bluuebunny/crossref_metadata_2025 huggingface dataset:
CO2 cooling in terrestrial planet thermospheres
Authors: Bougher S. W. , Hunten D. M. , Roble R. G.
Published: February 2004

The comparative approach to planetary problems is becoming increasingly fruitful as new information from various planet atmospheres is assimilated. In particular, it is clear that the important problem of CO2 cooling in the Earth's lower thermosphere is closely tied to the thermospheric heat budgets of Venus and Mars. CO2 cooling in each of these thermospheres is strongly impacted by collisions of CO2 and O, yielding vibrationally excited CO2 and enhanced 15‐μm emissions in regions where non‐local thermodynamic equilibrium conditions prevail. Both the relative abundance of atomic O and the CO2‐O relaxation rate affect the magnitude of this enhanced cooling process. We examine the recent progress in the debate on the CO2‐O relaxation rate, its temperature dependence, and its corresponding impact on the thermospheric heat budgets of Venus, Earth, and Mars. This comparative approach provides the broadest range of conditions under which a common CO2‐O relaxation rate should provide consistent results. New global mean calculations are presented for the heat budgets of these three planets using large CO2‐O relaxation rates that have been inferred recently from Earth CO2 radiance measurements and laboratory studies. Results indicate that available Venus and Mars data constrain the CO2‐O relaxation rate to be 2–4 × 10−12 cm3/s at 300 K. For Venus, this strong cooling serves as an effective thermostat that gives rise to a small variation of thermospheric temperatures over the solar cycle, just as observed. Conversely, CO2 cooling does not appear to be dominant in the dayside heat budget of the Mars thermosphere over most of the solar cycle. For the Earth, this strong cooling implies that the lower thermosphere does not typically require significant eddy diffusion or heat conduction. However, global‐scale dynamics or an additional heating mechanism may be needed to restore calculated temperatures to observed values when relaxation rates exceeding 2 × 10−12 cm3/s are employed.
Gravity and magnetic field modeling and structure of the Chicxulub Crater, Mexico
Authors: Pilkington Mark , Hildebrand Alan R. , Ortiz‐Aleman Carlos
Published: February 2004

The Chicxulub crater is an ∼180‐km‐diameter peak‐ring crater based on drill hole logs and samples, potential fields, seismic reflection profiles, and surface fracture patterns. A structural cross section produced based on these constraints has the features expected for a large complex impact crater. The Bouguer‐gravity anomaly consists of a broad ∼90‐km radius, ∼30‐mGal low with a central ∼20‐km radius, ∼20‐mGal high and two <5‐mGal concentric lows at ∼35‐ and ∼60‐km radius. The gravity anomaly is disrupted by large‐scale basement anomalies and possibly by large‐scale slumping and backwash erosion effects. The magnetic field anomaly over the crater consists of three zones, an outer zone from ∼45‐ to ∼90‐km radius of low‐amplitude, short‐wavelength anomalies with an irregular perimeter, a middle zone from ∼20‐ to ∼45‐km radius of high‐amplitude, short‐wavelength anomalies slightly elongated NNW‐SSE, and an inner ∼20‐km‐radius single large‐amplitude anomaly. Magnetic field modeling indicates that the melt pool averages ∼90 km in diameter and the melt volume in the crater is estimated at ∼20,000 km3. The melt pool size constrains the collapsed transient cavity diameter to ∼90 km. Gravity and magnetic field modeling indicate that the structural uplift is irregular in shape but ∼40 km in diameter and underlies or protrudes into the melt pool. The preliminary structural cross section indicates that the inferred peak‐ring is decoupled from the structural uplift. The geometry and Bouguer gravity signature of the crater indicate that no significant uplift of the Moho or relaxation of the crater has occurred.
A model of Triton's role in Neptune's magnetosphere
Authors: Decker R. B. , Cheng A. F.
Published: February 2004

Escape of neutral hydrogen (H) and nitrogen (N) from Triton maintains a large neutral cloud, called the Triton torus, in Neptune's magnetosphere. We have developed the first detailed Monte Carlo simulation model of the Triton torus that includes the collisionality, the complex geometry, the injection of two neutral species from Triton (H and N), and the combined effects of photoionization, electron impact ionization, and charge exchange. Ionization in Neptune's plasma sheet was modeled using Voyager plasma observations. Collisions cause both the H and N neutral clouds to become more radially extended, both toward Neptune and out beyond the magnetopause, as well as more extended in latitude, when compared with collisionless models. Moreover, collisions of H with the much more massive N greatly enhance the collisional ejection of H from the system and into Neptune's atmosphere. This effect decreases the probability of H ionization within the magnetosphere relative to that for N, and furthermore causes model results for two‐species injection from Triton to differ significantly from those for H injection alone. For a hydrogen escape rate from Triton of 5 × 1025 s−1, as given by photochemical models of Triton's upper atmosphere, a nitrogen escape rate of 5 × 1024 s−1 gives proton and N+ sources of 5.6 × 1024 s−1 and 3.3 × 1024 s−1, respectively, whose ratio is close to the observed ratio of protons to heavies. A nitrogen escape rate of 2 × 1025 s−1, for the same hydrogen escape rate of 5 × 1025 s−1, yields an N+ source more than twice that of protons, inconsistent with the Voyager data.
Venus cartography
Authors: Batson R. M. , Kirk R. L. , Edwards Kathleen , Morgan H. F.
Published: February 2004

