bluuebunny/crossref_metadata_2025 huggingface dataset:
Sensitivity of climate to late Cenozoic uplift in southern Asia and the American west: Numerical experiments
Authors: Kutzbach J. E. , Guetter P. J. , Ruddiman W. F. , Prell W. L.
Published: February 2008

Experiments with general circulation models with no mountains, half mountains and full (modern) mountains show the sensitivity of atmospheric circulation patterns to progressive amounts of uplift. The experiments were motivated by geologic evidence for large, kilometer‐scale uplift in the late Cenozoic in southern Asia and the American west, particularly within the last 10 m.y. In January the amplitude of the mid‐latitude, upper tropospheric planetary waves increased with uplift, and the low‐level winds were progressively blocked or diverted around the topographic features. In July the progressive uplift caused monsoonlike circulations to develop in the vicinity of the Colorado and Tibetan plateaus. Atmospheric heating rates, midtroposphere vertical motion, and upper‐tropospheric planetary wave amplitudes varied approximately linearly with progressive uplift. These results suggest that geologically recent kilometer‐scale uplift may have had climatic consequences comparable in magnitude and pattern to those of earlier stages of uplift. Comparison of the simulated climatic effects of uplift with the geologic evidence for late Cenozoic climatic change, especially for the last 10 m.y., is given in accompanying papers.
Regional validation of means, variances, and spatial patterns in general circulation model control runs
Authors: Santer B. D. , Wigley T. M. L.
Published: February 2008

The focus of this study is the control run performance of four general circulation models (GCMs): the Oregon State University (OSU) two‐layer atmospheric GCM (AGCM), the OSU coupled ocean‐atmosphere model (CGCM), the Goddard Institute for Space Studies (GISS) nine‐layer AGCM, and the European Centre for Medium‐Range Weather Forecasts (ECMWF) T21 model. The analysis variable is monthly mean sea level pressure (MSLP), and model validation is performed for a limited domain (North America/Atlantic/Europe). The first part of the investigation deals with the magnitude and gross spatial structure of model errors in means and interannual variability (for January and July only). These errors are examined with the aid of maps of time‐mean MSLP, difference fields, and local variance ratios. The significance of the local (grid point by grid point) differences in means and variances is then determined by performing univariate t‐ and F‐tests. This information on the spatial structure of large‐scale systematic errors is important for understanding the results of significance tests performed on the overall fields. In the second part of the investigation, the statistics recommended by Wigley and Santer (this issue) for use in model validation are applied to test the overall significance of observed/simulated differences in means, variances, and spatial patterns over the entire annual cycle. Significance levels are determined with the pool permutation procedure (PPP) introduced by Preisendorfer and Barnett (1983). Results indicate that all four models have highly significant errors in the mean field and spatial pattern over the entire annual cycle. Errors in the temporal variance are generally less significant, and significance levels for variance tests can depend critically on the choice of averaging period for observed validation data. The actual test statistic values show that there are considerable differences in model performance. The ECMWF T21 model simulates the spatial pattern and time‐mean MSLP field with greater fidelity than the other models considered here.
Five‐day mesospheric waves observed in Solar Mesosphere Explorer ozone
Authors: Rosenlof Karen H. , Thomas Ronald J.
Published: February 2008

Solar Mesosphere Explorer (SME) ozone data were analyzed for short‐term periodic oscillations in the altitude region from 50 to 90 km. A 5‐day oscillation was found to be a dominant feature at high southern latitudes near the December solstice in the altitude range from 50 to 80 km. The wave feature appears to travel westward with a speed consistent with that of a wave number one planetary‐scale oscillation circling the globe in 5 days. The amplitude also appears to be consistent with the temperature oscillations predicted by Geisler and Dickinson (1976). This oscillation was not found to be a dominant feature in SME ozone during northern hemisphere summer.
Clustered or regular cumulus cloud fields: The statistical character of observed and simulated cloud fields
Authors: Ramirez Jorge A. , Bras Rafael L.
Published: February 2008

The spatial distribution of cumulus clouds is assumed to be the result of the effects of convective activity on the thermodynamic environment. These effects can be parameterized in terms of a stabilization function representing the time rate of change of convective available potential energy. Using these results, a new inhibition hypothesis explaining the expected characteristics of the spatial distribution of cumulus clouds is postulated. This paper performs a verification of the inhibition hypothesis on real and simulated cloud fields. In order to do so, an objective measure of the spatial characteristics of cumulus clouds is introduced. Multiple cloud experiments are performed with a three‐dimensional numerical cloud model. Skylab pictures of real cumuli are also used in the verification. Results of applying this measure to simulated and observed cumulus cloud fields confirm the inhibition hypothesis.
Emissions of some trace gases from biomass fires
Authors: Hegg Dean A. , Radke Lawrence F. , Hobbs Peter V. , Rasmussen Rei A. , Riggan Philip J.
Published: February 2008

