Mar Biol (2014) 161:1593–1602
DOI 10.1007/s00227-014-2443-5
Original Paper
Stable isotope profiles from subtropical marine gastropods of the
family Fasciolariidae: growth histories and relationships to local
environmental conditions
Josiah Strauss · Anton Oleinik · Peter Swart Received: 22 July 2013 / Accepted: 5 April 2014 / Published online: 1 May 2014
© Springer-Verlag Berlin Heidelberg 2014
Abstract Oxygen and carbon stable isotope profiles were
constructed for two species of large subtropical gastropods
of the family Fasciolariidae—Triplofusus giganteus and
Fasciolaria tulipa—from the Florida Keys and the Bahamas, to evaluate their life history and to assess their potential as paleoenvironmental proxies. Oxygen isotope profiles revealed T. giganteus and F. tulipa grew their shells
for 6 and 3 years, respectively. Both mollusks show faster
growth rates during the first half of their lifespan. Mean
annual temperatures (MAT) derived from oxygen isotopes for T. giganteus were 26.5 °C and for F. tulipa were
26.7 °C, both matching instrumental MATs of 26.7 and
26.5 °C for the Florida Keys. Both shells, however, failed
to record entire mean annual temperature ranges (MART).
Fasciolaria tulipa yielded a calculated MART of 5.6 °C
compared with a measured MART of 9.3 °C, and T. giganteus showed a calculated MART of 6.9 °C compared with
Communicated by C. Harrod.
J. Strauss Nicholas School of the Environment, Duke University, Durham,
NC 27705, USA
J. Strauss · A. Oleinik Department of Geosciences, Florida Atlantic University,
Boca Raton, FL 33431, USA
J. Strauss (*) Dolan Integration Group, 2520 55th St, Suite 101, Boulder,
CO 80301, USA
e-mail: [email protected]
P. Swart Marine Geology and Geophysics, Rosenstiel School of Marine
and Atmospheric Science, University of Miami, 4600
Rickenbacker Causeway, Miami, FL 33149, USA
e-mail: [email protected]
a measured MART of 9.4 °C. Carbon isotopes of T. giganteus were ambiguous and reveal no significant relationships
with trends in nutrient concentrations (N and P), dissolved
oxygen, and dissolved organic carbon, although they did
exhibit more negative values concomitant with landfall
of Hurricane Irene and trended to increasing values with
ontogeny that could reflect migration. Carbon isotopes in
F. tulipa were lower during winters, possibly reflecting seasonal upwelling or seagrass-mediated carbon cycling.
Introduction
Oxygen and carbon stable isotopes of biogenic marine
carbonates (e.g., coral skeletons, mollusk and brachiopod shells and foraminiferal tests) have been widely used
as proxies for seawater temperatures and carbon cycling
dynamics, respectively, throughout the Phanerozoic (Veizer
et al. 1986; Hendry and Kalen 1997; Zachos et al. 2001;
Grossman 2012). In more recent geologic history, such
as the Quaternary to present, skeletal carbonates yielding
high-resolution, continuous records can be the most desirable paleoclimate proxies. For example, bivalve shells are
used to reconstruct seasonal climate variability (Patterson
et al. 2010; Wanamaker et al. 2011) and coral skeletons are
used evaluate periodicity of climatic events like the El Niño
Southern Oscillation (Cobb et al. 2013).
Mollusks and warm water corals are the most commonly
used high-resolution marine proxies (Richardson 2001;
Grottoli and Eakin 2007; Morrongiello et al. 2012). While
mollusk shells do not yield records as consistently long
as corals, they have a unique advantages in that they are
not restricted by a limited environmental range, are generally plentiful, and are easy to collect and sample. While
the majority of molluscan stable isotope studies focused
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on bivalves, gastropods have also shown to be useful, but
considerably less studied (Geary et al. 1992; Gentry et al.
