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U.S. Geological Survey Open-File Report 00-306: Chapter 9

Holocene Paleoclimate from Chesapeake Bay Ostracodes and Benthic Foraminifera from Marion-Dufresne core MD99-2209

by Thomas M. Cronin1 and Scott E. Ishman2


Ostracodes and benthic foraminifera from core MD99-2209 in the main channel of Chesapeake Bay off the Rhode River can be used to reconstruct early and late Holocene variability in salinity, water temperature, sedimentation and dissolved oxygen caused by climatic and anthropogenic processes. The early Holocene (7500-6000 yr BP) occurrence of marine species that today live on the continental shelf south of the bay's mouth suggest that Chesapeake Bay was relatively warm and saline compared to the late Holocene (2300-0 yr BP). Oscillations in the relative frequencies of Elphidium, Ammonia and Bolivina also indicate high frequency variability in salinity, and perhaps dissolved oxygen, most likely due to changes in regional precipitation and tributary discharge over centennial and decadal timescales.

During the late Holocene (2300 yr BP - present) Chesapeake Bay was inhabited by up to 20 % temperate ostracode species. A mid-Holocene climatic shift is indicated by the change from the mainly subtropical species Actinocythereis captionis to its congeneric temperate species, A. dawsoni, the abundance of Buccella frigida ~2,300- 600 yr BP, and the local extinction of Bolivina between ~5000-2000 yr BP.

The influence of 19th century land clearing in the Chesapeake Bay watershed resulted in significant environmental changes in Chesapeake Bay that are manifested by unprecedented changes in the ostracodes and benthic foraminifers. Among the most important are large declines in the relative frequencies of Cytheromorpha newportensis, Loxoconcha sp. and Elphidium sp., species which have inhabited the bay for more than 7,000 years. They were replaced by species tolerant of high turbidity (i.e., Cytheromorpha curta, Leptocythere nikraveshae, Ammobaculites exiguus) reflecting a fourfold increase in sedimentation rate most likely due to large-scale land clearance and direct runoff into the bay. Reduced levels of dissolved oxygen during the latter part of the 20th century due to increased anthropogenic nutrient influx and high levels of freshwater inflow are indicated by the progressive increase in abundance of Cytheromorpha curta and Ammonia.


Sediment coring by the Marion-Dufresne in Chesapeake Bay June 20-22, 1999 was carried out as part of an integrated study sponsored by the U.S. Geological Survey, the Maryland Geological Survey (MGS), the Naval Research Laboratory (NRL), and the U.S. Environmental Protection Agency (EPA) (Cronin and others, 1999a). These cores are providing excellent records of Holocene benthic foraminifera and ostracodes that reflect both the impact of paleoclimate change in the mid-Atlantic region over millennial to decadal timescales and human activity in the watershed during the last two centuries. The Marion-Dufresne recovered six cores from the middle part of Chesapeake Bay as part of the IMAGES V cruise. This report presents preliminary results of analyses of the two main calcareous microfossil groups preserved in Chesapeake sediment, benthic foraminifers and ostracodes. This paper focuses on faunas from core MD99-2209 and a kasten core RD-98-K2 taken at the same site in 1998 by the R/V Discovery. Although preliminary in nature, the results presented here indicate important long- and short-term environmental changes in Chesapeake Bay that can be attributed to climatic variability and human activity in the surrounding watershed.

Material and Methods

The coring and sampling procedures for the Marion-Dufresne IMAGES coring are described in the introductory chapter of this cruise report (Chesapeake Bay Shipboard members, this volume). Core MD99-2209 (1720 cm in length, 38°58.18' N, 76°23.68'W, 26 m water depth) was sampled in 2-cm thick intervals on board the Marion-Dufresne during its transit from Chesapeake Bay to Quebec City, Canada, June 22-29, 1999. Samples were kept refrigerated on the ship and during shipping from Quebec to the USGS laboratories in Reston, Virginia.