The entire surface of the planet Venus is being mapped at global and regional scales (1:50,000,000 through 1:1,500,000) with synthetic aperture radar (SAR), radar altimeter, and radiometer measurements of physical properties from the Magellan spacecraft. The mapping includes SAR image mosaics, shaded relief maps, and topographic contour overlays made from altimetry data and by radargrammetric methods. Methods used include new techniques of radar image processing that became operational as a result of the Magellan mission. The area being mapped (at a resolution of 75 m/pixel) is roughly equivalent to that of Earth, including seafloors. The mapping is designed to support geologic and geophysical investigations.
Evidence of ancient continental glaciation in the Martian northern plains
Authors: Kargel Jeffrey S. , Baker Victor R. , Begét James E. , Lockwood Jeffrey F. , Péwé Troy L. , Shaw John S. , Strom Robert G.
Published: February 2004

Whorled ridges, spaced about 2–6 km and forming lobate patterns with lobe widths of about 150 km, occur at many locations in the northern plains of Mars, commonly in close association with sinuous troughs that contain medial ridges. These landforms resemble moraines, tunnel channels, and eskers found in terrestrial glacial terrains, such as the midcontinent of North America. Some Martian landscapes may have formed by disintegration of continental glaciers that covered much of the northern plains into the early Amazonian (i.e., late in Martian geologic history). Meltwater processes apparently were important in the collapse of these hypothesized ice sheets; hence, the glaciers apparently were wet based in part. Whereas striking similarities exist among areas of the northern plains and some glaciated Pleistocene terrains on Earth, there are also important differences; notably, drumlin fields, such as those in many glacial landscapes on Earth, are rare, absent, or not yet resolved in images of the Martian northern plains. Another major difference is that postglacial fluvial and other water‐related modifications (especially erosion) of Pleistocene terrains are substantial, but similar modifications are not observed in the northern plains; a virtually complete and sudden decline in the activity of liquid surface water following glaciation in the northern plains seems to be implied. The climatic implications of the hypothesized Martian glaciers and their decline are unclear. We investigate two possibilities, alternatively involving a relatively warm paleoclimate and the modern Martian climate. The hypothesized ice sheets in the basins within the northern plains (generally at elevations lower than −1 km) suggest a relationship of these frozen bodies of water with former regional lakes or seas, which may have formed in response to huge discharges of water from Martian outflow channels. This possible relationship has been modeled. Glaciers may have evolved from seas by their progressive freezing and then grounding and sublimational redistribution of sea ice. The transition to glaciation may have taken several million years if the climate was very cold, comparable to today's, or tens of thousands of years if the climate was as warm as modern Antarctica. A glacierized sea may have involved an extended period of glaciolacustrine and ice shelf processes.
Deep circulation change linked to HEINRICH Event 1 and Younger Dryas in a middepth North Atlantic Core
Authors: Keigwin Lloyd D. , Lehman Scott J.
Published: February 2004

A core from the Mid‐Atlantic Ridge at 43.5°N and ∼3 km water depth shows distinct evidence of the deglacial events known as Heinrich event 1 (probably the marine equivalent of Oldest Dryas cooling in Europe) and the Younger Dryas. The Heinrich event, dated at three levels to between 14.3 and 15.0 ka, is marked by a minimum in foraminifera per gram, by maxima in rates of sedimentation, ice rafted debris per gram, and relative abundance of N. pachyderma (s.), and by a δ18O minimum in planktonic foraminifera. The Younger Dryas event is marked by peak abundance of N. pachyderma (s.) and a planktonic δ18O maximum. Benthic foraminiferal δ13C reaches minimum values during both the Heinrich event and the Younger Dryas. Our data indicate pronounced changes in surface water properties were coupled with reduced production of North Atlantic Deep Water at each of these times.
Astronomic timescale for the Pliocene Atlantic δ18O and dust flux records of Ocean Drilling Program Site 659
Authors: Tiedemann Ralf , Sarnthein Michael , Shackleton Nicholas J.
Published: February 2004

High‐resolution benthic oxygen isotope and dust flux records from Ocean Drilling Program site 659 have been analyzed to extend the astronomically calibrated isotope timescale for the Atlantic from 2.85 Ma back to 5 Ma. Spectral analysis of the δ18O record indicates that the 41‐kyr period of Earth's orbital obliquity dominates the Pliocene record. This is shown to be true regardless of fundamental changes in the Earth's climate during the Pliocene. However, the cycles of Sahelian aridity fluctuations indicate a shift in spectral character near 3 Ma. From the early Pliocene to 3 Ma, the periodicities were dominantly precessional (19 and 23 kyr) and remained strong until 1.5 Ma. Subsequent to 3 Ma, the variance at the obliquity period (41 kyr) increased. The timescale tuned to precession suggests that the Pliocene was longer than previously estimated by more than 0.5 m.y. The tuned ages for the magnetic boundaries Gauss/Gilbert and Top Cochiti are about 6–8% older than the ages of the conventional timescale. A major phase of Pliocene northern hemisphere ice growth occurred between 3.15 Ma and 2.5 Ma. This was marked by a gradual increase in glacial Atlantic δ18O values of 1‰ and an increase in amplitude variations by up to 1.5‰, much larger than in the Pacific deepwater record (site 846). The first maxima occured in cold stages G6‐96 between 2.7 Ma and 2.45 Ma. Prior to 3 Ma, the isotope record is characterized by predominantly low amplitude fluctuations (< 0.7‰.). When obliquity forcing was at its minimum between 4.15 and 3.6 Ma and during the Kaena interval, δ18O amplitude fluctuations were minimal. From 4.9 to 4.3 Ma, the δ18O values decreased by about 0.5‰, reaching a long‐term minimum at 4.15 Ma, suggesting higher deepwater temperatures or a deglaciation. Deepwater cooling and/or an increase in ice volume is indicated by a series of short‐term δ18O fluctuations between 3.8 and 3.6 Ma.
Nannofossils and superplumes: The Early Aptian “nannoconid crisis”
Authors: Erba Elisabetta
Published: February 2004

