bluuebunny/crossref_metadata_2025 huggingface dataset:
Carbon isotope discrimination of arctic and boreal biomes inferred from remote atmospheric measurements and a biosphere‐atmosphere model
Authors: Randerson J. T. , Still C. J. , Ballé J. J. , Fung I. Y. , Doney S. C. , Tans P. P. , Conway T. J. , White J. W. C. , Vaughn B. , Suits N. , Denning A. S.
Published: October 2002

Estimating discrimination against 13C during photosynthesis at landscape, regional, and biome scales is difficult because of large‐scale variability in plant stress, vegetation composition, and photosynthetic pathway. Here we present estimates of 13C discrimination for northern biomes based on a biosphere‐atmosphere model and on National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostics Laboratory and Institute of Arctic and Alpine Research remote flask measurements. With our inversion approach, we solved for three ecophysiological parameters of the northern biosphere (13C discrimination, a net primary production light use efficiency, and a temperature sensitivity of heterotrophic respiration (a Q10 factor)) that provided a best fit between modeled and observed δ13C and CO2. In our analysis we attempted to explicitly correct for fossil fuel emissions, remote C4 ecosystem fluxes, ocean exchange, and isotopic disequilibria of terrestrial heterotrophic respiration caused by the Suess effect. We obtained a photosynthetic discrimination for arctic and boreal biomes between 19.0 and 19.6‰. Our inversion analysis suggests that Q10 and light use efficiency values that minimize the cost function covary. The optimal light use efficiency was 0.47 gC MJ−1 photosynthetically active radiation, and the optimal Q10 value was 1.52. Fossil fuel and ocean exchange contributed proportionally more to month‐to‐month changes in the atmospheric growth rate of δ13C and CO2 during winter months, suggesting that remote atmospheric observations during the summer may yield more precise estimates of the isotopic composition of the biosphere.
Belowground factors mediating changes in methane consumption in a forest soil under elevated CO2
Authors: McLain Jean E. T. , Kepler Thomas B. , Ahmann Dianne M.
Published: October 2002

The sustained increase in atmospheric CO2 concentration observed over the past century, and projected to continue into the next, is of great significance for atmospheric CH4. Effects of elevated CO2 on microbial methane cycling are potentially mediated by its effects on plant physiology, which include enhancement of carbon assimilation, belowground carbon allocation, and water use efficiency. To determine the importance of such changes for methane cycling, belowground factors impacting soil CH4 consumption were investigated at the Free Air Carbon Transfer and Storage (FACTS)‐I site in the Duke Forest, North Carolina, in which plots have been exposed to ambient (370 ppm) or elevated (ambient + 200 ppm) CO2 since August 1996. CH4 fluxes at the soil surface, porespace concentrations of CH4, O2, and CO2, soil moisture, soil temperature, and soil pH were simultaneously measured over 24 months. Porespace CH4 concentrations were highest (1.98 ± 0.25 ppm) at the soil surface and decreased to 0.65 ± 0.22 ppm at 30 cm, indicating that methanotrophic activity was depleting CH4 in the upper soil layers and creating a gradient to draw atmospheric CH4 into the soil. This was confirmed by surface CH4 flux measurements, which averaged −1.54 ± 0.65 μmol/m2/h. Under elevated CO2, porespace CH4 was 25–30% higher in the upper 70 cm of soils; CH4 fluxes from the atmosphere into soil were diminished by ∼25%; soil CO2 increased by 10–70%; and volumetric soil moisture was greater by up to 40% during some seasons. Statistical modeling revealed that soil moisture strongly predicted variability in surface CH4 fluxes and that soil CO2 and soil moisture both predicted variability in soil CH4. Results also indicated that a portion of the net CH4 sink inhibition in elevated CO2 soils could be attributable to alterations in soil biological processes, suggesting that changes in the CH4‐cycling microbial ecology had taken place.
Silicic acid leakage from the Southern Ocean: A possible explanation for glacial atmospheric pCO2
Authors: Matsumoto Katsumi , Sarmiento Jorge L. , Brzezinski Mark A.
Published: October 2002

Using a simple box model, we investigate the effects of a reduced Si:N uptake ratio by Antarctic phytoplankton on the marine silica cycle and atmospheric pCO2. Recent incubation experiments demonstrate such a phenomenon in diatoms when iron is added [Hutchins and Bruland, 1998; Takeda, 1998; Franck et al., 2000]. The Southern Ocean may have supported diatoms with reduced Si:N uptake ratios compared to today during the dustier glacial times [Petit et al., 1999]. A similar reduction in the uptake ratio may be realized with an increased production of nondiatom phytoplankton such as Phaeocystis. Our model shows that reduced Si:N export ratios in the Southern Ocean create excess silicic acid, which may then be leaked out to lower latitudes. Any significant consumption of the excess silicic acid by diatoms that leads to an enhancement in their growth at the expense of coccolithophorids diminishes CaCO3 production and therefore diminishes the carbonate pump. In our box model the combination of a reduced carbonate pump and an open system carbonate compensation draw down steady state atmospheric CO2 from the interglacial 277 to 230–242 ppm, depending on where the excess silicic acid is consumed. By comparison, the atmospheric pCO2 sensitivity of general circulation models to carbonate pump forcing is ∼3.5–fold greater, which, combined with carbonate compensation, can account for peak glacial atmospheric pCO2. We discuss the importance of the initial rain ratio of CaCO3 to organic carbon on atmospheric pCO2 and relevant sedimentary records that support and constrain this “silicic acid leakage” scenario.
Comparison of algorithms for estimating ocean primary production from surface chlorophyll, temperature, and irradiance
Authors: Campbell Janet , Antoine David , Armstrong Robert , Arrigo Kevin , Balch William , Barber Richard , Behrenfeld Michael , Bidigare Robert , Bishop James , Carr Mary‐Elena , Esaias Wayne , Falkowski Paul , Hoepffner Nicolas , Iverson Richard , Kiefer Dale , Lohrenz Steven , Marra John , Morel Andre , Ryan John , Vedernikov Vladimir , Waters Kirk , Yentsch Charles , Yoder James
Published: October 2002

