Paul Baker - Academia.edu (original) (raw)
Papers by Paul Baker
The conventional high pressure diamond anvils were modified by growing an isotopically pure C-13 ... more The conventional high pressure diamond anvils were modified by growing an isotopically pure C-13 diamond layer by microwave plasma chemical vapor deposition using methane/hydrogen/oxygen chemistry. The isotopically pure C-13 nature of the culet of the diamond anvil was confirmed by the Raman spectroscopy measurements. This isotopically engineered diamond anvil was used against a natural abundance diamond anvil for high pressure experiments in a diamond anvil cell. Spatial resolved Raman spectroscopy was used to measure the stress induced shift in the C-13 layer as well as the undelying C-12 layer to ultra high pressures. The observed shift and splitiing of the diamond first order Raman spectrum was correlated with the stress distribution in the diamond anvil cell. The experimental results will be compared with the finite element modeling results using NIKE-2D software in order to create a mathematical relationship between sets of the following parameters: vertical (z axis) distance; horizontal (r axis) distance; max shear stress, and pressure. The isotopically enriched diamond anvils offer unique opportunities to measure stress distribution in the diamond anvil cell devices.
Carbonate cements and nodules from hydrothermal sediments of Middle Valley were analyzed for thei... more Carbonate cements and nodules from hydrothermal sediments of Middle Valley were analyzed for their mineralogical, chemical, and isotopic compositions. These diagenetic carbonates include calcite, high-magnesium calcite, and dolomite. The chemical sources for carbonate precipitation include alteration of basement rocks (calcium and strontium), diffusion from seawater (magnesium), recrystallization of biogenic calcite (calcium and carbon), oxidation of sedimentary organic matter (carbon), and
Nature, 2001
Tropical South America is one of the three main centers of the global, zonal overturning circulat... more Tropical South America is one of the three main centers of the global, zonal overturning circulation of the equatorial atmosphere (generally termed the "Walker" circulation 1 ). Although this area plays a key role in global climate cycles, little is known about South American climate history. Here we describe sediment cores and down-hole logging results of deep drilling in the Salar de Uyuni, on the Bolivian Altiplano, located in the tropical Andes. We demonstrate that during the past 50,000 years the Altiplano underwent important changes in effective moisture at both orbital (20,000-year) and millennial timescales. Long-duration wet periods, such as the Last Glacial Maximummarked in the drill core by continuous deposition of lacustrine sediments-appear to have occurred in phase with summer insolation maxima produced by the Earth's precessional cycle. Short-duration, millennial events correlate well with North Atlantic cold events, including Heinrich events 1 and 2, as well as the Younger Dryas episode. At both millennial and orbital timescales, cold sea surface temperatures in the high-latitude North Atlantic were coeval with wet conditions in tropical South America, suggesting a common forcing.
Climatic Change, 2002
Sediment cores from Lake Titicaca contain proxy records of past lake level and hydrologic change ... more Sediment cores from Lake Titicaca contain proxy records of past lake level and hydrologic change on the South American Altiplano. Large downcore shifts in the isotopic composition of organic carbon, C/N, wt. %C org , %CaCO 3 , and % biogenic silica illustrate the dynamic changes in lake level that occurred during the past 20,000 years. The first cores taken from water depths greater than 50 meters in the northern subbasin of the lake are used to develop and extend the paleolake-level record back to the Last Glacial Maximum (LGM). Quantitative estimates of lake level are developed using transfer functions based on the δ 13 C of modern lacustrine organic sources and the δ 13 C of modern sedimented organic matter from core-tops. Lake level was slightly higher than modern during much of the post-LGM (20,000-13,500 yr BP) and lake water was fresh under the associated outflow conditions. The Pleistocene/Holocene transition (13,500-7,500 yr BP) was a period of gradual regression, punctuated by minor trangressions. Following a brief highstand at about 7250 yr BP, lake level dropped rapidly to 85 m below the modern level, reaching maximum lowstand conditions by 6250 yr BP. Lake level increased rapidly between 5000 yr BP and 4000 yr BP, and less rapidly between 4000 yr BP and 1500 yr BP. Lake level remained relatively high throughout the latest Holocene with only minor fluctuations (<12 meters). Orbitally induced changes in solar insolation, coupled with long-term changes in El Niño-Southern Oscillation variability, are the most likely driving forces behind millennial-scale shifts in lake level that reflect regional-scale changes in the moisture balance of the Atlantic-Amazon-Altiplano hydrologic system. Glossary 1. Mesotrophic: Moderately productive; relating to the moderate fertility of a lake in terms of its algal biomass.
