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Surface-ocean variability in the northern Gulf of Mexico during the late Holocene
Get PDFThe surface waters of the Gulf of Mexico (GOM) are a major moisture source for North America and play an important role in modulating the hydroclimate of the region. Predictions of future changes in surface-ocean variability in the GOM and hydroclimatic changes in response to greenhouse gas forcing must be placed in context of past changes. However, the instrumental record of sea-surface temperature (SST) and salinity (SSS) observations in the GOM is too brief to examine climate variability on multidecadal-to-centennial timescales; thus, proxy records of SST and SSS variability as encoded in marine sedimentary archives must be used to extract information about climate change on these timescales. In this work, I produce a near-decadal-resolution record of SST and SSS variability in the northern GOM over the last 4,400 years. These paleo-records are based on the measurement of the stable isotopic and trace metal composition of planktic foraminifer Globigerinoides ruber (White) shells in a suite of multicores from the Garrison Basin, northern GOM (26º 40.19’N,93º 55.22’W). The fidelity of this proxy is assessed by performing geochemical measurements on in-situ samples from a nearby sediment trap and by performing statistical data-model comparisons with a foraminiferal forward model that can simulate different calcification depth habitats and seasonal productivity. Next, I construct a computational algorithm that characterizes uncertainty in foraminiferal reconstructions including age, analytical, calibration, ecological, sampling, and preservation errors. The utility of this algorithm is shown by applying it to several previously published records. It is also used to place the new Garrison Basin SST and SSS reconstructions in a quantitative uncertainty framework. I diagnose the controls of multidecadal-to-centennial-scale SST and SSS variability in the GOM (and establish its relationship with Atlantic Ocean circulation) by performing correlation analyses using observations, reanalysis datasets, and transient models. Several other marine and terrestrial proxy records spanning the last millennium are synthesized to document a coordinated linkage between Atlantic Ocean circulation and Western Hemisphere precipitation. This work hypothesizes that a century-scale linkage between ocean circulation and precipitation variability occurred over the last millennium, and perhaps the late Holocene, thereby providing a new perspective on long-term climate change.Earth and Planetary Science
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Quantifying changes in the El Niño-Southern Oscillation since the Last Glacial Maximum using climate simulations and fossil corals
Get PDFIncreases in El Niño-Southern Oscillation (ENSO) strength and extreme El Niño frequency since the late 20th century pose a major concern under anthropogenic warming. However, most coral archives of past ENSO extend only to 7,000 years ago, making it difficult to isolate signals forced by changes in background climate states. Here, I present deglacial and Holocene coral oxygen isotope records from Vanuatu and compare them with previously published coral data and new climate model simulations to analyze the ENSO response to changing background climate over the last 21,000 years. Results from both corals and simulations show a decrease in ENSO strength and extreme El Niño frequency 12 and 15 thousand years ago (ka) during the last deglaciation. Contrasts between past and future changes in simulated ENSO suggest exacerbated impacts of extreme El Niño events across the tropical Pacific under anthropogenic forcing. In addition, modeled changes in ENSO amplitude and pattern jointly influence changes in ‘pseudocorals’ (i.e., simulated coral records) at multiple sites across the tropical Pacific over the last 21 ka and into a future world forced by anthropogenic emissions. While ENSO shows large changes in amplitude from glacial times to the future, the model also shows pronounced changes in the spatial pattern of ENSO, particularly for El Niño events. The amplitude effect dominates at most sites; however, amplitude is significantly modulated by the pattern effect outside the central equatorial Pacific. These results provide confidence on the sign of ENSO amplitude changes reconstructed from coral records but call caution for the magnitude of changes for sites away from the central equatorial Pacific. Thus, I introduce a novel framework using pseudocorals to examine the detectability of ENSO strength changes across different scenarios of coral record size. The forced ENSO response is more detectable with longer records, but caution should be drawn regarding the magnitude of internal ENSO variability, which changes in time and space. Future work with tropical Pacific coral records from the data-sparse 21-7-ka interval and multi-model comparisons will further test how well models depict past ENSO, providing a measure of confidence in projections of stronger ENSO in a warmer future.Earth and Planetary Science
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Multidecadal rainfall variability in the South Pacific convergence zone using the geochemistry of stalagmites from the Solomon Islands
Get PDFThe instrumental record of hydroclimate variability in the South Pacific is sparse and of short duration; rainfall observations span on average 60-70 years, albeit, non-continuously. Therefore, proxy records of rainfall variability are needed to extend the hydroclimate record into the pre-instrumental period. In this study, we investigate the multidecadal timescale variability in rainfall using stable isotopic variations in three absolutely dated stalagmites (cave deposits) from the Central Province (CP) and Western Province (WP) of the Solomon Islands (SB) (~9.5°S, ~160°E). Hydroclimate variability in the SB is associated with the position and intensity of rainfall of the South Pacific Convergence Zone (SPCZ). The relatively high temporal resolution of the three stalagmites directs the focus on decadal-scale variability using oxygen isotope (δ18O) variations in the stalagmites, which are a proxy for rainfall variability in the tropics. We compare the three Solomon Island (SB) stalagmite δ18O records with previously published stalagmite δ18O records from Santo, Vanuatu and Guadalcanal, Central Province, Solomon Islands (CPSB) to assess the regional coherency amongst the various proxy rainfall records.
