Quantitative paleoenvironmental and paleoclimatic reconstruction ...
Quantitative paleoenvironmental and paleoclimatic reconstruction ...
Quantitative paleoenvironmental and paleoclimatic reconstruction ...
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ARTICLE IN PRESS<br />
16 N.D. Sheldon, N.J. Tabor / Earth-Science Reviews xxx (2009) xxx–xxx<br />
where the st<strong>and</strong>ard error (SE) is ±4.4°C, S is salinization (Table 3),<br />
<strong>and</strong> R 2 =0.37 for the empirical fit. The underlying principle is that<br />
alkali elements (K <strong>and</strong> Na) are typically accumulated in desert<br />
settings, which usually have relatively low MAT (even if they are<br />
seasonally or daily high). Thus, paleosols with high salinization ratios<br />
should have low MAT values. The relationship is applicable to lowl<strong>and</strong><br />
settings <strong>and</strong> moderate soil formation times (b100 Ka). It is not<br />
applicable to hillslope, montane, wetl<strong>and</strong>, or tropical (e.g., laterites)<br />
paleosols (Sheldon et al., 2002). A second Inceptisol-specific (~Protosol)<br />
relationship between MAT <strong>and</strong> “clayeyness” (Al/Si; Table 3) of<br />
the Bw or Bt horizon has also been proposed:<br />
Tð ○ CÞ =46:9C +4 ð25Þ<br />
where the SE is ±0.6 °C, C is clayeyness (Table 3), <strong>and</strong> R 2 =0.96<br />
(Sheldon, 2006c) for the empirical fit. Though Eq. (24) is relatively<br />
imprecise, the relationship is highly significant (t-test=8.5154,<br />
F-test = 72.58) <strong>and</strong> when applied to appropriate paleosols, gives<br />
results that are consistent with paleotemperature estimates based<br />
on leaf-margin <strong>and</strong> nearest living relative estimates from plants<br />
(Sheldon et al., 2002; Sheldon <strong>and</strong> Retallack, 2004; Sheldon, 2009). It<br />
has been applied to Cenozoic (Sheldon et al., 2002; Sheldon <strong>and</strong><br />
Retallack, 2004; Hamer et al., 2007b), Mesozoic (Retallack, 2005a),<br />
<strong>and</strong> Paleozoic (Sheldon, 2005) paleosols. The more precise Inceptisol-only<br />
Eq. (25) is potentially applicable over a wider range of<br />
estimated paleotemperatures, though Inceptisol-like paleosols are<br />
typically rare at very high or very low MATconditions. Nonetheless, it<br />
also gives paleotemperature results that are consistent with independent<br />
estimates based on fossil floras <strong>and</strong> faunas (Hamer et al.,<br />
2007a). The Inceptisol paleothermometer has been applied to<br />
Quaternary paleosols from Hawai'i (Sheldon, 2006c), Miocene<br />
paleosols (Hamer et al., 2007a), <strong>and</strong> Eocene–Oligocene transition<br />
paleosols (Sheldon, 2009). The primary weakness of both elementbased<br />
paleothermometers is that a variety of processes, including<br />
some not related to temperature, may alter chemical composition, so<br />
care is needed not to overinterpret results based solely on these<br />
approaches. As with many of the proxies that we are discussing, if<br />
possible, results should be verified using multiple proxies.<br />
5.5. Paleoprecipitation<br />
A variety of different quantitative proxies have been developed for<br />
estimating mean annual precipitation (MAP). Water availability is one of<br />
the key factors in determining the relative effectiveness of many<br />
pedogenic processes. For example, leaching of base cations is limited in<br />
arid settings (Retallack, 2001b) <strong>and</strong> enhanced in humid settings (e.g.,<br />
Sayyed <strong>and</strong> Hundekari, 2006). A variety of different paleoprecipitation<br />
proxies have been developed, all based on empirical relationships derived<br />
from modern soils. Each of the proxies that we will discuss is based on the<br />
properties of a single horizon or soil feature, so which proxy is most<br />
appropriate for a given setting will depend on what type of paleosol is<br />
being analyzed. Examples to be discussed include proxies based the iron<br />
content of Mn nodules in Vertisols, the depth to a Bk horizon below a<br />
paleosol surface, <strong>and</strong> the chemical composition of a Bw/Bt horizon.<br />
5.5.1. Content of Fe–Mn nodules in vertisols<br />
Stiles et al. (2001) studied a modern climosequence developed on<br />
the Texas Gulf Coastal Plain. Most of the soils there are Vertisols forming<br />
under modern MAP ranging from 800 to 1500 mm yr −1 (Stiles et al.,<br />
2001). They also observed that Fe–Mn nodules were common in the soils<br />
<strong>and</strong> hypothesized that the iron content corresponds to mean annual<br />
precipitation values (Fe TOT as a whole number % value, i.e., 20, not 0.2):<br />
<br />
P mm yr − 1<br />
= 654:4 + 31:5 Fe TOT ð26Þ<br />
where R 2 =0.92 for the empirical relationship (Stiles et al., 2001).<br />
They then applied the relationship to Fe–Mn nodules in Paleozoic<br />
paleosols, yielding inferred moisture regime results that correspond<br />
