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  • 1
    Language: English
    In: Geochimica et Cosmochimica Acta, 2011, Vol.75(13), pp.3741-3756
    Description: Marine sediments are the main sink in the oceanic phosphorus (P) cycle. The activity of benthic microorganisms is decisive for regeneration, reflux, or burial of inorganic phosphate (P ), which has a strong impact on marine productivity. Recent formation of phosphorites on the continental shelf and a succession of different sedimentary environments make the Benguela upwelling system a prime region for studying the role of microbes in P biogeochemistry. The oxygen isotope signature of pore water phosphate (δ O ) carries characteristic information of microbial P cycling: Intracellular turnover of phosphorylated biomolecules results in isotopic equilibrium with ambient water, while enzymatic regeneration of P from organic matter produces distinct offsets from equilibrium. The balance of these two processes is the major control for δ O . Our study assesses the importance of microbial P cycling relative to regeneration of P from organic matter from a transect across the Namibian continental shelf and slope by combining pore water chemistry (sulfate, sulfide, ferrous iron, P ), steady-state turnover rate modeling, and oxygen isotope geochemistry of P . We found δ O values in a range from 12.8‰ to 26.6‰, both in equilibrium as well as pronounced disequilibrium with water. Our data show a trend towards regeneration signatures (disequilibrium) under low mineralization activity and low P concentrations, and microbial turnover signatures (equilibrium) under high mineralization activity and high P concentrations. These findings are opposite to observations from water column studies where regeneration signatures were found to coincide with high mineralization activity and high P concentrations. It appears that preferential P regeneration in marine sediments does not necessarily coincide with a disequilibrium δ O signature. We propose that microbial P uptake strategies, which are controlled by P availability, are decisive for the alteration of the isotope signature. This hypothesis is supported by the observation of efficient microbial P turnover (equilibrium signatures) in the phosphogenic sediments of the Benguela upwelling system.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 2
    In: Canadian Journal of Zoology, June 2011, Vol.89(6), pp.453-465
    Description: The stable carbon isotope composition of animal tissues represents the weighted sum of the variety of food sources eaten. If sources differ in digestibility, tissues may overrepresent intake of more digestible items and faeces may overrepresent less digestible items. We tested this idea using whole blood and faeces of goats (Capra hircus L., 1758) fed different food mixtures of C3 lucerne (Medicago sativa L.) and C4 grass (Themeda triandra Forssk.). Although blood and faecal δ13C values were broadly consistent with diet, results indicate mismatch between consumer and diet isotope compositions: both materials overrepresented the C3 (lucerne) component of diets. Lucerne had lower fibre digestibility than T. triandra, which explains the results for faeces, whereas underrepresentation of dietary C4 in blood is consistent with low protein content of the grass hay. A diet switch experiment revealed an important difference in 13C-incorporation rates across diets, which were slower for grass than lucerne diets, and in fact equilibrium states were not reached for all diets. Although more research is needed to link digestive kinetics with isotope incorporation, these results provide evidence for nonlinear relationships between consumers and their diets, invoking concerns about the conceptual value of “discrimination factors“ as the prime currency for contemporary isotope ecology.
    Keywords: Animal Nutrition -- Research ; Carbon Isotopes -- Composition ; Herbivores -- Physiological Aspects;
    ISSN: 0008-4301
    E-ISSN: 14803283
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  • 3
    Language: English
    In: Geochimica et Cosmochimica Acta, Jan 15, 2014, Vol.125, p.519(9)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.gca.2013.10.010 Byline: Christian von Sperber, Hajo Kries, Federica Tamburini, Stefano M. Bernasconi, Emmanuel Frossard Abstract: Plants and microorganisms under phosphorus (P) stress release extracellular phosphatases as a strategy to acquire inorganic phosphate (P.sub.i). These enzymes catalyze the hydrolysis of phosphoesters leading to a release of P.sub.i. During the enzymatic hydrolysis an isotopic fractionation (I[micro]) occurs leaving an imprint on the oxygen isotope composition of the released P.sub.i which might be used to trace phosphorus in the environment. Therefore, enzymatic assays with acid phosphatases from wheat germ and potato tuber and alkaline phosphatase from Escherichia coli were prepared in order to determine the oxygen isotope fractionation caused by these enzymes. Adenosine 5' monophosphate and glycerol phosphate were used as substrates. The oxygen isotope fractionation caused by acid phosphatases is 20-30a[degrees] smaller than for alkaline phosphatases, resulting in a difference of 5-7.5a[degrees] in [delta].sup.18O of P.sub.i depending on the enzyme. We attribute the enzyme dependence of the isotopic fractionation to distinct reaction mechanisms of the two types of phosphatases. The observed difference is large enough to distinguish between the two enzymatic processes in environmental samples. These findings show that the oxygen isotope composition of P.sub.i can be used to trace different enzymatic processes, offering an analytical tool that might contribute to a better understanding of the P-cycle in the environment. Article History: Received 2 May 2013; Accepted 6 October 2013 Article Note: (miscellaneous) Associate editor: Jon Chorover
    Keywords: Adenosine ; Glycerol ; Enzymes ; Phosphates ; Enzymology ; Hydrolysis
    ISSN: 0016-7037
    Source: Cengage Learning, Inc.
