Although Chesapeake Bay has been the subject of numerous geological and biological investigations, there is still relatively little information about its late Pleistocene and Holocene geological and environmental history. With the exception of the uppermost 4-5 meters, the relatively thick (10 to >20 m) Holocene sedimentary sequence that has been identified in geophysical investigations (Colman and Halka, 1989; Hagen and Vogt, 1999; Vogt and Halka, unpublished data) had yet to be recovered and studied. Because Chesapeake sediments contain numerous micropaleontological and geochemical indicators of the environmental history of the bay and its watershed, they comprise an excellent source of information about climatic variability in the eastern United States (Cronin and others, 1999, 2000). In addition to its paleoclimatic record, bay sediments also provide a record of environmental disturbance by anthropogenic activity, such as 19th century land-clearing and 20th century fertilizer application, and thus provide useful targets for environmental restoration efforts (Karlsen and others, 2000). Chesapeake Bay is also an excellent site to study biogenic methane formation and migration in sediments.
Previous stratigraphic investigations of Chesapeake Bay have been limited by coring methods. With the exception of a 7.5 meter long core off the Rhode River taken in 1998 by Colman and King with a Mackereth coring device, prior piston coring had only recovered the upper 450 cm of bay sediments (see Kerhin and others, 1998). Depending on the sediment accumulation rate and erosional history at any particular site, the upper 450 cm usually represents only the last 2,000 years (In this report we use calendar years based in part on marine shell radiocarbon dates calibrated by CALIB Program of Stuiver and Reimer, 1993). In some cases piston coring recovered older Holocene deposits, but the sedimentary record was discontinuous and not well-suited for reconstructing a continuous paleoclimate history. Coring by the Marion-Dufresne provided the opportunity to test the potential of the giant piston corer, typically used in deep and mid-depth oceanic regions, to recover relatively long sedimentary records in extremely shallow estuarine environments.
To improve the recovery of Holocene sediments and answer the many questions they can provide, the U.S. Geological Survey (USGS), Naval Research Laboratory (NRL), Maryland Geological Survey (MGS) and U.S. Environmental Protection Agency (EPA) entered into a collaborative effort to investigate the sedimentary record of the bay. To do this, they joined the International Marine Global Change Study (IMAGES), a multi-nation program whose primary mission is to understand the role of the oceans in global climate change. From June to September, 1999, IMAGES sponsored a large part of an oceanographic cruise by the French coring ship Marion-Dufresne in the Caribbean, North Atlantic, Nordic and adjacent seas. As part of this larger effort, the U.S. consortium sponsored the part of the cruise that conducted coring in Chesapeake Bay. The purpose of this chapter is to describe the coring operations and preliminary shipboard findings of Chesapeake Bay coring by Marion-Dufresne. The following twelve chapters in this volume describe initial shore-based scientific results.
MD99-2204 and MD99-2207 Core Sites
The main goals at these sites were to obtain a complete stratigraphic record of the Holocene in the modern Chesapeake Channel off the mouth of the Potomac River, to determine the age of the pre-Holocene sediments, and to provide ground-truthing for geophysical "chirp" profiler records (Fig. 1.2). The Holocene sedimentary fill in this region is relatively thick (Colman and Halka, 1989) and it was hoped that a high-resolution sedimentary and paleoclimate record could be recovered. Pre-cruise geophysical surveys (Vogt and others, Chapter 3, this volume) showed that presumed early Holocene sediments were acoustically transparent and the mid-late Holocene consisted of acoustically layered units with approximately 10-15 distinct horizons that could be traced from the edge of the Cape Charles (Susquehanna) paleochannel toward the middle of the channel, where biogenic methane bubbles obscure the acoustic stratigraphy.
The MD99-2207 site was located near the center of the Susquehanna (Cape Charles) paleochannel, where methane gas obscures nearly all the stratigraphy. The site is south of site MD99-2204 and was chosen to obtain a detailed Holocene (and late glacial) paleoclimate record in a mid-bay part of the Chesapeake Bay estuary. This site contains records of terrestrial vegetation and, indirectly, precipitation because salinity variability reflects the offsetting influence of marine water flowing into the deep channel and freshwater runoff from the Susquehanna and Potomac Rivers and other tributaries which influences near surface salinity. The stratigraphy at site 2207 can be correlated by geophysical and micropaleontological data to sequences obtained at the gas-free site at MD99-2204 and in prior piston cores at nearby site PTMC-3 (Kerhin and others,1998; Cronin and others, 2000).
