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Oxygen isotopic evidence for greater seasonality in Holocene shells of Donax variabilis from Florida
Palaeogeography, Palaeoclimatology, Palaeoecology 228 (2005) 96 – 108 www.elsevier.com/locate/palaeo
Oxygen isotopic evidence for greater seasonality in Holocene shells of Donax variabilis from Florida Douglas S. Jones a,*, Irvy R. Quitmyer a, C. Fred T. Andrus b a
Florida Museum of Natural History, University of Florida, Gainesville, FL 32611, USA b University of Georgia, Savannah River Ecology Laboratory, Aiken, SC 29802, USA
Received 30 June 2004; received in revised form 28 October 2004; accepted 24 March 2005
Abstract Donax variabilis, the variable coquina clam, has been a common inhabitant of exposed sandy beach intertidal and shallow subtidal zones in the southeastern United States throughout the Pleistocene and Holocene. It is ideally suited for paleotemperature studies because it is restricted to environments of well-mixed, normal-marine seawater with a fairly uniform isotopic composition. As a result, oxygen isotopic variability in D. variabilis shells is largely explained by temperature variation. Although D. variabilis is small and short-lived, its shell represents an important paleoclimate archive because of its unique habitat preference. High-resolution sampling of individual D. variabilis shells and comparison of oxygen isotopic temperature profiles with historical seawater temperatures from the northeastern Florida coast indicate rapid shell growth over a brief life span of three to five or six months. Analysis of two modern shells reveals a close correspondence between isotopically determined water temperatures and historical water temperatures during the spring–summer growing season. Paleotemperature profiles from four archaeological shells, however, suggest a longer growth interval spanning summer–autumn. Two Preceramic Archaic Period shells (ca. 4240 and 5570 14C yr BP) and two Orange Period Archaic shells (ca. 3600 and 3760 14C yr BP), from four different archaeological sites, yield paleotemperatures that average 3.5 8C higher than present summer–autumn water temperatures. These warm paleotemperatures highlight seasonality differences associated with the mid-Holocene Hypsithermal climatic interval in this region. D 2005 Elsevier B.V. All rights reserved. Keywords: Oxygen isotopes; Donax variabilis; Shell growth; Paleotemperature; Florida; Holocene
1. Introduction The variable coquina clam, Donax variabilis Say, 1822, is one of the most common bivalve mollusks * Corresponding author. E-mail address: [email protected] (D.S. Jones). 0031-0182/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2005.03.046
inhabiting sandy beaches of the southeastern United States. This small bivalve, 1–2 cm in length, is found from Virginia to southern Florida and around the Gulf Coast to Texas (Morrison, 1971; Ruppert and Fox, 1988). It is often the dominant macro-invertebrate inhabiting the high-energy, sandy beach environment where it frequently occurs in extremely high densities
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(Pearse et al., 1942; Ansell, 1983; Brown and McLachlan, 1990; Wilson, 1999). Because Donax clams are excellent burrowers, they are ideally suited to life in the surf zone (Trueman and Ansell, 1969; McLachlan and Young, 1982). The wedge-shaped outline of the D. variabilis shell enhances substrate penetration. Its supreme ability as a rapid burrower has enabled this species to colonize the sands of exposed beaches which otherwise are fairly devoid of macrofauna (Wilson, 1999). The fossil record of D. variabilis suggests that it has occupied a similar ecological niche for hundreds of thousands to millions of years. It is widely reported from Pleistocene beach and barrier facies of several stratigraphic units exposed along the southeastern U.S. Atlantic Coastal Plain such as the Flanner Beach Fm. in North Carolina, the Canepatch Fm. in South Carolina, and the Satilla Fm. in coastal Georgia (DuBar et al., 1974). Whereas Donax shells are known back to at least the early Miocene Chipola Formation in Florida (Gardner, 1928), most Florida specimens of D. variabilis are known from Pleistocene and Holocene units (Dall, 1900). In northeastern Florida these include the Anastasia, Satilla, and Nashua formations with the Nashua possibly extending back to the late Pliocene (Huddlestun, 1988). In addition to Holocene occurrences of D. variabilis in raised beach deposits along the northeast coast of Florida, variable coquina clams are known from many archaeological sites in the region (Milanich, 1994). Carbon-14 dates from shell midden deposits attest to the fact that pre-Columbian people consumed vast quantities of coquina clams between the Middle Archaic and St. Johns periods (ca. 6000–400 yr BP). Middens range from nearly monotypic deposits of coquina clams to mixed assemblages containing brackish species such as oysters and hard clams, presumably collected from nearby estuaries or bays behind the barrier islands. This study of paleoclimate records in D. variabilis originated from an investigation of archaeological shells recovered from coastal middens in northeastern Florida to determine if there was a seasonal preference to ancient shellfish harvest (Quitmyer et al., in press; Jones et al., 2003). In addition to analyzing archaeological shells as bioarchives of past climates, livecollected clams were also studied in order to docu-
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ment the relationship between shell oxygen isotopes and water temperatures. 1.1. Bivalve shell records Bivalve mollusks grow their shells by means of the mantle-mediated addition of CaCO3 to the shell margin and by simultaneously thickening within the pallial line. This process is strongly influenced by environmental factors, many of which are rhythmic or periodic such as tides, diurnal cycles, and the annual change of the seasons. Such relationships are often so intimate that rhythmic accretionary fabrics within the shell such as growth lines, microgrowth increments, or macroscopic growth bands can be directly linked to the extrinsic factors responsible for their formation or to intrinsic factors such as spawning cycles or circadian rhythms (Jones and Quitmyer, 1996). As a mollusk shell grows, marking time by means of incremental growth features, the accretionary shell becomes a biogeochemical recorder of the environmental and climatic conditions experienced throughout the mollusk’s lifetime (Richardson, 2001). Annual growth increments represent the most pervasive pattern reported from bivalve shells (Jones, 1983, 1998). However, periodicities ranging from yearly down to sub-daily are recognized and the study of these features in skeletonized tissue is widely known as sclerochronology, analogous to the more familar dendrochronolgy (Hudson et al., 1976; Jones, 1983). In fossil or modern shells where banding patterns are indistinct or otherwise difficult to interpret, highresolution, sequential sampling for isotopic analyses often yields cyclical profiles, such as d 18O, that reflect annual environmental cycles (e.g., temperature). This approach can help evaluate or verify the timing and periodicity of shell growth increments (e.g., Jones et al., 1989; Witbaard et al., 1994; Scho¨ne et al., 2003) or it can offer independent evidence where rhythmic growth increments are lacking (e.g., Jones, 1983, 1998; Wefer and Berger, 1991). This approach has gained wide usage (e.g., Jones et al., 1983, 1989; Goodwin et al., 2001) and it is the strategy we use here because the D. variabilis shells lack prominent rhythmic banding patterns useful for age determination (Wilson, 1999).
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1.2. Longevity and shell growth of D. variabilis Despite its ecological importance, abundance and visibility on exposed sandy beaches, key life history parameters of D. variabilis such as growth rate and longevity are known only to a first approximation with widely divergent estimates in the literature. Longevity estimates for D. variabilis range from less than 1 year (e.g., Bonsdorff and Nelson, 1992) to upwards of three (e.g., Morrison, 1971). At least part of the explanation for the lack of accurate data is the traditional method for assessing growth and longevity in this species—size-frequency diagrams—an approach with many inherent problems (Mikkelsen, 1985). Other techniques such as mark-and-recapture studies are impractical because of the movements and high mortality of these clams (Mikkelsen, 1985) and growth rings on the shell are indistinct, not useful for age determination (Wilson, 1999). Microgrowth increments measuring a few microns to tens of microns in thickness are visible in shell cross-sections and may correspond to diurnal or tidal cycles. Counting such increments could provide an independent method of age determination; however, until their periodicity is ascertained, these increments are of limited value. In this study we use oxygen isotopic variation in shell carbonate to produce highresolution records of seasonal shell growth and generate longevity estimates for D. variabilis as a byproduct of our paleoclimate investigation. 1.3. Archaeological coquina shells in NE Florida The earliest evidence for human use of marine resources in northeastern Florida occurs in the numerous mid-Holocene shell middens scattered throughout the region (Milanich, 1994). Such deposits are widely recognized as important archives of cultural and paleoenvironmental data. The oldest date reported for one of these Atlantic coastal sites comes from Spencer’s Midden in Atlantic Beach, FL (5570 F 80 corr. 14C yr BP; calibrated to 6169–5759 yr BP), an arcuate oyster/coquina shell midden over 1 m deep and 50 m in length (Russo, 1996). A younger site at Crescent Beach, FL (4240 F 80 corr. 14C yr BP) located to the south (Fig. 1) was similarly occupied by preceramic Archaic peoples and contains abundant coquina shells (Russo, 1996). Overlying the precera-
mic midden components at several coastal sites are shell deposits containing fiber-tempered ceramics. These are characteristic of Orange Period peoples of the Late Archaic who emerged subsequently in this region (Milanich, 1994). Settlements identified on Fort George Island and neighboring barrier and marsh islands north of the St. Johns River indicate that extensive fiber-tempered ceramics appeared about 4200–3700 yr BP (Russo, 1996). Orange Period Archaic sites such as Rollins (3760 F 60 corr. 14C yr BP) and Guana Shell Ring (3600 F 50 corr. 14C yr BP) are typical of the many shell midden and ring sites of this and later periods known to contain large components of coquina clams (Newman and Weisman, 1992; Russo, 1992, 1996; Quitmyer et al., in press). Seasonal settlement patterns during these periods remain a subject of debate (Anderson, 1996; Cumbaa, 1976; Quitmyer et al., 1997). Middle Archaic through St. Johns II Period sites are also present in the interior of peninsular Florida. Freshwater fish and shellfish compose a major part of the refuse in these interior sites. Remains of marine bivalves, whelks, and sharks are also present and offer evidence for a connection with the coast (Quitmyer et al., in press). Some archaeologists have interpreted this to indicate that people seasonally migrated between the coast and Florida’s interior as part of their annual round of subsistence (Cumbaa, 1976). Zooarchaeologists have started to test this hypothesis against the alternative that coastal and interior people were permanent residents while trading resources (Quitmyer et al., 1997; Russo and Ste. Claire, 1992; Russo et al., 1992). Russo (1996 and references therein) notes that the almost exclusive use of small coquina clams, a beach resource, evidenced in some of the extensive sheet and mounded middens found on barrier islands and on the mainland coast, prompted speculation that coastal estuaries were not developed when the coasts were initially occupied because of lower sea-level stands. Therefore, permanent coastal settlement was not possible for the preceramic and early ceramic-producing cultures. In this view, coastal sites were seen as the winter seasonal occupations of interior groups who settled along the St. Johns River. Although more recent research suggests that both interior and coastal sites were occupied year-round (Quitmyer et al., 1997; Russo, 1996), the debate continues. The initial impetus for this study was to examine the issue of seasonal
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Fig. 1. Location map showing archaeological midden sites along the northeastern coast of Florida, including site of modern D. variabilis collection at Matanzas Beach, SAUFL station at Crescent Beach, and Florida Museum of Natural History (FLMNH).
occupation of coastal sites in northeastern Florida by using isotopic records in mollusk shells to determine season of harvest. We anticipated shell d 18O records might also shed light on climatic conditions that could have affected ancient settlement patterns.