2008; Tao and Grossman 2010). Gastropods represent a significant component of global Cenozoic fossil assemblages,
especially in the coastal plain Late Cenozoic sediments of
the southeastern United States and Florida Peninsula. However, only a few genera have been evaluated with regard to
their growth histories (i.e., growth rate and life-span) and
isotopically calculated temperatures using stable isotopes.
In this study, we examine the stable isotope chronologies
of two extant gastropods species collected from the Florida shelf and the Bahamas: Triplofusus giganteus (Kiener
1840) (also referred to as the “Florida Horse Conch” or
Pleuroploca gigantea (Kiener 1840)) and Fasciolaria
tulipa (Linnaeus 1758) (commonly known as the “True
Tulip”). These mollusks both belong to the family Fasciolariidae, subfamily Fasciolariinae, of which twelve species
of genera Fasciolaria and Cinctura and one species of the
genus Triplofusus are known from the western Atlantic
(Malacolog Version 4.1.1; Snyder 2003; Petuch 2013). All
mollusks of the family Fasciolariidae are carnivorous, feeding primarily on other gastropods and occasionally bivalves
(Paine 1963a, b). These gastropods, mostly due to their
thick shells and resistance to abrasion, represent one of
the most commonly sampled (Fasciolaria) and the largest
(Triplofusus) gastropods of Pleistocene shell deposits of the
Florida Peninsula (Petuch and Roberts 2007). Furthermore,
these genera and occupy a range of marginal environments
where other, more commonly used proxies (e.g., corals)
may not be available. T. giganteus is the largest known gastropod in the Atlantic Ocean (Abbott 1974), and although
both Triplofusus and Fasciolaria play important roles in
the benthic ecology of the Florida Keys, Bahamas, and the
Gulf of Mexico (Menzel and Nichy 1958; Paine 1963a),
very little is known about their longevity and characteristics of shell growth. Moreover, it is unknown whether
shells of these species are viable for paleoenvironmental
reconstruction. By comparing carbon and oxygen isotopic
records of chronologically calibrated modern specimens
with instrument-derived environmental records, we provide
a preliminary evaluation of the potential for each species as
an environmental proxy and shed insight into lifespans and
growth characteristics.
shallow, isolated carbonate bank located between the Florida Keys and Cuba. The modern T. giganteus (Tg) shell was
collected live in Hawk Channel off Tavernier Key, Florida
Keys (25°01′N, 80°29′W), in 4 m of water from seagrass
patches in March 2004 (Fig. 1). These sites were selected
because of their moderately stable oceanographic conditions without significant fluctuations of salinity for minimal interference with isotopic data. The shells of Ft and Tg
measured 148 mm and 340 mm in length, respectively, and
based on the average adult shell size of these species and
thickening of the portion of the outer lip in both specimens
(Abbott 1974). Both mollusks were identified as males
when extracted from their shells.
After removal of soft tissues, specimens were soaked in
6 % sodium hypochlorite for 12 h to remove the periostracum and any associated organic material. The sampling surface was then lightly polished with sandpaper and scrubbed
with 30 % hydrogen peroxide to remove any remaining
organic matter. Shells were then thoroughly rinsed with
distilled-deionized water and dried at 40 °C for 6 h.
High-spired gastropods, such as Fasciolariinae, cannot be mounted on a flat surface for conventional micromilling, as it is usually done with flat-spired gastropods
and bivalves (Goodwin et al. 2003; Kobashi and Grossman
2003; Wanamaker et al. 2011). Due to their shape and size,
Fasciolariine shells have to be sampled manually. Shells Ft
and Tg were sampled under binocular microscope, using a
multi-speed dental drill with a 0.5-mm carbide dental bur.
