Quaternary Geochronology 5 (2010) 519–532
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Quaternary Geochronology
journal homepage: www.elsevier.com/locate/quageo
Research Paper
Radiocarbon dating of small terrestrial gastropod shells in North America
Jeffrey S. Pigati a, *, Jason A. Rech b, Jeffrey C. Nekola c
a
U.S. Geological Survey, Denver Federal Center, Box 25046, MS-980, Denver CO 80225, USA
Department of Geology, Miami University, Oxford, OH 45056, USA
c
Department of Biology, University of New Mexico, Albuquerque, NM 87131, USA
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 May 2009
Received in revised form
20 January 2010
Accepted 21 January 2010
Available online 29 January 2010
Fossil shells of small terrestrial gastropods are commonly preserved in wetland, alluvial, loess, and glacial
deposits, as well as in sediments at many archeological sites. These shells are composed largely of
aragonite (CaCO3) and potentially could be used for radiocarbon dating, but they must meet two criteria
before their 14C ages can be considered to be reliable: (1) when gastropods are alive, the 14C activity of
their shells must be in equilibrium with the 14C activity of the atmosphere, and (2) after burial, their
shells must behave as closed systems with respect to carbon. To evaluate the first criterion, we conducted
a comprehensive examination of the 14C content of the most common small terrestrial gastropods in
North America, including 247 AMS measurements of modern shell material (3749 individual shells) from
46 different species. The modern gastropods that we analyzed were all collected from habitats on
carbonate terrain and, therefore, the data presented here represent worst-case scenarios. In sum, w78% of
the shell aliquots that we analyzed did not contain dead carbon from limestone or other carbonate rocks
even though it was readily available at all sites, 12% of the aliquots contained between 5 and 10% dead
carbon, and a few (3% of the total) contained more than 10%. These results are significantly lower than
the 20–30% dead carbon that has been reported previously for larger taxa living in carbonate terrain. For
the second criterion, we report a case study from the American Midwest in which we analyzed fossil
shells of small terrestrial gastropods (7 taxa; 18 AMS measurements; 173 individual shells) recovered
from late-Pleistocene sediments. The fossil shells yielded 14C ages that were statistically indistinguishable from 14C ages of well-preserved plant macrofossils from the same stratum. Although just one site,
these results suggest that small terrestrial gastropod shells may behave as closed systems with respect to
carbon over geologic timescales. More work on this subject is needed, but if our case study site is
representative of other sites, then fossil shells of some small terrestrial gastropods, including at least five
common genera, Catinella, Columella, Discus, Gastrocopta, and Succinea, should yield reliable 14C ages,
regardless of the local geologic substrate.
Published by Elsevier B.V.
Keywords:
Radiocarbon
Land snails
Limestone effect
Chronology
Quaternary
1. Introduction
Gastropods are one of the most successful animal groups on
Earth, with at least 70,000 extant species occupying terrestrial,
marine, and freshwater habitats. Globally, terrestrial gastropods
encompass at least 35,000 species (Barker, 2001), span 4–5 orders
of magnitude in shell volume, and represent a variety of trophic
levels, including polyphagous detritivores, herbivores, omnivores,
and carnivores (Kerney and Cameron, 1979; Burch and Pearce,
1990). They are so exceptionally diverse in their appearance,
ecology, and physiology that determining their phylogenetic relationships from conchological and/or anatomical characteristics
* Corresponding author. Tel.: þ1 303 236 7870; fax: þ1 303 236 5349.
E-mail address: jpigati@usgs.gov (J.S. Pigati).
1871-1014/$ – see front matter Published by Elsevier B.V.
doi:10.1016/j.quageo.2010.01.001
remains difficult and controversial (e.g., Ponder and Lindberg, 1997
and references therein). It is clear, however, that the preference for
terrestrial habitats of North American gastropods developed independently in three of six basal clades (Neritomorpha, Caenogastropoda, and Heterobranchia), with the informal group Pulmonata
representing more than 99% of the continental fauna. Of the Pulmonata, the most common size class1 in both modern and fossil
assemblages are individuals with adult shells that are <10 mm in
maximum dimension (Nekola, 2005) (Fig. 1).
1
Size classes of gastropods are categorized by the maximum shell dimension
(length or diameter) as follows: large (>20 mm), medium (10–20 mm), small (5–
10 mm), minute (2–5 mm), and micro (<2 mm). Although the size of the gastropods targeted in this study range from small to micro, for simplicity, we refer to
them collectively as ‘‘small’’.
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J.S. Pigati et al. / Quaternary Geochronology 5 (2010) 519–532
Fig. 1. Photographs of select small terrestrial gastropods included in this study (1 mm bar in each panel for scale). (a) Cochlicopa lubricella, (b) Columella columella alticola, (c) Discus
macclintockii, (d) Euconulus fulvus, (e) Hawaiia miniscula, (f) Hendersonia occulta, (g) Punctum minutissimum, (h) Pupilla muscorum, (i) Strobilops labyrinthica, (j) Vallonia gracilicosta,
(k) Vertigo elatior, and (l) Zonitoides arboreus.
Today, small terrestrial gastropods occupy and thrive in
incredibly diverse habitats, from marshes, wet meadows, and
grasslands to upland forests and tundra. Species are known from all
continents, save Antarctica, and occupy almost all climate regimes
except hyperarid deserts and the high Arctic. Their distribution in
the fossil record is equally diverse. Gastropod shells are commonly
preserved in wetland, alluvial, loess, and glacial deposits, as well as
within sediments at archeological sites worldwide (e.g., Evans,
1972). But even though their distribution is widespread and their
aragonitic shells contain w12% by weight carbon, terrestrial
gastropods are often avoided for 14C dating because many taxa
incorporate 14C-deficient (or ‘‘dead’’) carbon from limestone and
other carbonate rocks when building their shells. This phenomenon, referred to as the ‘‘Limestone Problem’’ by Goodfriend and
Stipp (1983), can cause 14C ages of gastropod shells to be as much
as w3000 yrs too old.
Despite the Limestone Problem, geochronologists have continued
to investigate the possibility of using terrestrial gastropod shells for
14
C dating because of their widespread occurrence and potential for
dating Quaternary sediments. Most 14C studies of gastropod shells
have found that gastropods consistently incorporate dead carbon
from limestone in their shells when it is available (Frye and Willman,
1960; Leighton, 1960; Rubin et al., 1963; Tamers, 1970; Evin et al.,
1980; Goodfriend and Hood, 1983; Goodfriend and Stipp, 1983;
Goslar and Pazdur, 1985; Yates, 1986; Goodfriend, 1987; Zhou et al.,
1999; Quarta et al., 2007; Romaniello et al., 2008). These studies,
however, were generally limited to a few individual gastropods
collected from a small number of sites, and were biased toward large
taxa and warm climates. Brennan and Quade (1997) analyzed
a number of small terrestrial gastropod taxa and found that small
shells generally yielded reliable 14C ages for late-Pleistocene paleowetland deposits in the American Southwest. Pigati et al. (2004)
followed by measuring the 14C activities of a suite of small gastropods living in alluvium dominated by Paleozoic carbonate rocks in
Arizona and Nevada and found that while some of the small gastropods did incorporate dead carbon from limestone when building
their shells, others did not.
