Dissolution kinetics of biogenic silica collected from the water column and sediments of three Southern California borderland basins     

Tao Cheng  Douglas E. Hammond  William M. Berelson  Janet G. Hering  Suvasis Dixit 《Marine Chemistry》2009,113(1-2):41-49

Understanding biogenic silica (bSi) dissolution kinetics in margin environments is important in assessing the global silicon cycle, a cycle closely linked to the global carbon cycle. This understanding is also essential to answer the question of whether bSi content in marine sediment is a valid indicator of productivity in the overlying surface ocean. In this study, plankton tow, sediment trap, and sediment samples were collected at sites in three Southern California borderland basins. Batch dissolution experiments with plankton tow and sediment trap materials (conducted in the laboratory at 22 °C) showed linear dissolution kinetics, from which mean dissolution rate constants of 0.05 d^? 1 for plankton tow samples and 0.07 d^? 1 for sediment trap samples could be calculated. The dissolution rate constants for both types of samples showed seasonal variability but not the same seasonal patterns. Faster dissolution was observed with sediment trap samples collected at 800 m than at 550 m. With sediment multi-core samples, non-linear dissolution kinetics was observed, which complicates the direct comparison of dissolution rates. Nonetheless, dissolution appeared to be slower for the sediments samples than for samples collected from the water column and to decrease with depth in the sediments. Rate constants for surface sediment (0–0.5 cm) were at least 3–5 times less, and sediments at depths > 2 cm had rate constants at least 6–13 times less than those for material sinking to the sediment surface at these sites. Dissolution experiments conducted with Santa Barbara Basin surface sediment samples amended with dissolved aluminum (Al) and San Pedro Basin trap samples amended with enriched detrital materials (obtained by leaching bSi from sediment samples) suggested that dissolution was inhibited by Al and that the sediments from the different basins varied in the extent of Al release.  相似文献   

Identifying the source of a strong fullerene envelope arising from laser desorption mass spectrometric analysis of meteoritic insoluble organic matter     

Matthew R. Hammond 《Geochimica et cosmochimica acta》2008,72(22):5521-5529

Insoluble organic matter (IOM) has been obtained from two carbonaceous chondrite meteorites and subjected to analysis by laser desorption mass spectrometry (LDMS) using standard operating conditions that were optimized for fullerene detection (3-6 μJ pulses at 337 nm focused to a spot size of approximately 100 μm in diameter). The preparation process yields no free C₆₀ in the IOM, and other experiments suggest that this material does not contain appreciable amounts of fullerenes. Nevertheless, a pronounced high-mass envelope is observed in LDMS, extending from 720 amu to about 4000 amu, with peaks spaced apart every 24 amu (corresponding to the gain or loss of C₂ units). We attribute this high-mass envelope to the existence of various fullerene molecules. The present work demonstrates that these fullerene molecules are created by the laser desorption laser ionization process under typical laser conditions used for studying free fullerenes in organic solvent extracts of natural samples (toluene and 1,2,4-trichlorobenzene). The implications of this false positive detection of fullerene molecules on the reports of fullerenes in other meteoritic samples have been investigated by introducing IOM into typical fullerene extraction procedures and examining the LDMS results. We found that IOM is capable of producing false positive signals in these experiments. The effect of ambient laboratory contamination producing fullerene signals is also described. It is found that extensive centrifugation of the meteoritic extracts is able to reduce the observed fullerene envelope, which points to an association of this envelope with IOM particulates that have passed through the filtering steps. We suggest the exercise of extreme caution in interpreting fullerene data from LDMS experiments.  相似文献   

Spectral distortion inherent in airborne profilometer measurements of ocean wave heights     

Donald L. Hammond  Charles R. McClain   《Ocean Engineering》1980,7(1):99-108

A theoretical analysis of the performance characteristics of an airborne profilometer is presented. The analytical characteristics are shown to agree with those of the prototype. Results show that both the wave spectra and the dominant wave heading can be determined using the airborne profilometer.  相似文献   

Bottom water circulation in Cascadia Basin     

Susan L. Hautala  H. Paul Johnson  Douglas E. Hammond 《Deep Sea Research Part I: Oceanographic Research Papers》2009,56(10):1688-1707

