bluuebunny/crossref_metadata_2025 huggingface dataset:
North Atlantic Intermediate to Deep Water circulation and chemical stratification during the past 1 Myr
Authors: Flower B. P. , Oppo D. W. , McManus J. F. , Venz K. A. , Hodell D. A. , Cullen J. L.
Published: February 2004

Benthic foraminiferal carbon isotope records from a suite of drill sites in the North Atlantic are used to trace variations in the relative strengths of Lower North Atlantic Deep Water (LNADW), Upper North Atlantic Deep Water (UNADW), and Southern Ocean Water (SOW) over the past 1 Myr. During glacial intervals, significant increases in intermediate‐to‐deep δ13C gradients (commonly reaching >1.2‰) are consistent with changes in deep water circulation and associated chemical stratification. Bathymetric δ13C gradients covary with benthic foraminiferal δ18O and covary inversely with Vostok CO2, in agreement with chemical stratification as a driver of atmospheric CO2 changes. Three deep circulation indices based on δ13C show a phasing similar to North Atlantic sea surface temperatures, consistent with a Northern Hemisphere control of NADW/SOW variations. However, lags in the precession band indicate that factors other than deep water circulation control ice volume variations at least in this band.
Variations in Miocene phytoplankton growth rates in the southwest Atlantic: Evidence for changes in ocean circulation
Authors: Pagani Mark , Arthur Michael A. , Freeman Katherine H.
Published: February 2004

Changes in ocean circulation are often credited as the primary control on large‐scale climate change during the Miocene. This study investigates the latest Oligocene to middle Miocene evolution of Southern Ocean circulation by evaluating stable isotopic trends of shallow‐ and deep‐dwelling planktonic foraminifera, as well as εprecords reconstructed from the carbon isotopic composition of diunsaturated alkenones in the southwestern Altantic Ocean (Deep Sea Drilling Project site 516). Changes in εpat site 516 closely paralleled the opening and deepening of the Drake Passage as inferred from seafloor magnetic anomalies. A large negative shift in εpat ∼20.3 Ma is interpreted to reflect an increase in upper water column nutrient concentrations, caused by the onset or strengthening of the Antarctic Circumpolar Current (ACC). Measurable alkenone concentrations disappear by ∼17 Ma, prior to a collapse in surface‐to‐thermocline δ18O and δ13C gradients. This is interpreted as reflecting a severe decrease in mixed layer nutrient concentrations and reduced proto‐Antarctic Intermediate Water influence. The δ18O gradient was reestablished by 14.5 Ma, coincident with the hypothesized East Antarctic ice sheet expansion, suggesting a direct relationship between increased strength of the ACC and the largest climate shift of the middle Miocene.
Stratospheric temperature trends: Observations and model simulations
Authors: Ramaswamy V. , Chanin M.‐L. , Angell J. , Barnett J. , Gaffen D. , Gelman M. , Keckhut P. , Koshelkov Y. , Labitzke K. , Lin J.‐J. R. , O'Neill A. , Nash J. , Randel W. , Rood R. , Shine K. , Shiotani M. , Swinbank R.
Published: February 2004

Trends and variations in global stratospheric temperatures are an integral part of the changes occurring in the Earth's climate system. Data sets for analyzing long‐term (a decade and more) changes in the stratospheric temperatures consist of radiosonde, satellite, lidar, and rocketsonde measurements; meteorological analyses based on radiosonde and/or satellite data; and products based on assimilating observations using a general circulation model. Each of these contain varying degrees of uncertainties that influence the interpretation and significance of trends. We review the long‐term trends from approximately the mid‐1960s to the mid‐1990s period. The stratosphere has, in general, undergone considerable cooling over the past 3 decades. At northern midlatitudes the lower stratosphere (∼16–21 km) cooling over the 1979–1994 period is strikingly coherent among the various data sets with regard to magnitude and statistical significance. A substantial cooling occurs in the polar lower stratosphere during winter‐spring; however, there is a large dynamical variability in the northern polar region. The vertical profile of the annual‐mean stratospheric temperature change in the northern midlatitudes over the 1979–1994 period is robust among the different data sets, with ∼0.75 K/decade cooling in the ∼20‐ to 35‐km region and increasing cooling above (e.g., ∼2.5 K/decade at 50 km). Model investigations into the cause or causes of the observed temperature trends are also reviewed. Simulations based on the known changes in species' concentrations indicate that the depletion of lower stratospheric ozone is the major radiative factor in accounting for the 1979–1990 cooling trend in the global, annual‐mean lower stratosphere (∼0.5 to 0.6 K/decade), with a substantially lesser contribution by the well‐mixed greenhouse gases. Ozone loss is also an important causal factor in the latitude‐month pattern of the lower stratospheric cooling trend. Uncertainties arise due to incomplete knowledge of the vertical profile of ozone loss near the tropopause. In the middle and upper stratosphere, both well‐mixed greenhouse gases and ozone changes contribute in an important manner to the cooling, but model simulations underestimate the observed decadal‐scale trend. While there is a lack of reliable information on water vapor changes over the 1980s decade, satellite measurements in the early to middle 1990s indicate increases in water vapor that could be a significant contributor to the cooling of the global lower stratosphere.
Influence of the spatial distribution of vegetation and soils on the prediction of cumulus Convective rainfall
Authors: Pielke Roger A.
Published: February 2004

