Climatic indicators in an ice core from the Yukon [abstract]
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Stable isotope data obtained from snow and ice cores retrieved from an altitude of 5340m on Mt. Logan (60°30'N; 140°36'W) indicate that isotopic seasons are not generally in phase with calendar seasons. The former are phase lagged with respect to the latter by up to several months and appear to be correlated with SST'S and ocean heat transfer curves and/or the position of the Aleutian low rather than with air temperature or the temperature difference between the ocean surface and the core site.Keywords:
Ice core
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Sea salt aerosol
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Antarctica is a major component in Earth’s climate system, as the equator to pole temperature gradient controls the characteristics of the general circulation of the atmosphere. Antarctica is also very useful to understand climate variability, as past climate information preserved in the ice may help extend the short observational records. However, the ice core drilling locations are unevenly spread across the glaciated continent, and the temperature reconstructions from the high elevation East Antarctic plateau suffer from poor temporal resolution, because low snow accumulation hampers our interpretation of water isotopes. Here, we present new temperature reconstructions from the Aurora Basin North (ABN, 77°S, 111°E, 2700 masl) ice core. First, we use the regional atmospheric model MAR to characterize the recent climate at ABN, and show that precipitation events are intermittent, and occur under temperature 2°C warmer than average. The large precipitation events are marked in the snow isotopes with δ18O values on par with summer levels, even during the winter, as attested by snow measurements and the isotope-enabled atmospheric model ECHAM5-wiso. Precipitations are consistently associated with a blocking on the Wilkes Land coast, North-East of ABN, and the blockings are more likely to occur during negative phases of the Southern Annular Mode (SAM), the main mode of variability in the southern hemisphere climate. Consequently, SAM positive phases are marked by cold temperatures at ABN, but not necessarily low δ18O, as precipitations may be weakened. The temperature reconstructed from the δ18O in the 300-m-deep, 2000-year ice core drilled at ABN supports stable conditions, with a temperature remaining within a ± 1°C range. We present a second temperature reconstruction from the same core, based on the inversion of borehole temperature and past firn temperature gradients, estimated with the stable isotope composition of Ar and N2 gases trapped in bubbles. This second temperature reconstruction, representative of changes in the snow, suggests that temperature at ABN was about 3°C colder during two periods of the last 2000 years: from 300 to 550 CE, and from 1000 to 1400 CE. This medieval cold anomaly is concurrent with a positive SAM phase, and could not be identified from the δ18O alone. This work highlights the importance of using multiple proxies to determine past temperature variability in Antarctica, as δ18O may be biased towards warm precipitation events.
Ice core
δ18O
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Ice core
δ18O
Firn
Paleoclimatology
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Abstract. A 181.8 m ice core was recovered from a borehole drilled into bedrock on the western plateau of Mt El'brus (43°20′53.9′′ N, 42°25′36.0′′ E; 5115 m a.s.l.) in the Caucasus, Russia, in 2009 (Mikhalenko et al., 2015). Here, we report on the results of the water stable isotope composition from this ice core with additional data from the shallow cores. The distinct seasonal cycle of the isotopic composition allows dating by annual layer counting. Dating has been performed for the upper 126 m of the deep core combined with 20 m from the shallow cores. The whole record covers 100 years, from 2013 back to 1914. Due to the high accumulation rate (1380 mm w.e. year−1) and limited melting, we obtained isotopic composition and accumulation rate records with seasonal resolution. These values were compared with available meteorological data from 13 weather stations in the region and also with atmosphere circulation indices, back-trajectory calculations, and Global Network of Isotopes in Precipitation (GNIP) data in order to decipher the drivers of accumulation and ice core isotopic composition in the Caucasus region. In the warm season (May–October) the isotopic composition depends on local temperatures, but the correlation is not persistent over time, while in the cold season (November–April), atmospheric circulation is the predominant driver of the ice core's isotopic composition. The snow accumulation rate correlates well with the precipitation rate in the region all year round, which made it possible to reconstruct and expand the precipitation record at the Caucasus highlands from 1914 until 1966, when reliable meteorological observations of precipitation at high elevation began.
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δ18O
Anomaly (physics)
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The snow isotopic composition (δ 18 O and δD) of two shallow cores from the high accumulation summit region of Law Dome, east Antarctica, was measured at monthly resolution over the 1980–1992 period. While the δ 18 O or δD signals clearly reflect the local temperature cycle, the deuterium excess ( d = δD ‐ 8δ 18 O) is shifted with respect to δ 18 O cycle by a 4 months lag. Interpretation of this phase lag is investigated using both an Atmospheric General Circulation Model (AGCM), which includes the water isotopic cycles, and a simple isotopic model, which better describes the microphysical processes within the cloud. Using this dual approach, we show that the seasonality of δ 18 O and d at Law Dome summit results from a combination of the southern ocean temperature cycle (shifted by 2–3 months with respect to the local insolation) and seasonal moisture origin changes due to a strong contribution of the local ocean when ice free. Both approaches are consistent with a dominant temperate to subtropical moisture origin. We thus demonstrate from our present‐day subseasonal study that the record of d in the Dome Summit South (DSS) deep ice core represents a potential tool for identifying changes in Southern Ocean temperatures and/or sea ice cover at the scale of the past thousand years.