Climatic Change, 2002
Sediment cores from Lake Titicaca contain proxy records of past lake level and hydrologic change ... more Sediment cores from Lake Titicaca contain proxy records of past lake level and hydrologic change on the South American Altiplano. Large downcore shifts in the isotopic composition of organic carbon, C/N, wt. %C org , %CaCO 3 , and % biogenic silica illustrate the dynamic changes in lake level that occurred during the past 20,000 years. The first cores taken from water depths greater than 50 meters in the northern subbasin of the lake are used to develop and extend the paleolake-level record back to the Last Glacial Maximum (LGM). Quantitative estimates of lake level are developed using transfer functions based on the δ 13 C of modern lacustrine organic sources and the δ 13 C of modern sedimented organic matter from core-tops. Lake level was slightly higher than modern during much of the post-LGM (20,000-13,500 yr BP) and lake water was fresh under the associated outflow conditions. The Pleistocene/Holocene transition (13,500-7,500 yr BP) was a period of gradual regression, punctuated by minor trangressions. Following a brief highstand at about 7250 yr BP, lake level dropped rapidly to 85 m below the modern level, reaching maximum lowstand conditions by 6250 yr BP. Lake level increased rapidly between 5000 yr BP and 4000 yr BP, and less rapidly between 4000 yr BP and 1500 yr BP. Lake level remained relatively high throughout the latest Holocene with only minor fluctuations (<12 meters). Orbitally induced changes in solar insolation, coupled with long-term changes in El Niño-Southern Oscillation variability, are the most likely driving forces behind millennial-scale shifts in lake level that reflect regional-scale changes in the moisture balance of the Atlantic-Amazon-Altiplano hydrologic system. Glossary 1. Mesotrophic: Moderately productive; relating to the moderate fertility of a lake in terms of its algal biomass.
On orbital timescales, lacustrine sediment records in the tropical central Andes show massive cha... more On orbital timescales, lacustrine sediment records in the tropical central Andes show massive changes in lake level due to mechanisms related to global-scale drivers, varying at precessional timescales. Here we use stable isotopic and diatom records from two lakes in the Lake Titicaca drainage basin to reconstruct multi- decadal to millennial scale precipitation variability during the last 7000 to 8000 years. The records are tightly coupled at multi-decadal to millennial scales with each other and with lake-level fluctuations in Lake Titicaca, indicating that the lakes are recording a regional climate signal. A quantitative reconstruction of precipitation from stable isotopic data indicates that the central Andes underwent significant wet to dry alternations at multi- centennial frequencies with an amplitude of 30 to 40% of total precipitation. A strong millennial-scale component, similar in duration to periods of increased ice rafted debris flux in the North Atlantic, is observed in both lake records, suggesting that tropical North Atlantic sea-surface temperature (SST) variability may partly control regional precipitation. No clear relationship is evident between these records and the inferred ENSO history from Lago Pallcacocha in the northern tropical Andes. In the instrumental period, regional precipitation variability on inter-annual timescales is clearly influenced by Pacific modes; for example, most El Ninos produce dry and warm conditions in this part of the central Andes. However, on longer timescales, the control of tropical Pacific modes is less clear. Our reconstructions suggest that the cold intervals of the Holocene Bond events are periods of increased precipitation in the central Andes, thus indicating an anti-phasing of precipitation variation in the southern tropics of South America relative to the Northern Hemisphere monsoon region.
ieeexplore.ieee.org
Page 1. xv TAIC PART 2007 Programme Committee ŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽ ŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽ... more Page 1. xv TAIC PART 2007 Programme Committee ŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽ ŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽ Academic Members •Giulio Antoniol, University of Sannio, Italy •Kirill Bogdanov, University of Sheffield, UK •David Binkley, Loyola College Maryland,USA •Jonathan Bowen, London South Bank University, UK •Myra Cohen, University of Nebraska-Lincoln, USA •John Clark, University of York, UK •Mireille Ducassé ...
Geophysical Monograph, 2002
Extending the work of Cadot, Kaesler, De Deckker, Chivas, and Corrège, we have measured the eleme... more Extending the work of Cadot, Kaesler, De Deckker, Chivas, and Corrège, we have measured the elemental chemistry of shells of marine ostracodes to evaluate the usefulness of ostracode shell chemistry as a paleoenvironmental proxy. Our work has focused primarily on Mg/Ca, Sr/Ca, and Na/Ca ratios of two common genera: deep-sea genus Krithe and shallow marine/estuarine genus Loxoconcha. We evaluated in vivo effects including genus, species, gender, ontogeny, shell size, intra-shell heterogeneity (Mg), water temperature, and salinity, and postmortem diagenetic effects including partial dissolution, recrystallization, and shell surface contamination. Analysis of modern (core-top), fossil, and laboratory-raised specimens across a wide range of temperature and salinity conditions confirms earlier work indicating that Krithe and Loxoconcha Mg/Ca ratios are dominantly controlled by water temperature. Sr/Ca and Na/Ca ratios co-vary with temperature in core-top Krithe, but not in cultured Loxoconcha suggesting that the Krithe Sr/Ca and Na/Ca correlation with temperature may be related to another variable that broadly co-varies with temperature. Phylogenetic and ontogenetic effects are also indicated, including different Mg-thermodependence and intra-shell Mg distribution between Krithe and Loxoconcha. Inter-specific effects are suggested for two species of Krithe. Magnesium uptake in eldest juvenile shells seems to be identical to that of adult shells, thus greatly increasing the amount of shell material available for paleoenvironmental studies. No salinity effects were observed. Shell Na/Ca ratios showed a dramatic decrease with increasing dissolution (natural and artificial) in waters that are undersaturated with respect to calcite, whereas Mg/Ca ratios displayed a minor decrease and Sr/Ca ratios showed no change. Of the ratios studied, Mg/Ca offers the most promise for Quaternary marine studies as a paleothermometer. Further calibration studies are needed to better understand the marine ostracode Mg/Ca-paleothermometer.
