Geomorphological evolution of a debris‐covered glacier surface

There exists a need to advance our understanding of debris‐covered glacier surfaces over relatively short timescales due to rapid, climatically induced areal expansion of debris cover at the global scale, and the impact debris has on mass balance. We applied unpiloted aerial vehicle structure‐from‐motion (UAV‐SfM) and digital elevation model (DEM) differencing with debris thickness and debris stability modelling to unravel the evolution of a 0.15 km2 region of the debris‐covered Miage Glacier, Italy, between June 2015 and July 2018. DEM differencing revealed widespread surface lowering (mean 4.1 ± 1.0 m a‐1; maximum 13.3 m a‐1). We combined elevation change data with local meteorological data and a sub‐debris melt model, and used these relationships to produce high resolution, spatially distributed maps of debris thickness. These maps were differenced to explore patterns and mechanisms of debris redistribution. Median debris thicknesses ranged from 0.12 to 0.17 m and were spatially variable. We observed localized debris thinning across ice cliff faces, except those which were decaying, where debris thickened. We observed pervasive debris thinning across larger, backwasting slopes, including those bordered by supraglacial streams, as well as ingestion of debris by a newly exposed englacial conduit. Debris stability mapping showed that 18.2–26.4% of the survey area was theoretically subject to debris remobilization. By linking changes in stability to changes in debris thickness, we observed that slopes that remain stable, stabilize, or remain unstable between periods almost exclusively show net debris thickening (mean 0.07 m a‐1) whilst those which become newly unstable exhibit both debris thinning and thickening. We observe a systematic downslope increase in the rate at which debris cover thickens which can be described as a function of the topographic position index and slope gradient. Our data provide quantifiable insights into mechanisms of debris remobilization on glacier surfaces over sub‐decadal timescales, and open avenues for future research to explore glacier‐scale spatiotemporal patterns of debris remobilization

EBV Promotes Human CD8+ NKT Cell Development

The reports on the origin of human CD8+ Vα24+ T-cell receptor (TCR) natural killer T (NKT) cells are controversial. The underlying mechanism that controls human CD4 versus CD8 NKT cell development is not well-characterized. In the present study, we have studied total 177 eligible patients and subjects including 128 healthy latent Epstein-Barr-virus(EBV)-infected subjects, 17 newly-onset acute infectious mononucleosis patients, 16 newly-diagnosed EBV-associated Hodgkin lymphoma patients, and 16 EBV-negative normal control subjects. We have established human-thymus/liver-SCID chimera, reaggregated thymic organ culture, and fetal thymic organ culture. We here show that the average frequency of total and CD8+ NKT cells in PBMCs from 128 healthy latent EBV-infected subjects is significantly higher than in 17 acute EBV infectious mononucleosis patients, 16 EBV-associated Hodgkin lymphoma patients, and 16 EBV-negative normal control subjects. However, the frequency of total and CD8+ NKT cells is remarkably increased in the acute EBV infectious mononucleosis patients at year 1 post-onset. EBV-challenge promotes CD8+ NKT cell development in the thymus of human-thymus/liver-SCID chimeras. The frequency of total (3% of thymic cells) and CD8+ NKT cells (∼25% of NKT cells) is significantly increased in EBV-challenged chimeras, compared to those in the unchallenged chimeras (<0.01% of thymic cells, CD8+ NKT cells undetectable, respectively). The EBV-induced increase in thymic NKT cells is also reflected in the periphery, where there is an increase in total and CD8+ NKT cells in liver and peripheral blood in EBV-challenged chimeras. EBV-induced thymic CD8+ NKT cells display an activated memory phenotype (CD69+CD45ROhiCD161+CD62Llo). After EBV-challenge, a proportion of NKT precursors diverges from DP thymocytes, develops and differentiates into mature CD8+ NKT cells in thymus in EBV-challenged human-thymus/liver-SCID chimeras or reaggregated thymic organ cultures. Thymic antigen-presenting EBV-infected dendritic cells are required for this process. IL-7, produced mainly by thymic dendritic cells, is a major and essential factor for CD8+ NKT cell differentiation in EBV-challenged human-thymus/liver-SCID chimeras and fetal thymic organ cultures. Additionally, these EBV-induced CD8+ NKT cells produce remarkably more perforin than that in counterpart CD4+ NKT cells, and predominately express CD8αα homodimer in their co-receptor. Thus, upon interaction with certain viruses, CD8 lineage-specific NKT cells are developed, differentiated and matured intrathymically, a finding with potential therapeutic importance against viral infections and tumors

