bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of photorespiration, the cytochrome pathway, and the alternative pathway on the triple isotopic composition of atmospheric O2
Authors: Angert Alon , Rachmilevitch Shimon , Barkan Eugeni , Luz Boaz
Published: May 2003

The triple isotopic composition of atmospheric O2 is a new tracer used to estimate changes in global productivity. To estimate such changes, knowledge of the relationship between the discrimination against 17O and the discrimination against 18O is needed. This relationship is presented as θ = ln(17α)/ln(18α). Here, the value of theta was evaluated for the most important processes that affect the isotopic composition of oxygen. Similar values were found for dark respiration through the cytochrome pathway (0.516 ± 0.001) and the alternative pathway (0.514 ± 0.001), and slightly higher value was found for diffusion in air (0.521 ± 0.001). The combined effect of diffusion and respiration on the atmosphere was shown to be close to that of dark respiration. The value we found for photorespiration (0.506 ± 0.005) is considerably lower than that of dark respiration. Our results clearly show that the triple isotopic composition of the atmosphere is affected by the relative rates of photorespiration and dark respiration. Also, we show that closing the current global isotopic balance will enable the estimation of the current global rate of photorespiration. Using the Last Glacial Maximum as a case study, we show that variations in global rate of photorespiration affected the triple isotopic composition in the past. Strong fractionations measured in illuminated plants indicated that the alternative pathway is activated in the same conditions that favor high rate of photorespiration. This activation suggests that the global rate of the alternative pathway is higher than believed thus far, and may help to close the gap between the calculated and measured Dole Effect.
Nutrient utilization ratios in the Polar Frontal Zone in the Australian sector of the Southern Ocean: A model
Authors: Wang Xiujun , Matear Richard J. , Trull Thomas W.
Published: May 2003

To investigate the non‐Redfield N/P depletion ratio in the Polar Frontal Zone (PFZ) of the Southern Ocean, we simulated the seasonal nitrate, phosphate, and silicate cycle in the upper ocean with a biophysical model. Total phytoplankton biomass was prescribed from the Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) estimates, and we included two phytoplankton types, diatoms and nondiatoms. We set the nondiatoms N/P uptake ratio to 16 while the diatoms N/P and N/Si ratios were determined by fitting the observed seasonal nitrate, phosphate, and silicate cycle in the mixed layer. The best model fit to the observations required an annual N/P utilization ratio of 13.2, but this low N/P ratio still overestimated the nitrate utilization during the summer. We considered three mechanisms for improving the simulated nitrate cycle: (1) seasonal variation in the N/P ratio of the horizontal nutrient supply to the PFZ, (2) different remineralization length scales for particulate organic nitrogen (PON) and particulate organic phosphorus (POP), and (3) seasonal accumulation and decomposition of labile dissolved and suspended organic matter (OM). Model simulations showed that the seasonal variability in the N/P ratio of horizontal supply failed to reduce the simulated excess nitrate utilization in summer. Preferential recycling of PON compared to POP below the mixed layer degrades the simulation and cannot produce results that satisfy both the observed seasonal nitrate and phosphate cycle in the mixed layer. The most realistic model simulation was obtained with preferential recycling of POP over PON, but this mechanism alone was incapable of satisfying the summer nitrate and phosphate data. With the inclusion of an OM pool in our model we were able to reproduce the observed seasonal mixed layer nitrate and phosphate cycles. Satisfactory results can be achieved through various combinations of the N/P ratio of OM and the lifetime of the OM. Seasonal observations of dissolved and suspended organic phosphorus, nitrogen and carbon are needed to confirm their role. The important conclusion of our model study is that in the PFZ the annual nutrient utilization ratio of nitrate to phosphate is considerably less than the classical Redfield value of 16.
How surface fire in Siberian Scots pine forests affects soil organic carbon in the forest floor: Stocks, molecular structure, and conversion to black carbon (charcoal)
Authors: Czimczik Claudia I. , Preston Caroline M. , Schmidt Michael W. I. , Schulze Ernst‐Detlef
Published: May 2003

In boreal forests, fire is a frequent disturbance and converts soil organic carbon (OC) to more degradation‐resistant aromatic carbon, i.e., black carbon (BC) which might act as a long‐term atmospheric‐carbon sink. Little is known on the effects of fires on boreal soil OC stocks and molecular composition. We studied how a surface fire affected the composition of the forest floor of Siberian Scots pine forests by comparing the bulk elemental composition, molecular structure (13C‐MAS NMR), and the aromatic carbon fraction (BC and potentially interfering constituents like tannins) of unburned and burned forest floor. Fire reduced the mass of the forest floor by 60%, stocks of inorganic elements (Si, Al, Fe, K, Ca, Na, Mg, Mn) by 30–50%, and of OC, nitrogen, and sulfur by 40–50%. In contrast to typical findings from temperate forests, unburned OC consisted mainly of (di‐)O‐alkyl (polysaccharides) and few aromatic structures, probably due to dominant input of lichen biomass. Fire converted OC into alkyl and aromatic structures, the latter consisting of heterocyclic macromolecules and small clusters of condensed carbon. The small cluster size explained the small BC concentrations determined using a degradative molecular marker method. Fire increased BC stocks (16 g kg−1 OC) by 40% which translates into a net‐conversion rate of 0.7% (0.35% of net primary production) unburned OC to BC. Here, however, BC was not a major fraction of soil OC pool in unburned or burned forest floor, either due to rapid in situ degradation or relocation.
Substantial advective iron loss diminishes phytoplankton production in the Antarctic Zone
Authors: Hoppema Mario , de Baar Hein J. W. , Fahrbach Eberhard , Hellmer Hartmut H. , Klein Birgit
Published: May 2003