Geochim Cosmochim Acta, 1976
Systematic variations in the isotopic composition of skeletal carbonate deposited by the Caribbea... more Systematic variations in the isotopic composition of skeletal carbonate deposited by the Caribbean reef-frame building coral Montastrea annularis are correlated with water depth, location of the corallites within the corallum, and polyp packing density, as is demonstrated by isotope ratio measurements for 426 samples collected at 4.6 m depth intervals between 0 and 27.4 m at St. Croix, U.S. Virgin Islands. These data support a model, based on a study of Indo-Pacific scleractinians, proposed earlier for stable isotope fractionation by corals. Of particular interest is the fact that, within this species, ecotypic differentiation into shallow-water and deep-water subpopulations, with a boundary close to 20m, is reflected by changes in skeletal ° 13C. Stable isotope geochemical studies of both modern and fossil coral-derived carbonate may contribute to the solution of several problems having geologic and paleontologic significance.
Spe Formation Evaluation, 1986
The Sechura (approx.6°S) and Pisco (approx.14°S) Basins of onshore Peru contain Miocene diatom ... more The Sechura (approx.6°S) and Pisco (approx.14°S) Basins of onshore Peru contain Miocene diatom and phosphate rich sediments which sharply contrast with underlying clastics. In the Sechura Basin the Miocene clastic Mancora, Heath and Montera Formations are overlain by the Zapallal Formation which grades upwards from a weakly biogenic base into fairly pure diatomites (biogenic silica >20%) and ore grade phosphorite (PâOâ>20%). Biogeneic content decreases in the eastern basin as clastic content increases. The base of the Zapallal Formation has been dated at between 12.2 and 14.0 mybp using radiolaria correlated to magnetic stratigraphy by Theyer, et al (1978), and the phosphatic section yields dates of between 8.0 and 11.2 my. In the Pisco Basin the Eocene clastic Paracas Formation is overlain unconformably by the Miocene Pisco Formation which contains a basal sequence of cross-bedded clastics, tuffs, and partially recrystallized diatomites; a phosphorite bearing middle sequence; and a diatom rich top. Current direction from the cross beds of the basal Pisco Formation indicate a generally southerly transport direction but with considerable directional variability. This may be related to the Peru under-current which shows highly variable strength and direction near 15°S.
Metal to insulator transitions can be effectively studied by application of pressure, specificall... more Metal to insulator transitions can be effectively studied by application of pressure, specifically in the case of the transition metal oxides. MnO is a Mott insulator at ambient conditions since splitting of the 3d-band, caused by Coulomb repulsion between electrons, is much larger than the 3d-bandwidth, resulting in correlation-induced localization of the 3d electrons. The width of the 3d subband, however, can be directly controlled with pressure. As pressure increases, bandwidth also increases until some critical value at which electron correlation effects are no longer dominant, and the electrons become delocalized. We use "designer" diamond anvils to measure the insulator to metal transition in MnO, which is complete by 106 GPa. Metallization is also confirmed by examining the temperature dependence of the resistance before and after the transition. We find the insulator to metal transition agrees well with structural transition data in the literature. Results on CoO will be presented as available.
Using microwave plasma-enhanced chemical vapor deposition (CVD), thin films of nano-crystalline d... more Using microwave plasma-enhanced chemical vapor deposition (CVD), thin films of nano-crystalline diamond were grown on 7 mm diameter polished titanium-alloy substrates. The films were monitored in situ by a laser reflectance interferometry (LRI) technique, in which both scattered and reflected laser light intensities were collected from the sample. A total of eight experiments were conducted. The gas flow rates were 499 sccm of hydrogen, 88 sccm of methane, and 8.8 sccm of nitrogen; average temperature during CVD was 830 degrees C. The periodic oscillatory patterns in the scattered and the reflected light intensities were analyzed for growth rate information about the diamond film. This information allowed diamond films of a controlled thickness to be grown. Ellipsometry measurements confirmed that these films ranged from approximately 100 nm to 500 nm in thickness.
Low fluorescence and stress free designer diamonds are needed in high pressure experimentation. N... more Low fluorescence and stress free designer diamonds are needed in high pressure experimentation. Nitrogen related defects in designer diamond anvils were studied by the low temperature photoluminescence (PL) spectroscopy at 80 K. Designer diamonds containing embedded micro-circuits were fabricated by the microwave plasma chemical vapor deposition method using a methane/oxygen/hydrogen chemistry at a substrate temperature of 1485 K. PL excitation in designer diamonds with a 514.5 nm laser shows the existence of zero phonon lines at 1.945 eV and 2.16 eV corresponding to nitrogen-vacancy pair and interstitial nitrogen-vacancy pair defects, respectively. Spatial distribution of this PL emission also shows significantly enhanced incorporation of nitrogen related defects in non (100) facets of designer diamonds. Subsequent annealing of designer diamonds at 1573 K in a pure hydrogen plasma led to a reduction of 2.16 eV defect center. An enhanced nitrogen incorporation gives rise to a higher...