MD99-2205, MD99-2206, and MD99-2208 Core Sites
Previous geophysical and coring studies (e.g., Reeburgh, 1969; Colman and Halka, 1989; Hill and others, 1992; Hagen and Vogt, 1999) have shown that methane gas is abundant in Chesapeake Bay Wisconsin-aged to early Holocene paleochannel fill, but mostly absent in sediments of similar age outside channel fills. The reverberant return signal produced in seismic imaging of methane-charged sediments prevents resolving the thickness of the gas bubble layer or any other details of its structure. The formation, migration, and distribution of methane and its relation to pore water circulation are still poorly known. Coring at sites MD99-2205 and 2206 on the western edge of the modern Chesapeake Channel (Fig. 1.3) was designed to obtain a pair of cores to examine the formation of biogenic methane in Chesapeake Bay sediments at two closely spaced sites, one with subbottom stratigraphy obscured by gas, and the other showing acoustically transparent section of Holocene sediments overlying earlier Quaternary channel fill. By sampling and measuring methane in sediment pore fluids from two long Calypso cores - one in the gassy area, and one adjacent but just outside it - we wanted to test multiple hypotheses that describe the vertical geometry, genesis, and migration of methane in Chesapeake Bay. The results of the methane studies are expected to have broad application to shallow-water methane studies in other regions as well as marine carbon cycling on a global scale. In addition to the objectives regarding methanogenesis, these cores also allow us to "calibrate" geophysical data from these sites, to examine the Holocene depositional history and sea level transgression in a region of Chesapeake Bay that has been intensely studied in terms of geophysical (seismic reflection, chirp and Sidescan Sonar) and sediment coring, and to study paleochannel development (Hill and others, 1992; Hagen and Vogt, 1999; Vogt and others, Chapter 2, this volume).
The MD99-2208 site is located at the southern, apparently gas-free, edge of the small Parker Creek paleochannel. The small gas-free segment of the chirp profile shows what appears to be the basal Holocene reflector at about 8.7 mbsf. The lower part of the Holocene is acoustically diffuse to stratified, whereas the upper half is acoustically transparent. The main goal at site MD99-2208 was obtain a Holocene paleoclimate record at a shallow-water, mid-bay site subjected to rainfall and discharge-induced salinity variability where geophysical data suggested that the Holocene was approximately 8-9 m thick. Although the paleoclimate record would not be as detailed nor as continuous as those from the more rapidly deposited deeper channel sites, this shallow-water site should be more sensitive to climatically-induced salinity variability than would deep regions under the influence of inflowing marine water (Cronin and others, 2000). This site also allows us to examine the paleoenvironmental history of a small tributary watershed much of whose remaining subaerial drainage has been preserved in its natural state.
MD99-2209 Core Site
This coring site was located near the center of the most northerly axial basin in the Chesapeake. This basin has a maximum depth of approximately 37 m and extends from just north of Annapolis near the Bay Bridge (39°N latitude) to near the mouth of the Choptank River (38.5°N latitude) (Fig. 1.4). The basin is usually the first part of the Chesapeake to show depletion of dissolved oxygen in the spring and the last area to return to fully oxygenated conditions in the fall; it should be the site most sensitive to recent and historical changes in oxygenation. Previous cores up to 4.5 m long (November 1998) showed that the site has an extremely high sedimentation rate (up to 3 cm/year) and preserves unbioturbated, banded sediments with abundant microfossils. Penetration to greater depth with the Calypso corer was intended to provide a continuous high resolution record of bay conditions in the centuries of land-use change following European colonization of the watershed, as well as a record of several thousand years of pre-Colonial background conditions. This core site was also chosen for comparison with a 7.8 m core (Baucom and others, 2000) collected previously from a nearby site in shallower water that penetrated basal sediments dated at ~7,800 14Ccal years.
On board, U.S. principal investigators Cronin, Vogt, Halka, and Colman worked with Leg 1 Co-chief Elisabeth Michel and Chief of Operations Yvon Balut to plan the coring sequence. During the prior week while the Marion-Dufresne was traveling from the Caribbean to Chesapeake Bay, Yvon Balut had prepared core tubes between 13 and 26 m length for the various sites. It was decided to first core in the main channel at sites off the mouth of the Potomac River, where all investigators had significant interest in the Holocene history, and at sites in the Parker Creek area, where methane/pore water studies were to be conducted. This plan would allow the ship to visit the northernmost Rhode River site and an additional site on the return trip from Portsmouth on June 21-22, while the ship was heading northward to exit Chesapeake Bay through the Chesapeake - Delaware Bay canal.