Powder samples weighing approximately 200 µg were
drilled from the upper portion of the whorl, near the suture,
where shell is the thickest. Sixty-nine and 114 samples
were collected from Ft and Tg, respectively (Fig. 2). Sampling of the shells for isotopic analyses adhered to methods
described by Wefer and Berger (1991), with special care
being taken to not penetrate deeper than 1 mm, to avoid the
inner layers of shell, which may have a different isotopic
composition. Shell Ft was sampled at an approximate resolution of 3 mm, while Tg, the much larger shell, was sampled every 6.2 mm.
Materials and methods
δ n X(0/00) =
Sampling
where n is the mass number of element X, and R is the ratio
of heavy to light isotopes in both the sample and standard.
Aliquots of powdered aragonite were analyzed for stable
oxygen and carbon isotopes using a Kiel II automated carbonate device attached to a Finnegan Delta Plus mass spectrometer in the Stable Isotope laboratory of the Rosenstiel
The modern F. tulipa (Ft) shell was collected live in 10
meters of water on the Thalassia testudinum seagrass
meadow near the Anguilla Cays of Cay Sal Bank, Bahamas
(23°30′N, 79°36′W), in May 2003. The Cay Sal Bank is a
13
Stable isotopes analysis
Stable isotope concentrations are expressed in the conventional delta notation:
Rsample − Rstandard
× 1,000
Rstandard
(1)
Mar Biol (2014) 161:1593–1602
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Fig. 1 Map illustrating sampling localities, shown by gray
stars, for Triplofusus giganteus
(Tg) in the Florida Keys and
Fasciolaria tulipa from the Cay
Sal Bank. Black rectangle (MR)
shows location of The Molasses
Reef NOAA Buoy, where ambient water temperatures were
recorded. Photographs of shells
Tg and Ft are shown, along with
individual shell lengths
Fig. 2 Sampling schematic for
gastropod shells. Shown is an
illustration of Fasciolaria tulipa
(Ft) with samples taken along
the growth axis of the shell
starting at the apex and ending
at the margin of the aperture
School of Marine and Atmospheric Sciences, University of Miami. Oxygen and carbon isotopic compositions
were reported in per mil units (‰) relative to the VPDB
(Vienna PeeDee Belemnite) carbonate standard. Analytical
precision for all samples was better than ±0.07 ‰ for δ18O
and ±0.03 ‰ for δ13C.
Calculated temperatures were derived from δ18O measurements using the paleotemperature equation developed
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by Grossman and Ku (1986) where coefficients have been
adjusted for Vienna standard mean ocean water (VSMOW):
T◦ C = 21.8 − 4.34 δ 18 Oar − δ 18 Osw
(2)
KNMS-CD/index.htm). We focused on stations 223, 224,
227, 232, and 233, which were proximal to the Tg collection site.
where δ18Oar is the isotopic composition (in ‰) of shell
aragonite and the δ18Osw is the assumed oxygen isotopic composition of seawater relative to Vienna Standard
Mean Ocean Water (VSMOW). For a modern seawater
δ18O value, we use a constant value of 1 ‰, derived from
regional observations within the NASA GISS Global Seawater Oxygen-18 database (Schmidt et al. 1999). Using
this equation, the analytical precision of the δ18O measurements represents a potential error of ±0.3 °C.
Results and discussion
Environmental temperatures, salinity, and water quality
Instrumental temperature observations for the Hawk
Channel locality were obtained from NOAA observation buoy MLRF1 at Molasses Reef (25.01″N, 80.38″W;
http://www.ndbc.noaa.gov/station_page.php?station=m
lrf1). Due to the lack of local detailed temperature, Molasses Reef temperature records were used for the Cay Sal
Bank locality as well. The Cay Sal Bank is situated 180 km
southeast of the Molasses Reef Buoy, and average seasonal
temperatures at the sea surface and 10 m depth show general uniformity between the Cay Sal Bank and the Florida
Keys (Locarnini et al. 2010) (Fig. 3).