Based in part on these initial positive results, small terrestrial
gastropod shells have been used recently to date Quaternary
wetland and lacustrine deposits in the Americas (e.g., Pedone and
Rivera, 2003; Placzek et al., 2006; Pigati et al., 2009). However, it
is unclear if the results obtained from modern gastropods collected
from a limited number of sites in the American Southwest can be
extrapolated to all geologic, ecologic, and climatic environments.
Moreover, it is not known if results for one taxonomic level (family,
genus, or species) can be extrapolated to other members of the
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J.S. Pigati et al. / Quaternary Geochronology 5 (2010) 519–532
same level living elsewhere, or even between individuals living
within the same population.
Here we report the results of a comprehensive analysis of the
Limestone Problem for small terrestrial gastropods from 163
localities in North America (Fig. 2). All samples that we analyzed
were collected from habitats on carbonate terrain and, therefore,
the data reported here represent worst-case scenarios. In addition,
we measured the 14C activities of a number of fossil shells recovered from well-dated sediments at a late-Pleistocene site in the
American Midwest as a case study to determine if the shells remain
closed systems with respect to carbon over geologic timescales.
Positive results for both tests for a particular taxon would allow us
to consider 14C ages derived from fossil shells of that taxon to be
reliable, regardless of the local geologic substrate.
2. Shell carbonate and
14
C dating
All materials (organic and inorganic) that yield reliable 14C ages
have two common characteristics. First, the initial 14C activity of the
material – a plant, for example – was in equilibrium with
atmospheric 14C at the time that it was alive. In other words, the 14C
activity of a plant that lived T yrs ago was the same as the 14C
activity of the atmosphere T yrs ago (after accounting for isotopic
fractionation). Second, after death, the material behaved as a closed
system; carbon was neither added to nor removed from the sample
material. If both of these criteria are met, then the measured 14C
activity is a function of only two parameters: the initial 14C activity
of the atmosphere and the amount of time elapsed since the death
of the organism.
The measured 14C activity and the 14C age of the material are
related by the familiar decay equation
A ¼ Ao elt
(1)
14
where A and Ao are the measured and initial C activities of the
material, respectively, l is the decay constant, and t is the time
elapsed since the death of the organism. Conventional radiocarbon
ages assume that the atmospheric 14C activity is invariant through
time (i.e., Ao ¼ 1). Radiocarbon ages can be converted to calendar
year ages to account for temporal variations in the 14C activity of the
atmosphere (Reimer et al., 2009).
Fig. 2. Locations of modern localities (red dots) and the fossil locality at the Oxford East outcrops in southwestern Ohio (star). Modern localities and collection information are listed
in Tables S1 and S2, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
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J.S. Pigati et al. / Quaternary Geochronology 5 (2010) 519–532
or granules which then dissolve in their stomach acid to produce
CO2. As before, the dead carbon from the rocks is introduced to the
hemolymph, passed on to the extrapallial fluid, and ultimately
incorporated in the shell carbonate. Dead carbon from limestone
can account for up to w30% of the total carbon in shells of large
terrestrial gastropods. (Goodfriend and Stipp, 1983).
2
Deviation from true 14C age (ka)
1
0
-1
-2
14
C ages of shell carbonate
-3
2.2. Effects of carbon sources on
-4
In most environments, the 14C activities of live plants are in
equilibrium with atmospheric carbon. Gastropods that obtain their
shell carbon from live plants and the air, therefore, should yield
reliable 14C ages, assuming they behave as closed systems after
burial (Fig. 3). Gastropods that consume organic detritus (i.e.,
decaying plant litter) typically do not pose a significant problem for
14
C dating because the time between plant death, its incorporation
into decomposition products, and consumption by gastropods is
usually quite short, on the order of a few yrs.
The 14C activity of water that is available for consumption by
terrestrial gastropods (e.g., dew, standing water, precipitation) is at
or near equilibrium with atmospheric 14C and, therefore, water is
unlikely to introduce a significant error to 14C ages of terrestrial
gastropod shells. Exceptions include gastropods living directly
adjacent to springs that discharge waters from deeply-circulating
carbonate aquifers, lakes or rivers with significant hard water
effects, or in active volcanic areas where 14C-deficient CO2 in
surface waters may be abundant (e.g., Riggs, 1984; Grosjean, 1994).
14
C ages of gastropods living in such areas should be evaluated
carefully.
The incorporation of 14C from limestone and other carbonate
rocks can present a significant problem for 14C dating of terrestrial
gastropod shells. The 14C activity of atmospheric carbon, plants, and
water consumed by gastropods is essentially the same, w100%
modern carbon (pMC). In contrast, because most carbonate rocks
are of pre-Quaternary age, their 14C activity is typically 0 pMC. Thus,
for 14C dating, the magnitude of the potential error introduced by
carbonate rocks is a direct function of the amount of carbon from
rocks that is incorporated in the gastropod shell (Fig. 3). Unfortunately, a simple correction that accounts for the incorporation of
14
C-deficient carbon in gastropod shells is not possible because we
cannot know a priori how much of the shell carbon was derived
from carbonate rocks versus other sources. Thus, to be confident in
14
C ages derived from terrestrial gastropod shells, it is imperative
that we identify and avoid taxa that incorporate dead carbon from
rocks altogether.
-5
-6
-7
-8
0
5
10
15
20
25
True 14C age (ka)
30
35
40
Fig. 3. Modeled deviation from the true 14C age for four scenarios: (1) closed-system
behavior and no dead carbon (thick solid line), (2) closed-system behavior and 10%
dead carbon (dashed line), (3) open-system behavior equivalent to 1% modern carbon
contamination and 10% dead carbon (thin solid line), and (4) open-system behavior
equivalent to 1% modern carbon contamination and no dead carbon (dotted line).
2.1. Sources of shell carbon
In order to evaluate the validity of a 14C age of a given sample,
the 14C contents of the original sources of the carbon and their
contribution to the total carbon content must be known. Carbon in
gastropod shell carbonate originates from as many as four different
sources: atmospheric CO2, food, water, and carbonate rocks.
Gastropods incorporate atmospheric CO2 in their shell
carbonate via respiration. Respired CO2 is introduced to the bicarbonate pool in the gastropod’s hemolymph and passed along to the
extrapallial fluid, from which the shell carbonate is ultimately
precipitated (Wilbur, 1972). Estimates of the contribution of
atmospheric CO2 to gastropod shell carbonate vary between
negligible (Stott, 2002), 16–48% (Romaniello et al., 2008), and 30–
60% (Goodfriend and Hood, 1983).
