10.6071/M3TT2J
Fraser, Danielle
Canadian Museum of Nature
Clementz, Mark
University of Wyoming
Welker, Jeffrey
University of Alaska System
Kim, Sora
0000-0002-4900-3101
University of California, Merced
Pronghorn (Antilocapra americana) enamel phosphate δ18O values reflect
climate seasonality: implications for paleoclimate reconstruction
Dryad
dataset
2021
FOS: Earth and related environmental sciences
Antilocapra americana
stable isotope analysis
climate variation
historical and contemporary
University of Wyoming Stable Isotope Facility*
Fullbright Fellowship*
Smithsonian Institute*
Peter Buck Fellowship
Natural Sciences and Engineering Research Council
https://ror.org/01h531d29
RGPIN-2018-05305
National Science Foundation
https://ror.org/021nxhr62
0847413, 0078433, 0899776
University of Chicago
https://ror.org/024mw5h28
T.C. Chamberlin Fellowship
2022-11-01T00:00:00Z
2022-02-28T00:00:00Z
en
https://doi.org/10.1002/ece3.8337
https://doi.org/10.5281/zenodo.5573002
36736 bytes
4
CC0 1.0 Universal (CC0 1.0) Public Domain Dedication
Stable oxygen isotope compositions from vertebrate tooth enamel are
commonly used as biogeochemical proxies for paleoclimate reconstructions.
However, the utility of enamel isotopic values across species varies due
to differences in rates of enamel deposition and mineralization as well as
sources of ingested water, body water residence times, and species’
physiology. We evaluate the use of stable oxygen isotope compositions from
pronghorn (Antilocapra americana Gray, 1866) enamel for the amplitude
reconstruction of terrestrial paleoclimate seasonality. We serially
sampled the third lower molars of pronghorn from Wyoming for oxygen
isotope composition in phosphate (δ18OPO4) and compared patterns to: (1)
interpolated and (2) measured yearly variation in central Wyoming
environmental waters (δ18Ow) as well as to (3) δ18O values from sagebrush
leaves and stems in the same region. Although we recognize the numerous
factors influencing the composition of mammal body water, our null
hypothesis was that δ18OPO4 enamel values reflect δ18Ow with a constant
offset due to mammalian physiology. We set up our null hypothesis by
converting δ18Ow values to δ18OPO4 using a published regression based on
empirical results from mammals. Pronghorn δ18OPO4 values from enamel are
consistently enriched in 18O relative to the predicted values. We
hypothesized that pronghorn δ18OPO4 values might also reflect dietary
water and therefore also converted δ18Oleaf values from plants into
predicted δ18OPO4 values. We infer that pronghorn obtain at least some of
their water from 18O-enriched plants because pronghorn enamel δ18OPO4
values are more similar to predicted δ18OPO4 values from plants than from
meteoric waters. Modeling of source body water δ18O values show amplitudes
between 70% and 95% of seasonal variation from Wyoming δ18Ow values.
Collectively, our findings establish that modern seasonality in source
water is reliably reflected in modern pronghorn enamel providing the basis
for exploring changes in the amplitude of seasonality of ancient climates
using archived tooth collections.
Modern and archaeological pronghorn specimens were acquired from the
University of Wyoming Anthropology museum and Department of Archaeology
(Appendix I). All specimens were collected from wild populations in
Wyoming during 1970-1972 and 2010 following deaths that were not related
to this study. The archaeological specimens date to 1720 ± 100 years
(Frison 1971), thus pre-dating the rapid climate change typical of the
late 20th century (Mann et al. 1998, Jones et al. 2001). The lower third
molar (m3) is one of the last to complete enamel mineralization and erupt
in pronghorn (Dow and Wright 1962), therefore, we included only
individuals with erupted third lower molars. To recover the most complete
isotopic time series, we included only individuals showing no or little
wear of the m3. We also excluded individuals with abscesses or obvious
abnormalities of the dentition or jaw bone. We extracted lower third
molars using a Dremel diamond cutting wheel and serially sampled the
enamel at ~2 mm intervals using a Dremel tool with a diamond taper point
bit (part #7144). We collected 2-3 mg of powdered enamel for each serial
sample. Further, we took bulk samples (~4-6 mg) of bone from the
mandibular angle just posterior to third lower molar for each individual.
