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).
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.
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.
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.
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).
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.
U.S. Department of Interior, U.S. Geological Survey
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