Carbon from food sources (e.g., living plants, fungi, organic
detritus) is incorporated into the extrapallial fluid through two
mechanisms: direct digestion and breakdown of urea. When
gastropods consume and digest food, carbon is introduced to the
hemolymph and passed along to the extrapallial fluid in the same
manner as atmospheric CO2. There it mixes with atmospheric
carbon before becoming incorporated in the shell carbonate
(Wilbur, 1972). Carbon derived from urea takes a more indirect
pathway. Urea that is not expelled by the gastropod breaks down
into CO2 and NH3 via a urease reaction (Stott, 2002). The resulting
CO2 is then reintroduced directly to the extrapallial fluid and ultimately incorporated into the shell carbonate. Estimates of the
amount of carbon derived from plants, either directly or indirectly
through urea, vary between 25 and 40% (Goodfriend and Hood,
1983), 36–73% (Romaniello et al., 2008), and w100% (Stott, 2002).
Terrestrial gastropods ingest water from multiple sources,
including dew, soil moisture, standing water, and precipitation, all
of which contain some amount of dissolved inorganic carbon (DIC).
Water is taken up through the foot of the gastropod by contact
rehydration (Balakrishnan and Yapp, 2004) and introduced to the
hemolymph before being passed on to the extrapallial fluid. Pigati
et al. (2004) found that aqueous carbon sources account for
w10% of the shell carbon for one species of Catinella, but it is not
known if this value is constant across the entire Succineidae family.
To our knowledge, data for other terrestrial taxa do not exist.
Finally, some terrestrial gastropods are able to scrape carbonate
rocks (limestone, dolomite, soil carbonate), and ingest the powder
2.3. Open-system behavior
Even if some terrestrial gastropods consistently manage to avoid
the Limestone Problem regardless of the local geologic substrate or
environmental conditions, there is another hurdle that must be
overcome before we can confidently use their shells for 14C dating.
That is, gastropod shells must remain closed systems with respect
to carbon after burial. For reliable 14C dating, the pool of carbon
atoms measured during the 14C dating process must consist solely
of carbon atoms that originally resided in the shell. Thus, following
burial, shells must resist the addition or exchange of 14C atoms with
the local environment. Shells that exhibit open-system behavior
typically yield 14C ages that are too young, and the magnitude of the
error depends upon the degree of such behavior (Fig. 3).
Previous work has suggested that 14C ages from small terrestrial
gastropod shells recovered from fossil deposits in arid environments may be reliable back to at least w30,000 14C yrs B.P., but
a small degree of open-system behavior appears to compromise 14C
dates obtained from shells older than this (Pigati et al., 2009). In
J.S. Pigati et al. / Quaternary Geochronology 5 (2010) 519–532
Fig. 4. Photographs of the three genera of the Succineidae family: Catinella (left
panels), Oxyloma (middle panels), and Succinea (right panels); all are w10 mm in
length. The simple shells of the three genera contain few diagnostic characteristics
and, therefore, species-level identification is based on soft-body reproductive organ
morphology, which is rarely preserved in the fossil record.
this study, we analyzed fossil shells from the American Midwest
because of their abundance in Quaternary deposits in the region,
the presence of multiple calcareous substrates (Paleozoic limestone, calcareous till, and loess), and the humid climate (annual
precipitation in southwestern Ohio is w100 cm yr1). If fossil shells
exhibit even a small degree of open-system behavior in arid environments, it may be exacerbated and, therefore, more easily
detected in humid environments where interaction between shells
and DIC in ground water is more prevalent.
3. Methods
3.1. Live gastropods
Previous ecological sampling by one of us (JCN) has resulted in
an extensive collection of modern terrestrial gastropods from North
America, constituting w250 taxa and over 470,000 individuals
from more than 1000 modern environments. Gastropods were
collected at each site from a representative 100–1000 m2 area by
hand collection of larger taxa and litter sampling of smaller taxa,
which provides the most complete assessment of site faunas
(Oggier et al., 1998; Cameron and Pokryszko, 2005). Collections
were made at places of high mollusc density, such as loosely
compacted leaf litter lying on top of highly compacted damp soil or
humus (Emberton et al., 1996). Litter was removed by hand and
sieved by shaking, tapping, or other agitation in the field using
a shallow sieve (ASTME #10; 2.0 mm mesh) nested inside a second
sieve (ASTME #30; 0.6 mm mesh). The process was continued for
15–60 min during which time 50–500 mg of material was collected
and retained.
Gastropods and detritus were dried at room temperature in the
laboratory and then hand-picked against a neutral background. All
shells, shell fragments, and slug plates were removed and identifiable material was assigned to species using JCN’s reference
collection. Nomenclature generally follows that of Hubricht (1985)
with updates and corrections by Nekola (2004).
523
We selected 247 aliquots of shell material (3749 individual
shells) from 163 sites across the United States and southern Canada
for 14C analysis. Nearly all of the specimens that we chose for
analysis were collected live, but at a few sites, only recently-dead
gastropods were available, which were identified by a translucent
appearance or the retention of color in the shells. Shells of small
gastropods that are dead for more than a year or so become
increasingly white and opaque with time (J. Nekola, unpublished
data), and were excluded from our study.
In the fossil record, species-level identification of fossil shells is
possible for most small terrestrial gastropods and, therefore, the
results of our investigation of modern gastropods can be applied
directly to the fossil record. An exception is the Succineidae family,
which is composed of three genera (Catinella, Oxyloma, and Succinea) that are difficult to differentiate in modern faunas, let alone the
geologic record (Fig. 4). Their simple shells exhibit few diagnostic
characteristics and, therefore, species-level identification is based
on soft-body reproductive organ morphology, which is rarely
preserved in the fossil record. This presents a significant problem for
geochronologists; that is, can we be confident in 14C ages derived
from shells from any taxon within the Succineidae family, or do we
need to target a specific genus or species? To address this issue, we
measured the 14C activity of 100 aliquots of gastropod shell material
(802 individual shells) for twelve species of the Succineidae family to
determine the level of identification (i.e., family, genus, or species)
required to apply our results to the fossil record.
We prepared aliquots of modern shell material for 14C analysis at
the University of Arizona Desert Laboratory (JSP) and Miami
University (JAR). We selected multiple shells at random for X-ray
diffraction (XRD) analysis using a Siemens Model D-500 diffractometer to verify that only shell aragonite remained prior to
preparation for 14C analysis. There was no evidence of primary or
secondary calcite in any of the shells that we analyzed. When
possible, shells were broken, the adhering soft parts were removed
using forceps, and the shells were treated with 6% NaOCl for 18–
24 h at room temperature to remove all remnants of organic matter.
Shells were not powdered during pretreatment to minimize the
potential for adsorption of atmospheric 14C (Samos, 1949). We
selectively dissolved some of the shells by briefly introducing dilute
HCl to remove secondary carbonate (dust) from primary shell
material. Shells were washed repeatedly in ASTM Type 1, 18.2 MU
(hereafter ‘‘ultrapure’’) water, sonicated for a few seconds to
remove adhered solution, washed again with ultrapure water, and
dried in a vacuum oven overnight at w70 C.
