Thermal fractionation of air in polar firn by seasonal temperature gradients

Air withdrawn from the top 5-15 m of the polar snowpack (firn) shows anomalous enrichment of heavy gases during summer, including inert gases.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 explore and 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) help calibrate a new technique for measuring temperature change in ice core gas records 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 sampling at the pole. We analyzed 15N/14N, 40Ar/36Ar, 40Ar/38Ar, 18O/16O of O2, O2/N2, 84Kr/36Ar, and 132Xe/36Ar. The results show the expected pattern 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 values of different gas pairs may. At Siple Dome, δ40Ar/4 is 66 ± 2% as sensitive to thermal diffusion as δ15N, in agreement with laboratory calibration; δ18O/2 is 83 ± 3%, and δ84Kr/48 is 33 ± 3% as sensitive as δ15N. The corresponding figures for summer South Pole are 64 ± 2%, 81 ± 3%, and 34 ± 3%. Accounting for atmospheric change, the figure for δO2/N2/4 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 a rectifier effect is seen

Abrupt changes in atmospheric methane at the MIS 5b–5a transition

New ice core analyses show that the prominent rise in atmospheric methane concentration at Dansgaard-Oeschger event 21 was interrupted by a century-long 20% decline, which was previously unrecognized. The reversal was found in a new ∼100-year resolution study of methane in the GISP2 ice core, encompassing the beginning of D-O event 21, which also corresponds to the transition from MIS 5b to 5a. Although a corresponding reversal (within age uncertainty) is observed in climate proxies measured in GISP2 ice, including δ¹⁸O[subscript ice], electrical conductivity, light scattering, and several ions, this feature has not been discussed previously. Abrupt changes in methane are paralleled by changes in δ¹⁵N of trapped air, a quantity that reflects local temperature change at Greenland summit. The reversal described here supports the hypothesis that climate can be unstable during major transitions, as was previously described for the last deglaciation.This is the publisher’s final pdf. The published article is copyrighted by the American Geophysical Union and can be found at: http://www.agu.org/journals/gl

Tree-Ring Structure of the Pinus sibirica and Pinus cembra Grafts as a Reflection of the Interaction of Scion and Rootstock

Проведен анализ анатомических характеристик годичных колец у подвоев P. sylvestris L. и привоев двух климатипов P. sibirica Du Tour (емельяновского и сургутского) и одного климатипа P. cembra L. (карпатского) за 10 лет их совместного роста. У привоев P. sibirica относительно контрольных деревьев увеличивается число трахеид, их радиальный диаметр и толщина клеточной стенки (кроме клеточной стенки сургутских привоев). В то же время в годичных кольцах подвоев P. sylvestris число трахеид и размеры данных характеристик уменьшаются относительно контрольных деревьев. Исключение составляет сосна обыкновенная, являющаяся подвоем для привоев сургутского климатипа, у которой радиальный диаметр трахеид переходной зоны увеличивается. Сезонная динамика радиального диаметра трахеид и толщины клеточной стенки привитых деревьев сохраняет свою видовую особенность, характерную для непривитых деревьев, что свидетельствует о стабильности генетической программы дифференциации ксилемы. Адаптация привоев к климатическим условиям их места происхождения не влияет на анатомическую структуру годичных колец подвоев в годы, близкие по условиям к климату региона происхождения привоевThe analysis of the anatomical characteristics of tree rings of P. sylvestris L. rootstocks and scions of P. sibirica Du Tour (Emelianovo and Surgut provenances) and P. cembra L. (Karpat provenance) has been carried out for 10 years of their joint growth. The number of tracheids, their radial diameter and the cell wall thickness for the P. sibirica scions increases relative to control trees (with the exception of the cell wall thickness for Surgut scions). At the same time, in the annual rings of the P. sylvestris rootstocks these characteristics as well as the number of tracheids decrease relative to the control trees. An exception is the Scots pine, which is the rootstock for Surgut provenance scion, for which the radial tracheid diameter of the transition zone increases. The seasonal dynamics of the radial tracheid diameter and the cell wall thickness of the grafted trees retains the species features typical for the ungrafted trees, suggesting the stability of the genetic program of xylem differentiation. The adaptation of scions to the climatic conditions of their place of origin does not affect the anatomical structure of the tree rings of the rootstocks during the years in which the conditions are close to the climate of the region of origin of the scion

Relative timing and variability of atmospheric methane and GISP2 oxygen isotopes between 68 and 86 ka