2. Materials and methods Particularly abundant in the shell middens described above are specimens of D. variabilis which often dominate the zooarchaeological component of any particular site. Collections of D. variabilis have been made at many of the significant excavation sites by archaeologists and students from the University of Florida (Quitmyer et al., in press). These are housed in the collections of the Environmental Archaeology Program at the Florida Museum of Natural History (FLMNH), University of Florida, Gainesville.
For this initial investigation four archaeological shells of D. variabilis were selected for oxygen isotopic analysis from among the largest specimens in the FLMNH collections. Two well-preserved shells from the Preceramic Archaic were selected, one each from the Spencer’s Midden and Crescent Beach sites (Fig. 1). The shell lengths of these specimens were 21.5 and 23.1 mm, respectively. In addition, two specimens were selected from Orange Period Archaic material, one each from the Rollins Site (21.3 mm) and the Guana Shell Ring (20.5 mm). XRD analyses of sample powders indicated that all shells were 100% aragonite. A modern analogue population for comparison with the archaeological coquina shell material was available at the FLMNH. This assemblage of 2493 D. variabilis specimens was live-collected at monthly intervals at Matanzas Beach between January–December 1988 by Dr. Michael Russo during the course of research in the Department of Anthropology at the
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University of Florida (Quitmyer et al., in press). The collection site on Anastasia Island is located just south of the beach access ramp from Fort Matanzas National Monument (N 29.71748; W 81.23108), north of the Matanzas Inlet, in the same vicinity where indigenous people could have harvested coquina clams thousands of years ago (Fig. 1). Two of the largest modern shells (13.1 and 13.2 mm), collected in early July 1988, were selected for stable isotopic analysis. When carefully sampled in ontogenetic sequence from the umbo to the ventral shell margin (Fig. 2), the pattern of oxygen isotopic variation across a bivalve shell can provide a wealth of information about seasonal growth, shell growth rates, longevity, and season of death (Jones, 1983; Wefer and Berger, 1991; Jones and Quitmyer, 1996; Richardson, 2001). In order to assess these parameters in D. variabilis and to look at possible changes through time, sequential samples of shell carbonate were recovered from the two modern and four archaeological shells. Prior to sampling, each valve was cleaned following Andrus and Crowe (2002), with a wire brush under distilled water, and then treated with a 30% solution of H2O2. After rinsing in distilled water and drying in a vacuum oven, the samples were mounted onto glass slides with epoxy and fixed to the sample stage of a Merchantek EO Micromill at the Savannah River Ecology Laboratory. Carbonate samples were milled from the outer surface of the valve in shallow grooves parallel to the growth lines. Each sampling
Fig. 2. Archaeological D. variabilis shell from Guana Shell Ring site showing sampling grooves from milling procedure to collect powders for oxygen isotopic analyses. Scale bar divisions are 2 mm.
groove was 25 Am deep and the width varied depending upon the location relative to the growth axis of the shell. Sampling grooves near the outer margin were the most narrow and became progressively wider near the umbo where the shell is smaller (Fig. 2). The isotopic analyses were conducted in the Light Stable Isotope Mass Spectrometry Laboratory, Department of Geological Sciences, University of Florida. The powdered CaCO3 samples were analyzed according to standard techniques (Jones and Quitmyer, 1996) which involved an initial reaction in vacuo with 100% orthophosphoric acid at 90 8C for 15 min. An on-line automated carbonate-preparation system facilitated the production and purification of the evolved CO2 gas. The isotopic differences between the derived CO2 gas and the VPDB standard were determined with a VG Isogas PRISM Series I mass spectrometer equipped with triple collectors and micro-inlet system. All values are reported in standard d notation where: . 18 d18 O ¼ 18 O=16 O O=16 O 1 sample
standard
103 permilðxÞ: The weight of individual micro-samples (20–50 Ag) was so small that replicates of unknowns could not be run and often required that adjacent samples be combined to produce a desired minimum sample weight of about 50 Ag. However, replicated NBS-19 standards run before and after sample strings varied by F0.04x (Table 1). The d 18O value of seawater from the Matanzas Beach collection site was measured via CO2 equilibration following Socki et al. (1992) at the University of Georgia, Geology Stable Isotope Laboratory. Five monthly samples were analyzed (Dec.–Mar. and Aug.), providing a measure of seasonal d 18O range. Precision was estimated based on analysis of the laboratory working standard of Athens tap water to be F 0.05 (1r). The d 18O values of seawater at the Matanzas Beach collection site were very consistent (Dec. 1.0x; Jan. 1.2x; Feb. 1.0x; Mar. 1.1x; and Aug. 1.1x), averaging 1.08x SMOW (F0.08x, 1r: maximum range = 0.2x). Mean salinity = 33.1 F 0.4 psu. We calculated the temperature of the water in which the shell carbonate formed using the paleotemperature equation of Grossman and Ku (1986) for the
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Table 1 Donax variabilis d 18O (permill relative to VPDB, x) and paleotemperatures Sample
Modern 1 18
Modern 2 18
Crescent Beach 18
Spencer’s Midden 18
Rollins Site 18
Guana Shell Ring
N
d O
8C
d O
8C
d O
8C
d O
8C
d O
8C
d 18O
8C
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
1.9 0.7 0.8 0.8 0.6 0.7 0.1 0.0
17.0 22.7 22.2 22.0 23.2 22.7 25.3 26.1
1.6 1.3 1.5 0.8 0.8 0.7 0.8 0.2 0.6 0.1 0.1 0.3 0.2
18.4 19.9 19.1 29.8 22.0 22.5 22.4 25.2 23.2 25.7 26.3 27.1 27.0
0.3 0.9 0.5 1.4 1.6 1.0 0.9 1.1 1.1 1.3 0.4 0.3 0.4 1.0 0.7 0.4 0.1 0.2
27.1 30.2 28.3 32.3 33.2 30.8 30.0 30.9 31.2 31.9 27.8 24.5 27.8 30.5 29.2 28.0 26.2 24.8
0.8 0.6 1.2 0.9 0.7 1.0 0.6 1.4 1.1 1.3 1.1 0.8 1.1 1.1 0.4 0.4 0.4 0.9 1.2 0.2
29.8 28.8 31.6 30.0 29.2 30.6 28.7 32.4 31.2 32.2 31.0 29.8 31.1 31.1 23.8 24.1 23.9 21.9 20.5 25.2
0.1 0.3 0.6 1.0 0.9 1.0 0.9 0.8 2.0 0.1 0.0 0.6 1.0 1.2 1.4 1.3 1.3 0.8 0.3 1.3
25.5 27.4 28.9 30.4 30.1 30.7 30.1 29.7 35.1 26.3 25.8 28.5 30.7 31.6 32.6 32.1 32.0 29.9 24.4 20.0
1.0 1.1 1.0 0.8 0.9 1.1 1.3 1.5 1.2 1.3 0.7 0.1 0.9
30.4 31.0 30.8 29.8 30.0 30.9 31.8 32.9 31.6 31.9 29.1 26.3 21.8
Replicates of NBS 19 standard: N = 13, mean = 2.19, and s.d. = 0.04.
temperature-dependent fractionation of aragonite in mollusks relative to seawater: T ð8CÞ ¼ 21:8 4:69 shell d18 OVPDB seawater d18 OSMOW 0:2x : The resulting paleotemperature data from the two modern shells were then graphed against the mean weekly seawater temperature curve based on historical measurements made in 1988 at the nearby St. Augustine National Data Buoy Center station, SAUFL (http:// www.ndbc.noaa.gov/Maps/Florida.shtml). The paleotemperature data from the four archaeological shells were graphed in similar fashion to the mean weekly seawater temperature curve averaged over a longer, 17year interval (1986–2002) from the SAUFL station. The sandy beach habitat preference of modern D. variabilis in northeastern Florida, characterized by fairly constant salinity and d 18Owater composition year-round, means that shell d 18O variations largely reflect changes in water temperature during the growing season. This greatly simplifies the interpretation of paleoenvironmental conditions recorded in D. variabilis shells as compared to the isotopic records of common estuarine bivalves like oysters or hard
clams. Both temperature and salinity–d 18O relationships must be factored into the interpretation of shell isotopic records in estuarine species such as Crassostrea virginica (Kirby, 2000; Surge et al., 2001, 2003) or Mercenaria mercenaria (Jones et al., 1989; Elliot et al., 2003). However, if it can be assumed that the open-marine, sandy beach habitat of D. variabilis did not change appreciably over the past several thousand years in terms of seawater chemistry, then fairly precise estimates of past water temperatures are possible from shell d 18O records. A sea level rise of 1–2 m in the past ~6000 years for the Florida coast (Davis, 1997; Gischler and Hudson, 2004) would yield only a minimal change in seawater d 18O of b 0.1x (Fairbanks, 1989). Therefore, we use the mean of our measured seawater-d 18O values (1.08x SMOW) in the aragonite fractionation equation of Grossman and Ku (1986) to solve for paleotemperatures.
