bluuebunny/crossref_metadata_2025 huggingface dataset:
Stratigraphic and geographic distribution of steep‐sided domes on Venus: Preliminary results from regional geological mapping and implications for their origin
Authors: Ivanov Mikhail A. , Head James W.
Published: December 2003

Analysis of the stratigraphic position of 44 steep‐sided domes in a global band of C1‐MIDRs at 30°N latitude shows that in a region representing over 11% of the surface of Venus, the vast majority of steep‐sided domes are closely associated stratigraphically with shield plains (Psh), a unit characterized by a very high abundance of small shields of apparent basaltic extrusive origin. Previously reported heterogeneities in areal and altitude distribution are now more plausibly interpreted in terms of formation in association with this unit and burial by subsequent regional plains units. Measurement of abundances of steep‐sided domes presently exposed in shield plains and extrapolation to areas below younger units suggest that over a thousand of these features may have formed originally. Interpretation of the mode of formation of the steep‐sided domes in the context of these new data favors hypotheses for their origin which call on remelting of basaltic crust to produce more silica‐rich magmas. Occurrences of some steep‐sided domes in association with summits of large shields (e.g., Tepev Mons and Sapas Mons) indicate that other mechanisms of dome formation (e.g., evolution of magma reservoirs) have also operated at other times in the history of Venus.
Experimental studies of Mars‐analog brines
Authors: Moore Jeffrey M. , Bullock Mark A.
Published: December 2003

Evaporite deposits may represent significant sinks of mobile cations (e.g., those of Ca, N, Mg, and Fe) and anions (e.g., those of C, N, S, and Cl) among the materials composing the Martian surface and upper crust. Carbon and nitrogen are especially interesting because of their role as atmospheric gases which can become incorporated into crustal rocks. However, the nature of evaporite precursor brines formed under Martian conditions is poorly understood. To date, only a very limited number of laboratory investigations have been reported which have any bearing on a better understanding of various processes related to brine or evaporite formation on Mars. Here we report on preliminary laboratory experiments that exposed igneous minerals analogous to those in Martian (Shergottites, Nakhlites, and Chassigny (SNC) group) meteorites to a simulated Martian atmosphere and pure, deoxygenated water. Analysis of the water over intervals of time approaching 1 year showed that atmospheric gases dissolved to form carbonate and nitrate ions while minerals dissolved to form sulfate and chloride along with various cations. On an annual basis, ion formation gave a carbonate/sulfate ratio that is comparable to the ratio found among salts in SNC meteorites. The sulfate/chloride ratio of the experimental brines is higher than in SNC meteorites but lower than in surface soils measured at the Viking and Pathfinder landing sites.
Evidence for rapid regional plains emplacement on Venus from the population of volcanically embayed impact craters
Authors: Collins G. C. , Head J. W. , Basilevsky A. T. , Ivanov M. A.
Published: December 2003

We have reexamined the population of craters on Venus believed to be embayed by lava flows and discuss the reliability of criteria for determining the volcanic embayment state of a crater, presenting nonvolcanic processes which may produce similar crater morphologies. We conclude that 29 craters have been embayed by lava flows from the exterior, and an additional 27 craters are ambiguous cases (3–6% of the total crater population). All of the embayed craters are embayed by the stratigraphically youngest lava flows except for five to 11 craters embayed by stratigraphically older regional plains deposits which are typically deformed by wrinkle ridges. Through geologic mapping of an area covering over 8% of the surface, we examined the spatial relationships between wrinkle‐ridged plains and stratigraphically older plains units, and estimate that a large portion of the wrinkle‐ridged plains are thinner than 500 m, which is thin enough for the rims of the largest half of preexisting craters to be preserved. There are not enough craters embayed by the wrinkle‐ridged plains to support a model in which the plains were erupted directly onto an ancient surface. Given the number of large craters embayed by thin deposits of wrinkle‐ridged plains, we develop a simple statistical model to overestimate the number of craters on the preplains surface. This model suggests that the average age of the surface beneath the plains plus the average length of time over which the plains were emplaced was of the order of tens of millions of years. The short exposure age for the preplains surface indicates that some event or events must have erased the ancient population of craters from the surface before the emplacement of the wrinkle‐ridged plains.
Morphometry of small shield volcanoes on Venus: Implications for the thickness of regional plains
Authors: Kreslavsky M. A. , Head J. W.
Published: December 2003

The morphometry of small volcanic shields on Venus is used to assess stratigraphic relationships and the thickness of regional volcanic plains. Comparison of shield width and height in three different regions supports the interpretation that plains with shields form a distinctive stratigraphic unit (Psh) that is subsequently embayed and flooded by younger regional plains with wrinkle ridges (Pwr), leaving lava shield kipukas among the plains near the contact. Differences in shield morphometry in the two units show that Pwr averages less than −150 m in these three embayed areas, and where the underlying shields disappear, the minimum thickness of Pwr is likely to be in excess of ∼400 m. This technique can be applied elsewhere on Venus in order to provide improved estimates of the total volume of regional plains.
Retention of an atmosphere on early Mars
Authors: Carr Michael H.
Published: December 2003