Designer Diamond Anvil Cells consist of lithographically deposited tungsten leads on a diamond cu... more Designer Diamond Anvil Cells consist of lithographically deposited tungsten leads on a diamond culet, followed by epitaxial diamond deposition to electrically insulate the leads from a metallic gasket. This technique can be used to measure the electrical resistance of materials up to megabar pressures, and the temperature dependance of the electrical resistance from 15 K up to room temperature. The metal-insulator transition of two materials will be discussed. First, GaAs undergoes an insulator to metal transition at 19 GPa at room temperature associated with a volume collapse. Second, the electrical resistivity of MnO has been measured in order to investigate the Mott-Insulator transition at 90 GPa.
Quaternary Research, 2001
A simple mass balance model provides insight into the hydrologic, isotopic, and chemical response... more A simple mass balance model provides insight into the hydrologic, isotopic, and chemical responses of Lake Titicaca to past climatic changes. Latest Pleistocene climate of the Altiplano is assumed to have been 20% wetter and 5°C colder than today, based on previous modeling. Our simulation of lacustrine change since 15,000 cal yr B.P. is forced by these modeled climate changes. The latest Pleistocene Lake Titicaca was deep, fresh, and overflowing. The latest Pleistocene riverine discharge from the lake was about 8 times greater than the modern average, sufficient to allow the expansion of the great paleolake Tauca on the central Altiplano. The lake δ 18 O value averaged about −13‰ SMOW (the modern value is about −4.2‰). The early Holocene decrease in precipitation caused Lake Titicaca to fall below its outlet and contributed to a rapid desiccation of paleolake Tauca. Continued evaporation caused the 100-m drop in lake level, but only a slight (1-2‰) increase (relative to modern) in δ 18 O of early Holocene lake waters. This Holocene lowstand level of nearly 100 m was most likely produced by a precipitation decrease, relative to modern, of about 40%. The lake was saline as recently as 2,000 cal yr B.P. The timing of these hydrologic changes is in general agreement with calculated changes of insolation forcing of the South American summer monsoon.
Palaeogeography, Palaeoclimatology, Palaeoecology, 2000
Magnesium/calcium (Mg/Ca) ratios were measured in the deep-sea ostracod (Crustacea) genus Krithe ... more Magnesium/calcium (Mg/Ca) ratios were measured in the deep-sea ostracod (Crustacea) genus Krithe from Chain core 82-24-4PC from the western mid-Atlantic Ridge (3427 m) in order to estimate ocean circulation and bottom water temperature (BWT ) variability over the past 200,000 years. Mg/Ca ratios have been used as a paleothermometer because the ratios are controlled primarily by ambient water temperatures at the time the organism secretes its adult carapace. Over the past two glacial-interglacial cycles, Mg/Ca values oscillated between about 7 mmol/mol and 12 mmol/mol, equivalent to a BWT range of 0 to >3.5°C. The lowest values were obtained on specimens from glacial marine isotope stages (MISs) 2, 4 and 6; the highest values were obtained from specimens from the early part of the Holocene interglacial (MIS 1), and also from MISs 5 and 7. These trends suggest that BWTs in the North Atlantic Ocean fluctuate over orbital time scales.
Nature, Jan 8, 2001
Tropical South America is one of the three main centres of the global, zonal overturning circulat... more Tropical South America is one of the three main centres of the global, zonal overturning circulation of the equatorial atmosphere (generally termed the 'Walker' circulation). Although this area plays a key role in global climate cycles, little is known about South American climate history. Here we describe sediment cores and down-hole logging results of deep drilling in the Salar de Uyuni, on the Bolivian Altiplano, located in the tropical Andes. We demonstrate that during the past 50,000 years the Altiplano underwent important changes in effective moisture at both orbital (20,000-year) and millennial timescales. Long-duration wet periods, such as the Last Glacial Maximum--marked in the drill core by continuous deposition of lacustrine sediments--appear to have occurred in phase with summer insolation maxima produced by the Earth's precessional cycle. Short-duration, millennial events correlate well with North Atlantic cold events, including Heinrich events 1 and 2, as w...
Journal of economic entomology, 2006
Control information for the desert subterranean termite Heterotermes aureus (Snyder) (Isoptera: R... more Control information for the desert subterranean termite Heterotermes aureus (Snyder) (Isoptera: Rhinotermitidae) is limited, despite being one of three major termite pests in the United States. Annual control information comes from field evaluations by the USDA-Forest Service. This partial assessment of termiticide efficacy is likely deficient without concurrent residual analysis and laboratory bioassays of termiticides. In this study, six termiticides were evaluated using exposed and covered field plots in Tucson, AZ, over a 5-yr period for persistence and efficacy by using both residue analysis and laboratory bioassays. All the termiticides degraded significantly during the study. Termiticide degradation seemed to be consistently slower in covered plots than in exposed plots, although this trend was not statistically supported. A comparison of yearly degradation rates showed the three classes of termiticides degraded at different rates. Chlorpyrifos, the organophosphate, degraded ...