Shell aragonite was converted to CO2 using 100% H3PO4 under
vacuum at either 50 or 75 C until the reaction was visibly complete
(w1 h). Water, SOx, NOx, and halide species were removed using
passive Cu and Ag traps held at w600 C and the resulting CO2 was
split into two aliquots. One aliquot was converted to graphite by
catalytic reduction of CO (modified after Slota et al., 1987) and
submitted to the Arizona-NSF Accelerator Mass Spectrometry
(AMS) facility for 14C analysis. The second aliquot was submitted for
d13C analysis in order to correct the measured 14C activity of the
shell carbonate for isotopic fractionation.
14
C data for modern gastropods are presented as D14C values in
per mil (Stuiver and Polach, 1977; Reimer et al., 2004) and analytical uncertainties are reported at the 2s (95%) confidence level. d13C
values are given in the usual delta (d) notation as the per mil
deviation from the VPDB standard. Analytical uncertainties for d13C
measurements are less than 0.1& based on repeated measurements of standards.
We also measured the 14C content of several gastropod bodies
for comparison to their corresponding shells. The bodies were
treated with 10% HCl for 15–30 min to remove any remnants of the
carbonate shell, rinsed repeatedly, and dried in a vacuum oven at
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J.S. Pigati et al. / Quaternary Geochronology 5 (2010) 519–532
C values of gastropod diets
To quantify the amount of carbon in a gastropod shell that was
derived from limestone or other carbonate rocks, it is necessary to
compare the measured D14C values of shells with D14C values of the
gastropod diet. D14C values of live plants consumed by gastropods
are identical to D14C values of the atmosphere (Fig. 5a), which were
calculated using 14C data averaged over the Northern Hemisphere
(Hua, 2004). We assigned a 5& uncertainty to the atmospheric
values to account for short-term, regional variations (Hsueh et al.,
2007) and small changes in atmospheric 14C values that occur at
a given site during the short (usually annual) lifespan of the small
gastropods (Manning et al., 1990; Meijer et al., 1995).
Plant detritus from previous years’ primary production has
a higher 14C activity than live plants because of the 14C ‘‘bomb-spike’’
(Hua, 2004). Simply comparing the D14C values of shells with the
D14C of the atmosphere or live plants during the year that the
gastropod was alive, therefore, would ignore the potential impact of
detritus in the gastropod diet. We estimated the amount of detritus
from a given year that was available for consumption using a wide
range of carbon turnover rates (0.2–0.002 yr1; Fig. 5b), which are
applicable to O horizons and the upper few centimeters of A horizons
in which small terrestrial gastropods typically live (Guadinski et al.,
2000; Torn et al., 2005; Brovkin et al., 2008). We intentionally
chose a wide range of carbon turnover rates, which are equivalent to
mean residence times for carbon of 5–500 yrs, to encompass the
potentially wide range of detritus ages at the 163 localities included
in our study. We then used Monte Carlo simulation to generate
10,000 values for the D14C of the gastropod diets for each of the past
13 yrs (the time span of our collections), which allowed two factors to
vary randomly: the carbon turnover rate (0.2–0.002 yr1) and the age
of the detritus fraction included in the diet (range of 0–1000 yrs). We
also ran simulations in which we let the age of the detritus vary up to
10,000 yrs, but the results did not change significantly.
We took the average and standard deviation of the 10,000
generated values as the gastropod diet D14C value for each year of
collection between 1996 and 2008 (Fig. 5c). All uncertainties are
reported at the 2s (95%) confidence level. As expected, the modeled
D14C value of the gastropod diet for a given year is slightly higher
than the atmospheric D14C value for the same year. Uncertainties
associated with the modeled values are relatively large, on the
order of w35%, because of the large range of detritus D14C values
that could be present at a given site.
Shells with D14C values that are lower than the modeled D14C
value of the gastropod diet during the year in which the gastropod
was alive indicate the presence of dead carbon from limestone or
other carbonate rocks. When applicable, the difference between the
D14C values was converted into 14C yrs to estimate the ‘‘limestone
effect’’, which represents the potential error introduced by the
incorporation of dead carbon in the shells. The magnitude of the
limestone effect should be considered a maximum value because
the calculation assumes that all of the dead carbon came from
carbonate rocks, rather than older (but not infinitely-aged) organic
matter. Because of the uncertainties associated with modeling the
D14C of the gastropod diet, we are unable to discern limestone
effects smaller than w300 14C yrs.
∆ 14C of atmosphere (‰)
14
250
200
150
100
50
0
1980
1985
1990
1995
2000
2005
2010
Year
b
1.0
0.9
Fraction of detritus available
for consumption
3.2. Modeling of
a
0.8
0.7
0.6
0.002 yr-1
0.5
0.2 yr-1
0.4
0.3
0.05 yr-1
0.2
0.02 yr-1
0.1
0.01 yr-1
0.0
0
200
400
600
800
1000
Time after plant death (years)
c
250
∆14C of gastropod diet (‰)
w70 C. The dried bodies were placed in 6 mm quartz tubes along
with w100 mg of cupric oxide (CuO) and a small piece
(1 mm 5 mm) of silver foil, all of which were pre-combusted at
900 C for 4–6 h. The tube was evacuated, sealed with a glassblower’s torch, and combusted offline at 900 C. The resulting CO2
gas was isolated and converted to graphite as above.
200
150
100
50
0
1993
1998
2003
2008
Date of collection
Fig. 5. Modeling results for (a) the 14C activity of the gastropod diet using measured
atmospheric values for the northern hemisphere (after Hua, 2004), (b) carbon turnover
rates (CTRs) ranging from 0.2 to 0.002 yr1, and (c) Monte Carlo simulation to generate
estimates of the D14C values of gastropod diets for each of the past 13 yrs.
3.3. Fossil gastropods
Fossil gastropod shells were collected from glacial deposits at
the Oxford East glacial outcrops in southwestern Ohio as a case
study to determine if the shells remain closed systems with respect
to carbon over geologic timescales. These outcrops contain a series
of glacial diamictons from the Miami Lobe of the Laurentide Ice
J.S. Pigati et al. / Quaternary Geochronology 5 (2010) 519–532
Sheet that are separated by thin (3–5 cm) units of calcareous
organic-rich silt that contain gastropods, plant macrofossils, and
rooted tree stumps. AMS 14C dating of plant macrofossils and in situ
tree stumps has shown that the age of this unit is between w20,100
and 21,400 14C yrs B.P. (Eckberg et al., 1993; Lowell, 1995).
Gastropod taxa identified previously from this unit include Columella columella, Discus cronkhitei, Euconulus fulvus, Hendersonia
occulta, Pupilla muscorum, Vertigo elatior, and multiple Succineidae
taxa (Dell, 1991).
Gastropod-bearing sediment was collected and placed in
deionized water with a deflocculant for several days to soften the
sediment enough to pass through a 0.5 mm sieve. A few samples
were placed in an ultrasonic bath for w1 h to further disaggregate
the sediment. Fossil shells were hand-picked from the retained
fraction, placed in a beaker of ultrapure water, subjected to an
ultrasonic bath for a few seconds, and then repeatedly dunked in
a second beaker of ultrapure water to remove sediment that
adhered to the shell surface or was lodged within the shell itself.