3. Results With respect to the modern shells, eight unique samples for specimen #1 (13.1 mm) and 13 unique
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samples for specimen #2 (13.2 mm) were analyzed. These samples produced d 18O values that ranged from 1.9x to 0.0x for specimen #1 and from 1.6x to 0.8x for specimen #2 (Table 1). Fig. 3 shows the isotopic paleotemperatures plotted against the sea surface temperatures. Four factors were considered in plotting these points: 1) the sample spacing of the milled grooves on each shell; 2) preserving the ontogenetic order of the samples; 3) the exponential shell growth pattern of most bivalves; and 4) the early July collection date which should coincide with the final sample from each shell margin. With these considerations in mind, the paleotemperature data were plotted in sequence by eye as closely as possible to matching water temperatures along the annual curve. The paleotemperature data exhibit an excellent fit to the segment of the curve corresponding to spring and early summer when water temperatures are rising. Only one data point (specimen #2, sample 4) shows excessive depletion and it is unclear if this is analytical (e.g., fractionation, contamination) or environmental (e.g., large storm with freshwater spike) in origin. The shell-edge paleotemperature values agree with the water temperature at the time of collection in early July 1988. The isotopic records indicate that shell growth in both specimens occurred over a 14–
16 week interval, approximately corresponding to weeks 12–27 of the year (early April–early July). In a similar manner, the four larger archaeological shells were sampled and analyzed for oxygen isotopic ratios. A total of 18 samples were analyzed from the Crescent Beach specimen. The d 18O values ranged from 0.3x to 1.6x. From the Spencer’s Midden shell a total of 20 samples were analyzed whose d 18O values ranged between 1.2x and 1.4x. A total of 20 samples from the Rollins Site shell yielded d 18O values that ranged between 1.3x and 2.0x. From the Guana Shell Ring specimen, 13 samples were analyzed. These produced d 18O values that ranged from 0.9x to 1.5x. The oxygen isotopic data were converted to paleotemperatures and these were overlain onto the mean weekly seawater temperature curve for the 17-year interval 1986–2002 from data at the SAUFL station (Fig. 4). The results are comparable to those achieved for the modern shells with a few important differences. The paleotemperature profiles for the two Preceramic Archaic shells from the Crescent Beach and Spencer’s Midden sites show the closest correspondence with weeks 25–45 of the annual temperature curve. This pattern suggests shell growth occurred for about 20 weeks, from late June into mid-November. The paleotemperature profiles for the two younger
40 35
Modern Donax variabilis
o
Temperature ( C)
30 25 20 15 10 Mean SST - 1988 (SAUFL Station)
5
Shell 1 Shell 2
0 1
4
7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52
Week Fig. 3. Weekly mean sea surface temperatures (small solid circles) from the SAUFL station for 1988 with isotopic paleotemperature profiles from two modern specimens of D. variabilis plotted in open symbols.
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40 Archaeological Donax variabilis Preceramic Archaic
35
Temperature (oC)
30 25 20 15 Mean SST - 1986-2002 (SAUFL Station)
10 Crescent Beach Shell (4240+/-80 14C yr BP)
5
Spencer's Midden Shell (3760+/-60 14C yr BP)
0 1
5
9
13
17
21
25
29
33
37
41
45
49
53
Week 40 Archaeological Donax variabilis
Temperature (oC)
35 30 25 20 15
Mean SST - 1986-2002 (SAUFL Station)
10 Rollins Site Shell (3760+/-60 14C yr BP)
5
Guana Shell Ring Shell (3600+/-50 14C yr BP)
0 1
5
9
13
17
21
25
29
33
37
41
45
49
53
Week Fig. 4. Seventeen-year average of weekly mean sea surface temperatures (small solid circles, bars = 1r) from the SAUFL station for 1986–2002 with isotopic paleotemperature profiles from two Preceramic Archaic shells (upper) and two Orange Period Archaic shells (lower) of D. variabilis plotted in open symbols.
shells from the Orange Period Archaic sites (Rollins and Guana Shell Ring) reveal a similar pattern although the umbonal region of the latter (early growth) was worn and not possible to sample. Shell growth over a period of about 23 weeks is indicated, approximately corresponding to weeks 24–47 of the year, mid-June to late November. All four archaeological shells were larger in size than any of the modern shells observed in this study. The results indicate that ancient clams achieved their larger size by growing slightly faster for a longer
period of time (i.e., 6–9 weeks longer) than their modern counterparts. Whereas maximum longevity among the modern shells was approximately three to four months, the archaeological specimens lived for five or six months. The paleotemperature records indicate that each of the four archaeological shells was harvested in autumn. Although the seasonal pattern of paleotemperature change recorded by the oxygen isotopes in the archaeological shells mirrors that of modern seawater at this site, the values reveal a consistent negative offset from modern conditions.
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The paleotemperature values from each archaeological shell reflect summer–autumn ocean water conditions that were warmer than today by about 3.5 8C.
4. Discussion 4.1. Shell growth and longevity The oxygen isotopic evidence from modern and archaeological shells of D. variabilis provides an accurate method of assessing seasonal shell growth and longevity in this species. These data serve to refine previous estimates that were based largely on size-frequency analyses. Ansell (1983) reported that most Donax species, like D. variabilis, are short-lived with life spans of 1– 2 years, rapid growth to maximum size, and early maturity. Morrison (1971) suggested that D. variabilis has a 2-year life span and that in some cases individuals may survive a third year. Mikkelsen (1985) estimated that D. variabilis in Florida grew at a rate of 3.0–3.7 mm/month in the summer months and that, bthe majority of individuals probably live for approximately one year, with a few entering a second yearQ (Mikkelsen, 1985, p.310). This conclusion generally agrees with Loesch (1957) and Pearse et al. (1942) who also employed shell length-frequency graphs to examine growth and longevity in this species. bBonsdorff and Nelson (1992) estimated a maximum growth rate of 3.43 mm/month for a Florida population of D. variabilis, at which rate the animals would grow to maturity in just two monthsQ (Wilson, 1999, p. 78). The oxygen isotope results achieved in this study support such rapid growth rate estimates and reduced longevity (i.e., months, not years). The growth rates calculated from our isotopic data are about the same, 3.3–3.7 mm/month, for the modern specimens and slightly higher, 4.1–4.6 mm/month, for the archaeological shells. The isotopic data also call into question whether these clams actually survive for one full year as the modern and archaeological shell isotope records indicated life spans of three to four and five to six months, respectively. Admittedly both the modern and archaeological specimens were live-collected by humans and therefore it could be argued that they still had the potential to grow larger and live longer. However,
both sets of shells were among the largest specimens out of the thousands recovered in either the modern or zooarchaeological collections, minimizing this possibility. Our results cast doubt on previous longevity estimates for this species that hypothesize growth and survival into a second or even a third year of life. In a study involving D. variabilis from the South Carolina coast, Wilson (1999) laments that growth rings on the shell were indistinct and it was not possible to verify the age of specimens by this method. Furthermore, constant movement of beach sands and contained clams, as well as the migratory ability of D. variabilis, prevented the use of a dmark-andrecaptureT technique for growth measurement. These same observations hold true regarding the populations in northeastern Florida and reinforce the value of the oxygen isotopic technique for growth rate and age determination. It is also applicable to archaeological and fossil shells. The close correspondence between paleotemperatures derived from oxygen isotopes and the measured seasonal variation in water temperature at the site made it possible to reconstruct the timing and duration of shell growth to a weekly level of resolution. Life spans that proved to be appreciably shorter than acknowledged in the literature, three to five or six months in duration, represent an unanticipated outcome of this study. Compared with the largest modern shells from the same region, the archaeological shells achieved a greater maximum size and longevity. The isotopic profiles suggest that the larger sizes of the ancient shells were achieved through slightly faster growth over an extended growing season. The spring–early summer growing season of the modern shells is clearly shifted from that of the archaeological shells (summer–autumn). Isotopic analyses of additional modern and archaeological shells will be necessary to document the extent of these differences and gauge changes through time. The possibility of size reduction from over-fishing, observed in many marine species (Jackson et al., 2001) and in other bivalves such as Mercenaria spp. from Florida midden sites (Quitmyer and Jones, 2000), must also be considered. 4.2. Season of harvest of archaeological specimens Several important differences were observed between the modern and archaeological D. variabilis
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Fig. 5. Worldwide record of glacial advances and retreats for the Holocene from Ro¨thlisberger (1986) with dashed horizontal lines corresponding to ages of archaeological shells used in this study (G=Guana Shell Ring; R=Rollins Site; C=Crescent Beach; and S=Spencer’s Midden).