The presence of valley networks and indications of high erosion rates in ancient terrains on Mars suggest that Mars was warm and wet during heavy bombardment. Various processes that could occur on early Mars were integrated into a self‐consistent model to determine what circumstances might lead to warm temperatures during and at the end of heavy bombardment. Included were weathering and burial of CO2as carbonates, impact erosion, sputtering, and recycling of CO2back into the atmosphere by burial and heating. The models suggest that despite losses from the atmosphere by weathering and impact erosion, Mars could retain a 0.5 to 1 bar atmosphere at the end of heavy bombardment partly because weathering temporarily sequesters CO2in the ground and protects it from impact erosion while the impact rate is declining and impact erosion is becoming less effective. Because of the low output of the early Sun, surface temperatures can be above freezing only for a very efficient greenhouse, such as that suggested byForget and Pierrehumbert[1997]. With weak greenhouse models, temperatures are below freezing throughout heavy bombardment, and such a large amount of CO2is left in the atmosphere at the end of heavy bombardment that it is difficult to eliminate subsequently to arrive at the present surface inventory. With strong greenhouse models, temperatures are well above freezing during heavy bombardment and drop to close to freezing at the end of heavy bombardment, at which time the atmosphere contains 0.5 to 1 bar of CO2. This can be largely eliminated subsequently by sputtering and low‐temperature weathering. Such a model is consistent with the change in erosion rate and the declining rate of valley formation at the end of heavy bombardment. Conditions that favor warm temperatures at the end of heavy bombardment are an efficient greenhouse, low weathering rates, low impact erosion rates, and a smaller fraction of heat lost by conduction as opposed to transport of lava to the surface.
Kilometer‐scale slopes on Mars and their correlation with geologic units: Initial results from Mars Orbiter Laser Altimeter (MOLA) data
Authors: Kreslavsky M. A. , Head J. W.
Published: December 2003

Martian surface slopes were calculated at baselines from 0.4 to 25 km using profiles obtained by the Mars Orbiter Laser Altimeter (MOLA) instrument during the aerobraking phase of the Mars Global Surveyor mission. Median slope is proposed as a characteristic measurement of the typical surface roughness at each corresponding scale. Median slope is favored over RMS slope because it is not influenced by the small number of higher slopes at the upper end of the slope‐frequency distribution tail. Median slope complements interquartile scale roughness characterization in that it is more sensitive to smaller baseline slopes. A map of the median slope of the northern hemisphere is presented. Median slopes and their scale dependences are used to characterize typical kilometer‐scale roughness for a set of geologic units mapped in the northern hemisphere. This analysis demonstrates that many individual units and groups of units are characterized by distinctive surface slopes and that these characteristics are sufficiently different that they hold promise for use in the definition and characterization of units. Characterization of the slope properties of geologic units provides information useful in the interpretation of their origin and evolution. For example, the generally smooth topography of the diverse Vastitas Borealis Formation subunits is dominated by about 3 km, 0.3° steep features almost indistinguishable in Viking images. The roughness characteristics of this unit differ from those of other geologic units on Mars and suggest some distinctive process(es) of formation and/or modification of kilometer‐scale topography common for all subunits. The similarity of roughness characteristics of the several highland plateau units suggests that kilometer‐scale topography was largely inherited from the period of heavy bombardment. The northern polar cap and layered terrains are largely very smooth at small scale. The long, steep‐sloped tails of the slope‐frequency distributions are compared for the dominant terrain types in the northern hemisphere of Mars and are compared with Earth continents. The Vastitas Borealis Formation found in the northern lowlands differs significantly from both cratered uplands and volcanic plains, and these latter two units can be distinguished from each other on the basis of median slope. Terrestrial continents are smoother than cratered highlands but rougher than the Vastitas Borealis Formation and most volcanic plains. An inventory of all observed slopes much steeper than the angle of repose is presented. Steep slopes occur in the upper parts of tectonic scarps, most likely representing bedrock exposures. The presence of extremely steep slopes in the presumably ductile polar cap provides evidence for geologically recent and/or ongoing formation of these slopes.
Primitive FeNi metal grains in CH carbonaceous chondrites formed by condensation from a gas of solar composition
Authors: Meibom Anders , Petaev Michail I. , Krot Alexander N. , Wood John A. , Keil Klaus
Published: December 2003

Some FeNi metal grains, ∼150 μm in apparent diameter, in CH carbonaceous chondrites are concentrically zoned in Ni (∼5–10 wt%), Co (0.2–0.4 wt%), and Cr (0.3–0.8 wt%). Silicon is present at the ∼0.1 wt% level. These observations are consistent with predicted gas‐solid condensation from a gas of solar composition at temperatures of ∼ 1370–1270 K and total pressure of ∼10−4 bar. Estimates of FeNi metal grain growth and cooling rates in this temperature range are consistent with brief and localized thermal episodes in the solar nebula. Compositionally similar FeNi metal grains have also been reported in CR and Bencubbin‐like chondrites. Because FeNi metal is highly susceptible to secondary alteration (i.e., metamorphism, melting, oxidation), the observed FeNi metal condensates in CH, Bencubbin‐like, and CR chondrites indicate that these meteorites experienced no thermal processing after their lithification and thus are among the most primitive meteorites in our collections.
Io's patchy SO2 atmosphere as measured by the Galileo ultraviolet spectrometer
Authors: Hendrix A. R. , Barth C. A. , Hord C. W.
Published: December 2003