Airborne measurements of 13 trace gases from seven forest fires in North America are used to determine their average emission factors. The emission factors are then used to estimate the contributions of biomass burning to the worldwide fluxes of these gases. The estimate for NH3 (∼7 Tg N yr−1) is about 50% of the global emissions of this gas. Combined NH3 and
emissions from biomass burning could be the most important component of the NH3 cycle. N2O from biomass burning (∼ 2 Tg N yr−1) is also significant worldwide. The estimate for NOx from biomass burning worldwide (∼ 19 Tg N yr−1), which is greater than previous estimates, is comparable to emissions from fossil fuel combustion. The estimate of the global flux of F12 (CF2Cl2) from biomass burning based on the complete data set (∼0.2 Tg yr−1) is ∼50% of the total global emission of F12. However, this estimate is strongly influenced by a very high emission of F12 from a fire in the Los Angeles Basin. Disregarding this fire yields a global flux of 0.06 Tg yr−1 (∼15% of total global emissions). The high emissions of NOx and F12 are due in whole or part to the resuspension of previously deposited pollutants. Since this can be the only source of F12 in the smoke from fires, deposition may be a significant sink for F12. Our estimate for NOx emissions from biomass burning in the South Coast Air Basin of California is much greater than previous estimates.
On the indirect determination of atmospheric OH radical concentrations from reactive hydrocarbon measurements
Authors: Mckeen S. A. , Trainer M. , Hsie E. Y. , Tallamraju R. K. , Liu S. C.
Published: February 2008

Measurements of nonmethane hydrocarbons (NMHCs) from urban plumes have previously been used to deduce average OH concentrations by monitoring the decay of the NMHCs from initial values and comparing their time rates of decay to the decay rate of a relatively inert tracer emitted from the same source. Under the assumption of no interaction between photochemistry and transport, these observationally derived indirect OH concentrations are lower for NMHCs with successively higher reactivities toward OH. In this study, analytical solutions to simple flow situations and a three‐dimensional mesoscale model are used to test this assumption. When turbulent transport is parameterized in terms of diffusion coefficients, the models yield results that are consistent with the observations and suggest that the assumption is not generally valid. Daytime vertical mixing within the planetary boundary layer (PBL) and horizontal diffusion are two transport processes that cannot be assumed to be separable from species reactivity. The net effect is that under most daytime conditions the OH concentrations can be underpredicted by more than a factor of 2 when highly reactive NMHCs are used to derive OH concentrations in the usual manner. However, these results only apply when the species used to derive OH concentrations are emitted from a continuous source. The assumption of separability between photochemistry and transport is valid when only instantaneous or puff emissions are considered. An explanation of these effects is presented within the context of the analytical results to some simple flow systems. Results from the three‐dimensional model are used to illustrate the interaction of species reactivity and transport for more realistic parameterizations of atmospheric transport. The results of this study have important implications for experiments designed to indirectly determine OH concentrations and also for the treatment of turbulent transport in Lagrangian and Eulerian photochemical models.
Cloud cover analysis with Arctic Advanced Very High Resolution Radiometer data: 2. Classification with spectral and textural measures
Authors: Key J.
Published: February 2008

The variation in cloud amount over polar ice sheets, sea ice, and ocean surfaces can have important effects on planetary albedo gradients and on surface energy exchanges, so that monitoring of polar cloud cover is crucial to studies of climate change. The spectral and textural characteristics of polar clouds and surfaces for a 7‐day summer series of advanced very high resolution radiometer (AVHRR) data in two Arctic locations are examined, and the results used in the development of a cloud classification procedure for polar satellite data. Since spatial coherence and texture sensitivity tests indicate that a joint spectral‐textural analysis based on the same cell size is inappropriate, cloud detection with AVHRR data and surface identification with passive microwave data are first done on the pixel level as detailed in part 1 (Key and Barry, 1989). Next, cloud patterns within (250 km)2 regions are described, then the spectral and local textural characteristics of cloud patterns in the image are determined and each cloud pixel is classified by statistical methods. Results indicate that both spectral and textural features can be utilized in the classification of cloudy pixels, although spectral features are most useful for the discrimination between cloud classes. This methodology provides a basis for future “objective” automated mapping of cloud types and amount over snow and ice covered surfaces.
An intercomparison of airborne nitrogen dioxide instruments
Authors: Gregory G. L. , Hoell J. M. , Carroll M. A. , Ridley B. A. , Davis D. D. , Bradshaw J. , Rodgers M. O. , Sandholm S. T. , Schiff H. I. , Hastie D. R. , Karecki D. R. , Mackay G. I. , Harris G. W. , Torres A. L. , Fried A.
Published: February 2008