Salinity, dissolved oxygen (DO), chlorophyll A, dissolved organic carbon (DOC), total nitrogen (TN), and
phosphorous (TP) concentrations for Hawk channel waters
were obtained from data made available by the Southeastern Environmental Research Center (SERC) at Florida
International University (http://serc.fiu.edu/wqmnetwork/F
Fig. 3 Temperature relationship between Florida Keys and Cay Sal
Bank sampling data. Seasonal temperature averages are from the
2009 World Ocean Atlas (Locarnini et al. 2010)
13
Environmental temperature and salinity
The Molasses reef buoy reported summer temperatures between 30 and 31 °C, with mean winter temperatures dropping to 22 °C and occasionally to 20 °C. The
mean annual temperature range (MART) in this region is
9–10 °C. Winter temperatures were considerably lower
than normal during early 1998, a result of the strong El
Niño contributing to cooler and wetter conditions over
southeastern North America (Changnon 1999). Wetter conditions also caused lower salinities during the 1997–1998
winter, with seafloor values near 34 psu. Lower salinities
between 32 and 33 psu were measured during the winters
of 1999–2000 and 2001–2002. Overall, salinities of the
Florida Keys locality averaged 36.1 ± 0.7 psu and ranged
from 32.3 to 37.8 psu from 1995 to 2011. The lowest salinities were measured during winter months and the highest
observed during summer.
Oxygen isotopes and shell growth
The oxygen isotope profiles for both species show cyclical trends, reflecting years of shell deposition throughout
the lifespan of each mollusk, where warmer temperatures
are indicated by lower δ18O values and cool temperatures
by higher δ18O values (Fig. 4). Shell Tg δ18O show a larger
range and higher average than Ft (Table 1). Tg δ18O values
range from −1.4 to 0.7 ‰ and exhibit mean of 0.4 ‰. The
δ18O values of Ft ranged from −1.0 to 0.5 ‰, with a mean
of −0.3 ‰.
Because both specimens were collected live, their δ18O
profiles could be chronologically calibrated to instrumental
records. The final samples (closest to the apertures) on both
shells were assigned a collection date, and the remainder
of growth was temporally calibrated by assigning the δ18O
maxima and minima to days of coldest and warmest water
temperature observations. This is similar to the profile tuning performed on Conus gastropods by Gentry et al. (2008).
Chronologically calibrated δ18O profiles (Fig. 5) closely
match profiles of seasonality. Intraseasonal increases in
temperature during the first quarters of 2001 and 2003
trends were also captured in the Tg record. The profiles revealed Tg had been precipitated over 6 years at an
approximate average rate of 0.4 mm d−1, and shell Ft had
been precipitated over nearly 3 years at approximately
0.3 mm d−1. Assuming that, with the exception of growth
Mar Biol (2014) 161:1593–1602
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Fig. 4 Cross-plots (left) and
stable isotope profiles (right) of
shells Tg and Ft. The negative relationship between δ13C
and δ18O is shown for Tg. No
relationship was found for Ft.
Inset on cross-plots shows shell
average values with error bars
of 1σ. The isotopic profiles of
Tg show seven summers and
six winters (marked as S or W),
while the Ft captures three summer and winter cycles
Table 1 Summary of δ18O and δ13C of gastropod shell carbonate
Shell Species
Spiral length (mm) Lifespan (years) Max δ18O Min δ18O Mean δ18O Max δ13C Min δ13C Mean δ13C
Tg
Triplofusus giganteous 700
6
0.7
Ft
Fasciolaria tulipa
220
3
0.5
−1.4
−1.0
−0.4
−0.3
3.0
2.1
−1.0
0.4
0.6
1.2
All values reported relative to the VPDB standard
cessations, these gastropods grew their shells in a nearly
continuous manner, as the δ18O curves suggest, then the
0.5 mm sampling diameter reflects roughly 2–3 days of
growth. Thus, T. giganteus and F. tulipa have the potential
to yield records at sub-weekly scales by hand sampling
with a standard dental bur, and higher resolution records
can likely be captured with more precise sampling.
