Browsing by Subject "Speleothem"

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    Changes in seasonal precipitation of East Central North America with connections to global climate.
    (2010-10) Hardt, Benjamin Fulper
    Research on oxygen isotopes in stalagmites collected in West Virginia caves has yielded several new insights into regional climate. Oxygen isotopes most likely represent changes in the mean annual isotopic composition of precipitation, a parameter determined locally by the seasonal distribution of precipitation (Hardt et al., 2010). Holocene samples indicate that summer precipitation represented a greater proportion of annual totals, consistent with lake level results (Shuman and Donnelly, 2006) and climate models (Braconnot et al., 2007; Diffenbaugh et al., 2006). During the Pleistocene, seasonal precipitation varies on precessional timescales, although the phasing appears unusual in that it is in-phase with September insolation rather than June. This offset could be due to several processes, most likely in conjunction with each other. These processes include: changes in Gulf of Mexico sea surface temperatures, partially modulated by glacial meltwater routed through the Mississippi (Oglesby et al., 1989; H Wang et al., 2010); changes in the mean state of the tropical Pacific ocean between to El Nino- and La Nina-like conditions (Timmermann et al., 2007); and a seasonally-lagged sea surface temperature response in the subtropical North Atlantic, which would enhance the anticyclonic circulation of the Bermuda High. These same processes also appear to influence isotopic behavior over the last glacial Termination. During the last glacial maximum, oxygen isotopic composition is enriched, consistent with model results indicating wetter summers near the southern margin of the ice sheet (Bromwich et al., 2005), but inconsistent with a temperature control. Antiphasing in summer precipitation between West Virginia and Florida support a control on seasonal precipitation by changes in the position or size of the Bermuda High (Donders and de Boer et al., 2009). Comparison with speleothems from Northeast Brazil (Cruz et al, 2009) indicates a role for the El Nino Southern Oscillation due to its effect on the position of the Bermuda High (Seager et al., 2005).
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    Evidence of Late Pleistocene Heinrich Stadials in Northeastern Brazil
    (2018-11) Berry, Akemi
    Quaternary paleoclimatology is an ever-expanding field. The last glacial period has been characterized by temporally small-scale periods of warming and cooling. Long-term climate oscillations have been attributed to solar forcing, while the cause of these more frequent oscillations are of greater debate. One such cycle is characterized by periods of extreme cooling that triggered freshwater and iceberg excursions into the North Atlantic. The massive release of freshwater and ice into the North Atlantic is thought to have caused an abrupt global reorganization of the ocean-atmosphere circulation. These extreme cooling events are named Heinrich Stadials. Paleoclimate studies have sought to better understand the climatic response to North Atlantic cooling events across the world. In the case of Heinrich Stadials, understanding the response of the hydroclimate throughout the world, and the relationship between local responses, is essential to further our understanding of Heinrich Stadials and their possible causes. In the ocean-atmospheric circulation changes related to Heinrich Stadials, the position of the Intertropical Convergence Zone (ITCZ) plays a pivotal role. Paleo-records suggest that the cooling in the North Atlantic associated with Heinrich Stadials triggered a southward migration in the ITCZ, a response consistent with altering ocean-atmospheric circulation due to influx hemispheric and ocean cooling. Likewise, a shift in the ITCZ could result in a shift in the location and amount of tropical precipitation. Here, we reconstruct one such paleo-record to continue the global reconstruction of climatic response to Heinrich Stadials. Semi-arid Northeastern Brazil is located south of the modern-day ITCZ position. However, large numbers of speleothems in Toca da Boa Vista (TBV) and Toca da Barriguda (TBR) caves in Northeastern Brazil suggest that there were periods of intense rainfall. One possible source of this past rainfall is a southern migration of the ITCZ. Previous studies have linked short-lived speleothem growth in both caves to Heinrich Stadials (X. Wang et al., 2004; Wendt, 2015). In the following chapters, we present chronologies from 5 distinct periods: 134.8 kyr-130.2 kyr, 72.5 kyr- 71.6 kyr, 66.5 kyr-59.7 kyr,54.9 kyr-54.8 kyr and 48.9 kyr-48.2 kyr. This high-resolution, absolute dated, multi-stalagmite oxygen isotope record considers the hydroclimate of Northeastern Brazil. The main record herein extends through the latter portion of Marine Isotope Stage 4, including HS-6, spanning from 66.4 to 59.7 kyr. We have dated three stalagmites using the U/Th method, with uncertainties in age of average ±0.2 kyr. Growth is continuous in TBV-63 from 62.7 to 59.7 kyr, in TBV-67 from 63.6 to 59.7 kyr and in TBV-34 from 66.4 to 63.0 kyr, with a growth hiatus between 64.8 and 64.0 kyr, within error of Greenland Interstadial (GIS) 18. Immediately following this hiatus, TBV-34 shows an abrupt drop in δ18O values, implying a rapid increase in rainfall likely related to southward migration of the ITCZ associated with stadial conditions in the North Atlantic. The δ18O record of these NE Brazil stalagmites is largely anti-phased with the East Asian Monsoon (EAM). Due to the steady decrease in δ18O values until 59.7 kyr and the correlation with the weak monsoon interval, we infer that rainfall over the cave site steadily increased during the prolonged stadial. An abrupt increase in δ18O values marks the end of growth in these samples and is synchronous within uncertainties with the strengthening EAM at 59.8 kyr, which marks the beginning of GIS-17 and Chinese Interstadial (CIS) 17. The anti-phase relationship between these distant, low-latitude caves supports the hypothesis of a southward migrating ITCZ sensitive to abrupt North Atlantic climate anomalies, and suggests a rapid teleconnection of atmospheric signals during HS-6.
