256 research outputs found
Viabilidade e teste de criopreservação de espermatozoides colhidos da cauda do epidídimo de equinos /
No full textOrientador: Romildo R. WeissCo-orientadores : Ivan Deconto e Luiz Ernandes KozickiDissertação (mestrado) - Universidade Federal do Paraná, Setor de Ciências Agrárias, Programa de Pós-Graduação em Ciencias Veterinárias. Defesa: Curitiba, 2007Inclui bibliografi
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Antarctic Ice Sheet stability during warm periods: Integrating numerical modeling with geologic data
No full textSea level rise is one of the major social and environmental challenges that threatens modern civilization, yet the response of polar ice sheets to future warming is deeply uncertain. Mass loss from the Antarctic Ice Sheet is projected to dominate global sea level rise in the near future, but how much, and when, remains a key unknown. The challenges associated with projecting Antarctica’s future sea level contribution are derived from a knowledge gap of physical ice sheet processes in a world warmer than today, and a lack of understanding of climatic thresholds that drive potentially irreversible retreat. Future and even modern climatic conditions are unprecedented within the last few million years; therefore, we must look to the geologic record for a glimpse of prospective Earth landscapes and climates. Past ‘warm periods’ (characterized by elevated atmospheric CO2 and surface temperatures) can provide a window into the feedbacks and instabilities that govern ice sheet dynamics under a fundamentally different climatic state. In this work, I integrate process-based ice sheet modeling, climate modeling, and remote sensing observations along with geologic data to explore the stability and behavior of the Antarctic Ice Sheet during past warm periods. In Chapter 3, I investigate Antarctic ice sheet and climate evolution during the mid-Miocene, a time period about 17 to 14 million years ago characterized by an epoch of peak global warmth followed by glacial expansion. Coupled ice sheet and climate model scenarios under varying boundary conditions provide continent-wide context for localized geologic paleoclimate and vegetation records. I combine model simulations with geologic constraints to make inferences about past CO2, tectonic uplift, and ice sheet fluctuations across this key time period. Chapter 3 has been published in EPSL (Halberstadt et al., 2021), with coauthors H. K. Chorley, R. H. Levy, T. Naish, R. M. DeConto, E. Gasson, and D. E. Kowalewski. In Chapter 4, I employ a similar modeling approach to address a long-standing data-based discrepancy regarding the stability of the Antarctic Ice Sheet during past warm periods. Marine data reconstruct periodic large-scale marine ice sheet fluctuations since the mid-Miocene (suggesting a dynamic ice sheet response to past increases in temperature and atmospheric CO2) while preserved terrestrial landforms reflect persistent cold conditions (implying that the Antarctic Ice Sheet was largely insensitive during past warm periods). I use high-resolution climate modeling under warm interglacial boundary conditions to reconcile terrestrial persistent cold conditions with receded or collapsed ice sheets during past warm periods. Chapter 4 will be published in Geology, with coauthors D. E. Kowalewski and R. M. DeConto. In Chapter 5, I focus on the modern ‘warm period’ using the satellite observational record. Increased surface meltwater generated on ice shelves fringing the Antarctic Ice Sheet can drive ice shelf collapse, so it is crucial to quantify the historical and current evolution of surface melt to better understand vulnerability of ice shelves. Because in situ observations of surface melt are very sparse given the vast size of Antarctica, I use remotely sensed data to map surface meltwater features from the satellite record. I developed a new methodology to identify meltwater from multispectral satellite images, using Google Earth Engine to train supervised image classification algorithms and identify surface lakes. This work paves the way for automating lake identification at a continental scale throughout the satellite observational record. Chapter 5 is published in Remote Sensing (Halberstadt et al., 2020), with coauthors C. J. Gleason, M. S. Moussavi, A. Pope, L. D. Trusel, and R. M. DeConto.GeosciencesDoctor of Philosophy (Ph.D.
