Firn

About: Firn is a research topic. Over the lifetime, 2181 publications have been published within this topic receiving 68705 citations.

Natural and anthropogenic changes in atmospheric CO2 over the last 1000 years from air in Antarctic ice and firn

Abstract: A record of atmospheric CO2 mixing ratios from 1006 A.D. to 1978 A.D. has been produced by analysing the air enclosed in three ice cores from Law Dome, Antarctica. The enclosed air has unparalleled age resolution and extends into recent decades, because of the high rate of snow accumulation at the ice core sites. The CO2 data overlap with the record from direct atmospheric measurements for up to 20 years. The effects of diffusion in the firn on the CO2 mixing ratio and age of the ice core air were determined by analyzing air sampled from the surface down to the bubble close-off depth. The uncertainty of the ice core CO2 mixing ratios is 1.2 ppm (1 σ). Preindustrial CO2 mixing ratios were in the range 275–284 ppm, with the lower levels during 1550–1800 A.D., probably as a result of colder global climate. Natural CO2 variations of this magnitude make it inappropriate to refer to a single preindustrial CO2 level. Major CO2 growth occurred over the industrial period except during 1935–1945 A.D. when CO2 mixing ratios stabilized or decreased slightly, probably as a result of natural variations of the carbon cycle on a decadal timescale.

Influence of the seasonal snow cover on the ground thermal regime: An overview

Abstract: [1] The presence of seasonal snow cover during the cold season of the annual air temperature cycle has significant influence on the ground thermal regime in cold regions. Snow has high albedo and emissivity that cool the snow surface, high absorptivity that tends to warm the snow surface, low thermal conductivity so that a snow layer acts as an insulator, and high latent heat due to snowmelt that is a heat sink. The overall impact of snow cover on the ground thermal regime depends on the timing, duration, accumulation, and melting processes of seasonal snow cover; density, structure, and thickness of seasonal snow cover; and interactions of snow cover with micrometeorological conditions, local microrelief, vegetation, and the geographical locations. Over different timescales either the cooling or warming impact of seasonal snow cover may dominate. In the continuous permafrost regions, impact of seasonal snow cover can result in an increase of the mean annual ground and permafrost surface temperature by several degrees, whereas in discontinuous and sporadic permafrost regions the absence of seasonal snow cover may be a key factor for permafrost development. In seasonally frozen ground regions, snow cover can substantially reduce the seasonal freezing depth. However, the influence of seasonal snow cover on seasonally frozen ground has received relatively little attention, and further study is needed. Ground surface temperatures, reconstructed from deep borehole temperature gradients, have increased by up to 4°C in the past centuries and have been widely used as evidence of paleoclimate change. However, changes in air temperature alone cannot account for the changes in ground temperatures. Changes in seasonal snow conditions might have significantly contributed to the ground surface temperature increase. The influence of seasonal snow cover on soil temperature, soil freezing and thawing processes, and permafrost has considerable impact on carbon exchange between the atmosphere and the ground and on the hydrological cycle in cold regions/cold seasons.

A 1000-year high precision record of δ 13 C in atmospheric CO 2

Abstract: We present measurements of the stable carbon isotope ratio in air extracted from Antarctic ice core and firn samples. The same samples were previously used by Etheridge and co-workers to construct a high precision 1000-year record of atmospheric CO 2 concentration, featuring a close link between the ice and modern records and high-time resolution. Here, we start by confirming the trend in the Cape Grim in situ δ 13 C record from 1982 to 1996, and extend it back to 1978 using the Cape Grim Air Archive. The firn air δ 13 C agrees with the Cape Grim record, but only after correction for gravitational separation at depth, for diffusion effects associated with disequilibrium between the atmosphere and firm, and allowance for a latidudinal gradient in δ 13 C between Cape Grim and the Antarctic coast. Complex calibration strategies are required to cope with several additional systematic influences on the ice core δ 13 C record. Errors are assigned to each ice core value to reflect statistical and systematic biases (between ± 0.025‰ and ± 0.07‰); uncertainties (of up to ± 0.05‰) between core-versus-core, ice-versus-firn and firn-versus-troposphere are described separately. An almost continuous atmospheric history of δ 13 C over 1000 years results, exhibiting significant decadal-to-century scale variability unlike that from earlier proxy records. The decrease in δ 13 C from 1860 to 1960 involves a series of steps confirming enhanced sensitivity of δ 13 C to decadal timescale-forcing, compared to the CO 2 record. Synchronous with a ‘‘Little Ice Age’′ CO 2 decrease, an enhancement of δ 13 C implies a terrestrial response to cooler temperatures. Between 1200 AD and 1600 AD, the atmospheric δ 13 C appear stable. DOI: 10.1034/j.1600-0889.1999.t01-1-00005.x