This paper uses published work to demonstrate the link between surface moisture and heat fluxes and cumulus convective rainfall. The Earth's surface role with respect to the surface energy and moisture budgets is examined. Changes in land‐surface properties are shown to influence the heat and moisture fluxes within the planetary boundary layer, convective available potential energy, and other measures of the deep cumulus cloud activity. The spatial structure of the surface heating, as influenced by landscape patterning, produces focused regions for deep cumulonimbus convection. In the tropics, and during midlatitude summers, deep cumulus convection has apparently been significantly altered as a result of landscape changes. These alterations in cumulus convection teleconnect to higher latitudes, which significantly alters the weather in those regions. The effect of tropical deforestation is most clearly defined in the winter hemisphere. In the context of climate, landscape processes are shown to be as much a part of the climate system as are atmospheric processes.
The role of cumulus convection in hurricanes and its representation in hurricane models
Authors: Smith Roger K.
Published: February 2004

This paper reviews our understanding of the role of cumulus convection in hurricanes as well as the various convective parameterization schemes that have been used in hurricane models. Elementary principles show that the primary (tangential) circulation of a vortex intensifies as rings of rotating air converge inward while conserving their (absolute) angular momentum. Thus intensification requires a mechanism to produce enough flow convergence above the surface boundary layer to counter the divergence induced there by the boundary layer itself. Typically, such convergence is associated with the inward branch of the secondary circulation and is produced by an unbalanced negative radial gradient of buoyancy above the boundary layer resulting from condensational heating in the inner region of the vortex. The fact that such buoyancy gradients are produced by all the parameterization schemes, as well as by explicit schemes for latent heat release, explains why all the models are able to simulate hurricane intensification with some degree of realism. In a weak vortex the secondary circulation is dominated by buoyant forcing, but as the vortex intensifies the contribution from surface friction increases until in the mature stage, the buoyantly induced convergence must closely balance the frictionally induced divergence just above the boundary layer. Only a handful of the more recent hurricane models and only two of those in which convection is parameterized represent the effects of convective downdrafts. These downdrafts cool and dry the subcloud layer, tending to suppress further convection, so that acting alone they would serve as a brake on hurricane intensification. The strength of downdrafts decreases when the middle‐tropospheric relative humidity increases as a result of sustained cumulus convection over an area. If surface wind speeds are high enough, surface fluxes of sensible and latent heat can more than counteract the cooling and drying effects of convective downdrafts, allowing continued warming of the troposphere in the inner region of the vortex so that vortex intensification can proceed. Accordingly, the surface fluxes provide the energy source for the hurricane, while convection transfers this energy vertically through the troposphere, creating a suitable radial gradient of buoyancy to drive the secondary circulation of the vortex.
Fast analysis of electromagnetic scattering from finite strip gratings on a grounded dielectric slab
Authors: Valero Alejandro , Rojas Roberto G.
Published: February 2004

A fast and accurate algorithm is explored for solving two‐dimensional electromagnetic scattering from electrically large finite strip arrays on grounded dielectric slabs. The induced current is computed efficiently using the novel spectral algorithm, a recently proposed fast single‐level matrix‐vector product algorithm that reduces the operation count to O(N), where N is the number of unknowns. This algorithm was originally cast into a forward‐backward method, but it can equally be used to accelerate the matrix‐vector products on conjugate gradient techniques. Both, transverse magnetic and transverse electric polarizations have been considered.
Effects of humidity and precipitation on VHF radar vertical beam echoes
Authors: Vaughan G. , Worthington R. M.
Published: February 2004

We investigate in this paper the variation in power of VHF radar vertical echoes as a function of atmospheric humidity. We find that the echoes are greatest in air of moderate (30–70%) humidity, and least in very dry or near‐saturated air. Compared to moderate humidities, the standard model for radar echoes, based on potential refractivity, overpredicts the echo power at high relative humidity by ∼10 dB. We propose that this is due to the effect of precipitation in suppressing small‐scale humidity gradients. A model which assumes complete suppression underpredicts the observed power drastically, showing that precipitation can only remove a part of the small‐scale structure in refractivity which is caused by fluctuations in humidity.
SOMARE‐99: A demonstrational field campaign for ultrahigh‐resolution VHF atmospheric profiling using frequency diversity
Authors: Chilson Phillip B. , Palmer Robert D. , Muschinski Andreas , Hooper David A. , Schmidt Gerhard , Steinhagen Hans
Published: February 2004

During May of 1999 the sounding system (SOUSY) VHF radar was operated in a demonstrational field campaign specially designed to test the applicability of range imaging (RIM) to radar studies of the atmosphere. The RIM technique utilizes frequency diversity to offer a novel method of improving radar range resolution over that which can be obtained with a conventional pulsed radar with the same bandwidth. During the field campaign, which is being called SOUSY Multifrequency Atmospheric Radar Experiment 1999 (SOMARE‐99), the application of RIM on a VHF radar has been demonstrated. The data from SOMARE‐99 are intended for investigating the dynamics and morphology of fine‐scale vertical structures in the troposphere. This paper gives an overview of SOMARE‐99 and provides some initial scientific results. A central and important result was obtained by comparing RIM‐processed radar observations and data from radiosondes. Profiles of the vertical gradient of the generalized refractive index and the so‐called RIM‐enhanced echo power are found to bear similarities during one particular case, for which the radiosonde was located directly over the radar. Furthermore, a spectral analysis of these two parameters has provided evidence that RIM is successfully identifying multiple structures within the radar sampling volume with scales smaller than the conventional range resolution.
Geophysical constraints on the location and geometry of the Las Vegas Valley Shear Zone, Nevada
Authors: Langenheim V. E. , Grow J. A. , Jachens R. C. , Dixon G. L. , Miller J. J.
Published: February 2004