Future climate response to Antarctic Ice Sheet melt caused by anthropogenic warming
No full text© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Sadai, S., Condron, A., DeConto, R., & Pollard, D. Future climate response to Antarctic Ice Sheet melt caused by anthropogenic warming. Science Advances, 6(39), (2020): eaaz1169, doi:10.1126/sciadv.aaz1169.Meltwater and ice discharge from a retreating Antarctic Ice Sheet could have important impacts on future global climate. Here, we report on multi-century (present–2250) climate simulations performed using a coupled numerical model integrated under future greenhouse-gas emission scenarios IPCC RCP4.5 and RCP8.5, with meltwater and ice discharge provided by a dynamic-thermodynamic ice sheet model. Accounting for Antarctic discharge raises subsurface ocean temperatures by >1°C at the ice margin relative to simulations ignoring discharge. In contrast, expanded sea ice and 2° to 10°C cooler surface air and surface ocean temperatures in the Southern Ocean delay the increase of projected global mean anthropogenic warming through 2250. In addition, the projected loss of Arctic winter sea ice and weakening of the Atlantic Meridional Overturning Circulation are delayed by several decades. Our results demonstrate a need to accurately account for meltwater input from ice sheets in order to make confident climate predictions.This research was supported by the NSF Office of Polar Programs through NSF grant 1443347, the Biological and Environmental Research (BER) division of the U.S. Department of Energy through grant DE-SC0019263, the NSF through ICER 1664013, and by a grant to the NASA Sea Level Science Team 80NSSC17K0698
A 40-million-year history of atmospheric CO<sub>2</sub>
No full textThe alkenone–pCO2 methodology has been used to reconstruct the partial pressure of ancient atmospheric carbon dioxide (pCO2) for the past 45 million years of Earth's history (Middle Eocene to Pleistocene epochs). The present long-term CO2 record is a composite of data from multiple ocean localities that express a wide range of oceanographic and algal growth conditions that potentially bias CO2 results. In this study, we present a pCO2 record spanning the past 40 million years from a single marine locality, Ocean Drilling Program Site 925 located in the western equatorial Atlantic Ocean. The trends and absolute values of our new CO2 record site are broadly consistent with previously published multi-site alkenone–CO2 results. However, new pCO2 estimates for the Middle Miocene are notably higher than published records, with average pCO2 concentrations in the range of 400–500 ppm. Our results are generally consistent with recent pCO2 estimates based on boron isotope-pH data and stomatal index records, and suggest that CO2 levels were highest during a period of global warmth associated with the Middle Miocene Climatic Optimum (17–14 million years ago, Ma), followed by a decline in CO2 during the Middle Miocene Climate Transition (approx. 14 Ma). Several relationships remain contrary to expectations. For example, benthic foraminiferal δ18O records suggest a period of deglaciation and/or high-latitude warming during the latest Oligocene (27–23 Ma) that, based on our results, occurred concurrently with a long-term decrease in CO2 levels. Additionally, a large positive δ18O excursion near the Oligocene–Miocene boundary (the Mi-1 event, approx. 23 Ma), assumed to represent a period of glacial advance and retreat on Antarctica, is difficult to explain by our CO2 record alone given what is known of Antarctic ice sheet history and the strong hysteresis of the East Antarctic Ice Sheet once it has grown to continental dimensions. We also demonstrate that in the Neogene with low CO2 levels, algal carbon concentrating mechanisms and spontaneous biocarbonate–CO2 conversions are likely to play a more important role in algal carbon fixation, which provides a potential bias to the alkenone–pCO2 method
Past extreme warming events linked to massive carbon release from thawing permafrost
No full textBetween about 55.5 and 52 million years ago, Earth experienced a
series of sudden and extreme global warming events (hyperthermals)
superimposed on a long-term warming trend1. The first and largest
of these events, the Palaeocene–Eocene Thermal Maximum (PETM),
is characterized by a massive input of carbon, ocean acidification2
and an increase in global temperature of about 5 6C within a few
thousand years3. Although various explanations for the PETM have
been proposed4–6, a satisfactory model that accounts for the source,
magnitude and timing of carbon release at the PETM and successive
hyperthermals remains elusive. Here we use a new astronomically
calibrated cyclostratigraphic record from central Italy7 to show that
the Early Eocene hyperthermals occurred during orbits with a com-
bination of high eccentricity and high obliquity. Corresponding
climate–ecosystem–soil simulations accounting for rising concen-
trations of background greenhouse gases8 and orbital forcing show
that the magnitude and timing of the PETM and subsequent
hyperthermals can be explained by the orbitally triggered de-
composition of soil organic carbon in circum-Arctic and
Antarctic terrestrial permafrost. This massive carbon reservoir
had the potential to repeatedly release thousands of petagrams
(1015 grams) of carbon to the atmosphere–ocean system, once a
long-term warming threshold had been reached just before the
PETM. Replenishment of permafrost soil carbon stocks following
peak warming probably contributed to the rapid recovery from each
event9, while providing a sensitive carbon reservoir for the next
hyperthermal10. As background temperatures continued to rise
following the PETM, the areal extent of permafrost steadily
declined, resulting in an incrementally smaller available carbon