Water flow through temperate glaciers

Abstract: Understanding water movement through a glacier is fundamental to several critical issues in glaci- ology, including glacier dynamics, glacier-induced floods, and the prediction of runoff from glacierized drainage basins. To this end we have synthesized a conceptual model of water movement through a temper- ate glacier from the surface to the outlet stream. Pro- cesses that regulate the rate and distribution of water input at the glacier surface and that regulate water movement from the surface to the bed play important but commonly neglected roles in glacier hydrology. Where a glacier is covered by a layer of porous, perme- able firn (the accumulation zone), the flux of water to the glacier interior varies slowly because the firn tempo- rarily stores water and thereby smooths out variations in the supply rate. In the firn-free ablation zone, in con- trast, the flux of water into the glacier depends directly on the rate of surface melt or rainfall and therefore varies greatly in time. Water moves from the surface to the bed through an upward branching arborescent net- work consisting of both steeply inclined conduits, formed by the enlargement of intergranular veins, and gently inclined conduits, spawned by water flow along the bottoms of near-surface fractures (crevasses). Engla- cial drainage conduits deliver water to the glacier bed at a limited number of points, probably a long distance downglacier of where water enters the glacier. Englacial conduits supplied from the accumulation zone are quasi steady state features that convey the slowly varying water flux delivered via the firn. Their size adjusts so that they are usually full of water and flow is pressurized. In contrast, water flow in englacial conduits supplied from the ablation area is pressurized only near times of peak daily flow or during rainstorms; flow is otherwise in an open-channel configuration. The subglacial drainage system typically consists of several elements that are distinct both morphologically and hydrologically. An up- glacier branching, arborescent network of channels in- cised into the basal ice conveys water rapidly. Much of the water flux to the bed probably enters directly into the arborescent channel network, which covers only a small fraction of the glacier bed. More extensive spatially is a nonarborescent network, which commonly includes cav- ities (gaps between the glacier sole and bed), channels incised into the bed, and a layer of permeable sediment. The nonarborescent network conveys water slowly and is usually poorly connected to the arborescent system. The arborescent channel network largely collapses during winter but reforms in the spring as the first flush of meltwater to the bed destabilizes the cavities within the nonarborescent network. The volume of water stored by a glacier varies diurnally and seasonally. Small, temper- ate alpine glaciers seem to attain a maximum seasonal water storage of ;200 mm of water averaged over the area of the glacier bed, with daily fluctuations of as much as 20 -30 mm. The likely storage capacity of subglacial cavities is insufficient to account for estimated stored water volumes, so most water storage may actually occur englacially. Stored water may also be released abruptly and catastrophically in the form of outburst floods.

A synchronized dating of three Greenland ice cores throughout the Holocene

Abstract: As part of the effort to create the new Greenland Ice Core Chronology 2005 (GICC05) a synchronized stratigraphical timescale for the Holocene parts of the DYE- 3, GRIP and NGRIP ice cores is made by using volcanic reference horizons in electri- cal conductivity measurements to match the cores. The main annual layer counting is carried out on the most suited records only, exploit- ing that the three ice cores have been drilled at locations with different climatic con- ditions and differences in ice flow. However, supplemental counting on data from all cores has been performed between each set of reference horizons in order to verify the valid- ity of the match. After the verification, the main dating is transferred to all records us- ing the volcanic reference horizons as tie points. An assessment of the mean annual layer thickness in each core section confirms that the new synchronized dating is consistent for all three cores. The data used for the main annual layer counting of the past 7900 years are the DYE- 3, GRIP and NGRIP stable isotope records. As the high accumulation rate at the DYE- 3 drill site makes the seasonal cycle in the DYE-3 stable isotopes very resistant to firn diffusion, an effort has been made to extend the DYE-3 Holocene record. The new syn- chronized dating relies heavily on this record of �75,000 stable isotope samples. The dat- ing of the early Holocene consists of an already established part of GICC05 for GRIP and NGRIP which has now been transferred to the DYE-3 core. GICC05 dates the Younger Dryas termination, as defined from deuterium excess, to 11,703 b2k; 130 years earlier than the previous GRIP dating.