An open source Bayesian Monte Carlo isotope mixing model with applications in Earth surface processes

The implementation of isotopic tracers as constraints on source contributions has become increasingly relevant to understanding Earth surface processes. Interpretation of these isotopic tracers has become more accessible with the development of Bayesian Monte Carlo (BMC) mixing models, which allow uncertainty in mixing end‐members and provide methodology for systems with multicomponent mixing. This study presents an open source multiple isotope BMC mixing model that is applicable to Earth surface environments with sources exhibiting distinct end‐member isotopic signatures. Our model is first applied to new δ18O and δD measurements from the Athabasca Glacier, which showed expected seasonal melt evolution trends and vigorously assessed the statistical relevance of the resulting fraction estimations. To highlight the broad applicability of our model to a variety of Earth surface environments and relevant isotopic systems, we expand our model to two additional case studies: deriving melt sources from δ18O, δD, and 222Rn measurements of Greenland Ice Sheet bulk water samples and assessing nutrient sources from ɛNd and 87Sr/86Sr measurements of Hawaiian soil cores. The model produces results for the Greenland Ice Sheet and Hawaiian soil data sets that are consistent with the originally published fractional contribution estimates. The advantage of this method is that it quantifies the error induced by variability in the end‐member compositions, unrealized by the models previously applied to the above case studies. Results from all three case studies demonstrate the broad applicability of this statistical BMC isotopic mixing model for estimating source contribution fractions in a variety of Earth surface systems.Key Points:Open source BMC model determines source contributions in Earth surface systemsEffectively applied to stable and radiogenic isotope systems in various settingsModel able to encompass end‐member uncertainties and multiple isotopic systemsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/111937/1/ggge20708.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/111937/2/ggge20708-sup-0001-2014GC005683-ts01.pd

The Greenland ice sheet through the last glacial-interglacial cycle

The evolution of the Greenland ice sheet during the last 150,000 years, in response to a climate history derived from a Greenland ice-margin oxygen-18 record,is simulated by means of a three-dimensional, time-dependent ice-sheet model. The calculations indicate that the ice sheet displayed considerable thinning andice-margin retreat during the last interglacial (the Eemian) and during a warm interstadial c. 100,000 yr B.P., resulting in splitting up of the ice sheet into acentral-northern and a southern part. However, the ice sheet in Central Greenland survived the warm stages with almost unchanged surface elevations ascompared with the present

The Greenland ice sheet and greenhouse warming

Increased melting on glaciers and ice sheets and rising sea level are often mentioned as important aspects of the anticipated greenhouse warming of the earth'satmosphere. This paper deals with the sensitivity of Greenland's ice mass budget and presents a tentative projection of the Greenland component of future sealevel rise for the next few hundred years. To do this, the 'Villach II temperature scenario' is prescribed, and output from a comprehensive mass balancemodel is used to drive a high-resolution 3-D thermomechanic model of the ice sheet.The mass balance model consists of two parts: the accumulation part is based on presently observed values and is forced by changes in mean annual airtemperature. The ablation model is based on the degree-day method and accounts for the daily and annual temperature cycle, a different degree-day factor forice and snow melting and superimposed ice formation. Under present-day climatic conditions, the following total mass balance results (in ice equivalent peryear): 599.3 109 m3 of accumulation, 281.7 109 m3 of runoff and assuming a balanced budget, 317.6 109 m3 of iceberg calving. A 1K uniform warming isthen calculated to increase the runoff by 119.5 109 m3. Since accumulation also increases by 32 109 m3, this leads to reduction of the total mass balance by87.5 109 m3 of ice, corresponding to a sea level rise of 0.22 mm/year. For a temperature increase larger than 2.7 K, runoff exceeds accumulation, and if icesheet dynamics were to remain unchanged, this would add an extra amount of 0.8 mm/year to the worlds' oceans.Imposing the Villach II scenario (warming up to 4.2 K) and accumulating mass balance changes forward in time (static response) would then result in aglobal sea level rise of 7.1 cm by 2100 AD, but this figure may go up to as much as 40 cm per century in case the warming is doubled. In a subsequentdynamic model run involving the ice flow, the ice sheet is found to produce a counteracting effect by dynamically producing steeper slopes at the margin,thereby reducing the area over which runoff can take place. This effect is particularly apparent in the northeastern part of the ice sheet, and is also morepronounced for the smaller temperature perturbations. Nevertheless, all these experiments certainly highlight the vulnerability of the Greenland ice sheet withrespect to a climatic warming

