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U.S. Geological Survey Open-File Report 00-306: Chapter 5
Sedimentology and Core Descriptions, Marion-Dufresne Cores MD99-2204 through - 2209, Chesapeake Bay
by Pattie C. Baucom1, John F. Bratton1, Steven M. Colman1, Julie Friddell2, and Andre Rochon3
Abstract
Coring of Chesapeake Bay sediments by the Marion-Dufresne in June 1999, recovered Holocene sediments and paleochannel fills up to 21 m thick. Sedimentary sequences sampled included basal Miocene deposits, basal fluvial/deltaic units underlain by ravinement surfaces, and overlying estuarine deposits. The stratigraphy and sedimentology of Chesapeake Bay described here is consistent with interpretation of previous geophysical studies of Chesapeake sediments and allow robust extension of new data from coring sites to the entire Chesapeake paleochannel system.
Introduction
Chesapeake Bay, which occupies the paleo-valley of the Susquehanna River, is a classic example of an estuarine system shaped by a complex history of Quaternary sea-level fluctuations. Previous seismic reflection surveys (Colman and others, 1990; Colman and Mixon, 1988) identified three alternating generations of the lowstand river valley and highstand bay, preserved as distinct paleochannel systems correlated to major glacial-interglacial cycles. Each of the paleochannel-fill sequences consists of two seismically distinguishable units. The lower fluvial unit is characterized by strong, irregular, discontinuous reflections, whereas the overlying restricted-estuarine to open bay unit is characterized by weak, long, smooth reflections (see also Vogt and others, this volume). These paleochannel systems were formed during periods of relative low sea level and backfilled with fine-grained estuarine sediment during subsequent sea-level highstands. The youngest paleochannel of the Susquehanna River preserved beneath Chesapeake Bay was carved approximately 18,000 years ago, and extended beyond Cape Charles to the edge of the continental shelf (Colman and others, 1990; Colman and Mixon, 1988). Similar sedimentary sequences from 7 to 21 m thick were penetrated during this coring operation that record important information about late Holocene sea-level history, sedimentation rates, and post-European colonization changes to Chesapeake Bay.
Previous scientific coring studies in the main stem of the Bay have been limited to the upper eight meters of sediment. Among the more significant efforts was a core transect project by the Maryland Department of Geology, Mines, and Water Resources in 1950 and 1951 (Ryan, 1953), a gridded coring program conducted by Biggs (1967) north of the Patuxent River, and a variety of core studies performed by Grace Brush and her students over the last few decades (e.g., Brush and others, 1982; Cooper and Brush, 1991). Additional sedimentary and paleontological studies were conducted by the USGS and collaborators beginning in 1996 as discussed by Cronin and Grinbaum (1996), Cronin (1997), Kerhin and others (1998), Cronin and others (1999a), Cronin and others (1999b), Baucom and Colman (1999), Cronin and others (2000), and Baucom and others (in review). Coring by the R/V Marion-Dufresne was successful in building on these previous efforts by the most complete record of Holocene sedimentation in Chesapeake Bay yet available. This paper describes the shipboard and post-cruise core descriptions, digital photography, spectrophotometry sampling, and water content.
Core Processing Methods
This section describes the shipboard and onshore description and sampling of six cores, numbered MD99-2204 through -2209, collected in June 1999 from the R/V Marion-Dufresne. Core locations and water depth for each sediment core collected (mbsl) are recorded in Chapter 1, Table 2, this volume. Cores were described and sampled on the ship or at the University of Rhode Island (URI) Graduate School of Oceanography laboratories and at USGS laboratories in Woods Hole, Massachusetts.
Lithological descriptions
A graphical key is presented in Figure 5.1 and lithological core logs are presented in Figures 5.2, 5.3, 5.4, 5.5, 5.6, and 5.7. One core collected June 20 (MD99-2205) and two cores collected June 22 (MD99-2208 and 2209) were described in the shipboard sediment laboratory (Fig. 5.8) prior to sampling. Working halves of cores were examined visually and with a 10X hand lens as soon as possible after splitting, photography, and spectrophotometry were performed to minimize color alteration. The color variables hue, value, and chroma were determined for sediments by matching with standard color chips on a Munsell® soil color chart. For example, a typical Chesapeake sediment color of hue 5GY, value 4, chroma 1, is noted as 5GY 4/1, a dark greenish gray. In addition to color, each sediment core was described in detail by noting grain size, density, sedimentary structures, biological components, and the nature of stratigraphic contacts. Whole cores offloaded during the port call in Virginia on June 21 were split, described, and sampled at URI and at USGS in Woods Hole later in July and August. These cores were kept refrigerated during storage and shipping (see Chapter 1, this volume).
