In order to improve our understanding of poorly studied Mexican Porphyry Copper Deposits in the SW regional metallogenetic province, a detailed study of the hydrothermal fluid evolution of La Caridad porphyry copper-molybdenum deposit, and its connection to a high sulfidation epithermal deposit, was performed using oxygen, hydrogen and sulfur stable isotopes combined with fluid inclusion studies. In addition, UPb and Re-Os geochronology from La Caridad, Milpillas and El Arco porphyry deposit were performed to constrain the timing of mineralization and magmatism in northwest Mexico. Uranium-lead zircon ages from La Caridad suggest a short period of magmatism, between 55.5 and 53.0 Ma. Re-Os molybdenite ages from potassic and phyllic hydrothermal veins yielded identical ages within error, 53.6 ± 0.3 Ma and 53.8 ± 0.3 Ma, respectively. Four stages of hypogene alteration and mineralization are recognized at La Caridad porphyry copper deposit. The isotopic composition of the water in equilibrium with hydrothermal alteration minerals is consistent with highly evaporated lacustrine waters mixed with magmatic waters or vapor separated from magmatic fluids, however, sulfur isotopes and fluid inclusions data support the lacustrine-magmatic water hypothesis. Milpillas porphyry copper deposit in the Cananea Mining District, yielded a crystallization age of 63.9 ± 1.3 Ma. Two Re-Os molybdenite ages yielded an identical age of 63.1 ± 0.4 Ma, Suggesting a restricted period of mineralization. Re-Os data indicate that mineralization in Cananea District, spanned ~4 m.y. in three discrete pulses at ~59 Ma, ~61 Ma and ~63Ma. El Arco porphyry copper deposit, Baja California, Mexico, yielded a Middle Jurassic crystallization age (U-Pb) of 164.7 ± 6.7 Ma and a Re-Os mineralization age of 164.1 ± 0.4 Ma and not ~100 Ma as previously determinated. Porphyry copper deposits in Mexico range in age from 164 Ma to 54 Ma and the mineralization in Sonora state occurred in two different periods, but magmatism overlaps in space and time.

One of the more robust 21st century projections from the most recent Intergovernmental Panel on Climate Change report (IPCC AR4) is a northward shift in the mean location of the Northern Hemisphere storm track. In the western United States, cool-season precipitation provides most of the water for domestic and industrial consumption, irrigation and power generation. In addition, winter precipitation is particularly effective in recharging soil moisture; as a result, it provides a strong control on the productivity of vegetation and on wildfire. Consequently, there is great interest in understanding 1) how changes in the storm track influence regional climate; 2) spatial and temporal variability in its impact; 3) how well general circulation models simulate the regional climate dynamics that bring precipitation to the West; and 4) whether errors in climate simulation might impact assessment of ecological changes. In order to investigate climate change in the western United States associated with shifts in the storm track, I analyzed the relationship between climate and the Northern Annular Mode (NAM). When the storm track is displaced to the north, there is an earlier transition to warm-season circulation patterns and weather conditions. However, the relationship between the winter NAM and climate is not stable over time. Further analysis identified changes in the correlations between the NAM and tree-ring width, precipitation and temperatures associated with changes in the phase of the Atlantic Multidecadal Oscillation and Pacific Decadal Oscillation. Examining 18 of the IPCC AR4 coupled climate models demonstrated cool-season precipitation errors approaching 300% over western North America. These errors are related to difficulties in representing orography, given the coarse resolution of models, and they may influence the quality of precipitation projections into the future. A simple test using the Köppen classification system found that these precipitation errors lead to underestimating the area of the United States in dryland ecosystem types by up to 89% and consequently allowed for much greater expansion of dryland in the future than is actually likely. These studies suggest that the West is likely to experience greater drought in the future. However, no single tool can yet quantify that change.

Volcanic rocks of the Pliocene Barroso formation in southern Peru were collected for a preliminary petrologic study. These volcanic rocks are typically found as aerially extensive 5-25 meter thick lava flows immediately west of Lake Titicaca and are interlayered with sedimentary continental and lacustrine deposits. Regionally, they represent the latest volcanic products in an area where various forms of subduction-related magmatism have generated distinct arc segments since the Cretaceous. Barroso flows are more mafic than previous volcanic products and formed at a time when the main arc was being formed significantly inboard; consequently, it is plausible that they are formed because of secondary processes operating on the plateau and not the main slab dehydration-related melting above the slab. On a TAS diagram the samples are high-K calc-alkaline series while some are shoshonitic and range from trachyte to basaltic trachyandesite. They are mafic to intermediate in silica concentration and have relatively high concentrations of MgO (<7%) and FeO (<9%). Trace elemental concentrations are indicative of a near-adakitic composition. Overall, these petrographic and geochemical characteristics are like other relatively small volume magmatic products found elsewhere on the plateau near local “bobber-type” basins (Ducea et al., 2013; Murray et. al., 2015) and where magmatism was interpreted to be triggered by small scale delamination events or heating overthickened crust. Thermodynamic forward modeling using major element composition suggest that these rocks were generated at pressure/temperature conditions corresponding to the lower part of the lithosphere, perhaps including the lowermost crust. The most likely source rocks are peridotites mixed with various pyroxenite-rich cumulates pre-existent in the crust (also found as rare xenoliths in some Barroso lavas). We speculate that these dense assemblages provide the negative buoyancy responsible for the topographic low represented by Lake Titicaca. The basin will presumably invert when the dense anomaly will detach and founder in the mantle.