The conventional high pressure diamond anvils were modified by growing an isotopically pure C-13 ... more The conventional high pressure diamond anvils were modified by growing an isotopically pure C-13 diamond layer by microwave plasma chemical vapor deposition using methane/hydrogen/oxygen chemistry. The isotopically pure C-13 nature of the culet of the diamond anvil was confirmed by the Raman spectroscopy measurements. This isotopically engineered diamond anvil was used against a natural abundance diamond anvil for high pressure experiments in a diamond anvil cell. Spatial resolved Raman spectroscopy was used to measure the stress induced shift in the C-13 layer as well as the undelying C-12 layer to ultra high pressures. The observed shift and splitiing of the diamond first order Raman spectrum was correlated with the stress distribution in the diamond anvil cell. The experimental results will be compared with the finite element modeling results using NIKE-2D software in order to create a mathematical relationship between sets of the following parameters: vertical (z axis) distance; horizontal (r axis) distance; max shear stress, and pressure. The isotopically enriched diamond anvils offer unique opportunities to measure stress distribution in the diamond anvil cell devices.
Carbonate cements and nodules from hydrothermal sediments of Middle Valley were analyzed for thei... more Carbonate cements and nodules from hydrothermal sediments of Middle Valley were analyzed for their mineralogical, chemical, and isotopic compositions. These diagenetic carbonates include calcite, high-magnesium calcite, and dolomite. The chemical sources for carbonate precipitation include alteration of basement rocks (calcium and strontium), diffusion from seawater (magnesium), recrystallization of biogenic calcite (calcium and carbon), oxidation of sedimentary organic matter (carbon), and
Nature, 2001
Tropical South America is one of the three main centers of the global, zonal overturning circulat... more Tropical South America is one of the three main centers of the global, zonal overturning circulation of the equatorial atmosphere (generally termed the "Walker" circulation 1 ). Although this area plays a key role in global climate cycles, little is known about South American climate history. Here we describe sediment cores and down-hole logging results of deep drilling in the Salar de Uyuni, on the Bolivian Altiplano, located in the tropical Andes. We demonstrate that during the past 50,000 years the Altiplano underwent important changes in effective moisture at both orbital (20,000-year) and millennial timescales. Long-duration wet periods, such as the Last Glacial Maximummarked in the drill core by continuous deposition of lacustrine sediments-appear to have occurred in phase with summer insolation maxima produced by the Earth's precessional cycle. Short-duration, millennial events correlate well with North Atlantic cold events, including Heinrich events 1 and 2, as well as the Younger Dryas episode. At both millennial and orbital timescales, cold sea surface temperatures in the high-latitude North Atlantic were coeval with wet conditions in tropical South America, suggesting a common forcing.
Climatic Change, 2002
Sediment cores from Lake Titicaca contain proxy records of past lake level and hydrologic change ... more Sediment cores from Lake Titicaca contain proxy records of past lake level and hydrologic change on the South American Altiplano. Large downcore shifts in the isotopic composition of organic carbon, C/N, wt. %C org , %CaCO 3 , and % biogenic silica illustrate the dynamic changes in lake level that occurred during the past 20,000 years. The first cores taken from water depths greater than 50 meters in the northern subbasin of the lake are used to develop and extend the paleolake-level record back to the Last Glacial Maximum (LGM). Quantitative estimates of lake level are developed using transfer functions based on the δ 13 C of modern lacustrine organic sources and the δ 13 C of modern sedimented organic matter from core-tops. Lake level was slightly higher than modern during much of the post-LGM (20,000-13,500 yr BP) and lake water was fresh under the associated outflow conditions. The Pleistocene/Holocene transition (13,500-7,500 yr BP) was a period of gradual regression, punctuated by minor trangressions. Following a brief highstand at about 7250 yr BP, lake level dropped rapidly to 85 m below the modern level, reaching maximum lowstand conditions by 6250 yr BP. Lake level increased rapidly between 5000 yr BP and 4000 yr BP, and less rapidly between 4000 yr BP and 1500 yr BP. Lake level remained relatively high throughout the latest Holocene with only minor fluctuations (<12 meters). Orbitally induced changes in solar insolation, coupled with long-term changes in El Niño-Southern Oscillation variability, are the most likely driving forces behind millennial-scale shifts in lake level that reflect regional-scale changes in the moisture balance of the Atlantic-Amazon-Altiplano hydrologic system. Glossary 1. Mesotrophic: Moderately productive; relating to the moderate fertility of a lake in terms of its algal biomass.