The calibrated δ18O profiles revealed ontogenetic
changes in the growth of each shell (Fig. 6). Tg growth
rates varied between approximately 0.2–0.6 mm d−1, with
the first 3 years (excepting the second half of the 2nd year)
deposited faster than the latter three. Ft showed a similar
growth trend with overall slower growth rates, but also
grew rapidly, to above 5 mm d−1, in its final year. The
overall growth patterns of Ft and Tg share similarities with
modern Strombus (Wefer and Killingley 1980), Conus
(Kobashi and Grossman 2003; Gentry et al. 2008), and fossil Clavilithes (Purton and Brasier 1997), where accelerated
growth for the first 2–3 years of life were observed. Climate, food availability, sex, and ecological stress may play
an important role in the growth habits of these mollusks
(G. Dietl, pers. comm.); therefore, limited conclusions on
growth can be drawn from this study since only single male
specimens were examined.
Calculated temperatures
Changes in the isotopic composition of seawater due freshwater mixing and evaporation can introduce error in isotopically calculated temperatures (Ingram et al. 1996; Strauss
et al. 2012a). Salinity variation on the Cay Sal Bank is
minimal due to limited freshwater influence. Conversely,
the southeastern Florida shelf experiences salinities as low
as 32 psu during winter months. Freshwater runoff from
Florida waters has a relatively high δ18O, so the impact of
freshwater discharge is not the same magnitude as other
localities (e.g., the northern Gulf of Mexico; Strauss et al.
2012b). An average of intercepts drawn from regressions
of δ18O and salinity for Florida Bay waters revealed the
δ18O of freshwater discharge to be roughly −1 ± 0.9 ‰
(Swart and Price 2002). If normal salinity is assumed to be
36 psu and δ18O equal to 1 ‰, a decline to 32 psu would be
reflected by a lowering of shelf δ18O to 0.8 ± 0.1 ‰. In the
context of temperature reconstruction, this would translate
to a “warming” effect of 1 ± 0.4 °C.
The tuned δ18O profiles (Fig. 5) reveal that summer temperatures are more accurately recorded in the shells than
winter temperatures. Isotopically calculated mean annual
temperature (MAT) values closely matched instrumental
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Fig. 6 Shell growth rates, calculated from chronologically tuned profiles, for shells Tg and Ft. Horizontal axis represents years of growth
beginning from each shells apex
Table 2 Oxygen isotope temperatures calculated using Grossman
and Ku (1986)
Shell
δ18Osw
Max °C
Min °C
MART °C
MAT °C
Tg
1.14
30.9 (31.5)
21.8 (20.3)
6.9 (9.4)
26.5 (26.7)
Ft
1.14
29.3 (30.4)
22.8 (20.5)
5.6 (9.3)
26.7 (26.5)
18
The value for δ Osw is relative to VSMOW. Calculated mean annual
temperature (MAT) and mean annual temperature range (MART) are
compared with instrumental values (shown in parentheses), calculated
over the span of individual shell growth
Fig. 5 Chronologically tuned isotopic profiles of Tg and Ft. Average daily water temperatures from Molasses Reef are illustrated
underneath shell δ18O values. Both Ft and Tg capture the warmest
summer temperatures, but fail to capture temperatures below 23 °C.
Carbon isotope profiles are shown separately. Tg δ13C values increase
with shell age, Ft exhibits higher values during summers. The bottom plot shows chronologically calibrated calculated temperatures
plotted against instrumentally measured temperatures. A 1:1 slope is
shown in gray. The high scatter is likely due to poor time calibration
between seasonal extremes, resulting in poorly matched temperatures
during seasonal transitions
13
observations, with the isotopic MAT of each shell within
0.2 °C of the instrumental MAT (Table 2). Both shells
underreport the MART by between 2.5 and 3.7 °C. Shell
Tg captured an average of 73 % of the seasonal temperature range, whereas Ft only captured an average of 60 % of
the seasonal range. These results are comparable to results
from Conus ermineus shells collected in the Gulf of Mexico that capture 69 % of the MART (Gentry et al. 2008).
