Glaciation, aridification, and carbon sequestration in the Permo ...

Palaeogeography, Palaeoclimatology, Palaeoecology 268 (2008) 222–233
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Palaeogeography, Palaeoclimatology, Palaeoecology
journal homepage: www.elsevier.com/locate/palaeo
Glaciation, aridification, and carbon sequestration in the Permo-Carboniferous:
The isotopic record from low latitudes
Ethan L. Grossman a, ⁎, Thomas E. Yancey a , Thomas E. Jones a , Peter Bruckschen b , Boris Chuvashov c ,
S.J. Mazzullo d , Horng-sheng Mii e
a Texas A&M University, Department of Geology and Geophysics, College Station, TX 77843, United States
b Bergheimer Steig 41, 45357 Essen, Germany
c Institute of Geology and Geochemistry, Russian Academy of Science, Urals Branch, Pochtovyi per. 7, Ekaterinburg, Russia
d Department of Geology, Wichita State University, Wichita, Kansas 67260, United States
e Department of Earth Sciences, National Taiwan Normal University, P.O. Box 97-62, Taipei, Taiwan 116, ROC
ARTICLE
INFO
ABSTRACT
Article history:
Accepted 26 March 2008
Keywords:
Paleoclimate
Permian
Carboniferous
Stable isotopes
Carbon cycling
Brachiopods
To evaluate the isotopic record of climate change and carbon sequestration in the Late Paleozoic, we have
compiled new and published oxygen and carbon isotopic measurements of more than 2000 brachiopod
shells from Carboniferous through Middle Permian (359–260 Ma) strata. We focus on the isotopic records
from the U.S. Midcontinent and the Russian Platform because these two regions provide well-preserved
marine fossils spanning a broad time interval. Both regions show a δ 18 O increase at the Mid-Carboniferous
boundary (ca. 318 Ma) that roughly correlates with geologic evidence for an expansion of Gondwanan
glaciers. Only the Russian Platform record shows a δ 18 O maximum during the glacial maximum in the
Asselian. In contrast to a previous study [Korte, C., Jasper, T., Kozur, H.W., and Veizer, J., 2005. δ 18 O and δ 13 Cof
Permian brachiopods: a record of seawater evolution and continental glaciation. Palaeogeogr. Palaeoclimatol.
Palaeoecol. 224, 333–351.], our data show no oxygen isotope evidence for glacial retreat in the early Permian,
but instead show increasing δ 18 O values related to aridification. Dissimilarity in the δ 18 O trends for the
epicontinental seas of North American and the Russian Platform suggests that at least one region experienced
periodic restriction that altered regional salinities and seawater δ 18 O values. These results highlight the need
for complementary proxies to identify restricted circulation in epicontinental seas.
Carbon isotopic compositions of carbonates exhibit substantial regional variation with low values in western
North America, intermediate values in the midcontinent, and high values in the Sverdrup Basin, Russian
Platform, and northern Spain. Nevertheless, both U.S. Midcontinent and Russian Platform records show a late
Serpukhovian minimum, a sharp increase across the Mid-Carboniferous boundary, and a minimum centered
on the Kasimovian. The correlative increase in brachiopod δ 13 C and δ 18 O values at the Mid-Carboniferous
boundary is our best isotopic evidence for a link between the carbon burial and glaciation in the Permo-
Carboniferous.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
The role of the carbon cycle in controlling climate is a critical and
contentious issue in Earth sciences (e.g., Veizer et al., 2000; Royer
et al., 2004). Phanerozoic evidence for this relationship may reside in
the carbon and oxygen isotopic compositions of carbonate fossils, with
carbon isotopes monitoring the carbon cycle and oxygen isotopes
serving as temperature and ice volume proxies. Ján Veizer and his
colleagues have conducted a comprehensive study of the isotopic
record of Phanerozoic climate and ocean chemistry (Veizer et al., 1997,
⁎ Corresponding author.
E-mail address: e-grossman@tamu.edu (E.L. Grossman).
1999). Using the “detrended” 18 O records, Veizer et al. (2000) contend
that paleotemperature and ice volume disagree with proxy records of
atmospheric CO 2 levels (pCO 2 ; Berner, 1994, 1998), arguing for a
decoupling of climate and pCO 2 . In contrast, Royer et al. (2004) and
Montañez et al. (2007) use these and other proxy records for pCO 2 ,
paleotemperature, and glaciation as evidence for a close correspondence
between glaciation and atmospheric CO 2 levels.
The principles behind application of oxygen isotope data to
Phanerozoic climate studies are straightforward, but the data are
not. For example, the compilation of Veizer et al. (1999), while
comprehensive and valuable, shows high variability and a dramatic
trend toward lower δ 18 O and δ 13 C with sample age not seen in more
recent studies (Mii et al., 2001; Wenzel et al., 2000; Van Geldern et al.,
2006; Joachimski et al., 2006). One step toward better understanding
0031-0182/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.palaeo.2008.03.053

E.L. Grossman et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 268 (2008) 222–233
223
of the drivers of Earth's climate is the development of more precise
and accurate isotopic records of Phanerozoic ocean temperatures and
carbon cycling. In this study, we use new and carefully-screened
published data to refine the isotopic record of climate change and
carbon cycling in the Permo-Carboniferous, the interval of Earth's last
greenhouse–ice house–greenhouse cycle, and to reexamine the link
between carbon cycling and climate change.
2. Samples and methods
Our record for Permo-Carboniferous paleoclimate and carbon
cycling is based on isotopic measurements of 2000 articulate
brachiopod shells, anchored with 1015 shells from the U.S. Midcontinent
and 400 shells from the Russian Platform. Articulate
brachiopod shells have a dense microstructure and low Mg chemistry
that make them resistant to post-depositional alteration (diagenesis)
(Compston, 1960; Popp et al., 1986); thick shells are especially
resistant to diagenesis. Specimens from the U.S. Midcontinent and
Russian Platform are especially useful for isotopic studies because of
their good preservation and wide geographic and temporal distribution.
During the Permo-Carboniferous, the U.S. Midcontinent and
Russian Platform were restricted to the tropics and subtropics, and
were moving slowly northward at a rate of roughly 2° latitude per
10 m.y. (Fig. 1; Scotese, 2001).
