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U.S. Geological Survey Open-File Report 00-306: Chapter 2

Geophysical Environment in Chesapeake Bay: Marion-Dufresne Sites MD99-2205, 2206 and 2208

by Peter R. Vogt1, Jeffrey P. Halka2, Rick A. Hagen3, and Thomas Cronin4


Seismoacoustic characterizations (primarily 2-15 kHz chirp subbottom profiles, but also 100 kHz sidescan sonar mosaics and archival bathymetry) comprise site survey information used to formulate scientific objectives and position Calypso piston cores for Marion-Dufresne (MD99-2205; 673 cm sediment recovered), MD99- 2206 (782 cm) and MD99-2208 (782 cm) in the Maryland portion of the Chesapeake Bay mainstem west of Parkers Creek. Core MD99-2205 is located in a patch of acoustically defined gassy sediment, 360 m west of a conjugate "no-gas" core in acoustically gas-free sediment.

Dissolved methane porewater analysis indicates that only the Holocene sediment, whose basal age is ca. 500 yr BP, contains significant concentrations of gas throughout. Underlying older Quaternary sediments, as well as the entire "no-gas" core to the west, lack significant dissolved methane. The young age of the oldest Holocene sediment at MD99-2205 shows that the methane in this gas patch is produced entirely from near-modern estuarine hemipelagic sediments. The young age of the oldest recovered Holocene sediment further demonstrates that Holocene sediments as young as a few centuries prior to European settlement were either removed by Holocene bottom currents or were never deposited. A strong seismic reflector interpreted as the Cape Charles Erosion Surface, underlying modern estuarine sediments, is present at 500 cm below sea floor (bsf), or 19 m bsl (below modern sealevel), at the no-gas site MD99-2206, and is correlated with the pebbly horizon cored at 350 cm bsf (1950 cm bsl) at the gas site MD99-2205. A 44,400 yr BP 14C age of wood from below the unconformity is consistent with the seismic stratigraphy and subaerial conditions during earlier Quaternary times, possibly marine isotope stage (MIS) 3. Core MD99-2208, taken in the Parkers Creek paleochannel less than 2 km from the presently eroding Miocene exposures along the Calvert Cliffs, seems from preliminary 14C dating (~5700 yr BP at 629 cm bsf (1629 cm bsl) and 7400 yr BP for oyster shells at 779 cm bsf (1779 cm bsl) to have recorded a possibly complete early-middle Holocene to present record of sedimentation in the Chesapeake well to the west of the Susquehanna (Cape Charles) paleochannel. The downward transition from acoustically transparent, laminated, locally gassy and reverberant sediments may reflect a ~4 ka transition from nearshore and narrow tributary stream environments to open estuarine environments like those characterizing the site today.

Introduction and Background

In this chapter we discuss the "geophysical" (seismoacoustic) data in the area of Marion-Dufresne cores MD99-2205, 2206 and 2208. These Calypso piston cores were collected during June 20-22, 1999 in the Maryland portion of the mainstem Chesapeake Bay, west of the Cape Charles (paleo-Susquehanna) paleochannel (Figs. 2.1 and 2.2), in a seismoacoustically well-mapped area (Figs. 2.3, 2.4, 2.5, 2.6, 2.7, and 2.8). As the chirp and older seismic reflection profiles only record round-trip reflection time, a sound speed profile has to be assumed to correlate core observations to seismic stratigraphy. Following Colman and Halka (1989), we shall assume 1500 m/s sound speed in water column and Holocene sediments, and 1600 m/s for the next older Quaternary unit, Qe.

Cores MD99-2205 and 2206 were taken respectively inside and west of a band of biogenic methane, to recover the porewater and thereby shed light on the factors which control the presence or absence, and the origin, of biogenic methane gas (See Hill and others, 1992, for an in-depth treatment of biogenic gas in Chesapeake Bay sediments). Our shipboard "working hypothesis" to be tested by these cores was that subbottom porewater convection is responsible for the gas/no gas boundary, with fresh water rising in the "gas" zone and Bay water, containing methane-oxidizing oxygen and sulfate in solution, descending nearby in the "no gas" zone. Alternatively, the sediment under the gas band contains more preserved organic matter, due to its greater water depth, hence lower average dissolved oxygen. These and other models or hypotheses will be the subject of later publications.

