Preliminary work has shown that biogenic carbonate from bivalves, foraminifera, and oysters are the most reliable fraction for radiocarbon dating in Chesapeake Bay (Cronin, 1999, 2000; Colman and others, 1999; Colman, unpublished data). For this study, samples of biogenic carbonate, mostly small clam shells, were analyzed for their radiocarbon content by AMS methods. A few samples of oysters and woody material were also analyzed (Table 6.1).
The radiocarbon ages were converted to calendar ages relative to 1950 using the calibration program CALIB 4.1 (Stuiver and Reimer, 1993). The wood samples were converted using the standard tree ring calibration data set of CALIB. The biogenic carbonate ages were converted using the marine calibration data set, which contains a reservoir correction of about 400 years. The reservoir correction for Chesapeake Bay probably varies from the standard marine reservoir age of 400 years because of mixing with river water. However, benthic organisms like bivalves tend to be in contact with more saline, marine water because of density effects. Analyses of three samples of oysters collected alive before atmospheric nuclear bomb testing suggest that the reservoir age for areas of Chesapeake Bay where cores were obtained is within analytical uncertainties of the standard marine reservoir age of 400 years (S.M. Colman and P. Vogt, unpublished data).
Because the upper part of the Holocene fill of the main channel was obtained in nearby core MD99-2207, only the lower part of MD99-2204 was sampled for radiocarbon. The lowermost part of the core consisted of compact, oxidized silty clay, which contained foraminifera and ostracodes of Miocene age. The ages of wood samples above this unit, apparently basal sediments above the unconformity formed by the late Pleistocene channel of the Susquehanna River, are as old as 12,750 years. (Fig. 6.1). These are some of the oldest radiocarbon ages ever obtained from sediments beneath the bay, although the possibility exits that the wood may have been transported to the site. The oldest age from marine/estuarine facies come from the site of MD99-2204: 10,310 yr BP at 464 cm.
For core MD99-2209, the ages form a remarkably well-behaved series that contains no age reversals with depth, except at the very base of the core (Figs. 6.1, 6.2). The upper and lower parts of the core appear to form distinct units, with an unconformity separating them at about 810 cm. The interval from about 2,200 to 5,800 yr BP appears to be missing, or represented by a very condensed section of sediment. Hiatuses have been observed in the seismic records of the Holocene fill within the late Pleistocene channel (Colman and Mixon, 1988; Vogt and others, this volume), where they appear to represent local shifts in the sites of rapid, progradational channel in-filling.
The ages for the upper part of MD99-2209 are entirely consistent with pollen stratigraphy from the core (Willard and others, this volume, Chapter 7), and with radioisotope data (A. Zimmerman, unpublished data) and radiocarbon ages (S. Colman, unpublished data) from a short core (RD98-1) taken at the same site. Together, the data suggest a major change in sedimentation at about the time of rapid land clearance in the area, here taken to be 1800 ± 40 AD (c.f. Brush 1984; Willard and others, this volume, Chapter 7). In terms of mass accumulation rate (Colman and others, 1999), the change amounts to an increase by a factor of about four.
The ages for core MD99-2207 also form a relatively well defined sequence, with only three anomalous samples (queried in Fig. 6.1). The ages of the oldest shells from the marine/estuarine facies (10,130 yr BP at 1161 cm) is similar to that in MD99-2204. In contrast to MD99-2209, no unconformity is apparent in the sequence, which spans the entire Holocene. For the last 2000 years, sediment accumulation rates at MD99-2207 are about half of those at MD99-2209. Like the latter site, the ages in the upper part of MD99-2207 are entirely consistent with pollen stratigraphy (Cronin and others, 2000), radioisotope data (Cronin and others, 2000), and radiocarbon ages (S. Colman, unpublished data) from a short core (PTMC-3) taken at the same site. The data indicate about a four-fold increase in mass accumulation rate (Colman and others, 1999), very similar to that at MD99-2209.
In the lower parts of the cores, there may be some disagreement between the radiocarbon ages and ages inferred from pollen stratigraphy (Willard and others, this volume; King and Heil, this volume). The basal ages of many of the cores come from samples of fluvial or fluvial-estuarine deposits, and may thus have been transported and be older than the sediments in which they occur.
Colman, S.M., and Mixon, R.B., 1988, The record of major sea-level fluctuations in a large Coastal Plain estuary - Chesapeake Bay, eastern United States: Palaeogeography, Palaeoclimatology, and Palaeoecology, v. 68, p. 99-116.
Colman, S.M., Halka, J.P., Hobbs, C.H., III, Mixon, R.B., and Foster, D.S., 1990, Ancient channels of the Susquehanna River beneath Chesapeake Bay and the Delmarva Peninsula: Geological Society of America Bulletin, v. 102, p. 1268-1279.
Colman, S.M., Cronin, T.M., Bratton, J.F., Baucom, P.C., and Poag, W.C., 1999, Chronology of sedimentation in the Chesapeake Bay from recent coring programs, including the 1999 R/V Marion-Dufresne IMAGES leg: Eos, American Geophysical Union Transactions, v. 80, no. 46, p. F1.
Cornwell, J.C., Conley, D.J., Owens, M., and Stevenson, J.C., 1996, A sediment chronology of the eutrophication of Chesapeake Bay: Estuaries, v. 19, p. 488-499.
Cronin, T., Colman, S.M., Willard, D., Kerhin, R., Holmes, C., Karlsen, A., Ishman, S., and Bratton, J., 1999, Interdisciplinary environmental project probes Chesapeake Bay down to the core: Eos, American Geophysical Union Transactions, v. 80, p. 237-241.
Cronin, T.M., Willard, D.A., Kerhin, R.T., Karlsen, A.W., Holmes, C., Ishman, S., Verardo, S., McGeehin, J., Colman, S., Zimmerman, A., 2000, Climatic variability in the eastern United States over the past millennium from Chesapeake Bay sediments: Geology, v. 28, p. 3-6.
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Ellison, R.L., and Nichols, M.M., 1976, Modern and Holocene foraminifera in the Chesapeake Bay Region, in Maritime Sediments Special Publication 1, p. 31-151.
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Officer, C.B., Lynch, D.R., Setlock, G.H., and Helz, G.R., 1984, Recent sedimentation rates in Chesapeake Bay, in Kennedy, V.S., The estuary as a filter: New York, Academic Press, p. 131-157.
Slota, P.J.J., Jull, A.J.T., Linick, T.W., and Toolin, L.J., 1987, Preparation of small samples for 14C accelerator targets by catalytic reduction of CO: Radiocarbon, v. 29, p. 303-306.
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