Results of a single‐blind round‐robin comparison of satellite primary productivity algorithms are presented. The goal of the round‐robin exercise was to determine the accuracy of the algorithms in predicting depth‐integrated primary production from information amenable to remote sensing. Twelve algorithms, developed by 10 teams, were evaluated by comparing their ability to estimate depth‐integrated daily production (IP, mg C m−2) at 89 stations in geographically diverse provinces. Algorithms were furnished information about the surface chlorophyll concentration, temperature, photosynthetic available radiation, latitude, longitude, and day of the year. Algorithm results were then compared with IP estimates derived from 14C uptake measurements at the same stations. Estimates from the best‐performing algorithms were generally within a factor of 2 of the 14C‐derived estimates. Many algorithms had systematic biases that can possibly be eliminated by reparameterizing underlying relationships. The performance of the algorithms and degree of correlation with each other were independent of the algorithms’ complexity.
Climate‐induced oceanic oxygen fluxes: Implications for the contemporary carbon budget
Authors: Bopp Laurent , Le Quéré Corinne , Heimann Martin , Manning Andrew C. , Monfray Patrick
Published: October 2002

Atmospheric O2 concentrations have been used to estimate the ocean and land sinks of fossil fuel CO2. In previous work, it has been assumed that the oceans have no long‐term influence on atmospheric O2. We address the validity of this assumption using model results and observations. Oceanic O2 fluxes for the 1860–2100 period are simulated using a coupled climate model in which is nested an ocean biogeochemistry model. Simulated oceanic O2 fluxes exhibit large interannual (±40 Tmol yr−1) and decadal (±13 Tmol yr−1) variability, as well as a net outgassing to the atmosphere caused by climate change (up to 125 Tmol yr−1 by 2100). Roughly one quarter of this outgassing is caused by warming of the ocean surface, and the remainder is caused by ocean stratification. The global oceanic O2 and heat fluxes are strongly correlated for both the decadal variations and the climate trend. Using the observed heat fluxes and the modeled O2 flux/heat flux relationship, we infer the contribution of the oceans to atmospheric O2 and infer a correction to the partitioning of the ocean and land CO2 sinks. After considering this correction, the ocean and land sinks are 1.8 ± 0.8 Pg C yr−1 and 0.3 ± 0.9 Pg C yr−1, respectively, for the 1980s (a correction of 0.1 from ocean to land) and are 2.3 ± 0.7 Pg C yr−1 and 1.2 ± 0.9 Pg C yr−1, respectively, in the 1990–1996 period (a correction of 0.5 from land to ocean). This correction reconciles the 1990s ocean sink estimated by the Intergovernmental Panel on Climate Change Third Assessment Report with ocean models.
Sea ice switch mechanism and glacial‐interglacial CO2 variations
Authors: Gildor Hezi , Tziperman Eli , Toggweiler J. R.
Published: October 2002

A physical mechanism is proposed for the glacial‐interglacial variations in the rate of vertical mixing and in sea ice cover in the Southern Ocean. Such variations were postulated without an explanation by Toggweiler [1999] and Stephens and Keeling [2000], who used them to explain the glacial‐interglacial CO2 variations. In the physical mechanism explored here, initially given by Gildor and Tziperman [2001b], changes in the stratification of the Southern Ocean due to the cooling of North Atlantic Deep Water (NADW) during glacial maxima reduce the rate of vertical mixing of the surface water with the deep water. The changed temperature of the NADW arriving to the Southern Ocean and the reduced vertical mixing there also increase the Southern Ocean sea ice cover during glacial maxima. These vertical mixing and sea ice cover changes are shown to be a natural result of the sea ice switch mechanism of the glacial cycles. A box model of the climate system is used to demonstrate the above physical mechanism and its effect on the atmospheric CO2. Because of the uncertainties in the exact dependence of the vertical mixing on vertical stratification, it is impossible to quantify the exact separate contribution of reduced vertical mixing and larger sea ice cover to the glacial CO2 variations. The CO2 variations are not essential to the existence of the glacial cycles in the sea ice switch mechanism, yet they are shown to amplify the glacial‐interglacial variability, consistent with the view of the role of CO2 deduced from proxy observations.
Experimental simulations with an ecosystem model of the Baltic Sea: A nutrient load reduction experiment
Authors: Neumann Thomas , Fennel Wolfgang , Kremp Christine
Published: October 2002