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shells. These are intriguing and warrant further study. The isotopic profiles from all four archaeological shells, from four separate sites and distinct Archaic Period time horizons, indicated coquina clam harvest in autumn. This preliminary result suggests a seasonal pattern of behavior by indigenous peoples that needs to be verified through the analysis of additional shells from these as well as other sites and time periods (Quitmyer et al., in press). 4.3. Paleoclimatic implications When plotted against a 17-year record of historical water temperatures (1986–2002), the majority of paleotemperatures recorded in the shell d 18O profiles of the four archaeological D. variabilis plot above the modern temperature curve (Fig. 4). The average difference is + 3.5 8C, suggesting warmer water temperatures in the mid-Holocene and possibly reflecting later stages of the Holocene thermal maximum (Haug et al., 2001), or bHypsithermalQ, in this region (Jones et al., 2003). The timing of these warm temperature episodes broadly correlates with worldwide records of Holocene glacial retreats (Fig. 5), although the correlation is far from perfect. Upon closer inspection the d 18O profiles indicate that around 5570, 4240, 3760 and 3600 14C yr BP mean water temperatures during summer–early autumn, the Northern Hemisphere warm season, were higher than modern values. It is less clear that the same was true of water temperatures leading up to the warm season (early summer) or away from it (late autumn). This dilemma arises from two sources—uncertainties concerning the growing season and longevity of archaeological D. variabilis and the likelihood that seasonality of a mid-Holocene year was greater than today. Both of these factors could influence the shape of the limbs of the convex d 18O profiles (Fig. 4). Higher seasonality (greater contrast between summer and winter) would yield an annual temperature curve with increased amplitude and steeper limbs. The higher warm-season temperatures indicated by the d 18O profiles and the convergence of the d 18O profiles with the modern temperature curve on either side of the warm season suggest that mid-late Holocene years were more seasonal. Testing this hypothesis will require sampling archaeological coquina clams that were har-
vested at different seasons of the year, if they exist, or other suitable taxa. Interestingly, Holocene climate models for the circum-Caribbean region predict greater seasonality during this interval. Hodell et al. (1991) suggested that long-term changes in the ratio of evaporation to precipitation (E / P) in the circum-Caribbean region are controlled by the intensity of the annual cycle, which in turn is controlled by long-term insolation changes forced by orbital mechanics. The intensity of the annual cycle is defined as the difference between August and February insolation at 108 N, with August and February corresponding to the months when the intertropical convergence zone (ITCZ) moves farthest north and south, respectively. Intervals of intense seasonality coincide with far northerly movement of the ITCZ during the Northern Hemisphere summer, bringing increased rainfall and higher temperatures to the northern American tropics. The intensity of the annual cycle in the northern tropics reached a maximum during the early Holocene and declined toward the present as a result of changes in seasonal insolation forced by Earth’s precessional cycle (Hodell et al., 1991). The higher warm-season temperatures seen in the archaeological coquina shell d 18O profiles (Fig. 4) are consistent with the increased intensity of the annual cycle described above for the early to middle Holocene in the circum-Caribbean region. Naturally it is difficult to establish causality in this case as northeastern Florida, at ~308 N, lies outside the zone of strongest influence by the annual migration of the ITCZ. It may be that climate change in the tropics, where much of Florida’s weather originates, exerts an important influence on the annual cycle of air and water temperature in the study area.
5. Concluding remarks Although long-lived species are usually preferred in stable isotopic and sclerochronologic analyses of shells (e.g., Jones et al., 1989; Marchitto et al., 2000), d 18O profiles from D. variabilis illustrate the value of this short-lived bivalve as a bioarchive of life history and paleoclimatic information. The exposed sandy beach habitat preference of D. variabilis, with its normal-marine seawater chemistry, simplifies the in-
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terpretation of the shell d 18O profiles and underscores the usefulness of this mollusk in paleoenvironmental reconstruction. By making additional oxygen isotopic analyses of D. variabilis shells from older as well as younger archaeological time periods, it should be possible to chart the history of water temperature change in northeastern Florida since the last glacial episode. If additional archaeological specimens are analyzed, particularly shells that grew during the winter and spring seasons, it should be possible to refine shell growth rate estimates, evaluate the likelihood of seasonal vs. year-round shellfish harvest by indigenous peoples, and test the hypothesis of greater seasonality during the mid-Holocene.
Acknowledgements We thank Michael Russo, National Park Service, Southeastern Center, Tallahassee, for sharing unpublished data. We also acknowledge the Invertebrate Paleontology and Environmental Archaeology Laboratories, Florida Museum of Natural History, for supplying essential resources. U.S. Department of Energy grant #DE-FC09-96SR18546 (CFTA) provided partial support of this research. We appreciate discussions with David Hodell and analytical assistance given by Jason Curtis of the Light Stable Isotope Mass Spectrometry Laboratory, Department of Geological Sciences, University of Florida. The manuscript was improved significantly by thoughtful reviews from Marie Balasse and Wolfgang Oschmann. References Anderson, D.G., 1996. Approaches to modeling regional settlement in the Archaic Period Southeast. In: Sassaman, K.E., Anderson, D.G. (Eds.), Archaeology of the Mid-Holocene Southeast. University Press of Florida, Gainesville, pp. 157 – 176. Andrus, C.F.T., Crowe, D.E., 2002. Alteration of otolith aragonite: effects of prehistoric cooking methods on otolith chemistry. J. Archaeol. Sci. 29, 291 – 299. Ansell, A.D., 1983. The biology of the genus Donax. In: McLachlan, A., Erasmus, T. (Eds.), Sandy Beaches as Ecosystems. W. Junk, The Hague, pp. 607 – 635. Bonsdorff, E., Nelson, W.G., 1992. The ecology of coquina clams Donax variabilis Say, 1822, and Donax parvula Philippi, 1849, on the east coast of Florida. Veliger 35, 358 – 365.
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Brown, A.C., McLachlan, A. (Eds.), 1990. Ecology of Sandy Shores. Elsevier, Amsterdam, p. 328. Cumbaa, S.L., 1976. A reconsideration of freshwater shellfish exploitation in the Florida Archaic. Fla. Anthropol. 29, 49 – 59. Dall, W.H., 1900. Contributions to the tertiary fauna of Florida. Wagner Free Inst. Sci. Trans. 3 (pt. 5), 949 – 1218. Davis, Jr. R.A., 1997. Geology of the Florida coast. In: Randazzo R.A., A.F., Jones, D.S. (Eds.), The Geology of Florida. University Press of Florida, Gainesville, pp. 155 – 168. DuBar, J.R., Johnson, Jr. H.S., Thom, B., Hatchell, W.O., 1974. Neogene stratigraphy and morphology, south flank of the Cape Fear Arch, North and South Carolina. In: Oaks Jr., R.Q., DuBar, J.R. (Eds.), Post-Miocene Stratigraphy, Central and Southern Atlantic Coastal Plain. Utah State University Press, Logan, Utah, pp. 139 – 173. Elliot, M., deMenocal, P.B., Linsley, B.K., Howe, S.S., 2003. Environmental controls on the stable isotopic composition of Mercenaria mercenaria: potential application to paleoenvironmental studies. Geochem. Geophys. Geosyst. 4 (7), 1 – 16. Fairbanks, R.G., 1989. A 17,000-year glacio-eustatic sea level record: influence of glacial melting rates on the Younger Dryas event and deep-ocean circulation. Nature 342, 637 – 642. Gardner, J., 1928. The molluscan fauna of the Alum Bluff group of Florida: Part V. Tellinacea, Solenacea, Mactracea, Myacea, Molluscoidea. U.S. Geol. Surv. Prof. Pap. 142-E, 185 – 249. Gischler, E., Hudson, J.H., 2004. Holocene development of the Belize Barrier Reef. Sediment. Geol. 164, 223 – 236. Goodwin, D.H., Flessa, K.W., Scho¨ne, B.R., Dettman, D.L., 2001. Cross-calibration of daily growth increments, stable isotope variation, and temperature in the Gulf of California bivalve mollusk Chione cortezi: implications for paleoenvironmental analysis. Palaios 16, 387 – 398. Grossman, E.L., Ku, T.-L., 1986. Oxygen and carbon isotopic fractionation in biogenic aragonite: temperature effects. Chem. Geol. 59, 59 – 74. Haug, G.H., Hughen, K.A., Sigman, D.M., Peterson, L.C., Ro¨hl, U., 2001. Southward migration of the intertropical convergence zone through the holocene. Science 293, 1304 – 1308. Hodell, D.A., Curtis, J.H., Jones, G.A., Higuera-Gundy, A., Brenner, M., Binford, M.W., Dorsey, K.T., 1991. Reconstruction of Caribbean climate change over the last 10,500 years. Nature 352, 790 – 793. Huddlestun, P.F., 1988. A revision of the lithostratigraphic units of the coastal plain of Georgia: the Miocene through Holocene. Georgia Geol. Surv. Bull. 104 (162 pp.) Hudson, J.H., Shinn, E., Halley, R., Lidz, B., 1976. Sclerochronology: a new tool for interpreting past environments. Geology 4, 361 – 364. Jackson, J.B.C., et al., 2001. Historical overfishing and the recent collapse of coastal ecosystems. Science 293, 629 – 638. Jones, D.S., 1983. Sclerochronology: reading the record of the molluscan shell. Am. Sci. 71, 384 – 391. Jones, D.S., 1998. Isotopic determination of growth rates and longevity in fossil and modern invertebrates. Paleontol. Soc. Pap. 4, 37 – 76.