The recovered shells were broken and examined under a dissecting
microscope to ensure that the interior whorls were free of
secondary carbonate and detritus. Fossil shells that were free of
detritus were then processed for 14C in the same manner as the
modern specimens, including random selection of shells for XRD
analysis. None of the fossil shells that we analyzed contained
measurable quantities of either primary or secondary calcite.
Organic samples, which included bark, charcoal, plant fragments, and wood, were subjected to a standard acid–base–acid
(ABA) chemical pretreatment with 1N HCl (1 h at 60 C), 1N NaOH
(18–24 h at 60 C), and 1N HCl again (2–4 h at 60 C) before
combustion at 900 C in the presence of cupric oxide and silver foil.
The resulting CO2 was purified and converted to graphite in the
same manner as above.
Conventional radiocarbon ages are reported in 14C yrs and, after
calibration, in calendar yrs. For calibration, we used the IntCal09.14C
dataset (CALIB 6.0.0, Stuiver and Reimer, 1993; Reimer et al., 2009).
4. Results
4.1. Modern shells
A few aliquots (11 of 247, or 4.5% of the total) yielded D14C values
that were higher than the modeled dietary D14C values of the
corresponding year of collection, which indicates these individuals
consumed unusually high amounts of bomb-spike carbon (Table S1,
Fig. 6). Data from these shells were excluded from further analysis.
For the remaining 236 aliquots of gastropod shells from the 46
different species that we analyzed, w78% did not contain any dead
carbon from limestone or other carbonate rocks even though it was
readily available at all sites, w12% contained between 5 and 10%
dead carbon, and a few (3% of the total) contained more than 10%
(Table S1, Fig. 6). D14C values for all taxa ranged from 97.5 to
158.4&, and limestone effects averaged only w180 14C yrs.
Dead carbon was not detected in the shells of at least 23
different species, including (number of shells in parentheses) Catinella avara (99), Catinella gelida (66), Catinella vermeta (39),
Cochlicopa lubricella (17), Cochlicopa morseana (16), Columella
columella (94), Discus catskillensis (40), Discus cronkhitei (47), Discus
macclintockii (5), Euconulus alderi (71), Euconulus polygyratus (57),
Gastrocopta pentodon (131), Nesovitrea binneyana (46), Punctum
minutissimum (542), Pupilla hebes (7), Strobilops affinis (36), Succinea bakeri (27), Succinea grosvernori (1), Succinea n. sp. ‘Minnesota
A’ (1), Succinea ovalis (50), Succinea strigata (14), Vertigo hubrichti
(144), and Vertigo modesta (73).
D14C values were most negative (i.e., contained the most dead
carbon) for shells from the Pupilla and Vallonia genera. Maximum
525
limestone effects for these genera ranged from 780 310 14C yrs for
P. muscorum to 1590 280 14C yrs for P. sonorana, and from
1010 380 14C yrs for Vallonia perspectiva to 1500 270 14C yrs for
V. cyclophorella, respectively (Table 1). The only other species that
exhibited a limestone effect that was greater than 1000 14C yrs was
H. occulta (1210 250 14C yrs).
These results can be applied directly to the fossil record if it is
possible to identify the taxa to the species-level based on shell
morphology. For the Succineidae family, the data must be evaluated
at the family or genus level to be applicable. Taking the Succineidae
family as a whole, 85% of the shell aliquots that we analyzed did not
contain measurable amounts of dead carbon; the remaining aliquots
contained an average of 5.2% dead carbon. Within the family, D14C
values for the genus Catinella ranged from 63.7 to 147.0&, Oxyloma
values ranged from 0.8 to 135.0&, and Succinea values ranged from
16.1 to 147.8&. Members of the Catinella genus incorporate little, if
any, dead carbon from limestone or other carbonate rocks in their
shells; 32 of 33 aliquots (97%) of Catinella shells did not contain
measurable amounts of dead carbon. The remaining sample
(DL-170) contained only a very small amount of dead carbon,
equivalent to a limestone effect of 320 310 14C yrs. Similarly, 36 of
39 aliquots (92%) of Succinea shells did not contain dead carbon. The
remaining three aliquots were all Succinea indiana; limestone effects
for these shells ranged from 430 370 to 610 380 14C yrs. For
Oxyloma, 15 of 24 aliquots (63%) did not contain measurable
amounts of dead carbon. Limestone effects for the remaining
samples ranged between 250 210 and 670 240 14C yrs.
4.2. Modern gastropod bodies
We also measured the 14C activity of the bodies of eight gastropods to determine the magnitude of the offset between the body
carbon and shell carbonate (Table 2). Ideally, we would have
preferred to measure the 14C content of the extrapallial fluid to
compare with the shell carbonate to determine if carbon isotopes are
fractionated when the shells are formed, but this was not feasible
because the gastropod bodies were simply too small. Regardless, the
measured D14C values of gastropod body carbon ranged from 42 to
101&, similar to the D14C values of shell carbonate from the same
sites, which ranged from 43 to 133&. However, we did not observe
a clear relation between the 14C activity of gastropod body carbon
and shell carbonate. D14C values of bodies of Oxyloma retusa
collected from Maquokata River Mounds, Iowa were indistinguishable from the D14C values of their corresponding shells
(D14Cbody ¼ 42 6&, D14Cshell ¼ 43 10&), as were values for S.
ovalis from Dave Pepin Homestead, Minnesota (D14Cbody ¼ 101 8&,
D14Cshell ¼ 110 8&). In contrast, D14C values for bodies of S. ovalis
were significantly lower than their shells from Brewer Boat Ramp,
Maine (D14Cbody ¼ 55 6&, D14Cshell ¼ 82 5&) and Zippel Bay
State Park, Minnesota (D14Cbody ¼ 84 6&, D14Cshell ¼ 133 12&).
The reason(s) for this difference is unclear.
4.3. Fossil shells
Well-preserved fossil organic material (bark, plant macrofossils,
and wood) recovered from sediments at the Oxford East outcrops
yielded calibrated ages that ranged from 24.60 0.40 to
25.28 0.55 ka, with an average of 24.93 0.30 ka (n ¼ 5; Table 3,
Fig. 7). Gastropod shells recovered from the same stratigraphic unit
yielded ages that ranged from 23.92 0.66 to 25.81 0.94 ka, and
averaged 24.73 0.44 ka (n ¼ 18). Average ages of six of the seven
fossil taxa were indistinguishable from the average age of the
organic matter: Discus shimeki (24.80 0.17 ka; n ¼ 3), P. muscorum
(24.34 0.32 ka; n ¼ 3), Vallonia gracilicosta (24.20 0.40 ka;
n ¼ 2), Vertigo hannai (24.90 0.77 ka; n ¼ 1), V. modesta
526
J.S. Pigati et al. / Quaternary Geochronology 5 (2010) 519–532
a
Fig. 6. Shell carbonate D14C values compared to modeled dietary D14C values for modern gastropods. Data points that fall on the solid black line in each panel represent gastropods
that obtained their carbon from live plants and the atmosphere. Data points that fall below the solid line indicate that dead carbon from limestone or other carbonate rocks was
incorporated during shell construction. The magnitude of this phenomenon, called the ‘‘limestone effect’’, depends upon the amount of shell carbon that was derived from rocks as
shown by the dashed lines.
