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Titolo:
Thermal fractionation of air in polar firn by seasonal temperature gradients
Autore:
Severinghaus, JP; Grachev, A; Battle, M;
Indirizzi:
Univ Calif San Diego, Scripps Inst Oceanog, La Jolla, CA 92093 USA Univ Calif San Diego La Jolla CA USA 92093 ceanog, La Jolla, CA 92093 USA Bowdoin Coll, Dept Phys & Astron, Brunswick, ME 04011 USA Bowdoin Coll Brunswick ME USA 04011 hys & Astron, Brunswick, ME 04011 USA
Titolo Testata:
GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS
, volume: 2, anno: 2001,
pagine: NIL_1 - NIL_24
SICI:
1525-2027(20010731)2:<NIL_1:TFOAIP>2.0.ZU;2-N
Fonte:
ISI
Lingua:
ENG
Soggetto:
ABRUPT CLIMATE-CHANGE; AGE-DIFFERENCES; ICE; O-2; DIFFUSION; MOVEMENT; RATIO; GASES; N-2; END;
Keywords:
thermal diffusion; firn air; isotope fractionation; gas isotopes; paleothermometer; thermal diffusion; sensitivity; troposphere-constituent transport and chemistry; global change-atmosphere; glaciology; paleoclimatology;
Tipo documento:
Article
Natura:
Periodico
Settore Disciplinare:
Physical, Chemical & Earth Sciences
Citazioni:
35
Recensione:
Indirizzi per estratti:
Indirizzo: Severinghaus, JP Univ Calif San Diego, Scripps Inst Oceanog, La Jolla, CA 92093 USA Univ Calif San Diego La Jolla CA USA 92093 CA 92093 USA
Citazione:
J.P. Severinghaus et al., "Thermal fractionation of air in polar firn by seasonal temperature gradients", GEOCH GEO G, 2, 2001, pp. NIL_1-NIL_24

Abstract

Air withdrawn from the top 5-15 m of the polar snowpack (fim) shows anomalous enrichment of heavy gases during summer, including inert gases. Following earlier work, we ascribe this to thermal diffusion, the tendency of a gas mixture to separate in a temperature gradient, with heavier molecules migrating toward colder regions. Summer warmth creates a temperature gradient in the top few meters of the firn due to the thermal inertia of the underlying firn and causes gas fractionation by thermal diffusion. Here we exploreand quantify this process further in order to (1) correct for bias caused by thermal diffusion in firn air and ice core air isotope records, (2) helpcalibrate a new technique for measuring temperature change in ice core gasrecords based on thermal diffusion [Severinghaus et al., 1998], and (3) address whether air in polar snow convects during winter and, if so, whether it creates a rectification of seasonality that could bias the ice core record. We sampled air at 2-m-depth intervals from the top 15 m of the firn at two Antarctic sites, Siple Dome and South Pole, including a winter samplingat the pole. We analyzed N-15/N-14, Ar-40/Ar-36, Ar-40/Ar-38, O-18/O-16 ofO-2, O-2/N-2, Kr-84/Ar-36, and Xe-132/Ar-36. The results show the expectedpattern of fractionation and match a gas diffusion model based on first principles to within 30%. Although absolute values of thermal diffusion sensitivities cannot be determined from the data with precision, relative valuesof different gas pairs may. At Siple Dome, delta Ar-40/4 is 66 +/- 2% as sensitive to thermal diffusion as delta N-15, in agreement with laboratory calibration; delta O-18/2 is 83 +/- 3%, and delta Kr-84/48 is 33 +/- 3% as sensitive as delta N-15. The corresponding figures for summer South Pole are64 +/- 2%, 81 +/- 3%, and 34 +/- 3%. Accounting for atmospheric change, the figure for deltaO(2)/N-2/4 is 90 +/- 3% at Siple Dome. Winter South Pole shows a strong depletion of heavy gases as expected. However, the data do not fit the model well in the deeper part of the profile and yield a systematic drift with depth in relative thermal diffusion sensitivities (except for Kr, constant at 34 +/- 4%), suggesting the action of some other process that is not currently understood. No evidence for wintertime convection or arectifier effect is seen.

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Documento generato il 06/04/20 alle ore 21:41:20