Digital photography
Cores were cut into 1.5-m sections and then split lengthwise on the ship's deck. The working half of each core section was brought to the core description laboratory and immediately photographed before interaction with the air changed the sediment's color. Using a digital Minolta SLR camera mounted above a platform that held the core sections in place (Fig. 5.8), three 50-cm segments were photographed for each 1.5-meter section. Each image was labeled a, b, and c to denote the top, middle, and bottom 50-cm segments of each section, respectively. Additionally, the core name (for example, MD99-2204) was placed at the bottom of each picture and the section number (1, 2, 3, etc.) was placed in the upper left corner beside the segment letter (see Figure 5.9). The platform was mobile and had centimeter-scale markings beside the trough where the core section was held so that the location and size of sediment features could be easily determined.
Appropriate lighting of the cores for photography was difficult because of the dark hue of most Chesapeake Bay sediment. Four small fluorescent lamps were suspended above the core to illuminate the photographed segment, and the overhead laboratory lights were turned off to avoid obscuring reflections and other lighting irregularities. The images were captured in color in high-resolution JPEG format on a personal computer in the core lab, and eventually stored on the hard drive of one of the ship's main computers. An electronic copy of each of the Chesapeake core image files was stored on a CD-ROM for additional post-cruise photo processing.
Spectrophotometry
In order to measure the color of the light reflected from sediment cores split on the ship, a Minolta CM-2002 handheld spectrophotometer was used in the core description laboratory. The instrument has an 8-mm diameter optical sensor that measures the wavelength of light reflected off the sediments. Measurements were made directly on the sediment surface as soon as possible after photography was completed and after sediment surfaces had been covered with transparent plastic wrap to minimize further oxidation and to keep the sensor clean. Spectral reflectance was measured at wavelengths of 400 to 700 nm divided into 31 channels of 10nm each. Plots of the 10-nm bands at the 400-410 nm and 690-700 nm for core MD99-2209 are show in the upper part of Figure 5.10. Measurements were made every 5 cm down the length of each core, and a white calibration was performed at the end of each 1.5 m section. Measurements were not made in voids or where gravel or shells made it impossible to place the optical sensor flat on the sediment surface. The reflectance measurements provide an estimate of the sediment color in Munsell® notation and in the L*a*b color difference system as established by the Commission Internationale d'Eclairage (CIE). L*a*b color consists of a luminance or lightness component (L) and two chromatic components: the "a" component (from green to red) and the "b" component from blue to yellow). A plot of the "L" component of the scan data for core MD99-2209 is shown in the lower part of Figure 5.10 (chromatic components "a" and "b" are not shown).
Sampling
Samples were collected on board from cores MD99-2208 and -2209 for analysis of water content, biogenic silica, trace metals, and stable isotopes (one sample from each interval; split after drying for separate analyses). Samples were collected in new polyethylene vials with screw caps that had been weighed prior to the cruise. Sample spacing was generally 10 cm and the interval sampled was approximately 0.5 cm thick yielding 710 g of wet sediment. In the upper banded interval of core MD99-2209, samples were collected from the center of each color band, rather than every 10 cm. Dedicated disposable sampling utensils were used for each water content sample. After initial 10-cm sampling was completed, working halves of cores were bulk sampled in 2-cm slices for microfossil analyses and later subsampling for other analyses (Fig. 5.11). Samples were refrigerated shortly after collection. Chilled samples were packed in coolers and transported from Quebec City to Lamont-Doherty Earth Observatory (LDEO) in Palisades, New York in a refrigerated truck. The coolers were sent by overnight courier from LDEO to the USGS laboratory in Woods Hole where they were refrigerated until further preparation began.