The inability of Tg and Ft to capture the complete MART
is likely due to of slowed or discontinued shell growth coupled with a too low sampling resolution. For shell Tg, it
may also be in part due to lower shelf water δ18O during
winters. More promising, summer temperatures were captured to within 1 °C in Tg, and Ft yielded similarly accurate temperatures during the second and third summers. Ft
underreported its first summer temperatures by approximately 2 °C, however, that is likely due to uncertainty on
the timing of first shell growth.
Regressions of chronologically calibrated calculated
temperatures against measured temperatures showed
Mar Biol (2014) 161:1593–1602
1599
significant relationships, but lacked the high level of correlation that might be expected based on visual comparison
of oxygen isotope profiles and temperature records (Fig. 5).
Linear regressions fall close to a slope of one, with Tg
yielding a regression model:
Tm = 0.94Ti + 1.91
(3)
with an R2 = 0.61, and Ft showing a similar relationship:
Tm = 0.99Ti + 0.32
(4)
2
with an R = 0.48, where Tm is the measured temperature,
and Ti is the isotopically calculated temperature (°C). The
scatter is most likely due to several factors: insufficient
chronologic calibration, winter growth cessation underreporting cooler temperatures, and influence of varying
δ18Osw. The latter occurred mainly in shell Tg from the
more marginal environments of the Florida Keys. Of note,
in both shells, calculated temperatures above approximately
28 °C are lower than measured values, while the majority of calculated temperatures below 28 °C are higher than
measured temperatures (Fig. 5). These results correspond
to the lower MART reported by both shells (Table 2).
Carbon isotopes
While mollusk shell δ13C has been shown to mainly reflect
the δ13C of the dissolved inorganic carbon (DIC) pool of
ambient water (δ13CDIC) (Gillikin et al. 2007; McConnaughey and Gillikin 2008), interpretation is often difficult.
This is because the δ13CDIC reflects carbon cycling (Hayes
et al. 1999), air–sea exchange (Broecker and Maier-Reimer
1992), and freshwater mixing (Mook and Tan 1991). Furthermore, metabolic (respired) CO2 can be incorporated
into the extrapallial fluid from which the shell is precipitated, resulting in δ13C values substantially lower than the
DIC. In filter-feeding bivalves, the influence of the metabolic CO2 has been estimated to represent, at the most,
10 % of the shell δ13C (McConnaughey and Gillikin 2008).
Though the trophic level may influence shell δ13C, both
grazing and carnivorous gastropods have yielded interpretable environmental information (e.g., seasonal upwelling)
from their δ13C records. (Geary et al. 1992; Strauss et al.
2012a, b; Tao et al. 2013). Both shells yield positive δ13C
values with similar δ13C ranges (Table 1). The δ13C values of Tg ranged from −1.0 to 3.0 ‰ and averaged 0.6 ‰.
Values showed little variation over the initial 275 mm of
shell growth. From 275 to 350 mm, a clear negative excursion exhibited values stabilizing at −0.8 ‰, following that,
values slowly increased for the remainder of shell growth.
The Ft δ13C values ranged from 0.4 to 2.1 ‰ with a mean
of 1.2 ‰. Three high-δ13C excursions from 75 to 90 mm,
120–135 mm, and 195–213 mm yielded values ranging
from 1.8 to 2.1 ‰.
The most notable behavior of the Tg δ13C curve was a
trend of increasing δ13C, to 3.0 ‰, toward the shell margin,
where peaks become antiphase with δ18O during years 2002
and 2003. This trend may be indicative of shifting metabolic influence (vital effect); however, ontogenetic trends
are reflected as decreased δ13C values in predatory Conus
shells (Gentry et al. 2008) and other mollusks (Krantz et al.