A combination of beta spiral and minimum length inverse methods, along with a compilation of historical and recent high-resolution CTD data, are used to produce a quantitative estimate of the subthermocline circulation in Cascadia Basin. Flow in the North Pacific Deep Water, from 900-1900 m, is characterized by a basin-scale anticyclonic gyre. Below 2000 m, two water masses are present within the basin interior, distinguished by different potential temperature-salinity lines. These water masses, referred to as Cascadia Basin Bottom Water (CBBW) and Cascadia Basin Deep Water (CBDW), are separated by a transition zone at about 2400 m depth. Below the depth where it freely communicates with the broader North Pacific, Cascadia Basin is renewed by northward flow through deep gaps in the Blanco Fracture Zone that feeds the lower limb of a vertical circulation cell within the CBBW. Lower CBBW gradually warms and returns to the south at lighter density. Isopycnal layer renewal times, based on combined lateral and diapycnal advective fluxes, increase upwards from the bottom. The densest layer, existing in the southeast quadrant of the basin below 2850 m, has an advective flushing time of 0.6 years. The total volume flushing time for the entire CBBW is 2.4 years, corresponding to an average water parcel residence time of 4.7 years. Geothermal heating at the Cascadia Basin seafloor produces a characteristic bottom-intensified temperature anomaly and plays an important role in the conversion of cold bottom water to lighter density within the CBBW. Although covering only about 0.05% of the global seafloor, the combined effects of bottom heat flux and diapycnal mixing within Cascadia Basin provide about 2-3% of the total required global input to the upward branch of the global thermohaline circulation.  相似文献   

“You know you can do this,right?”: developing geospatial technological pedagogical content knowledge and enhancing teachers’ cartographic practices with socio-environmental science investigations     

Thomas C. Hammond  Alec Bodzin  David Anastasio  Breena Holland  Kate Popejoy  Dork Sahagian 《制图学和地理信息科学》2018,45(4):305-318

Many barriers exist to K–12 classroom teachers’ adoption and implementation of geospatial technologies with their students. To address this circumstance, we have developed and implemented a geospatial curriculum approach to promote teachers’ professional growth with curriculum-linked professional development (PD) to support the adoption of socio-environmental science investigations (SESI) in an urban school environment that includes reluctant learners. SESI focus on social issues related to environmental science. The pedagogy is inquiry-driven, with students engaged in map-based mobile data collection and subsequent analysis with Web-based dynamic mapping software to answer open-ended questions. Working with four science and social studies teachers, we designed and implemented a sequence of three locally oriented, geospatial inquiry projects that were implemented with 140 9th grade students. We investigated how the geospatial curriculum approach impacted the teachers’ geospatial pedagogical content knowledge (PCK), their cartographic practices, and promoted geospatial thinking and analysis skills with their students. Findings revealed strong growth in teachers’ geospatial PCK, increased map use by teachers, use of maps as media for inquiry and not didactic instruction, and modeling to guide students’ geospatial analysis using GIS. Implications for PD to promote teachers’ geospatial PCK and in-class cartographic practices are discussed.  相似文献   

Thallium Mass Fraction and Stable Isotope Ratios of Sixteen Geological Reference Materials          下载免费PDF全文

Alex Brett  Julie Prytulak  Samantha J. Hammond  Mark Rehkämper 《Geostandards and Geoanalytical Research》2018,42(3):339-360

Thallium stable isotope ratio and mass fraction measurements were performed on sixteen geological reference materials spanning three orders of magnitude in thallium mass fraction, including both whole rock and partially separated mineral powders. For stable isotope ratio measurements, a minimum of three independent digestions of each reference material was obtained. High‐precision trace element measurements (including Tl) were also performed for the majority of these RMs. The range of Tl mass fractions represented is 10 ng g^?1 to 16 μg g^?1, and Tl stable isotope ratios (reported for historical reasons as ε²⁰⁵Tl relative to NIST SRM 997) span the range ?4 to +2. With the exception – attributed to between‐bottle heterogeneity – of G‐2, the majority of data are in good agreement with published or certified values, where available. The precision of mean of independent measurement results between independent dissolutions suggests that, for the majority of materials analysed, a minimum digested mass of 100 mg is recommended to mitigate the impact of small‐scale powder heterogeneity. Of the sixteen materials analysed, we therefore recommend for use as Tl reference materials the USGS materials BCR‐2, COQ‐1, GSP‐2 and STM‐1; CRPG materials AL‐I, AN‐G, FK‐N, ISH‐G, MDO‐G, Mica‐Fe, Mica‐Mg and UB‐N; NIST SRM 607 and OREAS14P.  相似文献   

The relationship between methane migration and shale-gas well operations near Dimock,Pennsylvania, USA     

Hammond  Patrick A. 《Hydrogeology Journal》2016,24(2):503-519

Migration of stray methane gas near the town of Dimock, Pennsylvania, has been at the center of the debate on the safety of shale gas drilling and hydraulic fracturing in the United States. The presented study relates temporal variations in molecular concentrations and stable isotope compositions of methane and ethane to shale-gas well activity (i.e., vertical/horizontal drilling, hydraulic fracturing and remedial actions). This was accomplished by analyzing data collected, between 2008 and 2012, by state and federal agencies and the gas well operator. In some cases, methane migration started prior to hydraulic fracturing. Methane levels of contaminated water wells sampled were one to several orders of magnitude greater than the concentrations due to natural variation in water wells of the local area. Isotope analyses indicate that all samples had a thermogenic origin at varying maturity levels, but from formations above the hydraulically fractured Marcellus Shale. The results from the initial water well samples were similar to annular gas values, but not those of production gases. This indicates that leakage by casing cement seals most likely caused the impacts, not breaks in the production casing walls. Remediation by squeeze cementing was partially effective in mitigating impacts of gas migration. In several cases where remediation caused a substantial reduction in methane levels, there were also substantial changes in the isotope values, providing evidence of two sources, one natural and the other man-induced. Sampling water wells while venting gas wells appears to be a cost-effective method for determining if methane migration has occurred.