Results from an airborne intercomparison of techniques to measure tropospheric levels of nitrogen dioxide (NO2) are discussed. The intercomparison was part of the National Aeronautics and Space Administration's Global Tropospheric Experiment and was conducted during the summer of 1986. Instruments intercompared included a two‐photon nitric oxide (NO) laser‐induced fluorescence system with laser photolysis of NO2 to NO, an NO/O3 chemiluminescence detector using FeSO4 for conversion of NO2 to NO, an NO/O3 chemiluminescence detector with arc lamp photolysis of NO2 to NO, and a tunable diode laser multipath absorption system. All intercomparisons were for NO2 mixing ratios of <200 pptv with most at mixing ratios of <100 pptv. The FeSO4 converter was found to convert peroxyacetyl nitrate (PAN) to NO, resulting in NO2 values a factor of 2–3 higher than reported by the other techniques. Thus the FeSO4 converter data are excluded from the analyses. Intercomparison data were analyzed in various mixing ratio ranges. Good correlation was observed between the remaining three instruments for those data sets which included mixing ratios to 100 or 200 pptv, showing on the average a 30–40% level of agreement among the techniques. However, when the data were restricted to mixing ratios of <50 pptv, little correlation among the measurements was observed. Even though correlations were poor at mixing ratios of <50 pptv, the tunable diode laser system tended to be high compared to data reported by the two‐photon laser and arc lamp chemiluminescence systems, and agreement between the latter two instruments was generally better than 20 pptv with an equal tendency for one to be high relative to the other.
Observed and model‐calculated NO2/NO ratios in tropospheric air sampled during the NASA GTE/CITE‐2 field study
Authors: Chameides W. L. , Davis D. D. , Bradshaw J. , Sandholm S. , Rodgers M. , Baum B. , Ridley B. , Madronich S. , Carroll M. A. , Gregory G. , Schiff H. I. , Hastie D. R. , Torres A. , Condon E.
Published: February 2008

Data gathered during the NASA GTE/CITE 2 airborne field campaign were analyzed and compared with diagnostically derived parameters to study the NOx photostationary state in the troposphere and the processes that control this photostationary state. Our analysis focussed on two sets of NO2/NO ratios derived from the data; these were based on overlapping NO and NO2 measurements made by two independent techniques; i.e., a chemiluminescent technique and a technique based on two‐photon, laser‐induced‐fluorescence. While for any given 6‐ to 10‐min time interval the two observed NO2/NO ratios often exhibited significant discrepancies, these discrepancies appeared to be mostly random rather than systematic, and as a result, the average difference for all time intervals with overlapping NOx measurements was only 12%. One notable exception, however, was the block of data gathered during the last three CITE 2 missions; during these three missions the ratios observed by the chemiluminescent technique were systematically larger than those observed by the laser‐induced fluorescence technique by a factor of 1.6. When the data from these three missions were omitted from the analysis, the averages of the observed ratios agreed to within 1%. In contrast to a number of previous studies, the ratios predicted from photochemical model calculations were found to be reasonably consistent with the observed ratios, although on average they tended to fall about 20–25% below the observations. This agreement between observations and theory provides strong evidence in support of the importance of peroxy radicals in the fast photochemical cycling of NOx (and the concomitant photochemical production of O3) in both the marine and continental troposphere.
A physical model of the bidirectional reflectance of vegetation canopies: 1. Theory
Authors: Verstraete Michel M. , Pinty Bernard , Dickinson Robert E.
Published: February 2008

An analytical expression for the bidirectional reflectance field of a vegetation canopy is derived from physical and geometrical considerations of the transfer of radiation through a porous medium. The reflectance pattern is shown to depend explicitly on the optical properties of the scatterers (for example, leaves), and on the structural parameters of the canopy, such as the statistical distribution of the orientation of these scatterers, the leaf area density, the size of the scatterers and their interspacing. This theory provides a simple and accurate way to understand the anisotropy of the radiation field over a vegetated surface. It can be useful for modeling applications (for example, the albedo is a by‐product which can be numerically estimated), as well as for extracting some of the structural and physical properties of the surface. These applications are discussed in the accompanying paper (Pinty et al., this issue).
Possible factors controlling global marine temperature variations over the past century
Authors: Wu Zhongxiang , Newell Reginald E. , Hsiung Jane
Published: February 2008

Data from the Global Ocean Surface Temperature Atlas (GOSTA), which includes 60 million ship reports for the 1856–1988 period, have been used to study global night marine air temperature (MAT) and sea surface temperature (SST) interannual variations. Linear regression has been used to establish trends over two periods: 1888–1988, a period often used for land station analyses, and 1856–1988. For global MAT the trends for the century beginning in 1888 computed for these periods are 0.49°C and 0.29°C, respectively. Stepwise linear regression is used to relate the marine time series to three physical variables thought to influence surface temperatures, namely, changes in solar radiation output, changes in atmospheric transmission for solar radiation, and changes in the global surface pressure field. Indices of these variables used are new computations of solar irradiance, atmospheric turbidity at Sonnblick, Austria, supported by sunshine records from Japan, and the Southern Oscillation Index (SOI). When MAT and SST series are examined for the globe, northern and southern hemisphere separately, the largest fraction of the variance (45% for the global MAT, 35% for global SST) is accounted for by turbidity changes, while the tropical east Pacific shows the largest fraction (45% for MAT, 49% for SST) associated with SOI changes. Modeled solar irradiance variations contribute the second most important factor in controlling marine temperature fluctuations in the southern hemisphere. The tropical west Pacific SST is relatively insensitive to the three parameters, in accordance with previous suggestions that the temperature there is limited by evaporation. When the residuals are examined after account is taken of these factors, the global MAT trend for 1888–1988 is reduced to 0.24°C. With the present approach the apparent twentieth century temperature increases can be viewed as partly due to a recovery from cooling at the turn of the century, probably associated with volcanic activity.
Bioclimatic distribution of vegetation for general circulation model studies
Authors: Prentice Katharine Culbertson
Published: February 2008