A group of calcareous nannoplankton named nannoconids experienced a crisis in the early Aptian and recovered only later in the late Aptian after a period of virtual absence. Although no extinctions occurred, the widespread nature of the “nannoconid crisis” suggests a global causal factor. This crisis is recorded within the Chiastozygus litterarius nannofossil and Globigerinelloides blowi planktonic foraminiferal zones, postdates magnetic chronozone M0 by approximately 300 kyr, and precedes the oceanic anoxic subevent 1a and associated δ13C anomaly by some 40–100 kyr. Selective dissolution and anoxia cannot explain the crisis, because nannoconids are dissolution‐resistant forms and their crisis clearly precedes the deposition of anoxic sediments. At least 1 m.y. prior to the “nannoconid crisis,” the onset of a nannoplankton speciation event may be the response of nannofloras to a major rise in relative sea level. The “nannoconid crisis” seems to be synchronous with the early Aptian volcanic eruptions in the Pacific Ocean. Hence calcareous nannoplankton were severely affected by the “superplume” volcanic episode. The coccolithophorid bloom/nannoconid crisis was possibly induced by the excessive CO2 levels in the atmosphere and/or caused by changes in nutrient content of oceanic surface waters. Fertility was enhanced by rapid turnover of nutrients due to the abnormal volcanic activity and accelerated transfer of nutrients from the continents into the oceans under warm and humid conditions of the mid‐Cretaceous greenhouse climate. The “nannoconid crisis” may represent a competition between phytoplankton groups for nutrients or, more likely, competition between different calcareous nannoplankton. The biologic affinity and mode of life of Nannoconus are unknown, because there is no modern analog of this genus. However, comparison of Lower Cretaceous nannofossil assemblages with modern nannoplankton cummunities suggests that nannoconids, like extant Florisphaera profunda, possibly inhabited the lower photic zone. Concentrations of nutrients in the upper euphotic zone may have triggered blooms of coccolithophorids and nannoconid depletion. This model implies that the “nannoconid crisis” is the result of an abrupt, major change in the structure of surface waters caused directly or indirectly by the “superplume.” The adjustments of the biosphere to the new paleoceanographic and climatic conditions required some 40–100 kyr before changing into abnormally high primary productivity and deposition of organic carbon‐rich sediments with dinoflagellates outcompeting nannoplankton.
The Himalayas, organic carbon burial, and climate in the Miocene
Authors: Raymo M. E.
Published: February 2004

Cooling ages of rock in the Himalayas imply that rapid exhumation between the Main Central thrust system and the South Tibetan detachment system occurred between 21 and 17 Ma. The generation of relief and enhanced weathering which followed this event may have resulted in a pronounced increase in the delivery of dissolved strontium, carbon, phosphorus, and other chemical weathering products to the ocean (Richter et al., 1992). The increased supply of nutrients stimulated productivity in oceanic upwelling zones and expansion of the oxygen minimum zone leading to enhanced burial and preservation of organic matter in the Monterey formation and other deposits from this interval. A downdraw of atmospheric CO2 associated with enhanced chemical weathering rates and organic matter burial may have led to global cooling and the expansion of the Antarctic ice sheet by 15 Ma. The above scenario differs from the “Monterey hypothesis” of Vincent and Berger in that CO2 downdraw is primarily via silicate weathering rather than organic carbon burial and that organic carbon burial is driven by increased delivery of nutrients to the ocean rather than by stronger upwelling. A carbon mass balance calculation which assumes that river fluxes have been increasing over the last 40 Ma predicts that absolute organic carbon burial increased over this interval while, at the same time, the fraction of carbon buried as organic matter versus carbonate decreased. This implies that the organic carbon cycle has acted as a net source of CO2 to the atmosphere over the late Cenozoic.
An apparent contradiction in the role of phosphorus in Cenozoic chemical mass balances for the World Ocean
Authors: Delaney Margaret Lois , Filippelli Gabriel M.
Published: February 2004