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Jones, D.S., Quitmyer, I.R., 1996. Marking time with bivalve shells: oxygen isotopes and season of annual increment formation. Palaios 11, 340 – 346. Jones, D.S., Williams, D.F., Arthur, M.A., 1983. Growth history and ecology of the Atlantic surf clam, Spisula solidissima (Dillwyn), as revealed by stable isotopes and annual shell increments. J. Exp. Mar. Biol. Ecol. 73, 225 – 242. Jones, D.S., Arthur, M.A., Allard, D.J., 1989. Sclerochronological records of temperature and growth from shells of Mercenaria mercenaria from Narragansett, Rhode Island. Mar. Biol. 102, 225 – 234. Jones, D.S., Quitmyer, I.R., Andrus, C.F.T., 2003. Stable isotope evidence of middle to late Holocene seasonality and temperature change in archaeological coquina clam shells from Florida. EOS, Trans. Am. Geophys. Un. 84 (52) (Ocean Sci. Mtg. Suppl., Abstr. OS51I-08). Kirby, M.X., 2000. Paleoecological differences between tertiary and quaternary Crassostrea oysters, as revealed by stable isotope sclerochronology. Palaios 15, 40 – 49. Loesch, H.C., 1957. Studies on the ecology of two species of Donax on Mustang Island, Texas. Publ. Inst. Mar. Sci., Univ. Tex. 4, 201 – 227. Marchitto, T.A., Jones, G.A., Goodfriend, G.A., Weidman, C.R., 2000. Precise temporal correlation of Holocene mollusk shells using sclerochronology. Quat. Res. 53, 236 – 246. McLachlan, A., Young, N., 1982. Effects of low temperature on the burrowing rates of four sandy beach molluscs. J. Exp. Mar. Biol. Ecol. 65, 275 – 284. Mikkelsen, P.S., 1985. A comparison of two Florida populations of the coquina clam, Donax variabilis Say, 1822 (Bivalvia: Donacidae): II. Growth rates. Veliger 27, 308 – 311. Milanich, J.T., 1994. Archaeology of Precolumbian Florida. Florida University Press, Gainesville, p. 476. Morrison, J.P.E., 1971. Western Atlantic Donax. Biol. Soc. Wash. Proc. 83, 545 – 568. Newman, C.L., Weisman, B.R., 1992. Prehistoric and historic settlement in the Guana Tract, St. Johns County, Florida. Fla. Anthropol. 45, 162 – 171. Pearse, A.S., Humm, A.J., Wharton, G.W., 1942. Ecology of sand beaches at Beaufort, North Carolina. Ecol. Monogr. 12, 135 – 190. Quitmyer, I.R., Jones, D.S., 2000. The over-exploitation of hard clams (Mercenaria spp.) from five archaeological sites in the southeastern United States. Fla. Anthropol. 53, 160 – 167. Quitmyer, I.R., Jones, D.S., Arnold, W.S., 1997. The sclerochronology of hard clams, Mercenaria spp., from the southeastern U.S.: a method of elucidating the zooarchaeological records of seasonal resource procurement and seasonality in prehistoric shell middens. J. Archaeol. Sci. 24, 835 – 840. Quitmyer, I.R., Jones, D.S., Andrus, C.F.T., in press. Seasonal collection of coquina clams (Donax variabilis Say, 1822) during the Archaic and St. Johns periods in coastal northeast Florida.
In: Bar-Yosef, D. (Ed.), Archaeomalacology: Molluscs in Former Environments of Human Behavior. Oxbow Books, Oxford, UK. Richardson, C.A., 2001. Molluscs as archives of environmental change. Oceanogr. Mar. Biol. Ann. Rev. 39, 103 – 164. Ro¨thlisberger, F., 1986. 10,000 Jahre Gletschergeschichte der Erde: Ein Vergleich zwischen Nord-und Su¨dhemispha¨re. Aarau, Verlag, Sauerla¨nder, p.416. Ruppert, E.E., Fox, R.S., 1988. Seashore Animals of the Southeast. University of South Carolina Press, Columbia, p. 429. Russo, M., 1992. Chronologies and cultures of the St. Marys region of Northeast Florida and Southeast Georgia. Fla. Anthropol. 45, 107 – 126. Russo, M., 1996. Southeastern mid-Holocene coastal settlements. In: Sassaman, K.E., Anderson, D.G. (Eds.), Archaeology of the Mid-Holocene Southeast. University Press of Florida, Gainesville, pp. 177 – 199. Russo, M., Ste. Claire, D., 1992. Tomoka stone: archaic period coastal settlement in east Florida. Fla. Anthropol. 45, 336 – 346. Russo, M., Purdy, B.A., Newson, L.A., McGee, R.M., 1992. A reinterpretation of late Archaic adaptations in central-east Florida: groves’ Orange Midden (8-VO-2601). Southeast. Archaeol. 11, 95 – 108. Scho¨ne, B.R., Tanabe, K., Dettman, D.L., Sato, S., 2003. Environmental controls on shell growth rates and (d 18O of the shallowmarine bivalve mollusk Phacosoma japonicum in Japan. Mar. Biol. 142, 473 – 485. Socki, R.A., Karlsson, H.R., Gibson, E.K., 1992. Extraction technique for the determination of O-18 in water using preevacuated glass vials. Anal. Chem. 64, 829 – 831. Surge, D., Lohmann, K.C., Dettman, D.L., 2001. Controls on isotopic chemistry of the American oyster, Crassostrea virginica: implications for growth patterns. Palaeogeogr. Palaeoclimatol. Palaeoecol. 172, 283 – 296. Surge, D., Lohmann, K.C., Goodfriend, G.A., 2003. Reconstructing estuarine conditions: oyster shells as recorders of environmental change, Southwest Florida. Estuar. Coast. Shelf Sci. 57, 737 – 756. Trueman, E.R., Ansell, A.D., 1969. The mechanism of burrowing into soft substrata by marine animals. Oceanogr. Mar. Biol. Ann. Rev. 7, 315 – 366. Wefer, G., Berger, W.H., 1991. Isotope paleontology: growth and composition of extant calcareous species. Mar. Geol. 100, 207 – 248. Wilson, J.G., 1999. Population dynamics and energy budget for a population of Donax variabilis (Say) on an exposed South Carolina beach. J. Exp. Mar. Biol. Ecol. 239, 61 – 83. Witbaard, R., Jenness, M.I., Van der Borg, K., Ganssen, G., 1994. Verification of annual growth increments in Arctica islandica L. from the North Sea by means of oxygen and carbon isotopes. Neth. J. Sea Res. 33, 91 – 101.
Oxygen isotopic evidence for greater seasonality in Holocene shells of Donax variabilis from Florida Douglas S. Jones a,*, Irvy R. Quitmyer a, C. Fred T. Andrus b a
Florida Museum of Natural History, University of Florida, Gainesville, FL 32611, USA b University of Georgia, Savannah River Ecology Laboratory, Aiken, SC 29802, USA
Received 30 June 2004; received in revised form 28 October 2004; accepted 24 March 2005
Abstract Donax variabilis, the variable coquina clam, has been a common inhabitant of exposed sandy beach intertidal and shallow subtidal zones in the southeastern United States throughout the Pleistocene and Holocene. It is ideally suited for paleotemperature studies because it is restricted to environments of well-mixed, normal-marine seawater with a fairly uniform isotopic composition. As a result, oxygen isotopic variability in D. variabilis shells is largely explained by temperature variation. Although D. variabilis is small and short-lived, its shell represents an important paleoclimate archive because of its unique habitat preference. High-resolution sampling of individual D. variabilis shells and comparison of oxygen isotopic temperature profiles with historical seawater temperatures from the northeastern Florida coast indicate rapid shell growth over a brief life span of three to five or six months. Analysis of two modern shells reveals a close correspondence between isotopically determined water temperatures and historical water temperatures during the spring–summer growing season. Paleotemperature profiles from four archaeological shells, however, suggest a longer growth interval spanning summer–autumn. Two Preceramic Archaic Period shells (ca. 4240 and 5570 14C yr BP) and two Orange Period Archaic shells (ca. 3600 and 3760 14C yr BP), from four different archaeological sites, yield paleotemperatures that average 3.5 8C higher than present summer–autumn water temperatures. These warm paleotemperatures highlight seasonality differences associated with the mid-Holocene Hypsithermal climatic interval in this region. D 2005 Elsevier B.V. All rights reserved. Keywords: Oxygen isotopes; Donax variabilis; Shell growth; Paleotemperature; Florida; Holocene
1. Introduction The variable coquina clam, Donax variabilis Say, 1822, is one of the most common bivalve mollusks * Corresponding author. E-mail address: [email protected] (D.S. Jones). 0031-0182/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2005.03.046
inhabiting sandy beaches of the southeastern United States. This small bivalve, 1–2 cm in length, is found from Virginia to southern Florida and around the Gulf Coast to Texas (Morrison, 1971; Ruppert and Fox, 1988). It is often the dominant macro-invertebrate inhabiting the high-energy, sandy beach environment where it frequently occurs in extremely high densities
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(Pearse et al., 1942; Ansell, 1983; Brown and McLachlan, 1990; Wilson, 1999). Because Donax clams are excellent burrowers, they are ideally suited to life in the surf zone (Trueman and Ansell, 1969; McLachlan and Young, 1982). The wedge-shaped outline of the D. variabilis shell enhances substrate penetration. Its supreme ability as a rapid burrower has enabled this species to colonize the sands of exposed beaches which otherwise are fairly devoid of macrofauna (Wilson, 1999). The fossil record of D. variabilis suggests that it has occupied a similar ecological niche for hundreds of thousands to millions of years. It is widely reported from Pleistocene beach and barrier facies of several stratigraphic units exposed along the southeastern U.S. Atlantic Coastal Plain such as the Flanner Beach Fm. in North Carolina, the Canepatch Fm. in South Carolina, and the Satilla Fm. in coastal Georgia (DuBar et al., 1974). Whereas Donax shells are known back to at least the early Miocene Chipola Formation in Florida (Gardner, 1928), most Florida specimens of D. variabilis are known from Pleistocene and Holocene units (Dall, 1900). In northeastern Florida these include the Anastasia, Satilla, and Nashua formations with the Nashua possibly extending back to the late Pliocene (Huddlestun, 1988). In addition to Holocene occurrences of D. variabilis in raised beach deposits along the northeast coast of Florida, variable coquina clams are known from many archaeological sites in the region (Milanich, 1994). Carbon-14 dates from shell midden deposits attest to the fact that pre-Columbian people consumed vast quantities of coquina clams between the Middle Archaic and St. Johns periods (ca. 6000–400 yr BP). Middens range from nearly monotypic deposits of coquina clams to mixed assemblages containing brackish species such as oysters and hard clams, presumably collected from nearby estuaries or bays behind the barrier islands. This study of paleoclimate records in D. variabilis originated from an investigation of archaeological shells recovered from coastal middens in northeastern Florida to determine if there was a seasonal preference to ancient shellfish harvest (Quitmyer et al., in press; Jones et al., 2003). In addition to analyzing archaeological shells as bioarchives of past climates, livecollected clams were also studied in order to docu-
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ment the relationship between shell oxygen isotopes and water temperatures. 1.1. Bivalve shell records Bivalve mollusks grow their shells by means of the mantle-mediated addition of CaCO3 to the shell margin and by simultaneously thickening within the pallial line. This process is strongly influenced by environmental factors, many of which are rhythmic or periodic such as tides, diurnal cycles, and the annual change of the seasons. Such relationships are often so intimate that rhythmic accretionary fabrics within the shell such as growth lines, microgrowth increments, or macroscopic growth bands can be directly linked to the extrinsic factors responsible for their formation or to intrinsic factors such as spawning cycles or circadian rhythms (Jones and Quitmyer, 1996). As a mollusk shell grows, marking time by means of incremental growth features, the accretionary shell becomes a biogeochemical recorder of the environmental and climatic conditions experienced throughout the mollusk’s lifetime (Richardson, 2001). Annual growth increments represent the most pervasive pattern reported from bivalve shells (Jones, 1983, 1998). However, periodicities ranging from yearly down to sub-daily are recognized and the study of these features in skeletonized tissue is widely known as sclerochronology, analogous to the more familar dendrochronolgy (Hudson et al., 1976; Jones, 1983). In fossil or modern shells where banding patterns are indistinct or otherwise difficult to interpret, highresolution, sequential sampling for isotopic analyses often yields cyclical profiles, such as d 18O, that reflect annual environmental cycles (e.g., temperature). This approach can help evaluate or verify the timing and periodicity of shell growth increments (e.g., Jones et al., 1989; Witbaard et al., 1994; Scho¨ne et al., 2003) or it can offer independent evidence where rhythmic growth increments are lacking (e.g., Jones, 1983, 1998; Wefer and Berger, 1991). This approach has gained wide usage (e.g., Jones et al., 1983, 1989; Goodwin et al., 2001) and it is the strategy we use here because the D. variabilis shells lack prominent rhythmic banding patterns useful for age determination (Wilson, 1999).