To analyze the oxygen isotope composition of phosphate (δ18OPO4), we
weighed 1.5-2.0 mg of enamel and 3-4 mg of bone from each specimen.
Preparation procedures for the modern specimens are from a combined
approach based on Bassett et al. (2007) and Weidemann-Bidlack et al.
(2008). We pre-treated all samples with 300 μl of 2.5% NaOCl for
approximately 20 hours to remove organics. Bone samples were usually
pre-treated twice to ensure complete organic removal (or more if there was
continued gas production). Samples were then rinsed with deionized (DI)
water 5 times and dried overnight at 50˚C. We then dissolved the remaining
powder in 100μl of 0.5M HNO3 overnight. To neutralize the solution and
precipitate CaF2, 75μl of 0.5 M KOH and 200μl of 0.36 M KF were added.
Samples were centrifuged to pellet the CaF2 and the supernatant was
transferred from the vials to reaction vessels. We precipitated silver
phosphate with 250µl of silver amine solution (0.2M AgNO3, 0.35M NH4NO3,
0.74M NH4OH) plus 3-6 drops of 0.1M AgNO3 to initiate the precipitation.
Samples were placed in a heat block at 50˚C overnight in a fume hood to
allow for maximum crystal growth. The silver phosphate crystals were
rinsed five times with ~2 mL of DI water to remove residual silver amine
solution. After the samples dried overnight at 50˚C, 200 – 300 μg were
weighed into pressed silver capsules and stored in an oven flushed with N2
until isotopic analysis. Preparation of historical specimens used a rapid
silver phosphate technique from Mine et al. (2018), which demonstrated
d18O value fidelity to the slow precipitation method in Weidemann-Bidlack
et al. (2008). Briefly, the historical samples were similarly treated for
organic removal, but hydroxyapatite was dissolved in 50 mL 2M HNO3 while
CaF2 precipitated with 30 mL 2.9M HF and neutralized with 50mL of 2M NaOH.
Steps to pellet and isolate CaF2 were similar between methods. To
precipitate silver phosphate, we added 180 mL silver amine solution (0.37M
AgNO3 and 1.09M NH4OH) and adjusted pH to 5.5-6.5 using dilute HNO3, which
was shown to maximize phosphate recovery (Mine et al., 2018). The silver
phosphate crystals were rinsed and dried similar to the modern samples as
outlined above. The δ18O value of silver phosphate [SK1] [DF2] was
measured after conversion to CO in a Temperature Conversion Elemental
Analyzer (TC/EA, Thermo Scientific) coupled with a Conflo IV (Thermo
Scientific) to a continuous flow isotope ratio mass spectrometer (CF-IRMS,
Thermo Scientific Delta V). Three in-house reference materials (two silver
phosphate and one benzoic acid) were used to normalize isotopic values and
check the effectiveness of pyrolysis within and between runs (modern
samples at UWSIF: ARCOS [N=4 per run], UWSIF21 [N=5 per run],
UWSIF33[SK3] [DF4] [N=6 per run]; historical samples at SIELO UCM: USGS80
[N=8], USGS 81 [N=8], IAEA601 [N=8]). Variation in d18O values exhibited
by these reference materials was < 0.4‰. In addition, we monitored
the potential for isotopic alteration during sample preparation by
precipitating silver phosphate from a synthetic hydroxyapatite and NIST
120c (N=3 and 3 with 1s < 0.3‰). All samples were analyzed in
triplicate. All d18OPO4 values are reported relative to the standard
V-SMOW. The spacing between enamel carbonate and phosphate δ18O values is
often used as a check for diagenesis (Koch et al. 1997) To check the
carbonate-phosphate isotopic spacing, we also analyzed a subset of enamel
samples for carbonate d18O values as per Koch et al. (1997) and Kohn and
Cerling (2002). We weighed 1 mg of enamel and 5 mg of bone for each sample
analysis of δ18OCO3 values. To remove organic matter from the bone
samples, we used 2-3% H2O2 at a ratio of 1 ml per 25mg of sample, leaving
the caps of the microcentrifuge tubes open to allow the escape of gas for
24 hours. We did not pre-treat the enamel samples to remove organic matter
due to the minimal organic content of enamel. Only the bone was
pre-treated for carbonate analysis. Similar to the phosphate preparation,
pre-treatment was repeated until gas production ceased. We rinsed the bone
samples 5 times with DI water to remove H2O2 from solution. We then added
1M CH3COOH with Ca acetate buffer (pH=4.5) to remove non-lattice bound
carbonates using the same ratio as the preceding step. Samples were rinsed
