Quantitative paleoenvironmental and paleoclimatic reconstruction ...

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N.D. Sheldon, N.J. Tabor / Earth-Science Reviews xxx (2009) xxx–xxx
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sub-metamorphic burial conditions. This is particularly important for
Paleozoic and older paleosols because smectite is meta-stable and
may be altered to illite in the presence of K-rich pore waters. Nonetheless,
unless there is evidence of K metasomatism or illitization (and
this is primarily of interest in metamorphosed areas or with very old, i.e.,
Precambrian, paleosols) from petrography or some other method, the
CIA is a good tool for evaluating chemical weathering and for comparing
different paleosols. Both the CIA-K of Maynard (1992) and PIA of Fedo
et al. (1995) of B horizons in soils are well-correlated with mean annual
precipitation (see Section 5.5; Sheldon et al., 2002), so the primary
recent use of the CIA-K has been for estimating paleoclimatic conditions
(e.g., Nordt et al., 2006; Hamer et al., 2007a,b).
One final consideration with this type of weathering index is that
there is one piece of nomenclatural confusion that has been propagated
through the literature. The “chemical index of weathering”
(CIW; Harnois, 1988) and CIA-K (Maynard, 1992) were developed for
different purposes (Harnois (1988) was concerned about the inconsistent
behaviour of K during pedogenesis, and Maynard (1992) was
concerned about K addition by metasomatism), but, nevertheless,
involve the same equation:
Al
CIWðCIA − KÞ = 100 ×
Al þ Ca þ Na
Retallack (various, e.g., 2001b; see Colman, 1982) has advocated a
slightly different weathering index because while CIA and its variants
give a good accounting of feldspar weathering, other types of minerals
are not necessarily captured by that index, in particular Mg-bearing
minerals. The ΣBases/Al ratio (Table 3) is designed to overcome this
and is calculated with the following expression (again, using
molecular values for each element):
P Bases
Al
Ca þ K þ Mg þ Na
=
Al
The ΣBases/Al ratio is susceptible to the same potential issues with
metasomatically altered paleosols as the CIA, but again, unless there is
some reason to suspect post-burial alteration, the ratio may be used to
describe weathering of paleosols. ΣBases/Al has been used for a
variety of purposes including characterizing weathering intensity
(e.g., Retallack, 1999; Bestland, 2000; Retallack and Krull, 1999;
Sayyed and Hundekari, 2006) and for taxonomic purposes (Sheldon
et al., 2002). Sayyed and Hundekari (2006) compared a variety of
weathering ratios including the ΣBases/Al ratio to examine differences
between near-modern and Cretaceous weathering on “red bole”
and “green bole” paleosols formed on Deccan Traps basalt flows. A
ΣBases/Al ratio of 0.5 is a statistically significant dividing line between
Alfisols (N0.5; dystric Argillisols) and Ultisols (b0.5; eutric Argillisols);
that criterion is roughly equivalent to the Soil Survey Staff
(2006) base saturation criteria of 35% to separate between base-rich
Alfisols and base-poor Ultisols (Sheldon et al., 2002). An example
where that criterion was applied can be found in Sheldon (2005), who
used it to confirm a field identification of Alfisol-like paleosols. In
addition, Clechenko et al. (2007) discuss a thick paleosol associated
with the Paleocene–Eocene boundary as being comparable to a
modern Ultisol. Using data from their paper (their Table 1), an average
ΣBases/Al of 0.3 is obtained for the paleosols, lending additional
support to this criterion as a means for distinguishing between paleo-
Alfisols and paleo-Ultisols. A further application of the ΣBases/Al ratio
can be found in Perez-Huerta and Sheldon (2006), who used it to
examine how short-term sea level (i.e., glacio-eustatic) changes
affected nutrient availability in shallow marine settings where the
ΣBases/Al ratio was used to represent the fluctuating continental
input derived from soils.
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Darmody et al. (2005) presented two other measurements of
chemical weathering that they applied to modern soil formation with
granitic parent materials:
WI − 1=
WI − 2=
Si þ Ca
Fe þ Ti
Si þ Ca
Fe þ Ti þ Al
Darmody et al. (2005) found strong positive correlations between
WI-1, WI-2, and Si/Al [1/“clayeyness” (Section 5.2.1)], and strong
negative correlations with the CIW of Harnois (1988), suggesting that
all of the ratios were capturing weathering intensity. To date, Eqs. (8)
and (9) have not been widely applied to paleosols. One exception is
Kahmann et al. (2008), who compared results between a number of
indices and found that WI-1 and WI-2 were in relatively poor
concordance with other weathering indices and also with estimates of
mean annual precipitation derived from CIA-K values (see Section
5.5.3). However, the parent material being studied by Kahmann et al.
(2008; sediments) was very different than that being studied by
Darmody et al. (2005). Thus, at this point, results using Eqs. (8)
and (9) are equivocal and more testing is needed to determine if the
relationships that Darmody et al. (2005) found are widely applicable
or only apply to cases with granitic parent materials.
5.2.3. Trace element ratios
Trace element abundances vary considerably among different
paleosol types and parent materials. In general, to be useful a trace
element must be present at the 10 s–100 s ppm level, because the
analytical uncertainty using either XRF or ICP is a couple of ppm. As
with major elements, ratios between trace elements are favoured both
as means of normalizing differences but also because different
elements are gained or lost due to different pedogenic processes.
Trace element geochemistry in paleosols has been used for a variety of
applications including examining weathering intensity (e.g., Kahmann
et al., 2008), evaluating leaching (Sheldon, 2006a; Retallack,
2001a,b, 1999), and for provenance (Hamer et al., 2007b; Sheldon
et al., 2009; Sheldon, 2006a). The most commonly used ratio is Ba/Sr,
which is thought to represent leaching behaviour during weathering
(Table 3; Sheldon, 2006a; Retallack, 2001b), where higher values
represented more leaching. Both are alkaline earth elements and have
similar atomic radii and similar chemical affinities, but Sr is
significantly more soluble than Ba (Vinogradov, 1959), so under
most conditions it will be leached more readily. Thus, a heavily
leached paleosol should have relatively lower Ba/Sr ratios near the top
of the profile and relatively high Ba/Sr ratios lower in the profile if a B
horizon is present (Sheldon, 2006a). The Ba/Sr ratio has been used to
characterize leaching behaviour in paleosols throughout the geologic
record including Precambrian (Retallack and Mindszenty, 1994),
Permian–Triassic (Krull and Retallack, 2000; Sheldon, 2006a), and
Cenozoic (Terry, 2001) examples. There are two potential weaknesses
of applying the Ba/Sr ratio. First, the chemical behaviour of Ba is less
well understood than Sr, particularly in soils. Second, different parent
materials have variable initial concentrations (and textures due to
different mineral composition), so relatively minor changes in absolute
abundance in samples with small initial amounts could be
interpreted as intensive leaching when the process was of relatively
little importance.
An additional trace element ratio that is potentially useful is U/Th
(Table 3). U and Th are both actinide elements; This typically more
abundant in most potential parent rocks except for limestones (e.g., Li,
2000). Th and U are both relatively immobile during weathering
except under intense conditions (Li, 2000), or where there is a strong
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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