Fig. 1. Paleogeographic maps for 360, 320, and 280 Ma with areas for which
comprehensive isotopic data are presented. USM = U.S. Midcontinent, UM = Ural
Mountains, MB = Moscow Basin (C. Scotese, 2001, pers. comm.).
Samples from the U.S. Midcontinent were collected in Texas,
Oklahoma, Arkansas, Kansas, Nebraska, Iowa, Missouri, Illinois, and
Indiana (Grossman et al., 1991, 1993; Mii et al., 1999; plusnewdata;
Appendix 1) (Fig. 2), representing the North American epicontinental sea
and interior basins. Nearly all of the brachiopod shells were from gray
shales or argillaceous and bioclastic limestone, often in shale interbeds in
the latter. New data are presented for 80 brachiopod shells collected from
shales, limestones, and argillaceous calcareous sandstones of the Kincaid
Formation and Fayetteville Shale (Serpukhovian: Chesterian), Hale and
Bloyd Formations (Bashkirian: Morrowan), and Tradewater Formation
(basal Moscovian: Atokan) from Oklahoma, Arkansas, and Illinois
(Appendix 1). Data from an additional 84 shells of early Permian age
were collected from calcareous shales and shaly limestones of the Council
Grove and Chase Groups in Kansas and Oklahoma and from the Cisco and
Wichita Groups of Texas. The U.S. Midcontinent data are supplemented by
data for Tournaisian and Kasimovian–Gzhelian age specimens from New
Mexico (Grossman et al., 1993; Stanton et al., 2002). During the
Carboniferous, this region experienced more open-ocean conditions
than the U.S. Midcontinent. Isotopic stratigraphies for North America are
based on brachiopods of the order Spiriferida (Anthracospirifer, Crurithyris,
Eridmatus, Neophricodothyris, Neospirifer Prospira, and Spirifer) and
Athyridida (Composita). These taxa exhibit superior resistance to diagenesis
because of their impunctate microstructure and thick shells, with
most having layers of coarse prismatic calcite.
Russian Platform samples are from limestones of the Moscow Basin
and the Ural Mountains (Bruckschen et al., 1999, 2001; Mii et al., 2001;
Grossman et al., 2002a; Korte et al., 2005; Fig. 1). Moscow Basin shells
range in age from mid-Visean through Gzhelian, with a hiatus in the late
Serpukhovian. The Uralian shells are of mid-Serpukhovian through
Moscovian and early Permian age. Nearly identical δ 18 Oandδ 13 C values
for the two regions during the Bashkirian show that the two areas are
fully comparable (Mii et al., 2001). Except for one interval in the
Tournaisian, no suitable data are available from Tournaisian through
early Visean sediments in Russia. Taxa analyzed from the Russian
Platform include Orthotetida (Orthotetes), Spiriferida (Brachythyrina,
Choristites, Martinia, Neospirifer, Spirifer), Athyridida (Composita), and
Productida (Chonetes, Gigantoproductus, Linoproductus, “Productus”,
Striatifera). These taxa provide shells resistant to diagenesis because of
their thickness and impunctate or pseudopunctate microstructure. For
the study of Bruckschen et al. (1999, 2001), only 48% of the specimens
analyzed were taxonomically identified. The specimens analyzed in
Korte et al. (2005) were not identified other than as brachiopods, with
many being small fragments.
Isotopic data for epicontinental seas may be influenced by riverine
influx and excess evaporation. While this potentiality cannot be
excluded, restriction of the study to articulate brachiopod shells at
least minimizes the likelihood of low salinity conditions because such
shells are typically found in deposits of stenohaline marine environments,
as indicated by coeval crinoids (Boardman et al., 1987).
Euryhalinity in modern brachiopods has been proposed based on
occurrences of some species in marginal marine or intertidal settings
(Fursich and Hurst, 1980; Theyer, 1981). Nevertheless, these appear to
be exceptions as brachiopods have a preference for normal marine
salinities (Richardson, 1997; Brand et al., 2003). As noted by Fursich
and Hurst (1980), no brachiopod species appears to have invaded very
hypersaline or truly brackish conditions.
Studies of modern specimens show that the main portion of the
brachiopod shell (interior secondary layer) is precipitated at or near
oxygen isotope equilibrium (e.g., Carpenter and Lohmann, 1995; Brand
et al., 2003; Parkinson et al., 2005). However, studies have also shown
disequilibrium fractionation (i.e., vital effect) in brachiopod shells. The
primary layer of modern articulate brachiopod shells (typically not
well preserved in fossils) and the outer secondary layer of punctate
Terebratalia transversa have been shown to exhibit vital effect
(Carpenter and Lohmann, 1995; Auclair et al., 2003). Comparisons of
dorsal and ventral shells of modern terebratulids show no significant

224 E.L. Grossman et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 268 (2008) 222–233
difference in δ 18 O(Curry and Fallick, 2002; E. Grossman, unpublished
data). Data for ancient brachiopods typically show little or no δ 18 O
difference between co-occurring taxa (Grossman et al., 1991; Lee and
Wan, 2000). Carbon isotope fractionation in brachiopod shells is not
well understood because the variability of bottom-water δ 13 C values
makes environmental characterization difficult. Nevertheless, it is
generally assumed that shell δ 13 C correlates with the δ 13 C of ambient
dissolved inorganic carbon, as is observed with other taxa that secrete
shells in oxygen isotope equilibrium (e.g., mollusks, foraminifera).
Despite their resistance to diagenesis, brachiopod shells can be
subject to oxygen and carbon exchange and must therefore be
carefully scrutinized for preservation of original microtexture and
chemistry (see Grossman, 1994, for a review). Researchers disagree,
however, as to best method to screen brachiopod shells for diagenesis.
Popp et al. (1986) used cathodoluminescence (CL) microscopy of thick
sections to recognize diagenesis and target the best-preserved areas
within the best-preserved specimens. Shells with post-depositional
Mn show distinctive CL patterns. We use the same approach except
that we thin-sectioned and imaged each shell in plane-polarized light
(PL) and CL (“TAMU” method; Grossman, 1994; Mii et al., 1999, 2001).