The Galileo ultraviolet spectrometer has observed Jupiter's volcanically active moon Io in the 2100–3200 Å wavelength range at 13.7 Å resolution. We find that both sulfur dioxide frost on the surface and sulfur dioxide gas in Io's atmosphere are detectable. Io's SO2 atmosphere is not hemispherically homogeneous, but is patchy, with both thick and thin regions. During an observation near 120–150°W longitude, we find that a thick SO2 atmosphere (column density N = 1.0 × 1019 cm−2) covers ∼35% of the observed region, while a thinner SO2 (N = 4.0 × 1017 cm−2) atmosphere covers 25% of the observed region. The thick atmospheric component is significant in the 2350–2400 Å range, while the thinner component is important particularly at wavelengths shorter than ∼2300 Å. Likely sources of the SO2 atmosphere are volcanic plumes and sublimation of SO2 frost, as well as outgassing associated with the several hotspots in the field of view.
Sources of nutrients and energy for a deep biosphere on Mars
Authors: Fisk Martin R , Giovannoni Stephen J
Published: December 2003

Hydrothermal vents appear to be the tip of the subsurface biosphere in the ocean crust. The primary producers in this biosphere are prokaryotes that tolerate a wide variety of physical and chemical conditions and are versatile in their use of inorganic compounds to drive metabolism. A synthesis of chemical and mineralogical data from Martian meteorites and measurements of the Martian surface suggest that conditions similar to those that make life possible in Earth's oceanic crust, namely, water, carbon, nutrients, appropriate temperatures, and gradients in redox conditions, also occur within Mars. Chemolithoautotrophic microorganisms capable of living below the seafloor on Earth would probably survive in some regions of the Martian subsurface.
Geology of mare deposits in South Pole‐Aitken basin as seen by Clementine UV/VIS data
Authors: Yingst R. Aileen , Head James W.
Published: December 2003

A multispectral analysis of 21 discrete lunar mare deposits within the South Pole‐Aitken basin was undertaken in order to model the general mineralogical composition of farside mare deposits so that the near‐ and farside may be compared in terms of magma generation and emplacement. Determination of each deposit's mineralogy was based upon albedo, the 0.75/0.95 μm ratio indicating the presence of a 1.0 μm absorption band, and the strength and shape of that band. A combination of improved spatial resolution and the availability of the general multispectral signature for each pond allowed deposits to be examined for homogeneity in order to assess the nature and number of flows represented. FeO and TiO2 abundances were also estimated. Based upon the resultant data, 19 out of 21 ponds were determined to be mare deposits. Of these 19, 15 had spectra well‐defined enough to classify general mineralogy, nine having high‐Ca pyroxene signatures indicative of basalt and six showing low‐Ca signatures suggesting a substantial noritic component. We believe these noritic signatures can plausibly be attributed to vertical/lateral soil mixing processes for the smallest ponds, although a unique lithology cannot be ruled out for the largest pond. A majority of ponds in this study region have homogeneous spectral signatures, interpreted to represent individual eruptive phases. This indicates that very large volumes for these regions may be typical. Also, because such volumes are comparable to the largest terrestrial eruptions, they may have formed by a similar mechanism of emplacement. The vast majority of mature soils are spectrally redder (and thus have a lower estimated FeO abundance) than the Mare Serenitatis soil standard (MS‐2) in particular, and nearside basalts in general. These soils also average low to medium estimated TiO2 abundance, similar to Apollo 12 and 15, and Luna 24. The observations made here are consistent with a scenario in which basalts with low FeO and low to medium TiO2 contents were deposited during the lunar peak volcanic period that occurred in the Late Imbrian (3.80–3.20 Ga) and because of their small areal extent were more susceptible to mixing processes than nearside basalts.
The gravitational and aerodynamic sorting of meteoritic chondrules and metal: Experimental results with implications for chondritic meteorites
Authors: Akridge D. Glen , Sears Derek W. G.
Published: December 2003

Chondritic meteorites appear to have undergone metal‐silicate fractionations and chondrule size sorting that resulted in unique distributions for each chondrite class. We present evidence from laboratory experiments for size sorting of meteoritic chondrules and metal by fluidization during parent body degassing caused by evaporation and sublimation of water in the parent bodies. Experiments using chondritic analogs in an upward flowing stream of air indicate that metal can travel upward in a fluidized bed, resulting in a metal‐rich surface and metal‐poor regions at depth. When the outgassing velocity drops below the minimum fluidization velocity for chondrules, metal grains still suspended in the gas travel upward through the pore spaces between chondrules, provided the size ratio between chondrules and metal is >3. These experiments also suggest that chondrule size sorting can occur but with less efficiency than metal‐silicate sorting. Fluidization can account for the sorting observed in enstatite chondrites, suggesting that only one parent body is required for EH and EL chondrites. Ordinary chondrite components are also sorted, with H chondrites deriving from locations near the surface of the parent body, while L and LL chondrites originate from greater regolith depths. The upward migration of metal to asteroid surfaces may help explain the reddened spectral features of asteroids likely to be the parent bodies of ordinary chondrites.
Assessing the Martian surface distribution of aeolian sand using a Mars general circulation model
Authors: Anderson F. S. , Greeley R. , Xu P. , Lo E. , Blumberg D. G. , Haberle R. M. , Murphy J. R.
Published: December 2003