Hagen and Vogt (1999) demonstrated from accurate repeat chirp profiles and 100 kHz sidescan sonar that where the methane gas top (acoustic reflector) approaches within one meter or less of the Bay floor, the depth to this top migrates vertically, approaching the Bay floor in late summer and fall, and descending by up to 50 cm in late winter and spring. This seasonal change in the acoustic character of bayfloor and subbottom methane gas was postulated to result from the changing sediment temperature, with warmer water promoting gas bubble formation, both by lower methane solubility and higher biogenic production rates. Cores MD99-2205 and 2206 were taken on 20-21 June, just prior to the average mid-summer high in water and immediate Bay floor sediment temperatures (Fig. 11 of Hagen and Vogt, 1999). At this time sediment from the Bay floor down to ca. 150-200 cm bsf would be expected to exceed its average annual temperature, while from ca. 200 to 500 cm bsf the sediment would be somewhat cooler, "remembering" the previous winter's cold.

MD99-2208 was taken in the Parkers Creek paleochannel as close to land as was practical, in this case 10 m of water. This core is the only one of the six MD99 Chesapeake Bay cores not collected in (or above) the Cape Charles paleochannel or its immediate margin. The main coring objective was to obtain a Holocene paleoclimate and sea-level rise record at a shallow-mid-Bay site subjected to rainfall and discharge-induced salinity variability. While this core was not expected to contain as detailed or continuous a record as at the deeper-water sites, the shallower-water record was expected to be more sensitive to climatically induced salinity variability (Cronin and others, 2000).

The area of these three MD cores (Figs. 2.3 and 2.4) is the most thoroughly mapped, geophysically, of any of the MD cores and corresponds to the paleochannel system of a small Chesapeake tributary, Parkers Creek. Geophysical data include the Ocean Research Equipment (ORE) Geopulse and 3.5 kHz profiles of Colman and Halka (1989), subsequently collected 3.5 kHz profiles (J. Halka, unpublished data), NRL Acoustic Sediment Classification System (ASCS) profiles (Fig. 2.3; Vogt and others, 1996), and very comprehensive Edgetech 2-15 kHz chirp profiler and 100 kHz sidescan sonar survey data (Figs. 2.3 and 2.4; Hagen and Vogt, 1999). The area off Parkers Creek had also been sampled with several shorter (3-4.5 m long ) piston cores prior to the Marion-Dufresne cruise ( prefixed "PRCK" ; Fig. 2.3; Kerhin and others, 1998; Cronin and others, 1999).

To the west of the MD and other cores lie the Calvert Cliffs, exposing middle Miocene fossiliferous silts and clays to shoreline erosion (Vogt and Eshelman, 1987). The MD cores were therefore expected to include cliff erosion products reworked into Holocene sediments. While pre-Wisconsinan Susquehanna paleochannels generally lie successively farther east of the Wisconsinan Cape Charles channel, this is not the case in the area off the central Calvert Cliffs, where the ca. 130-175 ka (?) Eastville paleochannel swings west of the younger Cape Charles paleochannel (Colman and Halka, 1989). As a result, the early to middle Holocene Cape Charles erosion surface is developed on post-Eastville channel fill rather than on Miocene marine sediments over much of the area off Parkers Creek. MD cores MD99-2205 and 2206 were recovered from the area interpreted as underlain by Holocene overlying Eastville channel fill, in turn overlying Miocene (Figs. 2.5 and 2.6), whereas the Holocene is interpreted to lie directly on Miocene at the site of MD99-2208. The Miocene sediments exposed in the Calvert Cliffs west of the MD core sites are semi-consolidated, variably fossiliferous clays, silts, and fine sands of the Calvert and Choptank formations, middle Miocene in age (Vogt and Eshelman, 1987).

