1,721,118 research outputs found
Enhanced phosphorus regeneration during formation of eastern Mediterranean sapropels (abstract of paper presented at AGU Fall Meeting, San Francisco, CA, 10-14 Dec 2001)
No full textControls on phosphorus regeneration and burial during formation of eastern Mediterranean sapropels
No full textThe carbon (C) and phosphorus (P) geochemistry of sapropels from four sites in the eastern Mediterranean Sea was determined to obtain more insight into the role of differences in sediment accumulation rates and bottom water anoxia on P regeneration and burial in sediments. Sediment Corg/Porg (with ORG=organic) ratios above Redfield indicate enhanced regeneration of P relative to C from organic matter during formation of the most recent sapropel S1 and a Pliocene sapropel (ODP-site 969E; i-282c). Release of P from Fe-oxides was relatively unimportant. Increased burial of Ca-P (authigenic carbonate fluorapatite and/or biogenic hydroxyapatite) occurred but was of minor importance compared to the enhanced P release from organic matter. For sapropel S1, Corg/Porg and Corg/Preac, (with REAC=reactive, defined as the sum of organic, Fe-bound, authigenic and biogenic Ca-P) ratios decreased with increasing sedimentation rate and oxygen exposure and decreasing water depth. Whether the water column in the deep basin was dysoxic/semi-euxinic (S1) or euxinic (Pliocene) does not appear to affect the estimated burial efficiencies of Porg and Preac. Enhanced Corg burial under the euxinic water column explains the much higher Corg/Porg and Corg/Preac ratios in the Pliocene sapropel (on average ~2860 and ~760, respectively) compared to sapropel S1 (on average ~530 and ~160, respectively
Enhanced regeneration of phosphorus during formation of the most recent eastern Mediterranean sapropel (S1)
No full textPhosphorus regeneration and burial fluxes during and after formation of the most recent sapropel S1 were determined for two deep-basin, low-sedimentation sites in the eastern Mediterranean Sea. Organic C/P ratios and burial fluxes indicate enhanced regeneration of P relative to C during deposition of sapropel S1. This is largely due to the enhanced release of P from organic matter during sulfate reduction. Release of P from Fe-bound P also increased, but this was only a relatively minor source of dissolved P. Pore-water HPO42? concentrations remained too low for carbonate fluorapatite formation. An increased burial of biogenic Ca-P (i.e., fish debris) was observed for one site. Estimated benthic fluxes of P during sapropel formation were elevated relative to the present day (~900 to 2800 vs. ~70 to 120 mol m?2 yr?1). The present-day sedimentary P cycle in the deep-basin sediments is characterized by two major zones of reaction: (1) the zone near the sediment-water interface where substantial release of HPO42? from organic matter takes place, and (2) the oxidation front at the top of the S1 where upward-diffusing HPO42? from below the sapropel is sorbed to Fe-oxides. The efficiency of aerobic organisms in retaining P is reflected in the low organic C/P ratios in the oxidized part of the sapropel. Burial efficiencies for reactive P were significantly lower during S1 times compared with the present day (~7 to 15% vs. 64 to 77%). Budget calculations for the eastern Mediterranean Sea demonstrate that the weakening of the antiestuarine circulation and the enhanced regeneration of P both contributed to a significant increase in deep-water HPO42? concentrations during sapropel S1 times. Provided that sufficient vertical mixing occurred, enhanced regeneration of P at the seafloor may have played a key role in maintaining increased productivity during sapropel S1 formation
Oxidation and origin of organic matter in surficial eastern Mediterranean hemipelagic sediments
No full textAerobic mineralisation of Corg in surface sediments of the deep (>2000 m water depth) eastern Mediterranean Sea has been quantified by analysis of detailed box core Corg concentration versus depth profiles and the modelling environment for early diagenetic problems MEDIA. The reactive fraction comprises 60–80% of the total Corg reaching the sediments and is largely oxidised within the surficial 10 cm. A non-reactive Corg fraction (GNR) dominates at depths >10 cm, and makes up 20–40% of the total Corg flux to the sediments. First-order rate constants for decomposition of the reactive fraction calculated from the Corg profiles range from 5.4 × 10-3 to 8.0 × 10-3 y-1 to 8.0 × 10-3 y-1. Total mineralization rates in the surface sediment are between 1.7 and 2.6 µmol C cm-2 y-1 and thus are typical for oligotrophic, deep-sea environments. The low fluxes and rapid remineralisation of Corg are accompanied by 210Pbexcess surface mixed layers which are only 2 cm deep, among the thinnest reported for oxygenated marine sediments. Model results indicate a mismatch between the Corg profiles and O2 microprofiles which were measured onboard ship. This can be attributed to a combination of decompression artefacts affecting onboard measurement of the O2 profiles or the leakage of oxygen into the core during handling on deck. Furthermore, the used Db values, based on 210Pb, may not be fully appropriate; calculations with higher Db values improve the O2 fits. The surficial sediment δ13Corg values of -22 become less negative with increasing depth and decreasing Corg concentrations. The major 13C change occurs in the top 3 to 4 cm and coincides with the interval where most of the organic carbon oxidation takes place. This indicates that the reactive fraction of organic matter, commonly assumed to be marine, has a more negative δ13Corg than the refractory fraction, usually held to be terrestrial. Palaeoproductivity estimates calculated from the sediment data by means of literature algorithms yield low surface productivities (12–88 gC m-2 y-1), which are in good agreement with field measurements of primary productivity in other studies. Such values are, however, significantly lower than those indicated by recent productivity maps of the area derived from satellite imagery (>100 gC m-2 y-1)
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Impact of biogeochemical processes on pH dynamics in marine systems
No full textUptake of anthropogenic carbon dioxide (CO2) from the atmosphere has resulted in a range of changes in ocean chemistry, including the lowering of pH, collectively referred to as ocean acidification. Rates of coastal-zone acidification exceed those of the open ocean since coastal-ocean pH is influenced by many other processes than absorption of CO2 alone. These processes do not only play a role in long-term acidification but also impact pH on seasonal timescales. Examples are enhanced atmospheric sulphur and nitrogen deposition, as well as eutrophication, the latter which can additionally result in the development of low-oxygen waters. The degree to which these processes induce a change in pH depends both on their rates and the extent to which the water can buffer acid production or consumption. This acid-base buffering capacity has been shown to decrease substantially in hypoxic waters, suggesting that low-oxygen conditions exacerbate ocean acidification. In this dissertation the key factors controlling the seasonal pH variability and longer-term pH changes in both the coastal and open ocean were examined, showing that buffering mechanisms play a crucial role in the impact of any biogeochemical or physical process on pH.