A sand transport model using White's [1979] sand flux equation and the Mars general circulation model [Pollack et al., 1990] was developed to understand the erosional sources, transport pathways, and depositional sinks of windblown sand on Mars. An initially uniform distribution of sand (4 mm over the entire surface) is regionally transported based on wind stress, saltation threshold, and percentage of topographic trapping, Results are consistent with the observed polar and Hellespontus dunes and Christensen's [1986] modeled block size distribution, but only for an extremely low saltation threshold (0.024 N/m2). Low thresholds generally result in transport of sand‐sized particles originating in the northern mid latitudes to the north pole, and transport from the northern lower latitudes to the southern hemisphere. Our results indicate that the polar dune fields could form in 50,000 years, consistent with the active polar dunes and lack of longitudinal dunes observed on the surface of Mars.
A geostrophic transport estimate for the Florida Current from the oxygen isotope composition of benthic foraminifera
Authors: Lynch‐Stieglitz Jean , Curry William B. , Slowey Niall
Published: December 2003

We present a new method for the quantitative reconstruction of upper ocean flows for during times in the past. For the warm (T>5°C) surface ocean, density can be accurately reconstructed from calcite precipitated in equilibrium with seawater, as both of these properties increase with decreasing temperature and increasing salinity. Vertical density profiles can be reconstructed from the oxygen isotopic composition of benthic foraminifera. The net volume transport between two vertical density profiles can be calculated using the geostrophic method. Using benthic foraminifera from surface sediment samples from either side of the Florida Straits (Florida Keys and Little Bahama Bank), we reconstruct two vertical density profiles and calculate a volume transport of 32 Sv using this method. This agrees well with estimates from physical oceanographic methods of 30–32 Sv for the mean annual volume transport. We explore the sensitivity of this technique to various changes in the relationship between temperature and salinity as well as salinity and the oxygen isotopic composition of seawater.
Sensitivity of paleonutrient tracer distributions and deep‐sea circulation to glacial boundary conditions
Authors: Winguth A. M. E. , Archer D. , Duplessy J.‐C. , Maier‐Reimer E. , Mikolajewicz U.
Published: December 2003

We use a carbon cycle model coupled to an ocean general circulation model to explore the links between sea surface boundary conditions, the deep‐sea circulation, and the distribution of paleonutrient tracers (δ13C and Cd/Ca) from Last Glacial Maximum (21,000 B.P) sediments. A glacial flow field with a shallower and 50% reduced North Atlantic overturning circulation generally reproduces the tracer data but cannot explain the strong glacial‐interglacial shift in δ13C in the Southern Ocean. Sensitivity experiments with changes of ±1 in salinity in the glacial salinity boundary fields show circulation patterns ranging from even stronger than the present day one to nearly a shutdown of the Atlantic deep‐sea circulation. Our model results indicate that the overturning in the North Atlantic is linearly related to the zonal wind forcing in the Southern Ocean but with half of the sensitivity of Toggweiler and Samuels [1993]. Atmospheric pCO2 appears to be insensitive to changing circulation and sea surface forcing; a tropical cooling of 4°C can only explain 8% of the glacial‐interglacial pCO2 change documented in ice cores.
Correlating high‐frequency climate variations
Authors: Crowley Thomas J.
Published: December 2003

There has been a great deal of recent interest in correlating high sedimentation rate marine sequences to high frequency climate fluctuations from the Greenland ice cores. However, conclusions about physical connections between far‐field records and the Greenland cores are often based on subjective comparisons. The possibility that high‐resolution marine records could be influenced by bioturbation and/or stochastic pulses in sedimentation is also sometimes insufficiently addressed. It is suggested that a protocol be established for a more formal approach to correlating high‐frequency events, with the software being made widely available to the research community. Such an approach should enable the climate community to more objectively assess claims concerning physical linkages between different regions.
Nitrogen isotopic variations in the Gulf of California since the Last Deglaciation: Response to global climate change
Authors: Pride Carol , Thunell Robert , Sigman Daniel , Keigwin Lloyd , Altabet Mark , Tappa Eric
Published: December 2003

High‐resolution records of the nitrogen isotopic composition of organic matter (δ15Norg), opal content, and opal accumulation rates from the central Gulf of California reveal large and abrupt variations during deglaciation and gradual Holocene changes coincident with climatic changes recorded in the North Atlantic. Homogenous sediments with relatively low δ15Norg values and low opal content were deposited at the end of the last glacial period, during the Younger‐Dryas event, and during the middle to late Holocene. In contrast, laminated sediments deposited in the two deglacial stages are characterized by very high δ15Norg values (>14‰) and opal accumulation rates (29–41 mg cm−2 yr−1). Abrupt shifts in δ15Norg were driven by widespread changes in the extent of suboxic subsurface waters supporting denitrification and were amplified in the central gulf record due to variations in upwelling, vertical mixing, and/or the latitudinal position of the Intertropical Convergence Zone.
Miocene evolution of atmospheric carbon dioxide
Authors: Pagani Mark , Arthur Michael A. , Freeman Katherine H.
Published: December 2003