Four global bioclimatic schemes which use climate to determine the distribution of vegetation are evaluated. Only 38% to 40% of the observed land surface, mapped as thirty‐one vegetation types, could be replicated by applying the four schemes to two global climate data sets. The simulations are significantly improved by further subdividing and regrouping the climates defined by the schemes. In addition, the observed vegetation types are regrouped to better reflect both similarity of climate and similarity of attributes which affect exchanges of atmospherically important gases. With these alterations, 77% of the predicted vegetative landscape corresponds with the distribution of present vegetation. Each observed vegetation type is analyzed in terms of the success with which it can be resolved from climate. This simple coupling of vegetation and climate allows the distribution of simulated vegetation to adjust to present or perturbed climate. It is a rudimentary interactive atmosphere‐biosphere in which the climatically determined distribution of vegetation forms the basis of predicting or prescribing atmosphere‐biosphere exchanges. The climate parameters used, the grouping of vegetation types, as well as the resolution and scope of the revised bioclimatic scheme, have been designed to be used with general circulation models.
Wind fluctuation measurements in the buoyancy range by stellar scintillation analysis
Authors: Caccia Jean‐Luc , Vernin Jean
Published: February 2008

Single‐star scintillation spatiotemporal cross‐correlation functions enable estimates of altitude, CN2 and horizontal wind velocity of each atmospheric turbulent layer between 2 and 20 km to be made. Furthermore, for the first time with such a technique, the standard deviations σv of the horizontal wind fluctuations are also estimated. The experimental results are compared with those obtained by other methods and good agreement is found. The σv estimations are interpreted in terms of the horizontal wind velocity spectrum and the measurements extend into the buoyancy range. These results encourage the belief that the use of a faster correlator will enable us to experimentally sample the buoyancy range for length scales ranging from 100 m to several tens of thousands of meters.
Reaction rate constant of SO3 + NH3 in the gas phase
Authors: Shen Gaunlin , Suto Masako , Lee L. C.
Published: February 2008

The reaction rate constant of SO3 + NH3 in the gas phase was measured using a photofragment emission flow tube technique. The reaction rate constant at room temperature is (6.9±1.5) × 10−11 cm3 molecule−1 s−1 at He pressures of 1–2 torr. This measured reaction rate is new information for studying the formation process of ammonium sulfates in the atmosphere. The reaction rate constant of SO3 + CH3NH2, which is (1.1 ±0.2) × 10−11 cm3 molecule−1 s−1, is also reported.
Stratospheric NO, NO2, and N2O5: A comparison of model results with Spacelab 3 Atmospheric Trace Molecule Spectroscopy (ATMOS) measurements
Authors: Allen Mark , Delitsky Mona L.
Published: February 2008

Photochemical model calculations utilizing updated reaction rate constants and photo‐cross sections and constrained by Spacelab 3 (May 1985) Atmospheric Trace Molecule Spectroscopy (ATMOS) experiment measurements of species abundances and atmospheric parameters yield NO/NO2 abundance ratios in excellent agreement (<25–30% differences: the measurement uncertainties) with the ATMOS 30°N sunset and 47°S sunrise results between ∼20 and ∼50 km. Over this altitude range the NO/NO2 ratio is varying by a factor of 100. At 25 km and above ∼40 km, difficulties in retrieving abundances from occultation spectra for the rapidly diurnally varying NO or NO2 may contribute to the larger discrepancies with the model values. The agreement between model and observations for most of the stratosphere serves to confirm key aspects of current stratospheric chemical models. In addition, model results for N2O5 at 47°S sunrise reproduce the ATMOS measurements, supporting the current simple scheme in models for sequestering NOx in the N2O5 nighttime reservoir. However, the model values at 30°N sunset are as much as a factor of 3.5 larger than the corresponding ATMOS N2O5 observations. Agreement between calculations and measurements occurs when the N2O5 photodissociation rate constants are increased by 50–75%.
Calculation of infrared limb emission by ozone in the terrestrial middle atmosphere: 2. Emission calculations
Authors: Mlynczak Martin G. , Drayson S. Roland
Published: February 2008

The spectrally integrated limb radiance is calculated for ozone and for carbon dioxide in the 9–11 μm spectral interval for tangent heights between 30 and 100 km for noon and midnight conditions using a high‐resolution, line‐by‐line radiative transfer model that incorporates nonlocal thermodynamic equilibrium (NLTE) energy level populations in the transmittance calculations and NLTE source functions. The radiances assuming local thermodynamic equilibrium (LTE) are compared to the NLTE radiances. The NLTE ozone limb radiance is equal to the LTE radiance below 55 km at noon and below 70 km at midnight. The largest difference between the LTE and NLTE spectrally integrated ozone radiances occurs near 96 km in tangent height at noon and near 93 km at midnight. The difference between radiances decreases significantly from noon to midnight and is dependent on the portion of the spectral interval that is observed. Because of the uncertainty in the chemical pumping process, limb radiances utilizing two statistical equilibrium models of ozone are compared, yielding a maximum 32% difference in the spectrally integrated radiance at a tangent height of 84 km. Overlap of ozone spectral lines from bands possessing different source functions is not significant in calculating the infrared limb emission from the mesosphere and lower thermosphere. Finally, carbon dioxide is predicted to be responsible for a significant fraction of the total limb radiance in the 9–11 μm spectral interval in the daytime upper mesosphere and lower thermosphere. The implications of NLTE radiative transfer on the remote sensing of ozone are discussed.
A numerical method for parameterization of atmospheric chemistry: Computation of tropospheric OH
Authors: Spivakovsky C. M. , Wofsy S. C. , Prather M. J.
Published: February 2008