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Diagenetic Ge-Si fractionation in continental margin environments: further evidence for a nonopal Ge sink     

James McManus  Douglas E. Hammond  Gary P. Klinkhammer 《Geochimica et cosmochimica acta》2003,67(23):4545-4557

We present data for dissolved germanic and silicic acids from several settings: sediment pore water profiles collected from the Peru-Chile continental margin, fluxes measured with in situ benthic flux chambers and shipboard whole-core incubations, and water column profiles from the California continental margin. Collectively, these data show that Ge and Si are fractionated in these continental margin sedimentary environments during diagenesis with ∼50% of the Ge released by opal dissolution being sequestered within the sediments. The areal extent of this diagenetic fractionation covers station depths from ∼100 m to >1000 m. Sediments from these sites typically have high pore water Fe²⁺ present in the upper ∼2 cm. At sites with low Fe²⁺ concentrations in the upper pore waters, the Ge:Si benthic regeneration ratio indicates little or no fractionation during diatom dissolution. Consistent with the sedimentary fractionation, water column dissolved Ge:Si ratios along the continental margin are on average lower (0.66 μmol/mol) than the global average ratio (0.72 μmol/mol, Mortlock and Froelich, 1996). This lower “average” ratio is driven by two distinct ΔGe:ΔSi data trends having similar slopes but different intercepts. Data from the upper ∼150 m has a Ge:Si slope of 0.74 ± 0.04 μmol/mol (2σ) and an intercept of 0.5 ± 0.4; whereas below ∼550 m the slope is 0.70 ± 0.06 μmol/mol, but the intercept is −5.0 ± 8.0. Assuming that the sediments sampled here are representative of all reducing marine environments, an assumption requiring further testing, our calculations indicate that sequestration of Ge into a nonopal phase throughout the global ocean in the depth range 0.2-1 km is sufficient to balance the Ge budget. Thus, we tentatively conclude that sequestering of Ge in reducing continental margin sediments is the “missing” Ge sink.  相似文献   

Offset caldera and crater collapse on Juan de Fuca ridge-flank volcanoes     

Stephen R. Hammond 《Bulletin of Volcanology》1997,58(8):617-627

 Forty-three volcanoes located along the flanks of the Juan de Fuca Ridge were selected to study relationships between their morphologies and off-axis magmatic processes. The volcanoes occur both in chains consisting of up to seven distinct cones and isolated edifices. Nearly all of the volcanoes are circular, truncated cones with steep flanks and large, relatively flat summit plateaus. In addition, most of these volcanoes also have prominent and distinctly offset calderas or craters. The most striking characteristic of the volcanoes' morphology is that nearly all of their collapse structures are located on the sides of the volcanoes which face the Juan de Fuca Ridge and many are breached with openings toward the ridge. A simple model based on these observations accounts for these ridge-facing features. As plate motion transports a volcano away from its magma source beneath the lithosphere, the volcano's magma supply conduits tend to lag behind. Eventually these conduits are abandoned and ridgeward collapse structures are formed. It can be inferred from the model that, on average, individual volcanoes were active for approximately 50 000 years and that most eruptions took place early in this interval. If most of the cone-building eruptions occurred during the first thousand years or so, associated hydrothermal activity may have temporarily rivaled the present-day yearly time-averaged hydrothermal output along the entire Juan de Fuca ridge axis. Received: 1 September 1996 / Accepted: 13 January 1997  相似文献   

Paleomagnetic polarity sequence and radiolarian zones,brunhes to polarity epoch 20     

F. Theyer  S.R. Hammond 《Earth and Planetary Science Letters》1974,22(4):307-319

Superposition of paleomagnetic polarity logs of seven chronologically overlapping piston cores from the central equatorial Pacific, using the established tropical radiolarian zonation as a stratigraphic reference, produced a nearly continuous correlation of magnetic and radiolarian events ranging from late Pleistocene to earliest Miocene. Twenty magnetic polarity epochs, and possibly as many as 30 polarity events, occur during this time span. Epoch 16 (reversed polarity) appears to be the longest interval (～ 14.8–17.6m.y. B.P.) among these Neogene magnetostratigraphic units. The middle/late Miocene boundary is shown to fall within latest Epoch 11 (normal) and its approximate age is between 10.5 and 11 m.y. B.P. The early/middle Miocene boundary occurs within the top of Epoch 16 at a suggested age of about 15 m.y. B.P.  相似文献