Little is known about the fluxes to and from the ocean during the Cenozoic of phosphorus (P), a limiting nutrient for oceanic primary productivity and organic carbon burial on geologic timescales. Previous studies have concluded that dissolved river fluxes increased worldwide during the Cenozoic and that organic carbon burial decreased relative to calcium carbonate burial and perhaps in absolute terms as well. To examine the apparent contradiction between increased river fluxes of P (assuming P fluxes behave like the others) expected to drive increased organic carbon burial and observations indicating decreased organic carbon burial, we determined P accumulation rates for equatorial Pacific sediments from Ocean Drilling Program leg 138 sites in the eastern equatorial Pacific and leg 130 sites on the Ontong Java Plateau in the western equatorial Pacific. Although there are site specific and depth dependent effects on P accumulation rates, there are important features common to the records at all sites. P accumulation rates declined from 50 to 20 Ma, showed some variability from 20 to 10 Ma, and had a substantial peak from 9 to 3 Ma centered at 5–6 Ma. These changes in P accumulation rates for the equatorial Pacific are equivalent to substantial changes in the P mass balance. However, the pattern resembles neither that of weathering flux indicators (87Sr/86Sr and Ge/Si ratios) nor that of the carbon isotope record reflecting changes in organic carbon burial rates. Although these P accumulation rate patterns need confirmation from other regions with sediment burial significant in global mass balances (e.g., the North Pacific and Southern Ocean), it appears that P weathering inputs to the ocean are decoupled from those of other elements and that further exploration is needed of the relationship between P burial and net organic carbon burial.
Ice‐rafted debris associated with binge/purge oscillations of the Laurentide Ice Sheet
Authors: Alley R. B. , MacAyeal D. R.
Published: February 2004

The North Atlantic sediment record suggests quasi‐periodic (7000‐ to 12,000‐year period) ice‐rafted debris (IRD) depositions during at least the last glacial period. The cause of these Heinrich events, as they are commonly known, is not fully understood; however, they may point to surges of the ice stream that drained the Hudson Bay/Hudson Strait region of the Laurentide Ice Sheet. We investigate a simple conceptual model of ice stream instability (the binge/purge model) to suggest ways in which the ice stream could have entrained sufficient debris to account for the estimated mass of IRD associated with a typical Heinrich IRD layer in the North Atlantic (1.0 ± 0.3 × 1015 kg). We find that freezing of debris‐laden ice at the bed of the ice stream during the brief (≈ 750 years) surge phase of the ice stream's hypothesized binge/purge cycle can incorporate up to 5.1 × 1015 kg. This amount is sufficient to meet the constraints of the North Atlantic sediment record but by no means verifies the binge/purge model as the cause of Heinrich events.
Benthic phosphorus regeneration, net primary production, and ocean anoxia: A model of the coupled marine biogeochemical cycles of carbon and phosphorus
Authors: Van Cappellen Philippe , Ingall Ellery D.
Published: February 2004

We examine the relationships between ocean ventilation, primary production, water column anoxia, and benthic regeneration of phosphorus using a mass balance model of the coupled marine biogeochemical cycles of carbon (C) and phosphorus (P). The elemental cycles are coupled via the Redfield C/P ratio of marine phytoplankton and the C/P ratio of organic matter preserved in marine sediments. The model assumes that on geologic timescales, net primary production in the oceans is limited by the upwelling of dissolved phosphorus to the photic zone. The model incorporates the dependence on bottom water oxygenation of the regeneration of nutrient phosphorus from particulate matter deposited at the water‐sediment interface. Evidence from marine and lacustrine settings, modern and ancient, demonstrates that sedimentary burial of phosphorus associated with organic matter and ferric oxyhydroxides decreases when bottom water anoxia‐dysoxia expands. Steady state simulations show that a reduction in the rate of thermohaline circulation, or a decrease of the oxygen content of downwelling water masses, intensifies water column anoxia‐dysoxia and at the same time increases surface water productivity. The first effect reflects the declining supply of oxygen to the deeper parts of the ocean. The second effect is caused by the enhanced benthic regeneration of phosphorus from organic matter and ferric oxyhydroxides. Sedimentary burial of organic carbon and authigenic calcium phosphate mineral (francolite), on the other hand, is promoted by reduced ocean ventilation. According to the model, global‐scale anoxia‐dysoxia leads to a more efficient recycling of reactive phosphorus within the ocean system. Consequently, higher rates of primary production and organic carbon burial can be achieved, even when the continental supply of reactive phosphorus to the oceans remains unchanged.
Quantitative estimation of surface ocean productivity and bottom water oxygen concentration using benthic foraminifera
Authors: Loubere Paul
Published: February 2004

An electronic supplement of this material may be obtained on adiskette or Anonymous FTP from KOSMOS.AGU.ORG. (LOGIN toAGU's FTP account using ANONYMOUS as the usemame andGUEST as the password. Go to the right directory by typing CDAPEND. Type LS to see what files are available. Type GET and thename of the file to get it. Finally, type EXIT to leave the system.)(Paper 94PA01624, Quantitative estimation of surface oceanproductivity and bottom water concentration using benthicforaminifera, by P. Loubere). Diskette may be ordered from AmericanGeophysical Union, 2000 Florida Avenue, N.W., Washington, DC20009; $15.00. Payment must accompany order.
Quantitative estimation of surface ocean productivity and bottom water oxygen concentration with benthic foraminifera was attempted using 70 samples from equatorial and North Pacific surface sediments. These samples come from a well defined depth range in the ocean, between 2200 and 3200 m, so that depth related factors do not interfere with the estimation. Samples were selected so that foraminifera were well preserved in the sediments and temperature and salinity were nearly uniform (T = 1.5° C; S = 34.6‰). The sample set was also assembled so as to minimize the correlation often seen between surface ocean productivity and bottom water oxygen values (r² = 0.23 for prediction purposes in this case). This procedure reduced the chances of spurious results due to correlations between the environmental variables. The samples encompass a range of productivities from about 25 to >300 gC m−2 yr−1, and a bottom water oxygen range from 1.8 to 3.5 ml/L. Benthic foraminiferal assemblages were quantified using the >62 µm fraction of the sediments and 46 taxon categories. MANOVA multivariate regression was used to project the faunal matrix onto the two environmental dimensions using published values for productivity and bottom water oxygen to calibrate this operation. The success of this regression was measured with the multivariate r² which was 0.98 for the productivity dimension and 0.96 for the oxygen dimension. These high coefficients indicate that both environmental variables are strongly imbedded in the faunal data matrix. Analysis of the beta regression coefficients shows that the environmental signals are carried by groups of taxa which are consistent with previous work characterizing benthic foraminiferal responses to productivity and bottom water oxygen. The results of this study suggest that benthic foraminiferal assemblages can be used for quantitative reconstruction of surface ocean productivity and bottom water oxygen concentrations if suitable surface sediment calibration data sets are developed and appropriate means for detecting no‐analog samples are found.
Photosynthetic fractionation of 13C and concentrations of dissolved CO2 in the central equatorial Pacific during the last 255,000 years
Authors: Jasper John P. , Hayes J. M. , Mix A. C. , Prahl F. G.
Published: February 2004