The study presents results of decadal simulations of the dynamics of the ecosystem of the Baltic Sea with a three‐dimensional coupled physical chemical biological model for the 1980s. The model consists of a circulation model, which is an implementation of the Modular Ocean Model (MOM 2) for the Baltic Sea and an embedded ecosystem model based on a nine‐component model (ERGOM [Neumann, 2000]). The model was driven with realistic atmospheric forcing, river loads, and atmospheric deposition of nutrients. A simulation with the realistic forcing was carried out as a control run in order to study in a further experimental simulation the response of the model ecosystem to a 50% reduction of riverine nutrient loads. It was found that the model food web reacts on the load reduction in a complex manner. The changes in dissolved nutrients and in phytoplankton display a spatial variability and different species‐specific reactions. While the total biomass and nutrient inventories are reduced, there are significant regional differences. In particular, the role of cyanobacteria, which can fix atmospheric nitrogen, is important, and the biomass of cyanobacteria increases in response to the reduced loads. Although the model system needs further improvements, the present study is a step toward an understanding of the quantitative effects of a reduction of nutrient river loads to the Baltic Sea ecosystem based on a full three‐dimensional coupled model.
A two‐dimensional nitrogen and carbon flux model in a coastal upwelling region
Authors: Ianson Debby , Allen Susan E.
Published: October 2002

Coastal upwelling regions are associated with high primary production and disproportionately large fluxes of organic matter relative to the global ocean. However, coastal regions are usually homogenized in global ocean carbon models. We have developed a carbon and nitrogen flux model including all major processes both within and below the euphotic zone over seasonal to decadal timescales for coastal upwelling regions. These fluxes control surface pCO2. The model is applied to the west coast of Vancouver Island, Canada (∼49°N, 126°W). Net annual air‐sea CO2 exchange and export flux of inorganic and organic carbon and nitrogen from the system to the rest of the ocean are estimated for different model scenarios. Model sensitivities are discussed. Results show strong biological drawdown of pCO2during summer and atmospheric CO2 invasion. However, this invasion is nearly balanced by gas evasion during winter. Therefore the region is a much smaller sink of atmospheric CO2 (6 g C m−2yr−1, or equivalently 200 kg C yr−1per m coastline) than the summer season predicts. More significantly, there is a large flux of inorganic carbon (3 × 104 kg C yr−1per m coastline) from intermediate depth ocean water to the surface ocean via the coastal system compared to a small export of organic carbon (all dissolved) (2 × 103 kg C yr−1 per m coastline) back into the lower layer of the open ocean. Thus we suggest that the dominant effect of coastal upwelling on the global ocean is providing a conduit for inorganic carbon to the surface ocean.
Remineralization ratios of carbon, nutrients, and oxygen in the North Atlantic Ocean: A field databased assessment
Authors: Thomas Helmuth
Published: October 2002

Remineralization ratios of carbon, nutrients, and oxygen have been assessed in the North Atlantic Ocean along the WOCE 1A/E section. The study is based on an extensive field data set comprising dissolved inorganic carbon (DIC), nitrate and nitrite (NO3/2), phosphate (PO4), and oxygen (O2) data as well as hydrographic data. A procedure has been introduced which normalizes DIC data to constant salinity and temperature and corrects for the contamination from anthropogenic CO2. The remaining variability on the normalized DIC values (DICbio) can be attributed to the remineralization of organic matter. DICbio can thus be seen as carbon‐analogy to the apparent oxygen utilization (AOU). The consecutive evaluation of the remineralization ratios obtains two different regimes separated at the density level ρ = 1027.7 kg m−3. In the shallower level the ratios (C/N = 4.5; C/P = 67, AOU/C = 2.0; N:P = 15; AOU:P = 134; AOU/N = 9.0) are shifted toward relatively higher nutrient release and higher oxygen consumption with respect to the Redfield ratios of particulate organic matter (POM). In contrast, in the deeper levels the ratios are shifted toward relatively higher carbon release and lower oxygen demand (C/N = 11; C/P = 152, AOU/C = 0.86; N:P = 13.9; AOU:P = 130; AOU/N = 9.4). The depth integrated inventories of the remineralization products (DIC, NO3/2, PO4, and AOU) provide water column averaged ratios for the investigation area (C/N = 8.8; C/P = 124, AOU/C = 1.1; N:P = 14.2; AOU:P = 131; AOU/N = 9.3) which imply a higher efficiency of the biological carbon pump in the North Atlantic Ocean than predicted with respect to the elemental composition of POM.
Modeling seasonal to annual carbon balance of Mer Bleue Bog, Ontario, Canada
Authors: Frolking Steve , Roulet Nigel T. , Moore Tim R. , Lafleur Peter M. , Bubier Jill L. , Crill Patrick M.
Published: October 2002

Northern peatlands contain enormous quantities of organic carbon within a few meters of the atmosphere and play a significant role in the planetary carbon balance. We have developed a new, process‐oriented model of the contemporary carbon balance of northern peatlands, the Peatland Carbon Simulator (PCARS). Components of PCARS are (1) vascular and nonvascular plant photosynthesis and respiration, net aboveground and belowground production, and litterfall; (2) aerobic and anaerobic decomposition of peat; (3) production, oxidation, and emission of methane; and (4) dissolved organic carbon loss with drainage water. PCARS has an hourly time step and requires air and soil temperatures, incoming radiation, water table depth, and horizontal drainage as drivers. Simulations predict a complete peatland C balance for one season to several years. A 3‐year simulation was conducted for Mer Bleue Bog, near Ottawa, Ontario, and results were compared with multiyear eddy covariance tower CO2flux and ancillary measurements from the site. Seasonal patterns and the general magnitude of net ecosystem exchange of CO2were similar for PCARS and the tower data, though PCARS was generally biased toward net ecosystem respiration (i.e., carbon loss). Gross photosynthesis rates (calculated directly in PCARS, empirically inferred from tower data) were in good accord, so the discrepancy between model and measurement was likely related to autotrophic and/or heterotrophic respiration. Modeled and measured methane emission rates were quite low. PCARS has been designed to link with the Canadian Land Surface Scheme (CLASS) land surface model and a global climate model (GCM) to examine climate‐peatland carbon feedbacks at regional scales in future analyses.
Biological oxygen productivity during the last 60,000 years from triple oxygen isotope measurements
Authors: Blunier Thomas , Barnett Bruce , Bender Michael L. , Hendricks Melissa B.
Published: October 2002

