296 research outputs found
Chemical weathering outputs from the flood plain of the Ganga
Transport of sediment across riverine flood plains contributes a significant but poorly constrained
fraction of the total chemical weathering fluxes from rapidly eroding mountain belts which has impor-
tant implications for chemical fluxes to the oceans and the impact of orogens on long term climate. We
report water and bedload chemical analyses from the Ganges flood-plain, a major transit reservoir of
sediment from the Himalayan orogen. Our data comprise six major southern tributaries to the Ganga,
31 additional analyses of major rivers from the Himalayan front in Nepal, 79 samples of the Ganga
collected close to the mouth below the Farakka barrage every two weeks over three years and 67 water
and 8 bedload samples from tributaries confined to the Ganga flood plain,. The flood plain tributaries
are characterised by a shallow d 18 O - dD array, compared to the meteoric water line, with a low dD excess
from evaporative loss from the flood plain which is mirrored in the higher dD excess of the mountain
rivers in Nepal. The stable-isotope data confirms that the waters in the flood plain tributaries are domi-
nantly derived from flood plain rainfall and not by redistribution of waters from the mountains. The
flood plain tributaries are chemically distinct from the major Himalayan rivers. They can be divided
into two groups. Tributaries from a small area around the Kosi river have 87 Sr/ 86 Sr ratios > 0.75 and
molar Na/Ca ratios as high as 6. Tributaries from the rest of the flood plain have 87 Sr/ 86 Sr ratios <0.74
and most have Na/Ca ratios <1. One sample of the Gomti river and seven small adjacent tributaries
have elevated Na concentrations likely caused by dissolution of Na carbonate salts. The compositions
of the carbonate and silicate components of the sediments were determined from sequential leaches of
floodplain bedloads and these were used to partition the dissolved cation load between silicate and car-
bonate sources. The 87 Sr/ 86 Sr and Sr/Ca ratios of the carbonate inputs were derived from the ace-
tic-acid leach compositions and silicate Na/Ca and 87 Sr/ 86 Sr ratios derived from silicate residues from
leaching. Modelling based on the 87 Sr/ 86 Sr and Sr/Ca ratios of the carbonate inputs and 87 Sr/ 86 Sr ratios
of the silicates indicates that the flood plain waters have lost up to 70% of their Ca (average ~ 50%) to
precipitation of secondary calcite which is abundant as a diagenetic cement in the flood plain sedi-
ments. 31% of the Sr, 8% of the Ca and 45% of the Mg are calculated to be derived from silicate miner-
als. Because of significant evaporative loss of water across the flood plain, and in the absence of hy-
drological data for flood plain tributaries, chemical weathering fluxes from the flood plain are best
calculated by mass balance of the Na, K, Ca, Mg, Sr, SO 4 and 87 Sr/ 86 Sr compositions of the inputs,
comprising the flood plain tributaries, Himalayan rivers and southern rivers, with the chemical dis-
charge in the Ganga at Farakka. The calculated fluxes from the flood plain for Na, K, Ca and Mg are
within error of those estimated from changes in sediment chemistry across the flood plain (Lupker et
al., 2012, Geochemica Cosmochimica Acta). Flood plain weathering supplies between 33 and 48% of
the major cation and Sr fluxes and 58% of the alkalinity flux carried by the Ganga at Farakka which
compares with 24% supplied by Himalayan rivers and 18% by the southern tributaries
Temporal controls on silicic acid utilisation along the West Antarctic Peninsula
The impact of climatic change along the Antarctica Peninsula has been widely debated in light of atmospheric/oceanic warming and increases in glacial melt over the past half century. Particular concern exists over the impact of these changes on marine ecosystems, not only on primary producers but also on higher trophic levels. Here we present a record detailing the historical controls on the biogeochemical cycling of silicic acid [Si(OH)4] on the west Antarctica Peninsula margin, a region in which the modern phytoplankton environment is constrained by seasonal sea-ice. We demonstrate that Si(OH)4 cycling through the Holocene alternates between being primarily regulated by sea-ice or glacial discharge from the surrounding grounded ice-sheet. With further climate-driven change and melting forecast for the 21st Century, our findings document the potential for biogeochemical cycling and multi-trophic interactions along the peninsula to be increasingly regulated by glacial discharge, altering food-web interactions
Iron, silicate, and light co-limitation of three Southern Ocean diatom species
The effect of combined iron, silicate, and light co-limitation was investigated in the three diatom species Actinocyclus sp. Ehrenberg, Chaetoceros dichaeta Ehrenberg, and Chaetoceros debilis Cleve, isolated from the Southern Ocean (SO). Growth of all species was co-limited by iron and silicate, reflected in a significant increase in the number of cell divisions compared to the control. Lowest relative Si uptake and drastic frustule malformation was found under iron and silicate co-limitation in C. dichaeta, while Si limitation in general caused cell elongation in both Chaetoceros species. Higher light intensities similar to SO surface conditions showed a negative impact on growth of C. dichaeta and Actinocyclus sp. and no effect on C. debilis. This is in contrast to the assumed light limitation of SO diatoms due to deep wind driven mixing. Our results suggest that growth and species composition of Southern Ocean diatoms is influenced by a sensitive interaction of the abiotic factors, iron, silicate, and light