In this report we adopt the nomenclature of Colman and Halka (1989) for the main Quaternary sediment bodies and erosion surfaces: the erosional unconformity at the base of the Holocene is called the Cape Charles erosion surface (CCES), and the Holocene sediments resting on the CCES are labeled Qx. Similarly, the Eastville erosion surface (EES) developed when the Susquehanna River flowed in the Eastville paleochannel. The paleochannel fill ,Qe, was deposited during the following sea-level rise. The oldest major paleochannel system (Exmore) described by Colman and Halka (1989) as underlying parts of the modern Chesapeake Bay is apparently not represented in any of the Marion-Dufresne cores, nor is it present in the subsurface at the core sites.

From the morphology of Parkers Creek and its relation to Battle Creek, which flows westward into the modern Patuxent River (estuary), Vogt (1991) inferred that Parkers Creek once flowed in a westward direction, forming the middle portion of a pre-Eastville drainage system. The headwaters of this watershed, located in the area of the present Chesapeake mainstem, has been subsequently eroded away, allowing the Chesapeake Bay to "capture" Parkers Creek by progressive westward headward erosion.

The lower part of modern Parkers Creek is occupied by a salt marsh, and organic-rich salt-marsh sediment, all deposited close to sea level, and extending to depths of ca. 3 mbsl (Froomer, 1979). The oldest (300 cmbsl) salt marsh deposits, ca 2 ka in age, overlie stiff Miocene clayey silts similar to those exposed in the Calvert Cliffs. Froomer (1979) studied cores from two small tidal tribuaries to the Potomac, besides Parkers Creek, and inferred a sea level ca. 70 cm below present values at the time of English settlement and land clearing in the second half of the 17th century. He identified this age horizon in his cores from the inorganic to organic sediment ratio, which increased rapidly due to land clearing and consequent erosion.

Geophysical Surveys

Following the seismic reflection (ORE Geopulse and 3.5 kHz) lines collected by the USGS and the Maryland Geological Survey (Colman and Halka, 1989; J. Halka, unpublished data) and the NRL ASCS (1994) and chirp (1995) reconnaissance lines (Vogt and others, 1996), systematic mapping of the area off Parkers Creek was carried out in June and September, 1997, and March, 1998 (Fig. 2.3; Hagen and Vogt, 1999). To examine the temporal variability of the acoustic returns from the biogenic methane gas, several of the lines were repeated at various times. One line was rerun during all three surveys, and a part of this line duplicated a portion of an October 1994 profile (reported by Vogt and others, 1996). The NRL EdgeTech system includes the EdgeTech-Subscan chirp profiler and a dual frequency (100 and 500 kHz) sidescan. (A Cesium magnetometer that can be attached to the back of the sonar towfish, feeding digital magnetic data into the sonar digital recording system, was not used in the Parkers Creek surveys). Prior to the 1997-1998 NRL surveys, sidescan sonar data had only been collected along the 1994 lines reported by Vogt and others (1996).(These tracks examined areas from near Pt. Lookout north to Holland Point, and only a few tracks cross the Parkers Creek area shown in Fig. 2.3). The sidescan used in 1994 was a 100 kHz Klein 595 system, and the data were collected on DAT tapes. Following the detailed 1997-1998 NRL surveys, some additional 3.5 kHz profiles (S. Colman, unpublished data) were collected, in part following NRL tracks, as "site surveys" for cores PRCK-1, PRCK-2, and PRCK-3 (Fig. 2.3).

To date there has been no comprehensive analysis and interpretation of the large seismoacoustic database now available for the area off Parkers Creek. Hagen and Vogt (1999) published selected NRL data (100 kHz sidescan and 2-15 kHz chirp profiler) relevant to the problem of temporal variability of methane gas. They did not analyze the 500 kHz sidescan sonar data collected on the 1997-1998 surveys. A more complete analysis of the NRL and other data has not been completed. Such an analysis will include non-gas-related problems such as the infilling of the Cape Charles paleochannel, the relation to sea-level rise, the problem of erosion by bottom currents, the character of the Cape Charles erosion surface, and the Eastville paleochannel with its own infill stratigraphy.