The oxygen isotope signature of atmospheric O2 is linked to the isotopic signature of seawater (H2O) through photosynthesis and respiration. Fractionation during these processes is mass dependent, affecting δ17O about half as much as δ18O. An “anomalous” fractionation process, which changes δ17O and δ18O of O2 about equally, takes place during isotope exchange between O2 and CO2 in the stratosphere. The relative rates of biologic O2 production and stratospheric processing determine the relationship between δ17O and δ18O of O2 in the atmosphere. Variations of this relationship thus allow us to estimate changes in the rate of mass‐dependent O2 production by photosynthesis versus the rate of O2‐CO2 exchange in the stratosphere with about equal fractionations of δ17O and δ18O. In this study we reconstruct total oxygen productivity for the last glacial, the last glacial termination, and the early Holocene from the triple isotope composition of atmospheric oxygen trapped in ice cores. With a box model we estimate that total biogenic productivity was only ∼76–83% of today for the glacial and was probably lower than today during the glacial‐interglacial transition and the early Holocene. Depending on how reduced the oxygen flux from the land biosphere was during the glacial, the oxygen flux from the glacial ocean biosphere was 88–140% of its present value.
Role of physical processes controlling oxycline and suboxic layer structures in the Black Sea
Authors: Oguz Temel
Published: October 2002

A one‐dimensional coupled physical‐biogeochemical model, together with reanalysis of the available data, are used to provide a new perspective for identification and interpretation of the suboxic layer (SOL) of the Black Sea. While the lower boundary appears to be stable at σt ≈ 16.15 ± 0.10 kg m−3, the upper boundary is found not to be isopycnally uniform as asserted previously. It is found to vary depending on the intensity of vertical diffusive and advective oxygen fluxes across the oxycline. Its position, therefore, does not always correspond to a distinct density surface irrespective of the circulation characteristics. Instead, it changes from σt ≈ 15.55 ± 0.1 in cyclonic to σt ≈ 15.9 ± 0.1 kg m−3 in anticyclonic regions, whereas its position in the peripheral Rim Current transition zone occurs at intermediate density values. These findings imply that vertical oxygen variations cannot be expressed in terms of density without taking into account physical characteristics of the water column. The presence of very strong density stratification is found to prevent ventilation of the SOL. Even intense turbulent mixing generated by a large buoyancy loss during an exceptionally cold winter season is unable to generate sufficiently dense water to overturn and temporarily supply oxygen into the suboxic zone.
Trends in North American net primary productivity derived from satellite observations, 1982–1998
Authors: Hicke Jeffrey A. , Asner Gregory P. , Randerson James T. , Tucker Compton , Los Sietse , Birdsey Richard , Jenkins Jennifer C. , Field Christopher
Published: October 2002

Net primary productivity (NPP) in North America was computed for the years 1982–1998 using the Carnegie‐Ames‐Stanford approach (CASA) carbon cycle model. CASA was driven by a new, corrected satellite record of the normalized difference vegetation index at 8‐km spatial resolution. Regional trends in the 17‐year NPP record varied substantially across the continent. Croplands and grasslands of the Central Plains and eastern Canadian forests experienced summer increases in NPP. Peak NPP trends in Alaska and western Canada occurred in late spring or early summer, suggesting an earlier onset of the growing season in these regions. Forests and woodlands of the southeastern United States showed NPP increases in spring and fall, also suggesting an increase in the length of the growing season. An analysis of climate variables showed that summer precipitation increased in the Central Plains, indicating that climate changes probably play some role in increasing NPP in this region, though intensive management of agricultural ecosystems has also increased productivity. Similarly, increased summer precipitation possibly increased NPP in eastern Canada, but another possible explanation is forest recovery after insect damage. NPP in the southeastern United States increased in the absence of climate variation. Much of this region consists of aggressively managed forests, with young stand ages and intensive silviculture resulting in increased NPP. The high latitudes of western Canada and Alaska experienced spring warming that could have increased NPP in late spring or early summer.
On the contribution of anthropogenic Cl to the increase in δ13C of atmospheric methane
Authors: Wang James S. , McElroy Michael B. , Spivakovsky Clarissa M. , Jones Dylan B. A.
Published: October 2002

A two‐dimensional model is used to evaluate the impact of stratospheric chlorine on δ13C of CH4 at the surface and to analyze sensitivity to different chemical and transport processes. It is found that the impact of Cl depends on the relative strength of the lower and upper branches of the stratospheric circulation, which emphasize regions of differing rates of methane fractionation. It is estimated that the increase in concentrations of Cl associated with human activity has contributed at most 0.54‰ to the 2‰ increase of δ13C observed over the past century. It follows that growth in the relative importance of heavier sources of CH4 (associated, for example, with fossil fuels and biomass burning) has been largely responsible for the trend.
An intensive field study on CO2, CH4, and N2O emissions from soils at four land‐use types in Sumatra, Indonesia
Authors: Ishizuka Shigehiro , Tsuruta Haruo , Murdiyarso Daniel
Published: October 2002