Carbon isotopically based estimates of CO2 levels have been generated from a record of the photosynthetic fractionation of 13C (≡ εp) in a central equatorial Pacific sediment core that spans the last ∼255 ka. Contents of 13C in phytoplanktonic biomass were determined by analysis of C37 alkadienones. These compounds are exclusive products of Prymnesiophyte algae which at present grow most abundantly at depths of 70–90 m in the central equatorial Pacific. A record of the isotopic compostion of dissolved CO2 was constructed from isotopic analyses of the planktonic foraminifera Neogloboquadrina dutertrei, which calcifies at 70–90 m in the same region. Values of εp, derived by comparison of the organic and inorganic δ values, were transformed to yield concentrations of dissolved CO2 (≡ ce) based on a new, site‐specific calibration of the relationship between εp and ce. The calibration was based on reassessment of existing εp versus ce data, which support a physiologically based model in which εp is inversely related to ce. Values of PCO2, the partial pressure of CO2 that would be in equilibrium with the estimated concentrations of dissolved CO2, were calculated using Henry's law and the temperature determined from the alkenone‐unsaturation index U37K′. Uncertainties in these values arise mainly from uncertainties about the appropriateness (particularly over time) of the site‐specific relationship between εp and 1/ce. These are discussed in detail and it is concluded that the observed record of εp most probably reflects significant variations in ΔpCO2, the ocean‐atmosphere disequilibrium, which appears to have ranged from ∼110 µatm during glacial intervals (ocean > atmosphere) to ∼60 µatm during interglacials. Fluxes of CO2 to the atmosphere would thus have been significantly larger during glacial intervals. If this were characteristic of large areas of the equatorial Pacific, then greater glacial sinks for the equatorially evaded CO2 must have existed elsewhere. Statistical analysis of air‐sea pCO2 differences and other parameters revealed significant (p < 0.01) inverse correlations of ΔpCO2 with sea surface temperature and with the mass accumulation rate of opal. The former suggests response to the strength of upwelling, the latter may indicate either drawdown of CO2 by siliceous phytoplankton or variation of [CO2]/[Si(OH)4] ratios in upwelling waters.
Impact of productivity events on the benthic foraminiferal fauna in the Arabian Sea over the last 150,000 years
Authors: Otto J. , Hermelin R. , Shimmield Graham B.
Published: February 2004

We have studied the sediment geochemistry and benthic foraminiferal fauna in a piston core from the northwest Arabian Sea (covering the last 150,000 years), and compared the results with the stable isotopic record from both benthic (Uvigerina peregrina) and planktonic (Globigerinoides sacculifer) foraminifera. The δ18O record of the planktonic foraminiferal species shows excursions towards heavier δ18O values when compared to the benthic record and the SPECMAP stack, particularly in stage 3, between approximately 30 and 45–50 kyr. We call this the “stage 3 event.” Species diversity and relative abundance data on benthic foraminifera reveal that some species, e.g. Cibicidoides wuellerstorfi, Oridorsalis umbonatus, Pullenia bulloides, Pyrgo spp., and the uvigerinids increased in relative abundance during glacial stages 2 and 4 and during the stage 3 event, whereas other (e.g., Astrononion novozealandicum, Cibicidoides bradyi, Eggerella bradyi, Gyroidina altiformis, Hoeglundina elegans, Pullenia subcarinata, and Sigmoilina edwardsii) decreased. Using correspondence analysis we determined that approximately 50% of the total sample variation may be accounted for by three first factors which all are interpreted as primarily related to surface water productivity and the quality of the organic matter that reaches the seafloor. The relative abundance of some species appears to vary on a precessional cycle, perhaps responding to monsoon‐driven upwelling and associated increased food supply, whilst the abundances of other species are related to the 100 kyr ice volume cycle, and may be responding to changes in intermediate and deep water chemistry. Geochemical paleoproductivity tracers, such as the Ba content of the sediment, have a very similar distribution to specific foraminiferal factors in both time and frequency domains. Certain planktonic foraminifera, (e.g., Globigerina bulloides), also display obvious maxima in the stage 3 event. The stage 3 event occurs at a time of reduced rate of amplitude change in both solar radiation and monsoon pressure index. We conclude, that there was a period 15 to 20 kyr duration during stage 3 with cooler‐than‐normal surface waters, enhanced productivity and rapid settling of biogenic matter.
Improved preservation of organic matter and pollen in eastern Mediterranean sapropels
Authors: Cheddadi Rachid , Rossignol‐Strick Martine
Published: February 2004

