Graduate Thesis Or Dissertation
A Refinement of Biomarker-Based Tools to Study The Pliocene-Pleistocene Climate Evolution of The Northern Tropical Andes Public Deposited
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I present quantitative Pliocene-Pleistocene terrestrial tropical temperature estimates along with the refinement of organic and stable isotope geochemical proxies in the northern tropical Andes of Colombia. The Pliocene epoch, 5 to ~2.6 million years ago (Ma), is often cited as the last time when Earth’s mean temperature was ~2.5-4°C warmer than today, and CO2 concentrations may have been higher than preindustrial levels. Although the tropics play an important role in regulating global climate, a recent summary of Pliocene temperature terrestrial records includes no site within 10º of the equator. The Sabana de Bogotá in the Eastern Cordillera of Colombia offers unique sedimentary archives from the tropics (~4ºN), including sediment from an extinct lake preserved in the Funza-II core that dates back to late Pliocene, which allows an opportunity to apply quantitative geochemical proxies for temperature reconstructions.
Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are bacterial cell membrane lipids that, when preserved in sedimentary archives, can be used to infer continental paleotemperatures. I present an in situ regional calibration of soil brGDGTs along altitudinal transects on both flanks of the Eastern Cordillera of Colombia that spans ~3,200 m in elevation for soil and air temperatures. These calibrations yield RMSEs of 1.5°C and 1.9°C, respectively, and allow for more precise and reliable reconstructions of past temperatures in the tropics than global calibrations. Along with refining brGDGTs in the northern tropical Andes, I also evaluate the efficacy of stable isotopes of precipitation and plant waxes as proxies for paleoaltimetry studies in the region. I use monthly hydrogen (δ2Hp) and oxygen (δ18Op) isotope values of precipitation for an annual cycle, as well as hydrogen isotope values of plant waxes (δ2Hwax) in top soils along the same elevation transects as for the brGDGTs calibrations. The δ2Hwax values along the eastern flank of the Eastern Cordillera follow a simple Rayleigh distillation, with the average δ2Hwax values of n-C29, n-C31, and n-C33 alkanes showing an R2 = 0.65 when regressed against elevation. In contrast, because of the lack of correlation with elevation in modern precipitation on the western flank, neither δ2Hp nor δ18Op, and therefore δ2Hwax, offer reliable estimates of past elevations.
Regressions of surface temperatures in the Eastern Cordillera of Colombia with sea-surface temperatures (SSTs) in the equatorial Pacific show that the Eastern Cordillera warms or cools by half of the amplitude of the variation of SSTs in the eastern Tropical Pacific. Because Pliocene SSTs in the eastern Tropical Pacific resemble those during major El Niño events, when SSTs warm by ~4°C, the Pliocene Eastern Cordillera warms by ~2ºC at both high and low elevations. To evaluate how temperature changed in the Sabana de Bogotá during the Pliocene-Pleistocene, I estimated brGDGTs-based temperatures in the Funza-II core. New geochronology based on zircon U-Pb dates from ash layers place the base of the core at around ~4 Ma. I show that Pliocene temperatures were ~2.2 ± 2.0°C warmer than mid-late Pleistocene temperatures. This ~2°C warm in temperature could be explained by a permanent El Niño-like teleconnection to the Eastern Cordillera of Colombia, rather than a pantropical change in temperature. These temperature estimates are the only terrestrial tropical record within 5° of the equator for Pliocene time.
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