We measured gas fluxes of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) from the soil surface to the atmosphere under various land uses in Sumatra, Indonesia, from September 1997. Four land‐use types, i.e., old‐growth forest, logged‐over forest, burned site after logging, and rubber plantation site, were selected. One logged‐over forest was clear‐cut and burned in the middle of the experiment. An incubation experiment was also performed to measure the potential of these three gases' emissions by using intact soil cores. The ranges of flux for 1 year for CO2, CH4, and N2O were 51.3–93.7 mg C m−2h−1, −21.2–4.2 μg C m−2h−1, and 0.74–26.34 μg N m−2h−1, respectively. The N2O and CO2fluxes were among the smallest values in all tropical regions. Clear‐cutting and burning of residual trees after logging caused an increase in N2O emissions. N2O emissions correlated highly with the nitrification rate at 0–10 cm soil layer (R2= 0.7834,p< 0.01). CH4fluxes correlated with the clay content of 0–10 cm soil layer (R2= 0.6071,p< 0.15). The results of flux measurements and core incubation strongly suggest that the regeneration of vegetation reduces the impact of land‐use/cover changes on trace gas emissions.
Organic carbon flux and organic carbon to calcite flux ratio recorded in deep‐sea carbonates: Demonstration and a new proxy
Authors: Mekik Figen A. , Loubere Paul W. , Archer David E.
Published: October 2002

Theoretical considerations indicate that degree of bottom water calcite saturation, organic carbon flux, and the ratio of that flux to the calcite flux all are reflected in the carbonate preservation record of deep‐sea sediments. Separating the influence of these various factors on the record depends first on estimating the percent of the calcite flux which is dissolved at the seabed. We have developed a new proxy for percent dissolved calcite by calibrating a foraminiferal fragmentation index to biogeochemical model‐derived data. Application of our proxy, combined with geochemical modeling, to the surface sediments of the Eastern Equatorial Pacific [EEP], demonstrates the need for all three of the variables listed above to account for the observations. Further, we used reverse modeling to map the ratio of the biotic fluxes across the EEP. We observe a coherent regional pattern with highest ratio values, as expected, associated with areas of upwelling, particularly along South America. Our proxy appears to be unaffected by carbonate sediment properties such as percent calcite in samples or the amount of coarse fraction (>63 μm) in the calcareous portion of the sediments; therefore it reflects primarily the geochemical environment rather than characteristics of seabed deposits.
Dissolved methane concentration profiles and air‐sea fluxes from 41°S to 27°N
Authors: Kelley Cheryl A. , Jeffrey Wade H.
Published: October 2002

Water column samples from a transect cruise from southern Chile through the Panama Canal to the Gulf of Mexico were used to determine dissolved methane depth profiles and air‐sea methane fluxes. In the Gulf of Mexico, surface concentrations were approximately 40% supersaturated with respect to the atmosphere, whereas near the equator and in the Peru upwelling region, 10–20% supersaturation generally occurred. These saturation ratios translate into an average flux of methane from the sea surface to the atmosphere of 0.38 μmol m−2 d−1. In addition, water column profiles of dissolved methane indicate that subsurface maxima in dissolved methane concentrations are a consistent feature of the open ocean, except near the equator. At the equator, the subsurface peak at the base of the mixed layer may be bowed down by the Equatorial Undercurrent. The highest methane concentration (12 nM) was observed in the Peru upwelling region.
Structure of uppermost fast‐spread oceanic crust exposed at the Hess Deep Rift: Implications for subaxial processes at the East Pacific Rise
Authors: Karson J. A. , Klein E. M. , Hurst S. D. , Lee C. E. , Rivizzigno P. A. , Curewitz D. , Morris A. R. , Miller D. J. , Varga R. G. , Christeson G. L. , Cushman B. , O'Neill J. M. , Brophy J. G. , Gillis K. M. , Stewart M. A. , Sutton A. L.
Published: October 2002

The uppermost 2 km of the oceanic crust created at the fast spreading (135 mm yr−1, full rate) equatorial East Pacific Rise (EPR) is exposed for tens of kilometers along escarpments bounding the Hess Deep Rift. Mosaics of large‐scale digital images from the remotely operated vehicle (ROV) Argo II and direct observations from the submersible Alvin document a degree of geological complexity and variability that is not evident from most studies of ophiolites or prevailing models of seafloor spreading. Dramatic variations in the thickness and internal structure are documented in both the basaltic volcanic and sheeted dike rock units. These rock units are characterized by extensive faulting, fine‐scale fracturing, and rotations of coherent crustal blocks meters to tens of meters across. The uppermost basaltic lavas are essentially undeformed and have overall gently inclined flow surfaces. Through most of the basaltic lava unit, however, lava flow contacts dip (20°–70°W) toward the EPR and generally increase in dip downward in the section. Dikes cutting the lavas and in the underlying sheeted dike unit generally dip (90°–40°E) away from the EPR. Deeper level gabbroic rocks show little evidence of the intense fracturing typical of the overlying units. We interpret this upper crustal structure as the result of subaxial subsidence within 1–2 km of the EPR that accommodated the thickening of the basaltic lava unit to ∼500 m. Variations in the thickness of lava and dike units and spatially related structures along the rift escarpments suggest temporal fluctuations in magma supply. These results indicate that substantial brittle deformation accompanied waxing and waning volcanism during the accretion of the crustal section exposed at the Hess Deep Rift. If this type of structure is typical of uppermost oceanic crust generated at the EPR, these processes may be common along fast spreading mid‐ocean ridges.
Alkenone distributions in the North Atlantic and Nordic sea surface waters
Authors: Sicre Marie‐Alexandrine , Bard Edouard , Ezat Ullah , Rostek Frauke
Published: October 2002

