Identifying outliers and assessing the accuracy of amino acid racemization measurements for geochronology: I. Age calibration curves   总被引：1，自引：1，他引：0  

Matthew A. Kosnik  Darrell S. Kaufman  Quan Hua   《Quaternary Geochronology》2008,3(4):308-327

Numerical ages derived from amino acid racemization (AAR) geochronology are typically based on calibration curves that relate the extent of AAR to the age of independently dated specimens. Here, we compare options for developing calibration curves and quantifying age uncertainties using AAR data from 481 late Holocene shells, and AMS ¹⁴C analyses of 36 shells of four molluscan taxa (Ethalia, Natica, Tellina, and Turbo) collected from shallow sediment cores from a back-reef lagoon of the central Great Barrier Reef. The four taxa differed substantially in the quality of their geochronogical results. Explicitly including data from specimens alive at the time of collection improves calibration curves, but weighting numerical ages based on their uncertainty has no effect. Calibration curve statistics do not adequately assess calibration uncertainty. The relation between ages inferred from different amino acids is recommended for identifying aberrant specimens and quantifying the uncertainty of inferred ages. For this study, the AAR ages based on two amino acids (aspartic acid and glutamic acid) exceed 200 yr or 20% of their mean inferred age in 15% of the specimens. Once these were removed, the mean age error (1σ) for individual specimens based on two amino acids analyzed in duplicate subsamples ranged from 53 to 142 yr for Tellina and Turbo, respectively, or about a 30% age error for these relatively young shells. This compares favorably with analytical errors estimated at 50 yr or 5%. The presence of notable outliers undetectable using data from single amino acids emphasizes the importance of analyzing multiple amino acids.  相似文献   

Diseases leading to accelerated decline of reef corals in the largest South Atlantic reef complex (Abrolhos Bank, eastern Brazil)     

Francini-Filho RB  Moura RL  Thompson FL  Reis RM  Kaufman L  Kikuchi RK  Leão ZM 《Marine pollution bulletin》2008,56(5):1008-1014

Although reef corals worldwide have sustained epizootics in recent years, no coral diseases have been observed in the southwestern Atlantic Ocean until now. Here we present an overview of the main types of diseases and their incidence in the largest and richest coral reefs in the South Atlantic (Abrolhos Bank, eastern Brazil). Qualitative observations since the 1980s and regular monitoring since 2001 indicate that coral diseases intensified only recently (2005–2007). Based on estimates of disease prevalence and progression rate, as well as on the growth rate of a major reef-building coral species (the Brazilian-endemic Mussismilia braziliensis), we predict that eastern Brazilian reefs will suffer a massive coral cover decline in the next 50 years, and that M. braziliensis will be nearly extinct in less than a century if the current rate of mortality due to disease is not reversed.  相似文献   

Earth tides observed by gravity and GPS in southeastern Alaska     

T. Sato  S. Miura  Y. Ohta  H. Fujimoto  W. Sun  C.F. Larsen  M. Heavner  A.M. Kaufman  J.T. Freymueller 《Journal of Geodynamics》2008,46(3-5):78

We analyzed gravity data obtained in Juneau and global positioning system (GPS) data obtained from three PBO sites in southeastern Alaska (SE-AK), which are part of a US research facility called ‘EarthScope’, and we compared the obtained tidal amplitudes and phases with those estimated from the predicted tides including both effects of the body tide and ocean tide. Global tide models predict the ocean tides in this region of complex coastline and bathymetry. To improve the accuracy of prediction, we developed a regional ocean tide model in SE-AK.Our comparison results suggest: (1) by taking into account the ocean tide effect, the amplitude differences between the observation and the predicted body tide is remarkably reduced for both the gravity and displacement (e.g. for the M₂ constituent, 8.5–0.3 μGal, and 2.4–0.1 cm at the AB50 GPS site in Juneau in terms of the vector sum of three components of the north–south, east–west and up–down), even though the ocean tide loading is large in SE-AK. (2) We have confirmed the precise point positioning (PPP) method, which was used to extract the tidal signals from the original GPS time series, works well to recover the tidal signals. Although the GPS analysis results still contain noise due to the atmosphere and multipath, we may conclude that the GPS observation surely detects the tidal signals with the sub-centimeter accuracy or better for some of the tidal constituents. (3) In order to increase the accuracy of the tidal prediction in SE-AK, it is indispensable to improve the regional ocean tide model developed in this study, especially for the phase.  相似文献   