1987; Lorrain et al. 2002, 2004). Thus, it is more likely that
increasing δ13C in Tg resulted from increasing δ13C of DIC.
Transects of δ13C within particulate organic carbon (POC)
have shown inshore POC to be enriched in δ13C by as much
as 4 ‰ relative to reef POC (Lamb and Swart 2008), and
through oxidation, this may be reflected in the DIC pool.
Thus, the increasing δ13C of Tg may possibly reflect the
gradual migration of the mollusk inshore to shallower
waters of Hawk Channel, where it was collected. Our field
observations also show that this is the area where the egg
capsules of the T. giganteus are most commonly encountered, so it is possible that specimens reaching sexual maturity move in these waters for spawning. Unfortunately,
besides predator–prey relationships, little detail is known
about the behavior of this species to further support this
interpretation.
The δ13C and δ18O of Tg exhibited a significant negative
correlation (p < 0.01, r2 = 0.13) where:
δ 13 C = −0.58δ 18 O + 0.4
(5)
Focusing only on the last 250 mm of shell growth, where
δ18O and δ13C peaks were antiphase, results in a slightly
improved model (r2 = 0.16), where
δ 13 C = −0.65δ 18 O + 1.0
(6)
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18
While positive correlations between δ C and δ O of
the shell have been observed in Gastropods from the Caribbean waters, where lowered salinity from freshwater runoff relate to lowered shell δ13C and δ18O, negative correlations have mainly been associated with upwelling of cooler,
low-δ13C waters (Bemis and Geary 1996) or the incorporation of deeper low-δ13C shelf waters caused by increased
mixing-depth during winter (Purton and Brasier 1997).
Although not seasonal, upwelling events do occur on the
eastern Florida shelf, and in the Florida Keys, it has shown
to drop water temperatures by about 6 °C along the reef
tract (Leichter and Miller 1999). Some of the intraseasonal
δ13C and δ18O variability within the shell may be related to
this high-frequency upwelling.
The negative correlation may also reflect increased
exchange of low-δ13C Florida Bay waters with the eastern
Florida shelf waters. Florida Bay mollusk shells typically
exhibit δ13C as low as −4 ‰ within enclosed bay waters
reflecting low δ13C of DIC caused by oxidation of terrestrial organic material (Lloyd 1964). Similarly, coral δ13C
records have revealed influence of Florida Bay waters on
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1600
the eastern Florida Keys shelf (Swart et al. 1996). We prefer this scenario because of the subdued salinity effects on
seawater δ18O by high-δ18O freshwater runoff (Swart and
Price 2002).
Comparison of Tg carbon isotope records with SERC
water quality data shows that salinities were lowest in
Hawk Channel during the winter months, with values
approaching 33 psu. These low-salinity periods corresponded to lower δ13C values in Tg during years 2002,
2003, and 2004. Winter months were relatively dry compared to summer months, limiting the influence of freshwater runoff. Average salinities of Florida Bay waters from
1998 to 2005 ranged from 24 to 42 ‰, with lowest values
associated with wet El Nino winters of 2002 and 2004 and
landfall of Hurricane Irene in late 1999. The arrival of Hurricane Irene to the shelf in fall of 1999 coincided with a
significant decrease in Tg δ13C. This was concurrent with
a reduction in salinity from 37 to 34 psu. Hurricane Irene
contributed to significant flooding on the mainland, leading
to large nutrient and labile organic carbon fluxes into Florida Bay (Davis and Yan 2004). The oxidation of hurricanefluxed organic carbon would have lowered δ13C of DIC in
Florida Bay waters. Hurricane Irene also induced eutrophication in Pamlico Sound of North Carolina (Paerl et al.
2001). The lower δ13C of Tg during this period may have
reflected hurricane-induced effects on the eastern Florida
shelf or the exchange of low-δ13C waters from Florida Bay.
