Quantitative paleoenvironmental and paleoclimatic reconstruction ...

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32 N.D. Sheldon, N.J. Tabor / Earth-Science Reviews xxx (2009) xxx–xxx
with low diffusivity occur only a few centimeters below the soil
surface, whereas low productivity soils with high diffusivity have
much deeper characteristic production depths, possibly as deep as 30–
50 cm below the soil surface (Wood and Petraitis, 1984; Quade et al.,
1989; Cerling and Quade, 1993; Yapp and Poths, 1992 Schroeder and
Melear, 1999). Furthermore, soil media with lower diffusivity will be
characterized by higher S(z) values than low diffusivity counterparts.
Moreover, due to variations in productivity of soil biota, soils from
warm climates will have higher S(z) values than cool climate counterparts,
and soils with abundant soil moisture will have higher S(z)
values than their drought-phase counterparts. For example, Breecker
et al. (2009) have recently monitored S(z) values for modern semiarid
soils in New Mexico and found that there was significant seasonal
variation between about 500 and 2000 ppmv, which was in turn
reflected by seasonal variations in δ 13 C and δ 18 O. Pedogenic carbonate
formation occurred simultaneously with S(z), δ 13 C, and δ 18 O minima,
which suggests that the carbonate is not recording mean growing
season conditions and that there is therefore a seasonal bias in the
formation of pedogenic carbonates.
Although modern 2-component soil CO 2 concentrations range from
little more than ambient tropospheric concentrations (~500 ppmV;
Wood and Petraitis, 1984) to as much as 21% of the soil atmosphere
(210,000 ppmV; Yamaguchi et al., 1967; Brook et al., 1983), soils that
are characterized by mixing of two soil CO 2 components, and which
contain in situ accumulations of calcite, appear to be limited to a range
of soil CO 2 concentrations from about 1000–8000 ppmV (de Jong and
Schappert, 1972; Brook et al., 1983; Wood and Petraitis, 1984; Soloman
and Cerling, 1987; Quade et al., 1989). This range of soil CO 2 concentrations
reflects the relatively low productivity of soils characterized
by calcite crystallization (Schlesinger, 1997). Note however, that
this very low range of soil pCO 2 also reflects that modern tropospheric
pCO 2 is low, and contributes very little (~300 ppmV) to total soil pCO 2 .
For times when tropospheric pCO 2 was significantly higher than
modern (e.g., 2000–3000 ppmV; Eocene, Paleocene, Cretaceous; Yapp
and Poths, 1996; Yapp, 2004; Tabor and Yapp, 2005a), soil CO 2 in
calcareous soils likely had a correspondingly higher concentration
(3000–11,000 ppmV).
Ekart et al. (1999) assumed soil CO 2 concentrations of 5000 ppmV
for 758 paleosol calcite samples representing ~400 million years time.
Nordt et al. (2002) assumed a range of soil CO 2 concentrations from
4000 to 6000 ppmV for paleosols from Upper Cretaceous and Lower
Paleogene strata in Alberta, Canada. Prochnow et al. (2006) assumed
soil CO 2 concentration to be 4000 ppmv for “arid to semiarid” paleosol
calcite and 5000 ppmv for “subhumid” paleosol calcite samples from
Triassic profiles in Utah, U.S.A. However, if the lower overall S(z)
values and seasonal bias recorded in modern semi-arid soils in terms
of S(z) was comparable in the geologic past, then it is likely that many
of the paleoatmospheric pCO 2 estimates in the literature are overestimates
(Breecker et al. (2009), so care is needed in considering
records from arid to semi-arid settings.
In their treatment of Permo-Carboniferous paleosols from the
southwestern United States, Montañez et al. (2007) assumed that
calcite samples taken from morphologies indicative of long-term net
soil moisture deficiency, such as Calcisols (Mack et al., 1993), were
characterized by generally low and invariant S(z) values, whereas
calcites from paleosol morphologies consistent with less soil moisture
deficiency, such as calcic Argillisols, were characterized by relatively
high and invariant S(z) values, and calcite samples collected from
paleosol morphologies that are indicative of significant variability in
soil moisture availability, such as calcic Vertisols, were characterized
by a larger range of S(z) values. Finally, the lithology of the paleosol
profiles (sandstone, siltstone, mudstone and claystone) was assumed
to have affected the soil pCO 2 . Montañez et al. (2007) used the
following ranges of S(z) values for paleoatmospheric pCO 2 estimates:
(1) sandstone-dominated Calcisols are lowest, ranging from 1000–
3000 ppmV, (2) siltstone-dominated Calcisols range from 1500 to
4000 ppmV, (3) mudstone-dominated Calcisols range from 2000 to
5000 ppmV, (4) mudstone-dominated Vertisols range from 3200 to
7500 ppmV, (5) claystone-dominated calcic Vertisols range from 4000
to 8000 ppmV, and (6) mudstone-dominated calcic Argillisols are
highest, ranging from 6000 to 9000 ppmV.