Monthly or seasonal water-column chemistry and process-rate measurements in a transiently hypoxic coastal marine basin indicate that, despite generally higher process rates in the surface water of the basin, the amplitude of pH variability as mainly governed by the balance between primary production and respiration is greater in the seasonally-hypoxic bottom water, due to a considerable reduction of its acid-base buffering capacity in summer. A proton budget, based on these measurements and set up for each season, shows that the net change in pH is much smaller than the flux of protons induced by each of the individual processes.
The interplay between absorption of atmospheric CO2 and atmospheric sulphur and nitrogen deposition in the coastal ocean was found to depend on the water-column concentration of CO2 relative to the atmosphere. If the atmospheric concentration surpasses that of the surface water, then this part of the coastal ocean is most sensitive to CO2-induced acidification, but least affected by additional acidification resulting from atmospheric acid deposition. Although coastal seas will become up to a factor 4 more sensitive to atmospheric deposition-induced acidification between the present-day and 2100, the annual change in proton concentration will only increase by 28% at most.
Finally, a set of general expressions describing the sensitivity of pH to a change in ocean chemistry was derived. These expressions, which can include as many acid-base systems as relevant and are thus generally applicable, were tested on several long-term open ocean data sets. For each of these sites, pH can be properly predicted if seasonal cycles of temperature, salinity, total alkalinity (TA) and the total concentrations of acid-base species are known. By the end of the 21st century a change in most acid-base parameters will induce a comparably greater pH excursion. This increased vulnerability is driven by enhanced CO2 concentrations and slightly moderated by the projected global warming
Deoxygenation and organic carbon burial in past oceans
No full textWithin the content of this thesis, I aim to qualitatively and quantitatively assess the driving mechanisms and consequences of marine deoxygenation and CORG burial in a greenhouse world (Fig. 1.5). The focus lies on instances of locally and globally enhanced deoxygenation (Fig. 1.6): the Holocene Baltic Sea (Chapter 2), three sapropels deposited in the Pliocene and Quaternary Mediterranean Sea (Chapter 3), the PETM (Chapters 3 and 4), OAE2 (Chapters 3 and 5) and Toarcian Oceanic Anoxic Event (Chapter 3). Key questions I aim to answer are the causes of deoxygenation on a local (Chapter 2) and global (Chapter 4) scale, the controls of P mineral formation on P recycling under anoxia (Chapter 3) and the global impact of enhanced redox-driven P recycling (Chapters 3 ‒ 5), and the effect of redox-driven CORG burial on the global carbon cycle (Chapters 4 and 5). In order to unravel the causes of extensive deoxygenation and its impact on CORG burial, I paired new geochemical and palynological sediment analyses, with the analysis of existing data sets and biogeochemical box modelling.
In summary, this thesis provides a framework for the causes and consequences of marine
deoxygenation and CORG burial. Warming, both regional (Chapter 2) and global (Chapters 3 and 4), promotes the loss of dissolved O2 from ocean waters and the accumulation of CORG in marine sediments. Deoxygenation enhances P recycling relative to CORG. This recycling was more pronounced during greenhouse periods than at present (Chapters 2 – 4). During the PETM (Chapter 4) and OAE2 (Chapter 5), the geochemical cascade leading to the buildup of CORG in marine sediments was caused by increased CO2 emissions. The burial of CORG had a profound effect on the exogenic carbon cycle, resulting in the drawdown of CO2 from the atmosphere.
The findings presented in this thesis provide indications for the possible future evolution of marine biogeochemical cycles. The projected increase in temperature (IPCC, 2013) will lead to loss of O2 from open ocean waters, while it may also hinder the recovery of coastal areas (e.g. the Baltic Sea) from deoxygenation, upon nutrient input reductions (Chapter 2). Acidification will not only affect calcifying organisms and marine alkalinity, but may enhance P recycling and, hence, eutrophication (Chapter 3). As the oceans warm and CaCO3 dissolution spreads into more areas of the deep ocean (e.g. Sulpis et al., 2018), P recycling
from deep marine sediments may increase. The impact of CORG burial on atmospheric CO2 (Chapters 4 and 5), even during a mild deoxygenation event such as the PETM (Chapter 4), underlines the importance of including this process and its causes in global biogeochemical and climate models when assessing the lifetime of our emissions in the atmosphere
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