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.
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: http://geology.er.usgs.gov/eespteam/ches/bayhome.html This site also contains scanning electron photomicrographs of foraminiferal and ostracode species from Chesapeake Bay from the report by Cronin and others (1999b).
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.
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 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.
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).
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.
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U.S. Department of Interior, U.S. Geological Survey
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