An efficient and stable computational scheme for parameterization of atmospheric chemistry is described. The 24‐hour‐average concentration of OH is represented as a set of high‐order polynomials in variables such as temperature, densities of H2O, CO, O3, and NOt (defined as NO + NO2 + NO3 + 2N2O5 + HNO2 + HNO4) as well as variables determining solar irradiance: cloud cover, density of the overhead ozone column, surface albedo, latitude, and solar declination. This parameterization of OH chemistry was used in the three‐dimensional study of global distribution of CH3CCl3 (Spivakovsky et al., this issue). The proposed computational scheme can be used for parameterization of rates of chemical production and loss or of any other output of a full chemical model. Coefficients for the polynomials are computed to provide the least squares fit to results of the full chemical model. Highly overdetermined systems are used with the sets of independent variables selected randomly in accordance with the distributions expected in the atmosphere. The least squares problem is solved using the Householder method of triangularization (by orthogonal transformations). The method allows detection and rectification of ill‐defined conditions (i.e., linear dependence among terms), as well as evaluation of the individual contribution of each term of the polynomial in reducing the residual vector. On the basis of that information the terms that have little bearing on the residual norm are discarded. Once the domain and the statistical distributions of independent variables are chosen, the entire parameterization procedure is implemented as a complete sequence of computer programs requiring no subjective analysis. The output of the procedure includes estimates of accuracy of the approximation against an independent sample of points, and computer written FORTRAN subroutines to compute the polynomials.
Identification of ions in continental air
Authors: Eisele F. L. , Tanner D. J.
Published: February 2008

The results of three tropospheric ion field studies are presented along with new ion identities and a discussion of ion variability. The ion identifications were made using a dual collision chamber tandem mass spectrometer. Included among the species recently identified are isobutylamine, 2‐dimethylaminoethanol and/or sarcosine, SO4NO−2, SO4NO−3, and a species that appears to be similar to glucose containing an amine and 0–3 methyl groups and possibly some fairly massive (masses > 200 amu) primary amines. The variability of most of the major ions in the positive spectrum is observed to be surprisingly small. For several of the more massive positive ions, this lack of variability is difficult to explain, since the atmospheric lifetime of the parent species of these ions should be short.
Measurements of light atmospheric hydrocarbons over the Atlantic in regions of low biological activity
Authors: Rudolph J. , Johnen F. J.
Published: February 2008

More than 200 in situ measurements of several selected nonmethane hydrocarbons (NMHCs) were made in the remote marine atmosphere over the Atlantic between 40°S and 50°N. Ethane, the by far longest lived of the NMHCs, showed southern hemispheric mixing ratios around 280 ppt, comparable in magnitude to most other previous measurements. The mixing ratios of ethene, propene, propane, and i‐ and n‐butane in the southern hemisphere were in the range of 10–30 ppt. For i‐pentane and n‐pentane the atmospheric mixing ratios in the southern hemisphere were below the detection limit of 15–20 ppt. These values are rather low compared to other published measurements in the marine atmosphere. This is due to the remoteness of the measuring locations and the very low biological activity in the surrounding ocean areas. For all alkanes the latitudinal profiles exhibit a considerable decrease from north to south. The relative hydrocarbon patterns show that the alkanes in the northern hemisphere are primarily due to long‐range transport from continental or coastal areas. In general the transport times exceed several days. Consequently, the observed northern hemispheric ethene and propene mixing ratios must be, with few exceptions, primarily the result of oceanic emissions. The average difference of these compounds by a factor of 2 between the southern and the northern hemisphere can be explained by change of the phytoplankton concentration in ocean water. There is little or no indication for the existence of significant diurnal cycles for these two alkenes.
Atmospheric OCS: Evidence for a contribution of anthropogenic sources?
Authors: Bingemer H. G. , Bürgermeister S. , Zimmermann R. L. , Georgii H.‐W.
Published: February 2008

Carbonyl sulfide (OCS) in the boundary layer of the marine atmosphere was measured by gas chromatography with flame photometric detection on board ship following a meridional transect in the Atlantic Ocean between 37°S and 51°N. Overall, OCS levels from 174 individual samples averaged 537 ppt with a standard deviation of 104 ppt. A pronounced northward increase in OCS of, on the average, 26 ppt per 10° latitude was found, together with a mean interhemispheric ratio of 1.25 between OCS in northern and southern hemisphere air masses. OCS correlated significantly with CH4 and CO values obtained during the cruise by an independent investigator.
Oligocene paleoceanography of the South Atlantic: Paleoclimatic implications of sediment accumulation rates and magnetic susceptibility measurements
Authors: Mead Gregory A. , Tauxe Lisa , LaBrecque John L.
Published: February 2008