The first four cores (MD99-2204 to 2207) were obtained June 20 and the morning of June 21, and were shipped back to Reston, Virginia from the dock in Portsmouth, Virginia by refrigerated (~40° C) truck, arriving in Reston in the afternoon of June 22. The last two Chesapeake cores (MD99-2208, MD99-2209) were taken on June 22 after docking in Portsmouth. They were logged, described and sampled on the Marion-Dufresne, held in a refrigerated container until docking June 29th in Quebec City, Canada, and then shipped to the USGS labs in Reston, Virginia. General information about each core site is given in Table 1.2.
Early in the afternoon June 20, after a 5-hour trip northward from the Norfolk area, the ship arrived at the Potomac site area. Due to the steep horizontal gradient in sub-bottom stratigraphy, it took ~ 1.5 hours to position the ship near the primary site off Point Lookout near the Potomac River mouth. The main goal was to obtain as complete a Holocene record as possible in a non-gassy region. Moreover, it was hoped that the stratigraphic record in this region would allow us to correlate the Holocene stratigraphy and environmental history to geophysically-defined units identified in chirp sonar surveys conducted by the R/V Discovery. The site is located on the western side of the main channel of the bay east of Point Lookout, Maryland between 1999 east-west tracklines 9 and 10 and a long north-south trackline in Vogt and others, (Chapter 3).
After the ship was positioned at the site, it drifted a short way from the destination area. Shipboard differential GPS was not working at this time. The chirp sonar showed that the site is located in a section of Holocene that was about 9 to 10 meters thick, a few meters less than the maximum thickness visible in the non-methanogenic region of the destination area. All reflectors in the mid-early Holocene were visible and traceable across the area and into the gassy zone (cored at site MD99-2207) a little farther to the south. MD99-2204 was taken in the no-gas acoustically clear section at ~18 meters water depth using an 18 m long core pipe and 2.9 ton weight. Vogt (unpublished data) has characterized the site in relation to the main channel of the modern bay, the paleochannel of the Susquehanna River, the seismic records of Colman and Halka (1989), the 1996 cores of Kerhin and others (1998), and the 1999 2-15 kHz chirp and 100 kHz sidescan sonar surveys.
Core MD99-2204 recovered about 770 cm of sediments below sea floor (bsf) consisting mostly of grey to grey-green silty muds. The core improved the recovery of sediment coring in this general area by about 70%. The bottom of the core consisted of clayey sand that apparently stopped the core barrel from penetrating further. MST logging could not be carried out because the logger was not operating properly. The core was cut into standard 150-cm long sections and split with a saw into working and archive halves. Ishman and Cronin examined core catcher sediment washed through a sieve and found common well-preserved calcareous microfossils of Miocene age. These included the ostracode (Cytheridea), unidentified planktic foraminifera, and benthic foraminifera (Nonionella-like) which suggest sediments correlative with the Calvert Group, which outcrops along the western shore of Chesapeake Bay.
This site was the first of a planned pair of cores to be taken in the gas-filled, acoustically opaque sediments, and in non-gassy, acoustically-transparent sediments in the Holocene on the western edge of the present mid-Chesapeake channel (Fig. 1.3). Vogt and others, (Chapter 2) discuss this site in detail. The site was located in the gas zone slightly east of the originally planned location because the differential GPS was not operating and it was necessary to be certain the core was taken in the acoustically opaque zone. A 13-m long pipe was used and the core bottomed out in our target horizon, which turned out to be a basal Quaternary brown, gravelly sand at a depth of about 673 cm bsf.
The core was lithologically heterogeneous with laminated methanogenic muds in the uppermost 50 cm, sandy mud in the 150-350 cm interval, sandier sediments between 350-450 cm, and clayey silts and silty clays from 450 to 670 cm. The core was cut into 50-cm sections in order to squeeze gas and pore water from 50-cm intervals. Initially, there were considerable problems setting up the pore water/methane lab operations (i.e., too rapid use of nitrogen, very slow squeezing), but by the next morning, these problems had been solved. Nine samples for pore water and methane analyses were obtained by Pohlman and Hill and taken back to NRL and MGS labs for analysis. Hill also stored some samples in the peat moss that had been brought on board in order to prevent chemical alteration.
