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We generated records of carbonate clumped isotopes and radiocarbon in deep-sea corals to investigate the role of the deep ocean during rapid climate change events. First we calibrated the carbonate clumped isotope thermometer in modern deep-sea corals. We examined 11 specimens of three species of deep-sea corals and one species of a surface coral spanning a total range in growth temperature of 2-25oC. We find that skeletal carbonate from deep-sea corals shows the same relationship of D47 to temperature as does inorganic calcite. We explore several reasons why the clumped isotope compositions of deep-sea coral skeletons exhibit no evidence of a vital effect despite having large conventional isotopic vital effects. We also used a new dating technique, called the reconnaissance dating method to investigate the ecological response of deep-sea coral communities in the North Atlantic and Southern Ocean to both glaciation and rapid climate change. We find that the deep-sea coral populations of D. dianthus in both the North Atlantic and the Southern Ocean expand at times of rapid climate change. The most important factors for controlling deep-sea coral distributions are likely climatically driven changes in productivity, [O2] and [CO32-]. We take 14 deep-sea corals that we had dated to the Younger Dryas (YD) and Heinrich 1 (H1), two rapid climate change events during the last deglaciation and make U-series dates and measure clumped isotopes in them. We find that temperatures during the YD and H1 are cooler than modern and that H1 exhibits warming with depth. We place our record in the context of atmospheric and marine benthic d13C, D14C, and d18O records during the deglaciation to understand the role of the deep North Atlantic during the deglaciation. We also investigated the role of climate change in the distribution of terrestrial megafauna. To help with this, we also developed a method for compound-specific radiocarbon dating of hydroxyproline extracted from bones in the La Brea Tar Pits. We find that the radiocarbon chronologies of megafauna from several locations around the world, including the La Brea Tar Pits, exhibit an increase in abundance of megafauna during Heinrich events.
The Last Glacial Maximum (LGM; ~23,000-19,000 years ago) and subsequent deglaciation (~19,000-11,000 years ago) represents the last major global climatological transition. In the Western United States, the LGM and deglacial were both characterized by increased effective moisture and expansive lake systems, with most lake growth and maximum lake extents achieved during the deglacial period. In stark contrast, the modern Great Basin is characterized by aridity and low effective moisture. The factors contributing to these large-scale changes in hydroclimates are critical to resolve, given this region is poised to undergo future anthropogenic-forced climate changes with large uncertainties in model simulations for the 21st century. Furthermore, there are ambiguous constraints on the magnitude and even the sign of changes in key hydroclimate variables between the LGM and present-day in both proxy reconstructions and climate model analyses of the Western United States. In this work, I present new stable and clumped isotope data from several ancient lakes, analyze this new data in concert with previously published data, and compare both new and existing results to climate model simulations. Radiocarbon dated samples from ancient lakes constrain lake elevation and the timing of lake level fluctuations. Using a hydrological modeling framework, clumped isotope data constrain several other hydroclimate variables including temperature, precipitation rate, and evaporation rate, which are all used to assess climate model simulations of the same hydrological variables. In Chapter 1, I compile new and existing radiocarbon ages from post-LGM lake basins, and provide an analysis of changing effective moisture through time and space. In Chapter 2, I provide a detailed analysis of our data from one specific basin, Lake Surprise, and provide evidence of evaporation depression as a key driver of lake growth. Finally, in Chapter 3, I use clumped and stable isotope analysis of samples collected across the Great Basin (by UCLA students and others) to provide evidence for spatial and temporal variation in hydroclimate. Concomitant analysis of proxy data and climate model simulations provides a robust means to understand past climate change, and by extension, predict how current hydroclimates may respond to expected future climate forcings.
The analyses of ice drilled in the Greenland ice sheet have revealed the occurrence of rapid climatic instabilities during the last glacial period, known as Dansgaard-Oeschger (DO) events. The imprint of DO events is also recorded at mid to low latitudes in different archives of the northern hemisphere. A detailed multiproxies study of the sequence of these rapid instabilities is essential for understanding the climate mechanisms at play. In this thesis, we combine water isotopes (d18O, d-excess and 17Oexcess) and air in Greenland ic cores (markers of temperature changes d15N, of land productivity d18Oatm, and also concentration and isotopic composition of methane) to quantify regional temperature changes in Greenland, to characterize the reorganization of the hydrological cycle and to better understand the role of the biosphere in climate feedbacks and the sequences of climatic events. Finally, we develop a multiproxies approach to identify in polar ice cores the fingerprint of Heinrich events (layers of ice rafted debris in North Atlantic marine cores, deposited by icebergs melt, caused by the collapse of Northern hemisphere ice sheets). Most of the new data presented in this thesis were obtained from the NEEM ice core, NW Greenland, drilled in 2008-2011 in the frame of an international project led by the Centre for Ice an Climate, Denmark.