Changes inpCO2or ocean circulation are generally invoked to explain warm early Miocene climates and a rapid East Antarctic ice sheet (EAIS) expansion in the middle Miocene. This study reconstructs late Oligocene to late MiocenepCO2from εpvalues based on carbon isotopic analyses of diunsaturated alkenones and planktonic foraminifera from Deep Sea Drilling Project sites 588 and 608 and Ocean Drilling Program site 730. Our results indicate that highestpCO2occurred during the latest Oligocene (∼350 ppmv) but decreased rapidly at ∼25 Ma. The early and middle Miocene was characterized by lowpCO2(260–190 ppmv). Lower intervals ofpCO2correspond to inferred organic carbon burial events and glacial episodes with the lowest concentrations occurring during the middle Miocene. There is no evidence for either highpCO2during the late early Miocene climatic optimum or a sharppCO2decrease associated with EAIS growth. Paradoxically,pCO2increased following EAIS growth and obtained preindustrial levels by ∼10 Ma. Although we emphasize an oceanographic control on Miocene climate, lowpCO2could have primed the climate system to respond sensitively to changes in heat and vapor transport.
Planktonic foraminiferal Cd/Ca: Paleonutrients or paleotemperature?
Authors: Rickaby R. E. M. , Elderfield H.
Published: December 2003

Records of Cd/Ca in planktonic foraminiferal calcite of Globigerinoides bulloides in cores from the Subantarctic region of the Southern Ocean show large glacial‐interglacial variations with lower Cd/Ca (by 0.06–0.10 µmol mol−1) at glacial times. Interpretation of these records in terms of lower dissolved phosphate and inferred higher glacial nutrient utilization has significant implications for glacial atmospheric carbon dioxide (pCO2) draw‐down. However, box core‐top data for G. bulloides in the North Atlantic suggest that the incorporation of Cd into planktonic foraminifera relative to seawater (DCd) is temperature sensitive (DCd=0.637 exp 0.15T). When the Subantarctic planktonic Cd/Ca records are corrected for this temperature dependence, they show little or no glacial‐interglacial diferences. If, as seems likely, this observation can be interpreted to indicate a minimal change (< 0.5 µmol kg−1) in surface water phosphate concentrations, then the explanation for lowered glacial pCO2 must be looked for elsewhere.
Error analysis of paleosalinity calculations
Authors: Schmidt Gavin A.
Published: December 2003

A detailed error analysis is performed for the calculation of open‐ocean salinity changes from carbonate oxygen‐18 proxy data. Measurement errors are generally negligible, however, errors due to the “goodness of fit” of various regressions used in the calculation are significant. Depending on the method used and on the proxies available, error bars (1σ) for the salinity range from 0.6 to 1.8. In particular, expected errors for the midlatitude and tropical salinity differences at the Last Glacial Maximum are around 1.1 to 2.5. The largest errors occur because of the differences between temporal and spatial seawater oxygen‐18:salinity relationships and through the use of nonconcurrent temperature measurements. Methods of improving paleosalinity estimates should focus on eliminating these large sources of uncertainty.
Foraminiferal faunal estimates of paleotemperature: Circumventing the No‐analog problem yields cool Ice Age tropics
Authors: Mix Alan C. , Morey Ann E. , Pisias Nicklas G. , Hostetler Steven W.
Published: December 2003

The sensitivity of the tropics to climate change, particularly the amplitude of glacial‐to‐interglacial changes in sea surface temperature (SST), is one of the great controversies in paleoclimatology. Here we reassess faunal estimates of ice age SSTs, focusing on the problem of no‐analog planktonic foraminiferal assemblages in the equatorial oceans that confounds both classical transfer function and modern analog methods. A new calibration strategy developed here, which uses past variability of species to define robust faunal assemblages, solves the no‐analog problem and reveals ice age cooling of 5° to 6°C in the equatorial current systems of the Atlantic and eastern Pacific Oceans. Classical transfer functions underestimated temperature changes in some areas of the tropical oceans because core‐top assemblages misrepresented the ice age faunal assemblages. Our finding is consistent with some geochemical estimates and model predictions of greater ice age cooling in the tropics than was inferred by Climate: Long‐Range Investigation, Mapping, and Prediction (CLIMAP) [1981] and thus may help to resolve a long‐standing controversy. Our new foraminiferal transfer function suggests that such cooling was limited to the equatorial current systems, however, and supports CLIMAP's inference of stability of the subtropical gyre centers.
Orbital controls on the El Niño/Southern Oscillation and the tropical climate
Authors: Clement A. C. , Seager R. , Cane M. A.
Published: December 2003

The global synchroneity of glacial‐interglacial events is one of the major problems in understanding the link between Milankovitch forcing and the climate of the late Quaternary. In this study we isolate a part of the climate system, the tropical Pacific, and test its sensitivity to changes in solar forcing associated with changes in the Earth's orbital parameters. We use a simplified coupled ocean‐atmosphere model that is run for the past 150,000 years and forced with Milankovitch changes in the solar insolation. This system responds primarily to the precessional cycle in solar forcing and is capable of generating a mean response to the changes in the seasonal distribution of solar radiation even while the annual mean insolation is roughly constant. The mean response to the precessional forcing is due to an interaction between an altered seasonal cycle and the El Niño/Southern Oscillation (ENSO). Changes in the ENSO behavior result in a mean tropical climate change. The hypothesis is advanced that such a change in the tropical climate can generate a globally synchronous climate response to Milankovitch forcing.
Major‐element analysis of cyclic black shales: Paleoceanographic implications for the Early Cretaceous deep western tethys
Authors: Wortmann Ulrich G. , Hesse Reinhard , Zacher Wolfgang
Published: December 2003