Here we investigate the processes leading to high organic content and high pollen concentration in pelagic marine sediments. Three marine cores near the base of the Nile River cone display the classical Quaternary sequence of sapropels and hemipelagic muds. Large variations in total pollen concentration, as well as in percentage composition of the pollen assemblage are clearly linked to the two lithological types. All pollen types have much higher concentration in sapropels than in muds, by 3 or 4 orders of magnitude. Within sapropels, the percentages of Gymnosperms (Pinus and Cedrus) pollen and Pteridophyte spores, are very low and the pollen spectrum is dominated by various Angiosperms. These variations in total pollen concentration as well as in percentages cannot be accounted for by variations in pollen production, or in efficiency of transport, because they similarly involve pollen of different areas and climates, transported to the sea by different means. The cause for these variations can only be a difference in preservation of pollen. Preservation is good in sapropels for all pollen types, but generally poor in muds. We believe that sapropels were deposited under reducing bottom waters which allowed a better preservation of the organic matter including pollen.
Possible role of oceanic heat transport in Early Eocene climate
Authors: Sloan L. Cirbus , Walker James C. G. , Moore T. C.
Published: February 2004

Increased oceanic heat transport has often been cited as a means of maintaining warm high‐latitude surface temperatures in many intervals of the geologic past, including the early Eocene. Although the excess amount of oceanic heat transport required by warm high latitude sea surface temperatures can be calculated empirically, determining how additional oceanic heat transport would take place has yet to be accomplished. That the mechanisms of enhanced poleward oceanic heat transport remain undefined in paleoclimate reconstructions is an important point that is often overlooked. Using early Eocene climate as an example, we consider various ways to produce enhanced poleward heat transport and latitudinal energy redistribution of the sign and magnitude required by interpreted early Eocene conditions. Our interpolation of early Eocene paleotemperature data indicate that an ∼30% increase in poleward heat transport would be required to maintain Eocene high‐latitude temperatures. This increased heat transport appears difficult to accomplish by any means of ocean circulation if we use present ocean circulation characteristics to evaluate early Eocene rates. Either oceanic processes were very different from those of the present to produce the early Eocene climate conditions or oceanic heat transport was not the primary cause of that climate. We believe that atmospheric processes, with contributions from other factors, such as clouds, were the most likely primary cause of early Eocene climate.
Glacial‐interglacial paleoenvironments of the eastern Atlantic Ocean: The Biogeochemical Ocean Flux Study (BOFS) Paleoceanography Program
Authors: Elderfield H. , Thomas E.
Published: February 2004

The papers in this special section discuss many aspects of paleoenvironmental analysis performed using various proxies on cores collected in the northeastern and tropical eastern Atlantic during two cruises in the framework of the NERC Biogeochemical Ocean Flux Study Program (BOFS). Coring of the northern BOFS leg was carried out on Leg 3 of the 1989 BOFS program (RRS Discovery cruise 184 [McCave, 1989]). Forty‐one cores were taken on a North‐South transect at about 20°W, from the flanks of the Mid‐Atlantic Ridge (47°N) to Gardar Drift (59°N) at water depths between 1150 and 4050 m. Of these, 15 cores were taken using a 3 m‐ long, large‐diameter kastenlot corer designed by Zangger and Mc Cave [199O]. Cruise and core data are given by Manighetti et al. [this issue]. The southern leg coring was carried out on Leg C of the 1990 BOFS program (RRS Charles Darwin cruise 53 [Shimmield, 199O]). Forty‐six cores, including 14 using the kastenlot corer, were taken along a transect at about 2O°W, between 18 and 42°N, at depths between 3000 and 4900 m and focusing on an area off northwest Africa .
Late Quaternary paleoceanography of the Eurasian Basin, Arctic Ocean
Authors: Cronin T. M. , Holtz T. R. , Stein R. , Spielhagen R. , Fütterer D. , Wollenburg J.
Published: February 2004

We reconstructed late Quaternary deep (3000–4100 m) and intermediate depth (1000–2500 m) paleoceanographic history of the Eurasian Basin, Arctic Ocean from ostracode assemblages in cores from the Lomonosov Ridge, Gakkel Ridge, Yermak Plateau, Morris Jesup Rise, and Amundsen and Makarov Basins obtained during the 1991 Polarstern cruise. Modern assemblages on ridges and plateaus between 1000 and 1500 m are characterized by abundant, relatively species‐rich benthic ostracode assemblages, in part, reflecting the influence of high organic productivity and inflowing Atlantic water. In contrast, deep Arctic Eurasian basin assemblages have low abundance and low diversity and are dominated by Krithe and Cytheropteron reflecting faunal exchange with the Greenland Sea via the Fram Strait. Major faunal changes occurred in the Arctic during the last glacial/interglacial transition and the Holocene. Low‐abundance, low‐diversity assemblages from the Lomonosov and Gakkel Ridges in the Eurasian Basin from the last glacial period have modern analogs in cold, low‐salinity, low‐nutrient Greenland Sea deep water; glacial assemblages from the deep Nansen and Amundsen Basins have modern analogs in the deep Canada Basin. During Termination 1 at intermediate depths, diversity and abundance increased coincident with increased biogenic sediment, reflecting increased organic productivity, reduced sea‐ice, and enhanced inflowing North Atlantic water. During deglaciation deep Nansen Basin assemblages were similar to those living today in the deep Greenland Sea, perhaps reflecting deepwater exchange via the Fram Strait. In the central Arctic, early Holocene faunas indicate weaker North Atlantic water inflow at middepths immediately following Termination 1, about 8500–7000 year B.P., followed by a period of strong Canada Basin water overflow across the Lomonosov Ridge into the Morris Jesup Rise area and central Arctic Ocean. Modern perennial sea‐ice cover evolved over the last 4000–5000 years. Late Quaternary faunal changes reflect benthic habitat changes most likely caused by changes in the import of cold, deepwater of Greenland Sea origin and warmer and middepth Atlantic water to the Eurasian Basin through the Fram Strait, and export of Arctic Ocean deepwater.