Paleoceanographic changes during the Oligocene in the South Atlantic have been examined using paleomagnetic, rock magnetic, carbonate content, and sediment accumulation rate (SAR) data. We show that magnetic susceptibility (MS) is largely controlled by carbonate content at Deep‐Sea Drilling Project site 522, which in turn is the primary control on SAR at the site. Over 5000 MS measurements were averaged over magnetostratigraphic polarity intervals for use as indicators of carbonate content and SAR proxies. The study produced three primary results: (1) a spectral analysis of the MS data from one upper Oligocene core reveals two dominant periodicities (44.7 ± 10.4 kyr and 26.4 ± 3.6 kyr) which approximate Milankovitch obliquity (41 kyr) and precession (23 kyr) periods; (2) four pulses of increased SAR lasting 1 to 2 m.y. punctuate the section, implying longer‐term climatic changes (these events are also seen in the Contessa quarry section of the Italian (Tethyan) Oligocene); and (3) two of the SAR maxima correspond to times of global cooling and possible ice volume increases, at 31 and 36 Ma.
The Varve microcosm: Propagator of cyclic bedding
Authors: Anderson Roger Y.
Published: February 2008

Changes in the proportion of seasonal components within varves accumulate over time to produce cyclic bedding. A varve couplet is therefore a microcosm of bedding at a larger scale within the same stratigraphic unit. Cyclic bedding arises from a differing physical response or a phase‐shifted temporal response to climatic forcing. An example from the Permian Castile formation suggests that the skipping of climatic cycles by the bedding response may be a common occurrence leading to erroneous estimates of the frequency of cyclic bedding. Seasonal changes that generate cyclic bedding occur in two domains of episodic sedimentation, separated by a megathreshold between pelagic and advective environments. Cumulative changes responsive to seasonality, within either domain, can produce several types of cyclic bedding over a broad range of frequencies, extending into the Milankovitch band.
Cyclic patterns in the Paleozoic and Mesozoic: Implications for time scale calibrations
Authors: Bayer Ulf , McGhee George R.
Published: February 2008

The analysis of both physical and biological data from the Devonian strata of New York State reveals the existence of two robust cyclic patterns: one cycle with a duration of 4.5–5.7 m.y. and a larger cycle of approximately twice this wavelength at 11.3–13.6 m.y. depending upon the absolute time scale used in the wavelength calculation. The analysis of sedimentation rates from the Jurassic likewise reveals a cycle with a temporal spacing of 4 to 6 m.y., a cycle with a periodicity virtually identical to the shorter cycle detected in the Devonian. The data further suggest the existence of cycles at 2.3 m.y. and 30 m.y. intervals, though these periodicities approach the lower and upper limits of resolution of the data base. The potential correlation of cyclic patterns has considerable implication for the development of new chronological time scales and global chronostratigraphic resolution. Current radiometric time scales have a resolution which is still above the stage level for most of the pre‐Cenozoic. In estimating the ages of stage boundaries, most time scales are based on either the assumption of equal stage durations or the assumption of equal biostratigraphic zone durations, and the subsequent linear interpolations between control tie‐points. Operating under the working hypothesis that a regular periodic signal exists in the data, the above listed Devonian cycles are brought into a more uniform periodicity by transforming the time scale itself, yielding an alternatively calibrated timescale which is neither equal stage nor equal zone in structure.
Upward shallowing platform cycles: A response to 2.2 billion years of low‐amplitude, high‐frequency (Milankovitch band) sea level oscillations
Authors: Grotzinger J. P.
Published: February 2008

Shallow‐water carbonate platforms, characterized by sequences of small‐scale upward shallowing cycles, are common in the Phanerozoic and Proterozoic stratigraphic record. Proterozoic small‐scale cycles are commonly 1 to 10 m thick, have asymmetrically arranged facies, and are strikingly similar to Phanerozoic platform cycles. In some platform sequences (eg. Rocknest, Wallace, and Helena formations of early to middle Proterozoic age), it can be demonstrated that the lateral distribution of facies within cycles relates to systematic variations in platform paleogeography and topography. In the Rocknest formation, cycles with intervals of tepees and pisolitic breccia formed on a topographic high (shoal complex) near the shelf edge rim, and provide evidence for eustatic falls in sea level at the end of each cycle. The presence of these facies in other Proterozoic cyclic platforms also suggests that eustatic sea level falls may have been important in the development of each cycle. Proterozoic upward shallowing cycles appear to have had periods of between 20,000 and 100,000 years, and probably formed during eustatic oscillations in sea level with amplitudes of less than 10 m. This suggests that cyclicity may have been regulated by Milankovitch band climatic forcing, perhaps influencing global sea level through minor changes related to small‐scale continental or alpine glaciation. It is possible, then, that Milankovitch band climatic forcing has occurred for at least the last 2.2 billion years of earth history.
Punctuated aggradational cycles: Implications for stratigraphic analysis
Authors: Goodwin P. W. , Anderson E. J. , Goodman W. M. , Saraka L. J.
Published: February 2008

