Through the last 60 million years (Ma) there were well-documented climate and habitat changes across North America. Continental and regional scale studies on ungulate (hooved mammals) tooth crown heights in relation to climate and habitat changes have documented an increase in tooth height as environments became more arid to combat a highly abrasive diet of fibrous plants and grit. Based on the relationships between crown height and climate, several studies have used taxon free methods to estimate past precipitation from fossil ungulates. Small mammals have also been shown to adapt to changing habitats, with specializations for life in open habitats appearing millions of years earlier than larger mammals. Utilizing taxon free methods, this study compared the crown heights of rodents and lagomorphs (rabbits and pika) from across North America to climate variables, including both temperature and precipitation. Studying small mammals allows examination of local changes to climate and can be informative of what is to be expected in regional communities if modern warming trends continue. Using ArcMap, modern climate data and mammal faunal lists for 100 random points were compiled and generated across North America. Crown heights of species were categorized and mean crown height for the community was calculated for each site. Linear regressions were used to examine the relationship between crown height and climate variables. Using ArcMap and ordinary kriging, maps were generated of predicted climate for all of North America and compared to known climate data. In general, the North-South gradient of temperature and the Northwest to Southeast gradient of precipitation was captured in predicted maps showing agreement with known climate maps. Regressions were also used to predict climate for 72 well-known fossil sites from the late Eocene (~37 Ma) to Pleistocene (~1.9 Ma) of North America, generating estimates of temperature and precipitation for many sites with no prior climate estimates. Results show strong correlation between rodent and lagomorph crown heights and temperature and precipitation variables. Temperature variables were more strongly correlated to tooth crown heights than precipitation. Overall, a decrease in precipitation and temperature was found across North America from 37 Ma to the present, consistent with prior paleoclimate records from other regional and global proxies. Application to detailed regional records from the Mid Miocene Climatic Optimum (around 15 Ma, during the early Barstovian land mammal age), which was a pronounced period of warming comparable to the warming we see today, showed regional variation in responses to warming. Tooth crown height increased in California and Nebraska before Oregon, showing climate did not change uniformly across North America similarly to what would be expected in modern warming. Using the method proposed in this study, past and future regional climate trends, as well as biotic responses to those changing climate trends can be better understood.

Late Middle Eocene time marks one of the most dynamic periods of the Paleogene in the western interior of North America. Analysis of an extensive, new collection of plant, invertebrate, and vertebrae fossils from the Uinta Formation in the Uinta Basin, south of Myton, Utah, USA, provides evidence of environmental change. Paleobotanical specimens are preserved in late stage Uinta Lake sediments and coarse-grained fluvial sediments which are stratigraphically 650 to 660 m above the Green River Formation. Deposition rates estimates of 18 to 55 cm/kyr, for Uinta Lake sediments in the Uinta Basin suggest a period of 1.18 Ma to 3.69 Ma to deposit the thick section of lacustrine and fluvial sediments that separates the well-documented Green River Flora from this new fossil leaf assemblage, the Wells Draw flora. Prolific invertebrate trace fossils and invertebrates have a preference for areas with high to fluctuating water tables and soil moistures. The trace fossils are similar to traces of extant invertebrates found in temperate to tropical climates. A variety of reptiles, namely eight species of turtles/tortoise, one lizard (first report of Saniwa from the Uinta Formation), and at least two crocodilian species, are indicative of warm-temperate to subtropical climatic zones. Flow direction data derived from sedimentary structures in sandstone channel-fills confirms previous studies which indicate the major sediment source area was to the east with a prevailing westward to northwestward flow direction. The presence of higher elevation plant material within the flora, however, implies possible hydraulic transport from the nearby Uinta Mountain highlands, located north of the basin. The first report of Palmoxylon from the Uinta Formation, included in this report, corroborates the faunal indicators of a tropical climate. Physiognomical analysis of the flora yields a mean annual temperature (MAT) of 16.1◦C and a mean annual precipitation (MAP) of 56 cm. Compared with older Green River flora, these new data suggest a slight drying and cooling trend toward the end of the Eocene. Leaf types also indicate possible seasonality with seasonally dry periods and an overall warm-temperate environment with extreme minima temperatures not much below freezing.