Unsplit cores MD99-2204, -2206, and -2207 were shipped in a refrigerated truck after docking in Portsmouth, Virginia, to USGS refrigerated storage facilities in Reston, Virginia on June 22, 1999. These cores were later shipped in a refrigerated truck to URI, where they were stored in a refrigerated section of a marine sample warehouse. Cores were split, described, and sampled for physical properties, micropaleontology, geochemistry and other parameters in July and August 1999. Cores split on shore were not photographed. Onshore sampling was performed in a similar manner to shipboard sampling. Samples for micropaleontology, water content, and other inorganic geochemical parameters were collected from cores MD99-2204 and -2207 at URI on July 21 and 27, 1999. Core MD99-2206 was split and described at URI on July 27; all sampling of this core was performed on August 4 and 5 at USGS in Woods Hole.
Water content
Water content samples were collected by the same protocol used in onboard sampling and transported to USGS-Woods Hole. Pre-weighed and capped vials containing aliquots of wet sediment were uncapped, weighed, and then placed in an oven maintained at a temperature of 60°C until the sediment was completely dried (usually 2-3 days). Vials were then reweighed to determine the weight of water lost by the sediment. Water content was calculated as the weight of the water lost divided by the initial wet sediment weight. No correction was made for salinity of porewater. Water content profiles are provided in Figure 5.12 for cores MD99-2204, -2206, -2207, 2208, and -2209.
Preliminary Results
The sediments preserved beneath Chesapeake Bay are composed of massive to horizontally laminated mud, sand, and gravel and were deposited in at least three depositional environments that may provide a complete record of a Holocene history of the bay. The oldest sediments are channel deposits that fill channels cut into green-gray, Miocene mud. Stratigraphically above the channel gravels and sands are restricted-estuarine sand and mud and open-estuarine mud. Channel deposits are preserved well in three cores: MD99-2204, -2205, and -2206. The channel deposits are 40 to 100 cm thick and consist of massive, poorly sorted sand, gravel, and cobbles as large as 10 cm in diameter. These coarse fluvial sediments were deposited above Miocene mud (in core 2204 and perhaps 2206) or interbedded with sand and mud deposits (in core 2205). The boundary between the fluvial and estuarine sediments and the Miocene is marked by a decrease in water content of about 20% from overlying open-estuarine deposits Fig. 5.12). Water content values are also less variable in the compact Miocene mud. Organic material consisting of mostly roots is abundant and well-preserved within the channel deposits.
Stratigraphically above and in sharp contact with channel sands and gravels are restricted-estuarine sediments. These fine-grained sediments are up to approximately 10 m thick and are composed of greenish-gray, generally fining-upward, massive, poorly-sorted muddy sand to sandy mud, or interbedded sand and mud, with scattered oyster beds and woody organic layers common. Filled burrows are well-preserved throughout this lithofacies. Gradually, these sediments fine upward and become more characteristic of true open-estuarine mud.
The youngest lithofacies, preserved in all cores, is open-estuarine mud. This unit is up to 7 m thick and is characterized as black to greenish gray, massive to diffusely laminated or banded, slightly sandy (trace to ~10%), clayey mud. Filled burrows are common in this lithofacies and broken and articulated bivalves (Mercenaria, Mulinia, Crassostrea) are abundant in shallow-water core sites, but are less common and occur in discrete layers in deeper water. Cores from most sites, especially deeper-water sites, contain intervals of banding or laminations without significant bioturbation, indicating high porewater concentrations of hydrogen sulfide and deposition in bottom waters with low dissolved oxygen. Layering and darker color characterize the top 20 to 100 cm of most cores, but extend to a depth of 280 cm in core MD99-2209. The transition from the dark core tops to lighter gray underlying sediments is typically sharp and is interpreted as marking the diagenetic oxidation and transformation of dark iron monosulfides (hydrotroilite, mackinawite, pyrrhotite) to the disulfide (pyrite) (Biggs, 1967).
Acknowledgements
We thank the captain, crew, and members of the scientific party of the R/V Marion-Dufresne, particularly Elisabeth Michel and Yvon Balut, for assistance with core collection and processing. We also thank John King and his associates at URI, Thomas Cronin and associates from USGS-Reston, and Jason Yonehiro for help during post-cruise sampling.
References Cited
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