After 1 decade of research it is a well‐established fact that iron limits photosynthetic CO2 fixation and phytoplankton growth in the Southern Ocean; intense blooms are scarce. However, the input of iron to the Southern Ocean is considerable. An important factor for diminished phytoplankton production refers to the meridional circulation of the Southern Ocean. Intense, spatially relatively homogeneous upwelling of Upper Circumpolar Deep Water (UCDW) causes a large iron flux into the surface layer. However, the main entrainment of upwelled UCDW into the surface layer occurs in autumn and winter, which strongly restricts the usefullness of iron supply for phytoplankton due to unfavorable light conditions. Moreover, the meridional transport within the Ekman layer is intense enough to export at least 25% of the iron input away from the Antarctic Zone before it can be used by phytoplankton. This also depresses the potential phytoplankton primary production by at least 25%. Most iron that crosses the Polar Front unused probably leaves the surface ocean north of the Polar Front because the surface water participates in Antarctic Intermediate Water/Mode Water formation.
Amplifying effects of land‐use change on future atmospheric CO2 levels
Authors: Gitz Vincent , Ciais Philippe
Published: May 2003

We constructed a model to analyze the interactions between land‐use change and atmospheric CO2 during the recent past and for the future. The primary impact of the conversion of forested lands to cultivated lands is to increase atmospheric CO2, via losses of biomass and soil carbon to the atmosphere. This increase is likely to continue in the next decades, but its magnitude can vary according to each land‐use scenario. We show that this first‐order effect is further amplified by the correlated diminution of terrestrial sinks, because when croplands replace forests, the turnover time of excess carbon in the biosphere decreases, and hence the sink capacity of terrestrial ecosystems decreases. This effect acts to further increase by up to 100 ppm the CO2 level reached by 2100, and it is of the same order of magnitude, although smaller, than climate‐carbon feedbacks. Uncertainties on the magnitude of this land‐use induced effect are large, because of uncertainties in the sink role of terrestrial ecosystems in the future and because of uncertainties inherent to the modeling of land‐use induced carbon emissions. Such an extra rise in atmospheric CO2 is however partially offset by the ocean reservoir and by sinks operating over undisturbed, pristine ecosystems, suggesting that conserving pristine forests with long turnover times might be efficient in mitigating the greenhouse effect.
Flux and bioavailability of Cape Fear River and rainwater dissolved organic carbon to Long Bay, southeastern United States
Authors: Avery G. B. , Willey Joan D. , Kieber Robert J. , Shank G. Christopher , Whitehead Robert F.
Published: May 2003

This study presents the first comparison of riverine and rainwater dissolved organic carbon (DOC) flux to a coastal region, and the first examination of rainwater DOC bioavailability. The bioavailability of DOC from these two sources was determined because its impact on coastal waters depends on whether DOC is labile or refractory. The study site, Long Bay, off the North Carolina and South Carolina coasts in the southeastern United States, receives external sources of DOC from the Cape Fear River system and rainwater. Conservative mixing of estuarine DOC, and a consistent DOC value for the river sources entering the Cape Fear Estuary (Cape Fear and Black rivers combined: 937 ± 178 μM, n = 12; Northeast Cape Fear River: 1266 ± 196 μM, n = 8) allowed for a calculation of DOC flux using river DOC values and U.S.G.S. river flow data. Rainwater DOC flux was determined by a deposition calculation at a rainwater collection station in Wilmington, N. C., 40 km from Long Bay. Riverine DOC flux was determined to be (77 × 109 gCyr−1) or roughly 4x the rainwater flux (21 × 109 gCyr−1) on an annual basis. The bioavailability of the riverine DOC (9.0 ± 4.5%) was much less than that of rainwater (63 ± 14%). Therefore, taking into account only bioavailable DOC, rainwater provides more labile DOC (13 × 109 gCyr−1) to Long Bay than riverwater (7 × 109 gCyr−1) meaning that rainwater is important for fueling secondary productivity whereas river DOC is important for long‐term storage of carbon.
Sources and sinks of carbon from land‐use change in China
Authors: Houghton R. A. , Hackler J. L.
Published: May 2003

Changes in land use contribute to the current terrestrial carbon sink in most regions of the northern midlatitudes but are poorly documented for China, the world's third largest country. We attempted to reconstruct the last 300 years of land‐use change in China, emphasizing changes in the area of forests. Changes in the area of croplands were inadequate for reconstruction of forest loss because the long‐term loss of forest area was more than twice the current area of croplands. We used historical information to reconstruct changes in forest area over time and the ecological literature to estimate the carbon stocks of the major natural ecosystems (vegetation and soil). We used a bookkeeping model to calculate the flux of carbon to or from living vegetation, dead vegetation, soils, and wood products under different types of land use. According to the data and assumptions, 180 (range: 80–200) × 106ha of forest were lost, and 17–33 PgC were released to the atmosphere between 1700 and 2000. About 25% of the loss was from soils. The accelerated clearing and logging of forests in northeastern and southwestern China led to emissions of carbon that reached peaks of 0.2–0.5 PgC yr−1from the late 1950s through the 1970s. Lower rates of deforestation since then, as well as expanding areas of tree plantations, reversed the net flux of carbon from a source to a sink during the 1990s.
Mercury emissions from the August 2001 wildfires in Washington State and an agricultural waste fire in Oregon and atmospheric mercury budget estimates
Authors: Friedli H. R. , Radke L. F. , Prescott R. , Hobbs P. V. , Sinha P.
Published: May 2003

