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The Pampas of Argentina contain a broad distribution of Pleistocene to Holocene loessic sediments and eolian dune deposits. Models describing the sediment provenance of this eolian system have, at times, conflicted. We address the provenance of these deposits through U-Pb detrital-zircon geochronology. Our results indicate broad similarity in age distributions between samples, with a dominant Permian-Triassic mode, and widespread but lesser Cenozoic, Devonian-Mississippian, Ediacaran-Cambrian, and Mesoproterozoic modes. These data are inconsistent with a large contribution of detritus from Patagonia as previously suggested. These data are consistent with very limited contribution of first cycle volcanogenic zircon to the Pampean eolian system, but abundances of older Neogene zircon indicate proto-sources in the Andes. The ríos Desaguadero, Colorado, and Negro contain populations that were likely within the dust production pathways of most of the loess, paleosol, and eolian dune deposits, but the derivation of the zircon ages in these sediments cannot be explained solely by these river systems. One statistical outlier, a loess sample from the Atlantic coast of the Pampa region, indicates quantitative similarity to the age spectra from the ríos Colorado and Negro, consistent with derivation from these subparallel rivers systems during subaerial exposure of the continental shelf under high global ice-volume. Another statistical outlier, a paleosol sample from the Río Paraná delta region, has zircon ages more closely associated with sediments in the Paraná region than in rivers south of the Pampa region. Collectively, these data point to the complexity of the Pampean eolian system and substantial spatial-temporal variation in this Pleistocene−Holocene eolian system.
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Unlocking the provenance of the Upper Miocene to Holocene southern South American loess record through U-Pb detrital zircon geochronology
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.
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- Award ID(s):
- 1911340
- PAR ID:
- 10553760
- Publisher / Repository:
- AGU Fall Meeting Abstracts
- Date Published:
- ISSN:
- 2023AGUFMEP53C1688P
- Page Range / eLocation ID:
- EP53C-1688
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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The Pampas of Argentina contain a broad distribution of Pleistocene to Holocene loessic sediments and eolian dune deposits. Models describing the sediment provenance of this eolian system have, at times, conflicted. We address the provenance of these deposits through U-Pb detrital-zircon geochronology. Our results indicate broad similarity in age distributions between samples, with a dominant Permian-Triassic mode, and widespread but lesser Cenozoic, Devonian-Mississippian, Ediacaran-Cambrian, and Mesoproterozoic modes. These data are inconsistent with a large contribution of detritus from Patagonia as previously suggested. These data are consistent with very limited contribution of first cycle volcanogenic zircon to the Pampean eolian system, but abundances of older Neogene zircon indicate proto-sources in the Andes. The ríos Desaguadero, Colorado, and Negro contain populations that were likely within the dust production pathways of most of the loess, paleosol, and eolian dune deposits, but the derivation of the zircon ages in these sediments cannot be explained solely by these river systems. One statistical outlier, a loess sample from the Atlantic coast of the Pampa region, indicates quantitative similarity to the age spectra from the ríos Colorado and Negro, consistent with derivation from these subparallel rivers systems during subaerial exposure of the continental shelf under high global ice-volume. Another statistical outlier, a paleosol sample from the Río Paraná delta region, has zircon ages more closely associated with sediments in the Paraná region than in rivers south of the Pampa region. Collectively, these data point to the complexity of the Pampean eolian system and substantial spatial-temporal variation in this Pleistocene−Holocene eolian system.more » « less
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Abstract The Tafí del Valle depression (~27° S) in the eastern Andes of Argentina provides a record of late Pleistocene dust deposition in the subtropics of South America. We present large-nU-Pb geochronology data for detrital zircons from upper Pleistocene loess-paleosol deposits. When compared to regional data, the age spectra from the Tafí del Valle samples are most like the southern Puna Plateau, supporting derivation largely from the west and northwest. This runs counter to hypotheses suggesting these loessic sediments were derived from the low elevation plains to the east or extra-Andean Patagonia. Mapping of linear wind erosion features on the Puna Plateau yield a mean orientation of 125.7° (1 s.d. = 12.4°). These new data and existing records are consistent with a westerly-northwesterly dominated (upper- and lower-level) wind system over the southern Puna Plateau (to at least ~27° S) during periods of high dust accumulation in Tafí del Valle.more » « less
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Abstract Wind-blown dust from southern South America links the terrestrial, marine, atmospheric, and biological components of Earth’s climate system. The Pampas of central Argentina (~33°–39° S) contain a Miocene to Holocene aeolian record that spans an important interval of global cooling. Upper Miocene sediment provenance based onn = 3299 detrital-zircon U-Pb ages is consistent with the provenance of Pleistocene–Holocene deposits, indicating the Pampas are the site of a long-lived fluvial-aeolian system that has been operating since the late Miocene. Here, we show the establishment of aeolian sedimentation in the Pampas coincided with late Miocene cooling. These findings, combined with those from the Chinese Loess Plateau (~33°–39° N) underscore: (1) the role of fluvial transport in the development and maintenance of temporally persistent mid-latitude loess provinces; and (2) a global-climate forcing mechanism behind the establishment of large mid-latitude loess provinces during the late Miocene.more » « less
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Quaternary dust production in southern South America is of particular interest because of its impacts on the global radiative forcing budget and on ocean chemistry. Characterizing dust production and delivery pathways is fundamental for understanding how dust-climate feedbacks in this region operate. A Holocene centered view of South American dust production emphases hyper arid basins on the Puna-Altiplano Plateau and the glaciated terrains of Patagonia as primary dust sources. Applying this Holocene-centric view to Pleistocene glacial-stadial period dust production is problematic as it largely ignores how glacial-inter-glacial variability impacts surficial processes and regional humidity, and thus dust production and transport. We sampled fluvial, loess and eolian dune deposits from central Argentina for U-Pb detrital zircon analysis in order better understand Pleistocene-Holocene dust production pathways and assess the relative importance of Andean rivers and dunes fields of the Pampa Plains as dust sources. Our data indicate that rivers draining the Andes south of the Pampa Plains and large vegetation-stabilized dune fields could have played a fundamental role in late Pleistocene glacial and stadial period dust production. If valid, dust production in this manner is at odds with the Holocene-centric view of South American dust production.more » « less
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