In addition to MD99-2209, foraminifera and ostracodes were analyzed from a 312 cm long kasten core (RD-98-K2, 38.8867°N, 76.3917°W, 26.5 m water depth) taken by John Bratton, Andrew Zimmerman, and Steve Colman at the same site in 1998 (Bratton and others, 2000). This core provides an excellent record of the last 200 years of sedimentation in this part of the bay and it was spliced into the record from MD99-2209, 300-1720 cm. At present samples from the combined MD99-2209/RD-98 sequence have been analyzed every 2-cm for the uppermost 800 cm of the section and every 10-cm for the interval 800 to 1720 cm. Several other cores obtained in 1996 as part of a USGS - MGS cooperative project (Kerhin and others, 1998) were also used in this study to establish biostratigraphic ranges of several species.

The following processing protocol was followed for all samples from MD99-2209. Approximately 40 grams of wet sediment was divided out for calcareous microfossil analysis. Sediment was washed in deionized water through a 63 micron sieve. A total of 100 foraminifera were picked with a fine brush from the >63 micron size fraction using a picking tray with 45 equally sized squares. All individuals in a particular square were picked using a random number system to select squares. Although it is preferable to study 300 individuals per sample, it was decided to examine 100 for several reasons. First, three times as many samples could be examined allowing much more detailed temporal resolution in reconstructing faunal and environmental trends. Second, species' relative frequencies can change on the order of 10% to >90 % within only ~10-15 cm of section (Cronin and others, 2000; Karlsen and others, 2000). With such large changes in relative frequencies, fewer individuals are required to obtain statistically significant trends of environmentally significant indicator species within 95 % confident limits (Buzas 1990). Third, because only a few species dominate Chesapeake Bay assemblages and "rare" species are almost never encountered, we have found that there is little difference between foraminiferal trends determined from 100 individuals from trends obtained from 300 individuals.

Ostracodes are usually less abundant than foraminifera in Chesapeake Bay sediments, so the entire >150 microns size fraction was picked for ostracodes. In order to eliminate "noise" and spikiness of trends in ostracodes due to small numbers of individuals in some samples, we combined species census data from two 2-cm samples into a single sample before computing relative frequencies. The same picking method was used for the RD-98-K2 Kasten core. Species census data are given for each core depth along with estimated age based on the age models described below in the USGS Chesapeake Bay website: This site also contains scanning electron photomicrographs of foraminiferal and ostracode species from Chesapeake Bay from the report by Cronin and others (1999b).

Holocene stratigraphy, sedimentation, and age models

Core MD99-2209 was taken in the deep modern channel of Chesapeake Bay where sediment filling the Cape Charles paleochannel may reach >20-25 m (Colman and Halka, 1989; Colman and Mixon, 1988). The sediments at this site consist of grey to black banded muds (280-0 cm) and massive muds with shelly layers (1720 - 280 cm). Core RD-98-K2 also consisted of light/dark banded layers in the upper ~250 cm. Based on 210Pb dating of RD-98-K2 by Andrew Zimmerman (Virginia Institute of Marine Science), 14C dating of MD99-2209 (see Colman and others, this volume), pollen stratigraphy in both cores (Willard and Korejwo, this volume) and the biostratigraphy of calcareous microfaunas described below, the sedimentary sequence at the 2209 site can be divided into four informal stratigraphic intervals: an early Holocene interval (1720-~900 cm), a zone of condensed sedimentation and/or a disconformity (900-800 cm), a late Holocene interval predating colonial times (800-300 cm), and a post-colonial interval when human activities (land clearance, agriculture, fertilization, urbanization) strongly influenced sedimentation and bay habitats (RD-98-K2 core, 312-0 cm).

