1,721,223 research outputs found
An oxygen isotope record fromthe Foscagno rock-glacier ice core,Upper Valtellina, Italian Central Alps
No full textNew high-resolution isotopic records (delta O-18, delta D and deuterium excess), from an ice core drilled in the Foscagno rock glacier ( Italian Central Alps), are presented. The delta O-18 data suggest a clear division between an upper part (2.5 and 4 m), showing relatively homogeneous values, and a middle part (4 - 7.65 m), showing seasonal variations of this parameter. The isotopic analyses confirm previous results (crystallographic and chemical analyses) suggesting a division of this relict glacier ice body into an upper part, between 2.5 and 4 m, where melting and refreezing processes occur, and a middle part, between 4 and 7.65 m, where the isotopic signal is preserved. Larger deuterium excess variations (d = delta D - delta*delta O-18) are found in the massive ice (below 4 m depth) rather than in the overlying ice. These are in antiphase with the delta O-18 but without any clear correspondence with the presence of the debris layers. Postdepositional processes could have affected, at least partially, the isotopic content of the original precipitation. The radiocarbon dating of a leaf (Salix spp.) found in the massive ice from another nearby borehole in the same rock glacier gave a calendar age ranging between AD 765 and 1260. The expected delta O-18 values of the present-day precipitation in the Foscagno valley are of the same order as those found in the massive ice (-12.4%). This similarity would suggest climate conditions not very different from present day, in good agreement with other available palaeoclimate reconstructions for this period. However, only more abundant precipitation would make the existence of a glacier possible in a climate not very different from that of the present
Oxygen isotopic composition of ice samples from the Hells Gate and Backstairs Passage ice shelves (Victoria Land, Antartica): evidence of bottom freezing
No full textIsotopic composition of Holocene shells from raised beaches and ice shelves at Terra Nova Bay (Northern Victoria Land, Antarctica).
No full textSeventy-six samples of calcium carbonate from shells
and skeletons of fossil marine. organisms collected in
the area of Terra Nova Bay (Victoria Land, Antarctica)
have been measured for their 13C/12C and 180 /160 ratios.
Solitary corals have been measured along with bryozoans,
cirripeds, serpulids and pelecypods coming from fossil
beaches, supraglacial debris and dirt cones. Age determinations
have been carried out by means of 14C measurements.
The isotopic results obtained may be summarized as
follows: the measurements carried out on corals agree well
with previous measurements clearly indicating that these
organisms do not precipitate the CaC03 of their skeleton
under isotopic equilibrium conditions with environmental
water. The 0 180 (COi-) are probably shifted by a constant
value when compared with the 0180(~Oi-) values at
equilibrium. The isotopic results obtained from bryozoans
do not agree with the data reported in the literature. Our
specimens show <S180 values away from isotopic equilibrium
condition and <S13Cvalues very close to those of "normal"
marine carbonate.
Four different species of pelecypods have been studied,
their age ranging from recent to about 7000 years B.P.
In the case of Adamussium colbecki, there seems to be: a;
relationship between age and 8180 (C0 32-), older specimens
yielding more negative < 5 180 values. The isotopic values
obtained from cirripeds suggest similar conclusions.
Finally, both the oxygen and, carbon isotopic values of
serpulids are not very far from equilibrium conditions.
The main interest of these preliminary data is represented
by the possibility of detecting relatively. small climatic
changes even under these extreme conditions and by the
possibility of obtaining information on the biological
behaviour of various marine organisms living in this peculiar
environment
Oxygen isotopic composition of ice samples from the Hells Gate and Backstairs Passage ice shelves (Northern Victoria Land, Antarctica): evidence for bottom freezing.
No full textDuring the 1986/87 and 1987/88 Italian Antarctic
expeditions a number of ice samples were col~ected f~om
different locations. Sixty-five samples coming mainly
from the Hells Gate and Backstairs Passage ice shelves
have been studied for their -oxygen isotopic composition.
Very negative <5 180 values are characteristic of continental
snowfall in Antarctica /(aroun.fl:-21±3 per mil for snow
deposited near the coastal area). The <5180 values obtained
from seawater edge ice and ice beneath dirt cones range
from -0.80 to +2.66 per mil, suggesting that the origin of
this ice is essentially from seawater freezing. This is also
proven by the finding of shells and skeletons of I?arine
organisms included in the ice and is in agreement with the
conclusions of previous studies on Antarctic ice shelves.
Ice samples from the upper section of dirt cones show
<5 180 values from -12.68 to -6.15 per mil, which are intermediate
between those of seawater ice and continental ice.
These values are probably explained by the mixing of
water from the melting of the snowdrift ice capping the
cones and the seawater ice forming their main body.
The ice core of the moraine of Corner Glacier, further
inland, shows <5 180 values from -31.81 to -26.40, in
agreement with a continental origin of the ice. This ice is
clearly formed by snow which fell at higher elevations, as
proven by the remarkable 180 depletion of these samples
as compared to local snowfall.
It is apparent that the origin of ice can be well identified
by isotope measurements, irrespective of sample
location and appearance
Ten years of oxygen and hydrogen isotopic composition of daily precipitation at Concordia station, East Antarctica
No full textHere we present a 10year
(20082017)
record of the isotopic composition of precipitation at
Dome C. The winterover
personnel at Concordia Station have been continuously collecting
daily precipitation since 2008: this represents a unique record of the isotopic composition of
precipitation from the Antarctic Plateau. Snow is collected on a raised platform (height: 1 m) in
the clean area of the station and analyzed for d18O, dD and deuterium excess. A significant positive
correlation between d18O (dD) of precipitation and 2m
air temperature is observed at different
temporal scale; the lowest temperature and isotopic values are preferentially observed
during winters affected by a strongly positive Southern Annular Mode index [Stenni et al., 2016].
We have compared the isotopic data of Concordia samples with meteorological data from the
Dome C AWS of the Programma Nazionale di Ricerche in Antartide (PNRA), as well as with the
isotopeenabled
atmospheric general circulation models ECHAM5wiso
and ECHAM6wiso,
in
order to improve the understanding of the mechanisms governing the isotopic composition of
precipitation on the Antarctic plateau, which are yet to be fully understood, as well as the postdepositional
effects altering the pristine isotopic signal of precipitation. The comprehension of
these physical processes is fundamental for reconstructing past temperature variations using
the isotopic records from ice cores drilled in low accumulation areas of Antarctica [Casado et
al., 2018], e.g., the upcoming Beyond EPICA drilling at Little Dome C. Very few temporal relationships
between temperature and snow isotopic composition, mostly limited in time or sampling
frequency, have been calculated, while spatial relationships have been traditionally used
to reconstruct past temperature from ice core isotopic records [MassonDelmotte
et al., 2008]
Non sorted polygons in Terra Nova Bay area (Victoria Land, Antarctica)
No full textIn the Terra Nova Bay fields of periglacial polygon are common. The develop on flat or gently sloping surfaces covered by a mantle of loose etherometric material, mainly of glacvial origin. The polygons have been described by topography, morphology and grain size analysis. Field data and interpretation suggest that the analized polygon fields are still covering ice bodies which represent the vanishing stage of previous larger glaciers, probably of Wisconsinian age
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
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