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Dome (geology)
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The Antarctic Peninsula region has exprienced a long-term warming trend over the twentieth century, with the 1971-90 mean at Faraday being 1.9°C warmer than the mean over 1903-41 based on expedition reports. For the period prior to 1900, there is conflicting evidence from different data sources. An initial interpretation of isotopic data from ice cores suggests that the nineteenth century was warmer than the twentieth century. In contrast, snow accumulation rate data for the nineteenth century from the same ice cores suggest lower temperatures. Here we investigate these facts by studying the links between atmospheric temperature over the Antarctic Peninsula, circulation parameters and isotopic data over the period of instrumental records. We show that the relationships between these variables are complex and highly spatially variable. In particular, the correlations between temperature and δ 18 O and δD are generally of the order r = 0.5 or less on timescales of one to five years. Conflicts between evidence from accumulation rate and isotopic data appear to reflect the influence of source-region effects on the isotope records. To unravel the complex isotopic records available for the Peninsula region better; additional cores must be analysed for both δ 18 O and 8D at the same site.
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Little ice age
Temperature record
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The IPCC 5th Assessment Report states that there are insufficient Southern
Hemisphere climate records to adequately assess climate change in much
of this region. Ice cores provide excellent archives of past climate, as they contain
a rich record of past environmental tracers archived in trapped air and
precipitation. However Antarctic ice cores, especially those from East Antarctica,
are limited in quantity and spatial coverage. To help address this, a 120
m ice core was drilled on Mill Island, East Antarctica (65° 30' S, 100° 40' E).
Mill Island is one of the most northerly ice coring sites in East Antarctica, and
is located in a region with sparse ice core data.
The specific project aims were: 1) To produce a high resolution, welldated
record of water stable isotopes (δ18O, δD), and trace ion chemistry (sea
salts, sulphate, methanesulphonic acid); 2) to investigate the seasonal and
interannual variability of sea salts, in order to reveal which climate factors
influence the Mill Island record; 3) to perform a regional comparison of δ18O
and snow accumulation rate with nearby existing climate records from ice
cores, observational stations, and atmospheric models, in order to seek the
optimal method for temperature reconstruction using the Mill Island ice core
record.
Hydrogen peroxide, water stable isotopes, and trace ion chemistry were
measured at high resolution throughout the entire core. The ice core was
dated using a combination of chemical species, but primarily using water stable
isotopes. The Mill Island ice core contains 97 years of climate record (1913
{ 2009), and has a mean snow accumulation of 1.35 metres (ice-equivalent)
per year (mIE/yr). Concentrations of trace ions were generally higher than
at other Antarctic ice core sites (e.g., mean sodium levels were 254 μEq/L).
The full trace ion record contained a mix of periods with well-defined seasonal
cycles and periods with weak seasonality and a higher baseline. An abrupt
change was observed in the sea salt record in the mid-1930s. This may be
related to a significant change in the local ice-scape. Sea salts were compared
with instrumental data, including atmospheric models and satellite-derived sea
ice concentration, to investigate influences on the Mill Island ice core record.
The mean annual sea salt record did not show a correlation with wind speed.
Instead, sea ice concentration to the east of Mill Island was found to be likely
influencing the annual mean sea salt record, at least during the period of 1979
to 2009. A mechanism involving formation of frost flowers on sea ice was proposed to explain the extremely high sea salt concentration. Post-depositional
migration of magnesium and methanesulphonic acid were observed in the trace
ion record, and for the first time, migration of sodium and chloride were observed.
Snow accumulation rate was compared with snow accumulation or precipitation
record from nearby sites. The Mill Island snow accumulation was
found to be influenced by local orography, i.e., the annual snow accumulation
record is not strongly related with precipitation in nearby sites. The Zonal
Wave Three (ZW3), large scale atmospheric mode, modulates precipitation at
nearby Law Dome, and to a lesser extent, modulates Mill Island precipitation.
Snow accumulation and δ18O were compared with precipitation and temperature
data from atmospheric models. The climatology of precipitation at
Mill Island shows evidence of higher snowfall during winter, consistent with
other Antarctic sites. The linear monthly ice core dating was adjusted using
the precipitation climatology, and the adjusted δ18O record resulted in a
warmer annual signal. This finding indicates that without this adjustment,
there is a small cold bias in annual temperature reconstructions from ice cores
that share this elevated winter precipitation. This bias should be considered
when reconstructing temperatures where climate trends differ with season and
when comparing with other temperature reconstructions (e.g., terrestrial or
ocean based records).