Smoke plumes from large wildfires burning in late August 2001 in the temperate forests along the eastern slopes of the Cascade Mountains in Washington State and from a wheat stubble fire on the northeastern plain of Oregon were sampled for mercury from the University of Washington's Convair 580 research aircraft. Total gaseous mercury (TGM) concentrations as high as 7.5 ng/m3, a six‐time enhancement over background, were measured. The TGM was linearly correlated with CO (R2= 0.805, whereRis a 95% confidence level regression coefficient). Mercury was speciated as elemental mercury (>95%) and as mercury contained in the particulate fraction of the smoke. Emission factors (EF) were calculated from TGM and CO emissions and from a combustion efficiency of 0.9, adjusted for mercury contained in particulates, to yield an EF for total mercury of (113 ± 59) μg Hg per kilogram consumed fuel. The amount of mercury released from temperate forests in North America was extrapolated from our EF measurements and from National Interagency Fire Center estimates of burn areas. The average release for the period 1997–2001 is estimated to be 3.7 t Hg/yr, which is small relative to anthropogenic emissions. More significant is the contribution from fires in boreal forests (∼22.1 t Hg/yr) because of their much larger burn area. A measurement in smoke from a wheat stubble fire in Oregon showed mercury emissions as well, although with a lower EF. However, since agricultural waste burning is large globally, mercury from this source is also important in the global atmospheric cycling of mercury.
Water and carbon cycles in the Mississippi River basin: Potential implications for the Northern Hemisphere residual terrestrial sink
Authors: Lee Dongho , Veizer Ján
Published: May 2003

The hydrologic cycle plays an important role in carbon cycling, due to the coupling of vapor release and CO2 uptake during photosynthesis. This coupling, expressed as Water Use Efficiency (WUE) or Transpiration Ratio, can provide an inexpensive alternative for estimating the Net Primary Productivity (NPP) of terrestrial ecosystems. The D/H and 18O/16O trends of river water in the Mississippi basin are mostly indistinguishable from those of precipitation. This, combined with isotopic mass balance relationships, suggests that direct evaporation of surface water is small and evapotranspiration (ET) flux from the basin therefore consists mostly of interception and transpiration, with interception approximated from field studies. The calculated water flux associated with transpiration is 1500.8 km3 (77.3% of the evapotranspiration flux). Utilizing the average WUE of 864 mol H2O for each mole of CO2, the NPP of the Mississippi River basin amounts to 1.16 Pg C/yr, similar to the model estimates of the heterotrophic soil respiration flux of 1.12 Pg C/yr. This does not favor the postulated existence of a major sink for atmospheric CO2 in the temperate Northern Hemispheric ecosystems of the conterminous United States, but due to uncertainties in the input parameters we cannot discount the possibility that these ecosystems act as a modest sink.
Holocene atmospheric CO2 increase as viewed from the seafloor
Authors: Broecker Wallace S. , Clark Elizabeth
Published: May 2003

Three scenarios have been proposed to explain the 20‐ppm post‐8000 BP rise in atmospheric CO2 content. Indermühle et al. [1999] call on a climate‐induced decrease in terrestrial biomass. W. F. Ruddiman (personal communication, 2002) calls on an anthropogenically induced decrease in terrestrial biomass. Broecker et al. [2001] suggest instead that this rise in CO2 was a response to a CaCO3 preservation event induced by an early Holocene increase in terrestrial biomass. The biomass decline hypothesis not only rests on shaky 13C data, but also requires an unreasonably large decrease in biomass (195 ± 40 GtC). While evidence for a decrease in deep sea carbonate ion concentration over the last 8000 years reconstructed from CaCO3 size index and foraminifera shell weight measurements appears to support the idea that the CO2 rise was caused by a change in the inventory of terrestrial biomass, the decrease appears to be too large to be explained solely in this way. Regardless, the CO3= decline cannot be used to distinguish between the late Holocene biomass decrease and early Holocene biomass increase scenarios. Only when a convincing 13C record for atmospheric CO2 has been generated will it be possible to make this distinction.
Production and flux of carbohydrate species in the Gulf of Mexico
Authors: Hung Chin‐Chang , Guo Laodong , Schultz Gary E. , Pinckney James L. , Santschi Peter H.
Published: May 2003

Carbohydrates are an important organic compound class in seawater and play an active role in the biogeochemical cycling of organic carbon and trace elements in the ocean, but are poorly characterized. To better understand the sources and role of carbohydrate species in marine environments, the concentrations and fluxes of particulate carbohydrates (CHO), total acid polysaccharides (APS), uronic acids (URA), phytoplankton composition and bacterial production were measured in the Gulf of Mexico in 2000 and 2001. A strong positive correlation between APS concentration and cyanobacteria abundance was found in 2000. In 2001, prymnesiophyte abundance correlated well with both concentrations of APS and URA. Bacterial production data, measured simultaneously in 2001, showed significant positive relationships with particulate organic carbon (POC), CHO, APS and URA concentrations, respectively. The average fluxes out of the euphotic zone of CHO, APS and URA in 2000 were 8.1, 1.3, and 0.7 mg C m−2 d−1, respectively. In 2001, the average fluxes of CHO, APS and URA were about 3 times higher than those in 2000, which was a time of lower nutrient concentrations, indicating that the fluxes of carbohydrate species are related to the nutrient status and phytoplankton composition. The results suggest that APS in the upper water column can be produced by cyanobacteria, prymnesiophytes, and heterotrophic bacteria. Most importantly, our data indicate that APS and CHO compounds are more resistant to biological degradation than other organic compounds, suggesting that the role of CHO compounds in carbon cycling in the ocean is more complex than previously thought.
Seasonal variability of N2O emissions and CH4 uptake by tropical rainforest soils of Queensland, Australia
Authors: Kiese Ralf , Hewett Bob , Graham Andrew , Butterbach‐Bahl Klaus
Published: May 2003

