Browsing by Autor "Jason B. Barnes"

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Spatiotemporal variability of modern precipitation <i>δ</i><sup>18</sup>O in the central Andes and implications for paleoclimate and paleoaltimetry estimates
(Wiley, 2015) Richard P. Fiorella; Christopher J. Poulsen; Ramiro Pillco Zolá; Jason B. Barnes; Clay Tabor; Todd A. Ehlers
Abstract Understanding the patterns of rainfall isotopic composition in the central Andes is hindered by sparse observations. Despite limited observational data, stable isotope tracers have been commonly used to constrain modern‐to‐ancient Andean atmospheric processes, as well as to reconstruct paleoclimate and paleoaltimetry histories. Here, we present isotopic compositions of precipitation ( δ 18 O p and δ D p ) from 11 micrometeorological stations located throughout the Bolivian Altiplano and along its eastern flank at ~21.5°S. We collected and isotopically analyzed 293 monthly bulk precipitation samples (August 2008 to April 2013). δ 18 O p values ranged from −28.0‰ to 9.6‰, with prominent seasonal cycles expressed at all stations. We observed a strong relationship between the δ 18 O p and elevation, though it varies widely in time and space. Constraints on air sourcing estimated from atmospheric back trajectory calculations indicate that continental‐scale climate dynamics control the interannual variability in δ 18 O p , with upwind precipitation anomalies having the largest effect. The impact of precipitation anomalies in distant air source regions to the central Andes is in turn modulated by the Bolivian High. The importance of the Bolivian High is most clearly observed on the southern Bolivian Altiplano. However, monthly variability among Altiplano stations can exceed 10‰ in δ 18 O p on the plateau and cannot be explained by elevation or source variability, indicating a nontrivial role for local scale effects on short timescales. The strong influence of atmospheric circulation on central Andean δ 18 O p requires that paleoclimate and paleoaltimetry studies consider the role of South American atmospheric paleocirculation in their interpretation of stable isotopic values as proxies.
Thermochronometer record of central Andean Plateau growth, Bolivia (19.5°S)
(Wiley, 2008) Jason B. Barnes; Todd A. Ehlers; Nadine McQuarrie; Paul O’Sullivan; S. Tawackoli
Quantifying the timing, magnitude, and rates of exhumation and deformation across the central Andes is a prerequisite for understanding the history of plateau rise. We present 23 new apatite and zircon fission track thermochronometer samples to chronicle the exhumation and deformation across the entire (∼500 km) Andean fold‐thrust belt at ∼19.5°S in Bolivia. Exhumation and deformation are constrained with inverse thermal modeling of the thermochronometer data, regional stratigraphy, geothermal gradients, and mass deficits inferred from a balanced section. Results suggest the following: (1) Initial exhumation of the Eastern Cordillera (EC) fore‐thrust and back‐thrust belts began in the late Eocene to early Oligocene (27–36 Ma) and continued in a distributed manner in the late Oligocene to early Miocene (19–25 Ma). Interandean zone (IA) exhumation began 19–22 Ma, followed by a third pulse of exhumation (11–16 Ma) in the EC back‐thrust belt and initial cooling in the westernmost Subandes (SA) 8–20 Ma. Finally, exhumation propagated eastward across the SA during the late Mio‐Pliocene (2–8 Ma). (2) Exhumation magnitudes are spatially variable and range from maximums of <8 km in the EC fore‐thrust belt to average values of ∼4–7 km across the EC, ∼2.5–3 km in the Altiplano, ∼4–6 km in the IA, and ∼3 km in the SA. (3) Exhumation rates range from ∼0.1 to 0.2 mm/a in the EC, from ∼0.1 to 0.6 mm/a in the IA, and from ∼0.1 to 0.4 mm/a to locally 1.4 mm/a or more in the eastern SA. We synthesize similar constraints with sediments throughout Bolivia and characterize plateau development by (1) distributed deformation throughout the Altiplano and EC regions from ∼20 to 40 Ma with minor deformation continuing until ∼10 Ma, (2) contemporaneous cessation of most EC deformation and exhumation of the IA ∼20 Ma implying establishment of the modern plateau width with significant, but unknown crustal thickness and elevation shortly thereafter by ∼15–20 Ma, and (3) dominantly eastward propagation of deformation from the IA since ∼20 Ma with minor out‐of‐sequence deformation in the central to eastern SA.