Effect of type and concentration of ballasting particles on sinking rate of marine snow produced by the Appendicularian Oikopleura dioica
Ballast material (organic, opal, calcite, lithogenic) is suggested to affect sinking speed of aggregates in the ocean. Here, we tested this hypothesis by incubating appendicularians in suspensions of different algae or Saharan dust, and observing the sinking speed of the marine snow formed by their discarded houses. We show that calcite increases the sinking speeds of aggregates by ~100% and lithogenic material by ~150% while opal only has a minor effect. Furthermore the effect of ballast particle concentration was causing a 33 m d(-1) increase in sinking speed for a 5Ă10(5) ”m(3) ml(-1) increase in particle concentration, near independent on ballast type. We finally compare our observations to the literature and stress the need to generate aggregates similar to those in nature in order to get realistic estimates of the impact of ballast particles on sinking speeds
Silica burial enhanced by iron limitation in oceanic upwelling margins
In large swaths of the ocean, primary production by diatoms may be limited by the availability of silica, which in turn limits the biological uptake of carbon dioxide. The burial of biogenic silica in the form of opal is the main sink of marine silicon. Opal burial occurs in equal parts in iron-limited open-ocean provinces and upwelling margins, especially the eastern Pacific upwelling zone. However, it is unclear why opal burial is so efficient in this margin. Here we measure fluxes of biogenic material, concentrations of diatom-bound iron and silicon isotope ratios using sediment traps and a sediment core from the Gulf of California upwelling margin. In the sediment trap material, we find that periods of intense upwelling are associated with transient iron limitation that results in a high export of silica relative to organic carbon. A similar correlation between enhanced silica burial and iron limitation is evident in the sediment core, which spans the past 26,000 years. A global compilation also indicates that hotspots of silicon burial in the ocean are all characterized by high silica to organic carbon export ratios, a diagnostic trait for diatoms growing under iron stress. We therefore propose that prevailing conditions of silica limitation in the ocean are largely caused by iron deficiency imposing an indirect constraint on oceanic carbon uptake
Suppression of charged particle production at large transverse momentum in central Pb-Pb collisions at TeV
Inclusive transverse momentum spectra of primary charged particles in Pb-Pb
collisions at = 2.76 TeV have been measured by the ALICE
Collaboration at the LHC. The data are presented for central and peripheral
collisions, corresponding to 0-5% and 70-80% of the hadronic Pb-Pb cross
section. The measured charged particle spectra in and GeV/ are compared to the expectation in pp collisions at the same
, scaled by the number of underlying nucleon-nucleon
collisions. The comparison is expressed in terms of the nuclear modification
factor . The result indicates only weak medium effects ( 0.7) in peripheral collisions. In central collisions,
reaches a minimum of about 0.14 at -7GeV/ and increases
significantly at larger . The measured suppression of high- particles is stronger than that observed at lower collision energies,
indicating that a very dense medium is formed in central Pb-Pb collisions at
the LHC.Comment: 15 pages, 5 captioned figures, 3 tables, authors from page 10,
published version, figures at http://aliceinfo.cern.ch/ArtSubmission/node/98
Two-pion Bose-Einstein correlations in central Pb-Pb collisions at = 2.76 TeV
The first measurement of two-pion Bose-Einstein correlations in central Pb-Pb
collisions at TeV at the Large Hadron Collider is
presented. We observe a growing trend with energy now not only for the
longitudinal and the outward but also for the sideward pion source radius. The
pion homogeneity volume and the decoupling time are significantly larger than
those measured at RHIC.Comment: 17 pages, 5 captioned figures, 1 table, authors from page 12,
published version, figures at
http://aliceinfo.cern.ch/ArtSubmission/node/388
Observation of associated near-side and away-side long-range correlations in âsNN=5.02ââTeV proton-lead collisions with the ATLAS detector
Two-particle correlations in relative azimuthal angle (ÎÏ) and pseudorapidity (Îη) are measured in âsNN=5.02ââTeV p+Pb collisions using the ATLAS detector at the LHC. The measurements are performed using approximately 1ââÎŒb-1 of data as a function of transverse momentum (pT) and the transverse energy (ÎŁETPb) summed over 3.1<η<4.9 in the direction of the Pb beam. The correlation function, constructed from charged particles, exhibits a long-range (2<|Îη|<5) ânear-sideâ (ÎÏâŒ0) correlation that grows rapidly with increasing ÎŁETPb. A long-range âaway-sideâ (ÎÏâŒÏ) correlation, obtained by subtracting the expected contributions from recoiling dijets and other sources estimated using events with small ÎŁETPb, is found to match the near-side correlation in magnitude, shape (in Îη and ÎÏ) and ÎŁETPb dependence. The resultant ÎÏ correlation is approximately symmetric about Ï/2, and is consistent with a dominant cosâĄ2ÎÏ modulation for all ÎŁETPb ranges and particle pT
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