Over 1 year we followed the seasonal variations of N2O and CH4 fluxes at a tropical rain forest site in Australia. In addition, meteorological parameters, litter fall and decomposition, plant species composition, and concentrations of NH4+/NO3− in the soil and N2O and CH4 in the soil atmosphere were measured. N2O emissions showed a pronounced seasonal pattern with highest rates in the wet season (108.6 μg N m−2 h−1) and significantly lower rates during dry season (mostly <10 μg N2O‐N m−2 h−1). N2O emissions were positively correlated to N2O‐concentrations in the soil profile and to moisture, but not to concentrations of NH4+ and NO3−. The annual emission of N2O (N = 6015) was 0.97 kg N ha−1 yr−1, and, thus, approximately 7 times lower than a previous estimate for the year 2000. The marked differences in N2O emissions between different years indicate that the interannual variability of N2O emissions from rain forest soils cannot be neglected. With regard to CH4 the soil functioned throughout the entire year as a significant sink. Rates of CH4 uptake during the dry period (35–68 μg CH4 m−2 h−1) were higher as compared to the wet period (4–45 μg CH4 m−2 h−1). A close linear correlation between soil moisture and magnitude of CH4 uptake was found. The calculated annual CH4 uptake (N = 6090) is 3.2 kg CH4 ha−1 yr−1. This implies that tropical rain forest soils function as significant sinks for atmospheric CH4 on a global scale.
PetroPlot: A plotting and data management tool set for Microsoft Excel
Authors: Su Yongjun , Langmuir Charles H. , Asimow Paul D.
Published: May 2003

PetroPlot is a 4000‐line software code written in Visual Basic for the spreadsheet program Excel that automates plotting and data management tasks for large amount of data. The major plotting functions include: automation of large numbers of multiseries XY plots; normalized diagrams (e.g., spider diagrams); replotting of any complex formatted diagram with multiple series for any other axis parameters; addition of customized labels for individual data points; and labeling flexible log scale axes. Other functions include: assignment of groups for samples based on multiple customized criteria; removal of nonnumeric values; calculation of averages/standard deviations; calculation of correlation matrices; deletion of nonconsecutive rows; and compilation of multiple rows of data for a single sample to single rows appropriate for plotting. A cubic spline function permits curve fitting to complex time series, and comparison of data to the fits. For users of Excel, PetroPlot increases efficiency of data manipulation and visualization by orders of magnitude and allows exploration of large data sets that would not be possible making plots individually. The source codes are open to all users.
Quasi‐continuous depth profiles of rock magnetization from magnetic logs in the HSDP‐2 borehole, Island of Hawaii
Authors: Steveling E. , Stoll J. B. , Leven M.
Published: May 2003

A quasi‐continuous magnetic log has been obtained in the Hawaii Scientific Drilling Project 2 (HSDP‐2) between 600 m and 1800 m, which corresponds to a time interval of approximately 350 ka to 480 ka. A tri‐axial borehole magnetometer was employed to measure the horizontal and vertical magnetic fields. Measurements were taken in downhole and uphole runs, with a good correlation between the two. In a first step the logs were corrected for the transfer function of the employed low‐pass filter and then for the logging depths. To calculate rock magnetizations from magnetic components, we used a multidisk cylindrical model for the penetrated rocks. The disk thickness corresponds with 0.1 m to the logging sampling rate. Magnetic borehole logging in the HSDP‐2 hole has established the following: Massive lava flows can be distinguished from those with prevailing hyaloclastites and enables us to supplement the lithology, especially in depth intervals with poor core recovery. The inclinations of rock magnetization derived from the magnetic log agree well with those measured in core samples from HSDP‐2 hole. The same applies to magnitudes of magnetizations from logging, as the sum of induced and remanent magnetizations, with laboratory determinations of the remanent magnetizations of core samples. We observe a distinct discrepancy between the local present‐day geocentric axial dipole (GAD) inclination of 35.6° and the mean logging inclination of 22.7°. Furthermore, a systematic inclination decrease with depth is observed. Logging and core inclinations in HSDP‐2 can be brought into agreement with core inclinations in the HSDP‐1 pilot hole by shifting depths of HSDP‐1 100 m downward. The correlation of inclination data between the two boreholes and the known age‐depth relation of HSDP‐1 is used to reexamine the “age versus depth” model curve for Mauna Kea of Sharp et al. (manuscript in preparation, 2003). We identify tentatively two logged inclination minima with excursion of the geomagnetic field in Brunhes, namely, “Levantine” at 360–370 ka and “Unknown” at 400–420 ka.
Crustal Evolution of the Mid‐Atlantic Ridge near the Fifteen‐Twenty Fracture Zone in the last 5 Ma
Authors: Fujiwara Toshiya , Lin Jian , Matsumoto Takeshi , Kelemen Peter B. , Tucholke Brian E. , Casey John F.
Published: May 2003