Application of the hypothesis of punctuated aggradational cycles (PACs), a model of episodic stratigraphic accumulation, has yielded results fundamentally in conflict with those predicted and generated by gradualistic models. The assumption that stratigraphic accumulation is controlled principally by rapid small‐scale base level fluctuations in the Milankovitch band leads to the adoption of the PAC, a thin time‐stratigraphic unit, as the fundamental unit of stratigraphic description and interpretation; the thicker generally diachronous formation is inadequate for stratigraphic analysis because it is an artifact of stratigraphic gradualism. Correlation of stratigraphic sections is accomplished by matching patterns of facies change, particularly the abrupt changes at the synchronous surfaces that mark PAC boundaries. This kind of physical event correlation is far more accurate than traditional biostratigraphic methods. Focus on synchronous PAC boundaries separating environmentally disjunct facies reveals discontinuities that preclude application of Walther's law across those surfaces. When viewed from this episodic perspective, the Manlius Formation of the Helderberg Group in New York state becomes a highly ordered set of time‐stratigraphic cycles rather than a disordered mosaic of facies representing short‐term autogenic processes in a single nearshore environment. Not only was Manlius deposition episodic with no gradual upward deepening as is predicted by stratigraphic gradualism, but also the Manlius‐Coeymans formational boundary is interpreted as a small‐scale unconformity rather than a paleoenvironmental boundary created by gradual transgression. Each of these interpretations is the direct result of assuming and applying a model of episodic stratigraphic accumulation.
Bryozoan bathymetric gradients within a Late Ordovician Epeiric Sea
Authors: Anstey R. L. , Rabbio S. F. , Tuckey M. E.
Published: February 2008

The large‐scale spatial zonation of the Late Ordovician epeiric sea in North America is reflected in the distributions of bryozoans and other invertebrates. Their major life zones (biomes) reflect changing patterns of sedimentation across the platform, from major siliciclastic to carbonate lithotopes. Deeper water within the epeiric sea most likely existed in an area centered on the Michigan Basin and between the clastic wedges of the Taconic Orogen and the extensive cratonic carbonate platform. Stratigraphic gradients in the Cincinnati Arch area indicate a first‐order cycle of transgression‐regression throughout the Late Ordovician, on which second‐order fluctuations in sea level have been superimposed. The second‐order fluctuations indicate three transgressive‐regressive subcycles, closely corresponding to the Eden, Maysville, and Richmond stages.
Estimating short‐term changes in Eustatic sea level
Authors: Moore T. C. , Loutit T. S. , Greenlee S. M.
Published: February 2008

The history of short‐term eustatic sea level change proposed by P. R. Vail and his coworkers was originally based on shifts through time in the landward pinchout of seismic sequences. In order to quantify the magnitude of these proposed sea level changes, histories of subsidence, sediment loading, sediment compaction and paleowater depth are required. Of these, accurate estimates of paleowater depth and subsidence are the most difficult to derive. This approach is applied to two example data sets. For one study, the Midway Atoll, we use the theoretical subsidence model of Detrick et al. (1981). The possible error in this model is approximately the same as the extreme differences in sea level highstands (50–100 m) proposed by previous investigators. The second example is from the Baltimore Canyon area. Here we estimate subsidence by an average angular tilt rate. Resulting estimates of sea level highstands for the past 30 Ma are similar to the long‐term sea level estimates of Kominz (1984); however, the overall variation in the highstands parallels that of Haq et al. (1987). Short‐term sea level falls are estimated to be similar in magnitude to those of Haq et al. (1987). Most of these short‐term fluctuations are within the range of reasonable glacioeustatic fluctuations (30–100 m). A correlation is found between the estimated short‐term changes in sea level and the Miocene oxygen isotope record. This correlation exists both prior to and after 14 Ma; however, there is a 1.08‰ isotopic shift in the data in the Middle Miocene. Because estimated highstands are within 10–15 m of being the same magnitude before and after this shift, we suggest that this shift is caused mainly by a cooling of deep waters.
Correction of accelerator mass spectrometry14C ages measured in planktonic foraminifera: Paleoceanographic implications
Authors: Bard Edouard
Published: February 2008

Carbon 14 dates obtained by accelerator mass spectrometry (AMS) on foraminiferal samples from deep‐sea sediment cores must be corrected for the difference in14C composition between the atmosphere and the sea surface. In the modern ocean, the “apparent age” of carbonate shells formed in surface waters varies between 300 and 1200 years and depends mainly on latitude. The time variation of this parameter during climate oscillations of the last 40,000 years may have been significant: there should have been small changes for most of the ocean between 40°S and 40°N, but an increase of the apparent age by several hundred years should be expected at high latitudes in response to subpolar/subtropical front movements. The North Atlantic is likely to have experienced the most significant changes, due to large variations in the mode and rate of North Atlantic Deep Water production. These hypothetical changes may be measured by coupled AMS14C dating of contemporaneous planktonic foraminifera and terrestrial organic matter (pollen, charcoal, wood, etc.) which occur in the same core or are stratigraphically linked by the same volcanic ash layer. The Δ(14C atmosphere,14C sea surface) can be viewed as a new paleoceanographic tracer which may provide additional information about high latitude surface waters complementary to those obtained with13C/12C and Cd/Ca ratios measured in planktonic foraminifera.
A different perspective on glauconite as a chronometer for geologic time scale studies
Authors: Obradovich John D.
Published: February 2008