In addition to the pore water/methane samples, the entire core was sampled at 2-cm intervals for micropaleontology, shells for radiocarbon dating, and other geochemistry. No archive half was kept for this core due to the requirements of methane sampling. As with site MD99-2204, MST logging could not be done because the motor in the machine continued to deteriorate, eventually stopping completely. Digital photography of the core was carried out on the entire core. Sediment samples were stored in cardboard boxes and refrigerated.
This site was located almost due east of MD99-2205 in the acoustically transparent zone on the western edge of the modern channel at about 14 m water depth (Fig. 1.3). The site was cored to a depth of about 782 cm bsl, bottoming out in a basal pre-Holocene (Eastville) fill. This recovery increased the sedimentary recovery from this area by previous coring by 175% over prior coring.
Due to the uncertainty in the pore water/methane operations at the time of coring, it was decided not to cut the core into 50-cm lengths but instead to cut into standard 150-cm sections. We also decided to seal the core until splitting it into working and archive halves at the University of Rhode Island (URI) lab in Narragansett, Rhode Island. Because the pore water group had improved their operations early on the morning of June 21, there was enough time for them to sample the 4 whole-core (unsplit) sections using a 10-cm long syringe inserted into the ends of the 1.5m-long sections. Care was taken to leave the remaining sediment intact for shore-based micropaleontology and geochemistry sampling. Pohlman and Hill took about eight samples for pore water/methane study from this core (Bratton and others, this volume). The core was stored in the cardboard boxes and was not sampled on board for radiocarbon dating and other analyses.
A decision was made that the highest priority before returning to dock in Norfolk was to attempt to core the deeper water site near the mouth of the Potomac south of MD-99-2204 and near site PTMC-3 (Kerhin and others, 1998; Cronin and others, 1999) (Fig. 1.2). The primary goal at this location was to attempt to recover fine-grained Holocene sediments in the main channel of Chesapeake Bay. We positioned ourselves very close to the site referred to in the April 1999 chirp sonar profiles as site POT#1 (Vogt and others, Chapter 3). This site was located in acoustically opaque sediments, about 0.4 nautical miles from acoustically transparent Holocene sediments to the east. The site thus provided a thick Holocene sequence of soft, penetrable, methanogenic sediments that could be correlated to the reflectors visible in the chirp sonar. The chirp profiles indicated intermittent reflectors below the top of the main gas reflector.
Using a 26-m long core pipe and additional weight, we recovered about 2070 cm of fine-grained sediments bottoming out in what is most likely a late Quaternary mud containing sparse shells. This is a 440 % increase in core recovery over that obtained from this area in 1996 coring at nearby site PTMC-3 (Kerhin and others, 1998). No shipboard micropaleontology was done to confirm the age. Post-cruise radiocarbon dating, however, has shown that the upper ~12 m are Holocene in age and the interval between 20 and 12 m is dated between ~12,000 and 8,000 14Ccal years.
The core was cut into 150-cm long sections but could not be logged due to the problems with the MST. Due to this and the limited time before docking in Portsmouth, we chose not to split the cores on board but to split, log, photograph and sample at URI. Because the core was located in a paleochannel gas zone, it was decided that Jim Hill would visit URI when the core was to be split and sample for acid-volatile sulfur and carry out Kodak Grey Scale analyses. The core was stored in cardboard boxes and placed in the on-board refrigerated container.
Upon docking in Portsmouth, Vogt, Willard, Weimer, Fagenholz, Hill, and Pohlman disembarked before the ship returned north again to take the final two cores.
Cores MD 99-2208 and MD 99-2209 were obtained after departure from Portsmouth, Virginia, a little after 2000 on June 21 after docking, a VIP luncheon, and sending off the first four cores in a refrigerated truck. While the vessel was docked, a new spectrometer was brought on board for core analyses, and the MST logging device was repaired by a technician sent from England.
With enough time left to take two additional cores, it was decided take a core in the Parker Creek paleochannel and another in the main channel at the Rhode River area site of Colman's June 1998 cruise. The justification for taking the shallow Parker Creek site rather than at an alternative site in the deep channel in the Patuxent River area was based on several factors. The Parker Creek core site would let us test the coring capabilities at 10 m water depth and provide a better understanding of the Parker Creek paleochannel development in this well-mapped area. The Parker Creek core would also provide a fairly complete Holocene record in a zone sensitive to precipitation-driven salinity variability in the modern Chesapeake. In theory, it might provide a shallow-water record comparable to, but possibly more continuous than the Rhode River site to the north. Finally, there was a good possibility the Parker Creek area would sample the pre-Holocene "basement" sediment. Previous cores and radiocarbon dates suggested that the Holocene stratigraphy in the deep channel in the Patuxent area provided a discontinuous depositional history, with a relatively thin late Holocene record and zones with sparse calcareous microfossils. It was felt that another core in the deep channel in the Patuxent area would provide relatively little new information about deep Chesapeake Bay oxygen depletion and faunal response to environmental change compared to those obtained from the Potomac area (MD99-2204, MD99-2207) and the Rhode River area (MD99-2209).