We model the basin configuration beneath Las Vegas Valley, Nevada, on the basis of gravity, drillhole, and seismic reflection data. We locate and characterize the various strands of the Las Vegas Valley Shear Zone (LVVSZ) by variations in basin thickness beneath the valley. The pre‐Tertiary bedrock surface is complex, with subbasins buried beneath the flat alluvial surface of the valley. We suggest that these basins are formed from transtensional strain. Subbasins elongated N70°W and N50°W are interpreted as strike‐slip basins. The deepest subbasin is 5 km west of Frenchman Mountain and strikes N40°E. This basin probably formed by combined movement on nonparallel strands of the LVVSZ and an earlier episode of normal faulting. The basin thickness map constrains the minimum depth of the inferred detachment fault beneath Las Vegas Valley to at least 4 km. Seismic reflection data do not image a detachment fault in the upper 10 km beneath Las Vegas Valley. Our results also illustrate the utility of gravity in determining basinal structures and providing a three‐dimensional perspective in areas with limited seismic reflection control.
Extension and basin formation in the southern Andes caused by increased convergence rate: A mid‐Cenozoic trigger for the Andes
Authors: Jordan Teresa E. , Burns W. Matthew , Veiga Ricardo , Pángaro Francisco , Copeland Peter , Kelley Shari , Mpodozis Constantino
Published: February 2004

The southern Andes between 33° and 45°S latitude are characterized by a series of complex basins that spanned the contemporaneous active continental margin, which itself was characterized by volcanic activity. The basins are filled with thick (up to 3000 m) accumulations of interbedded sedimentary and volcanic strata of late Oligocene‐early Miocene age. We interpret that these basins developed during a phase of moderate extension within the plate margin system, triggered by an increased rate of convergence of the Farallon (Nazca) and South American plates between 28 and 26 Ma. This history is inconsistent with models of convergence that link high rates of convergence of a continental margin and an oceanic plate to strong compressional coupling. Although extensional basins of this age are only well‐described in the southern Andes, the convergence history and volcanic chronology are similar farther north in the central Andes (18°–33°S), leading to the speculation that extension may have characterized the late Oligocene‐early Miocene interval regionally. We hypothesize that this extension was a necessary condition to subsequent building of the modern Andes Mountains.
Estimating the convective supply of nitrate and implied variability in export production over the North Atlantic
Authors: Williams Richard G. , McLaren Alison J. , Follows Michael J.
Published: February 2004

The maintenance and interannual variability of export production is partly controlled by the convective and Ekman supply of nutrients to the euphotic zone. This dynamical supply of nitrate to the euphotic zone is estimated over the North Atlantic from 1968 to 1993. First, a matrix of one‐dimensional mixed‐layer models are integrated over the North Atlantic forced by surface and Ekman fluxes of heat and freshwater. Second, nitrate fluxes to the euphotic zone are estimated by combining the mixed‐layer thickness cycle and wind stress data with climatological nitrate profiles. The model suggests that the convective supply of nitrate ranges from 0.1 mol N m−2 yr−1 in the subtropics to 1.4 mol N m−2 yr−1 over the subpolar gyre. The interannual variability in convective supply reaches ±0.2 mol N m−2 yr−1. This variability in nitrate supply is significantly correlated with the North Atlantic Oscillation index over parts of the central and western Atlantic but not over the eastern Atlantic. This convectively induced variability should modulate the levels of export production wherever the nitrate supply is limiting. The Ekman transfer is relatively small, reaching 0.1 mol N m−2 yr−1 over the subpolar gyre, but is important in maintaining nitrate concentrations within the seasonal boundary layer.
Calculation of global carbon dioxide emissions: Review of emission factors and a new approach taking fuel quality into consideration
Authors: Hiete Michael , Berner Ulrich , Richter Otto
Published: February 2004

Anthropogenic carbon dioxide emissions resulting from fossil fuel consumption play a major role in the current debate on climate change. Carbon dioxide emissions are calculated on the basis of a carbon dioxide emission factor (CEF) for each type of fuel. Published CEFs are reviewed in this paper. It was found that for nearly all CEFs, fuel quality is not adequately taken into account. This is especially true in the case of the CEFs for coal. Published CEFs are often based on generalized assumptions and inexact conversions. In particular, conversions from gross calorific value to net calorific value were examined. A new method for determining CEFs as a function of calorific value (for coal, peat, and natural gas) and specific gravity (for crude oil) is presented that permits CEFs to be calculated for specific fuel qualities. A review of proportions of fossil fuels that remain unoxidized owing to incomplete combustion or inclusion in petrochemical products, etc., (stored carbon) shows that these figures need to be updated and checked for their applicability on a global scale, since they are mostly based on U.S. data.
Modeling the volcanic signal in the atmospheric CO2 record
Authors: Jones Chris D. , Cox Peter M.
Published: February 2004