The importance of the assumed soil pCO 2 values (C S values) cannot
be overstated, as the resulting atmospheric pCO 2 estimates vary proportionately
with them (Cerling, 1991; Ekart et al., 1999; Royer et al.,
2001a,b; Tabor et al., 2004a,b). Therefore, until an independent means
of estimating soil CO 2 concentration (see Section 8.1 for a possible
method for this) at the time of calcite crystallization in a paleosol
profile becomes available, this parameter will remain an effectively
unknown value, and will continue to limit the utility of paleosol calcite
δ 13 C values as proxy of atmospheric pCO 2 .
Cerling (1991, 1992) were the first studies to demonstrate the
potential of paleosol calcite δ 13 C values as a proxy of atmospheric
pCO 2 , in which it was suggested that Cretaceous atmospheric pCO 2
was significantly higher than modern values. This was followed by
several important studies of Paleozoic paleosol calcites, which
appeared to delineate a first-order trend of decreasing atmospheric
pCO 2 from ~15–20 PAL in the Early Paleozoic to near PAL in the Late
Carboniferous (Mora and Driese, 1993; Mora et al., 1996). These
Paleozoic studies were especially important at the time of publication,
because they were among the first geochemically-based pCO 2 proxy
data for comparison with carbon mass flux model pCO 2 estimates
(Berner, 1991; Berner and Kothavala, 2001), and the two different
model estimates appeared to agree with one another.
Ekart et al. (1999) compiled the first temporally extensive dataset
of two-component paleosol calcite δ 13 C values. Ekart et al. (1999)
presented results for 758 δ 13 C analyses that represented each time
period since the advent of vascular plants (~400 million years BP;
Fig. 19). The results of that study largely agreed with estimated
paleoatmospheric pCO 2 from carbon mass flux models, but Ekart
et al. (1999) also noted a significant disparity between pCO 2
estimates from the two proxy methods for the Permian. There have
been dozens of additional studies of paleosol calcite δ 13 Cvaluessince
the compilation provided by Ekart et al. (1999). Fig. 19 presents 2615
pedogenic calcite δ 13 C values (black crosses), ranging in age from
Devonian to Recent (~415–0 My), which were compiled from the
literature. This compilation includes results only from those studies
that presented paleosol micrite δ 13 C values in a Table or Appendix
(see caption in Fig. 19 for sources). Also shown in Fig. 19 are the best
fit and 95% confidence interval for terrestrial vascular plant organic
matter δ 13 C values that is based upon analysis of 2148 different
samples, which range from Devonian to Late Cretaceous (Strauss and
Peters-Kottig, 2003). Because of complications related to the
evolution and prominence of C 4 photosynthesizers in landscapes
characterized by soils that accumulate calcite, the following discussion
is limited to pre-Cenozoic data. Furthermore, the authors
emphasize that the following discussion is intended only as a
demonstration of how models of 2-component soil CO 2 mixing can
be applied to paleosol calcite data, and it is not intended for any
specific inferences to be drawn about variations in paleoatmospheric
pCO 2 .
Assuming that organic matter δ 13 C values in Fig. 19 accurately
represent the δ 13 C values of organic matter in the soils from which
calcite δ 13 C values are derived, they can be used to define an effective
lower limit for calcite δ 13 C values that are permissive of twocomponent
soil CO 2 mixing. This lower limit calcite δ 13 C value (thick
red line marked by 0 PAL) is 14.8‰ more positive than contemporaneous
paleosol organic matter, which reflects 4.4‰ diffusive enrichment
of CO 2 δ 13 C values derived from oxidation of organic matter, as
well as an additional 10.4‰ carbon isotope enrichment from gaseous
CO 2 to calcite due to carbon isotope fractionation between carbonate
species (at mildly alkaline pH; Bottinga, 1968). Calcite δ 13 C values that
are more negative than this lower limit provide negative estimates of
Please cite this article as: Sheldon, N.D., Tabor, N.J., Quantitative paleoenvironmental and paleoclimatic reconstruction using paleosols, Earth-
Science Reviews (2009), doi:10.1016/j.earscirev.2009.03.004