Upon reaching the coring site, we located the site just off Calvert Cliffs in the southern branch of the Parker Creek paleochannel at about 10 meters water depth, the shallowest water cored by the Marion-Dufresne (Fig. 1.3). The no-gas zone in this channel is very narrow and we took the core at 0530, June 22 in the methanogenic zone.
The pipe length was about 13 meters, and penetration was about 1,000 cm, of which 782 cm of sediment was recovered, bottoming out in a sandy oyster shell hash with well-preserved Crassostrea virginica and other molluscan species. This improved the recovery of the sedimentary record at this area by about 175%. The basal oyster/sand unit may be the stratigraphic equivalent to the sands encountered in MD99-2205 and 2206 and the basal oyster unit found at the RR sites taken by Colman and Bratton in June, 1998. The age for this unit at the RR site was about 6,500 14Ccal years.
Core MD99-2208 was cut into 150-cm long sections and sampled for all major types of analyses, except pore water/methane, because the squeezer, glove box and other equipment had been taken off in Portsmouth. The MST logger worked well and an excellent record was obtained. The entire working half of the core was sampled in 2-cm, 10-cm, and 50-cm intervals for micropaleontology/isotopes, biogenic silica/metals, and organic geochemistry, respectively. Four samples were taken for radiocarbon dating (48-50, 75-76, 97-98, and 587-589 cm).
In early afternoon June 22, 1999 we cored in the deep part of the Chesapeake Bay channel near the site of Colman's RD-98-1 site just west of Kent Island and east of the Rhode River (Fig. 1.4). We did not have detailed chirp sonar records of this area, and the available 3.5 mHz records showed the area contained abundant methane gas obscuring the subbottom stratigraphy. Siting the core proceeded relatively quickly. The water depth and the core barrel were both 26 m with a core barrel penetration of 2,300 cm.
We recovered about 1,720 cm of sediment at this site, an increase in sediment recovery for this area of about 375% over the 450-cm long RD-98-1 core. The upper 250 cm were banded black and grey-green clays with a sharp color change near 250 cm bsf. This is similar to the sedimentary sequence obtained in the RD-98-1 core. Below 250 cm, the sediment lithologies were fairly homogeneous, consisting of slightly sandy grey/green muds with broken and whole bivalves throughout the entire core. The basal sediment recovered consisted of shelly sand.
Core MD99-2209 was described and sampled over the course of the next 4 days, mostly by Baucom and Friddell, sometimes by other shipboard staff. Twenty-seven radiocarbon samples were placed in plastic whirl pack bags. In addition, five black/grey couplets from the interval 240 to 251 cm were placed in separate plastic vials for diatom analysis. The MST logger provided an excellent shipboard record and is discussed in the chapter by King and Heil.
The prior core from this site (RD-98-1) had a radiocarbon chronology suggesting very high sediment accumulation rate. Radiocarbon dates from MD99-2209 confirm this, and yielded an excellent Holocene chronology, with relatively continuous sedimentation during the periods about 7500-6000 14Ccal years and 2000-0 14Ccal years, with an extended depositional hiatus during the middle Holocene.
One core collected June 20 (MD99-2205) and two cores collected June 22 (MD99-2208 and -2209) were described on board in the sediment laboratory prior to sampling. Working halves of cores were examined visually and with a 10X hand lens as soon as possible after splitting, as were photography, and spectrophotometry, 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.
Whole Core Physical Properties of cores MD99-2208 and 2209 were measured on the Marion-Dufresne. A GEOTEK Multi-Sensor Core Logging system (MST) was used for gamma density, P-wave velocity, and magnetic susceptibility whole core analyses on two sediment cores, MD99-2208 and MD99-2209, collected in the Chesapeake Bay. The same analyses using a similar system were conducted on the remaining cores at the University of Rhode Island.
P-wave velocity is measured using ultrasonic transducers to produce a pulsed compressional wave. The velocity at which the P-wave passes through the whole core is measured with an accuracy of approximately 3ms-1. The P-wave velocity for a core is greatly effected by gaps in the core, poor conductivity, and gaseous sediments.
