C51A-1015 0800h
Origin, Evolution and Age Considerations on a Hypogean Ice Deposit from a Cave in the Central Italian Alps
The LO LC 1650 "Abisso sul Margine dell'Alto Bregai" ice cave is located in the central Italian Alps east of the Como Lake at an altitude of 2030 m. One snow and two ice deposits are known in this cave, where ice has been found down to a depth of about 120 m below the entrance. We show the results of the integration of data about the cave area, the ice cave, its main hypogean ice deposit and the first core from the top surface of this ice block. The investigated ice deposit is located at a depth of about 80 m and is not reachable by snow. Ice crystals texture and c-axes fabric show the succession of clear ice layers to have formed by the freezing of shallow cave lakes one on top of the other. A relative chronology of three accumulation and three ablation events is summarised and some upper and lower bounds to the age of the ice block are discussed. Stable isotopes and chemical data point to an age younger than the end of the last glaciation and older than the onset of a detectable industrial signal. Dendrochronological evidences combine with these data in suggesting that probably the cored part of the ice deposit was already formed before the 13th century, which means before the Little Ice Age.
C51A-1016 0800h
Evidence From Tibetan Ice Core Records Concerning Climatic Influences on the Asian Monsoon During the 19th Century
Teleconnections between the North Atlantic and Asian monsoon regions on orbital and suborbital time scales have been documented in many proxy climate records. Here, the influence of this linkage on snow accumulation across the Tibetan Plateau during the latter part of the Little Ice Age (LIA) is examined using high-resolution ice core records from four sites. Since the end of the LIA ($\sim$1880), the net accumulation (A$_{n}$) time series from the Dasuopu ice cap ($28\deg$N, $85\deg$E) in the Himalayas has resembled the north central India summer monsoon rainfall (NCISMR) record. However, from $\sim$1810 to $\sim$1880 the ice core A$_{n}$ values were 50% higher than those of the NCISMR. This increase during the 19th century, which was contemporaneous with lower temperatures in the North Atlantic and a persistently negative early winter North Atlantic Oscillation (NAO) index, is argued to be the result of increased winter snowfall in the central Himalayas. In contrast to Dasuopu, the A$_{n}$ records from three sites to the north (Dunde at $38\deg$N, $96\deg$E, Puruogangri at $34\deg$N, $90\deg$E and Guliya at $35\deg$N, $81\deg$E) show lower rates during most of the 19th century. Although these glaciers receive a significant portion of their precipitation during the summer from a variety of competing weather systems besides the monsoon, such as mid-latitude westerlies and local thermally-induced storms, their accumulation records show similarities to the reconstructed all-India summer monsoon rainfall (AISMR) record over the last two centuries. The lower accumulation rates during the 19th century in these three ice core profiles and in the AISMR are possibly linked to the Dasuopu A$_{n}$ record (which in turn is linked to climate changes in the North Atlantic) by the inverse relationship between the Himalayan winter snow amounts and the subsequent summer monsoon intensity.
C51A-1017 0800h
Isotope/Air Temperature Relationships From Snow-Firn Core Time Series at the Northern and Southern Periphery of the Central Asian Mountain System
Spatial variability of stable isotope records from snow pits and snow-firn cores to depths of 21 m are used to examine isotope/air temperature relationships for three locations at the northern and southern periphery of the central Asian mountain system: the West Belukha glaciated Plateau in the Altai, the head of Inylchek Glacier in central Tien Shan, and the Bomi glacierized massif in southeastern Tibet. The $^{18}$O records show pronounced maxima and minima, interpreted to represent annual accumulation layers and verified with stratigraphic profiles. Mean $\delta$$^{18}$O from the Altai core is -13.6\permil, -16.0\permil for the Tien Shan core, and -16.3\permil for the southeastern Tibet core. Mean annual ranges of the $\delta$$^{18}$O signal vary from 9.5\permil in the Altai core, to 16.6\permil in the Tien Shan core, and 10.1\permil in the southeastern Tibet core. Mean deuterium excess varied from 10.9\permil for the Altai core to 23.0\permil for the Tien Shan core. Isotope/air temperature relationships are examined for variability of regression lines (slope and intercept) associated with internal and external moisture sources, which vary seasonally. Positive regression line slopes are observed for the Tien Shan and Altai Mountains, with minimum $\delta$$^{18}$O values associated with minimum winter temperatures. A negative regression line slope is observed at southeastern Tibet, with minimum $\delta$$^{18}$O values associated with heavy amounts of isotopically depleted precipitation occurring during the summer months at this location. These isotope records will aid in providing information important to the forecasting of global climate change linking regional responses with atmospheric circulation patterns from the North Atlantic to Pacific and Indian Oceans. These techniques can be applied to recently recovered deep (surface to bottom) ice cores from the central Tien Shan and Altai Mountains.