Net sedimentation rates varied greatly during each period; consequently, we developed separate age models for the early Holocene, mid-Holocene condensed zone, late Holocene, and post-colonial period. All ages given here are in calendar years before present and were obtained using the CALIB program to calibrate radiocarbon ages before 1950 adding 49 years (see Colman and others, this volume). The early Holocene (7500-6000 yr BP), late Holocene (2300-1800 yr BP) and the post-colonial periods all provided excellent, relatively continuous records with excellent age models. Ages for samples in the condensed zone (900-800 cm) were dated by a single radiocarbon date at about 4340 yr BP and are less certain. A more detailed picture of middle Holocene Chesapeake Bay environments will come from other Marion-Dufresne cores.

Results: Early Holocene

Benthic Foraminifera

The most important characteristics of early Holocene benthic foraminiferal trends are the frequent oscillations between Ammonia and Elphidium and the periodic occurrence of Bolivina (Fig. 9.1). Although further study is needed to establish the significance of these oscillations, there appear to be about 14 peaks in Ammonia between 7500 and 6000 yr BP (Fig. 9.1). This small, lightly calcified phenotype of Ammonia occurring in early Holocene sediments is grouped by Poag (1978) under Ammonia parkinsoniana. According to Poag's data on its distributions in the Gulf region, this form is typical of higher salinity and warmer water temperatures than other larger forms of Ammonia that occur in late Holocene interval of MD99-2209 and other Chesapeake cores (Karlsen and others, 2000).

Because these faunal cycles may signify important short-term climatic oscillations and 10-cm spaced samples are not adequate to fully understand their significance, we examined two cycles between 7300 and 7000 yr BP in detail by examining foraminifera at 2-cm spacing (Fig. 9.1, right side). These results show clearly that the large oscillations in Ammonia relative frequency from <10% to ~50 % are real events. Moreover, though rare, Bolivina consistently occurs in frequencies of 1-2 % with the Ammonia assemblage, but is almost always absent when Elphidium dominates and Ammonia is rare. The presence of Bolivina in early Holocene layers containing abundant Ammonia suggests higher salinity and perhaps also reduced oxygen levels.


Early Holocene ostracode assemblages are dominated (usually 95-100%) by Loxoconcha sp., a species that also dominates late Holocene assemblages, though in slightly lower relative frequencies (~60-95%) (Fig. 9.2). Loxoconcha sp. is the most abundant ostracode species in the lower, more saline part of Chesapeake Bay (Cronin unpublished data) and it also is occasionally found on the mid-Atlantic continental shelf (Valentine, 1971). This species was originally discovered as the dominant species in Newport Bay, Rhode Island (Williams 1966) and can be viewed as a dominant species in coastal bays and estuaries in water of salinity 20-35 ppt.

Early Holocene ostracode assemblages are also characterized by occasional warm water ostracode species and relatively low percentages (<3-4%) of cool temperate species (Fig. 9.2). This is in contrast to the abundance of cool temperate species in the middle Holocene condensed zone (900-800 cm) and some intervals of the late Holocene.

Results: Late Holocene

Benthic Foraminifera

Pre-colonial Late Holocene benthic foraminiferal assemblages are dominated by the genus Elphidium (>90%) with small amounts (~3-5%) of Ammonia (Fig. 9.1). Between ~2300 and 200 yr BP, these assemblages were relatively invariant, although future study is required to establish species level trends in Elphidium and Ammonia.

The upper 300 cm represents the past 200 years, a period which saw extensive changes in land use in the watershed (Brush and others, 1982; Brush, 1989). Two important events having a large impact on Chesapeake Bay benthos were extensive 19th century land clearing, leading to a fourfold increase in sedimentation rate, and late 20th century nutrient influx, causing increased oxygen depletion (Cooper and Brush 1991; Karlsen and others, 2000). These events are manifested in the sharp decrease in Elphidium and the increased abundance of Ammobaculites and Ammonia (Fig. 9.1, 9.3, and 9.4). Figure 9.3 shows that the decline in Elphidium can also be recognized in three other piston cores located south of the MD99-2209 site, from off Parker Creek in the Calvert Cliffs area (PRCK-3), and off the mouths of the Patuxent (PTXT-2) and Potomac Rivers (PTMC-3).