For the purposes of this report, we have combined Hagen and Vogt's (1999; their Fig. 3) map of the distribution of methane bubbles in the subbottom off Parkers Creek (Fig. 2.4) with a preliminary interpretation of the NRL 100 kHz sidescan imagery in terms of "backscatter provinces". The provinces shown in Figure 2.4 are defined as follows (from west to east): NG (no gas), gas-free near-shore portion of Parkers Creek paleochannel; DS (deltaic sediment), strong-backscatter lobes interpreted as delta or fan-like accumulations of sandy sediment, the southern lobe probably created by sediments deposited in the Chesapeake Bay by modern/late Holocene Parkers Creek; WBS1 (weak backscatter), acoustically transparent or slightly stratified Holocene sediments, whose base is the CCES , incised variously into Miocene "basement" or Qe deposits (Colman and Halka, 1989); G0 (methane gas), Parkers Creek paleochannel system, with the top of methane gas generally about 10 meters subbottom (Fig. 2.8); G1s (methane gas), band of strong backscatter, indicating methane gas bubbles at or close to Bay floor, especially in late summer or fall ( Figs. 2.5 and 2.6; Hagen and Vogt, 1999); G1c (methane gas), gradational western edge of methane band, with gas not reaching Bay floor , especially in late winter or spring, and subbottom reflectors locally visible beneath gas (Figs. 2.5 and 2.6); M, Miocene stiff clayey sands exposed by bottom currents at or near Bay floor; WBS2 (weak backscatter), Holocene sediments with complex acoustic stratigraphy (Fig. 2.6) on hummocky western flank of Cape Charles (paleo-Susquehanna) paleochannel (The two hummocks in Fig. 2.6 may also be deep methane gas returns); G2 (methane gas), main gas bubble "curtain" above Cape Charles paleochannel; WBS3 (weak backscatter), Holocene sediments above eastern flank of Cape Charles paleochannel; G3 (strong backscatter), methane gas band; and WBS4 (weak backscatter), acoustic window, with 10 m or more acoustically stratified, gas-free sediments on CCES. The nature and significance of the backscatter provinces shown in Fig. 2.6 will be discussed more completely in later publications. Chirp profiles C5/6-1 (Fig. 2.5), C5/6-2 (Fig.2.6), and C8-1 (Fig. 2.7) are discussed in terms of preliminary MD core results in the following section.

MD99-2205, 2206 Cores: Relation To Acoustic Stratigraphy

The 'gas/no-gas' core pair, MD99-2205 and 2206, was placed respectively at 38° 33.95' N, 76° 26.66' W ("gas" core) and at 38° 33.92' N, 76° 26.92' W ("no-gas core"), according to the ship's GPS positioning system. Water depths, given by the ship's log, were 16 m at the "gas" core and slightly shallower (14 m) at the "no-gas" core to the west. If the positions given above are plotted on the National Oceanic and Atmospheric Administration (NOAA) bathymetry, the corresponding water depths are 19 m and 13.5 m, respectively (Fig. 2.4). Core MD99-2205 penetrated 800 cm and recovered 673 cm sediment; MD99-2206 penetrated 900 cm and recovered 782 cm.