The three new SB stalagmite δ18O records from this study are dominated by abrupt and large amplitude changes (~1-2‰) on multidecadal timescales. Similar patterns of multidecadal stalagmite δ18O variability have previously been identified in the stalagmite δ18O from Guadalcanal (CPSB) and Vanuatu and implies that multidecadal rainfall variability occurred across the South Pacific Ocean over the last millennium. However, the multidecadal changes do not occur in phase with time, which is in part due to age errors, but is also greatly influenced by the different seasonality of rainfall across the sites that bias which months of rainfall change will be recorded by a stalagmite. Our results also show that large changes in δ18O may not necessarily translate to equally large changes in rainfall amount as the modern rainfall - δ18O relationships at the various sites do not have equivalent slopes on multidecadal timescales. After deriving rainfall- δ18O transfer functions for the different sites, we observe that South Pacific stalagmite δ18O records translate to roughly similar amounts of rainfall change on multidecadal time scales for Guadalcanal, CPSB, Suku, CPSB and Santo, Vanuatu. However, Western Province, Solomon Island (WPSB) site appears to have the largest rainfall amount change (~2.0m/year) on multidecadal timescale. We also demonstrate that it is challenging to precisely date decadal and multidecadal scale variability in young stalagmites (past 1000 years) that have very low 238U concentrations and are very short in length. These characteristics lead to larger temporal uncertainty than solely analytical error. This will be of importance to future studies that may overlook the difficulties with young and short stalagmites records with low 238U concentrations.Earth and Planetary Science
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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Hydroclimate changes over the Great Plains since the Last Glacial Maximum
Get PDFThe weather extremes associated with mesoscale convective systems (MCSs) over the Great Plains have substantial socio-economic impacts on the regions’ diverse and growing society. It remains unclear how anthropogenic warming will affect the climate of this region in the future due to large uncertainties in simulated precipitation changes by climate models and a lack of knowledge about the long-term variability of these storms. This dissertation seeks to improve our understanding of the climate dynamics of the Great Plains, focusing on using past changes in MCS activity and hydroclimate to understand their responses to external forcings. To do so, I generate a 20,000-year record of MCS activity and hydroclimate in central Texas using the D/H ratios of sedimentary leaf waxes coupled with trace element analysis and carbon isotope composition of bulk organic matter. To improve our confidence in the interpretation of hydrogen isotope data, I first investigate the meteorological controls on the isotopic composition of precipitation in the Great Plains, which suggests that the isotopic ratios of rainwater mainly reflect the magnitude of the dominating MCS storms over this region, with larger and stronger MCSs producing more negative isotopic values and vice versa. Based on our improved understanding of the regional isotope systematics, our multi-proxy record shows pronounced responses of MCS to external forcings (i.e., retreating ice sheets, solar insolation) on orbital to centennial timescales. Results from transient climate model simulations indicate that variations in the intensity of the Great Plains Low-Level Jet in response to springtime land surface warming can account for these changes in MCS activity. Differential springtime warming over the continent and the North Atlantic drives steeper zonal temperature and pressure gradients over the Great Plains, intensifying the Great Plains Low-Level Jet which consequently enhances the magnitude of Great Plains MCSs via increased moisture availability. These results provide important implications for the potential responses of Great Plains MCS activity to anthropogenic warming, where more pronounced land surface warming relative to the ocean may dynamically intensify future storms with increased risks for extreme weather in the Great PlainsEarth and Planetary Science
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Reconstructing past changes in El Niño-Southern Oscillation variability using geochemical proxies from corals
Get PDFThe El Niño-Southern Oscillation (ENSO) is the leading mode of interannual (>1-9 year) climate variability and has far-reaching impacts on global temperature and rainfall patterns. That said, the instrumental record of ENSO does not have the temporal coverage necessary to characterize the full range of natural variability. Uncertainties about how ENSO variability may change in the future with anthropogenic warming motivates the study of past climate conditions when the Earth experienced different climate conditions compared to the warm, rapidly changing climate of today. Climate scientists use two primary tools to study past climate. First, general circulation models use mathematical equations based on physical principles to simulate different aspects of the climate system, including the ocean, atmosphere, land, and ice sheets. Second, various geological archives like corals, cave stalagmites and lake and ocean sediment also provide clues about how the climate varied in the past. The paleoclimate proxy records generated from geological archives thus provide independent validation of paleoclimate model simulations. Geochemical proxy records from corals are particularly well-suited for investigating past (paleo) ENSO variability as corals can provide decades to hundreds of years of climate information from the tropics at sub-annual resolution.
This dissertation seeks to quantify changes in paleo-ENSO variability using geochemical proxies from corals. I first generate replicated coral records from Vanuatu in the southwest Pacific to investigate the range of ENSO variability during the 20th century and ~900 years during a time interval called the Medieval Climate Anomaly. The paleo-temperature records are based on the ratio of strontium to calcium (Sr/Ca) of massive Porites corals from Vanuatu. Next, I develop various computational algorithms to investigate how different uncertainties inherent to the coral archive impact the ability of a coral to capture changes in ENSO variability. The algorithms incorporate the impact of variable growth rates, analytical and calibration errors, and age model assumptions, and are collectively referred to a coral proxy system model (PSM). The utility of the coral PSM is demonstrated using unforced climate model output for the last millennium. I then use the PSM and apply the analytical techniques developed in the first two projects to investigate simulated and coral proxy-inferred changes in paleo-ENSO variability during the Holocene (11.65 thousand years ago to the present). The resulting model-data comparison provides a new perspective on long-term changes in ENSO variability.Earth and Planetary Science
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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