Appendix Tables Al‐A4 are available on microfiche. Orderfrom the American Geophysical Union, 2000 Florida Avenue,N.W., Washington, DC 20009. Document P94‐002; $2.50.Payment must accompany order.
Earliest Oligocene increase in South Atlantic productivity as interpreted from “rock magnetics” at Deep Sea Drilling Project Site 522
Authors: Hartl Paul , Tauxe Lisa , Herbert Timothy
Published: February 2004

The magnetic properties of the sediments (“rock magnetics”) at DSDP Site 522 in the South Atlantic exhibit clear differences between the latest Eocene and earliest Oligocene. Based on low temperature behavior of saturation remanence and hysteresis loops, we attribute the difference to a slightly greater proportion of the finest grained, so‐called “superparamagnetic” magnetite in the Eocene sediments. We believe that the lower proportion of very fine‐grained magnetite in the Oligocene sediments is a result of incipient reduction diagenesis caused by increased productivity and hence increased labile organic carbon transport to the sediments due to an early Oligocene increase in thermohaline circulation. The Eocene‐to‐Oligocene transition at Site 522 is also expressed by changes in microfossil assemblages, increased carbonate content, decreased insoluble residue, and decreased foraminiferal shell fragmentation. The increase in carbonate is synchronous with and parallels a change in the ratio of two of the rock magnetic parameters, a ratio that tracks the decrease in the very fine‐grained magnetite component. Also parallel to these is a trend toward heavier δ13C values in foraminiferal tests. The increase in organic carbon transport to the sediments led to chemical dissolution of the finest grain‐size fraction of magnetite in the Oligocene sediments, hence a reduction in the superparamagnetic component and the change in the rock magnetic ratio. In this way, rock magnetics can be sensitive indicators of environmental changes, such as fluctuations in organic carbon transport, which may leave little other trace in the sedimentary record.
Aridity of the Mediterranean Sea at the Last Glacial Maximum: A reinterpretation of the δ18O record
Authors: Bigg Grant R.
Published: February 2004

Paleoclimate data show conflicting evidence for aridity in the Mediterranean basin at the last glacial maximum (LGM). Two recent modeling studies of the LGM Mediterranean, using water‐balance/biome and ocean general circulation models, respectively, suggest that evaporation over the basin was reduced at the LGM compared with today. In apparent conflict with these results is the strongly enhanced δ18O signature of the LGM Mediterranean surface waters, particularly in the eastern basin, which has been ascribed to a much higher salinity. In an alternative interpretation presented here, a box model of the balance between paleoclimate model evaporation and rainfall and paleoceanographic model advective terms shows that the observed LGM δ18O signature can be reproduced without the need for an enhanced salinity, with advection playing the dominant role in the δ18O budget. The box model also shows that the δ18O data alone are insufficient to determine the type of exchange between the eastern and western basins. The results emphasize that regional paleoceanographic water balances should consider all relevant processes, and not just the evaporative flux.
Deep thermohaline circulation in the low‐latitude Atlantic during the Last Glacial
Authors: Beveridge N. A. S. , Elderfield H. , Shackleton N. J.
Published: February 2004

Present‐day low‐latitude eastern and western Atlantic basins are geochemically distinct below the sill depth of the Mid‐Atlantic Ridge. While Antarctic Bottom Water (AABW) circulates freely in the western Atlantic, flow into the eastern Atlantic is restricted below 4 km which results in filling the abyssal depths of this basin with water of geochemical similarity to nutrient depleted North Atlantic Deep Water. Using carbon isotopes and Cd/Ca ratios in benthic foraminifera we reconstruct the geochemistry of these basins during the last glacial maximum. Results indicate that deep eastern and western Atlantic basins became geochemically identical during the last glacial. This was achieved by shoaling of the upper surface of AABW above the sill depth of the Mid‐Atlantic Ridge, which allowed bottom waters in both basins to be filled with the same water mass. Although AABW became the dominant water mass in the deep eastern Atlantic basin during the glacial, Holocene‐glacial δ13C‐PO4 shifts in this basin are in Redfield proportions, unlike the disproportionate Holocene‐glacial δ13C‐PO4 shifts observed in the Southern Ocean. By examining the composition of deep and intermediate waters throughout the Atlantic, we show that this effect was induced by a change in gradient of the δ13C‐PO4 deepwater mixing line during glacial times. Evidence from high‐latitude planktonic data suggests that the change in gradient of the deepwater mixing line was brought about through a significant reduction in the thermodynamic effect on Southern Ocean surface waters. By using coupled δ13C‐PO4 data to constrain the composition of end member water masses in the glacial Atlantic, we conclude that deep waters in the low‐latitude glacial Atlantic were composed of a mixture of northern and southern source waters in a ratio of 1∶3.
Molecular stratigraphy of cores off northwest Africa: Sea surface temperature history over the last 80 Ka
Authors: Zhao M. , Beveridge N. A. S. , Shackleton N. J. , Sarnthein M. , Eglinton G.
Published: February 2004