(24.86 0.32 ka; n ¼ 3), and Succineidae (25.29 0.54 ka; n ¼ 3).
The average age of H. occulta (24.34 0.15 ka; n ¼ 3) was slightly
younger than the organic ages.
5. Discussion
5.1. Small terrestrial gastropods and the limestone problem
Approximately 78% of the modern shells that we analyzed did
not contain any dead carbon from limestone or other carbonate
rocks even though it was readily available at all sites, w12% of the
aliquots contained between 5 and 10% dead carbon, and a few (3%
of the total) contained more than 10%. Even at the high end, the
amount of dead carbon in the small shells is significantly less than
the 20–30% dead carbon that has been previously reported for
larger taxa (e.g., Goodfriend and Stipp, 1983).
If we extrapolate our results to the fossil record and assume that
the shells behave as closed systems with respect to carbon over
geologic timescales, then small terrestrial gastropod shells should
provide accurate 14C ages w78% of the time and ages that are
J.S. Pigati et al. / Quaternary Geochronology 5 (2010) 519–532
527
b
Fig. 6. (continued).
within w1000 14C yrs of the true age w97% of the time. Shells from
at least 23 different species did not contain dead carbon, and
therefore should yield reliable 14C ages if the modern shell data can
be applied directly to the fossil record.
For the Succineidae family as a whole, 85% of the shell aliquots
that we analyzed did not contain measurable amounts of dead
carbon; the remaining aliquots contained an average of 5.2% dead
carbon, equivalent to a limestone effect of 425 14C yrs. At the
genus level, shells of the genus Catinella should yield reliable 14C
ages w97% of the time, again assuming closed-system behavior,
and ages that are within w300 14C yrs of the true age every time.
Similarly, Succinea shells should yield reliable 14C ages w92% of
the time and ages that are within w600 14C yrs of the true age
every time. Results for the genus Oxyloma suggest that some
caution should be used when evaluating 14C ages derived from
these shells. AMS results for the Oxyloma shells show that nearly 1
in 3 aliquots contained at least some dead carbon. Although the
amount was relatively minor, <7% of the total, dead carbon was
present in Oxyloma shells more frequently than in either Catinella
or Succinea shells.
528
Table 1
Summary of
J.S. Pigati et al. / Quaternary Geochronology 5 (2010) 519–532
14
C results for modern gastropod shells.
Limestone effecta (14C yrs)
Family
Genus
Species
Aliquots
Shells
Negligibleb
Maximumc
Cochlicopidae
Cochlicopa
Discidae
Discus
Helicarionidae
Euconulus
Cochlicopa lubrica
Cochlicopa lubricella
Cochlicopa morseana
Discus catskillensis
Discus cronkhitei
Discus macclintockii
Discus shimeki
Euconulus alderi
Euconulus dentatus
Euconulus fulvus
Euconulus polygyratus
Hendersonia occulta
Punctum minutissimum
Columella columella
Gastrocopta pentodon
Gastrocopta tappaniana
Pupilla blandi
Pupilla hebes
Pupilla muscorum
Pupilla sonorana
Vertigo elatior
Vertigo hannai
Vertigo hubrichti
Vertigo modesta
Vertigo paradoxa
Strobilops affinis
Strobilops labyrinthica
Catinella avara
Catinella exile
Catinella gelida
Catinella vermeta
Oxyloma retusa
Oxyloma verrilli
Succinea bakeri
Succinea grosvernori
Succinea indiana
Succinea n.sp. ‘Minnesota A’
Succinea ovalis
Succinea strigata
Vallonia cyclophorella
Vallonia gracilicosta
Vallonia perspectiva
Hawaiia minuscula
Nesovitrea binneyana
Nesovitrea electrina
Zonitoides arboreus
3
3
4
6
6
3
7
3
3
8
3
13
3
3
3
3
9
1
8
2
3
3
3
3
3
3
3
9
15
9
4
17
7
8
1
4
1
19
6
6
7
5
3
3
4
4
17
17
16
40
47
5
33
71
34
107
57
13
542
94
131
105
90
7
80
29
113
123
144
73
124
36
43
99
316
66
39
136
47
27
1
6
1
50
14
129
155
221
152
46
33
20
53%
100%
100%
100%
100%
100%
97%
100%
68%
57%
100%
46%
100%
100%
100%
92%
67%
100%
38%
66%
38%
65%
100%
100%
74%
100%
65%
100%
97%
100%
100%
65%
57%
100%
100%
17%
100%
100%
100%
95%
60%
68%
29%
100%
73%
85%
650
–
–
–
–
–
430
–
670
730
–
1210
–
–
–
540
1000
–
780
1590
500
280
–
–
380
–
520
–
320
–
–
540
670
–
–
610
–
–
–
1500
1370
1010
340
–
710
530
Helicinidae
Punctidae
Pupillidae
Hendersonia
Punctum
Columella
Gastrocopta
Pupilla
Vertigo
Strobilopsidae
Strobilops
Succineidae
Catinella
Oxyloma
Succinea
Valloniidae
Vallonia
Zonitidae
Hawaiia
Nesovitrea
Zonitoides
390
240
290
300
250
360
270
310
280
280
270
240
320
310
360
240
380
270
270
380
270
240
240
a
Defined as the theoretical difference between the measured and true 14C ages for gastropods that incorporate the same amount of dead carbon in their shells as the
aliquots measured here. These values are based on the difference between the modeled diet and shell carbonate D14C values and converted into 14C yrs.
b
Percent of shells measured by AMS that did not contain dead carbon from limestone or other carbonate rocks (i.e., the D14C values for the shells were statistically
indistinguishable from the modeled diet D14C value).
c
Maximum limestone effect for a given taxon (given in 14C yrs). Uncertainties are given at the 2s (95%) confidence level.
5.2. Ca-limiting hypothesis
Large gastropod shells (>20 mm in maximum dimension)
routinely contain 20–30% dead carbon when living in habitats on
carbonate terrain (e.g., Evin et al., 1980; Goodfriend and Stipp, 1983),
whereas the small shells measured in this study rarely contained
more than w10%. We speculate that calcium may hold the clues to
determining the reasons for the difference. Gastropod shell carbonate
(aragonite) is composed of three elements – calcium, carbon, and
oxygen. The latter two elements are readily available in the environments in which gastropods live and, therefore, cannot be considered
as possible limiting factors for shell construction. In most settings,
however, calcium is present in plants and water in low concentrations
(typically parts per million). If small terrestrial gastropods can acquire
enough calcium from their ‘‘normal’’ diet (plants, detritus, and water),
then they may not have to consume carbonate rocks to supplement
their calcium intake when building their shells. Larger taxa may find it
more difficult to obtain enough calcium from these sources without
turning to carbonate rocks when they are available.
Our results support this hypothesis, but only on a gross scale.