There are advocates of the use of glauconite as a chronometer for geologic time scale studies who insist that if the geohistory of the sample in question is known in detail, one can ascertain before hand that the sample will yield a radiometric result that is accurate in the geological sense. Examples are presented indicating that while some glauconite samples may yield the correct age within their assigned analytical uncertainties when compared to high‐temperature minerals, others clearly do not and are too young by varying degrees. The outright acceptance of published results on glauconite can result in erroneous age estimates of various biostratigraphic levels and stage or epoch boundaries.
Tidal and sedimentary response to the Late Quaternary closure and opening of Cook Strait, New Zealand: Results from numerical modeling
Authors: Proctor Roger , Carter Lionel
Published: February 2008

Evidence from numerical modeling of the M2 tide, supplemented by geological data, reveals marked changes in the tidal and sedimentary regimes of Cook Strait over the last postglacial rise in sea level. Approximately 18,000 years B.P., when sea level was approximately −113 m, the strait was reduced to a narrow bay by the emergence of a land bridge connecting the North and South islands of New Zealand. High sediment supply and low, tidally induced bottom stress encouraged widespread deposition of fine sediment except at the bay mouth (the Narrows) where localized high stress resulted in the transport of sediment to the deep ocean via Cook Strait canyons. At 17,000 years B.P. (approximately −88 m), the bay reached maximum size and was near resonance with the oceanic tide resulting in high tidal amplitudes, bottom stress, and sediment movement at the Narrows and bay head. The land bridge was breached and Cook Strait was formed about 15,000 years B.P. (approximately −75 m). Both the Narrows and breach were subject to high bottom stress, whereas the intervening area was a zone of convergent low stress and fine sediment deposition. By 11,000 years B.P. (approximately −40 m) Cook Strait was well established and had a tidal response similar to the modern strait. The rising sea levels inundated remnants of the land bridge to form the broad continental shelf approaches to Cook Strait. Today, overall tidal stress is reduced, and mud deposition prevails except on the inner shelf where periodic forcing by wind‐induced currents maintains a sandy substrate, and in the Narrows where a tidal phase difference of approximately 120° maintains a zone of high sediment movement.
Late Pliocene variation in northern hemisphere ice sheets and North Atlantic deep water circulation
Authors: Raymo M. E. , Ruddiman W. F. , Backman J. , Clement B. M. , Martinson D. G.
Published: February 2008

High‐resolution records of δ18O, δ13O, and percent CaCO3from the late Pliocene North Atlantic (Deep Sea Drilling Project sites 607 and 609) are presented and oxygen isotope stages are formalized back to stage 116 at 2.73 Ma. From 2.8 to 1.6 Ma, the interval studied, variations in these records were dominated by the 41‐kyr component of orbital obliquity. Significant variation at the orbital frequencies of eccentricity (96‐kyr) and precession (23‐kyr) are observed in the δ18O record between 1.6 and 2.1 Ma, but not before. Prior to 2.4 Ma (stage 100), δ18O variations suggest ice sheet growth 1/4 to 1/2 as large as late Pleistocene ice volumes; however, these events are below the threshold needed to result in extensive ice‐rafting to the open North Atlantic Ocean. After 2.4 Ma, ice sheets appear to be, on average, 1/2 as large as those of the late Pleistocene. The δ18O record indicates that some glacial suppression of North Atlantic Deep Water occurred both before and after 2.4 Ma and that glacial‐interglacial transfers of12C between the continents and oceans appear to have been larger in the late Pliocene relative to the late Pleistocene. In addition, the strong 23‐kyr power observed in δ18O between 2.75 and 2.10 Ma suggests that deep‐sea circulation (or changes in biomass) is controlled, in part, by climatic variations unrelated to ice sheets.
Variations in high‐latitude oceanographic fronts in the southern Indian Ocean: An estimation based on faunal changes
Authors: Morley Joseph J.
Published: February 2008

A quantitative estimate of the magnitude of late Pleistocene/Holocene variations in the migration of siliceous microfauna is calculated from fossil assemblages preserved in marine sediments from piston cores located in the southeast Indian Ocean sector of the Subantarctic. Results of this modern analog technique indicate that fauna characteristic of colder waters today occupied surface and near‐surface waters overlying the core sites for much of the last half million years. Warm‐water fauna inhabited these waters for only brief time periods, usually during the early portion of interglacial isotopic stages. The estimated faunal migration pattern from the most southern sites is similar in most respects to that of atmospheric temperatures over Antarctica recorded in the Vostok ice core. A model of changes in the positions of the Subtropical Convergence and the Antarctic Polar Front, constructed from time series of faunal variations, indicates that although southward migrations of these oceanographic fronts in this region of the southern ocean were limited to approximately 4° of latitude, northward displacements were of the order of 5° to 7° of latitude. Fauna at the Antarctic Polar Front react earlier than either the fauna at the Subtropical Convergence or the global ice volume signal to major global warming (terminations), providing further documentation of the early response of the high‐latitude southern ocean relative to other components of the climate system to the various factors forcing Pleistocene climate.