The molecular stratigraphy of Biogeochemical Oceanic Flux Study core 31K (19°N, 20°10′W) and Ocean Drilling Program Hole 658C (20°45′N, 18°35′W;) has been studied for C37 alkenone abundances over the past 80 ka at high resolution (∼circa 200–500 years). The derived Uk37′ sea surface temperature record for both cores shows a range of temperatures from about 18°C during the last glacial to 21.5°C during the early Holocene. Both records also reveal changes in sea surface temperature as much as 2°‐4°C over a few hundred years, which correlate well with similar abrupt climatic changes observed in cores from elsewhere in the NE Atlantic, associated with “Heinrich events.” Our results indicate that meltwater produced by these ice‐rafting events was transmitted southward by the Canary Current, where it had considerable impact on sea surface temperatures in the subtropical eastern Atlantic.
Can seismology tell us anything about convection in the mantle?
Authors: Montagner Jean‐Paul
Published: February 2004

The understanding of mantle convection is one of the most puzzling problems of modern geophysics. Among the different approaches used by geophysicists to investigate mantle convection, seismic tomography is the only one able to visualize, at the same time, temperature, petrological anomalies, and flow directions from seismic velocity and anisotropy heterogeneities. In order to enable a comparison with other geophysical observables, tomographic models are expanded into spherical harmonics. Most tomographic models agree that down to 300–400 km, deep structure is closely related to plate tectonics and continental distribution. Its corresponding spectral content regularly decreases with decreasing wavelength. At greater depths in the transition zone, degree 2 and (to a lesser extent) degree 6 distributions become predominant. A degree 2 pattern is also present in the lower mantle and is strongly correlated with the geoid but offset with respect to the degree 2 pattern of the transition zone. A simple flow pattern with two upgoing and two downgoing large‐scale flows can be invoked to simply explain the predominance of degrees 2 and 6 for seismic velocity and degree 4 for radial anisotropy. Therefore below the apparent complexity of plate tectonics, it turns out that mantle convection is surprisingly simply organized in the transition zone. Between 400 and 1000 km, these large‐scale flows are not independent from the circulation in the first 400 km but are connected to some of the most tectonically active zones (fast ridges and slabs). It is also suggested that the degree 6 which seems to be a marker of the hotspot distribution is not independent of the deep degree 2 but might be the consequence of this simple flow pattern. The good correlation between seismic tomography degree 6 and hotspot degree 6 favors an origin at depth of hotspots in the transition zone. Generally, the mantle cannot be divided into independent convecting cells but is characterized by imbricated convection, where different scales coexist and where exchange of matter is possible. Therefore seismic tomography is able to provide very strong constraints on possible models of mantle convection, but many features are still unexplained. Only very long spatial wavelengths are well resolved so far, and a complete understanding of mantle dynamics necessitates relating the different scales present in convective processes.
Mechanisms of Earth differentiation: Consequences for the chemical structure of the mantle
Authors: Carlson Richard W.
Published: February 2004

Compared to the mantles of the Moon and perhaps Mars, the Earth's mantle is much less differentiated chemically. Both the Moon and Mars appear to have undergone a major differentiation accompanying planet formation. The only clear signature of a similar event on the Earth is core formation. While this might imply that the Earth did not experience extensive melting and differentiation during planet formation, the higher pressures and temperatures present in the Earth could have led to a distinctly different chemical evolution for this initial differentiation. The most significant potential outcome of early differentiation on the Earth's mantle is formation of chemically distinct upper and lower mantles distinguished by Mg/Si higher and lower than chondritic, respectively. Plate tectonics on Earth provides a continuing mechanism for planet differentiation that forms crust at the expense of chemical differentiation of the mantle. Plate tectonics, however, also offers a mechanism to return the chemically distinct materials of the crust back into the mantle. Mixing of subducted crustal material into the mantle through the stirring provided by mantle convection can serve to negate the effects of crust formation on the chemical composition of the mantle. Similarly, mixing within the mantle could serve to destroy evidence of early differentiation, if such differentiation occurred on Earth. Completely efficient operation of the plate tectonic cycle would result in remixing of crust and differentiated mantle, with the end result being a homogenous mantle with composition identical to that of the bulk earth minus the materials segregated into the core. In part, this may explain the relatively undifferentiated nature of the Earth's mantle. Plate tectonics has not been completely efficient on Earth, however. Both oceanic and continental crust exist, and there is widespread evidence for chemical variability in the mantle. At least four chemically and isotopically distinct components are observed in mantle‐derived rocks. The nature of these components points to the importance of crust formation and recycling in determining the chemical variability of the mantle. Mapping of the surface expression of chemical heterogeneity in the mantle is providing new views of the chemical structure of the mantle and the geodynamic processes that operate in the Earth's interior.