The two cores are located respectively 75 m and 100 m south of east-west chirp profile C5/6-2 (Fig. 2.6). The gas core, MD99-2205, was placed ca. 200 m east of the north-south chirp profile C5/6-1, while the no-gas core, MD99-2206, was taken 125 m west of this profile (Fig. 2.5). While only the 'target' core locations were on the NS chirp profile, these small displacements are of no significance to the scientific issue being addressed. The principal concern was that the cores be far enough into the "gas" and "no-gas" acoustic provinces, respectively, that navigation errors could not have allowed their accidental placement in the "wrong" acoustic province. The projected locations of the two cores place them in the "correct" provinces on the east-west profile (Fig. 2.6), while they both appear to plot in the "gas" province on the north-south profile (Fig. 2.5). However, inspection of the cores plotted relative to the acoustic backscatter provinces (Fig. 2.4) shows that core MD99-2206 (the "no-gas" core) has simply been artificially projected into the gas province. That a mere 125 m can move the core from the 'gas' to the 'no gas' province illustrates the sharpness of the boundary between these provinces. On the east-west profile (Fig. 2.6) the boundary appears abrupt. However, the north-south profile (Fig. 2.5) cuts the boundary at a small angle, showing that a finite transition zone exists. We estimate the width of this transition zone to be about 50 m, measured perpendicular to the province boundary. Proceeding eastward from the gas-free zone, we first encounter isolated pockets of gas, which act as 'point scatterers', whose actual dimensions, except for subbottom depth to the top of each pocket, cannot be determined from the chirp data. Farther to the east, a continuous "curtain" of gas appears, its top surface two meters below the Bay floor. The gas zone top then rises progressively toward the east, until it finally is shallow enough (a few decimeters) to appear as strong backscatter in 100 kHz sidescan images (Hagen and Vogt, 1999). In the western part of the "gas" zone the methane bubble concentration and/or bubble zone thickness are small enough to allow 2-15 kHz sound to penetrate the gas curtain and register reflections from the underlying stratigraphy (between m 250 and 350 in Fig. 2.5).

The acoustic stratigraphy at the "gas" core site is entirely obscured by methane gas, but the short separation between the two cores (ca. 360 m) and the relatively horizontal, continuous character of the subbottom reflectors near the "no-gas" core give us some confidence that the stratigraphy at the two sites is similar, save for porewater methane. The east-west profile (Fig. 2.6) and other nearby chirp profiles reveal a strong reflector corresponding to the CCES, 5 meters below the Bay floor, at the no-gas core site. The Holocene sediments above this reflector are acoustically transparent save for a relatively weak reflector ca. 1 to 1.2 m below the Bay floor. At historical sedimentation rates, this reflector must arise from within historically deposited sediments, so we labelled it "HI" (historical) in Figure 2.5. This reflector (or one or more similar ones) is seen in a number of NRL chirp profiles from this region. Below the prominent CCES reflector is a relatively weaker, less continuous horizon we interpret, following Colman and Halka (1989), as the EES. Based on the NRL chirp data and the previous work of Colman and Halka (1989), we suppose the EES to be developed on stiff Miocene clayey silts, similar to those exposed in the Calvert Cliffs updip to the northeast (e. g., Vogt and Eshelman, 1987). According to our interpretation prior to the Marion-Dufresne coring, the top ca 5 m of sediment at the "no-gas" core site, and a comparable thickness at the "gas" site (see below), should comprise mid- to late Holocene estuarine muds (Qc) deposited subsequent to the overtopping of the Cape Charles paleochannel by the widening Chesapeake Bay. Similarly, the 4.5 to 5 m sediment between the CCES and the EES reflectors comprise Quaternary (ca. 130-120 ka ) estuarine muds deposited after rising sea level caused overtopping of the Eastville (next older Susquehanna) paleochannel. Thus, both the CCES and the EES should be terrestrially formed erosion surfaces with probable coarser lag deposits and plant materials. The original thickness of Chesapeake estuarine muds (Qe) laid down on the EES during high sea level (~ + 6-7 m relative to present sea level) of marine isotope stage (MIS) 5e, and eroded during the subsequent colder stages, is unknown, but must obviously be less than height of MIS 5e sea level above the EES, i.e., less than ca. 30 m. If the erosion during these stages was instead small, then this penultimate Chesapeake Bay must have been about 25 m deep at the site of the cores.