There is clearly a significant difference in the amount of dead
carbon incorporated in the shells of large taxa previously studied
and the small taxa studied here. However, shell size alone is not the
only factor to consider when evaluating the results within the small
body size class. For example, in the present study, we did not
observe a significant correlation between shell size and measured
D14C values (R2 ¼ 0.015). The largest taxon that we included in our
analysis, H. occulta, averaged 15.6 mg per shell and contained
approximately the same amount of dead carbon as P. muscorum and
Vallonia shells, which averaged only 1.4 and 0.7 mg per shell,
respectively. Similarly, we did not find a clear correlation between
shell size and measured D14C values even within a single family. For
Succineidae, Catinella shells were generally the smallest, averaging
1.1 mg per shell and contained the least amount of dead carbon, and
529
J.S. Pigati et al. / Quaternary Geochronology 5 (2010) 519–532
Table 2
14
C results for modern gastropod bodies and corresponding shells.
Lab #
AA #
Taxon
Sitea Lat ( N) Long ( W) Mass (mg) d13C (vpdb) F14Cb
Shell D14C Atmos D14C Diet D14C Limestone Effectc (14C yrs)
Bodies
MU-109
MU-114
MU-108
MU-111
MU-117
MU-115
MU-112
MU-113
80177
80181
80176
80178
80183
80182
80179
80180
Succinea ovalis
Succinea ovalis
Succinea ovalis
Succinea ovalis
Oxyloma retusa
Succinea ovalis
Succinea strigata
Succinea ovalis
1
2
3
4
5
6
7
8
44.819
48.410
48.906
47.874
42.559
50.264
64.858
48.866
68.723
94.819
96.027
96.422
90.713
66.411
147.862
94.843
4.22
8.64
3.79
11.05
9.04
12.88
4.01
6.88
24.9
24.8
25.1
26.0
26.1
25.0
24.2
23.5
1.0547
1.0996
1.0946
1.0877
1.0419
1.0485
1.0740
1.0833
0.0064
0.0082
0.0062
0.0062
0.0064
0.0072
0.0060
0.0060
55 6
101 8
96 6
89 6
42 6
49 7
74 6
84 6
70
80
80
80
52
57
54
80
5
5
5
5
5
5
5
5
96 33
107 36
107 36
107 36
73 25
79 27
75 26
107 36
330 280
50 300
90 300
150 300
250 210
250 230
0 210
180 300
Shells
MU-162
MU-128
MU-179
MU-141
80928
80194
80942
80910
Succinea ovalis
Succinea ovalis
Oxyloma retusa
Succinea ovalis
1
2
5
8
44.819
48.410
42.559
48.866
68.723
94.819
90.713
94.843
8.80
11.72
13.08
8.91
10.0
9.8
11.0
9.6
1.0817
1.1090
1.0425
1.1316
0.0048
82 5
0.0084 110 8
0.0100 43 10
0.0116 133 12
70
80
52
80
5
5
5
5
96 33
107 36
73 25
107 36
110 270
0 300
250 220
0 300
Uncertainties are given at the 2s (95%) confidence level.
a
Key to sites: 1 ¼ Brewer Boat Ramp, Maine; 2 ¼ Dave Pepin Homestead, Minnesota; 3 ¼ Duxby, Minnesota; 4 ¼ Huot Forest WMA, Minnesota; 5 ¼ Maquokata River
Mounds, Iowa; 6 ¼ September Islands, Quebec; 7 ¼ University of Alaska – Fairbanks; 8 ¼ Zippel Bay State Park, Minnesota.
b
F14 C values are derived from the measured 14C activity, corrected for fractionation, and account for decay that occurred between the time of collection and the AMS
measurement.
c
Defined in Table 1.
Oxyloma shells averaged 2.1 mg per shell and contained the most
dead carbon. Succinea shells were the largest, averaging 5.8 mg per
shell, but were between Catinella and Oxyloma in terms of the
amount of dead carbon in their shells (Table S1).
The results presented here suggest that the Limestone Problem
for small terrestrial gastropods is often negligible and always
much less than the 20–30% dead carbon for larger taxa. However,
there are additional factors that apparently influence the dietary
intake of carbonate rocks of small terrestrial gastropods living
side by side, which may include opportunistic behavior, variations
in microhabitats, and the dietary needs or wants of individual
gastropods.
Table 3
14
C results for the Oxford East outcrops.
Lab #
AA #
Taxon
Na
Organics
MU-212
MU-213
MU-214
MU-211
MU-210
82584
82585
82586
82583
82582
bark
bark
twig
wood
wood chip
Average
–
–
–
–
–
Gastropod shells
MU-194
82567
MU-195
82568
MU-196
82569
MU-188
MU-189
MU-190
82561
82562
82563
MU-199
MU-200
MU-201
82572
82573
82574
MU-191
MU-192
MU-193
82564
82565
82566
MU-202
MU-203
82575
82576
MU-205
MU-206
MU-207
MU-208
82577
82578
82579
82580
Discus shimeki
Discus shimeki
Discus shimeki
Average
Hendersonia occulta
Hendersonia occulta
Hendersonia occulta
Average
Pupilla muscorum
Pupilla muscorum
Pupilla muscorum
Average
Succineidae
Succineidae
Succineidae
Average
Vallonia gracilicosta
Vallonia gracilicosta
Average
Vertigo hannai
Vertigo modesta
Vertigo modesta
Vertigo modesta
Average
d13C (vpdb)
F14C
4.47
3.28
3.80
3.82
4.29
24.6
25.1
24.9
22.6
23.6
0.0731
0.0767
0.0721
0.0743
0.0772
2
2
2
9.20
10.94
14.99
6.4
6.9
6.4
0.0758 0.0052
0.0762 0.0052
0.0745 0.0053
20.72 0.55
20.68 0.54
20.87 0.57
1
1
1
12.41
10.29
18.42
6.4
6.1
7.1
0.0805 0.0051
0.0787 0.0055
0.0777 0.0055
20.24 0.51
20.42 0.56
20.52 0.57
8
9
10
10.86
11.88
14.13
6.5
6.4
6.4
0.0760 0.0052
0.0805 0.0052
0.0809 0.0051
20.71 0.55
20.24 0.52
20.20 0.51
8
4
15
10.58
9.32
9.53
5.3
5.5
5.8
0.0712 0.0052
0.0689 0.0056
0.0759 0.0055
21.23 0.59
21.49 0.65
20.71 0.58
15
15
10.72
11.08
6.0
6.2
0.0776 0.0058
0.0824 0.0058
20.53 0.60
20.05 0.56
30
15
15
20
10.46
11.47
14.14
15.28
6.5
6.7
7.0
6.7
0.0750
0.0751
0.0771
0.0729
20.81
20.80
20.59
21.04
Mass (mg)
14
C age (ka)
0.0026
0.0025
0.0028
0.0042
0.0025
0.0052
0.0052
0.0053
0.0053
21.02
20.62
21.13
20.88
20.58
0.28
0.27
0.32
0.45
0.26
0.55
0.55
0.55
0.58
Calendar age (ka)b
Pc
25.15
24.64
25.28
24.97
24.60
24.93
0.34
0.37
0.55
0.63
0.40
0.30
1.00
1.00
1.00
1.00
1.00
24.73
24.67
25.00
24.80
24.18
24.37
24.47
24.34
24.71
24.18
24.13
24.34
25.33
25.81
24.73
25.29
24.49
23.92
24.20
24.90
24.88
24.54
25.18
24.86
0.75
0.74
0.79
0.17
0.67
0.68
0.70
0.15
0.74
0.68
0.67
0.32
0.80
0.94
0.78
0.54
0.76
0.66
0.40
0.77
0.77
0.71
0.77
0.32
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
0.98
1.00
1.00
1.00
1.00
Uncertainties for the raw and calibrated 14C ages are given at the 2s (95%) confidence level.
a
Number of shells per aliquot.
b
Calibrated ages were calculated using CALIB v. 6.0.0, IntCal09.14C dataset; limit 50.0 calendar ka B P. Calibrated ages are reported as the midpoint of the calibrated range.