Climatic Change, 2002
Sediment cores from Lake Titicaca contain proxy records of past lake level and hydrologic change ... more Sediment cores from Lake Titicaca contain proxy records of past lake level and hydrologic change on the South American Altiplano. Large downcore shifts in the isotopic composition of organic carbon, C/N, wt. %C org , %CaCO 3 , and % biogenic silica illustrate the dynamic changes in lake level that occurred during the past 20,000 years. The first cores taken from water depths greater than 50 meters in the northern subbasin of the lake are used to develop and extend the paleolake-level record back to the Last Glacial Maximum (LGM). Quantitative estimates of lake level are developed using transfer functions based on the δ 13 C of modern lacustrine organic sources and the δ 13 C of modern sedimented organic matter from core-tops. Lake level was slightly higher than modern during much of the post-LGM (20,000-13,500 yr BP) and lake water was fresh under the associated outflow conditions. The Pleistocene/Holocene transition (13,500-7,500 yr BP) was a period of gradual regression, punctuated by minor trangressions. Following a brief highstand at about 7250 yr BP, lake level dropped rapidly to 85 m below the modern level, reaching maximum lowstand conditions by 6250 yr BP. Lake level increased rapidly between 5000 yr BP and 4000 yr BP, and less rapidly between 4000 yr BP and 1500 yr BP. Lake level remained relatively high throughout the latest Holocene with only minor fluctuations (<12 meters). Orbitally induced changes in solar insolation, coupled with long-term changes in El Niño-Southern Oscillation variability, are the most likely driving forces behind millennial-scale shifts in lake level that reflect regional-scale changes in the moisture balance of the Atlantic-Amazon-Altiplano hydrologic system. Glossary 1. Mesotrophic: Moderately productive; relating to the moderate fertility of a lake in terms of its algal biomass.
On orbital timescales, lacustrine sediment records in the tropical central Andes show massive cha... more On orbital timescales, lacustrine sediment records in the tropical central Andes show massive changes in lake level due to mechanisms related to global-scale drivers, varying at precessional timescales. Here we use stable isotopic and diatom records from two lakes in the Lake Titicaca drainage basin to reconstruct multi- decadal to millennial scale precipitation variability during the last 7000 to 8000 years. The records are tightly coupled at multi-decadal to millennial scales with each other and with lake-level fluctuations in Lake Titicaca, indicating that the lakes are recording a regional climate signal. A quantitative reconstruction of precipitation from stable isotopic data indicates that the central Andes underwent significant wet to dry alternations at multi- centennial frequencies with an amplitude of 30 to 40% of total precipitation. A strong millennial-scale component, similar in duration to periods of increased ice rafted debris flux in the North Atlantic, is observed in both lake records, suggesting that tropical North Atlantic sea-surface temperature (SST) variability may partly control regional precipitation. No clear relationship is evident between these records and the inferred ENSO history from Lago Pallcacocha in the northern tropical Andes. In the instrumental period, regional precipitation variability on inter-annual timescales is clearly influenced by Pacific modes; for example, most El Ninos produce dry and warm conditions in this part of the central Andes. However, on longer timescales, the control of tropical Pacific modes is less clear. Our reconstructions suggest that the cold intervals of the Holocene Bond events are periods of increased precipitation in the central Andes, thus indicating an anti-phasing of precipitation variation in the southern tropics of South America relative to the Northern Hemisphere monsoon region.
ieeexplore.ieee.org
Page 1. xv TAIC PART 2007 Programme Committee ŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽ ŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽ... more Page 1. xv TAIC PART 2007 Programme Committee ŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽ ŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽ Academic Members •Giulio Antoniol, University of Sannio, Italy •Kirill Bogdanov, University of Sheffield, UK •David Binkley, Loyola College Maryland,USA •Jonathan Bowen, London South Bank University, UK •Myra Cohen, University of Nebraska-Lincoln, USA •John Clark, University of York, UK •Mireille Ducassé ...
Geophysical Monograph, 2002
Extending the work of Cadot, Kaesler, De Deckker, Chivas, and Corrège, we have measured the eleme... more Extending the work of Cadot, Kaesler, De Deckker, Chivas, and Corrège, we have measured the elemental chemistry of shells of marine ostracodes to evaluate the usefulness of ostracode shell chemistry as a paleoenvironmental proxy. Our work has focused primarily on Mg/Ca, Sr/Ca, and Na/Ca ratios of two common genera: deep-sea genus Krithe and shallow marine/estuarine genus Loxoconcha. We evaluated in vivo effects including genus, species, gender, ontogeny, shell size, intra-shell heterogeneity (Mg), water temperature, and salinity, and postmortem diagenetic effects including partial dissolution, recrystallization, and shell surface contamination. Analysis of modern (core-top), fossil, and laboratory-raised specimens across a wide range of temperature and salinity conditions confirms earlier work indicating that Krithe and Loxoconcha Mg/Ca ratios are dominantly controlled by water temperature. Sr/Ca and Na/Ca ratios co-vary with temperature in core-top Krithe, but not in cultured Loxoconcha suggesting that the Krithe Sr/Ca and Na/Ca correlation with temperature may be related to another variable that broadly co-varies with temperature. Phylogenetic and ontogenetic effects are also indicated, including different Mg-thermodependence and intra-shell Mg distribution between Krithe and Loxoconcha. Inter-specific effects are suggested for two species of Krithe. Magnesium uptake in eldest juvenile shells seems to be identical to that of adult shells, thus greatly increasing the amount of shell material available for paleoenvironmental studies. No salinity effects were observed. Shell Na/Ca ratios showed a dramatic decrease with increasing dissolution (natural and artificial) in waters that are undersaturated with respect to calcite, whereas Mg/Ca ratios displayed a minor decrease and Sr/Ca ratios showed no change. Of the ratios studied, Mg/Ca offers the most promise for Quaternary marine studies as a paleothermometer. Further calibration studies are needed to better understand the marine ostracode Mg/Ca-paleothermometer.