The Mid‐Atlantic Ridge around the Fifteen‐Twenty Fracture Zone is unique in that outcrops of lower crust and mantle rocks are extensive on both flanks of the axial valley walls over an unusually long distance along‐axis, indicating a high ratio of tectonic to magmatic extension. On the basis of newly collected multibeam bathymetry, magnetic, and gravity data, we investigate crustal evolution of this unique section of the Mid‐Atlantic Ridge over the last 5 Ma. The northern and southern edges of the study area, away from the fracture zone, contain long abyssal hills with small spacing and fault throw, well lineated and high‐amplitude magnetic signals, and residual mantle Bouguer anomaly (RMBA) lows, all of which suggest relatively robust magmatic extension. In contrast, crust in two ridge segments immediately north of the fracture zone and two immediately to the south is characterized by rugged and blocky topography, by low‐amplitude and discontinuous magnetization stripes, and by RMBA highs that imply thin crust throughout the last 5 Ma. Over these segments, morphology is typically asymmetric across the spreading axis, indicating significant tectonic thinning of crust caused by faults that have persistently dipped in only one direction. North of the fracture zone, however, megamullions are that thought to have formed by slip on long‐lived normal faults are found on both ridge flanks at different ages and within the same spreading segment. This unusual partitioning of megamullions can be explained either by a ridge jump or by polarity reversal of the detachment fault following formation of the first megamullion.
Alteration of hyaloclastites in the HSDP 2 Phase 1 Drill Core 1. Description and paragenesis
Authors: Walton Anthony W. , Schiffman Peter
Published: May 2003

The core from the Hawaii Scientific Drilling Project 2 Phase 1 provides a unique opportunity for studying the low‐temperature alteration processes affecting basalt in suboceanic‐island environments. In hyaloclastites, which make up about one half of the lower 2 km of this core (the portion that accumulated below sea level), these processes have resulted in zones of incipient, smectitic, and palagonitic alteration. The alteration of sideromelane in these hyaloclastites has four distinct outcomes: dissolution, replacement by two different textural varieties of smectite (i.e., reddened and green grain‐replacive), and conversion to palagonite. All samples show evidence of the incipient stage of alteration, suggesting that every sample passed through that zone. However, most samples that show palagonitic alteration do not also show evidence of smectitic alteration and vice versa, suggesting these two outcomes represent divergent paths of alteration. Incipient alteration (1080 to 1335 m depth) includes fracturing and mechanical reduction of porosity from 40–45% to about 20–30%; growth of one form of pore‐lining smectite; dissolution of sideromelane; and formation of sideromelane‐grain replacements consisting of Fe‐hydroxide‐strained smectite, titaniferous nodules, and tubules. DNA‐specific stains and morphological features indicate that tubules are the result of microbial activity. Smectitic alteration (1405 to 1573 m) includes growth of a second variety of pore‐lining smectite, pore‐filling and grain‐replacing smectite, and cements of phillipsite and Ca‐silicate minerals. Palagonitic alteration (1573 m to the deepest samples) includes replacement of margins of shards with palagonite and growth of pore‐filling chabazite. The porosity is reduced by cementation to less than 4% at 1573 m. Porosity does not decrease further down hole, nor does the thickness of palagonite rims on shards increase through the zone of palagonitic alteration. In these samples, palagonite is not an intermediate alteration product in the development of smectite. Rather, in hyaloclastites from the HSDP core, palagonite has formed after all observed smectites. Current downhole temperatures at the boundaries between the three alteration zones are in the range from 12° to 15°C, suggesting that geochemical thresholds or vital effects, not temperature conditions, control different outcomes of alteration.
Melt extraction from partially molten peridotites
Authors: Renner Jörg , Viskupic Karen , Hirth Greg , Evans Brian
Published: May 2003

We measured the rate of melt extraction from partially molten olivine aggregates into a porous reservoir as a function of melt content (ϕ), melt viscosity, grain size, and pressure difference. Samples were prepared using mixtures of olivine and either lithium silicate, MORB glass, or albite glass to obtain a range in melt viscosities. Experiments were conducted at 1473 K with a confining pressure (Pc) of 300 MPa. The melt pressure (Pm) was controlled using a pore fluid actuator to adjust the pressure of Ar gas (Pf) in contact with the melt. Compaction rates decrease with decreasing ϕ and increase linearly with the pressure difference ΔP = Pc − Pf. For the two lower viscosity melts, no gradients in melt fraction were observed in samples with melt fractions as low as 0.03. By contrast, the melt fraction increases with distance away from the reservoir for samples with the highest viscosity albitic melt. These observations indicate that the compaction length for the lower viscosity melt experiments (i.e., olivine + MORB and olivine + Li‐silicate) was greater than the sample length. However, the compaction rates of these samples were limited by two different processes. Melt transport is relatively easy in the coarse‐grained Li‐silicate samples where melt viscosity is lower. In this case, melt extraction is limited by viscous compaction of the solid matrix. In the olivine + MORB samples, the fine grain size and higher melt viscosity inhibits melt transport and limits extraction rates. The olivine + MORB data constrain the permeability of the partially molten samples. Permeability is proportional to ϕ3 for melt fractions from 0.05 to 0.35. In addition, assuming that permeability scales with the square of the grain size, the magnitude of the permeability compares very well with that measured for Fontainebleau sandstone and fine‐grained quartz aggregates with similar pore‐space topology. Calculations of the compaction length using previously published continuum models agree with the estimates derived from the experiments.
Increased input of circumpolar deep water‐borne detritus to the glacial SE Atlantic Ocean
Authors: Bayon Germain , German Christopher R. , Nesbitt Robert W. , Bertrand Philippe , Schneider Ralph R.
Published: May 2003