Late Holocene ostracode contrast with those from the early Holocene in the lower proportions of Loxoconcha sp., the greater proportions of Cytheromorpha newportensis and several cool temperate species (Fig. 9.2). In addition, during the 20th century, there is a large increase in the species Cytheromorpha curta, a species tolerant of high turbidity and low levels of dissolved oxygen. The rise of C. curta coincided with decline in the dominance of Loxoconcha sp. in the deep channel of the mid-bay (Fig. 9.3).

Foraminiferal and Ostracode Biostratigraphy

The MD99-2209/RD cores provided a Holocene record of ostracode and foraminifer that can be combined with stratigraphic data from prior cores to establish the ages of biostratigraphic events in Chesapeake Bay (Table 9.1). Figures 9.4 and 9.5 provide plots of the relative frequencies of four ostracodes and four foraminifera that have diagnostic stratigraphic ranges in the main channel of Chesapeake Bay. Although the absence of a species in a particular sample may be due to a bias imposed by low abundance in that sample, the dense sampling (every 2 to 10 cm) and records from multiple cores, suggest that these data are adequate to identify long-term biostratigraphic events. Useful marker horizons include local extinction events for several species designated as last occurrernce events (LOs). In addition to LOs, major declines or increases in the abundance of Elphidium, Ammobaculites and Loxoconcha during the 19th and 20th centuries (Fig. 9.3) can also serve as useful age markers in the way pollen stratigraphy is useful.


Foraminiferal and ostracode assemblages from the 1710-cm long MD99-2209 core provide the first data on early Holocene benthic communities from Chesapeake Bay and, because of the rapid, continuous sedimentation, the most detailed record of late Holocene/post colonial Chesapeake benthos yet available. The preliminary results presented here reveal a strong contrast between early and late Holocene bay environments that most likely signifies different temperature and salinity regimes. The Early Holocene bay was relatively warmer and more saline, although there were frequent high amplitude cycles in benthic assemblages probably due to salinity and perhaps dissolved oxygen. Based on the excellent series of radiocarbon dates between 7500 and 6000 years, the fourteen cycles would have a frequency of about one every century.

In contrast to the early Holocene, the middle and late Holocene was characterized by assemblages with varying amounts of cooler water immigrants into Chesapeake Bay. Although the lack of a continuous mid-Holocene sedimentation precludes precise dating of the faunal shift, it appears to have occurred sometime between 5000 and 3000 yr BP. Climatic cooling during the late Holocene is known from many paleoclimate records during the time referred to as the Neoglacial period. However, the late Holocene Chesapeake record also exhibits a step-wise decline in the abundance of cool temperate ostracode and foraminiferal taxa between 2300 and 200 yr BP. Two notable examples reflecting this apparent warming trend are the disappearance Buccella frigida and the decrease in cool water ostracodes. These trends are being examined in more detail using quantitative faunal and geochemical (isotopic and trace element) methods.


We are grateful to Yvon Balut, Elisabeth Michel, and G. Faubert, Captain of the Marion-Dufresne and his staff and crew, for their support in obtaining cores in Chesapeake Bay. The 1999 cruise of the Marion-Dufresne was part of the IMAGES V program to study the role of the ocean in climate change. The Chesapeake Bay portion of the cruise was jointly funded by the U.S. Geological Survey, the Naval Research Laboratory, the Maryland Geological Survey, and the U.S. Environmental Protection Agency. We thank Michelle Cangelosi for picking foraminifera, Patti Baucom for core sampling, Robert Wagner and Moira Slattery for processing samples, and Clair Carroll for sample curation. Harry Dowsett, Curt Larsen, and Bill Orem provided useful reviews of the paper.


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