C51A-1018 0800h
Thirty-year Records Of Glacier Melting In The Nepal Himalayas
Two net balance records of neighboring glaciers under different conditions are compared to examine temporal variations in glacier melting in the Himalayas. No melt occurred at one site due to its high elevation, whereas a significant melt was observed every year at the other site. The difference between the net balances of the two glaciers is calculated as the _emelt index_Eof glacier-melting conditions. Comparing instrumental meteorological records at a rural station suggests that the calculated melt index is a good proxy of the summer mean air temperature at high elevations, where measured meteorological records are few. The melt index shows a decadal fluctuation with a major amplitude never before seen in previous ice core studies in the Himalayas. Ice cores from melting sites have been considered unreliable in reconstructing past climates since climatic signals in ice are disturbed by meltwater infiltration. However, we suggest a new use of a so-called bad ice core to glean information not obtainable from a good quality ice core alone.
C51A-1019 0800h
Recent Climate Change as Documented by Ice Core Evidence, Glacier Retreat, and Borehole Temperatures for the Gregoriev Ice Cap, Central Asia
From 1990 to 2003 five shallow ice cores were recovered from the summit of the Gregoriev Ice Cap ($41.98\deg$N; $77.916\deg$E; 4609 m asl) in the Tien Shan, Central Asia. The 1990 ice core was analyzed for stable isotopes, microparticle concentrations, major ions and total Beta-activity. The 2001 and 2003 cores were analyzed for stable isotopes and concentrations of radionuclides. The cores were dated using major both reference horizons and annual microparticle concentrations. The detailed stratigraphic records for the top sections of the 1990 and 2001 cores indicate a mean annual net accumulation of 0.35 m ice equivalent (i.e.) for that period compared with the net accumulation from 1963 to 1990 of 0.42 m i.e. At the 4450 m site a decrease of the firn pack depth from 9 to 6 meters has been observed between 1962 and 2003 and over the same period the concentration of infiltration ice increased as a result of greater melt water percolation. Stable isotopic enrichment is evident in the top sections of both the 1990 and 2003 cores. The local meteoric water line is compared with the global one and the relationship between $\delta$$^{18}$O in the cores and mean summer air temperature is compared with that from other glacier areas. Borehole temperatures measured on the top of the Gregoriev Ice Cap (4609 m) in 1990, 2001 and 2003 reveal a $\sim$$1\deg$C warming at 10 m depth over that period. Likewise from 1962 to 2003 boreholes drilled at 4450 m indicate temperature increases of $\sim$$2.5\deg$C at 10m and $0.5\deg$C at 30 m depth. A comparison of airborne images from 1943, 1956 and 1988 along with a satellite image from 2001 shows considerable retreat of the glacier terminus. The rate of retreat is compared to that determined from LIA moraine positions. These glaciological observations, suggesting that a regional warming is underway, are consistent with meteorological observations from the Tien Shan Meteorological Station.
C51A-1020 0800h
Evolution Of The Gregoriev Ice Cap (Tien Shan, Central Asia) For 1980-2050 Revealed By Mathematical Modeling
We develope two-dimensional flowline model to study an ice flow of the Gregoriev Ice Cap (Tien-Shan, Central Asia). The model, which takes into account the transverse change of glacier width, is tuned by data on measured surface velocity at the glacier front and the mass balance measurements in 1987 and 1988. The ice flow velocity varies from 0 to 3 m/year along glacier flowline. The velocity maximum is shifted from the lowest part toward the middle part of the Gregoriev Ice Cap for the studied period 1980-2050. The flow velocity near the front of glacier is decreased because of two reasons. The first is the negative mass balance that results in thinning of the glacier tongue. The second is the deeper penetration of the cold temperatures from the surface to the bed because of the glacier thinning in its low part. As a result, an internal heating decreases in this part of the glacier and moderates the flow. the seasonal temperature variations moderate/accelerate the horizontal ice flow near the surface. Due to that the additional significant longitudinal stresses arise in places of the surface slope change. These stresses can exceed the compressive/tensile strength and produce crevasses. The calculated changes of the glacier shape show the degradation and decrease of glaciation at the South slope of the Terskey Ala-Tau (Tien Shan).
C51A-1021 0800h
Mercury Concentrations In A 200-year Snow/Ice Core Record from Mont Blanc
Mercury pollution is a global phenomenon of increasing importance due to its strong toxicity in the environment. Quantifying the scale of Hg pollution since the onset of the industrial era, and distinguishing natural from anthropogenic sources of Hg, is a major scientific challenge. One means of resolving the historical Hg levels in the atmosphere is through the examination of polar and alpine ice cores. There are exceedingly few studies of Hg concentrations in the ice record. We present here the results of the Hg record extracted from a 140 m snow/ice core drilled in 1994 into the east slope of Dome du Gouter (4250 m elevation), approximately 1.5 km northwest of the Mont Blanc summit in the French-Italian Alps. Hg concentrations in the late 20th century segment of the core are approximately 10 times higher than the earliest samples (from approx. 1790 AD) and 3 times higher than concentrations in much of the 19th and early 20th centuries. Most of the concentrations are in general agreement with a mid-latitude Hg ice core study from the Rocky Mountains in the United States, though peak Hg concentrations are higher in this Alpine core, perhaps reflecting its closer proximity to industrial sources.