Porewater squeezed from nine 50 cm long sediment plugs spaced every 1 meter were analyzed for dissolved methane at NRL and for other dissolved constituents (Pohlman and others, this volume). Dissolved methane concentrations of 0.3 to 1.1 microMoles in the "no-gas" core (MD99-2206) samples confirm that, indeed, there is practically no methane in the porewater, the low measured values keeping the porewater very undersaturated. By contrast, the "gas" core (MD99-2205) contains abundant dissolved methane throughout the Holocene portion of the core, ranging from 142 to 491 microMoles, the highest concentration being at 145 cm depth. Given the uncertainties in the depth to the actual highest methane concentration, it is possible that the shallow chirp reflector "HI" (Fig. 2.5) represents an "enhanced reflector" (due to small amounts of methane bubbles) or has sediment properties suitable for trapping methane bubbles. The lower part of core MD99-2205 is essentially gas-free. A pebbly horizon at 350 cm bsf (1950 cm bsl) probably marks the CCES, the boundary between the overlying, gassy Holocene sediment, and the underlying Eastville paleochannel fill, which according to Colman and Halka (1989) and the newer NRL chirp data has already degassed. If pebbles are common along the CCES, this would qualitatively explain why the CCES forms a strong seismoacoustic reflector (e.g., Figs. 2.5 and 2.6).

If the lowest sediments in core MD99-2205 are from at or near the EES, the entire unit Qe (323 cm) must have been sampled. That the two Quaternary units are thinner at site 2205 (350 and 323 cm, respectively), compared to their thicknesses at the no-gas site (500 and 480 cm, respectively) is not unexpected, as the entire Quaternary package thins from west to east of the methane gas band (Fig. 2.6). (As noted in the introduction, we assume 1500 and 1600 m/s, respectively, for the average sound speeds of units Qc and Qe).

Preliminary 14C ages are available for two samples from core 2205. A shell from 280-282 cm bsf was dated at 780+/-55 yr BP (WW-2719) uncorrected (ca 434 yr BP calibrated age), suggesting an age of ca. 500 yr BP for the oldest Holocene sediment, just above the lithologic unconformity interpreted as the Cape Charles Erosion Surface. This unexpectedly young date implies that the CCES was being actively eroded by modern bottom currents almost into historic times. Evidently the area of bottom erosion (exposed Miocene "basement" labeled "M" in Fig. 2.4) also covered this site at the time, implying the erosion area was either larger or in a somewhat different location than today. If we assume that sedimentation began abruptly at a constant rate 500 years ago, that rate has been about 7 mm/yr, nearly an order of magnitude higher than the average rate for core 2208 in the Parkers Creek paleochannel to the west (see below). The young age of the sediments just above the CCES at 2205 also implies that the methane being generated from these sediments is formed from organic matter deposited and preserved under essentially modern, open estuarine conditions. The high sedimentation rate, in addition to low oxygen levels, probably explain the preservation and burial of the organic matter. Assuming constant sedimentation rates throughout the last 500 years, we estimate the age of the weak reflector at no more than ca. 100 yr BP. If sedimentation rates have increased through time, and if compaction effects are included, this reflector is still younger.

The second 14C date from core 2205 was obtained from a wood fragment sampled from the basal part of the core. The age of 44,400 +/- 2500 years (WW-2722) is so great that carbon may be 14C "dead." We suppose that this forest was growing at the site during a time of low sea level, prior to the late Wisconsinan. It is uncertain whether the 14C age is "infinite," i.e. , of great but indeterminate age, or if this date, near the limit of reliable 14C dating, places the woody remains in the mid-Wisconsinan, in the "moderately cold" MIS 3. In either case the date is consistent with pre-cruise interpretations of the seismoacoustic data (e.g., Colman and Halka, 1989).