Recent successful drilling operations at Lake El'gygytgyn, NE Russia have recovered sediment cores back to 3.6Ma, representing the longest time-continuous sediment record of past climate change in the terrestrial Arctic. Analysis of the hydrogen isotopic ratio (δD) of specific organic biomarkers allows reconstruction of past hydrological conditions, thereby providing a powerful tool for reconstructing past Arctic climate changes. Compound specific isotopic analysis of sedimentary lipids from this remote basin provides new insights into the climate evolution of the Arctic, capturing the mechanisms and dynamics of the last two glacial-interglacial transitions, potentially enhancing the accuracy of modeled future climate change projections and presenting an opportunity to estimate past polar amplification of climate change. The results of this research document the first continuous, high fidelity continental record of reconstructed δD in precipitation from terrestrial plant leaf waxes in the High Arctic spanning the last 120 ka. The hydrogen isotopic composition of lipid biomarkers were determined from previously obtained Lake El'gygytgyn sediment cores and compared with other multi-proxy evidence of past climate change within the lake basin. The modern isotope hydrology and controls on the δD lipid signal were first established within the El'gygytgyn Basin from modern precipitation, stream and lake waters, ice cover as well as modern vegetation, water column and lake bottom surface sediments in order to provide a modern context to properly constrain and interpret paleoclimatic proxy data. Reconstructed δD records of paleoprecipitation and temperature at Lake El'gygytgyn lead other northern hemisphere climate records (e.g. North Greenland Ice Core Project, NGRIP δ18O records) and are in phase with other continental and Antarctic climate records, suggesting early high northern latitude continental warming prior to established glacial-interglacial transitions. The data set generated here leads to multiple avenues of future work and provides critical insights into Arctic paleoclimate and paleohydrology, contributing to our understanding of high latitude environmental change over geological timescales. Collectively, the results of this dissertation research will provide a context for paleoclimate reconstructions and future organic geochemical and stable isotope analysis. Future application of compound-specific H isotope analyses to long drill cores (recovered in 2009; ~315m of sediment) will potentially provide a quantitative high-resolution record of paleoclimatic and paleoenvironmental changes spanning the last 3.6 Ma.

Speleothems (cave carbonate deposits) have been recognized as a multi-proxy paleoclimate archive. Variations in carbon and oxygen isotopes in speleothems can record past climate changes (e.g., temperature, rainfall and vegetation) under isotopic equilibrium conditions. However, non-climatic noises caused by in-cave processes may affect these stable isotope records under non-equilibrium isotopic conditions. The identification of equilibrium and non-equilibrium isotopic conditions in speleothems is still disputed in the speleothem research community; however, this is a prerequisite for the interpretation of carbon and oxygen isotope records in speleothems as paleoclimate proxies. In this Ph.D. thesis, a series of laboratory experiments under cave-analogue conditions were performed to simulate the formation of speleothems in natural caves. The results of these experiments demonstrate that stable isotope equilibrium in speleothems is achieved under slow carbonate precipitation in pool-like settings (pool carbonates). On the basis of these pool carbonates, equilibrium carbon and oxygen isotope fractionation factors between calcite and water (or DIC for carbon) were determined. Our experiments show larger carbon and oxygen isotope non-equilibrium fractionations between calcite and water (or DIC for carbon) in stalagmite-like settings (fast carbonate precipitate) than those determined in pool-like settings. The flow rate of drip water above the surface of stalagmite appears to control the magnitude of these non-equilibrium isotope effects which increase with decreasing the flow rate. Furthermore, a natural speleothem sample was examined as a paleoclimate archive in this thesis. The growth of a double stalagmite (WS-5d) in Wadi Sannur Cave from the Northeastern Sahara was used to infer the greening of the Sahara (intensive rainfall and vegetation). The U/Th dating in the WS-5d stalagmite suggests that greening conditions extended widely in the Sahara during the interglacial Marine Isotope Stages MIS 5.5, MIS 7.3, and the early MIS 9. Based upon oxygen isotope compositions from the WS-5d, we attributed the source of these greening periods to long-traveling rains from the Atlantic Ocean that were delivered via the West African monsoon system. Our study suggests that the two youngest greening periods were concurrent with the arrival of Homo sapiens in the Levant and an earlier possible change in human population at 244 ka, indicating a key role of the Sahara route in early human dispersal out of Africa. Finally, clumped isotope measurements (Δ47) on carbonate-derived CO2 have been shown to reflect the formation temperature of the carbonate minerals. The absolute Δ47 values of these isotopic measurements seem to be sensitive to the standardization methods (heated CO2 gases and water-equilibrated CO2 gases) that are used to normalize the raw Δ47 measurements. Neither the hypothetical base for the heated CO2 gas standardization method nor the theoretical base for the water-equilibrated CO2 gas standardization method has been experimentally tested. A series of CO2 gases were heated in pre-dehumidified quartz tubes to obtain equilibrium Δ47 values of these CO2 gases at temperature range of 50 – 1100 °C. Consequently, the first experimentally derived Δ47 – T calibration in a CO2 gas phase was proposed. This experimental calibration provides a validated base for the standardization of the raw Δ47 data. Moreover, heating CO2 in a pre-humidified quartz tube enables us to easily prepare a CO2 standard gas of a similar Δ47 value to the CO2 sample (i.e., similar Δ47 correction matrix). This will lead to an improvement in the correction scheme of the carbonate clumped isotope thermometry and reliably adjust the absolute Δ47 scale. / Thesis / Doctor of Philosophy (PhD)

No description available.

No description available.

No description available.

