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