Gamma density is used to determine the density/porosity of core sediments. Gamma photons are emitted from a source (137Cs) and passed through the core sediments either attenuated by Compton scattering or unscattered. Gamma density is determined by measuring the number of unscattered gamma photons.
Magnetic susceptibility of the cores sediments was measured using a Bartington loop sensor. The sensor uses an oscillating circuit to generate a low-intensity unsaturating magnetic field that is altered by magnetically susceptible materials. This change in oscillating frequency is detected by the sensor and converted to magnetic susceptibility values (SI). The magnetic susceptibility (MS) records from cores MD99-2208 and MD99-2209 have been corrected for drift.
Appropriate lighting of the cores for photography was quite difficult because of the dark hue of most Chesapeake Bay sediment. Four small (~50 cm) fluorescent lamps were suspended above the core in order 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, instantly downloaded onto an Apple Macintosh 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 and paper printouts.
To extract pore fluid, 50 to 100 cubic cm of wet sediment were extruded from a subcorer into the stainless steel cylinder of a Manheim-type sediment squeezer (Manheim and others, 1994). The chamber was then sealed with a Teflon® disk and butyl rubber gasket, and a stainless steel piston was inserted into the top of the cylinder. The chamber was then placed in a hydraulic press and compressed, forcing pore fluid, methane gas, and a small amount of air through a filter and conduit and into a 30-ml syringe attached to the squeezing apparatus. The syringe was removed after no more fluid was produced from the squeezer, and fitted with a stainless steel needle. The gas and liquid contents of the syringe were injected through a rubber septum into a pre-evacuated serum vial. All samples were preserved by adding 0.4 ml saturated HgCl2 per ml of sample and stored at room temperature. Typically, 20 ml of water and <5 ml of gas were recovered from each sample. Duplicate samples were collected from samples that produced especially large volumes of pore fluid. Samples were transported to NRL on June 21 and were analyzed within 48 hours.
Because of difficulties in getting the squeezer operational and operation of the squeezer once it became functional, pore water samples were not collected under an inert atmosphere. Splits of the pore water samples were taken back to the Maryland Geological Survey labs for analyses of major anions and cations, but nutrient and metals analyses were not performed as originally planned.
Sediment analyses are also planned for biomarkers (chlorin steryl esters or CSEs; King and Repeta, 1994) that are produced during grazing of Chesapeake Bay phytoplankton by zooplankton. Certain CSEs produced by discrete phytoplankton groups (e.g., chlorin dinosteryl ester is produced from dinoflagellates) are preserved in relative proportion to their abundance at the time they were deposited. These compounds act as chemical fossils that persist in anoxic sediments for thousands of years, allowing us to reconstruct the makeup of the phytoplankton assemblages in the Chesapeake Bay of the past. Other biomarkers (free sterols, Zimmerman and Canuel, 2000; alkenones, Mercer and others, 1999) may provide additional information on phytoplankton, nutrients, and water temperature.
Stable isotopes of carbon, oxygen, nitrogen, and sulfur preserved in sediment and microfossils make it possible to recreate changes in cycling of materials in the estuary through time (Bratton and others, 1998).
Dried Chesapeake Bay sediments typically contain 5 to 10 percent biogenic silica by weight. The balance of the sediment consists of organic matter (0.5 to 3 percent), shells (about 5 percent except in shell beds), and mineral grains (silt, clay, and sand; typically 80 to 95 percent). Under conditions of constant detrital sediment input, increases in percent biogenic silica indicate increased diatom blooms usually associated with higher nutrient supplies (esp. nitrate), more spring runoff, and/or lower salinity (Baucom and others, 1999).
This project received unwavering support from James Quick, Bruce Wardlaw, Wayne Newell, Scott Phillips, Elliott Spiker, S. Jeffress Williams, Dave Russ, Debbie Hutchinson, and Sarah Gerould, all of the USGS. We are also grateful to Drs. James Latimer and Hal Walker of the U.S. Environmental Protection Agency, Emery Cleaves of the Maryland Geological Survey, and the Office of Naval Research for their support of this research.
Finally, all those investigating Chesapeake Holocene history owe much to the dedication and spirit of the late Randy Kerhin of the Maryland Geological Survey, who freely shared his knowledge of the bay with all of us.
Robert Stamm deserves all of the participants thanks for text editing.
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U.S. Department of Interior, U.S. Geological Survey
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