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Isotopic dynamics of C, O and U in soils and their relation to climate

  • Author(s): Oerter, Erik
  • Advisor(s): Amundson, Ronald
  • et al.
Abstract

The development of soil is driven by environmental conditions, and soil properties are reflective of those conditions. Pedogenic carbonate is widespread in arid and semi-arid soils and its stable C and O isotope composition is often used as a paleoclimate indicator. However, questions remain about the precise meaning of those signals, and most importantly the season(s) that the isotopic composition most closely reflects. Chapter 1 examines the soil conditions that lead to carbonate formation, and describes research on a climosequence of four sites on Holocene soils in Fish Lake Valley, Nevada (western USA), where Mean Annual Precipitation ranges from ~80mm yr-1 to ~220mm yr-1. Continuous measurements of soil temperature, soil volumetric water content, air temperature, and air humidity were recorded in 0.5hr intervals. Precipitation and samples of soil atmosphere from 10, 25, 50 and 100cm were collected at various intervals for CO2 concentration, δ13C and δ18O values (which are a proxy for soil water δ18O). This climosequence indicates that the C and O stable isotope composition of soil CO2, H2O and carbonate change systematically with elevation and climate. At the lowest elevation site, soil carbonate δ13C and δ18O values reflect constant conditions nearly year round, especially in the 25-50cm soil depth interval. At the mid-elevation sites, spring through summer conditions appear to be recorded in δ13C and δ18O values of pedogenic carbonate. At the highest elevation site of 2602m, the C and O isotopes of carbonate never reflected the measured soil CO2 and H2O isotopes. These results indicate that there is inherent difficulty or complexity in assigning a definitive timing of carbonate formation based on short-term (one or two year) field measurements of soil CO2 and H2O, but also suggests that short-term monitoring, despite its weaknesses, helps to reveal the nature of the long term record isotopically recorded in soil carbonate.

Chapter 2 presents the first continuous millennial-scale paleoclimate reconstruction based on O, C and U isotopes in dense, laminated pedogenic carbonate. The record is from the Wind River Basin of Wyoming, and provides isotopic proxies for climate and vegetation conditions in the North American mid-continent for the last 120ka. Accurate and precise 230Th/U ages were obtained from laser ablation ICP-MS spots of 93um diameter size. The resulting carbonate growth calculations indicate that ~100μm sampling regions represent 10ka to 0.1ka at growth rates of 10 to 1000μm ka-1. The U-series transects were coupled to δ13C and δ18O values obtained by ion probe on 10um diameter spots. Modern soil carbonate in the region has δ18O and δ13C values that are similar to the youngest (early Holocene) samples acquired in the laminations, strengthening the interpretation that the carbonate reflects soil isotopic conditions at the time of formation. The carbonate lamination record from 120 to 80ka is somewhat sparse due to slowly forming carbonate, but records conditions that change in phase with summer solar insolation, and indicate that relatively warm and dry conditions prevailed during the penultimate deglaciation and last interglacial period. The paleoclimate signal in the soil carbonate δ18O values during this time period are similar to that recorded in the well-studied Devils Hole phreatic carbonate δ18O record from the Mojave Desert in southwestern North America. From 75 to 55ka, carbonate δ18O values sharply increase by ~2‰, a change also recorded by speleothems from central North America, but not seen in the Devil’s Hole record. Atmospheric circulation during this time appears to have changed from a state dominated by westerly winds to one influenced by meridional flow from the south along the western limb of a strong and west-based North Atlantic Subtropical High, a condition that increased the proportion of summer to winter precipitation by 20-33%. δ13C and 234U/238Ui values from 75 to 55ka also indicate a shift towards wetter conditions and are supported by regional fossil pollen records suggesting a change to prairie-like ecosystems with wetter summers during this time. Subsequently, the carbonate record suggests that climate in mid-latitude central North America became progressively more arid during the onset of the last glaciation (both δ13C and 234U/238Ui values become more positive), culminating in peak aridity during the Last Glacial Maximum. Carbonate formation rates during these colder conditions increased and the temporal resolution of the Wind River record increased as a result, allowing it to capture sub-millenial abrupt climate changes associated with Dansgaard-Oeschger cycles 6 through 3, Heinrich Stadials 2 through 0 (Younger Dryas), as well as the dramatic Bolling-Allerod warming at 14.5ka. The Wind River pedogenic carbonate record shows the potential to develop similar high temporal resolution paleoclimate records form soil carbonate deposits that are relatively common worldwide.

The Cenozoic paleoclimate of the Atacama Desert is not well known. Chapter 3 presents the results of a study of 14 Oligocene to early Miocene paleosols exposed in the El Tesoro Mine, near Calama, Chile. The paleosols developed on an aggrading alluvial fan system, and lie above mineralized gravels that host a copper ore body. Soil forming conditions that oscillated between chemical weathering and clay production (humid: analogous to modern Alfisols) to environments favoring the accumulation of pedogenic carbonate (arid to semi-arid: analogous to modern Aridisols) are indicated. In contrast, the region is presently hyperarid, and soils accumulate sulfate, chlorides, and nitrates. While total chemical analyses clearly reveal the accumulation of Ca by the carbonate rich paleosols, none of the soils revealed significant losses of elements by leaching. The δ18O values of the carbonates range from -13.81‰ to -3.16‰ (VPDB). The O isotope data, when combined with published data from the region, revealed a significant divergence in the O isotope composition of precipitation in the eastern and western margins of the Andean plateau since the Oligocene, suggesting that simple interpretations of declines in δ18O values of carbonate with increasing elevation may not be appropriate. These paleosols clearly indicate that wetter conditions prevailed in what is now the Atacama Desert during the Oligocene to early Miocene.

Chapter 4 presents findings from a study examining a special case of interactions between soil water oxygen isotopes and cations adsorbed to clay minerals in the soil matrix. In isotope-enabled hydrology, soil and vadose zone sediments have been generally considered to be isotopically inert with respect to the water they host. This is inconsistent with knowledge that clay particles possessing an electronegative surface charge and resulting cation exchange capacity (CEC) interact with a wide range of solutes which, in the absence of clays, have been shown to exhibit δ18O isotope effects that vary in relation to the ionic strength of the solutions. To investigate the isotope effects caused by high CEC clays in mineral-water systems, we created a series of monominerallic-water mixtures at gravimetric water contents ranging from 5 to 32%, consisting of pure deionized water of known isotopic composition with homoionic (Mg, Ca, Na, K) montmorillonite. Similar mixtures were also created with quartz to determine the isotope effect of non-, or very minimally-, charged mineral surfaces. The δ18O value of the water in these monominerallic soil analogs was then measured by isotope ratio mass spectrometry (IRMS) after direct headspace CO2 equilibration. Mg- and Ca-exchanged homoionic montmorillonite depleted measured δ18O values up to 1.55‰ relative to pure water at 5% water content, declining to 0.49‰ depletion at 30% water content. K-montmorillonite enriched measured δ18O values up to 0.86‰ at 5% water content, declining to 0.11‰ enrichment at 30% water. Na-montmorillonite produces no measureable isotope effect. The isotope effects observed in these experiments may be present in natural, high-clay soils and sediments. These findings have relevance to the interpretation of results of direct CO2-water equilibration approaches to the measurement of the δ18O value of soil water. The adsorbed cation isotope effect may bear consideration in studies of pedogenic carbonate, plant-soil water use and soil-atmosphere interaction. Finally, the observed isotope effects may prove useful as molecular scale probes of the nature of mineral-water interactions.

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