Lower Cretaceous sediments are frequently characterized by a well expressed cyclicity. While the processes influencing environments above the carbonate compensation depth (CCD) are reasonably well understood, almost nothing is known about the deep ocean. Cretaceous sub‐CCD sediments from the Tethys and Atlantic Oceans typically show rhythmic black/green shale successions. To gain insight into the nature of these black/green shale cycles, we performed detailed geochemical analyses (X‐ray fluorescence, Rock‐Eval and reactive iron analysis) on a 3 m long section of latest Aptian age. The major‐element distribution of the analyzed shale sequence indicates a periodic change from a high‐productivity and well‐oxygenated green shale mode to a low‐productivity oxygen‐deficient black shale mode. It is proposed here that the preservation of organic matter was dependent on the strength of salinity‐driven deepwater generation. Furthermore, the data show that the Corg content covaries with changes in the detrital composition. Therefore we hypothesize that Tethyan deepwater circulation was sensitive to changes in the monsoonal system. Time series analysis suggests that these changes are periodic in nature, although we are currently unable to prove that the dominant periodicity is related to the precession component of the Milankovitch frequencies.
CaCO3 size distribution: A paleocarbonate ion proxy?
Authors: Broecker W. S. , Clark E.
Published: December 2003

Lysocline reconstructions play an important role in scenarios purporting to explain the lowered atmospheric CO2 content of glacial time. These reconstructions are based on indicators such as the CaCO3 content, the percent of coarse fraction, the ratio of fragments to whole foraminifera shells, the ratio of solution‐susceptible to solution‐resistant species, and the ratio of coarse to fine CaCO3. All assume that changes with time in the composition of the input material do not bias the result. However, as the composition of the input material does depend on climate, none of these indicators provides an absolute measure of the extent of dissolution. In this paper we evaluate the reliability of the ratio of >63 µm CaCO3 to total CaCO3 as a dissolution indicator. We present here results that suggest that in today's tropics this ratio appears to be determined solely by CO3= ion concentration and water depth (i.e., the saturation state of bottom waters). This finding offers the possibility that the size fraction index can be used to reconstruct CO3= ion concentrations for the late Quaternary ocean to an accuracy of ±5 µmol kg−1.
Marine evidence for episodic Holocene megafloods in North America and the northern Gulf of Mexico
Authors: Brown Paul , Kennett James P. , Ingram B. Lynn
Published: December 2003

Hemipelagic muds deposited during the past 5.3 cal kyr in the northern Gulf of Mexico (Orca Basin) contain seven intervals punctuated by relatively coarse siliciclastic grain‐size peaks, planktonic faunal turnovers, and negative δ13C excursions. We believe these episodes represent megaflood deposits reflecting historically unprecedented outfall of North American floodwater and terrigenous mud plumes into the gulf, resulting in collapse of the open‐ocean pelagic ecosystem. The deposits record multidecadal episodes of high continental precipitation and large Mississippi River floods at ∼4.7, 3.5, 3.0, 2.5, 2.0, 1.2, and 0.3 cal ka (500–1200‐year recurrence interval). Variations in tropical plankton frequencies define submillenial warming intervals that culminate in these fluvial episodes. Strengthened tropical currents in the gulf at these times appear to have increased sea surface temperatures and associated flow of moist gulf air to the midwest. Terrestrial paleohydrologic records support the marine evidence for millennial‐scale changes in recurrence of large midwest flood episodes.
Is the spectral signature of the 100 kyr glacial cycle consistent with a Milankovitch origin?
Authors: Ridgwell Andy J. , Watson Andrew J. , Raymo Maureen E.
Published: December 2003

Milankovitch theory seeks to explain the Quaternary glaciations via changes in seasonal insolation caused by periodic changes in the Earth's obliquity, orbital precession, and eccentricity. However, recent high‐resolution spectral analysis of δ18O proxy climate records have cast doubt on the theory [Muller and MacDonald, 1997a, b]. The spectral signature of the “100 kyr” component, which dominates the climate record over the past 0.6–0.8 Myr does not match the frequencies of the eccentricity variation. Muller and MacDonald [1997b, c] have therefore argued that a more likely pacemaker for the climate cycles is the variation in inclination of the Earth's orbit relative to the invariant plane of the solar system. Here we show that the spectral signature of δ18O records are entirely consistent with Milankovitch mechanisms in which deglaciations are triggered every fourth or fifth precessional cycle. Such mechanisms may involve the buildup of “excess” ice due to low summertime insolation at the previous precessional “high.”
Radiocarbon age differences between coexisting foraminiferal species
Authors: Broecker Wallace , Matsumoto Katsumi , Clark Elizabeth , Hajdas Irka , Bonani Georges
Published: December 2003