Geochim Cosmochim Acta, 1976
Systematic variations in the isotopic composition of skeletal carbonate deposited by the Caribbea... more Systematic variations in the isotopic composition of skeletal carbonate deposited by the Caribbean reef-frame building coral Montastrea annularis are correlated with water depth, location of the corallites within the corallum, and polyp packing density, as is demonstrated by isotope ratio measurements for 426 samples collected at 4.6 m depth intervals between 0 and 27.4 m at St. Croix, U.S. Virgin Islands. These data support a model, based on a study of Indo-Pacific scleractinians, proposed earlier for stable isotope fractionation by corals. Of particular interest is the fact that, within this species, ecotypic differentiation into shallow-water and deep-water subpopulations, with a boundary close to 20m, is reflected by changes in skeletal ° 13C. Stable isotope geochemical studies of both modern and fossil coral-derived carbonate may contribute to the solution of several problems having geologic and paleontologic significance.
Spe Formation Evaluation, 1986
The Sechura (approx.6°S) and Pisco (approx.14°S) Basins of onshore Peru contain Miocene diatom ... more The Sechura (approx.6°S) and Pisco (approx.14°S) Basins of onshore Peru contain Miocene diatom and phosphate rich sediments which sharply contrast with underlying clastics. In the Sechura Basin the Miocene clastic Mancora, Heath and Montera Formations are overlain by the Zapallal Formation which grades upwards from a weakly biogenic base into fairly pure diatomites (biogenic silica >20%) and ore grade phosphorite (PâOâ>20%). Biogeneic content decreases in the eastern basin as clastic content increases. The base of the Zapallal Formation has been dated at between 12.2 and 14.0 mybp using radiolaria correlated to magnetic stratigraphy by Theyer, et al (1978), and the phosphatic section yields dates of between 8.0 and 11.2 my. In the Pisco Basin the Eocene clastic Paracas Formation is overlain unconformably by the Miocene Pisco Formation which contains a basal sequence of cross-bedded clastics, tuffs, and partially recrystallized diatomites; a phosphorite bearing middle sequence; and a diatom rich top. Current direction from the cross beds of the basal Pisco Formation indicate a generally southerly transport direction but with considerable directional variability. This may be related to the Peru under-current which shows highly variable strength and direction near 15°S.
Metal to insulator transitions can be effectively studied by application of pressure, specificall... more Metal to insulator transitions can be effectively studied by application of pressure, specifically in the case of the transition metal oxides. MnO is a Mott insulator at ambient conditions since splitting of the 3d-band, caused by Coulomb repulsion between electrons, is much larger than the 3d-bandwidth, resulting in correlation-induced localization of the 3d electrons. The width of the 3d subband, however, can be directly controlled with pressure. As pressure increases, bandwidth also increases until some critical value at which electron correlation effects are no longer dominant, and the electrons become delocalized. We use "designer" diamond anvils to measure the insulator to metal transition in MnO, which is complete by 106 GPa. Metallization is also confirmed by examining the temperature dependence of the resistance before and after the transition. We find the insulator to metal transition agrees well with structural transition data in the literature. Results on CoO will be presented as available.
Using microwave plasma-enhanced chemical vapor deposition (CVD), thin films of nano-crystalline d... more Using microwave plasma-enhanced chemical vapor deposition (CVD), thin films of nano-crystalline diamond were grown on 7 mm diameter polished titanium-alloy substrates. The films were monitored in situ by a laser reflectance interferometry (LRI) technique, in which both scattered and reflected laser light intensities were collected from the sample. A total of eight experiments were conducted. The gas flow rates were 499 sccm of hydrogen, 88 sccm of methane, and 8.8 sccm of nitrogen; average temperature during CVD was 830 degrees C. The periodic oscillatory patterns in the scattered and the reflected light intensities were analyzed for growth rate information about the diamond film. This information allowed diamond films of a controlled thickness to be grown. Ellipsometry measurements confirmed that these films ranged from approximately 100 nm to 500 nm in thickness.
Low fluorescence and stress free designer diamonds are needed in high pressure experimentation. N... more Low fluorescence and stress free designer diamonds are needed in high pressure experimentation. Nitrogen related defects in designer diamond anvils were studied by the low temperature photoluminescence (PL) spectroscopy at 80 K. Designer diamonds containing embedded micro-circuits were fabricated by the microwave plasma chemical vapor deposition method using a methane/oxygen/hydrogen chemistry at a substrate temperature of 1485 K. PL excitation in designer diamonds with a 514.5 nm laser shows the existence of zero phonon lines at 1.945 eV and 2.16 eV corresponding to nitrogen-vacancy pair and interstitial nitrogen-vacancy pair defects, respectively. Spatial distribution of this PL emission also shows significantly enhanced incorporation of nitrogen related defects in non (100) facets of designer diamonds. Subsequent annealing of designer diamonds at 1573 K in a pure hydrogen plasma led to a reduction of 2.16 eV defect center. An enhanced nitrogen incorporation gives rise to a higher...