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    Multi-Proxy Evidence Of Southward Migration Of The Intertropical Convergence Zone During Heinrich Stadial 1
    (2023-08) Hansen, Wyatt
    Heinrich event 1 (H1) and its associated stadial period (HS1) resulted in an aridification in the northern hemisphere subtropics and wetter conditions in the Southern Hemisphere subtropics (Deplazes et al.,2013; Wang et al., 2001). Pluvial periods in Northeast (NE) Brazil (Auler et al., 2004) associated with Heinrich Stadials are characterized by a two-phased increase in rainfall amount and a transition to a wetter and more diverse vegetation regime (Dupont et al., 2010) attributed to a southward shift of the Intertropical Convergence Zone (ITCZ). This two-phased structure is well documented from palynological and sediment core records but lacks the high-precision dating required to fully understand the exact timing and duration of the pluvial period. This issue is shared with IPCC models which have a difficult time characterizing the precipitation response in the tropics to modern climate change. By fully understanding past pluvial responses to climate forcings, climate models and mitigation strategies can be improved.Heinrich tuned growth intervals of speleothems in Toca da Barriguda cave in NE Brazil capture past pluvial periods. This study presents the U-Th dated speleothem record, TBR-14, from Toca Da Barriguda in NE Brail during HS1. The main growth interval of TBR-14 extended from 17,029 ±76 years B.P. (before 1950 A.D.) to 15,850 ±90 years B.P. The record consists of oxygen isotopes (δ18O), trace elements, and confocal fluorescence during the pluvial period. The resulting record reveals the timing and structure of the NE Brazil pluvial anomaly associated with HS1 at an unprecedented resolution and precision. δ18O displays a two-phased shift in values related to the latitudinal range of the ITCZ. The oxygen isotope structure is characterized by an initial wet period lasting until 16,166 ± 61 years B.P (-4 to -6 ‰ ?18O), followed by a wetter period ending at 15,850 ± 90 years B.P (-6 to -8 ‰ ?18O). The onset of the wetter period occurred abruptly with a dramatic drop in ?18O (~1.4‰) in two years. The fluorescent banding structure, prior calcite precipitation (PCP) indices, and phosphorus/calcium (P/Ca) ratios indicate that the transition resulted in the wettest period of NE Brazil during HS1. The isotopic drop occurs, within error, at the same time and rate that China shifts abruptly to a drier climate (Treble et al., 2007). On a broader timescale, PCP indices, P/Ca, and confocal banding depict decreased aridity and precipitation changes. These changes are consistent with the ecological bridge hypothesis, which attributes vegetational changes recorded in regional records for the cause of periodic species exchange between the Atlantic and Amazon Rainforests.
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    Quantification of magnetic components in sediments with applications in paleoenvironmental studies.