These photographs are available on-line at our Digital Gallery of
Paleochemical Specimens and Thin-sections (DiGPaST; http://geoweb.
tamu.edu/faculty/grossman/SHELL_IMAGES/index.html). Shell areas
that appear dark or cloudy (secondary inclusions) in plane-polarized
light, show faint or dull CL, or have fine-scale CL microfractures are
avoided. The CL character is noted for every sample analyzed. Using
photomicrographs as a guide, 50 to 150 µg of non-luminescent shell
are microsampled from thin-sections or complementary billets using
dental pick or drill. We analyze at least three shell areas per thinsection
or billet, and one area with cement or matrix when available.
An alternative method employed by Ján Veizer and his colleagues
involves crushing the shell, hand-picking 4–6 mg of calcite fragments
with a binocular microscope, and isotopically and chemically analyzing
the fragments (e.g., Bruckschen and Veizer, 1997). These
researchers rely on trace element analyses and SEM observations on
select samples to evaluate samples for diagenesis (“Ruhr method”).
However, Bruckschen et al. (1999) and Veizer et al. (1999) did not cull
samples that contained Sr and Mn outside the range of values found in
modern brachiopods. Korte et al. (2005), whose Permian data are
discussed later, culled samples with Mn and Sr values ≥250
and ≤400 ppm, respectively.
The Ruhr and TAMU methods are compared in Bruckschen et al.
(1999, 2001). Mid-Carboniferous brachiopod specimens from the
Donets Basin, Ukraine were screened, sampled, and analyzed by the
Ruhr method and then by the TAMU method (Bruckschen et al., 1999).
Initial screening and analysis by the Ruhr method yielded values much
lower and more variable than expected from typical marine environments
(−15 to −1‰), highly suggestive of diagenetic alteration in
meteoric (low δ 18 O) water. The δ 18 O values of samples prepared
using the TAMU method were equal to or higher than those previously
sampled and analyzed by the Ruhr method, with an average difference
of 3.1±3.7‰ (N=15;fromdatainBruckschen et al.,1999)(Fig. 3A). These
results imply that fine-scale sampling guided by CL and PL photomicrographs
is more effective than the Ruhr method in avoiding
diagenetic material. A similar test with Russian Platform samples
showed no significant difference between methods (Fig. 3B; Bruckschen
et al., 2001), suggesting that sample screening and sampling techniques
are not critical with more pristine samples. As for δ 13 C, no significant
difference was seen in Donets brachiopods analyzed using the two
different methodologies (4.9±1.1‰ versus 4.6±1.4‰ for TAMU versus
Ruhr methodologies).
To minimize error resulting from diagenesis, we focus on data
produced through petrographic screening and cathodoluminescence
(e.g., Popp et al.,1986; Grossman et al.,1991,1993; Mii et al.,1999, 2001),
and Russian Platform data processed by the Ruhr method (Jasper, 1998;
Bruckschen et al., 2001; Grossman et al., 2002a; Korte et al., 2005) that
have been shown to yield equivalent results as samples processed using
petrographic screening and cathodoluminescence. This treatment
deemphasizes data from brachiopods collected in Belgium, Germany,
the Ukraine, and China reported in Bruckschen and Veizer (1997),
Fig. 2. Map of sample localities in the U.S. Midcontinent (Mii et al., 1999; this study) compared with location of Missourian and Meramecian evaporite deposits (Johnson, 1989).

E.L. Grossman et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 268 (2008) 222–233
225
Bruckschen et al. (1999) and Jasper (1998), and compiled in Veizer et al.
(1999). As seen in the histogram in Fig. 4, these data are highly variable
and much lower in δ 18 O than U.S. Midcontinent and Russian Platform
data (see later discussion).
New data presented in this paper are derived from brachiopod
shells prepared by the TAMU method discussed above. Powdered
carbonate (150±50 µg) is reacted with “100%” phosphoric acid at 70 °C
using a Kiel II automated carbonate reaction system. The resulting CO 2
is analyzed on a Finnigan MAT 251 isotope ratio mass spectrometer.
Isotope data are calibrated to the Peedee belemnite standard (PDB)
using NBS-19 (δ 13 C=1.95‰, δ 18 O=−2.20‰; i.e., calibrated to VPDB).
Precision for the analyses averages 0.05‰ and 0.08‰ for δ 13 C and δ 18 O
respectively (1σ).
3. Results and discussion
3.1. The oxygen isotopic record
The oxygen isotopic records of previous studies have been
augmented with new data and updated in light of revised stratigraphic
correlations and a new time scale (Gradstein et al., 2004)
(Appendix 1). The Carboniferous δ 18 O record for midcontinental North
America was presented in Grossman et al. (1991, 1993) and Mii et al.
(1999), and further discussed in Mii et al. (2001). New data from
Arkansas and Illinois help constrain δ 18 O and δ 13 C shifts near the Mid-
Carboniferous boundary (Appendix 1). The earliest Mississippian
samples from the Glen Park formation (Illinois) yield an average δ 18 O
of −5.5‰ (Fig. 5A) that appears to continue a Late Devonian trend of
low δ 18 O values in well-preserved brachiopods (Joachimski et al.,
2004; Van Geldern et al., 2006). The Mississippian data increase in the
earliest Tournaisian (Kinderhookian) to −0.8‰ in the late Tournaisian.
The δ 18 O values further increase to a maximum of −0.4‰ in the early
Visean (Osagean–Meramecian), then decrease irregularly to about
−2.6‰ in the late Serpukhovian (late Chesterian). This is followed by a
0.9‰ increase across the Mid-Carboniferous boundary to −1.7‰. The
δ 18 O values for the remainder of the Pennsylvanian average between
−2 and −3‰ with no systematic trends.
The increase is not seen in data for unaltered brachiopods from
Arrow Canyon (Brand and Brenckle, 2001), the global boundary
stratotype section and point (GSSP) for the Mid-Carboniferous
boundary. Mississippian brachiopods near the boundary average
−2.4±0.8‰ in δ 18 O, similar to values for the near-boundary Pennsylvanian
brachiopods (−2.2±0.7‰). It is not known why δ 18 O values do
not increase across the boundary. Perhaps values increase lower or
higher in the section.
New data for the early Permian of Kansas and northern Oklahoma
(KS/OK) continue the Pennsylvanian trend, with minor fluctuations
around a mean of −2.1‰ and a small increase in the Artinskian
(Mazzullo et al., 2007). In contrast, new δ 18 O data for the early
Permian and latest Pennsylvanian of Texas average −3.1‰ (Jones et al.,
2003), about 1‰ lower than KS/OK samples.