MD99-2208 Core: Relation To Acoustic Stratigraphy

MD99-2208 targeted the Parkers Creek paleochannel at a shallow site where part of the apparent paleochannel thalweg reflector is present in a chirp profile. Figure 2.7 shows the chirp profile and the intended (PRK-3) and actual MD99-2208 core locations, projected slightly east onto the profile. MD99-2208 was taken at 38° 32.24' N, 76° 29.19' W, in 10 m water according to the ship's log. The target location (PRK-3), located on the chirp profile (Fig. 2.7) is 38° 32.197' N, 76° 29.160' W, in 10.5 meters of water. This represents the shallowest water in which the 6-m draft Marion-Dufresne has cored with its Calypso corer, and required dropping the core barrel from above the water. Two cores were taken at the site, one worked up on board, the other archived. The 10 m long core barrel penetrated its entire length into the sediment and recovered 782 cm sediment. Oyster shells in the core bottom suggest that sediment recovery in the core barrel stopped when the oyster horizon was reached. As the core barrel penetrated its full length, it probably did not quite reach the Miocene "basement" (Fig. 2.7). Several strong reflectors ca. 600-700 cm bsf (1600-1700 cm bsl) (labeled "X" in Fig. 2.7) occur at a level at PRK-3 that, if extrapolated to the actual core site, may correspond to the oyster horizon cored at 782 cm bsf (1782 cm bsl). The strong reverberant reflector at 870 cm bsf (1870 cm bsl) at PRK-3 may be due to methane gas bubbles but may also represent coarse shell hash and gravelly debris, or perhaps gassy salt marsh deposits in the paleochannel bottom. Such salt marsh deposits were cored in nearby Parkers Creek (Froomer, 1979) and are ca. 3 m thick below the modern stream.

Preliminary 14C dates were obtained from shell material recovered from core 2208 at 628-630 cm depth (WW-2705, 6055 +/- 65 yr BP uncorrected, 6452 calibrated age) and from the core bottom (oysters at 778-780 cm; WW-2706, 7740 +/- 55 yr BP uncorrected, 8177 yr BP calibrated age). Although it is hazardous to interpolate ages from just these two 14C dates and the "zero" age of the Bay floor, we note that the ages are consistent with a relatively constant long-term average sedimentation rate of ~0.9 mm/yr, with a rate several times higher starting at the beginning of the historical period. On the basis of this estimate, the weak reflector at ca 100-120 cm bsf would represent some change in later post-European settlement time. If a constant sedimentation rate (uncorrected for compaction) is assumed for the entire core, the weak reflector dates from ~1 ka, i.e., late pre-settlement times. Until more dates are available these results are offered only as preliminary and approximate age estimates. The reflector may represent some diagenetic effect instead of the record of a depositional event or change.

Because most oysters live near sealevel, and because nearshore sediments tend to be sandy, we consider it likely that the oysters (mixed with sand) recovered at the bottom Core MD99-2208 represent a very shallow-water depositional environment. If this is the case, local sealevel must have risen by approximately or slightly less than the depth of oyster recovery, 17.8 m bsf. The sealevel rise curve derived for Barbados and corrected for local uplift there (Fig. 2 of Fairbanks, 1989) indicates a sealevel rise of ~18.5 m since 7740 14C yr BP , the uncorrected radiocarbon age of the oysters at MD99-2208. Thus, within the various uncertainties, we conclude that if the oysters were growing near sealevel, the Chesapeake Bay sealevel history for the early Holocene to present has been approximately the same as at Barbados. If the Parkers Creek paleochannel oysters were living significantly below sealevel (we consider this unlikely), the local sealevel rise must have been less than at Barbados.

If the 0-6000 yr BP submergence of 10 m (ave., 1.7 mm/yr) derived by Ellison and Nichols (1976, their Fig.7) for the James and Rappahannock estuaries is applied to site MD99-2208, and the sealevel rise prior to 6000 yr BP is assumed to have followed the Fairbanks (1989) Barbados curve, local sealevel must have risen by ~21 m since 7740 14C yr BP, inconsistent with the presence of oysters at 17.8 mbsl. We conclude that if the 14C ages for the oysters are correct, local sealevel cannot have risen as much as implied by the Ellison and Nichols (1976) data.

Although the chirp profile across the Parkers Creek paleochannel (Fig. 2.7) does not show an unambiguous reflection from the CCES, chirp and older data (Colman and Halka, 1989) suggest that if the oysters were living near sealevel at the time, this sealevel was probably below the CCES on either side of the paleochannel. That is, while brackish water had already flooded into the channel of Parkers Creek by 7740 yr BP (8177 yr BP calendar age), there was still dry land bordering the channel. Not long thereafter, however, the rising sealevel overtopped the channel edge, probably reducing the speed of currents in the channel, promoting the deposition of mud in a quieter environment, less favorable for oysters.