Steady-state characteristics of the Greenland ice sheet under different climates

The Greenland ice sheet is modelled to simulate its extent and volume in warmer climates, and also to find out whether the ice sheet would re-form on theice-free bedrock under present climatic conditions. The ice sheet model is a three-dimensional thermo-mechanical model with a fine resolution grid. Thebedrock surface beneath the ice sheet was mapped using radio-echo-sounding measurements by the Electromagnetic Institute, Copenhagen. The modelexperiments show that increased temperature will result in ice-margin retreat, but the ice sheet is relatively stable; it takes a rather high temperature rise of atleast 6¡C for the ice sheet to disappear completely, which indicates that the ice sheet probably survived the last interglacial. Also, it appears that the Greenlandice sheet is not a mere relict ice mass from a previously colder climate but that the ice sheet will still re-form on the bare bedrock under the present, or evenslightly warmer, climatic conditions

Past and forecast fluctuations of Glacier Blanc (French Alps)

In 1980 the new advance of Glacier Blanc covered a trail leading to a mountain hut and threatened the footbridge that crosses its melt-water stream. This event prompted the Laboratoire de Glaciologie et Géophysique de l’Environnement (LGGE) to undertake a new study of the glacier. With the old Eaux et Forêts measurements, and the more recent LGGE ones, it has been possible to forecast the probable fluctuations in glacier length by two different methods: a statistical method based on a correlation between the variations in length and mass balance, and a one-dimensional numerical ice-flow model. Both methods show that a substantial glacier retreat is unlikely during the 1985–90 period.</jats:p

Monitoring glacier albedo as a proxy to derive summer and annual surface mass balances from optical remote-sensing data

Less than 0.25 % of the 250 000 glaciers inventoried in the Randolph Glacier Inventory (RGI V.5) are currently monitored with in situ measurements of surface mass balance. Increasing this archive is very challenging, especially using time-consuming methods based on in situ measurements, and complementary methods are required to quantify the surface mass balance of unmonitored glaciers. The current study relies on the so-called albedo method, based on the analysis of albedo maps retrieved from optical satellite imagery acquired since 2000 by the MODIS sensor, on board the TERRA satellite. Recent studies revealed substantial relationships between summer minimum glacier-wide surface albedo and annual surface mass balance, because this minimum surface albedo is directly related to the accumulation–area ratio and the equilibrium-line altitude. On the basis of 30 glaciers located in the French Alps where annual surface mass balance data are available, our study conducted on the period 2000&ndash;2015 confirms the robustness and reliability of the relationship between the summer minimum surface albedo and the annual surface mass balance. For the ablation season, the integrated summer surface albedo is significantly correlated with the summer surface mass balance of the six glaciers seasonally monitored. These results are promising to monitor both annual and summer glacier-wide surface mass balances of individual glaciers at a regional scale using optical satellite images. A sensitivity study on the computed cloud masks revealed a high confidence in the retrieved albedo maps, restricting the number of omission errors. Albedo retrieval artifacts have been detected for topographically incised glaciers, highlighting limitations in the shadow correction algorithm, although inter-annual comparisons are not affected by systematic errors

Monitoring glacier albedo as a proxy to derive summer and annual surface mass balances from optical remote-sensing data

Less than 0.25% of the 250 000 glaciers inventoried in the Randolph Glacier Inventory (RGI V.5) are currently monitored with in situ measurements of surface mass balance. Increasing this archive is very challenging, especially using time-consuming methods based on in situ measurements, and complementary methods are required to quantify the surface mass balance of unmonitored glaciers. The current study relies on the so-called albedo method, based on the analysis of albedo maps retrieved from optical satellite imagery acquired since 2000 by the MODIS sensor, on board the TERRA satellite. Recent studies revealed substantial relationships between summer minimum glacier-wide surface albedo and annual surface mass balance, because this minimum surface albedo is directly related to the accumulation-area ratio and the equilibrium-line altitude. On the basis of 30 glaciers located in the French Alps where annual surface mass balance data are available, our study conducted on the period 2000-2015 confirms the robustness and reliability of the relationship between the summer minimum surface albedo and the annual surface mass balance. For the ablation season, the integrated summer surface albedo is significantly correlated with the summer surface mass balance of the six glaciers seasonally monitored. These results are promising to monitor both annual and summer glacier-wide surface mass balances of individual glaciers at a regional scale using optical satellite images. A sensitivity study on the computed cloud masks revealed a high confidence in the retrieved albedo maps, restricting the number of omission errors. Albedo retrieval artifacts have been detected for topographically incised glaciers, highlighting limitations in the shadow correction algorithm, although inter-annual comparisons are not affected by systematic errors