    (2011-12) Lascu, Ioan
    The present dissertation is a collection of papers investigating the magnetic properties of sediments. The main aim of the work presented here is to study the magnetic characteristics of sedimentary deposits by using a methodology that efficiently quantifies the contributions of various ferrimagnetic components in sediments, and to exemplify how this model can be used to make inferences about past climatic and environmental variability. Magnetic minerals in sediments have long been used as indicators of variability in the factors controlling sediment deposition, and sediment-magnetic properties can be interpreted in terms of the processes controlling the fluxes of various magnetic components. Ferrimagnetic minerals, such as magnetite, are strong magnetically, and tend to dominate the signal from bulk measurements. Two sedimentary ferrimagnetic components that play a major role in shaping the magnetic record with time: a detrital component and a biogenic component. The detrital component of magnetic assemblages probably accounts for the greater proportion of the magnetic signal in many records, and therefore has been the focus of most environmental magnetism studies. The processes that control detrital records are mostly tied to local hydrology, climate, and vegetation cover. However, there is strong evidence that many magnetic assemblages are dominated by autochthonous magnetic particles, which in most cases are produced as a result of direct biologic control. Knowing the contribution of each of these components to the total mass of ferrimagnetic material becomes important when making inferences about past climatic or environmental conditions. The theoretical mixing model devised here using the characteristics of detrital and biogenic end members was tested on lake sediments from Minnesota. The analysis incorporates both spatial and temporal effects on magnetic record. We have investigated the history of sediment flux to Deming Lake, Minnesota, for the past 1000 years. Our results reveal several episodes of reduced precipitation, during which less sediment is mobilized from the catchment by overland flow and runoff. The most prominent episode occurred at the end of the Little Ice Age, indicating that this time period was not only cold but might have been drier than previously thought. The spatial control on sediment-magnetic properties was established via a survey of the magnetic properties of surface sediments from several Minnesota lakes. The magnetic properties are controlled by the competing fluxes of detrital and biogenic particles, according to location in the basin, while the position of the oxic-anoxic interface controls whether biogenic magnetite is formed in the sediment or in the water column, with implications in the preservation of intact versus collapsed bacterial chains. The thesis concludes with an incursion into the magnetic properties of chemical sediments from caves, or speleothems. The magnetic recordings preserved in calcite speleothems hold enormous potential for paleomagnetic and paleoenvironmental reconstructions. Speleothems lock in magnetization instantly, are not affected by post-depositional effects, and can be dated with high precision. The natural remanence in speleothems is carried mainly by magnetite, and the main remanence acquisition mechanism is depositional, through physical alignment of detrital magnetic grains parallel to the Earth's magnetic field. Future speleothem magnetism studies should benefit from increasingly sensitive magnetometers, operating at high spatial resolution, that are able to resolve short-term geomagnetic variability, and characterize events such as geomagnetic excursions at an unprecedented scale.
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    Understanding Tropical South American Rainfall Response to Global Climate Dynamics: A Speleothem Multi-Proxy Approach
    (2024-05) Parmenter, Dylan
    One of the greatest challenges facing climate scientists today is predicting large scale tropical rainfall response to climate change. One of the goals of speleothem paleoclimatology has been focused on using stalagmite oxygen isotopes to reconstruct tropical rainfall response to global climate processes on millennial and orbital timescales, in the hope that proxy enabled models may improve rainfall predictability. In South America, oxygen isotope records in the Andes and Amazon Basin have helped to paint a picture of large-scale rainfall response to glacial/interglacial cycles, greenhouse gasses, and ocean circulation. These studies interpret changes in oxygen isotope composition as reflecting rainout along a give moisture trajectory. While this type of analysis is informative in terms of inferring large-scale rainfall changes, complimentary proxies may help to constrain changes to specific regions, especially in cases where the moisture reaching a given site travels a long distance, or where the moisture source changed over time. In this dissertation, we have extended existing oxygen isotope records in the Eastern Amazon and Central Peruvian Andes deeper in time, with the new Amazon record pushing another 25,000 years into the Last Glacial, and the new Peru record extending another 55,000 years, now covering the entire Last Glacial Period and part of the Last Interglacial. In order to constrain rainfall to these specific regions, we analyzed Metal to Calcium ratios, which can be used as a proxy for local aridity, for both our extended portions of the records and for multiple intervals where only oxygen isotopes were published. We also replicated previously published oxygen isotope ratios with a new sample from the Central Peruvian Andes that grew over a precessional cycle during the Penultimate Glacial, and obtained Metal/Calcium ratios to test our hypotheses further in time. Our multi-proxy findings suggest that the Eastern Amazon oxygen isotope record does in fact reflect regional rainfall, and that high-latitude forcing is the primary control for Amazon rainfall variability during the Last Glacial on millennial timescales. In the Central Peruvian Andes, however, our record indicates that the majority of high-latitude forced millennial scale variability occurred further upstream. The lack of end-member shifts in our carbon isotope records from both regions suggests that vegetation did not undergo any major changes on millennial or orbital timescales over the last glacial/interglacial cycles. On orbital timescales, our records indicate that a rainfall dipole exists between the two regions, controlled by the regional Walker Circulation. During periods of higher orbital variability such as the Holocene, Last Interglacial, and Penultimate Glacial, our results suggest that this circulation pattern is controlled by regional insolation. Over the Last Glacial period when eccentricity was low, however, the pattern appears locked and does not respond to insolation, with sustained rainout in the Peruvian Andes, and higher subsidence causing overall drier conditions in the Eastern Amazon. Possible mechanisms causing this locked regional Walker Circulation include ice volume and greenhouse forcing, the latter of which also seems to have exerted a direct control on Eastern Amazon rainfall across Termination 1 and the MIS 4/3 boundary.