The C37–C39 alkenones were quantified in suspended particulate matter obtained from the surface waters of the North Atlantic, including the Nordic seas, over a temperature range of 4°–20°C. U37K′ values were linearly correlated to temperature over the entire range covered by our data set, i.e., 4°–20°C (U37K′ = 0.027T + 0.036, r2 = 0.97). The compilation of water column data from the Atlantic, Pacific, and Southern Ocean and Mediterranean Sea suggests that the linear model may not be the best function for fitting U37K′ and in situ temperature values. It also shows that suspended matter data from warm waters (T > 20°C) are still needed to constrain the equation of the global curve. High abundances of C37:4 were found in the coldest polar waters. Around 4°C, %C37:4 represented up to 35%, a value that dropped to zero at temperatures above 10°C. Values of %C37:4 were linearly correlated to temperature (%C37:4 = −3.7T + 43.7, r2 = 0.50) and salinity (%C37:4 = −48.1S‰ + 1691, r2 = 0.78). Further investigations from other oceanic basins are necessary to confirm these findings.
Abyssal peridotite osmium isotopic compositions from cr‐spinel
Authors: Standish J. J. , Hart S. R. , Blusztajn J. , Dick H. J. B. , Lee K. L.
Published: October 2002

Abyssal peridotites, fragments of residual upper oceanic mantle, are believed to have less radiogenic Os compositions and higher Os concentrations than primitive upper mantle (PUM, 187Os/188Os = 0.129 [Meisel et al., 1996]). We have measured 187Os/188Os in 10 whole rock abyssal peridotites representing non‐plume‐influenced mid‐ocean ridge segments. The 187Os/188Os ratios range from 0.1183–0.1582. This large range in Os composition, to both less radiogenic and more radiogenic values than primitive upper mantle, can be attributed to ancient melting and subsequent sequestering of isotopic signatures, melt‐rock reaction, or secondary alteration, or a combination of any of the three. Petrographic, electron microprobe, and hand‐sample inspection show the peridotites to have experienced varying amounts of serpentinization and weathering, accompanied by heterogeneity in 187Os/188Os values. In addition, a majority of the peridotites studied here are spatially associated with gabbros having equally, if not more highly, heterogeneous Os isotopic compositions and N‐MORBs with homogeneous Sr, Nd, and Pb isotopic signatures. This suggests secondary seawater alteration as the dominant influence on present‐day Os signatures. We show that careful separation, leaching, and analysis of Cr‐spinel from abyssal peridotite largely removes the radiogenic seawater Os isotopic signature, allowing a more accurate assessment of the Os signature of depleted mid‐ocean ridge basalt mantle (DMM). Cr‐spinel is a highly refractory mantle mineral and commonly host to tiny sulfide inclusions, which are carrier phases for Os. These sulfides are well protected by the Cr‐spinel from high‐temperature serpentinization and low‐temperature seafloor weathering, thereby preserving nonradiogenic DMM‐like Os isotopic signatures. Our analyses of treated Cr‐spinel fractions finds 187Os/188Os ratios that are dominantly less radiogenic (0.1238–0.1482) than corresponding whole rock compositions. Nonmagnetic Cr‐spinels show more highly variable Os compositions and concentrations than magnetic Cr‐spinels, overlapping with the more radiogenic whole rock values. Magnetic Cr‐spinels are dominantly less radiogenic than PUM, with high but variable Os concentrations, possibly resulting from a sulfide “nugget” effect. The systematically lower 187Os/188Os in Cr‐spinels compared to whole rocks shows that whole rock abyssal peridotites are largely compromised by radiogenic seawater interaction, often obscuring the mantle signature. Measurements of treated abyssal peridotite Cr‐spinels in some cases can circumvent this seawater alteration problem allowing more straightforward interpretation of isotopic values and thus further constraining the Os isotopic signature for the depleted upper mantle. Our results suggest that the 187Os/188Os for the upper mantle lies in the range 0.120–0.125.
Strong heterogeneity caused by deep mantle layering
Authors: Tackley Paul J.
Published: October 2002

Numerical simulations are used to characterize the volumetric heterogeneity associated with proposed chemical layering in the deep mantle. A globally continuous, undulating layer generates enormous amounts of volumetric heterogeneity, which is caused by lateral variations in the layer boundary depth and by strong hot upwellings arising from the top of the layer, in the middle lower mantle region. In contrast, the heterogeneity in the deepest mantle is very weak. Partial cancellation of the temperature and compositional contributions to seismic velocity could mask the layer itself, but the strong signatures of the thermochemical boundary layer and induced upwellings remain, even when fields are filtered to approximate the resolution of global seismic tomographic models. This distinctive heterogeneity is not observed in global seismic tomographic models. Dense material that is restricted to discontinuous “piles” generates heterogeneity that is more consistent, although not entirely, with seismic observations, with strong heterogeneity at the core‐mantle boundary. Radial correlation functions display a characteristic narrowing in the vicinity of any chemical layer but are very sensitive to the applied “seismic tomographic” filtering.
Volatiles in basaltic glasses from the Easter‐Salas y Gomez Seamount Chain and Easter Microplate: Implications for geochemical cycling of volatile elements
Authors: Simons Kyla , Dixon Jacqueline , Schilling Jean‐Guy , Kingsley Richard , Poreda Robert
Published: October 2002