Regional isotopic differences such as that mentioned above are
common in the U.S. Midcontinent data. A high δ 18 O for KS–OK
specimens relative to Texas brachiopods was observed in late
Pennsylvanian data (Grossman et al., 1993). Furthermore, Fayetteville
formation (∼320 Ma) specimens from Oklahoma are 1.4‰ higher in
δ 18 O than Arkansas specimens, and Verdigris Cycle (∼308 Ma)
specimens from Oklahoma are 2‰ higher than coeval samples from
Texas. On the other hand, other Oklahoma samples do not show this
enrichment. These isotopic differences may represent differences in
depositional environment; unfortunately, detailed sedimentologic
studies are not available to evaluate this possibility. A simple
compilation of North American Permian data implies that δ 18 O values
decrease in the Guadalupian. However, considering the Texas and
Kansas–Oklahoma data sets separately reveals a δ 18 O increase in the
Artinskian and Kungarian. The regional differences highlight the need
to develop isotopic records for different cratons and different localities
on the same craton to produce a representative global record.
Isotopic data for Russian Platform seas are available from Popp et al.
(1986), Bruckschen et al. (1999, 2001), Mii et al. (2001),andKorte et al.
(2005), along with new Permian data from this study (Appendix 1). The
Carboniferous data are examined in Grossman et al. (2002a). Fig. 5B
shows the Permo-Carboniferous δ 18 O data for the Russian Platform. The
discussion below highlights trends seen in the running mean with a
Fig. 3. Oxygen isotopic compositions of brachiopod shells sampled and analyzed by the Ruhr method versus the TAMU method. Data from Bruckschen et al. (1999; Donets Basin) and
Bruckschen et al. (2001; Moscow Basin). Donets brachiopod data produced using the TAMU method average 3.1‰ higher in δ 18 O. No significant difference is seen for the Moscow
Basin specimens.

226 E.L. Grossman et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 268 (2008) 222–233
Fig. 4. Histogram showing Carboniferous δ 18 O data for different regions (data from Grossman et al.,1991,1993; Mii et al.,1999, 2001; Veizer et al.,1999; Bruckschen et al.,1999, 2001;thispaper).
3 Ma window and 1 Ma step. Variability in δ 18 OishigherinBruckschen
et al. (1999, 2001) data compared with other data, especially for samples
in which brachiopod taxa are not identified. Nevertheless, average
values obtained by different studies are, in most cases, similar.
Moscow Basin and Uralian data have similar values and trends for
both δ 18 Oandδ 13 C where samples overlap (e.g., Bashkirian–Moscovian),
thus both data sets are combined. Except for a sole specimen from the
Tournaisian (−3.9‰), there are few reliable Carboniferous data older
than mid-Visean for the Russian Platform. Mid-Visean samples average
about −3‰ in δ 18 O and appear to decrease in the Serpukhovian. Late
Serpukhovian samples from the Askyn section in the Urals yield values
averaging −4.8‰. Bruckschen et al. (2001) suspected that these
Serpukhovian samples might be influenced by fresh water influx during
deposition. However, the strata contain exposure features such as smallscale
vadose vug-and-channel system (microkarst) cemented by late
diagenetic blocky spar, evidence for exposure (P. Kabanov, pers. comm.,
2007). Thus, the possibility of diagenetic influence cannot be excluded.
Oxygen isotope values increase across the Serpukhovian–Bashkirian
(Mid-Carboniferous) boundary to values averaging −1.4‰. Theδ 18 O
progressively decreases through the Bashkirian and Moscovian to a
minimum of −3.5‰ in the Kasimovian, followed by a Gzhelian increase
to −1.3‰ in the Asselian. Beyond the early Asselian, the Permian record
differs between studies. Our data and those of Popp et al. (1986) show
δ 18 O values increasing irregularly in the Permian to average values as
high as 0.1‰. Korte et al. (2005) show a decrease to about −3.6‰ in the
Artinskian, which is not seen in our Russian Platform and U.S.
Midcontinent records.
Fig. 6A compares the oxygen isotopic record for North America and
the Russian Platform with Permo-Carboniferous data for other
localities, including other European and North American localities.
Because Popp et al. (1986) and Morante (1996) used cathodoluminescence
to sample NL parts of shell, and Veizer, Brand, and their
coworkers used the Ruhr/Ottawa methodology, these data are
differentiated in Fig. 6. Much of the data for samples outside the U.S.
Midcontinent and Russian Platform are highly variable and low in
δ 18 O. Note that Belgian Visean and Ukrainian Bashkirian samples show
ranges of more than 8‰ within narrow time intervals (Bruckschen
and Veizer, 1997; Bruckschen et al., 1999). Less variable data were
obtained for Pennsylvanian samples from northern Spain (Popp et al.,
1986), with mean values similar to those of the U.S. Midcontinent and
the Russian Platform samples. Both Russian Platform and northern
Spain samples yield an early Kasimovian δ 18 O minimum, however, the
samples from Spain show an early Moscovian δ 18 O maximum not seen
in the Russian Platform and U.S. Midcontinent records.
Australian samples analyzed by Compston (1960) and Morante
(1996) provide a rare glimpse of conditions in Gondowanan basins in
temperate latitudes. The oxygen isotopic composition of Cisuralian
brachiopods from the Bowan Basin averages −1.6 ± 1.3‰ (N=34),
slightly higher than U.S. Midcontinent values for the same time
interval (−2.1±0.7‰ (N=70) (Morante, 1996). The higher δ 18 O values

E.L. Grossman et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 268 (2008) 222–233
227
Fig. 5. Oxygen isotopic data for brachiopod shells from U.S. craton (A) and the Russian Platform (B). Heavy gray lines are the running mean for 3 Ma window and 1 Ma steps. The light
gray bands represent ±2 standard errors. Dotted line is used for a significant gap in the record. Data for U.S. Midcontinent are from Grossman et al. (1991, 1993), Mii et al. (1999), Korte
et al. (2005), and this study; data for the Russian Platform are from Popp et al. (1986), Bruckschen et al. (1999, 2001), Mii et al. (2001), Korte et al. (2005), and this study. The timing of
Glacial events I, II, and III is from Isbell et al. (2003a) as modified by Montañez et al. (2007). The glacial events of Fielding et al. (2008) are shown as bars with the height representing
relative ice volume. Oxygen isotope paleotemperatures are calculated from the Hays and Grossman (1991) reformulation of O'Neil et al. (1969).
of the Australian brachiopods likely reflect cooler average temperatures
in the temperate waters.