There is significant interannual variability in the atmospheric concentration of carbon dioxide even when the effect of anthropogenic sources has been accounted for. It has been shown that this variability is correlated with the El Nino Southern Oscillation (ENSO) cycle [Bacastow, 1976; Keeling et al., 1995]. However, there are periods during the atmospheric CO2 record when this correlation does not hold and CO2 levels are much lower than can be explained by the correlation with ENSO. These periods coincide with major volcanic eruptions. It has been well documented that a major eruption has a cooling effect on the surface and lower troposphere [McCormick, 1992; Hansen, et al., 1996]. Here we show that it is likely that this cooling has a significant and measurable effect on the carbon cycle. We use a coupled general circulation climate‐carbon cycle model to study the mechanisms involved. The model simulates the observed temperature and CO2 response of the climate to the 1991 eruption of Mount Pinatubo. The surface cooling due to the eruption leads to reduced soil and plant respiration globally and increased gross primary productivity in the tropics. The result is significant uptake of carbon (1‐2 GtC yr−1) by the terrestrial biosphere for several years after the eruption. There is no significant variation in uptake or release of carbon by the oceans.
Comparison of global net primary production trends obtained from satellite‐based normalized difference vegetation index and carbon cycle model
Authors: Ichii Kazuhito , Matsui Yohei , Yamaguchi Yasushi , Ogawa Katsuro
Published: February 2004

The global terrestrial net primary production (NPP) trend was estimated from two independent methods, satellite observation data and a carbon cycle model, and the results were compared for validation. The satellite‐based NPP trend was estimated from the incoming surface solar radiation data set and a National Oceanic and Atmospheric Administration/ Advanced Very High Resolution Radiometer data set that was corrected by normalized difference vegetation index in areas of desert and dense vegetation. The increase in NPP from the Goddard Institute for Space Studies solar radiation data set and from the LaRC solar radiation data set over 10 years in the 1980s was estimated to be 1.8 and 4.4%, respectively. The NPP trend based on a carbon cycle model was estimated from a simple carbon cycle model that was established for the period 1850–1990 with biospheric and oceanic carbon cycle history constraints. The historical trend obtained from the model correlates well with the time variation of not only the observed atmospheric CO2 but also the biospheric and oceanic carbon cycle history. Terrestrial NPP shows an increasing trend beginning in 1930 and is estimated to increase at a rate of 1.1% over the 10‐year period in the 1980s. Although all these methods show a recent increase in NPP, satellite‐based estimation using the LaRC data set shows a larger trend than the others. A comparison of he trends estimated by these methods indicates that it is necessary to improve the accuracy of incoming surface radiation data, CO2 emission history from changes in land‐use change and model structure.
Volatile organic trace gases emitted from North American wildfires
Authors: Friedli H. R. , Atlas E. , Stroud V. R. , Giovanni L. , Campos T. , Radke L. F.
Published: February 2004

Trace gases from wildfires and prescribed burns were collected from the National Science Foundation (NSF)/National Center for Atmospheric Research (NCAR) C‐130 aircraft at fire locations in temperate forests (Montana, Colorado) and sage scrub (California). Comprehensive compositions for hydrocarbons, partially oxidized hydrocarbons (POHC), halocarbons, alkyl nitrates, and some sulfur compounds were determined in 99 samples. All of these trace gas emission groups correlated linearly with CO emissions. Fires in temperate forests emitted ∼28% more nonmethane hydrocarbons (NMHC) and 120% more POHC than the California fire and the molecular weight range of the emitted hydrocarbons was higher. The POHC contribution relative to NMHC emissions is significant: 26 and 38% for California and temperate forests, respectively. Since the observed POHCs (mostly ketones, aldehydes, and furans) typically react faster than many NMHC and undergo cascades of photochemical degradations, they should be included in calculations of the effect that fires have on the local and global oxidative capacity of the atmosphere. The composition of the hydrocarbons and POHCs in the plumes varied with combustion efficiency, displaying a distinct pattern, which reflected fire chemistry. Interfire differences were evident among fires in temperate forests. The dominant halocarbon emissions from all fires were methyl halides. The emission ratio was always largest for methyl chloride and was dependent on vegetation and fire location. C1‐C5alkyl nitrates were found in all fire emissions and showed little dependence on the location of the fire. The major organic sulfur compound measured was carbonyl sulfide. Minor amounts of dimethyl sulfide and dimethyl disulfide were observed. Budget estimates for emissions from fires in temperate forests were made based on the measured emission ratios and published carbon monoxide estimates.
Budgets of soil erosion and deposition for sediments and sedimentary organic carbon across the conterminous United States
Authors: Smith S. V. , Renwick W. H. , Buddemeier R. W. , Crossland C. J.
Published: February 2004