Analysis of radiogenic isotopes in marine sediments can provide useful information on the provenance and transport of detrital material, directly relevant to paleoceanographic investigations. Here we show that the detrital Nd isotopic composition of recent SE Atlantic marine sediments matches the complex modern‐day hydrography. In these same cores, glacial‐interglacial isotopic variations are consistent with previous investigations (using different paleoceanographic proxies), which have shown that the relative influence of North Atlantic Deep Water (NADW) into the South Atlantic was reduced during glacial periods. In a novel departure, however, we also calculate the mass accumulation rates of terrigenous material delivered by each of Circumpolar Deep Water (CDW) and NADW to demonstrate that the accumulation of detritus delivered by CDW was enhanced significantly in the glacial South Atlantic. This enhanced transport flux could be explained by an increased flow of CDW into the glacial South Atlantic and/or an increased concentration of suspended terrigenous material transported by glacial CDW.
Subduction fluxes of water, carbon dioxide, chlorine, and potassium
Authors: Jarrard Richard D.
Published: May 2003

The alteration of upper oceanic crust entails growth of hydrous minerals and loss of macroporosity, with associated large‐scale fluxes of H2O, CO2, Cl−, and K2O between seawater and crust. This age‐dependent alteration can be quantified by combining a conceptual alteration model with observed age‐dependent changes in crustal geophysical properties at DSDP/ODP sites, permitting estimation of crustal concentrations of H2O, CO2, Cl−, and K2O, given crustal age. Surprisingly, low‐temperature alteration causes no net change in total water; pore water loss is nearly identical to bound water gain. Net change in total crustal K2O is also smaller than expected; the obvious low‐temperature enrichment is partly offset by earlier high‐temperature depletion, and most crustal K2O is primary rather than secondary. I calculate crustal concentrations of H2O, CO2, Cl−, and K2O for 41 modern subduction zones, thereby determining their modern mass fluxes both for individual subduction zones and globally. This data set is complemented by published flux determinations for subducting sediments at 26 of these subduction zones. Global mass fluxes among oceans, oceanic crust, continental crust, and mantle are calculated for H2O, Cl−, and K2O. Except for the present major imbalance between sedimentation and sediment subduction, most fluxes appear to be at or near steady state. I estimate that half to two thirds of subducted crustal water is later refluxed at the prism toe; most of the remaining water escapes at subarc depths, triggering partial melting. The flux of subducted volatiles, however, does not appear to correlate with either rate of arc magma generation or magnitude of interplate earthquakes.
Stable isotopic composition of upper oceanic crust formed at a fast spreading ridge, ODP Site 801
Authors: Alt Jeffrey C.
Published: May 2003

ODP Legs 129 and 185 sampled the upper 474 m of ∼170 Ma ocean crust in the western Pacific in order to investigate alteration processes in fast spread crust and to determine inputs to subduction. Fourteen composite bulk samples of altered upper oceanic crust from Site 801 have δ18O = 8.7–25.7‰, δD = −69.4‰ to −90.4‰, and δ13C = −2.7‰ to 1.8‰. The intensity of alteration and the amount of sediment within the basement decrease with depth, leading to corresponding decreases in δ18O and δD. A SUPER composite, constructed to estimate the bulk composition of the upper crust, has δ18O = 12.0‰, δD = −87.0‰, and δ13C = 0.7‰. Compared to core descriptions and geophysical logs, the SUPER composite contains too much 18O‐rich sediment (δ18O = 25.7‰), and a corrected δ18O value of 10.8‰ is more reasonable for the upper crust at Site 801. These values are higher than those for other bulk upper oceanic basement sections (δ18O = 8.0–10.0‰) and result in part from: (1) intense low‐temperature (<100°C) hydrothermal alteration of the upper 100 m of tholeiitic basement at Site 801 that may not be representative of material subducting in the western Pacific and (2) an aging effect, whereby progressive addition of 18O‐rich secondary carbonate in veins and breccia cements contributed to the high bulk δ18O of this old upper crustal section.
Caldera collapse and the generation of waves
Authors: Gray J. P. , Monaghan J. J.
Published: May 2003

The aim of this paper is to begin a study of the waves produced by the collapse of a caldera connected to the sea. An example is the bronze age collapse of the caldera of Santorini (Thera), which is thought to have involved an area of approximately 70 km2 subsiding to a depth close to the present 390 m. In this paper, we concentrate on the purely mechanical aspects of the flow and adopt a simple geometry that replicates some of the features of the pre‐bronze age caldera of Santorini. By combining laboratory experiments with computer simulations, we have been able to determine the amplitude of the waves for a wide range of cavity parameters. For cavities with a width comparable to the depth of water entering the cavity, we have determined a scaling relation for the amplitude in terms of the geometry of the system. In the case of wider cavities, the flow begins like a breaking dam flow; it then becomes similar to a classical bore before breaking up into waves. The computer simulations agree well with experiment and will allow us to simulate more complicated geometries.
Record of forearc devolatilization in low‐T, high‐P/T metasedimentary suites: Significance for models of convergent margin chemical cycling
Authors: Sadofsky Seth J. , Bebout Gray E.
Published: May 2003