3.2. The carbon isotope record
The Mississippian–early Pennsylvanian δ 13 C record for the U.S.
Midcontinent shows shifts roughly covariant with the δ 18 O record
(Fig. 7A; Appendix 1). Starting with low δ 13 C (1.2‰) in the earliest
Carboniferous, values increase 2‰ during the Tournaisian to a mean of
5.4‰ before attaining relatively constant values of about 3.5‰ in the late
Tournaisian and early Visean. The δ 13 C values decrease about 1‰ in the
late Visean to the lowest values since the earliest Tournaisian, then
increase abruptly at the Mid-Carboniferous boundary. This sharp Mid-
Carboniferous increase was first reported by Popp et al. (1986) and later
refined by Mii et al. (1999, 2001). Arrow Canyon brachiopods show
slightly higher δ 13 C values above the Mid-Carboniferous boundary (2.4±
0.8‰ [N=26]) versus below (2.0±0.8‰ [N=39]) (Brand and Brenckle,
2001). Preliminary results for brachiopods higher in the section, however,
show a return to lower values (Jones et al., 2003).
The δ 13 C record for the remainder of the Carboniferous and early
Permian is complicated by the fact that Pennsylvanian Composita
subtilita shells are 1‰ higher in δ 13 C relative to other taxa (Grossman
et al., 1993). The δ 13 C values for the Pennsylvanian remain relatively
constant at ∼4.8‰ for Composita and 3.7‰ for other taxa, but both
data sets show a small (0.5‰) decline centered on the Kasimovian.
The Permian isotopic record for the U.S. Midcontinent is based
mainly on Composita, except for the unidentified samples from the
Guadalupe Mountains (TX) reported in Korte et al. (2005). There is no
significant difference in δ 13 C between Composita and other taxa for
the Permian, or between Texas and KS/OK specimens. Earliest Permian
δ 13 C values are similar to those of the latest Pennsylvanian (∼4.6‰),
then decrease irregularly from the late Pennsylvanian–early Permian
maximum to an Artinskian minimum (∼3.5‰). Brachiopod data from
the Guadalupe Mountains suggest a δ 13 C increase to 4.1±0.3‰ in the
Wordian–Capitanian.
The Permo-Carboniferous record for the Russian Platform starts with a
low δ 13 C value for the early Tournaisian (0.8‰). There is a gap in the record
until the mid-Visean, where values average 2.3‰ into the early
Serpukhovian. Immediately below the Serpukhovian–Bashkirian boundary,
values dip to b− 2‰. These anomalously low δ 13 C values are consistent
with the hypothesized exposure and diagenesis discussed earlier. Values
increase to a mean of 4.7‰ in the early Bashkirian, remain high in the
Bashkirian and Moscovian, decline in the early Kasimovian to a low of
2.8‰, then recover to a maximum in the Gzhelian and Asselian (∼4.7‰).

228 E.L. Grossman et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 268 (2008) 222–233
Fig. 6. Comparison of oxygen and carbon isotopic trends for U.S. craton and the Russian Platform (Fig. 5) with data from other low-latitude sites (e.g., Canada, Great Britain, Spain, France,
Belgium, and Germany). Data are differentiated by location and research group, the latter because different sampling methodologies can yield different results (see text for discussion).
Sources of data: (1) Brand and Veizer (1981), Veizer et al. (1986), Brand (1989), Brand and Legrand-Blain (1993), Bruckschen and Veizer (1997), Bruckschen et al. (1999),andVeizer et al.
(1999); (2)Popp et al. (1986) as reported in Popp (1986); (3)Morante (1996). Updated age assignments to GTS 2004 for Veizer/Brand/Bruckschen data are from Jan Veizer's updated
database (www.science.uottawa.ca/geology/isotope_data/).
For the remainder of the Permian, δ 13 C fluctuates between lows of 4±1‰
and highs of 5.5±1‰. Interpretation of these fluctuations is complicated
by the sparse data and the fact that different studies and stratigraphic
sections account for the maxima (this study) and minima (Popp et al.,
1986; Korte et al., 2005).
The δ 13 C records for North America and the Russian Platform are
convergent with the carbon isotope records from other localities, in
contrast to the divergent δ 18 O records. Remarkably, all data for the
Visean show the same average data and trend. Also, all data show an
increase in δ 13 C in the Mid-Carboniferous and higher values in the
Pennsylvanian than in the Mississippian. However, δ 13 C values for the
Donets Basin (Ukraine) increase at the Visean–Serpukhovian boundary,
rather than at the Serpukhovian–Bashkirian boundary. These data must
be evaluated in the context that the Donets Basin samples show
extraordinarily low δ 18 O values, explainable only by post-depositional
alteration. Though carbon isotopic compositions are much less
susceptible to diagenesis than oxygen isotopic compositions, we cannot
be certain that these samples provide primary carbon isotope signals.
3.3. Do sediments from epicontinental seas record open open-ocean
conditions?
Carbon isotope variability between and within Permo-Carboniferous
epicontinental seas has called attention to the potential to

E.L. Grossman et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 268 (2008) 222–233
229
Fig. 7. Carbon isotopic data for brachiopod shells U.S. craton (A) and the Russian Platform (B). Heavy gray lines are the running mean for 3 Ma window and 1 Ma steps. The light gray
bands represent ±2 standard errors. Dotted line is used for a significant gap in the record. Data for U.S. Midcontinent are from Grossman et al. (1991, 1993), Mii et al. (1999), Korte et al.
(2005), and this study; data for the Russian Platform are from Popp et al. (1986), Bruckschen et al. (1999, 2001), Mii et al. (2001), Korte et al. (2005), and this study. Timing of Glacial
events I, II, and III is from Isbell et al. (2003a) as modified by Montañez et al. (2007).
misinterpret regional differences as global change (Beauchamp et al.,
1987; Grossman et al., 1991, 1993). Furthermore, recent studies of the
neodymium and carbon isotopic composition of Ordovician sediments
from North America provide evidence that circulation between
epicontinental seas and the open ocean can be restricted, resulting
in local and regional variation in the geochemical proxy records
(Holmden et al., 1998; Panchuk et al., 2005, 2006).