The subbottom adjoining the Parkers Creek paleochannel margins returns a strong reflection at 150-250 cm bsf (1150-1250 cm bsl). This reflector appears to obscure the CCES, which should lie at greater depth. Only at the left (northern side) of the chirp profile is there a hint of a Miocene "basement" reflector. We consider it likely that the acoustically transparent sediments, ~450 cm thick at PRK-3, and 150-250 cm thick outside the paleochannel (Fig. 2.7), represent estuarine muds deposited offshore under modern, open estuarine conditions. The underlying Holocene sediments are probably coarser and were deposited close to the eroding paleo-shoreline and in the confined Parkers Creek valley, and may therefore include larger amounts of Calvert Cliffs Miocene and post-Miocene sediments (Vogt and Eshelman, 1987). Based on the two 14C dates from the 2208 core, we estimate the transition to modern, offshore estuarine sedimentation to have occurred ~4 ka in this area.

Summary and Conclusions

Although cores MD99- 2205, 2206, and 2208 are just in their initial stages of study, we venture these preliminary conclusions: 1) The gas/no-gas cores verify earlier interpretations that biogenic methane is being generated in the acoustically "gassy" site (Pohlman and others, this volume) (Figs. 2.5 and 2.6). The cores further demonstrate that the gas is being produced only from sediments which, from extrapolation of chirp profiles, are of Holocene age (Qc of Colman and Halka, 1989) and estuarine in character, not derived from marshy paleochannel fill. 2) It is likely, although not proven, that both MD99-2205 and 2206 recovered both units Qc and Qe at both sites. Verification requires demonstration of reworked Miocene in both core bottoms. The difference in Quaternary cover thickness west (thick) vs east (thin) of the methane band in Fig. 2.6 probably explains why Qc and Qe are thinner at MD99-2205 than they appear to be in the chirp data at site 2206. 3) The recovery of a pebbly horizon at 350 cm bsf (1950 cm bsl) at 2205 at the probable (although gas-obscured) CCES would account for the strong acoustic returns generally observed for this reflecting horizon. 4) The recovery of oyster shells at ~782 cm bsf (1782 cm bsl) at site 2208 may explain the strong reflections seen at that approximate depth (Reflector X, Fig. 2.7). 5) The transparent upper unit at 2208 likely represents modern estuarine conditions out of influence of littoral processes, although an abrupt change implied by the reflectors seems inconsistent with a gradual migration of the core site farther offshore. It is also possible that the reflector represents the time when rapid shoreline erosion of Quaternary (Qe) sediments was replaced by slow erosion of fine-grained Miocene Calvert Cliffs deposits. Future studies of MD99-2208 sediments in relation to those exposed in the nearby Calvert Cliffs may serve to test these possibilities. 6) The recovery of 8180 yr BP (7740 yr BP uncorrected 14C age) oyster shells and sand at 17.8 mbsl in core MD99-2208 is consistent with these oysters having lived at or just below prevailing sealevel, provided local sealevel has risen by about the same amount (~18.5 m) it has over the same time period at Barbados (based on Fig. 2 of Fairbanks, 1989).


P. Vogt and R. Hagen were supported by the Office of Naval Research. Most of the data reported here were collected as part of a National Research Council Research Associateship award to R. Hagen. The Naval Research Laboratory Platform Support Fund helped pay for the use of the vessel R/V Discovery. We thank Rick Younger, captain of Discovery, for his skills in carrying out the geophysical surveys. Irene Jewett and Jan Morrow helped prepare the illustrations. J. Gardner, W.-Y. Jung, S. McDonnell, A.N. Shor, C. Nishimura, and particularly M. Czarnecki aided in the field work. J. McGeehin helped prepare samples for radiocarbon analyses at the USGS in Reston, and run on accelerator mass spectrometer at the University of Arizona. We thank C. Larsen for his review.

References Cited

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