Workshop on the Delphi Method     

G. G. Baxter  S. M. Cargill  A. H. chidester  P. E. Hart  G. M. Kaufman  F. Urquidi-Barrau 《Mathematical Geology》1978,10(5):581-587

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A Bayesian machine learning model for estimating building occupancy from open source data     

Robert Stewart  Marie Urban  Samantha Duchscherer  Jason Kaufman  April Morton  Gautam Thakur  Jesse Piburn  Jessica Moehl 《Natural Hazards》2016,81(3):1929-1956

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Energy release in solar flares     

P. A. Sturrock  P. Kaufman  R. L. Moore  D. F. Smith 《Solar physics》1984,94(2):341-357

We examine observational evidence concerning energy release in solar flares. We propose that different processes may be operative on four different time scales: (a) on the sub-second time scale of sub-bursts which are a prominent feature of mm-wave microwave records; (b) on the few-seconds time scale of elementary bursts which are a prominent feature of hard X-ray records; (c) on the few-minutes time scale of the impulsive phase; and (d) on the tens-of-minutes or longer time scale of the gradual phase.We propose that the concentration of magnetic field into magnetic knots at the photosphere has important consequences for the coronal magnetic-field structure such that the magnetic field in this region may be viewed as an array of elementary flux tubes. The release of the free energy of one such tube may produce an elementary burst. The development of magnetic islands during this process may be responsible for the sub-bursts. The impulsive phase may be simply the composite effect of many elementary bursts.We propose that the gradual phase of energy release, with which flares typically begin and with which many flares end, involves a steady process of reconnection, whereas the impulsive phase involves a more rapid stochastic process of reconnection which is a consequence of mode interaction.In the case of two-ribbon flares, the late part of the gradual phase may be attributed to reconnection of a large current sheet which is being produced as a result of filament eruption. A similar process may be operative in smaller flares.Also, Department of Applied Physics, Stanford University.  相似文献   

Late Quaternary tephrostratigraphy,Ahklun Mountains,SW Alaska     

Darrell S. Kaufman  Britta J. L. Jensen  Alberto V. Reyes  Caleb J. Schiff  Duane G. Froese  Nicholas J. G. Pearce 《第四纪科学杂志》2012,27(4):344-359

Radiocarbon‐dated sediment cores from six lakes in the Ahklun Mountains, south‐western Alaska, were used to interpolate the ages of late Quaternary tephra beds ranging in age from 25.4 to 0.4 ka. The lakes are located downwind of the Aleutian Arc and Alaska Peninsula volcanoes in the northern Bristol Bay area between 159° and 161°W at around 60°N. Sedimentation‐rate age models for each lake were based on a published spline‐fit procedure that uses Monte Carlo simulation to determine age model uncertainty. In all, 62 ¹⁴C ages were used to construct the six age models, including 23 ages presented here for the first time. The age model from Lone Spruce Pond is based on 18 ages, and is currently the best‐resolved Holocene age model available from the region, with an average 2σ age uncertainty of about ± 109 years over the past 14.5 ka. The sedimentary sequence from Lone Spruce Pond contains seven tephra beds, more than previously found in any other lake in the area. Of the 26 radiocarbon‐dated tephra beds at the six lakes and from a soil pit, seven are correlated between two or more sites based on their ages. The major‐element geochemistry of glass shards from most of these tephra beds supports the age‐based correlations. The remaining tephra beds appear to be present at only one site based on their unique geochemistry or age. The 5.8 ka tephra is similar to the widespread Aniakchak tephra [3.7 ± 0.2 (1σ) ka], but can be distinguished conclusively based on its trace‐element geochemistry. The 3.1 and 0.4 ka tephras have glass major‐ and trace‐element geochemical compositions indistinguishable from prominent Aniakchak tephra, and might represent redeposited beds. Only two tephra beds are found in all lakes: the Aniakchak tephra (3.7 ± 0.2 ka) and Tephra B (6.1 ± 0.3 ka). The tephra beds can be used as chronostratigraphic markers for other sedimentary sequences in the region, including cores from Cascade and Sunday lakes, which were previously undated and were analyzed in this study to correlate with the new regional tephrostratigraphy. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

10Be ages of late Pleistocene deglaciation and Neoglaciation in the north‐central Brooks Range,Arctic Alaska     