Radiocarbon‐age measurements on single species of foraminifera from a core on the Ceara Rise demonstrate the importance of the joint effect of bioturbation and variable rain abundance of foraminifera. The relatively high mixed layer ages for Pulleniatina obliquiloculata reflect, at least in part, an early Holocene peak in its abundance while the relatively young ages for Globorotalia menardii reflect the delay until mid Holocene of its reappearance in the Atlantic Ocean. These results clearly demonstrate that core‐top sediment samples need not be representative foraminifera falling from today's surface ocean. Rather, at least on the Ceara Rise, such samples consist of a composite of changing species groupings. These results also reconfirm the pitfalls associated with attempts to reconstruct the radiocarbon age of deep ocean water on the basis of benthic‐planktonic foraminiferal age differences.
Magnetic characterization of Holocene sedimentation in the South Atlantic
Authors: Schmidt Andrea M. , von Dobeneck Tilo , Bleil Ulrich
Published: December 2003

Surface sediment samples representative for the tropical and subtropical South Atlantic (15°N to 40°S) were investigated by isothermal magnetic methods to delineate magnetic mineral distribution patterns and to identify their predominant Holocene climatic and oceanographic controls. Individual parameters reveal distinct, yet frequently overlapping, regional sedimentation characteristics. A probabilistic (“fuzzy c‐means”) cluster analysis was applied to five concentration independent magnetic properties assessing magnetite to hematite ratios and diagnostic of bulk and fine‐particle magnetite grain size and coercivity spectra. The resultant 10 cluster structures establish an oceanwide magnetic sediment classification scheme tracing the major terrigenous eolian and fluvial fluxes, authigenic biogenic magnetite accumulation in high‐productivity areas, transport by ocean current systems, and effects of bottom water velocity on depositional regimes. Distinct dissimilarities in magnetic mineral inventories between the eastern and western basins of the South Atlantic reflect prominent contrasts of both oceanic and continental influences.
Late Ordovician glaciation under high atmospheric CO2: A coupled model analysis
Authors: Poussart Pascale F. , Weaver Andrew J. , Barnes Christopher R.
Published: December 2003

The analysis of the geologic record has revealed a question concerning how the Late Ordovician glaciation could have occurred simultaneously with high CO2 levels (10–18x). Sensitivity studies using a coupled atmosphere‐ocean‐sea ice model show that it is possible to maintain a permanent snow cover (which corresponds to 60% of all the glacial deposits found on Gondwana) under 10x CO2 levels, warm fall/cool spring orbital parameters, a 4.5% reduction in solar luminosity, a length of day of 21.5 hours, and an enhanced snow/sea ice albedo of 0.3. A cold summer orbit experiment with 10x CO2 and a reduced snow/sea ice albedo of 0.1 also sustains a permanent (albeit less extensive) snow cover. The geographic configuration of the Late Ordovician results in an up to ∼42% increase in the global ocean poleward heat transport in the Southern Hemisphere relative to present‐day and a significant asymmetry relative to the equator.
Fluctuations in the ocean environment within the Western Pacific Warm Pool during Late Pleistocene
Authors: Kawahata Hodaka
Published: December 2003

The mass accumulation rate (MAR) of organic carbon in the western sector of the western Pacific warm pool (WPWP), a currently high primary productivity region, clearly responded to glacial‐interglacial alternations, although oceanic surface temperature fluctuated within a narrow range of 2°C. Wind‐related mixing or upwelling would have increased primary productivity during glacial periods when the Asian winter monsoon was more developed. Higher MARs of lithogenic matter generally occurred during relatively low sea level periods. The increased terrestrial inputs would have been linked with the enhanced biological activity in the western sector of the WPWP. In contrast, more intense surface mixing or upwelling driven by stronger winds might have made little or no difference to the low to medium productivity in the central and eastern sectors of the WPWP because a deep thermocline prevented nutrient‐bearing water from upwelling even during the glacial times.
Reliability of coral isotope records from the Western Pacific Warm Pool: A comparison using age‐optimized records
Authors: Guilderson Thomas P. , Schrag Daniel P.
Published: December 2003

Stable isotope analysis of two separate corals from Nauru Island in the warm pool of the western tropical Pacific confirm that coral oxygen isotopes are a robust recorder of environmental variables. Coral δ18O faithfully records interannual variability even when strongly influenced by kinetic effects. Interannual to decadal variability of coral δ13 remains enigmatic; only one of the two corals exhibits the expected light‐δ13C response in the interannual band. Coral δ18O from this region has a great similarity to global instrumental temperature records reflecting the impact of the Pacific warm pool and variations of the Indonesian Low in concert with El Niño‐Southern Oscillation on global climate. Since 1896, coral δ18O has exhibited a cumulative decrease of 0.7‰, which reflects a combination of warming and more frequent El Niño events (precipitation) affecting the surface waters of this region.
Forward modeling of carbonate proxy data from planktonic foraminifera using oxygen isotope tracers in a global ocean model
Authors: Schmidt Gavin A.
Published: December 2003

The distribution and variation of oxygen isotopes in seawater are calculated using the Goddard Institute for Space Studies global ocean model. Simple ecological models are used to estimate the planktonic foraminiferal abundance as a function of depth, column temperature, season, light intensity, and density stratification. These models are combined to forward model isotopic signals recorded in calcareous ocean sediment. The sensitivity of the results to the changes in foraminiferal ecology, secondary calcification, and dissolution are also examined. Simulated present‐day isotopic values for ecology relevant for multiple species compare well with core‐top data. Hindcasts of sea surface temperature and salinity are made from time series of the modeled carbonate isotope values as the model climate changes. Paleoclimatic inferences from these carbonate isotope records are strongly affected by erroneous assumptions concerning the covariations of temperature, salinity, and δ18Ow. Habitat‐imposed biases are less important, although errors due to temperature‐dependent abundances can be significant.
Iron‐stimulated changes in 13C fractionation and export by equatorial Pacific phytoplankton: Toward a paleogrowth rate proxy
Authors: Bidigare Robert R. , Hanson Kristi L. , Buesseler Ken O. , Wakeham Stuart G. , Freeman Katherine H. , Pancost Rich D. , Millero Frank J. , Steinberg Paul , Popp Brian N. , Latasa Mikel , Landry Michael R. , Laws Edward A.
Published: December 2003