The fate of soil organic matter during erosion and sedimentation has been difficult to assess because of the large size and complex turnover characteristics of the soil carbon reservoir. It has been assumed that most of the carbon released during erosion is lost to oxidation. Budgets of bulk soil and soil organic carbon erosion and deposition suggest that the primary fates of eroded soil carbon across the conterminous United States are trapping in impoundments and other redeposition. The total amount of soil carbon eroded and redeposited across the United States is ∼0.04 Gt yr−1. Applying this revision to the U. S. carbon budget byHoughton et al.[1999] raises their net sequestration estimate by 20–47 %. If comparable rates of erosion and redeposition occur globally, net carbon sequestration would be ∼1 Gt yr−1.
Controls on soil nitrogen oxide emissions from forest and pastures in the Brazilian Amazon
Authors: Garcia‐Montiel Diana C. , Steudler Paul A. , Piccolo Marisa C. , Melillo Jerry M. , Neill Christopher , Cerri Carlos C.
Published: February 2004

Soil N dynamics and water content are important controls of nitrous (N2O) and nitric (NO) oxide emissions from tropical soils. We used a chronosequence of one forest and six pastures to investigate how soil N availability and soil moisture content affect emissions of N2O and NO. Forest soils had high N availability and large N oxide emissions. Forest conversion to pasture decreased N availability and N oxide emissions. In the forest where N availability was high, seasonal changes in soils moisture led to higher N2O fluxes during the wet season, but higher NO fluxes during the dry season. Soil water content exerted the largest control on N2O emissions, which seemed to be determined by a threshold value of approximately 29–30%, below which N2O emissions were nearly constant and above which they increased (Figure 5a). In pastures, where low N availability constrains N2O and NO fluxes, soil water content has a minor influence on N oxide emissions.
Introduction to Special Section: Ocean Measurements and Models of Carbon Sources and Sinks
Authors: Wallace Douglas W. R.
Published: February 2004

This issue of Global Biogeochemical Cycles contains a remarkable set of papers, which critically evaluate a variety of model‐ and observation‐based approaches addressing the oceanic distribution, storage, and transport of CO2. Three of the papers are concerned with observation‐based estimates of excess (or anthropogenic) CO2 [Coatanoan et al., this issue; Sabine and Feely, this issue; Chen, this issue]. They focus on the approaches, assumptions, and uncertainties involved in detecting the excess CO2 signal above the ocean's large and variable natural dissolved inorganic carbon (DIC) background.
Self‐consistent generation of tectonic plates in time‐dependent, three‐dimensional mantle convection simulations 2. Strain weakening and asthenosphere
Authors: Tackley Paul J.
Published: February 2004

In a previous paper [Tackley, 2000b] it was shown that a combination of temperature‐dependent viscosity and viscoplastic yielding is sufficient to give rudimentary plate tectonic‐like behavior in three‐dimensional models of mantle convection. Here the calculations are extended to include two complexities that have been suggested as being important in localizing deformation at plate boundaries: strain weakening and the presence of a low‐viscosity asthenosphere. Introducing an asthenosphere by reducing the viscosity by a factor of 10 where material reaches a solidus (1) dramatically improves plate quality, even if the asthenosphere is restricted to regions around spreading centers, (2) gives good, smoothly evolving platelike behavior over a wide range of yield stress values spanning an order of magnitude, and (3) gives bimodal stable solutions over a range of yield stresses; either immobile‐lid or plate behavior may be obtained, depending on initial condition. By contrast, introducing an asthenosphere by using depth‐dependent viscosity has a much smaller effect on system behavior. Increasing strain weakening (1) improves localization at spreading centers but leads to an increasingly complex network of spreading centers fragmenting the plates, (2) weakens convergent zones and can make downwellings highly episodic, and (3) does not lead to pure transform boundaries in these calculations. Time‐dependent “damage” evolution and instantaneous strain‐rate weakening give very similar results. All cases that display platelike behavior have very long wavelength mantle heterogeneity, consistent with Earth's.
Changes in North Atlantic deep‐sea temperature during climatic fluctuations of the last 25,000 years based on ostracode Mg/Ca ratios
Authors: Dwyer Gary S. , Cronin Thomas M. , Baker Paul A. , Rodriguez‐Lazaro Julio
Published: February 2004

We reconstructed three time series of last glacial‐to‐present deep‐sea temperature from deep and intermediate water sediment cores from the western North Atlantic using Mg/Ca ratios of benthic ostracode shells. Although the Mg/Ca data show considerable variability (“scatter”) that is common to single‐shell chemical analyses, comparisons between cores, between core top shells and modern bottom water temperatures (BWT), and comparison to other paleo‐BWT proxies, among other factors, suggest that multiple‐shell average Mg/Ca ratios provide reliable estimates of BWT history at these sites. The BWT records show not only glacial‐to‐interglacial variations but also indicate BWT changes during the deglacial and within the Holocene interglacial stage. At the deeper sites (4500‐ and 3400‐m water depth), BWT decreased during the last glacial maximum (LGM), the late Holocene, and possibly during the Younger Dryas. Maximum deep‐sea warming occurred during the latest deglacial and early Holocene, when BWT exceeded modern values by as much as 2.5°C. This warming was apparently most intense around 3000 m, the depth of the modern‐day core of North Atlantic deep water (NADW). The BWT variations at the deeper water sites are consistent with changes in thermohaline circulation: warmer BWT signifies enhanced NADW influence relative to Antarctic bottom water (AABW). Thus maximum NADW production and associated heat flux likely occurred during the early Holocene and decreased abruptly around 6500 years B.P., a finding that is largely consistent with paleonutrient studies in the deep North Atlantic. BWT changes in intermediate waters (1000‐m water depth) of the subtropical gyre roughly parallel the deep BWT variations including dramatic mid‐Holocene cooling of around 4°C. Joint consideration of the Mg/Ca‐based BWT estimates and benthic oxygen isotopes suggests that the cooling was accompanied by a decrease in salinity at this site. Subsequently, intermediate waters warmed to modern values that match those of the early Holocene maximum of ∼7°C. Intermediate water BWT changes must also be driven by changes in ocean circulation. These results thus provide independent evidence that supports the hypothesis that deep‐ocean circulation is closely linked to climate change over a range of timescales regardless of the mean climate state. More generally, the results further demonstrate the potential of benthic Mg/Ca ratios as a tool for reconstructing past ocean and climate conditions.
Review of alkenone calibrations (culture, water column, and sediments)
Authors: Herbert T. D.
Published: February 2004