five times with DI water, dried for 24 hours in a freeze drier, and ~1 mg
weighed into exetainer tubes for isotopic analysis. Once all samples and
reference materials were weighed into exetainer tubes, they were dried
overnight at 50°C, the headspace flushed with He, and 100-200 mL of
>100% H3PO4 was added to react for 24 hours (room temperature). The
CO2 within the headspace was sampled for isotopic composition measurement
using a gas bench (Thermo Scientific) coupled to a CF-IRMS (Thermo
Scientific Delta Plus) at UWSIF. Three in-house carbonate reference
materials were used to normalize isotopic values and check for variation
within and between runs (UWSIF18 [N=4 per run], UWSIF06 [N=4 per run],
UWSIF17 [N=4 per run]).Variation in d18OCO3 values of these reference
materials was < 0.2‰. In addition, we monitored the potential
isotopic alteration during sample preparation with one lab bioapatite
(MSW0479, ashed manatee bone, N = 1 per run, d18OCO3 = 19.0‰) and
variation in d18OCO3 values of a phosporite reference standard (NIST 120c,
N = 5, d18OCO3 = 28.4 ± 0.3‰). All d18OCO3 values are reported relative
to the standard V-SMOW. Recovering primary isotope time series Modelling
is the only means by which the primary isotopic time series can be
recovered (Green et al. 2018), even for mammal taxa with relatively fast
rates of enamel mineralization and low rates of isotopic overprinting. To
recover the primary isotopic input signals from our pronghorn tooth
isotopic time series, we used the mathematical model from Passey and
Cerling (2002) and Passey et al. (2005) as a transfer function. The Passey
and Cerling (2002) model takes primary isotope series using measured d18O
values from body water and reconstructs tooth enamel d18O values, while
also accounting for time averaging due to amelogenesis and variation in
enamel maturation from the crown to the root. The Passey et al. (2005)
method incorporates the Passey and Cerling (2002) time-averaging model and
uses an inverse linear system to recover the input signals from enamel
isotopic time series (see Matlab code associated with Passey et al.,
2005). The model terms for the Passey and Cerling (2002) and Passey et al.
(2005) models are 1a and 1m, which are the length of apposition (distance
along the tooth from where a new enamel layer contacts the enamel-dentine
junction and the external layer of the tooth; 7 mm) and length of
maturation (the length of the tooth that is mineralizing at a given time;
13.3 mm assuming an enamel maturation time of one month as in small bovids
such as sheep; Zazzo et al. 2010), respectively. The la parameter was
estimated using values reported for sheep, which range from 2 mm to 12 mm,
depending on whether la was measured on the buccal, mesial, or lingual
side of the tooth and crown height (Zazzo et al. 2012). We used the
midpoint of these values (7 mm) as a conservative estimate of la in
pronghorn. We used a constant sample depth (ls) of 75% of enamel thickness
because we drilled through approximately 75% of the enamel when serial
sampling. The Passey and Cerling (2002) and Passey et al. (2005) models
were run in R version 3.5.1 (Appendix I). To compare pronghorn enamel
δ18OPO4 values to seasonality of environmental waters, we downloaded
interpolated monthly average isotope values for precipitation from central
Wyoming (43˚N, 107.5 ˚W at 6,700 ft of elevation) from waterisotopes.org
(Bowen et al. 2005, Bowen 2014). These interpolations for the entire
United States are based in large part on Welker’s USNIP (United States
Network for Isotopes in Precipitation; Welker 2000, 2012) for the years
1989-1994 and scarce IAEA GNIP (Global Network for Isotopes in
Precipitation) data from a few years in the 1960’s for 6 sites across the
United States (Rozanski et al. 1993). Environmental water δ18Ow values
were converted into predicted phosphate values using the following
equation from Kohn and Cerling (2002), which is based on empirical data
from mammalian enamel: δ18OPO4 = (0.9 x δ18Ow) + 23 (2) We use equation 2
as a means of setting up our hypothesis, that pronghorn δ18OPO4 values
reflect only δ18Ow and physiological offsets. Given that equation 2 was
originally derived for evaporation insensitive species (i.e., those that
rely primarily on meteoric waters), underestimation relative to measured
δ18O values can reveal whether a focal species is evaporation sensitive.
