Paleobotanical proxies for the Eastern Cordillera of Colombia suggest a rapid rise in elevation of 1.5?2.5 km over the past 6 million years. Independent paleoaltimetry proxies are needed to determine the timing and rate of uplift in the northern Andes. As in many tropical locations, the appropriate carbonate or silicate minerals needed to apply oxygen isotope paleoaltimetry are missing. Fortunately, extensive preservation of organic-rich deposits and paleosols affords an excellent opportunity to test two new paleoaltimeters. The first paleoaltimeter exploits the variation in deuterium isotopes preserved in leaf waxes to reconstruct the lapse rate of stable isotopes with elevation. The second, the MBT/CBT paleo-thermometer (paleothermometer based on the Cyclisation of Branched Tetraether (CBT) index and Methylation of Branched Tetraether (MBT) index), utilizes the proportions of distinct moites of tetraether lipids produced by bacteria in soils (which are a sensitive function of mean annual temperature) to reconstruct the temperature versus elevation lapse rate. The two independent paleoaltimeters will be applied at both high- and low-elevation sites, providing multiple records that can be compared to determine if variations are the result of surface uplift or climate change. These data will be combined with chronostratigraphic results to determine the timing and rate of surface uplift.
Recent models based on oxygen isotope and paleobotanical proxies posit rapid surface uplift of approximately 2.5 km for the Altiplano plateau in the central Andes between about 10 and 6 Ma. This rapid rate of uplift has led to suggestions that punctuated uplift resulted from delamination or convective removal of lower lithosphere previously thickened by extensive horizontal shortening. Estimates of total shortening in the central Andes are about 200?350 km. In the northern Andes, paleobotanical paleoaltimetry estimates show a similarly rapid late Cenozoic rise in elevation, although crustal shortening in the Eastern Cordillera of Colombia does not exceed 60 km. Nevertheless, like the central Andes, deformation in the Eastern Cordillera was ongoing since the Eocene, which would suggest a more gradual isostatic uplift history for both the central and northern Andes. Determining whether rapid uplift occurred in the absence of extensive horizontal shortening will shed light on the ongoing debate over the mechanisms of uplift and support of elevated orogens.
Intellectual Merit. This project addressed ongoing debates over the timing of uplift in the Andes Mountains by focusing on the Eastern Cordillera of Colombia, where paleobotanical studies suggest a rapid rise in elevation from <1.25 to 2.5 km 6 to 3 million years (Myr) ago. The project merged geochronology, stratigraphy, petrology, and basin analysis with emerging independent methods designed to assess the timing and rate of surface uplift. Emerging techniques include two new paleoaltimeters employing geochemical signatures preserved in organic-rich sedimentary deposits. The first paleoaltimeter exploits the variation in deuterium isotopes preserved in leaf waxes to reconstruct past elevation based on systematic changes in oxygen and hydrogen isotopes with elevation. The second, the MBT/CBT paleothermometer, utilizes proportions of distinct moites of tetraether lipids produced by bacteria in soils (which are a sensitive function of mean annual temperature) to reconstruct past elevation. The two independent paleoaltimeters have been applied at both high- and low-elevation sites. The project also generated data defining the chronology of basin evolution in the Eastern Cordillera, principally in the Bogota Plateau, where results from magnetostratigraphy and U-Pb geochronology provide absolute age control for late Miocene to Quaternary sedimentation. Field-based analyses provided sedimentological evidence for depositional patterns and evolving trends in the sediment dispersal. These results provide a foundation for the paleoaltimeters, affording an independent chronological context and record of relative topographic variations. Specifically, the magnetostratigraphic results help pinpoint sediment accumulation at 8 to 0.5 Ma. Other results provide additional age control and insights into the provenance and dispersal of sediment. Integration of organic-geochemical paleoaltimetry with new geochronologic, stratigraphic, and compositional findings provide details on the record of surface uplift. The data support initial uplift at 8-6 Myr ago, with accelerated uplift at 4-1 Myr ago. The latter interval of rapid uplift is substantially younger than many other regions of the Andes. This requires variability in uplift processes within the Andes Mountains and renews questions on the range of mechanisms responsible for surface uplift in large mountain belts. Broader Impacts. Methods. This project utilized existing techniques and designed and developed additional field and laboratory methods for careful collection of geochemical and geochronological data, including setting up protocols, processing sample materials, and making lab measurements. Several novel aspects include new methods for paleomagnetic field sampling of poorly consolidated materials, automated analyses of glass-tube cylinders in the paleomagnetics laboratory, installation and maintenance of soil-temperature monitoring stations, and extraction, isolation, and measurement of isotopic and temperature signatures in soil and plant materials from a humid, tropical setting. Personnel and Training. This project afforded training of one postdoctoral associate, one graduate research assistant, and ten undergraduate researchers who received hands-on laboratory and field training in organic geochemistry, paleomagnetism, and sedimentary tectonics laboratories. At UT-Austin, both postdoc Saylor and PhD student Anderson gained valuable research experience as well as an introduction to mentoring with their interactions with the undergraduate students who worked on this project. During the course of this proposal, PI and postdoc Saylor started a tenure-track faculty position at the University of Houston. Graduate assistant Anderson also participated in a 2011 summer school offered by international climate/tectonic researchers at the University of Colorado at Boulder. The 10 undergraduate students involved in the project icluded six research assistants from UT-Austin, two summer REU students (both from non-research universities and one from an underrepresented group), and two field research assistants, from the Universidad de Medellin, Colombia. Partnerships. The project strengthened collaborative ties with international partner Ecopetrol-ICP (Instituto Colombiano del Petróleo), a research laboratory within the national oil company of Colombia. Ecopetrol and UT Austin personnel have been involved in a scientific exchange program designed to provide mutual access to research facilities and local experts in Colombia and the USA. Ecopetrol personnel have helped provide access to restricted sites, helped in obtaining maps and aerial photographs, and offered discussions of local and regional geology, as well as logistical insights regarding safe travel in different parts of the country. Instruction. The PIs teach large undergraduate and graduate classes, where they routinely expose students to hot topics and cutting-edge techniques in the geosciences. These include large-enrollment undergraduate courses for geoscience majors (>100 students/semester) and non-majors (>250-300 students/semester). The PIs include several undergraduate and graduate students as integral parts of their research programs. Outreach. PI Horton has been involved in workshops targeted at prospective and incoming graduate students as part of the Academic English Program administered by the UT-Austin International Office. PI Shanahan has participated in UT events encouraging undergraduate students to discover earth science, including the Research Experience for Undergraduate (REU) Program. Graduate research assistant Anderson served as a mentor for an REU participant working in the stable isotope laboratories. PI Saylor has been engaged in outreach to a local elementary and middle school and has led activities introducing students to sedimentology and tectonics.