I review the status of alkenone unsaturation parameters as proxies for growth temperature and the potential for pCO2 estimates using the carbon isotopic composition of alkenones. Culture studies allow investigators to manipulate parameters that could influence the relative proportions of C37 and C38 ketones in producing organisms and to assess the possible influence of genetic variability on the calibration of unsaturation to growth temperature. Culture studies suggest large variability in the relation of unsaturation indices to growth temperature and hint that growth rate may play a role along with temperature in controlling unsaturation. Water column studies match the unsaturation index measured in particulate matter to ambient temperature, providing a snapshot of unsaturation‐temperature relationship. In comparison to culture work, assemblages of regional water column data sets give more consistent relations of the unsaturation index to temperature, although the argument can be made for regional rather than basin‐wide or global calibrations. A much simpler picture emerges from analyzing a global array of core top sediments. Regressions show that the sedimentary unsaturation indices follow a linear trend that is best correlated to mean annual temperature in the upper 10 m of the overlying water column. The regression is indistinguishable from the original Prahl et al. [1988] culture calibration of the U37k′ index to growth temperature. Exceptions occur at high latitudes, where the effects of highly seasonal production and the possible influence of salinity may affect the unsaturation index in sediments. Core top analyses also fail to reveal the variability of other alkenone parameters (for example, the ratio of C37 to C38 ketones) which have been found in culture studies. Much of the variance that appears in culture and water column calibrations is thus apparently removed in sediments, perhaps by temporal averaging. The promising paleo‐pCO2 approach based on the isotopic composition of alkenones awaits the arrival of a significant number of core top and stratigraphic studies that will determine the extent to which physiological factors compromise the alkenone paleobarometer.
Alkenone temperature and carbon isotope records: Temporal resolution, offsets, and regionality
Authors: Schneider Ralph
Published: February 2004

Over the last decade the number of paleoceanographic studies presenting and discussing alkenone‐derived sea surface temperature (SST) records and carbon isotope ratios of alkenones (δ13Calkenones) has increased rapidly. This short review paper summarizes the state of knowledge about SST variability determined by alkenone paleothermometry in sediment trap studies and investigations of marine sediments for timescales of single to millions of years. In particular from the evaluation of Quaternary records, characteristic temperature pattern have emerged that mismatch with SST curves estimated by other methods, e.g., foraminiferal stable oxygen isotopes and transfer functions. In future studies the spatial distribution of such variance patterns typically observed in alkenone SST curves should be checked in order to better understand their meaning for ocean circulation changes and a possible relationship to global climate change. On the other hand, certain physical and biological processes have the potential to put the reliability of alkenone SST curves into question. Accordingly, examples for possible effects of bioturbational mixing, lateral transport of remote alkenones, diagnetic alteration and degradation of alkenone composition, biogeographic and temporal diversity changes in alkenone‐producing algae, as well as the influence of changes in salinity and nutrient conditions on the alkenone paleotemperature signal, are briefly mentioned, highlighting the need for more detailed future studies on these issues. Finally, a short summary on the status of the use of the δ13Calkenones signal as a proxy for past variations in the carbon dioxide pressure (PCO2) in ocean surface waters and related problems is given.
Paleomagnetism and 40Ar/39Ar ages from La Palma in the Canary Islands
Authors: Tauxe Lisa , Staudigel Hubert , Wijbrans Jan R.
Published: February 2004

The structure of the time‐averaged geomagnetic field has been known forcenturies to be approximately dipolar. Significant departures of the time‐averagedfield from that of an axial geocentric dipole, however, have been reportedfor decades. The data on which time‐averaged field models are based must beof the highest quality in order to document subtle long‐term features. Wepresent here new paleomagnetic data and 40Ar/39Ar ages for the island of La Palma, The Canary Islands.Paleomagnetic samples were obtained from 28 lava flows. Of these, 21 met ourminimum acceptance criteria for use in time‐averaged field models. The 40Ar/39Ar age determinationswere successfully carried out on samples from eight of the flows. Our isotopicages and paleomagnetic polarities are consistent with the currently acceptedgeomagnetic reversal timescales. The reversed data in the updated databaseare antipodal to the normal data within the uncertainties, and the time‐averageddirection is indistinguishable from that expected from a geocentric axialdipole.
Major element, rare earth element, and sulfur isotopic composition of a high‐elevation firn core: Sources and transport of mineral dust in central Asia
Authors: Kreutz Karl J. , Sholkovitz Edward R.
Published: February 2004