The Franciscan Complex (Coast Ranges and Diablo Range, California) and the Western Baja Terrane (WBT; Baja California, Mexico) were metamorphosed along high‐P/T paths like those experienced in many active subduction zones, recording peak conditions up to ∼1 GPa and 300°C. Franciscan and WBT metasedimentary rocks are similar in lithology and geochemistry to clastic sediments outboard of many subduction zones. These metamorphic suites provide evidence regarding devolatilization history experienced by subducting sediments, information that is needed to mass‐balance the inputs of materials into subduction zones with their respective outputs. Analyzed samples have lower total volatile contents than their likely protoliths. Little variation in LOI among similar lithologies at differing metamorphic grades, suggests that loss of structurally bound water occurred during early clay‐mineral transformations. Finely disseminated carbonate is present in the lowest‐grade rocks, but absent in all higher‐grade rocks. δ13CVPDB of reduced‐C is uniform in the lower‐grade Franciscan samples (mean = −25.1‰, 1σ = 0.4‰), but varies in higher‐grade rocks (−28.8 to −21.9‰). This likely reflects a combination of devolatilization and C‐isotope exchange, between organic and carbonate reservoirs. Nitrogen concentration ranges from 102 to 891 ppm, with δ15Nair of +0.1 to +3.0‰ (n = 35); this organic‐like δ15N probably represents an efficient transfer of N from decaying organic matter to reacting clay minerals. The lowest‐grade rocks in the Coastal Belt have elevated carbonate contents and correlated N‐δ15N variations, and exhibit the most uniform δ13C and C/N, all consistent with these rocks having experienced less devolatilization. Most fluid‐mobile trace elements are present at concentrations indistinguishable from protoliths. Suggesting that, despite apparent loss of much clay‐bound H2O and CO2 from diagenetic cements (combined, <5–10 wt. %), most fluid‐mobile trace elements are retained to depths of up to ∼40 km. Organic‐like δ15N, lower than that of many seafloor sediments, is consistent with some loss of adsorbed N (perhaps as NO3−) during early stages of diagenesis. The efficient entrainment of fluid‐mobile elements to depths of at least 40 km in these relatively cool subduction zone settings lends credence to models invoking transfer of these elements to the subarc mantle.
Observations and models of lateral hydrothermal circulation on a young ridge flank: Numerical evaluation of thermal and chemical constraints
Authors: Stein Joshua S. , Fisher Andrew T.
Published: May 2003

We used a two‐dimensional coupled heat and fluid flow model to investigate large‐scale, lateral heat and fluid transport on the eastern flank of the Juan de Fuca Ridge. Cool seawater in the natural system is inferred to enter basement where it is exposed close to the spreading center and flow laterally beneath thick sediments, causing seafloor heat flow to be depressed relative to that input at the base of the plate. The flow rate, and thus the properties of permeable basement (the flow layer), controls the efficiency of lateral heat transport, as quantified through numerical modeling. We simulated forced flow in this layer by pumping water through at a fixed rate and quantified relations between flow rate, thickness of the permeable basement, and the extent of suppression of seafloor heat flow. Free flow simulations, in which fluid flow was not forced, match heat flow constraints if nonhydrostatic initial conditions are used and flow layer permeabilities are set to the high end of observed values (10−11 to 10−9 m2). To match seafloor heat flow observations, the models required lateral specific discharge of 1.2 to 40 m/yr for flow layer thicknesses of 600 to 100 m, respectively. The models also replicate differences in fluid pressures in basement, and the local distribution of pressures above and below hydrostatic. Estimated lateral flow rates are 10× to 1000× greater than estimates based on radiocarbon ages of basement pore waters. Estimated lateral flow rates based on thermal and chemical constraints can be reconciled if diffusion from discrete flow zones into less permeable stagnant zones in the crust is considered.
Paleomagnetic record at ODP Site 980 (Feni Drift, Rockall) for the past 1.2 Myrs
Authors: Channell J. E. T. , Raymo M. E.
Published: May 2003

A detailed record of variations in the direction and intensity of the geomagnetic field over the past 1.2 Myrs has been obtained from an ∼120‐m‐thick sedimentary section cored at ODP Site 980 on the Feni Drift (North Atlantic). The record has high resolution due to high mean sedimentation rates (11.3 cm/kyr for the Brunhes Chron and 5.5 cm/kyr for the Matuyama Chron), paleomagnetic measurements every 1 cm along u‐channel samples, and high quality isotopic age control. The Iceland Basin Event is manifest by virtual geomagnetic poles crossing the equator at ∼190 ka. The base of the recovered section lies immediately below the Cobb Mountain Subchronozone that occurs within marine isotope stage 35. Normalized remanence data can be correlated to paleointensity records from ODP Sites 983 and 984 (∼700 km to the NW from Site 980) and with lower resolution paleointensity data from the Pacific Ocean. Differences between the Site 980 paleointensity record and the Pacific records are attributed to variable sedimentation rates, variable quality age control, and inadequacies in the normalization procedure used to derive the paleointensity proxies.
Sedimentary record of disintegrating ice shelves in a warming climate, Antarctic Peninsula
Authors: Gilbert Robert , Domack Eugene W.
Published: May 2003

