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Aeolian dust deposition is an important phosphorus (P) input to terrestrial ecosystems, but its influence on P dynamics during long-term ecosystem development remains poorly understood. In this study, we characterized P speciation using P K-edge XANES spectroscopy in surface soils (0–15 cm, A horizon) and contemporary aeolian dust collected at each site of a 3000-ky volcanic soil chronosequence in a cool, semi-arid environment. Phosphorus speciation in dust was dominated by calcium-bound P (Ca-P; 54–74%), with 11–23% iron and aluminum-bound P [(Fe + Al)-P] and 7–25% organic P (Po). In soils, Po contributed 1–23% of total P, being greater in older soils; however, the proportions of Ca-P (16–39%) and (Fe + Al)-P (48–82%) fluctuated with increasing weathering over the soil chronosequence. These soil fluctuations resulted from the accumulation and preservation of alkaline aeolian dust during pedogenesis in the semi-arid climate, which significantly increased soil Ca-P while decreasing the total amounts and relative abundances of soil (Fe + Al)-P. We suggest that the effects of an aeolian dust input on soil P transformations are functions of the relative magnitude and chemical composition of the dust input and the soil weathering intensity. For a given source of dust, when the net dust flux is greater than the weathering rate, dust accumulates and thus alters the pattern of P transformations during pedogenesis; otherwise, the dust influence on soil P transformations is negligible. By accurately identifying the chemical nature of P pools, our work highlights the advantage of P K-edge XANES spectroscopy over chemical extractions in examining soil P dynamics, and demonstrates how dust inputs can modify the Walker and Syers model of pedogenic P transformations in semi-arid environments. Overall, this work provides a foundation for understanding how dust influences P cycling during soil and ecosystem development, and indicates that dust inputs and composition, and the soil weathering rate, all must be considered for developing integrated climate-biogeochemical models with predictive power in terrestrial ecosystems.
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Relict soil evidence for post-Miocene aridification in the Atacama Desert of South America
Abstract Deep exposures of soil profiles on Miocene or Mio-Pliocene alluvial deposits were studied along a 500 km N-S transect in the Atacama Desert. These ancient deposits, with excellent surface preservation, now stand many meters above a broad incised Plio-Pleistocene alluvial terrain. Total geochemical analyses and mass balance calculations allowed the establishment of elemental gains, losses, and redistribution in the soils. From north to south (presently hyperarid to arid), the ancient soils reveal an increase in losses of rock-forming elements (Si, Al, Fe, K, Mg). Additionally, rare earth elements (REE) show losses with increasing southerly latitude and systematic patterns with soil depth. Some REEs appear to be unique chemical tracers of exogenous dust and aerosol additions to the soils. The removal of major elements and REEs is impossible in the present climate (one of salt and dust accumulation), revealing that for a significant period following the deposition of the alluvium, soils were exposed to rainfall, chemical weathering, and mass loss—with a geographical pattern that mirrors the present rainfall gradient in the region. Following the cessation of weathering, the pre-weathered soils have undergone enormous dust and salt accumulations, with the rates and types of salt accumulation consistent with latitude: (1) carbonate in the south and (2) sulfate, chlorides, and nitrates to the north. The quantity, and apparent rates, of salt accumulation have a strong latitudinal trend. Isotopes of sulfate have predictable depth patterns based on isotope fractionation via vertical reaction and transport. The relict hyperarid soils are geochemically similar to buried Miocene soils (ca. 10–9 Ma) in the region, but they differ from older Miocene soils, which formed in more humid conditions. The overall soil record for the Atacama Desert appears to be the product of changes in Pacific Ocean sea surface temperatures over time, and resulting changes in rainfall. The mid-Miocene was relatively humid based on buried soil chemistry and evidence of fluvial activity. The mid to late Miocene cooling (ca. 10–5.5 Ma) appears to have aridified the region based on paleosol soil chemistry. Pliocene to earliest Pleistocene conditions caused weathering of the relict soils examined here, and regional fluvial activity. Since the earliest Pleistocene, the region has largely experienced the accumulation of salts and, except for smaller scale oscillations (glacial-interglacial), has experienced protracted hyperaridity.