Radioactive isotopes can be used in paleoceanography both for dating samples and as tracers of ocean processes. Here I use radiocarbon and uranium series isotopes to investigate the ocean's role in climate change over the last deglaciation. I present a new method for rapid radiocarbon analyses as a means of age-screening deep-sea corals for further study. Based on age survey results, I selected forty corals from the Drake Passage and thirteen from the Reykjanes Ridge off Iceland and dated them with uranium series isotopes. The uranium series dates give independent ages that allow radiocarbon to be used as a tracer of circulation and carbon cycle changes. The radiocarbon records generated from the Drake Passage corals show increased stratification in the Southern Ocean during the last glacial maximum (LGM) that disappeared during the start of the deglaciation as atmospheric CO2 began to rise during Heinrich Stadial 1 (H1). Considering these data and using a simple mass budget calculation, I show that the drop in atmospheric radiocarbon activity during H1 can be explained given direct carbon exchange between the radiocarbon-depleted deep ocean and atmosphere, e.g. through the Southern Ocean. The Drake Passage radiocarbon records also show evidence for decreased air-sea gas exchange in the Southern Ocean during the Antarctic Cold Reversal/Bølling-Allerød coincident with the hiatus in the deglacial CO2 rise. During this time period in the North Atlantic, radiocarbon reconstructions from deep-sea corals collected from off Iceland show a similar ventilation rate to that observed today and during the Holocene. To further investigate changes in North Atlantic ventilation over the last deglaciation, I used an inverse model to assess the consistency of sedimentary 231Pa/230Th ratios from the Holocene, H1, and the LGM with the modern circulation. Although sedimentary 231Pa/230Th has been used to infer changes in the strength of the meridional overturning circulation in the past, I find that published data are consistent with the modern circulation during the LGM and H1. These findings highlight the importance of giving due regard to the uncertainties in the behavior and spatial distribution of paleoceanographic tracers.
Paleoclimate reconstructions can help us learn the evolution of climate mean state and variability in the past, understand mechanisms for climate change, and test the climate models that are extensively used for future climate projections. However, reconstructions have one major disadvantage that usually they are measurements of proxy variables (e.g., oxygen isotopes in calcium shells of foraminifera) instead of climate state variables (e.g., ocean temperature), which are directly simulated in traditional coupled climate models. This require that the model-data comparisons are indirect in paleoclimate studies and making it extremely difficult to address model-data discrepancies, especially when both models and reconstructions are subject to substantial biases and uncertainties. To overcome this obstacle, my PhD work involves developing a fully coupled water isotope-enabled Community Earth System Model (iCESM) in collaboration with other scientists. The physical climate of the iCESM is one of the best state-of-the-art fully coupled earth system models. In addition to the regular hydrologic cycle, iCESM can explicitly simulate the transport and transformation of water isotopes (e.g., H218O) in its components--the atmosphere, land, ocean, sea ice and river runoff. The iCESM can well-reproduce the major features of water isotopes in observations, including the tropical amount effect and high latitude temperature effect, as well as the continental and altitude effects in precipitation-d18Op in present day observations. The simulated d18O in seawater (d18Ow) also closely resemble the pattern in observations. Moreover, a simulation of the LGM (21,000 ka before present) shows that the model is able to simulate the glacial-interglacial changes of d18Op in ice cores, d18Ow in porefluid reconstructions, and d18Oc in ocean sediments, suggesting the model is suited for paleoclimate studies. With the water-isotope capability of the iCESM, I have investigated the following scientific questions: (1) How and why the water isotope-temperature relationship in Greenland ice cores varies during abrupt climate changes; (2) Whether the El Niño-Southern Oscillation (ENSO) was stronger or weaker at the LGM than the present day. (1) Isotope-temperature relationship. For more than 50 years, water isotope values (e.g., d18O) in ice cores have provided a tremendous amount of information about the Earth climate history during the late Quaternary. Initially based on a "modern analogue method", d18O changes in ice cores were directly interpreted as variations in local temperature, which was challenged later by independent reconstructions. Although it is now becoming clear that the temporal d18O-temperature relationship could vary both spatially and temporally in ice-core records, how the temporal slope could vary during abrupt climate changes and what is causing these changes still remain unclear. In my PhD study, I have quantitatively studied the changes in d18O-temperature relationship over Greenland in response to varied climatic forcings using the iCESM. I found that the temporal slope in Greenland increases significantly with the amount of meltwater discharged into