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1.2. Longevity and shell growth of D. variabilis Despite its ecological importance, abundance and visibility on exposed sandy beaches, key life history parameters of D. variabilis such as growth rate and longevity are known only to a first approximation with widely divergent estimates in the literature. Longevity estimates for D. variabilis range from less than 1 year (e.g., Bonsdorff and Nelson, 1992) to upwards of three (e.g., Morrison, 1971). At least part of the explanation for the lack of accurate data is the traditional method for assessing growth and longevity in this species—size-frequency diagrams—an approach with many inherent problems (Mikkelsen, 1985). Other techniques such as mark-and-recapture studies are impractical because of the movements and high mortality of these clams (Mikkelsen, 1985) and growth rings on the shell are indistinct, not useful for age determination (Wilson, 1999). Microgrowth increments measuring a few microns to tens of microns in thickness are visible in shell cross-sections and may correspond to diurnal or tidal cycles. Counting such increments could provide an independent method of age determination; however, until their periodicity is ascertained, these increments are of limited value. In this study we use oxygen isotopic variation in shell carbonate to produce highresolution records of seasonal shell growth and generate longevity estimates for D. variabilis as a byproduct of our paleoclimate investigation. 1.3. Archaeological coquina shells in NE Florida The earliest evidence for human use of marine resources in northeastern Florida occurs in the numerous mid-Holocene shell middens scattered throughout the region (Milanich, 1994). Such deposits are widely recognized as important archives of cultural and paleoenvironmental data. The oldest date reported for one of these Atlantic coastal sites comes from Spencer’s Midden in Atlantic Beach, FL (5570 F 80 corr. 14C yr BP; calibrated to 6169–5759 yr BP), an arcuate oyster/coquina shell midden over 1 m deep and 50 m in length (Russo, 1996). A younger site at Crescent Beach, FL (4240 F 80 corr. 14C yr BP) located to the south (Fig. 1) was similarly occupied by preceramic Archaic peoples and contains abundant coquina shells (Russo, 1996). Overlying the precera-
mic midden components at several coastal sites are shell deposits containing fiber-tempered ceramics. These are characteristic of Orange Period peoples of the Late Archaic who emerged subsequently in this region (Milanich, 1994). Settlements identified on Fort George Island and neighboring barrier and marsh islands north of the St. Johns River indicate that extensive fiber-tempered ceramics appeared about 4200–3700 yr BP (Russo, 1996). Orange Period Archaic sites such as Rollins (3760 F 60 corr. 14C yr BP) and Guana Shell Ring (3600 F 50 corr. 14C yr BP) are typical of the many shell midden and ring sites of this and later periods known to contain large components of coquina clams (Newman and Weisman, 1992; Russo, 1992, 1996; Quitmyer et al., in press). Seasonal settlement patterns during these periods remain a subject of debate (Anderson, 1996; Cumbaa, 1976; Quitmyer et al., 1997). Middle Archaic through St. Johns II Period sites are also present in the interior of peninsular Florida. Freshwater fish and shellfish compose a major part of the refuse in these interior sites. Remains of marine bivalves, whelks, and sharks are also present and offer evidence for a connection with the coast (Quitmyer et al., in press). Some archaeologists have interpreted this to indicate that people seasonally migrated between the coast and Florida’s interior as part of their annual round of subsistence (Cumbaa, 1976). Zooarchaeologists have started to test this hypothesis against the alternative that coastal and interior people were permanent residents while trading resources (Quitmyer et al., 1997; Russo and Ste. Claire, 1992; Russo et al., 1992). Russo (1996 and references therein) notes that the almost exclusive use of small coquina clams, a beach resource, evidenced in some of the extensive sheet and mounded middens found on barrier islands and on the mainland coast, prompted speculation that coastal estuaries were not developed when the coasts were initially occupied because of lower sea-level stands. Therefore, permanent coastal settlement was not possible for the preceramic and early ceramic-producing cultures. In this view, coastal sites were seen as the winter seasonal occupations of interior groups who settled along the St. Johns River. Although more recent research suggests that both interior and coastal sites were occupied year-round (Quitmyer et al., 1997; Russo, 1996), the debate continues. The initial impetus for this study was to examine the issue of seasonal
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Fig. 1. Location map showing archaeological midden sites along the northeastern coast of Florida, including site of modern D. variabilis collection at Matanzas Beach, SAUFL station at Crescent Beach, and Florida Museum of Natural History (FLMNH).
occupation of coastal sites in northeastern Florida by using isotopic records in mollusk shells to determine season of harvest. We anticipated shell d 18O records might also shed light on climatic conditions that could have affected ancient settlement patterns.
2. Materials and methods Particularly abundant in the shell middens described above are specimens of D. variabilis which often dominate the zooarchaeological component of any particular site. Collections of D. variabilis have been made at many of the significant excavation sites by archaeologists and students from the University of Florida (Quitmyer et al., in press). These are housed in the collections of the Environmental Archaeology Program at the Florida Museum of Natural History (FLMNH), University of Florida, Gainesville.
For this initial investigation four archaeological shells of D. variabilis were selected for oxygen isotopic analysis from among the largest specimens in the FLMNH collections. Two well-preserved shells from the Preceramic Archaic were selected, one each from the Spencer’s Midden and Crescent Beach sites (Fig. 1). The shell lengths of these specimens were 21.5 and 23.1 mm, respectively. In addition, two specimens were selected from Orange Period Archaic material, one each from the Rollins Site (21.3 mm) and the Guana Shell Ring (20.5 mm). XRD analyses of sample powders indicated that all shells were 100% aragonite. A modern analogue population for comparison with the archaeological coquina shell material was available at the FLMNH. This assemblage of 2493 D. variabilis specimens was live-collected at monthly intervals at Matanzas Beach between January–December 1988 by Dr. Michael Russo during the course of research in the Department of Anthropology at the
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University of Florida (Quitmyer et al., in press). The collection site on Anastasia Island is located just south of the beach access ramp from Fort Matanzas National Monument (N 29.71748; W 81.23108), north of the Matanzas Inlet, in the same vicinity where indigenous people could have harvested coquina clams thousands of years ago (Fig. 1). Two of the largest modern shells (13.1 and 13.2 mm), collected in early July 1988, were selected for stable isotopic analysis. When carefully sampled in ontogenetic sequence from the umbo to the ventral shell margin (Fig. 2), the pattern of oxygen isotopic variation across a bivalve shell can provide a wealth of information about seasonal growth, shell growth rates, longevity, and season of death (Jones, 1983; Wefer and Berger, 1991; Jones and Quitmyer, 1996; Richardson, 2001). In order to assess these parameters in D. variabilis and to look at possible changes through time, sequential samples of shell carbonate were recovered from the two modern and four archaeological shells. Prior to sampling, each valve was cleaned following Andrus and Crowe (2002), with a wire brush under distilled water, and then treated with a 30% solution of H2O2. After rinsing in distilled water and drying in a vacuum oven, the samples were mounted onto glass slides with epoxy and fixed to the sample stage of a Merchantek EO Micromill at the Savannah River Ecology Laboratory. Carbonate samples were milled from the outer surface of the valve in shallow grooves parallel to the growth lines. Each sampling
Fig. 2. Archaeological D. variabilis shell from Guana Shell Ring site showing sampling grooves from milling procedure to collect powders for oxygen isotopic analyses. Scale bar divisions are 2 mm.
