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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certainty, fossils collected from different paleoenvironments including
paleosols should be distinguishable.
In general, the difficulties with applying different REE geochemical
proxies are the same. First, though many REEs may be obtained by
relatively conventional means (XRF or ICP-MS), there is additional
cost relative to major element geochemistry. Second, many potential
soil and paleosol parent materials are relatively low in REE prior to
pedogenesis, so only a long formation time or relatively intense
weathering will result in easily decipherable REE patterns. Third,
though elemental ratios show some promise, individual REEs are not
useful because it is the pattern of their distribution both between
samples, and relative both to some standard (NASC or chondrites) and
to the parent material chemistry.
5.3. Mass-balance calculations
5.3.1. Pedogenesis and diagenesis
Many geologic and pedogenic processes can be most easily
discussed in terms of which elements are involved in a given process
and how their abundance and distribution changed as the soil
developed from the protolith. One common method of assessing
gains and losses of various elements in soils is through constitutive
mass balance (see Brimhall and Dietrich (1987); Chadwick et al.
(1990)). Mass balance calculations have also been used extensively
with paleosols to understand pedogenesis (various, e.g., Driese et al.,
2000, 2007; Bestland, 2002; Sheldon, 2003, 2005; Hamer et al.,
2007b). Mass balance can be reduced to two concepts, strain (ε) ofan
“immobile” element and transport (τ) of a second element with
respect to the immobile element. While a more thorough discussion
may be found in any of the original references mentioned above, the
basic concept is that if elements that were immobile during weathering
can be identified then it is possible to assess losses and gains of
mobile elements compared to the immobile element. This has in it the
underlying assumption of open system transport, that is, that mass
may be lost or gained by the system. In real terms, the gains and losses
of elements could be due to a variety of physical and chemical
processes. For example, the loss of a given element could represent
pedogenic processes while the addition of another element could be
from aeolian processes. The open system mass-transport function for
element j in the weathered sample (w) isdefined as follows:
"
τ j; w=
ρ w C #
h i
j;w
e
ρ p C i;w +1 − 1
j;p
ð11Þ
where ρ w is the density of the weathered material, C j,w is the chemical
concentration (weight percentage) of element j in the weathered
material, ρ p is the density of the parent material, and C j,p is the
chemical concentration (weight percentage or molar mass) of
element j in the parent material. In many cases, ρ w values must either
be assumed based on modern analogues or adjusted from measured
values to take into account compaction of the paleosol (Eq. (4);
Sheldon and Retallack, 2001) after its formation. If τ j,w =0 (i.e.,
element w was immobile), then ε i,w can be solved for separately, thus
bypassing volume (as in the classical definition of strain) as follows:
where ε i,w is the strain on immobile element i in the weathered
sample. Selection of immobile elements is often made on the basis of
theory rather than observations: Ti, Nb, Al, and Zr are typically assumed
to be immobile during weathering. However, it is possible to
assess the immobility by comparing the relative mobilities of a
supposedly immobile element assuming that another element is immobile
(Chadwick et al., 1990). For example, one could plot τ Ti,w,ε(Zr)
against ε Zr,w and τ Zr,w,ε(Ti) against ε Ti,w to determine which element
was truly immobile during weathering. If more than one element
shows similar immobility, then the usual convention is to use the
more abundant element. Thus, while Nb is immobile in most settings,
if Zr is similarly immobile, it would be the element of choice for the
calculations because it is typically 5+ times as abundant. An
additional consideration with the selection of putatively immobile
elements for use in mass balance calculations is the texture of the
paleosols. For example, Stiles et al. (2003) found lower ε Ti values than
ε Zr values in a modern climosequence, where the Zr resided almost
exclusively within the sand and coarse-size fractions of the soils while
Ti resided preferentially in smaller size fractions. There was little
chemical weathering of the Zr-bearing zircons evident in SEM as
compared with Ti-bearing minerals, so the lower ε Zr values represent
physical, rather than chemical weathering, and preferential removal of
larger grain sizes. Thus, Stiles et al. (2003) advocate using Ti as the
immobile element in clay-dominated soils and paleosols and Zr as the
immobile element in coarser-grained soils and paleosols.
To illustrate some of these concepts, an example using data from
Sheldon (2003) is presented. The middle Miocene Picture Gorge
Subgroup is part of the Columbia River flood basalt province; between
Picture Gorge flows, a variety of paleosols have been preserved
including Alfisol-like (Argillisol) and Histosol-like paleosols (Sheldon,
2003). For the Picture Gorge paleosols, Zr was determined to be the
least mobile of the typically immobile elements (Fig. 8; as compared
with Ti and Nd). ε Zr values indicate slight addition of Zr, but significant
loss of Ti (τ Ti ). This pattern indicates that Zr is more immobile than Ti
because an element (Zr in this case) that is chemically immobile and
only redistributed by physical weathering processes should accumulate
during regular pedogenesis. If the plot is reversed (ε Ti versus τ Zr ),
unrealistic addition (200% addition) of both elements is indicated,
because there are virtually no Ti- or Zr-bearing minerals observed in
thin section (Sheldon, 2003, 2006d). Thus, for other mass balance
calculations Zr is used as the immobile element. The gains or losses of
alkali (K, Na, Rb) and alkaline earth elements (Ca, Mg, Sr) can then be
calculated and plotted as a function of depth for a typical Picture Gorge
paleosol (Fig. 9). Ca and Na were lost extensively throughout the
profile with more than 80% of Ca and 60% of Na removed relative to
the parental basalt (Fig. 9A), a pattern similar to modern basalts
(Chadwick et al., 1999) and other basalt-parented paleosols (Sheldon,
2006c). In contrast, both K and Rb were added to the paleosols relative
to the basaltic parent, except deep in the profile (Fig. 9B). Rb addition
is systematically higher than K addition, a pattern that was interpreted
by Sheldon (2003) as indicating airborne addition of volcanic ash from
a local source. Given that both elements have the same chemical
affinities, if the addition of both elements was instead due to metasomatism,
the τ values would be equal (Sheldon, 2003). Plants use K
as an important cellular electrolyte, whereas Rb is not a biologically
important cation, so the difference between the two should represent
"
e i;w =
ρ p C #
j;p
− 1
ρ w C j;w
ð12Þ
Fig. 8. Immobile element determination.
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