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As anthropogenic emissions of greenhouse gasses continue to alter the Earth's climate, it becomes increasingly vital to understand how the Earth system has responded to high temperatures and pCO2 in the past. The Cenozoic era in particular offers unique insights into climate systems equilibrated with modern to near-future radiative forcing and comparable global geographic boundary conditions. Isotopes in precipitation ([delta] D and [delta] 18O) are one of the more ubiquitous tools used to investigate Cenozoic climate, as they are sensitive to a number of hydro-climatic factors including rainout patterns, terrestrial moisture recycling, evaporative source conditions, and atmospheric mixing by transient eddies. While this sensitivity allows for the potential characterization of large-scale hydrologic dynamics, separating out these disparate effects remains a major challenge for interpreting proxy records. This dissertation aims to address this challenge by using a reactive transport model of isotopes in precipitation to develop a framework for testing hypotheses of past conditions against proxy records. This framework is then applied along with other established methods to investigate a number of Cenozoic climate questions including: (1) migration of the Pacific storm track in response to changes in the structure of tropical Pacific SST's and initiation of Northern Hemisphere glaciation through the Plio-Pleistocene; (2) peak eustatic Pliocene sea level and East Antarctic Ice Sheet stability; and (3) global latent heat transport under Early Eocene hothouse climate conditions.
Our ability to forecast future climate scenarios and manage our water resources requires an understanding of the fluxes of transpired water in forested areas. In addition to quantifying the amounts of water moving through trees, we also need to know where different species of trees get their water and how responsive species are in changing their source water to drought or other climatic factors. The critical zone (CZ), which on hillslopes can be defined as the area between the tree tops and the top of underlying fresh bedrock, has emerged as an important multidisciplinary framework by which to study the flux of water up into trees, or down into runoff generating pathways. A common approach to defining the water sources used by the vegetation employs the use of different stable isotope ratios of water (2H/H and 18O/16O) in both the various reservoirs and in the trees; such information can also be exploited as a hydrologic tracer for any given catchment or in the CZ. This study was conducted at "Rivendell", an intensively instrumented site at the South Fork Eel Critical Zone Observatory. Rivendell spans a north-south divide. The North Slope is an unchanneled 32° hillslope draining to Elder Creek, upstream from the confluence with the South Fork Eel. The South Slope is 25°, and is a large head scarp of a deep-seated landslide draining directly to the South Fork Eel. Soils are thin (10-70 cm), and are underlain by saprolite (a friable, soil-like material of variable thickness with relict rock properties), vertically dipping weathered bedrock of argillite and sandstone above fresh bedrock. The depth of the transition from weathered bedrock to fresh bedrock varied from 6 m near Elder Creek to approximately 26 m at the hillslope divide. The site is within the Coastal Belt of the Franciscan Formation. The climate is Mediterranean, and the bulk of the 1535 mm of annual rainfall that occurred during the study period fell between October and May. I used nearly 2000 samples of natural abundance water isotopes to quantify the variation in potential sources of water used by the vegetation and to trace precipitation inputs through the different materials of the critical zone: soil, saprolite, and weathered bedrock and to follow subsurface mobile waters through the weathered zone to a seasonally dynamic groundwater that then drained to Elder Creek. All runoff occurred as groundwater discharge. Most samples were collected bi-weekly from June 2011-January 2013. The isotopic composition of the shallow soil showed strong seasonally dynamics, shifting from evaporatively enriched (heavier than "average" rainfall) in the summer (detected by having more positive H and O stable isotope ratios) to depleted (lighter than "average" rainfall or the mobile water fraction collected with lysimeters) in the winter (detected by having more negative H and O stable isotope ratios). In contrast, the isotopic composition of the saprolite behaved oppositely: it was lightest in the summer, and shifted toward the average annual rainfall values during the winter, only to return to very light values progressively through the summer. Deep weathered bedrock was always distinct in being isotopically light (negative). Mobile water sampled from lysimeters passed through the soil in winter but due to mixing within the saprolite and weathered bedrock of the critical zone, by the time the water reached the groundwater it was isotopically invariant and appears to have become homogenized such that its value was equal to the average annual precipitation isotope values. As a consequence, the groundwater and resulting Elder Creek flow were also isotopcially nearly invariant. Surprisingly, water held more tightly within the interior of argillite (but not sandstone) is very negative isotopically, falling well to the left of the Local Meteoric Water Line (LMWL)-the precipitation inputs that fell on to the site during the study period. The persistently