We present H2O, CO2, and Cl concentrations in 66 basaltic glasses from the Easter Microplate (EMP) and Easter‐Salas y Gomez Seamount Chain (ESC) system in the southeastern Pacific. The EMP‐ESC system is characterized by binary mixing between a depleted mid‐ocean ridge basalt (MORB) mantle source (DMM) and an incompatible element and radiogenic isotope enriched source, the Salas y Gomez mantle plume (SyG). Plume material is channeled toward the ridge crest centered at ∼27°S on the east rift of the microplate. Water concentrations on the EMP are highest on the east rift at ∼27°S and become progressively lower to the north and south, following the spatial pattern of other geochemical tracers. EMP basalts have not lost H2O to degassing but have assimilated variable quantities of a Cl‐rich hydrothermal component. In contrast, some seamount basalts have lost water by shallow degassing, but very few have gained Cl, indicating little assimilation of Cl‐rich materials. Several ESC seamount glasses may have assimilated a hydrous component, for example, serpentinized harzburgite, during magma ascent through the lithosphere. On the basis of samples unaffected by shallow processes, the main plume component has H2O/Ce of ∼210 ± 20 and is neither preferentially enriched nor depleted in H2O relative to other similarly incompatible elements. The depleted MORB source has H2O/Ce of ∼150 ± 10. Estimated mantle volatile concentrations are 750 ± 210 ppm H2O and 40 ± 11 ppm Cl for the SyG source, 120 ± 27 ppm H2O and 4.5 ± 1.4 ppm Cl for an average EPR source, and 54 ± 12 ppm H2O and 1.7 ± 0.4 ppm Cl for the DMM source. The coupled behavior of H2O and Cl with similarly incompatible elements, coupled with elevated 3He/4He ratios, suggests that the volatiles are dominantly juvenile, representative of a component common to mantle plumes, with minor contribution from recycled lithospheric components.
Satellite observations of bloom events in the Strait of Ombai: Relationships to monsoons and ENSO
Authors: Moore Thomas S. , Marra John
Published: October 2002

Persistent phytoplankton bloom events, surface cooling, and associated fronts in the Strait of Ombai were seen by Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS), Advanced Very High Resolution Radiometer (AVHRR), and Synthetic Aperture Radar (SAR) imagery. It is suggested that the cooling events were caused by variable flow through the strait and its interaction with complex bathymetry. During the strongest cooling events a sharp, midstrait frontal feature was seen with a cross‐front gradient in sea surface temperature (SST) of the order of 4°C and a gradient in chlorophyll concentration of over 1 mg/m3. The frontal feature, associated with increased ocean color signal, extended, during major events, 100s of kilometers downstream of the strait and at times influenced nearly all of the Savu Sea. We show that variability in the bloom events can be tied to seasonal monsoons, ENSO, and their effect on thermocline depth.
Behavior of Li and its isotopes during serpentinization of oceanic peridotites
Authors: Decitre Sylvie , Deloule Etienne , Reisberg Laurie , James Rachael , Agrinier Pierre , Mével Catherine
Published: October 2002

Analyses of Li and Li isotopes in serpentinized peridotites have been performed using Thermo‐Ionisation Mass Spectrometry (TIMS) and Secondary Ion Mass Spectrometry (SIMS) techniques on samples collected from the southwest Indian Ridge (SWIR). In the bulk samples, Li concentrations range from 0.6 to 8.2 ppm, while whole rock δ6Li values range from −2.9 to −14‰. In situ analyses display a greater range in both Li concentration (0.1–19.5 ppm) and Li isotopic composition (−27 to +19‰), with the serpentinized portions having higher Li concentrations than the associated relict phases. These variations may reflect changes in Li partitioning and isotopic fractionation between serpentine and fluid with temperature and water/rock ratio. They may also be explained by changes in the composition of the serpentinizing fluid over the course of serpentinization. As the serpentine forms by interaction with a circulating fluid, it preferentially removes 6Li, causing the Li in the fluid to become isotopically heavier. The isotopic composition of the initial hydrothermal fluid is dominated by basalt‐derived Li, which easily overwhelms the very low Li content originally present in seawater. As this fluid circulates through ultramafic rocks, it induces the formation of serpentine that incorporates this mantle‐derived Li. Hence, Li in serpentine is mainly derived from oceanic crust rather than from seawater and serpentinization involves Li recycling within this crust. Consequently, Li isotopes are good tracers of the hydrothermal contribution in serpentinizing fluid. These results imply that serpentinized peridotites are probably only a minor sink of oceanic Li.
A high‐temperature (3000 K) assembly for piston cylinder experiments
Authors: Wade J. , Wood B. J.
Published: October 2002

In order to reach temperatures above 2200 K in the piston cylinder apparatus, thermal conduction to the pressure vessel walls must be minimized, and all low melting point components need to be excluded. We present a simple arrangement for performing experiments up to ∼3000 K. A modified form, with a larger sample size, is also shown for experiments up to around 2200 K.
Shallow remineralization in the Weddell Gyre
Authors: Usbeck Regina , Rutgers van der Loeff Michiel , Hoppema Mario , Schlitzer Reiner
Published: October 2002