To validate the interpolated δ18Ow values we used 1000+ measured δ18O
values from precipitation at 9 sites in Wyoming (Appendix II) from USNIP
for the entire USNIP record 1989-2006 (Welker 2000, Vachon et al. 2010b,
Welker 2012). The high density modern precipitation network (USNIP)
provides the only site, sub-state, regional and continental record of
actual meteoric water values that are becoming increasing valuable in
revealing the range of seasonality in modern precipitation (Vachon et al.
2007, Vachon et al. 2010b, a). We made comparisons between the
interpolated δ18Ow values and measured values using monthly averages
across all sample sites and at the site closest to the Laramie and Rawlins
region (Albany site), the site closest to the area where our modern
pronghorn teeth were collected. To evaluate the relative contributions of
meteoric and river waters, we also extrapolated maximum and minimum δ18O
values from river waters in Wyoming (Kendall and Coplen 2001). As with the
meteoric waters, we converted river water δ18O values to enamel phosphate
values. We also obtained published δ18O values for water in sagebrush
leaves and stems, rabbitbrush leaves, and pronghorn incisor enamel from
Fenner and Frost (2008) for comparison to δ18OPO4 values from pronghorn
molar enamel (this study) as well as δ18Ow values (from interpolated and
measured meteoric precipitation). All plant tissues were sampled by Fenner
and Frost (2008) during the months of June and July; we converted these
δ18O values into phosphate enamel values using equation 2. This conversion
of plant leaf water values to enamel phosphate values allowed us to set up
our alternative hypothesis: pronghorn δ18OPO4 values from enamel reflect a
combination of environmental water, evaporated leaf water, and
physiological mechanisms. Similarity of pronghorn δ18OPO4 values to δ18Ow
from precipitation, lakes, and rivers or δ18Oleaf values should provide
information on their relative inputs.
Appendix I. Raw δ18O data from serial samples of modern and archaeological
pronghorn from Fraser et al. (2021) Pronghorn (Antilocapra Americana)
enamel phosphate δ18O values reflect climate seasonality: implications for
paleoclimate reconstruction. Ecology and Evolution, In Press. [Not sure if
2021 will be the correct year or if you want to put the entire citation
here] Appendix II. R translation of the function Emeas1_1 from Passey et
al. (2005) from Fraser et al. (2021) Pronghorn (Antilocapra Americana)
enamel phosphate δ18O values reflect climate seasonality: implications for
paleoclimate reconstruction. Ecology and Evolution, In Press.. Appendix
III. R translation of the function mSolv1_1 from Passey et al. (2005) from
Fraser et al. (2021) Pronghorn (Antilocapra Americana) enamel
phosphate δ18O values reflect climate seasonality: implications for
paleoclimate reconstruction. Ecology and Evolution, In Press.. Appendix
IV. Running the R translations of Emeas1_1 and mSolv1_1 with example data
from Passey et al. (2005) from Fraser et al. (2021) Pronghorn (Antilocapra
Americana) enamel phosphate δ18O values reflect climate seasonality:
implications for paleoclimate reconstruction. Ecology and Evolution, In
Press.. Appendix V. Summary data of Wyoming sites for which USNIP data
were extracted from Fraser et al. (2021) Pronghorn (Antilocapra Americana)
enamel phosphate δ18O values reflect climate seasonality: implications for
paleoclimate reconstruction. Ecology and Evolution, In Press..