This interpretation for Ordovician seas may apply to the Carboniferous.
The Visean (Meramecian–Osagean) δ 18 O maximum in U.S.
Midcontinent sediments, interpreted by Mii et al. (1999) as evidence
for glaciation, may reflect 18 O-enrichment of the water resulting from
excess evaporation. Evaporation in the Red Sea, for example, results in
δ 18 O values of 2‰ relative to Vienna Standard Mean Ocean Water
(VSMOW; Al-Rousan et al., 2003). The eastern U.S. Midcontinent
deposits approximately coincide in space and time with aridity and
the deposition of evaporites (Johnson, 1989; Cecil, 1990; Fig. 2). In
contrast, the centers of evaporite deposition shifted to the west during
the Pennsylvanian, a time of more moderate δ 18 O values despite
evidence for Gondwanan glaciation (Isbell et al., 2003a). This
interpretation is supported by general circulation model (GCM)
simulations that show that during the mid-Mississippian, the U.S.
Midcontinent was located on the subtropical high pressure zone
(Grossman et al., 2002b; Grossman et al., in prep.).
As further evidence of isotopic differences between water masses,
Pennsylvanian brachiopods from New Mexico are low in δ 18 O relative
to midcontinental specimens by about 1‰ (Grossman et al., 1993).
Grossman et al. (1993) attributed this to higher temperatures on the
shallow platform, but Stanton et al. (2002) argued that the restricted
seas of the U.S. Midcontinent allowed evaporative 18 O enrichment of
the water, in contrast to the ocean-facing New Mexico site. This
argument was used by Stanton et al. (2002) to explain low oxygen
isotope compositions in Tournaisian brachiopods from the Lake Valley
formation in New Mexico. However, recent reevaluation of the
correlation scheme has brought the data for Lake Valley formation
in better agreement with U.S. midcontinent results.
3.4. Comparison with oxygen isotope studies of conodonts
Recent oxygen isotope studies of conodont phosphate provide an
opportunity to confirm the paleotemperatures and trends derived
from brachiopod shell studies. Oxygen isotopic records based on
conodont phosphates have the advantage of being less susceptible to

230 E.L. Grossman et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 268 (2008) 222–233
diagenesis (Wenzel et al., 2000; Joachimski et al., 2004) and potential
pH influences. Furthermore, conodonts are more mobile than
brachiopods and can be found in deep- as well as shallow-water
facies (Wenzel et al., 2000; Joachimski et al., 2006). On the other hand,
the mobility of the conodont leads to uncertainty in depth and
location of growth. To better constrain the life habitat of conodonts,
Joachimski et al. (2006) measured the δ 18 O of four co-occurring
genera. They found no significant difference between three, and 0.6‰
enrichment in the fourth, Gondolella, a genus known to indicate
deeper waters. This provides evidence for a common life habitat for at
least three conodont genera.
Brachiopods and conodonts from the same Pennsylvanian cycles in
Kansas and Missouri were analyzed by Grossman et al. (1993;
brachiopods) and Joachimski et al. (2006; conodonts). Though sample
localities differed, precluding direct comparisons between data,
similar temperature ranges were obtained with each method (20 to
26 °C assuming paleoseawater δ 18 O=0‰ VSMOW). Similar isotopic
paleotemperatures have also been obtained for Late Permian
brachiopods (Korte et al., 2005) and conodonts (Korte et al., 2004)
from different Iranian sections.
A new conodont δ 18 O stratigraphy for the Mississippian agrees
with brachiopod data in showing a δ 18 O increase in the Tournaisian
and early Visean, but disagrees in that conodont δ 18 O remains
constant in the Visean and increases in the Serpukhovian (Buggisch
et al., this volume). Brachiopod δ 18 O values decrease in the Visean
and remain low in the Serpukhovian. Buggisch et al. (this volume)
interpret the conodont data as cooling and glaciation in the
Tournaisian, followed by intensified glaciation in the Serpukhovian.
These differences between conodont and brachiopod data may result
from differences in preservation, habitat (benthic for brachiopods and
nektonic for conodonts), or stratigraphic sections. Future isotopic
studies of co-occurring conodonts and brachiopod shells will address
this issue.
Conodont and brachiopod data within cyclothems also differ in that
conodont δ 18 O correlates negatively with inferred paleodepth (Joachimski
et al., 2006), whereas brachiopod δ 18 O shows either a positive correlation
or no correlation (Adlis et al., 1988; Grossman et al., 1991, 1993). The
negative correlation of conodont data is attributed as higher seawater δ 18 O
with greater ice volume and lower sea level, while the positive correlation
of brachiopod data is interpreted as cooler seafloor temperatures as the
site deepened. Again, future studies will endeavor to explain and perhaps
utilize these differences.
3.5. Seawater δ 18 O, pH effects, and paleoclimate
Permo-Carboniferous brachiopod shells from the U.S. Midcontinent
and Russian Platform (−2.3±1.1‰, N=1412) yield paleotemperatures
equivalent to modern low-latitude temperatures assuming a constant
hydrospheric δ 18 O and ice-free (17–27 °C for δ 18 O sw =−1‰ VSMOW) or
moderately glaciated conditions (19–29 °C for δ 18 O sw =−0.5‰ VSMOW).
By contrast, the average Carboniferous δ 18 O value for “Ottawa/Bochum
data” reported in Veizer et al. (1999) is −4.8±2.6‰ (N=444), equivalent
to temperatures that are ∼12 °C warmer. We believe that these low
values reflect the influence of diagenesis as discussed previously. Veizer
et al. (1999; 2000) contend that low Paleozoic δ 18 O values reflect lower
seawater δ 18 O, arguing for a long-term evolution of seawater δ 18 O. We
believe that the strong δ 18 O-age trend observed by Veizer et al. (1999)
mostly reflects the increased influence of diagenesis with the age of their
samples. Our data do not show unusually low δ 18 O values for the Permo-
Carboniferous, and suggest that the average δ 18 O of the hydrosphere
remained relatively constant since the early Carboniferous. This
supports studies that contend that seawater δ 18 O has remained
relatively constant through much of the Phanerozoic (Muehlenbachs
and Clayton, 1976; Gregory, 1991; Joachimski et al., 2004). This
hypothesis has been strengthened recently by the application of the
“clumped isotope” paleothermometer to the Paleozoic (Came et al.,
2007), which yields seawater δ 18 O values of −1.2±0.5‰ VSMOW for the
Early Silurian and −1.6±0.1‰ VSMOW for the Middle Pennsylvanian.