Here we analyze climate variability using instrumental, paleoclimate (proxy), and the latest climate model data to understand more about the sources and impacts of low-frequency climate variability. Understanding the drivers of climate variability at interannual to century timescales is important for studies of climate change, including analyses of detection and attribution of climate change impacts. Additionally, correctly modeling the sources and impacts of variability is key to the simulation of abrupt change (Alley et al., 2003) and extended drought (Seager et al., 2005; Pelletier and Turcotte, 1997; Ault et al., 2014). In Appendix A, we employ an Earth system model (GFDL-ESM2M) simulation to study the impacts of a weakening of the Atlantic meridional overturning circulation (AMOC) on the climate of the American Tropics. The AMOC drives some degree of local and global internal low-frequency climate variability (Manabe and Stouffer, 1995; Thornalley et al., 2009) and helps control the position of the tropical rainfall belt (Zhang and Delworth, 2005). We find that a major weakening of the AMOC can cause large-scale temperature, precipitation, and carbon storage changes in Central and South America. Our results suggest that possible future changes in AMOC strength alone will not be sufficient to drive a large-scale dieback of the Amazonian forest, but this key natural ecosystem is sensitive to dry-season length and timing of rainfall (Parsons et al., 2014). In Appendix B, we compare a paleoclimate record of precipitation variability in the Peruvian Amazon to climate model precipitation variability. The paleoclimate (Lake Limón) record indicates that precipitation variability in western Amazonia is ‘red’ (i.e., increasing variability with timescale). By contrast, most state-of-the-art climate models indicate precipitation variability in this region is nearly'‘white' (i.e., equally variability across timescales). This paleo-model disagreement in the overall structure of the variance spectrum has important consequences for the probability of multi-year drought. Our lake record suggests there is a significant background threat of multi-year, and even decade-length, drought in western Amazonia, whereas climate model simulations indicate most droughts likely last no longer than one to three years. These findings suggest climate models may underestimate the future risk of extended drought in this important region. In Appendix C, we expand our analysis of climate variability beyond South America. We use observations, well-constrained tropical paleoclimate, and Earth system model data to examine the overall shape of the climate spectrum across interannual to century frequencies. We find a general agreement among observations and models that temperature variability increases with timescale across most of the globe outside the tropics. However, as compared to paleoclimate records, climate models generate too little low-frequency variability in the tropics (e.g., Laepple and Huybers, 2014). When we compare the shape of the simulated climate spectrum to the spectrum of a simple autoregressive process, we find much of the modeled surface temperature variability in the tropics could be explained by ocean smoothing of weather noise. Importantly, modeled precipitation tends to be similar to white noise across much of the globe. By contrast, paleoclimate records of various types from around the globe indicate that both temperature and precipitation variability should experience much more low-frequency variability than a simple autoregressive or white-noise process. In summary, state-of-the-art climate models generate some degree of dynamically driven low-frequency climate variability, especially at high latitudes. However, the latest climate models, observations, and paleoclimate data provide us with drastically different pictures of the background climate system and its associated risks. This research has important consequences for improving how we simulate climate extremes as we enter a warmer (and often drier) world in the coming centuries; if climate models underestimate low-frequency variability, we will underestimate the risk of future abrupt change and extreme events, such as megadroughts.

I assessed the paleoclimatic significance of SD values of pition pine (Pinus edulis and P. monophylla) cellulose nitrate (cn) by developing, testing and applying deterministic and empirical models, in the context of the soil-plant-atmosphere continuum. Stable isotope values of precipitation, soil water, xylem sap, leaf water, atmospheric vapor, annual and sub-annual samples of tree-ring and needle cellulose, and climatic parameters, were measured along a gradient of decreasing summer rain in the southwestern U.S. Stable isotope composition of sap indicated depth of moisture extraction. Over the growing season in New Mexico and Arizona, where monsoon rains are important, trees shifted their water use to shallower depths. In Nevada, where summer rain is scarce, trees shifted to deeper moisture late in the growing season. Evaporation altered δD and δ¹⁸O values of precipitation inputs to soil. Only after heavy monsoons did soil water and sap isotopically resemble recent precipitation. Average precipitation δD values set the baseline for δD(cn). values at each site, but interannual variations in relative humidity and precipitation amount altered wood and leaf δD(cn) values, via leaf water effects. Leaf water (1w) was evaporatively enriched by seasonal moisture stress. δD(1w). and δ¹⁸O(1w) values were strongly correlated with relative humidity on a seasonal basis, but not on a diurnal basis. Measured δ¹⁸O(1w) values fit a steady-state model, with an offset attributable to relative humidity. Measured δD(1w) values were more depleted than predicted by the model, suggesting leaf water - organic matter isotopic exchange. Biochemical fractionation (ε(B)) of hydrogen isotopes between leaf water and cellulose was inversely correlated with relative humidity. Empirical models based on linear regressions demostrated significant correlations between δD(en) values and precipitation seasonality. An El Nirio-Southern Oscillation signal (wood δD(en) values inversely related to winter precipitation amount) was found in New Mexico and Arizona. A summer rain signal (leaf δD(en) values inversely related to summer humidity) was found at all sites. δD(en) values of pirion needles in packrat middens from Sevilleta LTER, New Mexico, suggest that late Pleistocene summers were as wet as today's, and/or that storm tracks could have shifted, bringing in more tropical moisture than currently.