Uncertainties are reported as the difference between the midpoint and either the upper or lower limit of the calibrated age range, whichever is greater. Multiple ages are
reported when the probability of a calibrated age range exceeds 0.05.
c
P ¼ probability of the calibrated age falling within the reported range as calculated by CALIB.
530
J.S. Pigati et al. / Quaternary Geochronology 5 (2010) 519–532
Fig. 7. Photograph of the section at the Oxford East outcrops and the calibrated ages obtained from the organic material and fossil gastropod shells.
5.3. Small terrestrial gastropods and open-system behavior
The present study includes only a single fossil locality, the Oxford
East outcrops of southwestern Ohio, which we present here as
a case study. Average ages of organic materials and fossil gastropod
shells from the Oxford East outcrops were statistically indistinguishable; 24.93 0.30 ka for the organics and 24.73 0.44 ka for
the shells. Six of the seven taxa that we analyzed (D. shimeki, P.
muscorum, Succineidae, V. gracilicosta, V. hannai, and V. modesta)
yielded average 14C ages that are indistinguishable from the organic
ages; the seventh (H. occulta) yielded ages that were slightly
younger than the organic ages. The range of ages of the shell
material, 1.8 ka, is significantly larger than the range of ages of the
organics, 0.8 ka. If the dispersion of shell ages was related to opensystem behavior, then we would expect the ages to be systematically younger than the organic matter ages, which they are not. It
may be that the small number of organic samples fails to adequately
capture the full range of time represented by the sampled stratum.
More work needs to be done on this subject, including additional
comparisons of shell and organic ages from other sites, but the
results from the Oxford East outcrops suggest that small terrestrial
gastropod shells may behave as closed systems with respect to
carbon in the American Midwest for at least the past w25 ka.
6. Summary and conclusions
Fossil shells of small terrestrial gastropods are commonly
preserved in Quaternary sediment across North America, including
loess, wetland, glacial, and alluvial deposits, as well as in sediments at
many archeological sites. Their aragonitic shells contain w12% by
weight carbon, and therefore contain sufficient carbon for 14C dating.
However, terrestrial gastropod shells in carbonate terrains are often
avoided for 14C dating because large taxa are known to incorporate
dead carbon from limestone or other carbonate rocks when building
their shells, which can cause their 14C ages to be up to 3000 yrs too
old. Previous studies suggested that small terrestrial gastropod shells
may yield reliable 14C ages in arid environments, but a systematic and
comprehensive analysis was needed before ages derived from their
shells could be considered reliable outside of the Desert Southwest.
To this end, we measured the 14C activity of 247 aliquots of
modern shell material (3749 individual shells) from 163 localities
across North America. Approximately 78% of the aliquots did not
contain measurable amounts of dead carbon even though limestone
or other carbonate rocks were readily available at all sites, w12 of the
aliquots contained between 5 and 10% dead carbon, and the
remaining few (3% of the total) contained more than 10%. The
average Limestone Effect for these samples was only w180 14C yrs,
which is significantly less than the 2000–3000 14C yrs that previous
researchers found for larger taxa. Assuming that the small gastropod
shells behave as closed systems with respect to carbon after burial,
they should yield reliable 14C ages w78% of the time, and ages that
are within w1000 yrs of the true age w97% of the time, regardless of
the taxon analyzed, local bedrock type, climate, or environmental
conditions. If fossil shells can be identified to the species level, then
at least 23 different species should yield reliable 14C ages if the
modern shell data can be applied directly to the fossil record.
The terrestrial gastropod family Succineidae is one of the most
common gastropod taxa in North America. Unlike the other
J.S. Pigati et al. / Quaternary Geochronology 5 (2010) 519–532
gastropods studied here, our 14C data for Succineidae must be
evaluated at the genus or even family level because species-level
identification is based on soft-part morphology, which is rarely
preserved in the fossil record. Based on the data from modern
shells, the Succineidae family as a whole should yield reliable 14C
ages w85% of the time, and ages that are within w700 14C yrs every
time. At the genus level, Catinella should yield reliable 14C ages
w97% of the time, again assuming closed-system behavior, and
ages that are within w300 14C yrs of the true age every time.
Similarly, Succinea shells should yield reliable 14C ages w92% of the
time and ages that are within w600 14C yrs of the true age every
time. Caution should be used when evaluating shells of the genus
Oxyloma, however, as nearly 1 in 3 aliquots contained dead carbon,
equivalent to a limestone effect of up to w700 14C yrs.
Fossil shells of small terrestrial gastropods recovered from welldated, late-Pleistocene sediments in the Midwest yielded ages that
were statistically indistinguishable from ages obtained from wellpreserved plant macrofossils (wood, bark, plant remains).
Although just one site, these results suggest that small terrestrial
gastropod shells may behave as closed systems with respect to
carbon over geologic timescales. More work on this subject is
needed, but if our case study site is representative of other sites,
then fossil shells of some small terrestrial gastropods, including at
least five common genera, Catinella, Columella, Discus, Gastrocopta,
and Succinea, should yield reliable 14C ages, regardless of the local
geologic substrate. Fossil shells of these and other small terrestrial
gastropods are common in a wide range of Quaternary deposits in
North America and, therefore, our results may have broad chronologic applications to Quaternary geology and New World
archeology.
Acknowledgments
We thank K. Gotter for converting carbon dioxide samples to
graphite at the JAR’s lab at Miami University and J. Quade and the
University of Arizona Desert Laboratory for access to their vacuum
extraction systems. We also thank J. Rosenbaum and D. Van Sistine
of the U.S. Geological Survey for their help with Figs. 1 and 2,
respectively. This manuscript benefited from constructive reviews
from E. Ellis, G. Hodgins, D. Muhs, M. Reheis, J. Southon, and two
anonymous reviewers. This project was funded by the National
Science Foundation Sedimentary Geology and Paleobiology
Competition, award #EAR 0614840. Additional support was
provided to JSP by the U.S. Geological Survey’s Mendenhall postdoctoral research program.
Appendix. Supporting information
Supplementary data associated with this article can be found in
the online version at doi:10.1016/j.quageo.2010.01.001.
Editorial Handling by: A. Hogg
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