While this low-δ13C event did correlate with an increase in
chlorophyll A from 0.4 to 1 mg L−1, DOC, TN, and TP did
not show any similar shifts. Overall, the Tg δ13C record did
not share any trends with SERC DO, DOC, and nutrient
data.
In shell Ft, high-δ13C excursions were separated by low13
δ C values near or below 1 %, with heavier δ13C values
persisting during the summers and lighter values during
the winter. The Cay Sal Bank is isolated from the mainland, so that it is practically free of freshwater effects on
δ13CDIC. As a result, the δ13C of Ft may reflect upwelling,
in situ carbon cycling, and/or potential metabolic effects.
Upwelling primarily occurs on the northwestern region of
the Cay Sal Bank bordering the Straits of Florida and is the
subject to the fluctuations within the Florida Current (Lee
et al. 1995). The influence of cold, low-δ13C waters would
lower both δ18O and δ13C of shell carbonate. The δ18O and
δ13C values showed no significant relationship, suggesting
limited or no upwelling influence. Because this specimen
was collected at 10 m depth in expansive Thallasia testudinum meadows, the δ13C values may track T. testudinum—
mediated carbon cycling. Seagrass communities are some
of the most productive marine ecosystems, comparable
with mangroves in net primary productivity (Mateo 2006).
During the autumn continuing into winter, seagrass litter is
generally highest (Mateo and Romero 1997). The microbial
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Mar Biol (2014) 161:1593–1602
oxidation of Thallasia leaf litter may have contributed to
the lower δ13C values of Ft shell during winter months.
Conclusions
We present oxygen and carbon isotope profiles of two
modern fasciolariid shells from the Florida Keys and
Cay Sal Bank. Oxygen isotopes revealed the T. giganteus sampled grew its shell for at least 6 years, and the F.
tulipa grew its shell over 3 years. Both mollusks exhibited faster growth rates during the first half of the shell
growth span. Oxygen isotope calculated temperatures
of both shells closely matched instrumentally measured
means, but failed to capture the entire seasonal temperature ranges. We found the F. tulipa and T. giganteus shells
examined to capture 61 and 73 % of measured seasonal
temperature ranges, respectively, which is comparable to
isotopic temperature records of other gastropods. Both
species examined in the present study yielded accurate
calculated temperatures. While regressions of chronologically tuned isotopically calculated temperatures with
measured temperatures show significant scatter, this is
likely related to limitations in the chronologic calibration
during seasonal transitions, winter growth cessations,
and variability of the oxygen isotopic composition of the
water. Of the two specimens we examined, T. giganteus
provided a longer and more precise paleotemperature
record than F. tulipa. It must be noted that this finding is
limited by the sampling and study of single specimens,
as growth rates and habits may be influenced by environment and physiology.
Interpretation of carbon isotopes of both shells were less
certain. The δ13C record of F. tulipa was almost in antiphase
with δ18O values, possibly suggesting seasonal variations to
the carbon cycle mediated by the expansive seagrass meadows from which it was collected. However, we found no
correlation between T. giganteus δ13C and nutrient, and
DOC and DO concentrations, a singular event of decreased
values correlates with the landfall of Hurricane Irene, most
likely due to a high flux of freshwater from the Everglades
and Florida Bay. The T. giganteus shell also shows a trend
to increasing values with ontogeny, possibly reflecting
migration to more inshore environments.
Acknowledgments We thank Amel Saied and Corey Schroeder of
the University of Miami RSMAS stable isotope laboratory for their
assistance with stable isotope measurements and Ethan Grossman for
providing a review of an early draft of the manuscript. This manuscript was also improved by helpful comments from Gregory Dietl,
Chris Harrod, and two anonymous reviewers. We thank the Department of Fisheries of the Commonwealth of the Bahamas for issuing
a permit to collect in Cay Sal Bank. This study was supported by a
student grant provided by the Conchologists of America and in part
by National Science Foundation Grant OPP 0095095.
Mar Biol (2014) 161:1593–1602
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