The arid region extending from the Caspian Sea into China contains some of the most extensive dust aerosol production areas in the world. Major element (Al, Fe, Ca, and S), rare earth element (REE), and isotopic (δ34S and δ18O) data from a 14.4‐m‐long firn core recovered on the Inilchek Glacier (Tien Shan Mountains, Kyrgyzstan; 5100‐m elevation) are presented and used to investigate the composition, sources, and transport of mineral dust to this central Asian site. The combination of geochemical compositions points to three distinct types of mineral deposits (loess, calcium carbonate, and gypsum), which do not covary in a linear fashion during the period recorded in the firn core, (1992–1998). Loess, identified by its REE composition, is characterized by distinct large‐scale deposition events and by ubiquitous background deposition at other times. Significant interannual‐scale variability is evident, with the two largest loess events occurring within a single accumulation year, (1997) and background conditions prevailing during 1994–1996. REE compositions, specifically Nd/Yb ratios, point to multiple sources and transport directions of loess to Inilchek Glacier. Nonloess sources provide a significant fraction of the total Ca and S deposited at the Inilchek Glacier. Hence the downcore profiles of Ca and S reflect variability in the deposition of calcium carbonate and gypsum. Differences in the sulfur isotopic composition (δ34S) of high dust period (+15.0‰) and low (background) dust period (+5.4‰) samples provide strong evidence for two sources of sulfate. The former isotopic composition reflects a marine evaporite source of which there are many to the west of the Tien Shan Mountains. The latter composition indicates that a major portion of the atmospheric SO42− was derived from anthropogenic emissions. While the deposition of non‐loess Ca (calcite) and non‐loess S (gypsum) is also high during the periods of high loess deposition, there is pronounced Ca and S variability during background loess periods, (1994–1996). We suggest that the observed geochemical variability in the firn core can be explained by changes in local‐ to regional‐scale atmospheric circulation and associated transport from multiple dust sources (western Kyrgyzstan/Kazakhstan versus the Taklimakan Desert, China).
Young upper crustal chemical composition of the orogenic Japan Arc
Authors: Togashi Shigeko , Imai Noboru , Okuyama‐Kusunose Yasuko , Tanaka Tsuyoshi , Okai Takashi , Koma Takeshi , Murata Yasuaki
Published: February 2004

A new geochemical estimate of the young (mainly Paleozoic age to present) upper crust of the Japan Arc shows a dacitic composition in contrast to the idea that andesite is predominant in active orogenic arcs. Temporal changes in composition are not significant from the Paleozoic age to the present for the Japan Arc. The major element composition is similar to previous models of old cratonic upper crusts. The coincidence in the major elements between young and old crusts indicates that essential mechanisms during crust formation have not changed from the Archean era to the present. In trace element compositions the average young upper crust of the Japan Arc has higher Sb and As concentrations and lower concentrations of alkaline, light rare earth, and high field strength elements with respect to previous models of continental upper crusts. The large degree of constancy of trace element composition in marine sedimentary rocks is in contrast to the large variety in igneous rocks. However, the averages for both accretionary and nonaccretionary sedimentary rocks are almost identical to the average for the igneous rocks of the Japan Arc, with the exceptions of high Sb and As concentrations in unmetamorphosed sedimentary rocks. The compositional homogeneity among different types of rocks on an arc scale implies that recycling processes mechanically mix the arc‐derived igneous materials to homogenize the chemical composition during erosion, transportation, sedimentation, accretion, and uplifting. Since the contribution of oceanic crust to the composition of arc crust is small, the recycling processes have not changed the bulk upper crustal composition of the active continental margin except increase the Sb and As from sediments. Instead, the influx of differentiated acidic rocks from depth is essential to characterize the orogenic crust formation of the young Japan Arc. The characteristically low incompatible element content of the Japanese upper arc crust appears inherited from parental magmas derived from a mantle source depleted during a long‐term evolution.
Metasomatism of oceanic gabbros by late stage melts and hydrothermal fluids: Evidence from the rare earth element composition of amphiboles
Authors: Gillis K. M. , Meyer P. S.
Published: February 2004

We report new compositional and textural observations on amphiboles hosted in oceanic gabbros recovered from the slow spreading Mid‐Atlantic (MARK area) and Southwest Indian (Hole 735B) ridges and the fast spreading East Pacific Rise (Hess Deep) in an effort to find a set of reliable criteria that unequivocally distinguishes magmatic from hydrothermal amphibole. We find that there is no simple set of criteria that may be applied to distinguish between these parageneses. Rather, it is necessary to assess both a magmatic and hydrothermal origin for all amphibole grains in order to establish appropriate criteria for a particular sample suite. The rare earth element (REE) content of ∼75% of our amphibole grains may be accounted for by crystallization from residual melts; some of these melts were enriched in the light rare earth elements by the crystallization of apatite and/or amphibole. Of significance, only a small subset of these grains crystallized at magmatic temperatures (>825°C). We conclude that the grains that equilibrated at lower temperatures were initially magmatic but that their major elements were reset during subsolidus, hydrothermal alteration, and deformation. This decoupling of the major and rare earth elements was most prevalent in the Hess Deep and MARK suites, both of which were tectonically exhumed at amphibolite to greenschist facies conditions. The remaining REE data identify hydrothermal amphibole that formed by mineral‐scale reactions involving clinopyroxene, fluids, and ± plagioclase at subsolidus conditions. Evaluation of fluids in equilibrium with amphibole shows that deep hydrothermal fluids are unlike those venting at the seafloor in terms of REE contents and/or patterns.
A molecular recorder of methane hydrate destabilization
Authors: Hinrichs Kai‐Uwe
Published: February 2004