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  • 4
    Language: English
    In: Geochimica et Cosmochimica Acta, 2011, Vol.75(15), pp.4216-4227
    Description: Phosphorus (P) availability limits productivity in many ecosystems worldwide. As a result, improved understanding of P cycling through soil and plants is much desirable. The use of the oxygen isotopes associated to phosphate can be used to study the cycle of P in terrestrial systems. However, changes with time in the oxygen isotopes associated to available P have not yet been evaluated under field conditions. Here we present the variations in available-P oxygen isotopes, based on resin extractions, in a semi-arid site that included plots in which the amount of rainfall reaching the soil was modified. In addition, the oxygen isotopes in the less dynamic fraction which is extractable by HCl, were also measured. The δ O of the HCl-extractable phosphate shows no seasonal pattern and corresponds to the average value of the available phosphate of 16.5‰. This value is in the expected range for equilibration with soil water at the prevailing temperatures in the site. The δ O values of resin-extractable P showed a range of 14.5–19.1‰ (SMOW), and evidence of seasonal variability, as well as variability induced by rainfall manipulation experiments. We present a framework for analyzing the isotopic ratios in soil phosphate and explain the variability as mainly driven by phosphate equilibration with soil water, and by the isotopic effects associated with extracellular mineralization. Additional isotopic effects result from fractionation in uptake, and the input to the soil of phosphate equilibrated in leaves. These results suggest that the δ O of resin-extractable P is an interesting marker for the rate of biological P transformations in soil systems.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 5
    Language: English
    In: Geology, June, 2011, Vol.39(6), p.563(4)
    Description: Early diagenetic dolomite formation in methanogenic marine sediments is enigmatic because acidification by C[O.sub.2], a by-product of methanogenesis, should lead to carbonate dissolution and not precipitation. However, petrographic relationships indicate that dolomite breccia layers with [[delta].sup.13]C values of ~+15 [per thousand], recovered from the lower slope of the Peru continental margin (Ocean Drilling Program Site 1230), formed deep in the methanogenic zone during tectonic activity of a decollement. Based on radiogenic Sr isotope ratios ([sup.87]Sr/[sup.86]Sr 〉 0.711) and positive [[delta].sup.18]O values (+6[per thousand]), we present evidence that the dolomite breccias mainly formed from fluids originating from deep sedimentary units within the accretionary prism, where they interacted with continental crust and/or siliciclastic rocks of continental affinity. Due to silicate alteration and dehydration, such fluids are likely alkaline and thus have the potential to neutralize the acidification imposed by the high dissolved C[O.sub.2] concentrations. This scenario provides a potential mechanism by which dolomite formation can be induced deep in a highly active methanogenic zone. doi: 10.1130/G31810.1
    Keywords: Breccia -- Research ; Breccia -- Natural History ; Marine Sediments -- Natural History ; Marine Sediments -- Research ; Dolomite -- Research ; Dolomite -- Natural History
    ISSN: 0091-7613
    E-ISSN: 19432682
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  • 6
    Language: English
    In: Geochimica et Cosmochimica Acta, May 1, 2013, Vol.108, p.125(16)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.gca.2012.12.049 Byline: Anna-Lena Grauel (a), Thomas W. Schmid (a), Bin Hu (a)(b), Caterina Bergami (c), Lucilla Capotondi (c), Liping Zhou (b)(d), Stefano M. Bernasconi (a) Abstract: The reconstruction of past ocean temperatures is fundamental to the study of past climate changes, therefore considerable effort has been invested in developing proxies for seawater temperatures. One of the most recent and promising new proxy is carbonate 'clumped isotope' thermometry, in particular because it is based on thermodynamic equilibrium and not on biogeochemical proxies. Here, we present a new calibration of the 'clumped isotope' thermometer to foraminifera based on seven species of planktic and benthic foraminifera spanning a growth temperature range of [approximately equal to]2-28[degrees]C. We used a newly developed technique for the measurements of small samples to improve the applicability of this method to paleoceanography. Our data have a comparable precision ([approximately equal to]0.005-0.013a[degrees]) and confirm previous calibration studies based on biogenic and inorganic calcite. We discuss possible sources of uncertainty such as over-/underestimation of the calcification temperatures, species-specific vital effects, pH variations between the seawater and the vacuole water of the species and possible kinetic effects on the 'clumped isotope' calibration. To validate our calibration study and test the applicability of our measuring technique to paleoclimate and paleoceanographic studies we measured the isotope composition of Globigerinoides ruber (white) at high-resolution in a sediment core covering the last 700years in the Gulf of Taranto (Mediterranean Sea). The results show that it is necessary to average a relatively large number of analyses to achieve a consistent temperature signal for the detection of small sea surface temperature changes. Although with the current analytical system, 'clumped isotope' thermometry is only applicable to the analysis of relatively large SST changes in marine sediments, further technical improvements may make this a very powerful technique for paleoceanographic studies. Author Affiliation: (a) Geological Institute, ETH Zurich, 8092 Zurich, Switzerland (b) Department of Geography, Peking University, 100871 Beijing, China (c) CNR - National Research Council of Italy, ISMAR - Institute of Marine Sciences, Bologna, Italy (d) Centre for Ocean Research, Peking University, 100871 Beijing, China Article History: Received 12 May 2012; Accepted 29 December 2012 Article Note: (miscellaneous) Associate editor: Edwin Schauble
    Keywords: Measuring Instruments ; Climate
    ISSN: 0016-7037
    Source: Cengage Learning, Inc.