groove was 25 Am deep and the width varied depending upon the location relative to the growth axis of the shell. Sampling grooves near the outer margin were the most narrow and became progressively wider near the umbo where the shell is smaller (Fig. 2). The isotopic analyses were conducted in the Light Stable Isotope Mass Spectrometry Laboratory, Department of Geological Sciences, University of Florida. The powdered CaCO3 samples were analyzed according to standard techniques (Jones and Quitmyer, 1996) which involved an initial reaction in vacuo with 100% orthophosphoric acid at 90 8C for 15 min. An on-line automated carbonate-preparation system facilitated the production and purification of the evolved CO2 gas. The isotopic differences between the derived CO2 gas and the VPDB standard were determined with a VG Isogas PRISM Series I mass spectrometer equipped with triple collectors and micro-inlet system. All values are reported in standard d notation where: . 18 d18 O ¼ 18 O=16 O O=16 O 1 sample
standard
103 permilðxÞ: The weight of individual micro-samples (20–50 Ag) was so small that replicates of unknowns could not be run and often required that adjacent samples be combined to produce a desired minimum sample weight of about 50 Ag. However, replicated NBS-19 standards run before and after sample strings varied by F0.04x (Table 1). The d 18O value of seawater from the Matanzas Beach collection site was measured via CO2 equilibration following Socki et al. (1992) at the University of Georgia, Geology Stable Isotope Laboratory. Five monthly samples were analyzed (Dec.–Mar. and Aug.), providing a measure of seasonal d 18O range. Precision was estimated based on analysis of the laboratory working standard of Athens tap water to be F 0.05 (1r). The d 18O values of seawater at the Matanzas Beach collection site were very consistent (Dec. 1.0x; Jan. 1.2x; Feb. 1.0x; Mar. 1.1x; and Aug. 1.1x), averaging 1.08x SMOW (F0.08x, 1r: maximum range = 0.2x). Mean salinity = 33.1 F 0.4 psu. We calculated the temperature of the water in which the shell carbonate formed using the paleotemperature equation of Grossman and Ku (1986) for the
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Table 1 Donax variabilis d 18O (permill relative to VPDB, x) and paleotemperatures Sample
Modern 1 18
Modern 2 18
Crescent Beach 18
Spencer’s Midden 18
Rollins Site 18
Guana Shell Ring
N
d O
8C
d O
8C
d O
8C
d O
8C
d O
8C
d 18O
8C
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
1.9 0.7 0.8 0.8 0.6 0.7 0.1 0.0
17.0 22.7 22.2 22.0 23.2 22.7 25.3 26.1
1.6 1.3 1.5 0.8 0.8 0.7 0.8 0.2 0.6 0.1 0.1 0.3 0.2
18.4 19.9 19.1 29.8 22.0 22.5 22.4 25.2 23.2 25.7 26.3 27.1 27.0
0.3 0.9 0.5 1.4 1.6 1.0 0.9 1.1 1.1 1.3 0.4 0.3 0.4 1.0 0.7 0.4 0.1 0.2
27.1 30.2 28.3 32.3 33.2 30.8 30.0 30.9 31.2 31.9 27.8 24.5 27.8 30.5 29.2 28.0 26.2 24.8
0.8 0.6 1.2 0.9 0.7 1.0 0.6 1.4 1.1 1.3 1.1 0.8 1.1 1.1 0.4 0.4 0.4 0.9 1.2 0.2
29.8 28.8 31.6 30.0 29.2 30.6 28.7 32.4 31.2 32.2 31.0 29.8 31.1 31.1 23.8 24.1 23.9 21.9 20.5 25.2
0.1 0.3 0.6 1.0 0.9 1.0 0.9 0.8 2.0 0.1 0.0 0.6 1.0 1.2 1.4 1.3 1.3 0.8 0.3 1.3
25.5 27.4 28.9 30.4 30.1 30.7 30.1 29.7 35.1 26.3 25.8 28.5 30.7 31.6 32.6 32.1 32.0 29.9 24.4 20.0
1.0 1.1 1.0 0.8 0.9 1.1 1.3 1.5 1.2 1.3 0.7 0.1 0.9
30.4 31.0 30.8 29.8 30.0 30.9 31.8 32.9 31.6 31.9 29.1 26.3 21.8
Replicates of NBS 19 standard: N = 13, mean = 2.19, and s.d. = 0.04.
temperature-dependent fractionation of aragonite in mollusks relative to seawater: T ð8CÞ ¼ 21:8 4:69 shell d18 OVPDB seawater d18 OSMOW 0:2x : The resulting paleotemperature data from the two modern shells were then graphed against the mean weekly seawater temperature curve based on historical measurements made in 1988 at the nearby St. Augustine National Data Buoy Center station, SAUFL (http:// www.ndbc.noaa.gov/Maps/Florida.shtml). The paleotemperature data from the four archaeological shells were graphed in similar fashion to the mean weekly seawater temperature curve averaged over a longer, 17year interval (1986–2002) from the SAUFL station. The sandy beach habitat preference of modern D. variabilis in northeastern Florida, characterized by fairly constant salinity and d 18Owater composition year-round, means that shell d 18O variations largely reflect changes in water temperature during the growing season. This greatly simplifies the interpretation of paleoenvironmental conditions recorded in D. variabilis shells as compared to the isotopic records of common estuarine bivalves like oysters or hard
clams. Both temperature and salinity–d 18O relationships must be factored into the interpretation of shell isotopic records in estuarine species such as Crassostrea virginica (Kirby, 2000; Surge et al., 2001, 2003) or Mercenaria mercenaria (Jones et al., 1989; Elliot et al., 2003). However, if it can be assumed that the open-marine, sandy beach habitat of D. variabilis did not change appreciably over the past several thousand years in terms of seawater chemistry, then fairly precise estimates of past water temperatures are possible from shell d 18O records. A sea level rise of 1–2 m in the past ~6000 years for the Florida coast (Davis, 1997; Gischler and Hudson, 2004) would yield only a minimal change in seawater d 18O of b 0.1x (Fairbanks, 1989). Therefore, we use the mean of our measured seawater-d 18O values (1.08x SMOW) in the aragonite fractionation equation of Grossman and Ku (1986) to solve for paleotemperatures.
3. Results With respect to the modern shells, eight unique samples for specimen #1 (13.1 mm) and 13 unique
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samples for specimen #2 (13.2 mm) were analyzed. These samples produced d 18O values that ranged from 1.9x to 0.0x for specimen #1 and from 1.6x to 0.8x for specimen #2 (Table 1). Fig. 3 shows the isotopic paleotemperatures plotted against the sea surface temperatures. Four factors were considered in plotting these points: 1) the sample spacing of the milled grooves on each shell; 2) preserving the ontogenetic order of the samples; 3) the exponential shell growth pattern of most bivalves; and 4) the early July collection date which should coincide with the final sample from each shell margin. With these considerations in mind, the paleotemperature data were plotted in sequence by eye as closely as possible to matching water temperatures along the annual curve. The paleotemperature data exhibit an excellent fit to the segment of the curve corresponding to spring and early summer when water temperatures are rising. Only one data point (specimen #2, sample 4) shows excessive depletion and it is unclear if this is analytical (e.g., fractionation, contamination) or environmental (e.g., large storm with freshwater spike) in origin. The shell-edge paleotemperature values agree with the water temperature at the time of collection in early July 1988. The isotopic records indicate that shell growth in both specimens occurred over a 14–
16 week interval, approximately corresponding to weeks 12–27 of the year (early April–early July). In a similar manner, the four larger archaeological shells were sampled and analyzed for oxygen isotopic ratios. A total of 18 samples were analyzed from the Crescent Beach specimen. The d 18O values ranged from 0.3x to 1.6x. From the Spencer’s Midden shell a total of 20 samples were analyzed whose d 18O values ranged between 1.2x and 1.4x. A total of 20 samples from the Rollins Site shell yielded d 18O values that ranged between 1.3x and 2.0x. From the Guana Shell Ring specimen, 13 samples were analyzed. These produced d 18O values that ranged from 0.9x to 1.5x. The oxygen isotopic data were converted to paleotemperatures and these were overlain onto the mean weekly seawater temperature curve for the 17-year interval 1986–2002 from data at the SAUFL station (Fig. 4). The results are comparable to those achieved for the modern shells with a few important differences. The paleotemperature profiles for the two Preceramic Archaic shells from the Crescent Beach and Spencer’s Midden sites show the closest correspondence with weeks 25–45 of the annual temperature curve. This pattern suggests shell growth occurred for about 20 weeks, from late June into mid-November. The paleotemperature profiles for the two younger
40 35
Modern Donax variabilis
o
Temperature ( C)
30 25 20 15 10 Mean SST - 1988 (SAUFL Station)
5
Shell 1 Shell 2
0 1
4
7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52
Week Fig. 3. Weekly mean sea surface temperatures (small solid circles) from the SAUFL station for 1988 with isotopic paleotemperature profiles from two modern specimens of D. variabilis plotted in open symbols.
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103
40 Archaeological Donax variabilis Preceramic Archaic
35
Temperature (oC)
30 25 20 15 Mean SST - 1986-2002 (SAUFL Station)
10 Crescent Beach Shell (4240+/-80 14C yr BP)
5
Spencer's Midden Shell (3760+/-60 14C yr BP)
0 1
5
9
13
17
21
25
29
33
37
41
45
49
53
Week 40 Archaeological Donax variabilis
Temperature (oC)
35 30 25 20 15
Mean SST - 1986-2002 (SAUFL Station)
10 Rollins Site Shell (3760+/-60 14C yr BP)
5
Guana Shell Ring Shell (3600+/-50 14C yr BP)
0 1
5
9
13
17
21
25
29
33
37
41
45
49
53
Week Fig. 4. Seventeen-year average of weekly mean sea surface temperatures (small solid circles, bars = 1r) from the SAUFL station for 1986–2002 with isotopic paleotemperature profiles from two Preceramic Archaic shells (upper) and two Orange Period Archaic shells (lower) of D. variabilis plotted in open symbols.
shells from the Orange Period Archaic sites (Rollins and Guana Shell Ring) reveal a similar pattern although the umbonal region of the latter (early growth) was worn and not possible to sample. Shell growth over a period of about 23 weeks is indicated, approximately corresponding to weeks 24–47 of the year, mid-June to late November. All four archaeological shells were larger in size than any of the modern shells observed in this study. The results indicate that ancient clams achieved their larger size by growing slightly faster for a longer
period of time (i.e., 6–9 weeks longer) than their modern counterparts. Whereas maximum longevity among the modern shells was approximately three to four months, the archaeological specimens lived for five or six months. The paleotemperature records indicate that each of the four archaeological shells was harvested in autumn. Although the seasonal pattern of paleotemperature change recorded by the oxygen isotopes in the archaeological shells mirrors that of modern seawater at this site, the values reveal a consistent negative offset from modern conditions.