The global biogeochemical cycle of methane has received wide attention because of methane's role as a greenhouse gas. Measurements of methane in air trapped in Greenland ice cores provide a high-resolution record of methane levels in the atmosphere over the past ~100 ka, providing clues about what controls the methane cycle on geologic timescales. Remarkable similarity between local temperature recorded in Greenland ice cores and changes in global methane concentrations has been noted in previous studies, with the inference that the local temperature variations have global significance, but the resolution of sampling and measurement precision limited fine-scale comparison of these variables. In this work a higher-precision (~2 ppb) methane data set was obtained from the Greenland Ice Sheet Project 2 (GISP2) ice core for the time interval between 86 and 68 ka, encompassing three large abrupt warming events early in the last glacial period: Dansgaard-Oeschger (D-O) events 19, 20, and 21. The new data set consists of duplicate measurements at 158 depths, with average time resolution of 120 years. Such detailed measurements over D-O 21, the longest in Greenland records, have not yet been reported for other ice cores. The new data set documents short-term variability (~20 ppb typical amplitude), which is remarkably persistent, and in many cases similar features are observed in the most detailed published δ¹⁸O[subscript ice] record. High-precision GISP2 δ¹⁵N data show that changes in Greenland temperature are synchronous with the methane variations at the onset of D-O events 19, 20, and 21, supporting previous results from the Greenland Ice Core Project ice core for D-O 19 and 20. Cross-spectral analysis quantifies the extremely close similarity between the new methane record and the δ¹⁸O[subscript ice] record. Because methane sources are widely distributed over the globe, this work further validates δ¹⁸O[subscript ice] at Greenland summit as a geographically broad climate indicator on millennial to multicentennial timescales.Keywords: methane, Dansgaard-Oeschger events, ice core

Annual Sedimentary Record From Lake Donguz-Orun (Central Caucasus) Constrained by High Resolution SR-XRF Analysis and Its Potential for Climate Reconstructions

Bottom sediments of the proglacial Lake Donguz-Orun situated at ∼2500 m a.s.l. in the Elbrus Region (Central Caucasus) reveal regular laminae, characteristic of proglacial varved lakes. This is the first laminated sediment sequence recorded in the region. However, visual counting of the layers was restricted due to partial indistinctness of the lamination. In order to confirm the annual sedimentary cyclicity and proceed with annually resolved data, in addition to the visual identification we used high-resolution geochemical markers. The upper 160 mm of the sediment core were scanned at 200 μm intervals using synchrotron radiation X-ray fluorescence analysis (SR-XRF). Additional ultrahigh resolution scanning at 30 μm increments was employed for the upper 20 mm of the core. The Rb/Sr and Zr/Rb ratios are interpreted to record annual changes in grain-size. Based on this geochemical assessment, we identified 88 annual layers covering the interval between 1922 and 2010, while visually we have been able to identify between 70 and 100 layers. The correctness of the geochemical results is confirmed by mean accumulation rates assessed by 137Cs and 210Pb dating. Cross-correlation between the ring width of local pine chronology and the layer thickness, identified as a distance between the annual Rb/Sr peaks, allowed for the accurate dating of the uppermost preserved year of the sediment sequence (AD 2010). Annually averaged elemental data were then compared with regional meteorological observations, glacier mass balance and tree-ring chronologies. The comparison revealed notable conformities: content of bromine is positively correlated with annual temperatures (r = 0.41, p < 0.01), content of terrigenous elements (major elements with the origin in watershed rocks) is positively correlated (up to r = 0.44, p < 0.01) with annual precipitation. A high statistically significant negative relationship is observed between the concentrations of terrigenous elements and tree-ring width of local pine chronology (up to r = -0.56, p < 0.01). Taken together, these data point to a common composite climatic signal in the two independent records (lake sediments and tree rings) and confirm that the laminae represent annual layers (i.e., varves). These findings open opportunities for high-resolution multiproxy climate reconstructions 300–350 years long using the longer sediment core and tree-ring records

Methodology for Development of a 600-Year Tree-Ring Multi-Element Record for Larch from the Taymir Peninsula, Russia