Africa's climate became progressively drier and more variable in the last few million years (e.g., deMenocal, 2004). Of particular interest, is a shift to drier and more variable conditions in the Olorgesailie Formation (Kenya) between 500 and 300 thousand years ago (ka) in which Potts et al. (2018) observed a turnover of ~85% of large-body mammalian fauna to smaller-body related taxa, suggesting that the shift was an evolutionary response to better adapt to the changing climate. However, a hiatus in the Olorgesailie record means that the cause of this faunal shift is still an outstanding question. Here, we analyze Lake Malawi drill core MAL 05–1 (~11ºS, 34ºE) to investigate if a specific climatic event stands out as a possible driver of the dramatic change observed in the East African mammal community. We use organic geochemical proxies including branched glycerol diaklyl glycerol tetraethers (GDGTs), leaf wax carbon and deuterium isotopic records to develop high-resolution temperature, vegetation, and precipitation records, respectively, between 600 and 200 ka. Results show a dramatic and abrupt temperature increase of ~6°C occurring in less than 3000 years during Glacial Termination V, which is the Marine Isotope Stage (MIS) 12 to MIS 11 transition at ~430 ka. Notably, this deglacial warming coincides with enriched leaf wax deuterium isotopic values suggesting a shift to more arid conditions in interglacial MIS 11 than in glacial MIS 12. Results also show another abrupt warming period in which temperature increased ~9°C around MIS 7 (~240 ka). We propose that the major warming and drying during Termination V in East Africa represents a significant abrupt change in the climate of eastern Africa and was a likely driver of the major faunal turnover noted in the Olorgesailie Basin.

Hydroclimate evolution in the northern Caribbean region has been investigated by several studies, but they offer contradicting results and none of them cover a full glacial-interglacial transition to examine orbital climate forcing. Here we present refined U-Th dating and oxygen isotope (δ18O) data from a stalagmite named Katún collected from Rio Secreto cave in the Yucatan Peninsula that discontinuously spans the time interval between 198 thousand years (kyr) before present (BP) to 320 kyr BP. Stalagmite δ18O is interpreted to reflect precipitation amount variability as established by previous studies from this cave. On orbital timescales, we identified marine isotope stages (MIS) 7, 8, and 9 in the Katún stalagmite δ18O record. The average stalagmite δ18O value during MIS 7 and 9 interglacial intervals is ~-4.5‰, suggesting wetter conditions than the glacial MIS 8 with an average δ18O value of ~-3‰. The stalagmite δ18O record exhibits higher amplitude δ18O and thus precipitation variability, and an increase in the frequency and intensity of drought events during the drier glacial interval. The record suggests a strong link between atmospheric greenhouse gases, CO2 andCH4, and precipitation variability in the Caribbean, which affects hydroclimate via changes in tropical Atlantic sea surface and air temperature. We propose that changes in North Atlantic high latitude climate are connected to regional hydroclimate, as evidenced by concurrent Caribbean precipitation and atmospheric methane variability, similar to Greenland D-O events. The broader implication of this study is that human activities may alter the fundamental relationship between Caribbean hydroclimate and greenhouse gases, which have likely sustained tropical wetlands and modulated methane production for millennia.

Calcified skeletons are produced within complex assemblages of proteins and polysaccharides whose roles in mineralization are not well understood. Researchers have long postulated that living organisms utilize the macromolecules of organic matrices to actively guide the formation of crystal structures. The timing and placement of the subsequent minerals that form are most easily controlled during nucleation; however, a physical and chemical picture of how organic functional group chemistry influences the initial stages of nucleation is not yet established. These processes are further complicated by the realization that carbonate biominerals can form by an amorphous to crystalline transformation process, which has prompted the question of how chemical signatures are recorded during mineralization. Investigations of mineralization processes such as the kinetics of nucleation and the transformation of amorphous calcium carbonate (ACC) to crystalline products are critical to building a better understanding of biomineral formation. Only from that fundamental basis can one begin to decipher changes in climate and seawater chemistry over geologic time and by recent anthropogenic effects. This dissertation presents the findings from experimental studies of the thermodynamics and kinetics of multiple mineral formation processes, including nucleation and transformation from an amorphous phase. The kinetics of calcite nucleation onto a suite of high-purity polysaccharide (PS) substrates were quantified under controlled conditions. Nucleation rates were measured as a function of 1) supersaturation extending above and below ACC solubility and 2) ionic strength extending to seawater salinity. These conditions decipher the chemical interactions between the PS substrate, calcite crystal, and solution. These investigations show the energy barrier to calcite formation is regulated by competing interfacial energies between the substrate, crystal, and liquid. The energy barriers to nucleation are PS-specific by a systematic relationship to PS charge density and substrate structure that is rooted in minimization of the competing substrate-crystal and substrate-liquid interfacial energies. The data also suggest ionic strength regulates nucleation barriers through substrate-liquid and crystal-liquid interfacial energetics. In a second experimental study, stable isotope labeling was used to directly probe the transformation pathway. Four processes were considered: dissolution-reprecipitation, solid-state, or combinations of these end member processes. Isotope measurements of calcite crystals that transform from ACC have signatures that are best explained by dissolution-reprecipitation. The extent of isotopic mixing correlates with the amount of ACC transferred and the time to transformation, suggesting the calcite crystals are recording the changing local solution environment during the transformation. These investigations into different mineralization mechanisms build a framework for how functional group chemistries of organic molecules regulate mineralization and the resulting isotopic and elemental signatures in the calcite. This may provide useful insights to interpreting chemical signatures of carbonate biominerals in fossil record and understanding ocean chemistry changes throughout geologic time. / Ph. D.