negative isotopic compositions of critical zone materials suggest the possibility of either inherited paleo-meteoric water or post-precipitation isotope effects could be influencing the isotope values of water at depth and that these post-precipitation isotope effects may be common, yet are largely unaccounted for, in eco-hydrologic applications. Despite a transient isotopic response due to precipitation inputs, the soil moisture, rock moisture, and mobile water have distinct isotopic compositions that lead to a structured heterogeneity that persist over multiple years. Thus, the material properties in the critical zone impart an isotopic 'fingerprint' on their waters that should be traceable into the xylem of vegetation growing on the site. During the study period (2011-2013), I continually sampled tree xylem from 33 Douglas-fir (Pseudotsuga menziesii), 8 madrone (Arbutus menziesii), 16 live oak (Quercus wislizenii), and 5 tanoak (Notholithocarpus densiflora) trees to determine the source water of different species of vegetation growing across the north-south divide. The [delta]D of the Douglas-fir matches that of the saprolite and weathered bedrock moisture, but not the soil. In order to determine the source of Douglas-fir water, we conduced a targeted tracer experiment in which D2O enriched water was slowly added via 12 pipes to the saprolite in early June, 2014. I measured the isotope composition of xylem water of 17 trees and found the "spiked" isotope tracer in two Douglas-fir but not in the hardwoods. This tracer experiment showed that Douglas-firs do use rock moisture. However, the long term monitoring shows that the [delta]18O composition of Douglas-fir xylem water is commonly more positive than rock moisture and no mixing line of observed sources can explain this seemingly deviant heavy [delta]18O. Based on these combined results I hypothesize that the dominant water source used by Douglas-fir xylem may have experienced fractionation in the subsurface before being taken up by the trees. This fractionation may occur in the critical zone due to uptake of rock moisture through mycorrizhae (that are symbiotically connected to Douglas-fir roots), or (less likely) there may be mixing within the bole of the Douglas-fir that leads to a positive [delta]18O offset. If true, the isotope composition of Douglas fir xylem water would not be a reliable indicator of source water. In contrast to the Douglas fir, hardwoods on this site appear to be opportunistic water users. Hardwoods use mobile moisture and bulk soil water in the winter, and the evaporatively- enriched bulk soil moisture, and, to a lesser extent, non-evaporatively enriched saprolite moisture, in the summer. An analysis of an adjacent Douglar fir and madrone tree showed that even the roots from the different trees that crossed each other in the subsurface and the adjacent boles of the trees both in summer and after winter rains held isotopically distinct water. The roots of an individual tree, especially in summer, are isotopically diverse and size dependent, indicating that the bole values record a mixture of sources. South Slope hardwoods may show signs of greater reliance on deeper, non-evaporatively enriched saprolite moisture in late summer. If this finding is supported with further observations it would suggest that these trees compete with Douglas-firs for rock moisture. The use of mobile water by the hardwoods on our study site, the strong summer diurnal oscillations in stream flow in Elder Creek, and the documented rise in stream runoff (including summer baseflow) after clear cut logging in nearby experimental watersheds all demonstrate that forests use water that would otherwise contribute to streamflow. Subsurface isotopic dynamics of the critical zone moisture causes the retained moisture to be isotopically distinct from streamflow, but the mobile that drains to depth and mixes enroute results in a groundwater (and resulting streamflow) that is nearly invariant isotopically (and similar to annual rainfall values). Thus, forests do use water that contributes to runoff and yet can be isotopically distinct from streamflow. These observations challenge the recently proposed concept of only 'two water worlds': a hypothesis that argues that plant available moisture is hydrologically disconnected from stream flow. The forest canopy at our study site and in much of the northern California landscape is mixed hardwoods and Douglas-fir. The opportunistic hardwoods can extract moisture to lower water potentials, giving them access to different water sources than conifers like Douglas-fir can use and thus, in severe drought, may effectively compete with Douglas-fir that prefer, and may require, using water sources that are at a higher water potential. Such a competition needs to be investigated as it matters to future forest composition under drier and warmer future climates in the Northern California Coast Ranges. Collectively, the results of my investigations point toward the importance of material properties in influencing the isotopic composition of the waters they hold, and to vegetative water-source use that is species-specific. The observed isotope effects require future study to determine the mechanisms responsible for the chronically negative rock moisture of the soil and weathered bedrock, the cause of the invariant Douglas-fir xylem water that lacks similarity with any subsurface reservoir, and the possibility of hardwood use of rock moisture and subsequent competition with Douglas-fir trees at some places and at some times in the watershed.