well with field characteristics of the paleosols <strong>and</strong> with qualitative<br />
observations based on comparisons to modern analogues. Stiles et al.<br />
(2001) did not find that their results correspond well to estimates<br />
based on depth to the Bk horizon (see Section 5.5.2) <strong>and</strong> found the Bk<br />
depth generally underestimated the modern known precipitation<br />
values. This method seems to be viable for paleo-Vertisols, however, it<br />
is unclear how common authigenic Fe–Mn nodules are given that<br />
there are relatively few reported occurrences (see references in Stiles<br />
et al., 2001), so its applicability may be somewhat limited.<br />
5.5.2. Depth to Bk horizon<br />
CaCO 3 is highly soluble <strong>and</strong> both Ca 2+ <strong>and</strong> CO 3 2− /HCO 3 − are readily<br />
mobilizedbothingroundwater<strong>and</strong>insoilsolutions,sothepresenceof<br />
CaCO 3 in a soil or paleosol indicates at least some degree of aridity<br />
(perhaps just seasonally), because under wetter conditions carbonate<br />
would not be stable or form authigenically. Jenny <strong>and</strong> Leonard (1935) <strong>and</strong><br />
Jenny (1941) first recognized that there was a relationship between the<br />
depth below the surface of modern soils to their Bk horizon (carbonatebearing)<br />
<strong>and</strong> MAP in Great Plains soils. Arkley (1963) further exp<strong>and</strong>ed<br />
observations of the relationship by considering more arid Mojave Desert<br />
settings <strong>and</strong> found the same relationship. Retallack (1994) combined<br />
Jenny's (1941) <strong>and</strong> Arkley's (1963) data <strong>and</strong> other literature data (n=317<br />
in total) <strong>and</strong> fit a regression line to it that related depth to the Bk horizon to<br />
MAP. That relationship was widely applied to paleosols ranging in age<br />
from Paleozoic to the Quaternary (e.g., examples in Retallack et al., 2000;<br />
Retallack, 2001b). Factors to consider before applying the relationship to<br />
paleosols include erosion of the paleosols prior to burial, post-burial<br />
compaction, <strong>and</strong> the effect of elevated atmospheric CO 2 levels (models of<br />
Bk horizon formation suggest that they will form deeper in the profile at<br />
significantly elevated CO 2 levels; McFadden et al., 1991). Erosion may be<br />
dealt with through careful stratigraphy <strong>and</strong> consistent application of the<br />
proxy (e.g., tops of soils are always measured from the surface that the<br />
highest root traces emanate down from). Compaction may be dealt with<br />
as described above (see Section 2.2.1). There are many means of<br />
reconstructing past CO 2 levels including using isotopic data from paleosols<br />
(see Section 7.4.3.1). In general though, this is only a concern for<br />
Ordovician–Silurian <strong>and</strong> Jurassic–Cretaceous paleosols (e.g., Berner <strong>and</strong><br />
Kothavala, 2001), <strong>and</strong> for some Precambrian paleosols (Sheldon, 2006b).<br />
Royer (1999) questioned whether the proxy worked by looking at<br />
1168 Bk-bearing soils from the NRCS database <strong>and</strong> finding only a weak<br />
relationship between Bk depth <strong>and</strong> MAP. He proposed a simpler test,<br />
namely that soils receiving less than 760 mm yr− 1 would be Bk-bearing<br />
<strong>and</strong> those receiving more than 760 mm yr −1 would not have Bk horizons.<br />
Retallack (2000) pointed out a number of issues in Royer (1999) that<br />
merit reconsideration of those findings. The soils used in Retallack's<br />
(1994) study were tightly constrained to have similar characteristics (e.g.,<br />
horizonation, age, degree of development), a factor ignored by Royer<br />
(1999), who simply used presence/absence of a Bk horizon as his sole<br />
criterion. For example, according to Retallack (1994), the relationship<br />
only applies to moderately developed soils (i.e., with carbonate nodules,<br />
not wisps or caliche layers) formed on unconsolidated parent material<br />
(e.g., alluvium or loess) in lowl<strong>and</strong> settings, <strong>and</strong> which were undisturbed<br />
by human activity. Hundreds of Royer's (1999) soils did not conform to<br />
one or more of these criteria, so it is unsurprising that he did not find the<br />
same relationship. Royer's (1999) second result (a Bk-horizon isohyet at<br />
760 mm yr −1 ) probably also gives only weak guidance because the Bkbearing/Bk-free<br />
isohyet is at different precipitation values in many<br />
modern settings on different continents (Retallack, 2000). Nonetheless,<br />
in part due to that criticism, Retallack (2005b) substantially exp<strong>and</strong>ed his<br />
database to include 807 soils with a slightly revised relationship:<br />
<br />
P mm yr − 1 = − 0:013D 2 +6:45D + 137:2 ð27Þ<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