section was subjected to an alternating field demagnetization at 10.0 mT and measured again. The resulting data is reported as declination, inclination, and intensity.

U-channel sub-samples were taken from core MD99-2207 and taken to the Laboratoire des Sciences du Climat et de l'Environment in Gif-sur-Yvette, France in order to obtain higher resolution mineral magnetic and paleomagnetic data. The U-channels were subjected to several demagnetization steps for NRM, anhysteretic remanant magnetization (ARM), and isothermal remanant magnetization (IRM). The inclination data from these U-channels is then plotted against age and used to correlate the Chesapeake Bay sediment to well-dated regional secular variation curves.

In order to compare the paleomagnetics data from core MD99-2207 to the regional secular variation curves, the data was fit to an age model. The control points for the age model were obtained from Steve Colman of the USGS in Woods Hole, Massachusetts. The age model was made using 7 radiocarbon ages, 6 from bivalves and one from wood. The model consisted of the age control points and their corresponding depths. The points were connected and the equation of the line between each set of points was calculated (Figure 11.14). These equations were used to calculate the corresponding age for each depth along that line. Once an age had been assigned to each depth, the inclination data could be plotted versus age and compared to the dated regional secular variation curves. In addition, pollen studies of MD99-2207 (Willard and Korejwo, this volume) identified a regional pollen datum, the Tsuga (hemlock) decline at ~ 890 cm. This regional isochronous pollen feature (Webb, 1982; Davis, 1981) has also been identified in the regional SV curve and has an age of 4600 ± 100 years B.P. for the regional curve (King, 1983).

A second age model was constructed using the tie points from Figure 11.13. By visually matching troughs and the Tsuga decline in the regional secular variation curves to troughs and the hemlock decline in the U-channel data, new ages could be assigned to depths in the sediment. These new magnetically and pollen dated control points and corresponding depths are plotted next to the Colman age model in Figure 11.14.

Results

The GEOTEK® logging results for susceptibility and GRAPE density are presented in Figures 11.1, 11.2, 11.3, 11.4, 11.5, and 11.6. P-wave velocity data are not presented in this report because of problems encountered with use of the pad-type transducer. As a result, the P-wave analyses are being rerun. The data show distinctive patterns of downcore variations that may be used for core correlation. In general, higher values of susceptibility and density are associated with sandy lithologies, whereas lower values are associated with muddy lithologies.

The GEOTEK® logging results for RGB color analyses of digital core images are shown in Figures 11.7, 11.8, 11.9, 11.10, 11.11, and 11.12. The data show distinctive patterns of downcore color variations.

Geomagnetic secular variation studies of U-channel samples obtained from core MD99-2207 produced a high-quality inclination curve. We used radiocarbon ages obtained from MD99-2207 (Colman et al., this volume) to construct a radiocarbon dated inclination curve for this site. A comparison of the MD99-2207 inclination curve with a regional inclination curve using an "old carbon" corrected radiocarbon time scale (King and Peck, in press) is shown in Figure 11.13. The regional Tsuga decline provides a biostratigraphic constraint for the correlation. The radiocarbon ages for 2207 of Colman and others (this volume) are ~1000 14C years too old for the late Holocene (0-4500 years B.P.) and the difference increases to ~2000 14C years too old for the mid to early Holocene 4500-9000 years B.P.). A comparison of the age-depth curves obtained using the MD99-2207 radiocarbon ages (Colman et al., this volume) and the regional inclination curves are shown in Figure 11.14 and Table 11.1. The estimated basal age for MD99-2207 using the inclination comparison approach is ~9100 B.P., which is ~3200 14C years younger than the basal age estimated by Colman and others (this volume).

We believe that the secular variation (SV) 14C age estimates are more accurate than the radiocarbon ages of Colman and others (this volume) because the SV age estimates are consistent with the pollen stratigraphy of Willard and Korejwo (this volume) for core MD99-2207 and the regional pollen stratigraphy. Both the basal age estimate and the estimated age of the Tsuga decline (~4600 14C years) obtained by the SV approach are consistent with the interpretation of Willard and Korejwo (this volume) and previous regional pollen studies.

Conclusions

1. The GEOTEK® logging results provide a basis for core correlation. After calibration with other paleoenvironmental proxies, they may provide a basis for paleoenvironmental interpretations.

2. The Holocene radiocarbon ages obtained from MD99-2207 are too old. They are ~1000 14C years too old for the interval 1000-4500 yr B.P. and ~2000 14C years too old for the interval 4500-9000 yr B.P.

3. The basal age of core MD99-2207 is estimated to be approximately 9100 yr B.P. using SV correlation, which is ~3200 14C years younger than the radiocarbon age of Colman and others (this volume).

References Cited

Colman, S.M., Bratton, J.F., and Baucom, P.C., this volume, Radiocarbon dating of Marion Dufresne cores MD99-2204, -2207, and -2209, Chesapeake Bay, in Cronin, T.M., ed.: U.S. Geological Survey Open-File Report 00-306.

Davis, M.B., 1981, Outbreaks of forest pathogens in Quaternary history, in International Palynological Conference, 4th, Lucknow, 1976-77, v. 3, p. 216-217.

King, J., 1983, Geomagnetic secular variation curves for Northeastern North America for the last 9,000 Years B.P.: University of Minnesota, Ph.D. dissertation .

King, J., and Peck, J., in press, Use of paleomagnetism in studies of lake sediments, in Developments in Paleoenvironmental Research, J. Paleolimnology.

Webb, T., III., 1982, Temporal resolution in Holocene pollen data, in North American Paleontological Convention, 3rd, Proceedings 2, p. 569-572.

Willard, D.A. and Korejwo, D.A., this volume, Holocene palynology from Marion-Dufresne Cores MD99-2209 and 2207 from Chesapeake Bay - impacts of climate and historic land-use change, in Cronin, T.M., ed.: U.S. Geological Survey Open-File Report 00-306.

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