the northern North Atlantic Ocean, due to the reduced moisture from the nearby oceans and the tracer effect from depleted meltwater (e.g., about -30 ‰). Otherwise, the d18O-temperature relationships (spatial and temporal) are relatively stable in response to greenhouse gas (GHG), ice sheets and mid-Holocene orbital forcing. It is also found that part of the d18O signal in ice cores during meltwater events can be simply attributed to the tracer effect--the propagation of depleted meltwater in the hydrological cycle--instead of any changes in the climate state. These important findings imply that abrupt temperature changes during meltwater events previously inferred from ice cores could have been significantly overestimated. (2) ENSO variability at the LGM. Despite its paramount importance in climate system, the response of ENSO to anthropogenic global warming is still inconclusive in recent climate models. Studying the ENSO strength in the past can serve as a testbed for these climate models used for future projections and provide us the opportunity to investigate possible relationships between ENSO variability and the mean climate states. But, the ENSO strength at the LGM is inconclusive both in current climate models and paleoclimate reconstructions, including those records using the individual foraminifera analysis (IFA) in the eastern equatorial Pacific (EEP). Here, for the first time, I have directly compared modeled water isotopes in the iCESM with the IFA records. Synthesizing evidence from both models and reconstructions, it is found that ENSO at the LGM is most likely weaker than that of the preindustrial, because of the weakened Bjerknes feedbacks. The iCESM suggests that total variance of the IFA records may only reflect changes in the annual cycle instead of ENSO variability as previously assumed. Furthermore, the interpretation of subsurface IFA records can be substantially complicated by the habitat depth of thermocline-dwelling foraminifera and their vertical migration with a temporally varying thermocline. The model suggests an inverse relationship between ENSO variability and zonal SST gradient, thermocline depth and surface winds in equatorial Pacific, consistent with previous theoretical or observation based studies. Results indicate that ENSO variability could be stronger in response to the future anthropogenic global warming.
Surveys atmospheric, oceanic and cryospheric processes, present and past conditions, and changes in polar environments.
Increasing concerns over future anthropogenic effects on climate change as a result of increasing greenhouse gases generate concomitant efforts to better characterize recent climate in order to more accurately predict climate in the future. To this end, a multiproxy study of climate variability in western Ireland from lacustrine sediment was undertaken. The interpretation of paleoclimate records derived from lacustrine carbonate minerals is difficult without a good understanding of the mechanisms that generate variation in isotope values of modern surface waters. Variation in surface waters are ultimately incorporated into lacustrine sediment records conflated by temperature. Therefore, a study of the spatial distribution of ä18O and äD values of lake and river waters from 144 locations in Ireland has been conducted to provide insight into the behavior of lakes and rivers in Ireland, including source, recycling and loss through evapotranspiration. A 7.6 m sediment core was recovered from Lough Inchiquin that provides evidence for rapid and long-term climate change from the Late Glacial to the Holocene. This was determined using carbon and oxygen isotope analyses of lacustrine calcite as well as carbon from bulk organic sediment fractions. Several significant climate perturbations were identified in the ä18Ocalcite record such as the Oldest Dryas, Younger Dryas, and the 8.2 ka cold event. A previously undescribed climate anomaly between 7,300 to 6,700 cal. yr B.P. characterized by low ä18O calcite values with high frequency variability. Variations in carbon isotopes of calcite and bulk organics from the Late Glacial to the Holocene are significant in magnitude (~12) and have similar trends that record temporal shifts in the relative contributions of carbon from the weathering of limestone versus the weathering of terrestrial organic matter. ä13Ccalcite and ä13Corg
This book honors the career of Professor Elizabeth Gierlowski-Kordesch who was a pioneer and leader in the field of limnogeology since the 1980s. Her work was instrumental in guiding students and professionals in the field until her untimely death in 2016. This collection of chapters was written by her colleagues and students and recognize the important role that Professor Gierlowski-Kordesch had in advancing the field of limnogeology. The chapters show the breadth of her reach as these have been contributed from virtually every continent. This book will be a primary reference for scientists, professionals and graduate students who are interested in the latest advances in limnogeologic processes and basin descriptions in North and South America, Europe, Africa, and China. *Free supplementary material available online for chapters 3,11,12 and 13. Access by searching for the book on link.springer.com
A synthesis of the environmental and climatic history of every major desert and desert margin, for researchers and advanced students.