Studies have shown that pH can influence oxygen isotopic
fractionation in planktonic foraminifera (Spero et al., 1997). Royer
et al. (2004) incorporated this pH effect in paleotemperature
determinations based on the Phanerozoic δ 18 O curve of Veizer et al.
(1999), after correction for the long-term trend (i.e., “detrended”)
(Veizer et al., 2000). We have not considered pH in our isotopic
paleotemperature estimates because a pH dependence on δ 18 O has
not been demonstrated for macrofauna.
What do oxygen isotope records for the U.S. Midcontinent and the
Russian Platform reveal about Gondwanan glaciation? Though the
timing and magnitude of Gondwanan glaciation is still debated, it is
generally accepted that the Late Paleozoic experienced three major
glacial intervals: roughly latest Devonian–Tournaisian, late Visean–
Serpukhovian–Bashkirian, and Asselian–Artinskian (e.g., Frakes et al.,
1992; Isbell et al., 2003a [Glacial I, II, and III]; Montañez et al., 2007).
Fielding et al. (2008) present evidence for eight discrete glacial intervals
in the Permo-Carboniferous based on eastern Australian strata (Fig. 5).
Their results principally differ from the consensus view in the lack of
evidence for latest Devonian–Tournaisian glaciation, and in the
persistence of small Gondwanan glaciers through the Guadalupian.
The U.S. Midcontinent shows convincing evidence for high δ 18 O
during glacial intervals only with regard to the increase at the Mid-
Carboniferous boundary (Fig. 5). The mid-Mississippian high in δ 18 O,
as discussed previously, probably reflects excess evaporation in a
restricted sea rather than glaciation as proposed by Mii et al. (1999).
The δ 18 O record for the Russia Platform, while less complete than the
North American record, shows a trend similar to the Glacial II–
interglacial–Glacial III pattern noted in Isbell et al. (2003a). Unfortunately,
the oxygen isotopic record lacks the resolution for comparison
to the detailed record of Fielding et al. (2008).
There are two inconsistencies between brachiopod δ 18 O and sedimentological
records for glaciation. First, δ 18 O values are low for the
Visean and Serpukhovian. Sedimentologic data suggest that significant
ice volume existed in the Serpukhovian (e.g., González, 1990, Frakes et
al., 1992) and perhaps the late Visean (Smith and Read, 2000; Wright
and Vanstone, 2001). Second, many researchers contend (e.g., Dickins
(1996), Isbell et al. (2003a, b), among others) that Glacial II represents
alpine glaciation and low ice volume, so high δ 18 O values might not be
expected. Note that the Bashkirian δ 18 O values are equal to or higher
than those of the Asselian, when Gondwanan glaciers are believed to
be at their acme. Aridification cannot explain the high δ 18 O of
Bashkirian brachiopods because, at least for eastern North America,
the Mid-Carboniferous boundary coincided with a shift toward more
humid conditions (Cecil, 1990). Cyclothem deposition in the middle
Carboniferous supports large ice volume changes, but as mentioned
above, the timing appears to differ from the isotopic shift (Smith and
Read, 2000; Wright and Vanstone, 2001).
Korte et al. (2005) found evidence in Urals samples for a
Sakmarian–Artinskian decrease in δ 18 O, the trend expected with an
Artinskian decline of the glaciers (e.g., Isbell et al., 2003a); however,
our North American and Uralian data show an increase. We ascribe the
North American δ 18 O increase to aridification. Tabor et al. (2002) have
drawn the same conclusion based on δ 18 O analyses of pedogenic
phyllosilicates. Korte et al.'s Urals samples are from the Usolka and
Dalnij Tyulkas sections, where brachiopods occur as small fragments
in slump deposits in a basinal environment. Clearly these important
results require confirmation with data from other areas.
Local effects may confound the oxygen isotopic records of
Laurussian epicontinental seas. Furthermore, the brachiopod and
conodont isotopic records are biased toward the recording of high
stands when marine sediments are more widely distributed. Accepting
these limitations, the oxygen isotopic records for the U.S.
Midcontinent and the Russian Platform suggest a major increase in
ice volume in the earliest Bashkirian. The isotopic record for the

E.L. Grossman et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 268 (2008) 222–233
231
Russian Platform is consistent with major glaciation during the
Gzhelian–Asselian, but the post-Asselian record is sparse and greater
coverage is needed to test for the isotopic record of deglaciation.
3.6. The east–west gradient in Laurussian carbon isotopes
In their study of late Pennsylvanian and early Permian limestones
from Canada, Beauchamp et al. (1987) noted no temporal δ 13 C
gradient but a strong spatial one. Carbon isotope values decreased
from the Sverdrup Basin (5–7‰) to the Yukon Territory (3–5‰) and
British Columbia (1–3‰). This was interpreted as a gradient from high
δ 13 C values within a stagnant basin (Sverdrup Basin) toward open
open-ocean conditions with lower values (British Columbia). Grossman
et al. (1991, 1993) and Mii et al. (1999) also observed an east–west
δ 13 C difference for Laurussia, interpreting low U.S. Midcontinent
values and high Russian Platform–Cantabrian (Spain) values as
reflecting upwelling in eastern Panthalassa and downwelling in the
western Paleotethys respectively. Building upon this hypothesis,
Saltzman (2003) suggested that restricted circulation in the epicontinental
seas enhanced the eastern Panthalassan–western Paleotethyan
gradient. Saltzman's low δ 13 C values for Arrow Canyon sediments in
Nevada are similar to those for British Columbian samples hypothesized
by Beauchamp et al. (1987) to be open ocean.
These results imply that different surface surface-water masses
covered western Laurussia (Panthalassan) and north and east
Laurussia (Sverdrup Basin, Spain, Russian Platform). Though data
are sparse, high δ 13 C values for early Pennsylvanian brachiopods
from China (Mii, 1999) and early Permian brachiopods from Australia
(Morante, 1996; Fig. 6) suggest that surface-water δ 13 Cwashighin
the Paleotethys and perhaps southwestern Panthalassia. The development
of distinct Panthalassan and Paleotethyan water masses is
believed to coincide with the Mid-Carboniferous boundary, where
the δ 13 C records for the U.S. Midcontinent and Russian Platform
diverge, presumably marking circulation changes associated with
the closing of the Rheic Ocean (Grossman et al., 1991, 1993; Mii et al.,
1999).
