Quantitative paleoenvironmental and paleoclimatic reconstruction ...

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30 N.D. Sheldon, N.J. Tabor / Earth-Science Reviews xxx (2009) xxx–xxx
Fig. 18. δ 18 O values of meteoric precipitation (black line), soil moisture at 10 cm, and soil moisture at 60 cm depth in a modern soil profile in Ellensburg, Washington U.S.A., versus progressive
days of analysis (Robertson and Gazis, 2005). The shallowest sample level (10 cm) of the profile has soil water δ 18 Ovaluessignificantly more positive (~9‰) δ 18 O values than that of mean
annual precipitation values, whereas the deepest sample level (60 cm) of the profile has δ 18 O values very near that that of the mean δ 18 O value of precipitation. See text for discussion.
(Tabor and Montañez, 2005; Tabor 2007). Because of all of these factors,
paleosol calcite δ 18 O values should probably be avoided as a proxy of
paleotemperature.
7.3.2. Pedogenic siderite as a proxy for soil moisture δ 18 O values
Pedogenic sphaerosiderites exhibit δ 18 O values ranging from ~0‰ to
−18‰ (PDB; Baker et al.,1996; Ludvigson et al.,1998; White et al., 2001;
Ufnar et al., 2002, 2004). However, siderite δ 18 O values typically exhibit
a narrow range of intra-profile values (b2‰), which is interpreted to
reflect relatively stable (invariant) temperatures and mean δ 18 Ovalueof
local groundwater that flood the soil profile (Ludvigson et al., 1988;
White et al., 2001; Ufnar, 2002, 2004). Nevertheless, because the δ 18 O
value of siderite is related to two variables (T and δ 18 OofH 2 O), one
variable must be assumed, or determined by an independent means, in
order to determine the other (as is the case with pedogenic calcite). In
their treatment of sphaerosiderite δ 18 O values from paleosols that
defined a mid-Cretaceous paleolatitudinal transect from ~35° to 55°N,
White et al. (2001; Ludvigson et al., 1998; Ufnar et al., 2002, 2004)used
the predicted mean annual surface air temperatures from General
Circulation Models (GCM) for Middle Cretaceous time (Barron, 1989;
Barron et al., 1989; Poulsen,1999) in order to estimate groundwater and
meteoric precipitation δ 18 O values. Sphaerosiderite δ 18 O values indicate
significantly more negative δ 18 O values (4.0–4.3‰) of mid-Cretaceous
meteoric precipitation than modern isolatitudinal sites. Based upon the
well-known relationship in the modern hydrological cycle, that coastal
precipitation shows ~− 2‰ shift for every additional 100 mm of
precipitation per month, White et al. (2001) hypothesized that these
sphaerosiderite-producing soils formed in the presence of ~2400 to
~2600 mm of precipitation per yr along the coastlines of the North
American Cretaceous interior seaway.
Other examples of sphaerosiderite in paleosol profiles have been
described from high-latitude Permo-Carboniferous (Brown and Kingston,
1993; Faure et al., 1995) and Triassic strata (Retallack, 1976) as
well as tropical Paleogene strata (Jacobs et al., 2005; Fig. 17). They
typically occur in clastic strata that are associated with coal measures.
In this regard, sphaerosiderite may be an abundant paleopedogenic
mineral in both time and space, which is limited to poorly drained
environments, and that has gone largely unnoticed as a useful proxy of
paleoclimate. If so, δ 18 O values of paleopedogenic sphaerosiderites
have the potential to become an important paloeclimate proxy which
represents pedogenic environments that are generally exclusive of
pedogenic calcite.
7.4. δ 13 C values of soil carbonate
7.4.1. Calcite from one-component of soil CO 2
The closed-system, one-component soil CO 2 model of calcite
crystallization is not normally considered in soil systems, but is
frequently associated with early diagenetic marine and lacustrine
calcites that are characterized by very negative δ 13 C values (e.g., Irwin
et al., 1977; Gluyas, 1984; Scotchman, 1991). Calcites that precipitate
via one component, chemically closed or semi-closed, conditions will
have δ 13 C values no more than 14.8‰ heavier than in situ soil organic
matter. This reflects that open-system oxidation of soil organic matter,
in the absence of contribution of isotopically heavier CO 2 from the
global troposphere, results in (at least) 4.4‰ diffusive enrichment,
and 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).
Evidence for calcite formation in the presence of one soil CO 2
component is sparse. However, a study of waterlogged soils in the
Atchafalaya Swamp, Louisiana, U.S.A. demonstrated that calcite δ 13 C
values can be less than − 20‰, and therefore did not crystallize under
open-system exchange with tropospheric CO 2 (Whelan and Roberts,
1973). In this regard, calcite precipitation in the presence of one soil
CO 2 component may be a reasonably common occurrence in swampy,
waterlogged soil and paleosol profiles. Given the sedimentary bias
toward preservation of low-lying and poorly-drained landscapes,
there may be also a bias toward preservation of paleosol calcite that
forms via one component of soil CO 2 .
As mentioned above, calcite crystallization in the presence of one
CO 2 component is related to dysoxic or anoxic conditions in poorly
drained soil profiles. Therefore, at field scale, one-component soil
calcites may be associated with profiles that exhibit drab or redoximorphic
soil matrix colors and weak or no development of soil structure
(Vepraskas, 1992). However, soil and paleosol colors are strongly
susceptible to change after burial (e.g., Retallack, 1991), therefore very
negative calcite δ 13 C values may be the most diagnostic characteristic of
a one-component soil CO 2 system. Examples of paleosol calcite that have
been interpreted to form in the presence of one component of soil CO 2
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