Seafloor sediments from beneath the former Larsen‐A and Prince Gustav ice shelves document the recent breakup of the shelves and provide evidence for interpretation of previous events. At three of five sites, sediment texture coarsens upward with up to 40% sand at the surface. Radiometric 210Pb dating shows this to have occurred between 1985 and 1993, several years before breakup of the shelf, and that rates of accumulation of sediment on the seafloor doubled to quadrupled during this period. These events are related to the release of eolian sediment in periodic rapid draining of small lakes and crevasses on the ice shelf before breakup. X‐ray radiographs of sediment cores also document the recent influx of coarse particles (gravel) related to ice rafting during ice shelf disintegration. Because sediment is released irregularly in time and space from well‐separated point sources on the ice shelf during at least several years before final disintegration, only a portion of the seafloor is affected.
Quantitative bedrock geology of Alaska and Canada
Authors: Peucker‐Ehrenbrink Bernhard , Miller Mark W.
Published: May 2003

We quantitatively analyze the area‐age distribution of sedimentary, igneous, and metamorphic bedrock on the basis of data from the most recent geologic maps of Alaska [Beikman, 1980] (1:2,500,000) and Canada [Douglas, 1969] (updated; 1:5,000,000), made available in digital form by the U.S. Geologic Survey and the Geologic Survey of Canada (National Resources Canada), respectively. Sediments cover 72.9% and 52.4% of the surface of Alaska and Canada. Volcanic rocks comprise 11.6% and 6% of the surface area in Alaska and Canada, respectively, whereas intrusive rocks cover 6.6% and 24% of the surface, respectively. Ultramafic rocks account for 0.20% and 0.08% of the bedrock area, whereas metamorphic rocks cover 3.4% and 16.1%. The average ages of major lithologic units, weighted according to bedrock area, are significantly younger in Alaska than in Canada: marine sediments (stratigraphic age of 206 Myr in Alaska versus 599 Myr in Canada), continental sediments (stratigraphic age of 81 Myr versus 150 Myr), volcanic rocks (126 Myr versus 1377 Myr), intrusive rocks (114 Myr versus 2109 Myr), ultramafic rocks (261 Myr versus 874 Myr), and metamorphic rocks (474 Myr versus 2545 Myr). This age difference reflects the contrast between the young active margin influence on the geology of Alaska and the old cratonic nature of much of the bedrock exposed in Canada. Glacial erosion, and the resulting exposure of Precambrian bedrock, primarily in Canada, contributes to this age contrast. The average temporal and spatial resolution of the digital data is sufficiently high to perform age‐area analyses on individual river basins larger than ∼20,000 km2 and to evaluate the relationship between bedrock geology and river chemistry.
A precise, high‐throughput method for determining Sr/Ca, Sr/Ba, and Ca/Ba ratios in marine barite
Authors: Averyt Kristen B. , Paytan Adina , Li Guangchao
Published: May 2003

Recent evidence suggests that marine barite, an inorganic phase preserved in oxic, deep‐sea sediments, may record seawater Sr and Ca concentrations. Here we describe a novel method for determining Sr/Ca, Sr/Ba, and Ca/Ba ratios of discrete marine barite separates. Marine barite, separated from deep‐sea sediments, is dissolved in a basic, ethylene diamine tetra acetic acid (EDTA) solution and subsequently analyzed using an inductively coupled plasma optical emission spectrometer (ICP‐OES). Since dissolution occurs within 2–5 days, and >15 samples can be analyzed on the ICP‐OES per hour, data acquisition is rapid compared with other procedures. The method yields precisions of <0.7% (Sr/Ca), <0.6% (Sr/Ba), and <1.0% (Ca/Ba), which are reproducible within 1.7% (Sr/Ca), 1.7% (Sr/Ba), and 2.5% (Ca/Ba). Precise determination of trace element incorporation in marine barite has implications for paleoceanography (development of seawater Sr and Ca concentration curves) and calculation of barite solubility in both the modern and ancient ocean.
Fractal Plate Tectonics
Authors: Sornette Didier , Pisarenko Vladilen
Published: May 2003

We analyze in details the statistical significance of the claim by Bird [2002] of a power law distribution of plate areas covering the Earth and confirm that the power law with exponent 0.25 ± 0.05 is the most robust and parsimonious model for all plates, including the very largest plates, when taking into account the constraint that the plates areas must sum up to 4 π steradians. We propose a general class of fragmentation models that rationalize this observation and discuss the implications for the Earth dynamics and the general self‐organization of tectonic deformations at multiple scales.
Organic and black carbon 13C and 14C through the Santa Monica Basin sediment oxic‐anoxic transition
Authors: Masiello C. A. , Druffel E. R. M.
Published: May 2003

Black carbon (BC) is a significant percentage of sedimentary organic carbon (SOC) at abyssal ocean sites, but its presence in shelf sediments is not well studied. Approximately 1600AD, Santa Monica Basin bottom waters shifted from oxic to very low oxygen (dysoxic) deposition conditions. Under oxic deposition conditions BC was 11 ± 4% of SOC, whereas after the overlying water became dysoxic (and sediments became anoxic), BC was 5.2 ± 1.2% of SOC. This shift may reflect the preferential remineralization of non‐black SOC under oxic conditions. There is an offset between BC and SOC 14C ages which changes with oxidation conditions, suggesting that BC storage is related to oxygen exposure and confirming a previously published report of the vulnerability of BC to sedimentary oxidation [Middelburg, 1999]. Terrestrial carbon is 17 ± 5% of total SOC in this core's anoxic region, and 31 ± 11% of this terrestrial carbon is BC.