3.7. Carbon isotope trends and paleoclimate
Though regional variation complicates the search for a global trend
in carbon isotopes, several features of the record stand out in both
North America and the Russian Platform. These include the late
Tournaisian maximum, a late Serpukhovian decline followed by a
sharp increase at the Mid-Carboniferous boundary, and a subtle
minima centered on the Kasimovian (Fig. 7). The carbon isotope
record of micritic limestone at Arrow Canyon, Nevada also shows
features in common with the U.S. Midcontinent and Russian Platform
records, including the Tournaisian spike, the decline before the
Serpukhovian–Bashkirian boundary, and the increase across the
boundary (Saltzman, 2003) (Fig. 8). Arrow Canyon and U.S. Midcontinent
records also show a small maximum in the mid-Visean.
Interpretation of Arrow Canyon results is complicated by the presence
of exposure surfaces near the Mid-Carboniferous boundary. Diagenesis
during exposure could account for the sharp decline prior to the
Mid-Carboniferous boundary, like we hypothesize for the Serpukhovian
δ 13 C minimum in the Urals (Fig. 8). Interestingly, brachiopod data
for the GSSP at Arrow Canyon do not show a minimum below the
boundary, nor the δ 13 C increase above (Brand and Brenckle, 2001;
Jones et al., 2003), suggesting that there may be a brachiopod δ 13 C
shift higher or lower in the section.
In Fig. 8 we compile all brachiopod carbon isotope data shown in
Fig. 6. We have not excluded data based on preservation, location, or
taxonomy. Thus, small small-scale features, especially those for the
Permian, may be biased by the spatial, temporal, and taxonomic
distribution of samples (Figs. 6, 7). The main features of the
compilation are the δ 13 C increase in the early Mississippian, the
sharp increase at the Mid-Carboniferous boundary, and the retention
of high values throughout the Pennsylvanian and early Permian.
The carbon isotope record for terrestrial organic carbon in the Late
Paleozoic shows a general increase from late Silurian through the
Permian (Peters-Kottig et al., 2006). The Mississippian record for organic
carbon has some of the same features of the carbonate record (Fig. 8),
with higher values in the mid-Mississippian and a decrease in the late
Fig. 8. Comparison of carbon isotopic data for (1) brachiopod shells from U.S. Craton and the Russian Platform (from Fig. 7), (2) micritic limestone from Arrow Canyon, Nevada (Saltzman,
2003), and (3) terrestrial organic matter (Peters-Kottig et al., 2006; moving average based on 20 Ma window and 5 Ma step. 95% confidence interval shown as light band), and glacial events.
The chronology of the Saltzman (2003) data is based on stage boundary ages from Gradstein et al. (2004) and assumes constant sedimentation rate within stages. Timing of Glacial events I,
II, and III is from Isbell et al. (2003a) as modified by Montañez et al. (2007). The glacial events of Fielding et al. (2008) are shown as bars with the height representing relative ice volume. The
red curve represents the running average of all brachiopod shells (4 Ma window, 2 Ma steps). The lines above and below represent ±2 standard errors of themean.

232 E.L. Grossman et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 268 (2008) 222–233
Mississippian. However, no δ 13 C increase is seen at the Mid-Carboniferous
boundary, and the subtle features of the Pennsylvanian are not
reproduced. The isotopic data for Permian brachiopods are too sparse for
a rigorous comparison. The current record provides no clear relationship
between the δ 13 C of carbonates and organic matter (Fig. 8).
We observe an interesting relationship between the carbon isotope
record and glacial events in the latest Devonian–early Mississippian,
middle Carboniferous, and earliest Permian (Isbell et al.'s (2003a)
glacial events I, II, and III; Fig. 8). All three glacial events include or
follow well documented δ 13 C rises. Glacial I occurs during the early
Mississippian rise in δ 13 C and presumably a decline in pCO 2 . Glacial II
includes the Mid-Carboniferous boundary δ 13 C increase, and Glacial III
follows the late Pennsylvanian minimum. Considering the uncertainty
in the ages and magnitudes of these glacial intervals, it is possible that
glacial advances were triggered or promoted by increased drawdown
of carbon and a concomitant lowering of pCO 2 . However, the oxygen
isotope, carbon isotope, and geologic evidence for a linkage between
ice volume expansion and carbon sequestration converge only for the
Mid-Carboniferous (Glacial II).
4. Conclusions
The following conclusions are based on the oxygen and carbon
isotopic records for brachiopod shells:
1. Oxygen and carbon isotopic records for the U.S. Midcontinent and
the Russian Platform suggest that a major increase in ice volume
coincides with the Mid-Carboniferous boundary, and that this
event was linked to carbon sequestration.
2. Excess evaporation in the U.S. Midcontinent accounts for higher
δ 18 O during the mid-Mississippian and early-middle Permian in the
region. Similar 18 O enrichment is seen for early Permian Russian
Platform samples.
3. Carbon isotope data for Laurussia show a westward δ 13 C decrease,
marking differences in ocean circulation presumably initiated with
the closing of the Rheic Ocean.
4. The construction of global oxygen- and carbon isotope records for the
Paleozoic is complicated by diagenesis, restricted circulation within
epicontinental seas, and the lack of global and temporal coverage.
Future studies need to focus on broadening sample coverage,
improving evaluation of sample preservation, expanding the application
of conodonts in isotopic studies, and developing and applying
proxies for circulation (e.g., Nd isotopes) and paleosalinity.
Acknowledgments
We thank Chris Scotese for providing paleogeographic maps and
PointTracker software and Christoph Korte, Dan Murphy, and Anne
Raymond for helpful discussion. The manuscript has benefited from
editorial handling by Isabel Montañez and reviews by Christoph Korte,
Michael Joachimski, and an anonymous reviewer. This work was
supported by National Science Foundation (NSF) grant EAR-0003596
and the Mollie B. and Richard A. Williford Professorship (Texas A&M
University).
Appendix 1. Supplementary data
Data from this study are available at the CHRONOS website (http://
www.chronos.org/resources/GrossmanYanceyApp_1_PPP.xls).
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