Michael E. Badding  Jason P. Briner  Darrell S. Kaufman 《第四纪科学杂志》2013,28(1):95-102

We present a chronology of late Pleistocene deglaciation and Neoglaciation for two valleys in the north‐central Brooks Range, Alaska, using cosmogenic ¹⁰Be exposure dating. The two valleys show evidence of ice retreat from the northern range front before ～16–15 ka, and into individual cirques by ～14 ka. There is no evidence for a standstill or re‐advance during the Lateglacial period, indicating that a glacier advance during the Younger Dryas, if any, was less extensive than during the Neoglaciation. The maximum glacier expansion during the Neoglacial is delimited by moraines in two cirques separated by about 200 km and dated to 4.6 ± 0.5 and 2.7 ± 0.2 cal ka BP. Both moraine ages agree with previously published lichen‐inferred ages, and confirm that glaciers in the Brooks Range experienced multiple advances of similar magnitude throughout the late Holocene. The similar extent of glaciers during the middle Holocene and the Little Ice Age may imply that the effect of decreasing summer insolation was surpassed by increasing aridity to limit glacier growth as Neoglaciation progressed. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

A continuous multi-millennial record of surficial bivalve mollusk shells from the São Paulo Bight,Brazilian shelf     

Troy A. Dexter  Darrell S. Kaufman  Richard A. Krause Jr.  Susan L. Barbour Wood  Marcello G. Simões  John Warren Huntley  Yurena Yanes  Christopher S. Romanek  Michał Kowalewski 《Quaternary Research》2014

To evaluate the potential of using surficial shell accumulations for paleoenvironmental studies, an extensive time series of individually dated specimens of the marine infaunal bivalve mollusk Semele casali was assembled using amino acid racemization (AAR) ratios (n = 270) calibrated against radiocarbon ages (n = 32). The shells were collected from surface sediments at multiple sites across a sediment-starved shelf in the shallow sub-tropical São Paulo Bight (São Paulo State, Brazil). The resulting ¹⁴C-calibrated AAR time series, one of the largest AAR datasets compiled to date, ranges from modern to 10,307 cal yr BP, is right skewed, and represents a remarkably complete time series: the completeness of the Holocene record is 66% at 250-yr binning resolution and 81% at 500-yr binning resolution. Extensive time-averaging is observed for all sites across the sampled bathymetric range indicating long water depth-invariant survival of carbonate shells at the sediment surface with low net sedimentation rates. Benthic organisms collected from active depositional surfaces can provide multi-millennial time series of biomineral records and serve as a source of geochemical proxy data for reconstructing environmental and climatic trends throughout the Holocene at centennial resolution. Surface sediments can contain time-rich shell accumulations that record the entire Holocene, not just the present.  相似文献   

Oxygen isotope composition of annually banded modern and mid-Holocene travertine and evidence of paleomonsoon floods, Grand Canyon, Arizona, USA     

Gary R. O'Brien  Darrell S. Kaufman  Warren D. Sharp  Viorel Atudorei  Roderic A. Parnell  Laura J. Crossey 《Quaternary Research》2006,65(3):366-379

Holocene and modern travertine formed in spring-fed Havasu Creek of the Grand Canyon, Arizona, was studied to determine the factors governing its oxygen-isotope composition. Analysis of substrate-grown travertine indicates that calculated calcite-formation temperatures compare favorably with measured water temperatures, and include silt-rich laminae deposited by monsoon-driven floods. Ancient spring-pool travertine is dated by U-series at 7380 ± 110 yr and consists of 14 travertine-silt couplets of probable annual deposition. One hundred eighty high-resolution δ¹⁸O analyses of this mid-Holocene sample average −11.0‰ PDB. The average value for modern travertine is 0.5‰ lower, perhaps because mid-Holocene temperature was higher or there was proportionally greater summer recharge. δ¹⁸O cyclicity in the mid-Holocene travertine has average amplitude of 1.9 ± 0.5‰ PDB, slightly less than the inferred modern-day annual temperature range of Havasu Creek. The annual temperature range might have been reduced during the 14-yr interval compared to present, although other non-temperature factors could account for the muted annual variation. Silt-rich laminae within isotopically lower calcite in the modern and mid-Holocene travertine verifies the seasonal resolution of both samples, and suggests that similar temperature-precipitation conditions, as well as monsoon-generated summer floods, prevailed in the mid-Holocene as they do throughout the Grand Canyon region today.  相似文献