We developed a long (600-year) dataset for the concentrations of 26 elements in tree rings of larch from the Taymir Peninsula, the northernmost region in the world (ca. 72°N) where trees grow. Tree rings corresponding to the time period from 1300 to 1900 A.D. were studied. Eleven wood strips, each from a different larch tree, were cut into ca. 100 mg samples usually consisting of ten consecutive tree rings (but occasionally five). Between 19 and 40 consecutive samples resulted from each tree, yielding a total of 277 samples. The replication of each time interval ranged from three (for periods 1300-1400 A.D. and 1600-1700 A.D.) to six (for 1450-1600 A.D.). Wood samples were digested with concentrated HNO 3 for measurement of Li, B, Na, Mg, Al, Si, P, Cl, K, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Rb, Sr, Y, Zr, Nb, Mo, Ag, Cd, Sn, Sb, I, Ba, La, Ce, Nd, W, Au, Pb, Bi, Th, and U using solution Inductively Coupled Plasma Mass Spectrometry (ICPMS). Fourteen elements (V, Co, As, Y, Nb, Mo, Sb, La, Ce, Nd, W, Au, Th, and U) with extremely low concentrations were eliminated from consideration as unreliable. Here we report our sample preparation and measurement procedure, as well as the observed concentrations in tree rings, emphasizing considerations for developing representative and reliable denrodochemical datasets.Нами был получен длительный массив данных (600 лет) концентраций 26 элементов в годичных кольцах лиственницы с полуострова Таймыр, самого северного региона в мире (около 72° с.ш.), где возможен рост деревьев. Изучались годичные кольца, соответствующие промежутку времени с 1300 по 1900 год н.э. Одиннадцать древесных выпилов, по одному для каждой лиственницы, нарезались на образцы массой около 100 мг, которые, как правило, состояли из десяти годичных колец (но в некоторых случаях из пяти). Из каждого дерева было получено от 19 до 40 последовательных образцов, что дало в общей сложности 277 образцов. Повторность для каждого временного интервала варьировала от трех (для периодов 1300-1400 г.н.э. и 1600-1700 г.н.э.) до шести (для периода 1450-1600 г.н.э.). Древесные образцы растворяли в концентрированной HNO 3 для последующего измерения Li, B, Na, Mg, Al, Si, P, Cl, K, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Rb, Sr, Y, Zr, Nb, Mo, Ag, Cd, Sn, Sb, I, Ba, La, Ce, Nd, W, Au, Pb, Bi, Th и U при помощи масс-спектрометрии с индуктивно связанной плазмой (ICP-MS) для растворов. Четырнадцать элементов (V, Co, As, Y, Nb, Mo, Sb, La, Ce, Nd, W, Au, Th и U) с очень низкими концентрациями были исключены из рассмотрения как недостоверные. В данной статье, основной целью которой являлась отработка методики получения репрезентативных и достоверных дендрохимических данных, приводится использованная нами процедура пробоподготовки и измерений, а также полученные концентрации в годичных кольцах

A method for precise measurement of argon 40/36 and krypton/argon ratios in trapped air in polar ice with applications to past firn thickness and abrupt climate change in Greenland and at Siple Dome, Antarctica

International audienc

Precise timing and characterization of abrupt climate change 8200 years ago from air trapped in polar ice

International audienceHow fast and how much climate can change has significant implications for concerns about future climate changes and their potential impacts on society. An abrupt climate change 8200 years ago (8.2 ka event) provides a test case to understand possible future climatic variability. Here, methane concentration (taken as an indicator for terrestrial hydrology) and nitrogen isotopes (Greenland temperature) in trapped air in a Greenland ice core (GISP2) are employed to scrutinize the evolution of the 8.2 ka event. The synchronous change in methane and nitrogen implies that the 8.2 ka event was a synchronous event (within ±4 years) at a hemispheric scale, as indicated by recent climate model results [Legrande, A. N., Schmidt, G. A., Shindell, D. T., Field, C. V., Miller, R. L., Koch, D. M., Faluvegi, G., Hoffmann, G., 2006. Consistent simulations of multiple proxy responses to an abrupt climate change event. Proceedings of the National Academy of Sciences 103, 837–842]. The event began with a large-scale general cooling and drying around 8175±30 years BP (Before Present, where Present is 1950 AD). Greenland temperature cooled by 3.3±1.1 °C (decadal average) in less than 20 years, and atmospheric methane concentration decreased by 80±25 ppb over 40 years, corresponding to a 15±5% emission reduction. Hemispheric scale cooling and drying, inferred from many paleoclimate proxies, likely contributed to this emission reduction. In central Greenland, the coldest period lasted for 60 years, interrupted by a milder interval of a few decades, and temperature subsequently warmed in several steps over 70 years. The total duration of the 8.2 ka event was roughly 150 years