pool and smaller hyperthermals at each successive orbital forcing
maximum. A mechanism linking Earth’s orbital properties with
release of soil carbon from permafrost provides a unifying model
accounting for the salient features of the hyperthermals
Mountain uplift and the glaciation of North America – a sensitivity study
No full textThe Miocene (24 to 5 million years ago) wasa period of relative global warmth compared to the Quaternary(2 million years ago to present; e.g. Zachos et al.,2001) and was characterised by the intermittent glaciationof Antarctica only. Paradoxically, the majority of availableproxy data suggest that during the Miocene, pCO2 was similar,or even lower, than the pre-industrial levels (280 ppmv;Pagani et al., 1999; Pearson and Palmer, 2000; K¨urschneret al., 1996, 2008) and at times probably crossed the modelledthreshold value required for sustained glaciation in theNorthern Hemisphere (DeConto et al., 2008). Records ofice rafted debris and the oxygen isotope composition of benthicforaminifera suggest that at several times over the last25 million years substantial amounts of continental ice didbuild up in the Northern Hemisphere but none of these ledto prolonged glaciation. In this contribution, we review evidencethat suggests that in the Miocene the North AmericanCordillera was, at least in parts, considerably lower than today.We present new GCM simulations that imply that smallamounts of uplift of the North American Cordillera resultin significant cooling of the northern North American Continent.Offline ice sheet modelling, driven by these GCMoutputs, suggests that with a reduced topography, inceptionof the Cordilleran ice sheet is prohibited. This suggests thatuplift of the North American Cordillera in the Late Miocenemay have played an important role in priming the climate forthe intensification of Northern Hemisphere glaciation in theLate Pliocene
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Model data for 'The Paris Agreement and climate justice: inequitable impacts of sea level rise associated with temperature targets'
No full textThis is the data repository associated with the manuscript "The Paris Agreement and climate justice: inequitable impacts of sea level rise associated with temperature targets". The data contained here is related to the sea level rise fingerprints generated for the study. These include the projected sea level rise fingerprint data at years 2100, 2200, and 2300 under emissions scenarios RCP4.5 and RCP8.5. For RCP4.5 data are from ice sheet simulations which include marine ice sheet instability. For RCP8.5 data is presented for two scenarios- one which includes only marine ice sheet instability and one that includes both marine ice sheet instability as well as marine ice cliff instability. Details about the content of the data repository can be found in the readme.txt file located in the repository.National Science Foundation grant 166401
Bedrock erosion surfaces record former East Antarctic Ice Sheet extent
No full textEast Antarctica hosts large subglacial basins into which the East Antarctic Ice Sheet (EAIS) likely retreated during past warmer climates. However, the extent of retreat remains poorly constrained, making quantifying past and predicted future contributions to global sea level rise from these marine basins challenging. Geomorphological analysis and flexural modeling within the Wilkes Subglacial Basin is used to reconstruct the ice margin during warm intervals of the Oligocene–Miocene. Flat‐lying bedrock plateaus are indicative of an ice sheet margin positioned >400–500 km inland of the modern grounding zone for extended periods of the Oligocene–Miocene, equivalent to a 2 meter rise in global sea level. Our findings imply that if major EAIS retreat occurs in the future, isostatic rebound will enable the plateau surfaces to act as seeding points for extensive ice rises, thus limiting extensive ice margin retreat of the scale seen during the early EAIS
Working Group Report: Chemistry. Synthesis and Modelling Project, Ocean Biogeochemical Response to Climate Change, Durham, NH, 13-17 July 1998
No full textPotential seaways across West Antarctica
No full textThe West Antarctic ice sheet (WAIS) has long been considered vulnerable to rapid retreat and today parts are rapidly losing ice. Projection of future change in WAIS is, however, hampered by our poor understanding of past changes, especially during interglacial periods that could be analogs for the future, but which undoubtedly provide an opportunity for testing predictive models. We consider how ice-loss would open seaways across WAIS; these would likely alter Southern Ocean circulation and climate, and would broadly define the de-glacial state, but they may also have left evidence of their existence in the coastal seas they once connected. We show the most likely routes for such seaways, and that a direct seaway between Weddell and Ross seas, which did not pass through the Amundsen Sea sector, is unlikely. Continued ice-loss at present rates would open seaways between Amundsen and Weddell seas (A-W), and Amundsen and Bellingshausen seas (A-B), in around one thousand years. This timescale indicates potential future vulnerability, but also suggests seaways may have opened in recent interglacial periods. We attempt to test this hypothesis using contemporary bryozoan species assemblages around Antarctica, concluding that anomalously high similarity in assemblages in the Weddell and Amundsen seas supports recent migration through A-W. Other authors have suggested opening of seaways last occurred during Marine Isotope Stage 7a (209 ka BP), but we conclude that opening could have occurred in MIS 5e (100 ka BP) when Antarctica was warmer than present and likely contributed to global sea levels higher than today
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