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- Award ID(s):
- 2300559
- PAR ID:
- 10575407
- Publisher / Repository:
- Geological Society of America
- Date Published:
- Journal Name:
- Geological Society of America Bulletin
- Volume:
- 137
- Issue:
- 1-2
- ISSN:
- 0016-7606
- Page Range / eLocation ID:
- 465 to 480
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Aeolian dust deposition is an important phosphorus (P) input to terrestrial ecosystems, but its influence on P dynamics during long-term ecosystem development remains poorly understood. In this study, we characterized P speciation using P K-edge XANES spectroscopy in surface soils (0–15 cm, A horizon) and contemporary aeolian dust collected at each site of a 3000-ky volcanic soil chronosequence in a cool, semi-arid environment. Phosphorus speciation in dust was dominated by calcium-bound P (Ca-P; 54–74%), with 11–23% iron and aluminum-bound P [(Fe + Al)-P] and 7–25% organic P (Po). In soils, Po contributed 1–23% of total P, being greater in older soils; however, the proportions of Ca-P (16–39%) and (Fe + Al)-P (48–82%) fluctuated with increasing weathering over the soil chronosequence. These soil fluctuations resulted from the accumulation and preservation of alkaline aeolian dust during pedogenesis in the semi-arid climate, which significantly increased soil Ca-P while decreasing the total amounts and relative abundances of soil (Fe + Al)-P. We suggest that the effects of an aeolian dust input on soil P transformations are functions of the relative magnitude and chemical composition of the dust input and the soil weathering intensity. For a given source of dust, when the net dust flux is greater than the weathering rate, dust accumulates and thus alters the pattern of P transformations during pedogenesis; otherwise, the dust influence on soil P transformations is negligible. By accurately identifying the chemical nature of P pools, our work highlights the advantage of P K-edge XANES spectroscopy over chemical extractions in examining soil P dynamics, and demonstrates how dust inputs can modify the Walker and Syers model of pedogenic P transformations in semi-arid environments. Overall, this work provides a foundation for understanding how dust influences P cycling during soil and ecosystem development, and indicates that dust inputs and composition, and the soil weathering rate, all must be considered for developing integrated climate-biogeochemical models with predictive power in terrestrial ecosystems.more » « less
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South America, from southernmost Bolivia through central Argentina, contains a useful Late Miocene to Holocene record of eolian sedimentation that can be used to advance our understanding of atmospheric circulation and dust production pathways over that interval. Our research indicates that loess provinces in the eastern Andes, Chaco Plains, and Pampean Plains had quasi-independent dust production pathways. A summary of our findings is as follows. 1) Detrital zircon crystals in the high-elevation upper Pleistocene loess deposits in the eastern Andes area of Tafí del Valle were primarily derived from the Puna Plateau to the west. At a latitude of ~27° S, this necessitates a several-degree equatorward shift in the upper- and lower-level westerlies during intervals with high dust accumulation in Tafí del Valle. 2) Upper Pleistocene to Holocene eolian sand deposits of the Pampean Sand Sea and loessic strata in the central and eastern Pampas contain detrital zircon U-Pb age spectra indicating derivation from the Río Desaguadero, Río Colorado, and Río Negro which drain the central Andes. Although the present-day Puna-Altiplano Plateau is hyperarid, the presence of major Argentine river systems in the dust production pathways of the Pampas is important for identifying the relative importance of precipitation and river courses on dust production, which parallels the relationship between the Yellow River and Chinese Loess Plateau in East Asia. 3) Upper Miocene strata of the Cerro Azul Formation, deposited between ~8.9 and ~5.5 Ma, include loess and aggradational paleosols. These eolian strata yield detrital zircon U-Pb age spectra that are consistent with the present-day Río Colorado and Río Negro, and similar to the Upper Pleistocene to Holocene deposits of the Pampas. This suggests a Late Miocene establishment of the Pampean eolian system. Interestingly, the Pampean eolian system and Chinese Loess Plateau both cover the same latitudes (~33°-39°) but in different hemispheres, and both were established at roughly the same time during the Late Miocene. These observations point to bihemispheric intensification of Hadley circulation in forcing the establishment of these two large eolian provinces.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1130/B37356.1