We present δ13C values for phytol, an algal biomarker, which document up to 7 per mil isotopic enrichment during the IronEx II iron fertilization experiment. We evaluate these data using a laboratory‐derived 13C fractionation model and show this variability is largely the result of elevated growth rates. Isotopic enrichment and stimulation of growth rate were accompanied by a sevenfold increase in the export of particulate organic carbon as estimated from 234Th activities. This is the first direct evidence that enhanced productivity following iron enrichment can lead to both increased export of organic matter and an associated isotopic signal in an algal biomarker. On the basis of these results, we propose biomarker isotopic data be used in conjunction with paleo‐CO2 records to reconstruct paleogrowth rates. This approach provides a means to test for iron‐stimulated changes in algal growth in sedimentary records.
Northeast Atlantic sea surface circulation during the past 30‐10 14C kyr B.P.
Authors: Lassen Susanne , Jansen Eystein , Knudsen Karen Luise , Kuijpers Antoon , Kristensen Margrethe , Christensen Karen
Published: December 2003

A study was made of three cores from the Faeroe‐Shetland gateway, based on planktonic foraminifera, oxygen isotopes, accelerator mass spectrometry 14C dates, magnetic susceptibility, and counts of ice rafted debris (IRD). The data, covering the period 30–10 ka, show that during the Last Glacial Maximum the Arctic Front occupied a position close to the Faeroes, allowing a persisting inflow of Atlantic surface water into the Faeroe‐Shetland Channel. The oceanographic environment during deposition of two IRD layers is influenced by Atlantic surface water masses during the lower IRD layer, with transport of icebergs from N‐NW. Polar surface water conditions prevailed only during deposition of the upper IRD layer. There is no indication of surface meltwater influence in the region during the deglaciation, but there is a persistent influence of Atlantic surface water masses in the region. Thus we conclude that during almost the entire period (30‐10 ka) the Faeroe‐Shetland Channel was a gateway for transport of Atlantic surface water toward the Norwegian Sea.
Regional patterns of Pleistocene ice‐rafted debris flux in the North Pacific
Authors: St. John Kristen E. K. , Krissek Lawrence A.
Published: December 2003

IRD mass accumulation rate (MAR) records are presented for Deep Sea Drilling Project site 580 and Ocean Drilling Program sites 882 and 887 and are combined with results from previous IRD MAR studies to provide a synthesis of North Pacific Pleistocene ice rafting. Important regional differences in the ice rafting histories of the NW and NE Pacific are highlighted by this synthesis and include (1) significant variations in IRD fluxes across the North Pacific, which identify two major IRD sources, coastal Alaska and coastal Siberia/Kamchatka Peninsula, and (2) significant variations in the timing of IRD supply across the North Pacific, particularly in response to global‐scale climatic forcing. These regional differences indicate that local and regional controls on iceberg input, transport, and melting exert a major influence on ice rafting throughout the North Pacific. This suggests that a single generalized ice rafting record for the North Pacific does not adequately describe the complexity of this basin's Pleistocene history.

Data used to calculate LSRs and IRD MARs for sites 380, 882, and 887 are available on diskette or via anonymous FTP from kosmos.agu.org, directory APEND (Usemame = anonymous, Password = guest). Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009 or by phone at 800‐966‐2481; $15.00. Payment must accompany order.
New evidence for subtropical warming during the Late Paleocene thermal maximum: Stable isotopes from Deep Sea Drilling Project Site 527, Walvis Ridge
Authors: Thomas Deborah J. , Bralower Timothy J. , Zachos James C.
Published: December 2003

The late Paleocene thermal maximum (LPTM) was a dramatic, short‐term global warming event that occurred ∼55 Ma. Warming of high‐latitude surface waters and global deep waters during the LPTM has been well documented; however, current data suggest that subtropical and tropical sea surface temperatures (SSTs) did not change during the event. Conventional paradigms of global climate change, such as CO2‐induced greenhouse warming, predict greater warming in the high latitudes than in the tropics or subtropics but, nonetheless, cannot account for the stable tropical/subtropical SSTs. We measured the stable isotope values of well‐preserved late Paleocene to early Eocene planktonic foraminifera from South Atlantic Deep Sea Drilling Project (DSDP) Site 527 to evaluate the subtropical response to the climatic and environmental changes of the LPTM. Planktonic foraminiferal δ18O values at Site 527 decrease by ∼0.94‰ from pre‐LPTM to excursion values, providing the first evidence for subtropical warming during the LPTM. We estimate that subtropical South Atlantic SSTs warmed by at least ∼1°–4°C, on the basis of possible changes in evaporation and precipitation. The new evidence for subtropical SST warming supports a greenhouse mechanism for global warming involving elevated atmospheric CO2 levels.