Designer Diamond Anvil Cells consist of lithographically deposited tungsten leads on a diamond cu... more Designer Diamond Anvil Cells consist of lithographically deposited tungsten leads on a diamond culet, followed by epitaxial diamond deposition to electrically insulate the leads from a metallic gasket. This technique can be used to measure the electrical resistance of materials up to megabar pressures, and the temperature dependance of the electrical resistance from 15 K up to room temperature. The metal-insulator transition of two materials will be discussed. First, GaAs undergoes an insulator to metal transition at 19 GPa at room temperature associated with a volume collapse. Second, the electrical resistivity of MnO has been measured in order to investigate the Mott-Insulator transition at 90 GPa.
Quaternary Research, 2001
A simple mass balance model provides insight into the hydrologic, isotopic, and chemical response... more A simple mass balance model provides insight into the hydrologic, isotopic, and chemical responses of Lake Titicaca to past climatic changes. Latest Pleistocene climate of the Altiplano is assumed to have been 20% wetter and 5°C colder than today, based on previous modeling. Our simulation of lacustrine change since 15,000 cal yr B.P. is forced by these modeled climate changes. The latest Pleistocene Lake Titicaca was deep, fresh, and overflowing. The latest Pleistocene riverine discharge from the lake was about 8 times greater than the modern average, sufficient to allow the expansion of the great paleolake Tauca on the central Altiplano. The lake δ 18 O value averaged about −13‰ SMOW (the modern value is about −4.2‰). The early Holocene decrease in precipitation caused Lake Titicaca to fall below its outlet and contributed to a rapid desiccation of paleolake Tauca. Continued evaporation caused the 100-m drop in lake level, but only a slight (1-2‰) increase (relative to modern) in δ 18 O of early Holocene lake waters. This Holocene lowstand level of nearly 100 m was most likely produced by a precipitation decrease, relative to modern, of about 40%. The lake was saline as recently as 2,000 cal yr B.P. The timing of these hydrologic changes is in general agreement with calculated changes of insolation forcing of the South American summer monsoon.
Palaeogeography, Palaeoclimatology, Palaeoecology, 2000
Magnesium/calcium (Mg/Ca) ratios were measured in the deep-sea ostracod (Crustacea) genus Krithe ... more Magnesium/calcium (Mg/Ca) ratios were measured in the deep-sea ostracod (Crustacea) genus Krithe from Chain core 82-24-4PC from the western mid-Atlantic Ridge (3427 m) in order to estimate ocean circulation and bottom water temperature (BWT ) variability over the past 200,000 years. Mg/Ca ratios have been used as a paleothermometer because the ratios are controlled primarily by ambient water temperatures at the time the organism secretes its adult carapace. Over the past two glacial-interglacial cycles, Mg/Ca values oscillated between about 7 mmol/mol and 12 mmol/mol, equivalent to a BWT range of 0 to >3.5°C. The lowest values were obtained on specimens from glacial marine isotope stages (MISs) 2, 4 and 6; the highest values were obtained from specimens from the early part of the Holocene interglacial (MIS 1), and also from MISs 5 and 7. These trends suggest that BWTs in the North Atlantic Ocean fluctuate over orbital time scales.
Nature, Jan 8, 2001
Tropical South America is one of the three main centres of the global, zonal overturning circulat... more Tropical South America is one of the three main centres of the global, zonal overturning circulation of the equatorial atmosphere (generally termed the 'Walker' circulation). Although this area plays a key role in global climate cycles, little is known about South American climate history. Here we describe sediment cores and down-hole logging results of deep drilling in the Salar de Uyuni, on the Bolivian Altiplano, located in the tropical Andes. We demonstrate that during the past 50,000 years the Altiplano underwent important changes in effective moisture at both orbital (20,000-year) and millennial timescales. Long-duration wet periods, such as the Last Glacial Maximum--marked in the drill core by continuous deposition of lacustrine sediments--appear to have occurred in phase with summer insolation maxima produced by the Earth's precessional cycle. Short-duration, millennial events correlate well with North Atlantic cold events, including Heinrich events 1 and 2, as w...
Journal of economic entomology, 2006
Control information for the desert subterranean termite Heterotermes aureus (Snyder) (Isoptera: R... more Control information for the desert subterranean termite Heterotermes aureus (Snyder) (Isoptera: Rhinotermitidae) is limited, despite being one of three major termite pests in the United States. Annual control information comes from field evaluations by the USDA-Forest Service. This partial assessment of termiticide efficacy is likely deficient without concurrent residual analysis and laboratory bioassays of termiticides. In this study, six termiticides were evaluated using exposed and covered field plots in Tucson, AZ, over a 5-yr period for persistence and efficacy by using both residue analysis and laboratory bioassays. All the termiticides degraded significantly during the study. Termiticide degradation seemed to be consistently slower in covered plots than in exposed plots, although this trend was not statistically supported. A comparison of yearly degradation rates showed the three classes of termiticides degraded at different rates. Chlorpyrifos, the organophosphate, degraded ...