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  • 7
    In: Nature, 2004, Vol.428(6979), p.130
    ISSN: 0028-0836
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  • 8
    Language: English
    In: Plant and Soil, 2011, Vol.341(1), pp.179-192
    Description: The integration of multipurpose legumes into low-input tropical agricultural systems is needed because they are a nitrogen (N) input through symbiotic fixation. The drought-tolerant cover legume canavalia ( Canavalia brasiliensis ) has been introduced for use either as forage or as a green manure into the crop-livestock system of the Nicaraguan hillsides. To evaluate its impact on the subsequent maize crop, an in-depth study on N dynamics in the soil-plant system was conducted. Microplots were installed in a 6-year old field experiment with maize-canavalia rotation. Direct and indirect 15 N-labelling techniques were used to determine N uptake by maize from canavalia residues and canavalia-fed cows’ manure compared to mineral fertilizer. Litter bags were used to determine the N release from canavalia residues. The incorporation of N from the amendment into different soil N pools (total N, mineral N, microbial biomass) was followed during the maize cropping season. Maize took up an average of 13.3 g N m −2 , within which 1.0 g N m −2 was from canavalia residues and 2.6 g N m −2 was from mineral fertilizer, corresponding to an amendment N recovery of 12% and 32%, respectively. Recoveries in maize would probably be higher at a site with lower soil available N content. Most of the amendment N remained in the soil. Mineral N and microbial N were composed mainly of N derived from the soil. Combined total 15 N recovery in maize and soil at harvest was highest for the canavalia residue treatment with 98% recovery, followed by the mineral fertilizer treatment with 83% recovery. Despite similar initial enrichment of soil microbial and mineral N pools, the indirect labelling technique failed to assess the N fertilizer value of mineral and organic amendments due to a high N mineralization from the soil organic matter.
    Keywords: Canavalia brasiliensis ; N ; Indirect and direct labelling techniques ; Microplot study ; Organic amendments
    ISSN: 0032-079X
    E-ISSN: 1573-5036
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  • 9
    Language: English
    In: Geochimica et Cosmochimica Acta, 15 January 2014, Vol.125, pp.519-527
    Description: Plants and microorganisms under phosphorus (P) stress release extracellular phosphatases as a strategy to acquire inorganic phosphate (P ). These enzymes catalyze the hydrolysis of phosphoesters leading to a release of P . During the enzymatic hydrolysis an isotopic fractionation (ε) occurs leaving an imprint on the oxygen isotope composition of the released P which might be used to trace phosphorus in the environment. Therefore, enzymatic assays with acid phosphatases from wheat germ and potato tuber and alkaline phosphatase from were prepared in order to determine the oxygen isotope fractionation caused by these enzymes. Adenosine 5′ monophosphate and glycerol phosphate were used as substrates. The oxygen isotope fractionation caused by acid phosphatases is 20–30‰ smaller than for alkaline phosphatases, resulting in a difference of 5–7.5‰ in δ O of P depending on the enzyme. We attribute the enzyme dependence of the isotopic fractionation to distinct reaction mechanisms of the two types of phosphatases. The observed difference is large enough to distinguish between the two enzymatic processes in environmental samples. These findings show that the oxygen isotope composition of P can be used to trace different enzymatic processes, offering an analytical tool that might contribute to a better understanding of the P-cycle in the environment.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 10
    Language: English
    In: Geochimica et Cosmochimica Acta, 01 January 2018, Vol.220, pp.535-551
    Description: The oxygen isotope composition of siderites can be used to deduce the temperature and/or oxygen isotope composition of the fluids from which they precipitated. Previous siderite-water oxygen isotope fractionation calibrations are not well constrained at temperatures below 33 °C where most of the siderite forms at the Earth’s surface. Moreover, the few experimental low temperature calibration points available are possibly inaccurate as the corresponding siderites may not have formed in equilibrium with the solution. In this study, we synthesized siderite in the laboratory from 8.5 to 62 °C, using both active-degassing experiments and microbial cultures. We used the enzyme carbonic anhydrase, which significantly reduces the equilibration time of oxygen isotopes among all dissolved inorganic carbon (DIC) species and water to minimize siderite formation out of equilibrium. Our calibration is based on many more data points than previous calibrations and significantly reduces the uncertainty in siderite-water oxygen isotope fractionation in natural siderites formed at low temperatures. The best fit equation is where α (1000 +  O /1000 +  O ) is the fractionation factor and is the temperature in Kelvin.
    Keywords: Oxygen Isotope Fractionation ; Siderite ; Ambient Temperature ; Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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