C51A-1022 0800h
Application Of Pollen Analysis For Seasonal Dating Of Alpine Glacier Ice Cores With Seasonal Missing Layers
Ice core studies from middle and low latitude glaciers have a problem that a reliable dating method has proved difficult due to wind erosion and vanishingly small amount of seasonal precipitation. They obscure the annual signal from the seasonal variations in chemical concentrations and oxygen isotope ratios that are typically used to date for ice cores. On the other hand, the ice cores from alpine glaciers contain many species of pollen grains that have different pollen seasons. Here we show that these pollen peaks allow us to decide four seasonal layers of a snow pit, and examine whether some seasonal missing layers due to wind erosion or little seasonal precipitation exist. In addition, they accordingly allow us to give an accurate measure of annual snow deposition even from the glacier where some seasonal layers are eroded by wind, or missing due to little seasonal precipitation.
C51A-1023 0800h
Borehole Logging to Reconstruct Past Annual Accumulation Rates in High Altitude Firn Areas
Our understanding of climatic changes essentially stems from knowledge of past annual accumulation rates. Ice cores give an encrypted review over past precipitation. To decipher the observed layer thickness from ice cores, firn compaction and deformation of annual ice layers have to be taken into account. A new method for measuring the vertical strain-rate was developed which entails the scratching of horizontal grooves directly into the wall of a borehole. No other type of strain markers (disturbing other simultaneous measurements) need to be installed. By tracking the vertical position of these grooves with a caliper probe, the vertical strain-rate including firn compaction can be determined from which the depth-dependent thinning-rate is calculated. In combination with measured annual layer thicknesses, the original accumulation rate can be reconstructed (without resorting to the knowledge of the density profile). Results obtained from an ice core drilling site on Fiescherhorn in the Swiss Alps are compared with model computation.
http://www.glaciology.ch
C51A-1024 0800h
A new Dating Method for Glacier ice Using Radiocarbon Analysis of Carbonaceous Particles
In the deepest parts, ice cores from high elevation glaciers are difficult or even impossible to date with conventional dating techniques due to strong layer thinning. Traditional radiocarbon applications by means of plant or insect fragments are of limited use since such material is rarely found in glacier ice. We are therefore developing a new method for radiocarbon dating, using the organic carbon fraction (OC) of carbonaceous particles contained in ice. Carbonaceous particles are produced by incomplete combustion of carbon containing material and are long-range transported even to very remote locations. For the time period before the industrial use of fossil fuels, OC is believed to originate from biogenic sources only and therefore radiocarbon dating with its 14C/12C ratio should be possible. Today, Accelerator Mass Spectrometry measurements of this ratio are possible with as little as 0.02 - 0.1 mg carbon (corresponding to 500 - 1000 g of ice) with expected uncertainties for future dating of 200 years. Additionally, the 14C/12C ratio of carbonaceous particles from younger layers gives a unique and absolute measure of the contemporary to fossil carbon source ratio. This can be used for source apportionment of biogenic and anthropogenic emissions during the industrial time period (from around 1870 to the present), and can give evidence for reconstruction of biomass burning events in the past. First analysis of ice from an Alpine glacier has been performed, indicating that the oldest ice is about 1500 years old, which is younger than predicted by 3D glacier flow modelling.
C51A-1025 0800h
Climate Signals and Trends in the Dasuopu Snow Accumulation Time Series Associated with the Indian Summer Monsoon, the Regional Hadley and Walker Circulations
A new climate record of potential interest in understanding climate variability is the snow accumulation time series from the Dasuopu ice core that is situated at an elevation of 7020m above sea level in the southern Himalayas ($28\deg$23'N, $85\deg$43'E). Snow accumulation at the site is thought to occur predominately during the summer season. In addition, snow accumulation is sufficiently high as to allow for the recovery of an annually resolved time series. In this talk, we will discuss the climate signals contained in this time series. Despite its proximity to the Indian subcontinent, we find no statistically significant relationship between the Dasuopu snow accumulation (DSA) and the Indian monsoon rainfall index (IMR) for the period 1956-1993. Furthermore, we find a statistically significant negative trend in the DSA during this period, while the IMR exhibited no such trend. We attribute these differences to the extreme height of the ice core site that effectively decouples snow accumulation at the site from the surface confined moisture transport that is associated with the Indian summer monsoon. On the other hand, we find statistically significant relationships between DSA and various indicators of the upper-level circulation associated with the monsoon. These results suggest that the DSA may contain an expression of the regional Hadley and Walker circulations. To confirm this hypothesis, we investigate the climate signal in the DSA that is associated with the global divergent circulation that contains information on the Hadley and Walker circulations. We find that there exist trends in the regional Hadley and Walker circulations that are consistent with the observed trend in the DSA. These changes are widespread and appear to be associated with reductions in West African and North China summer rainfall that have occurred during the latter half of the 20th century. These similarities lead us to conclude that the decreasing trends in the three precipitation time series during the latter half of the 20th century are the result of large-scale changes in the global overturning circulation that we propose are associated with large-scale changes in tropical sea surface temperatures.