The region influenced by the Polar Front in the Southern Ocean is characterized by relatively high productivity, which is mirrored in strong depletions of 234Th in the surface water, a good tracer of export production, and by high accumulation rates on the underlying seabed. Farther south, the Weddell Sea is generally considered a low productivity region with very low export fluxes. This finding is based on satellite observations, sediment accumulation rates, trap deployments, and phytoplankton distribution. If this would be true, 234Th should be close to equilibrium with its parent. However, in a series of high‐resolution transects of 234Th/238U across the Antarctic Circumpolar Current (ACC), 234Th was found to be depleted by 10–15% throughout the clear Weddell Gyre, only to reach equilibrium in sea‐ice covered regions of the coastal zone. Vertical profiles showed that the depletion was limited to the upper mixed layer and was balanced by an enrichment of similar magnitude at 100–250m depth. This implies that the export of particles below 250 m is negligible. Such shallow remineralization is in line with the discrepancies between biogenic silica production rates and sediment trap data observed in the Weddell and Ross Seas. These observations in the Weddell Sea are fully consistent with our inverse modeling results for both organic carbon and opal, and they are not inconsistent with TCO2 and oxygen sections that show a TCO2 enriched, oxygen reduced shallow subsurface layer. This blue ocean, characterized by upwelling of CO2 ‐enriched deep waters, supports sufficient productivity to be a net sink for CO2 to abyssal depths [Hoppema et al., 1999]. No record of this productivity and export is stored in the underlying sediment, which has important palaeoceanographic consequences.
Melt depletion and subsequent metasomatism in the shallow mantle beneath Koolau volcano, Oahu (Hawaii)
Authors: Ducea Mihai , Sen Gautam , Eiler John , Fimbres Jennifer
Published: October 2002

Xenoliths from Pali (Oahu, Hawaii) include samples of the mantle lithosphere underlying Koolau shield volcano. Most such xenoliths are spinel peridotites, the remainder being plagioclase‐spinel peridotites, and garnet‐free pyroxenites. Clinopyroxene separates from Pali peridotite xenoliths have relatively depleted 87Sr/86Sr (0.70309–0.70346) and 143Nd/144Nd (0.512967–0.513206). The mantle lithosphere beneath the Koolau volcano has a range of present‐day εNd = 6.9–11. On a 143Nd/144Nd versus 147Sm/144Nd diagram, they define a 61 ± 20 Ma errorchron, within error of the estimated age of ∼80–85 Ma of the Pacific lithosphere at Oahu, and an intercept of εNd = 6.8. We interpret the Pali spinel peridotites as samples of the Pacific lithosphere residual to melt extraction of the Pacific crust. These rocks were not metasomatized by melts having isotopic composition similar to the shield‐building Koolau volcano, but they could have been metasomatized by melts related to the Honolulu Volcanic Series. Plagioclase mineral separates from two peridotites are in 87Sr/86Sr isotopic disequilibrium with coexisting clinopyroxenes. The plagioclases are light rare earth element depleted and have “enriched” 87Sr/86Sr ratios (0.70436 and 0.70443). The δ18O of olivine (5.09–5.12‰) and clinopyroxene (5.32–5.33‰) from spinel peridotites are typical for oceanic upper mantle rocks. In contrast, δ18O of olivine, pyroxene, and plagioclase in plagioclase‐bearing peridotites are enriched by ∼0.5‰ relative to the spinel peridotites (e.g., δ18O ratios of olivines from plagioclase peridotites are 5.4–5.5‰). It is here interpreted that the plagioclases represent phases that formed by reaction with or precipitation from a melt that intruded the Pacific lithospheric mantle. The Pacific lithospheric isotopic composition is reflected in the clinopyroxene separates. The high δ18O and 87Sr/86Sr of the plagioclase resemble Koolau lavas and suggest that the melt passing through the lithosphere was plume‐related. The elevated δ18O of the plagioclase indicates that the melt from which it fractionated had a component of recycled crustal material not acquired from local crustal contamination. It is also estimated here that the duration of the porous melt flow of the Koolau‐like melt though the mantle lithosphere was short‐lived, ∼102–104 years.
The relationship between shell size and Mg/Ca, Sr/Ca, δ18O, and δ13C of species of planktonic foraminifera
Authors: Elderfield H. , Vautravers M. , Cooper M.
Published: October 2002

The relationship between shell size and Mg/Ca, Sr/Ca, δ18O, and δ13C of planktonic foraminiferal calcite has been investigated using seventeen species in six different size fractions. Mg/Ca increases and Sr/Ca decreases with increasing size, except for two globorotaliid species which show the opposite trend. The changes in Mg/Ca broadly follow δ18O calcification temperatures except that surface and near‐surface dwelling species show larger changes in Mg/Ca than can be accounted for by differences in calcification temperature derived from δ18O. The increases in Mg/Ca and decreases in Sr/Ca vary linearly with the well‐established increase in δ13C with size. This is consistent with smaller individuals calcifying faster than larger individuals and larger individuals forming calcite that more closely reflects seawater temperature and composition. It appears that variations in calcification rate affect Mg and Sr shell chemistry. The observations may account for part of the temporal variability in foraminiferal Sr/Ca within a single size fraction that has been attributed to changes in seawater Sr/Ca.