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The paleotemperature values from each archaeological shell reflect summer–autumn ocean water conditions that were warmer than today by about 3.5 8C.
4. Discussion 4.1. Shell growth and longevity The oxygen isotopic evidence from modern and archaeological shells of D. variabilis provides an accurate method of assessing seasonal shell growth and longevity in this species. These data serve to refine previous estimates that were based largely on size-frequency analyses. Ansell (1983) reported that most Donax species, like D. variabilis, are short-lived with life spans of 1– 2 years, rapid growth to maximum size, and early maturity. Morrison (1971) suggested that D. variabilis has a 2-year life span and that in some cases individuals may survive a third year. Mikkelsen (1985) estimated that D. variabilis in Florida grew at a rate of 3.0–3.7 mm/month in the summer months and that, bthe majority of individuals probably live for approximately one year, with a few entering a second yearQ (Mikkelsen, 1985, p.310). This conclusion generally agrees with Loesch (1957) and Pearse et al. (1942) who also employed shell length-frequency graphs to examine growth and longevity in this species. bBonsdorff and Nelson (1992) estimated a maximum growth rate of 3.43 mm/month for a Florida population of D. variabilis, at which rate the animals would grow to maturity in just two monthsQ (Wilson, 1999, p. 78). The oxygen isotope results achieved in this study support such rapid growth rate estimates and reduced longevity (i.e., months, not years). The growth rates calculated from our isotopic data are about the same, 3.3–3.7 mm/month, for the modern specimens and slightly higher, 4.1–4.6 mm/month, for the archaeological shells. The isotopic data also call into question whether these clams actually survive for one full year as the modern and archaeological shell isotope records indicated life spans of three to four and five to six months, respectively. Admittedly both the modern and archaeological specimens were live-collected by humans and therefore it could be argued that they still had the potential to grow larger and live longer. However,
both sets of shells were among the largest specimens out of the thousands recovered in either the modern or zooarchaeological collections, minimizing this possibility. Our results cast doubt on previous longevity estimates for this species that hypothesize growth and survival into a second or even a third year of life. In a study involving D. variabilis from the South Carolina coast, Wilson (1999) laments that growth rings on the shell were indistinct and it was not possible to verify the age of specimens by this method. Furthermore, constant movement of beach sands and contained clams, as well as the migratory ability of D. variabilis, prevented the use of a dmark-andrecaptureT technique for growth measurement. These same observations hold true regarding the populations in northeastern Florida and reinforce the value of the oxygen isotopic technique for growth rate and age determination. It is also applicable to archaeological and fossil shells. The close correspondence between paleotemperatures derived from oxygen isotopes and the measured seasonal variation in water temperature at the site made it possible to reconstruct the timing and duration of shell growth to a weekly level of resolution. Life spans that proved to be appreciably shorter than acknowledged in the literature, three to five or six months in duration, represent an unanticipated outcome of this study. Compared with the largest modern shells from the same region, the archaeological shells achieved a greater maximum size and longevity. The isotopic profiles suggest that the larger sizes of the ancient shells were achieved through slightly faster growth over an extended growing season. The spring–early summer growing season of the modern shells is clearly shifted from that of the archaeological shells (summer–autumn). Isotopic analyses of additional modern and archaeological shells will be necessary to document the extent of these differences and gauge changes through time. The possibility of size reduction from over-fishing, observed in many marine species (Jackson et al., 2001) and in other bivalves such as Mercenaria spp. from Florida midden sites (Quitmyer and Jones, 2000), must also be considered. 4.2. Season of harvest of archaeological specimens Several important differences were observed between the modern and archaeological D. variabilis
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Fig. 5. Worldwide record of glacial advances and retreats for the Holocene from Ro¨thlisberger (1986) with dashed horizontal lines corresponding to ages of archaeological shells used in this study (G=Guana Shell Ring; R=Rollins Site; C=Crescent Beach; and S=Spencer’s Midden).
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shells. These are intriguing and warrant further study. The isotopic profiles from all four archaeological shells, from four separate sites and distinct Archaic Period time horizons, indicated coquina clam harvest in autumn. This preliminary result suggests a seasonal pattern of behavior by indigenous peoples that needs to be verified through the analysis of additional shells from these as well as other sites and time periods (Quitmyer et al., in press). 4.3. Paleoclimatic implications When plotted against a 17-year record of historical water temperatures (1986–2002), the majority of paleotemperatures recorded in the shell d 18O profiles of the four archaeological D. variabilis plot above the modern temperature curve (Fig. 4). The average difference is + 3.5 8C, suggesting warmer water temperatures in the mid-Holocene and possibly reflecting later stages of the Holocene thermal maximum (Haug et al., 2001), or bHypsithermalQ, in this region (Jones et al., 2003). The timing of these warm temperature episodes broadly correlates with worldwide records of Holocene glacial retreats (Fig. 5), although the correlation is far from perfect. Upon closer inspection the d 18O profiles indicate that around 5570, 4240, 3760 and 3600 14C yr BP mean water temperatures during summer–early autumn, the Northern Hemisphere warm season, were higher than modern values. It is less clear that the same was true of water temperatures leading up to the warm season (early summer) or away from it (late autumn). This dilemma arises from two sources—uncertainties concerning the growing season and longevity of archaeological D. variabilis and the likelihood that seasonality of a mid-Holocene year was greater than today. Both of these factors could influence the shape of the limbs of the convex d 18O profiles (Fig. 4). Higher seasonality (greater contrast between summer and winter) would yield an annual temperature curve with increased amplitude and steeper limbs. The higher warm-season temperatures indicated by the d 18O profiles and the convergence of the d 18O profiles with the modern temperature curve on either side of the warm season suggest that mid-late Holocene years were more seasonal. Testing this hypothesis will require sampling archaeological coquina clams that were har-
vested at different seasons of the year, if they exist, or other suitable taxa. Interestingly, Holocene climate models for the circum-Caribbean region predict greater seasonality during this interval. Hodell et al. (1991) suggested that long-term changes in the ratio of evaporation to precipitation (E / P) in the circum-Caribbean region are controlled by the intensity of the annual cycle, which in turn is controlled by long-term insolation changes forced by orbital mechanics. The intensity of the annual cycle is defined as the difference between August and February insolation at 108 N, with August and February corresponding to the months when the intertropical convergence zone (ITCZ) moves farthest north and south, respectively. Intervals of intense seasonality coincide with far northerly movement of the ITCZ during the Northern Hemisphere summer, bringing increased rainfall and higher temperatures to the northern American tropics. The intensity of the annual cycle in the northern tropics reached a maximum during the early Holocene and declined toward the present as a result of changes in seasonal insolation forced by Earth’s precessional cycle (Hodell et al., 1991). The higher warm-season temperatures seen in the archaeological coquina shell d 18O profiles (Fig. 4) are consistent with the increased intensity of the annual cycle described above for the early to middle Holocene in the circum-Caribbean region. Naturally it is difficult to establish causality in this case as northeastern Florida, at ~308 N, lies outside the zone of strongest influence by the annual migration of the ITCZ. It may be that climate change in the tropics, where much of Florida’s weather originates, exerts an important influence on the annual cycle of air and water temperature in the study area.
5. Concluding remarks Although long-lived species are usually preferred in stable isotopic and sclerochronologic analyses of shells (e.g., Jones et al., 1989; Marchitto et al., 2000), d 18O profiles from D. variabilis illustrate the value of this short-lived bivalve as a bioarchive of life history and paleoclimatic information. The exposed sandy beach habitat preference of D. variabilis, with its normal-marine seawater chemistry, simplifies the in-
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terpretation of the shell d 18O profiles and underscores the usefulness of this mollusk in paleoenvironmental reconstruction. By making additional oxygen isotopic analyses of D. variabilis shells from older as well as younger archaeological time periods, it should be possible to chart the history of water temperature change in northeastern Florida since the last glacial episode. If additional archaeological specimens are analyzed, particularly shells that grew during the winter and spring seasons, it should be possible to refine shell growth rate estimates, evaluate the likelihood of seasonal vs. year-round shellfish harvest by indigenous peoples, and test the hypothesis of greater seasonality during the mid-Holocene.
Acknowledgements We thank Michael Russo, National Park Service, Southeastern Center, Tallahassee, for sharing unpublished data. We also acknowledge the Invertebrate Paleontology and Environmental Archaeology Laboratories, Florida Museum of Natural History, for supplying essential resources. U.S. Department of Energy grant #DE-FC09-96SR18546 (CFTA) provided partial support of this research. We appreciate discussions with David Hodell and analytical assistance given by Jason Curtis of the Light Stable Isotope Mass Spectrometry Laboratory, Department of Geological Sciences, University of Florida. The manuscript was improved significantly by thoughtful reviews from Marie Balasse and Wolfgang Oschmann. References Anderson, D.G., 1996. Approaches to modeling regional settlement in the Archaic Period Southeast. In: Sassaman, K.E., Anderson, D.G. (Eds.), Archaeology of the Mid-Holocene Southeast. University Press of Florida, Gainesville, pp. 157 – 176. Andrus, C.F.T., Crowe, D.E., 2002. Alteration of otolith aragonite: effects of prehistoric cooking methods on otolith chemistry. J. Archaeol. Sci. 29, 291 – 299. Ansell, A.D., 1983. The biology of the genus Donax. In: McLachlan, A., Erasmus, T. (Eds.), Sandy Beaches as Ecosystems. W. Junk, The Hague, pp. 607 – 635. Bonsdorff, E., Nelson, W.G., 1992. The ecology of coquina clams Donax variabilis Say, 1822, and Donax parvula Philippi, 1849, on the east coast of Florida. Veliger 35, 358 – 365.
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