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