In the 1990s Richard B. Alley and his colleagues made headlines with the discovery that the last ice age came to an abrupt end over a period of only three years. In The Two-Mile Time Machine, Alley tells the fascinating history of global climate changes as revealed by reading the annual rings of ice from cores drilled in Greenland. He explains that humans have experienced an unusually temperate climate compared to the wild fluctuations that characterized most of prehistory. He warns that our comfortable environment could come to an end in a matter of years and tells us what we need to know in order to understand and perhaps overcome climate changes in the future. In a new preface, the author weighs in on whether our understanding of global climate change has altered in the years since the book was first published, what the latest research tells us, and what he is working on next.
Warming has caused widespread changes to the Arctic hydrologic cycle, indicated by sea ice reductions, the Greenland Ice Sheet (GIS) mass loss, and permafrost degradation. Understanding Arctic hydrologic processes is essential for quantifying hydrological responses to climate change. A valuable tool to study these responses is the hydrogen and oxygen isotope ratios of water. Studies presented here aim to both innovatively apply water isotopes with existing understanding, and gain new knowledge in isotope systematics. I present several studies here. First, I show that Arctic precipitation increases with enhanced evaporation due to sea ice reduction; each 100,000 km2 loss in sea ice area increases the fraction of Arctic sourced moisture in total precipitation by 11 to 18%. Second, I argue that vapor sublimated from the GIS significantly contributes to summer precipitation at Summit, Greenland. This conclusion is first supported by isotopic variations in the daily precipitation collected at Summit for three years, and then further verified by 30 annual isotopic cycles in a shallow ice core. The result is not only important for quantifying the current ice sheet mass balance, but also for inferences of paleoclimate from ice cores. Third, I demonstrate that local scale atmospheric circulation in the glacier-free strip of West Greenland is dominated by convergence of dry glacial air masses from the east and moist marine air masses from the west. The dynamics of this convergence are affected by both regional radiation balance differences and broader circulation patterns such as the North Atlantic Oscillation. Humidity variations associated with these air masses control local precipitation and lake evaporation. Finally, along the east-west moisture gradient in West Greenland, lake evaporation also exhibits systematic changes in rate and isotopic enrichment, a result that is important for lake sediment core research. I have made advances in understanding water isotope systematics, mostly related to deuterium excess. In particular, variations in the [delta]D-[delta]18O slope, both for meteoric water and for lake water, are shown to contain interpretable environmental information. I also show that simple equilibrium Rayleigh distillation alters deuterium excess, an effect that was underappreciated in previous work.
'Deep-Sea Sediments' focuses on the sedimentary processes operating within the various modern and ancient deep-sea environments. The chapters track the way of sedimentary particles from continental erosion or production in the marine realm, to transport into the deep sea, to final deposition on the sea floor.
The 20th century has experienced environmental changes that appear to be unprecedented in their rate and magnitude during the Earth's history. For the first time, Stable Isotopes as Indicators of Ecological Change brings together a wide range of perspectives and data that speak directly to the issues of ecological change using stable isotope tracers. The information presented originates from a range of biological and geochemical sources and from research fields within biological, climatological and physical disciplines covering time-scales from days to centuries. Unlike any other reference, editors discuss where isotope data can detect, record, trace and help to interpret environmental change. - Provides researchers with groundbreaking data on how to predict the terrestrial ecosystems response to the ongoing rapid alterations - Reveals how ecosystems have responded to environmental and biotic fluctuations in the past - Includes examples from research by a wide range of biological and physical scientists who are using isotopic records to both detect and interpret environmental change
This two-volume book provides a comprehensive, detailed understanding of paleoclimatology beginning by describing the “proxy data” from which quantitative climate parameters are reconstructed and finally by developing a comprehensive Earth system model able to simulate past climates of the Earth. It compiles contributions from specialists in each field who each have an in-depth knowledge of their particular area of expertise. The first volume is devoted to “Finding, dating and interpreting the evidence”. It describes the different geo-chronological technical methods used in paleoclimatology. Different fields of geosciences such as: stratigraphy, magnetism, dendrochronology, sedimentology, are drawn from and proxy reconstructions from ice sheets, terrestrial (speleothems, lakes, and vegetation) and oceanic data, are used to reconstruct the ancient climates of the Earth. The second volume, entitled “Investigation into ancient climates,” focuses on building comprehensive models of past climate evolution. The chapters are based on understanding the processes driving the evolution of each component of the Earth system (atmosphere, ocean, ice). This volume provides both an analytical understanding of each component using a hierarchy of models (from conceptual to very sophisticated 3D general circulation models) and a synthetic approach incorporating all of these components to explore the evolution of the Earth as a global system. As a whole this book provides the reader with a complete view of data reconstruction and modeling of the climate of the Earth from deep time to present day with even an excursion to include impacts on future climate.
New York : Wiley, c1985.