The magnitude and timing of temporal variation of δ13C recorded in benthic and planktic foraminifera from the Santa Barbara Basin strongly suggest episodic events of methane hydrate destabilization during the Late Quaternary [Kennett et al., 2000]. However, this and similar evidence from earlier in the geologic past are indirect and therefore not unambiguous. Here I report a direct geochemical response to elevated levels of methane in the Late Quaternary Santa Barbara Basin. Molecular paleontological data provide clear‐cut evidence for strongly elevated concentrations of methane in bottom waters at times coinciding with diagnostic foraminiferal assemblages and carbon isotopic compositions.
Determination of molybdenum isotope fractionation by double‐spike multicollector inductively coupled plasma mass spectrometry
Authors: Siebert Christopher , Nägler Thomas F. , Kramers Jan D.
Published: February 2004

Molybdenum isotopic compositions are precisely determined by MC‐ICP‐MS measurements using a Mo double spike. The double spike is added prior to chemical purification, so that laboratory and instrumental mass fractionations are separated from natural mass‐dependent fractionation. Fractionation is determined on four Mo mass ratios, providing an internal consistency check. The external standard reproducibility is at 0.06 per mil on the 98Mo/95Mo ratio (2 standard deviation)). Using a normal microconcentric nebuliser with a cyclonic spray chamber, the minimum quantity of Mo is ∼1 μg for high‐precision results. A hydrothermal molybdenite shows fractionation of −0.3 per mil on the 98Mo/95Mo ratio relative to our standard (Johnson Matthey, 1000 μg/mL (±0.3%) ICP standard solution, lot 602332B). Fine‐grained sediments show fractionation of 0.1 and −0.3 per mil on the 98Mo/95Mo ratio. The observed Mo isotope fractionation is small but resolvable with the presented high‐resolution technique.
Oblique plate convergence and interseismic strain accumulation
Authors: Bevis Michael , Martel Stephen J.
Published: February 2004

A simple, half‐space elastic model is used to investigate interseismic strain accumulation above a subduction zone associated with oblique plate convergence. The horizontal surface velocity field near the leading edge of the upper plate is found to rotate as distance from the trench increases. Above the locked portion of the plate interface, surface velocity is more oblique than the plate convergence vector. Farther back, toward the interior of the upper plate, the surface velocity is less oblique than the plate convergence vector. Surface velocity fields emphasize the strike‐slip component of plate convergence near the locked zone and emphasize the dip‐slip component farther back. This interseismic “strain partitioning” is purely elastic: interseismic straining will be reversed during the eventual rupture of the locked zone.
Hydrothermal plumes along segments of contrasting magmatic influence, 15°20′–18°30′N, East Pacific Rise: Influence of axial faulting
Authors: Baker Edward T. , Cormier Marie‐Helene , Langmuir Charles H. , Zavala Karina
Published: February 2004

Vertical profiles of light backscattering and temperature recorded on 133 rock cores and dredge hauls between the Orozco and Rivera transform faults on the East Pacific Rise (EPR) (15°20′–18°30′N) provide an opportunity to compare the hydrothermal environment of three adjacent but distinctly different segments that span the maximum range of axial cross section at a relatively constant spreading rate. Contrary to predictions based on data from other Pacific ridges, hydrothermal plumes over the inflated 16°N segment were less extensive and weaker than along the narrower, rifted 17°N segment. Remarkably, the 17°N segment has a plume incidence equal to the mean of superfast spreading segments from the southern EPR. The data suggest that the local permeability environment in this region controls the expression of hydrothermal activity in the water column. The 16°N segment, which has little or no indication of faulting, may have its hydrothermal activity presently suppressed by widespread volcanic flows that act as an impermeable cap over much of the segment. Activity on the 17°N segment may be tectonically enhanced, with hydrothermal fluids circulating through deep faults to a cracking front. Within each segment, intense hydrothermal plumes characteristic of focused discharge seem associated with clearly rifted areas, while weaker water column signals characteristic of diffuse discharge are associated with unrifted portions of the ridge axis that appear dominated by magmatism. Previous studies at intermediate‐ to‐superfast spreading ridges have emphasized a positive correlation between local magmatic budget and hydrothermal activity. Our data suggest, however, that even at fast rates local tectonics can control the extent and nature of hydrothermal activity, as documented for several sites on the slow‐spreading Mid‐Atlantic Ridge. Despite the segment‐scale incongruity between hydrothermal activity and magmatic budget, the fraction of total ridge length between 15°20′ and 18°30′N overlain by plumes (0.39) follows the existing global correlation between plume incidence and spreading rate.