In situ temperature data (e.g., a co-located Automatic Weather Station)
are not available at Mill Island. Instead, the annual mean δ18O record was
compared with atmospheric reanalysis model output temperature at Mill Island.
The correlation was found to be statistically insignificant. To attempt
a more accurate palaeothermometer reconstruction, the annual record was divided
into summer and winter \windows with the maximum δ18O value set
as the summer window centre, and the minimum δ18O value set as the winter
window centre. It was found that when using narrow summer and winter windows,
the δ18O value was significantly correlated with December to April mean
model temperatures and May to July mean model temperatures, respectively.
Ice core
δ18O
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We examine the quality of atmospherically deposited ion and isotope signals in an ice core taken from a periodically melting ice field, Lomonosovfonna in central Spitsbergen, Svalbard. The aim is to determine the degree to which the signals are altered by periodic melting of the ice. We use three diagnostics: (1) the relation between peak values in the ice chemical and isotopic record and ice facies type, (2) the number of apparent annual cycles in these records compared with independently determined number of years represented in the ice core, and (3) a statistical comparison of the isotopic record in the ice core and the isotope records from coastal stations from the same region. We find that during warm summers, as much as 50% of the annual accumulation may melt and percolate into the firn; in a median year this decreases to ∼25%. As a consequence of percolation, the most mobile acids show up to 50% higher concentrations in bubble‐poor ice facies compared with facies that are less affected by melt. Most of the other chemical species are less affected than the strong acids, and the stable water isotopes show little evidence of mobility. Annual or biannual cycles are detected in most parameters, and the water isotope record has a comparable statistical distribution to isotopic records from coastal stations. We conclude that ice cores from sites like Lomonosovfonna contain a useful environmental record, despite melt events and percolation and that most parameters preserve an annual, or in the worst cases, a biannual atmospheric signal.
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Firn
δ18O
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Abstract. Water stable isotope ratios and net snow accumulation in ice cores are commonly interpreted as temperature or precipitation proxies. However, only in a few cases has a direct calibration with instrumental data been attempted. In this study we took advantage of the dense network of observations in the European Alpine region to rigorously test the relationship of the annual and seasonal resolved proxy data from two highly resolved ice cores with local temperature and precipitation. We focused on the time period 1961–2001 with the highest amount and quality of meteorological data and the minimal uncertainty in ice core dating (±1 year). The two ice cores were retrieved from the Fiescherhorn glacier (northern Alps, 3900 m a.s.l.), and Grenzgletscher (southern Alps, 4200 m a.s.l.). A parallel core from the Fiescherhorn glacier allowed assessing the reproducibility of the ice core proxy data. Due to the orographic barrier, the two flanks of the Alpine chain are affected by distinct patterns of precipitation. The different location of the two glaciers therefore offers a unique opportunity to test whether such a specific setting is reflected in the proxy data. On a seasonal scale a high fraction of δ18O variability was explained by the seasonal cycle of temperature (~60% for the ice cores, ~70% for the nearby stations of the Global Network of Isotopes in Precipitation – GNIP). When the seasonality is removed, the correlations decrease for all sites, indicating that factors other than temperature such as changing moisture sources and/or precipitation regimes affect the isotopic signal on this timescale. Post-depositional phenomena may additionally modify the ice core data. On an annual scale, the δ18O/temperature relationship was significant at the Fiescherhorn, whereas for Grenzgletscher this was the case only when weighting the temperature with precipitation. In both cases the fraction of interannual temperature variability explained was ~20%, comparable to the values obtained from the GNIP stations data. Consistently with previous studies, we found an altitude effect for the δ18O of −0.17‰/100 m for an extended elevation range combining data of the two ice core sites and four GNIP stations. Significant correlations between net accumulation and precipitation were observed for Grenzgletscher during the entire period of investigation, whereas for Fiescherhorn this was the case only for the less recent period (1961–1977). Local phenomena, probably related to wind, seem to partly disturb the Fiescherhorn accumulation record. Spatial correlation analysis shows the two glaciers to be influenced by different precipitation regimes, with the Grenzgletscher reflecting the characteristic precipitation regime south of the Alps and the Fiescherhorn accumulation showing a pattern more closely linked to northern Alpine stations.
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Proxy (statistics)
δ18O
Orographic lift
Seasonality
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Oxygen isotope variations in ice cores from Bolivia and Peru are highly correlated with sea surface temperatures (SSTs) across the equatorial Pacific Ocean, which are closely linked to ENSO variability. Circulation anomalies associated with this variability control moisture flux from the equatorial and tropical Atlantic Ocean and Amazon Basin to the ice core sites. Below average SSTs lead to higher accumulation rates and isotopically lighter snow; such conditions are also associated with lower atmospheric freezing levels. During warm events, opposite conditions prevail. Oxygen isotope variations in an ice core from the Himalayas also reflect SST variations in the equatorial Pacific Ocean, pointing to the prospect of reconstructing low latitude circulation anomalies from a network of ice cores in selected locations.
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Atmospheric Circulation
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