Modern climate and erosion in the Himalaya

Between June and September each year, the Indian monsoon typically delivers about 80% of the Nepalese Himalaya's annual precipitation. Topography on the windward (southern) flank of the range modulates persistent spatial variations in precipitation along the length of the range. Where topography is stepped with an initial abrupt rise as the Himalaya abut the foreland and a second rise as topography ascends toward the high peaks, two bands of high precipitation prevail, each where relief passes a threshold value. In contrast, more uniform, northward rising topography localizes a single high-rainfall band near the front of the range. A dense meteorological network that was operated in the Marsyandi catchment from 1999 to 2004 in central Nepal gives more insight on spatial variability in precipitation. Annual precipitation decreases ten-fold between the rainfall peak (of ∼4. m/yr) on the southern flank of the Himalaya and the semi-arid rain shadow on its northern flank (40-50. km farther north). Modest contrasts in rainfall between ridges versus valleys during the monsoon are replaced by strong altitude-dependent precipitation contrasts in the winter. Strikingly, above ∼4. km altitude, ∼40% of the total precipitation arrives as winter snowfall. Four years of daily discharge and suspended sediment measurements on the main-stem and on several tributaries of the Marsyandi during the monsoon document a strong north-south gradient in average erosion rates. Based on a suspended-to-bedload ratio of 2:1 (as estimated from grain-sizes in a landslide-dammed paleo-lake), erosion rates range from ∼0.1. mm/yr in the northern rain shadow to ∼2. mm/yr in the monsoon-drenched south. This strong modern spatial gradient in erosion rates mimics the precipitation gradient across the same area and broadly scales with specific discharge. In the wetter regions, nearly a meter of rain is required before significant sediment fluxes occur. After this initial meter of rain, the daily rainfall required to trigger sediment pulses (attributable to landsliding) gradually decreases during the remaining monsoon season from ∼40. mm to 10. mm. In the higher altitude rain shadow to the north, water discharge is more closely linked to temperature than to precipitation: a linkage suggesting that melting of snow and ice, rather than rainfall, modulates the runoff. The sediment flux in the rain shadow during the monsoon season displays a marked temporal hysteresis: fluxes broadly scale with discharge during the first two months of the monsoon, but decouple from discharge later in the monsoon. This behavior suggests that the sediment flux is supply limited. We interpret that much of the sediment is subglacially derived and that its transport into the river network is restricted either by limited bedrock erosion or subglacial hydrology. © 2012 Académie des sciences

Modern climate and erosion in the Himalaya

Between June and September each year, the Indian monsoon typically delivers about 80% of the Nepalese Himalaya's annual precipitation. Topography on the windward (southern) flank of the range modulates persistent spatial variations in precipitation along the length of the range. Where topography is stepped with an initial abrupt rise as the Himalaya abut the foreland and a second rise as topography ascends toward the high peaks, two bands of high precipitation prevail, each where relief passes a threshold value. In contrast, more uniform, northward rising topography localizes a single high-rainfall band near the front of the range. A dense meteorological network that was operated in the Marsyandi catchment from 1999 to 2004 in central Nepal gives more insight on spatial variability in precipitation. Annual precipitation decreases ten-fold between the rainfall peak (of ∼4. m/yr) on the southern flank of the Himalaya and the semi-arid rain shadow on its northern flank (40-50. km farther north). Modest contrasts in rainfall between ridges versus valleys during the monsoon are replaced by strong altitude-dependent precipitation contrasts in the winter. Strikingly, above ∼4. km altitude, ∼40% of the total precipitation arrives as winter snowfall. Four years of daily discharge and suspended sediment measurements on the main-stem and on several tributaries of the Marsyandi during the monsoon document a strong north-south gradient in average erosion rates. Based on a suspended-to-bedload ratio of 2:1 (as estimated from grain-sizes in a landslide-dammed paleo-lake), erosion rates range from ∼0.1. mm/yr in the northern rain shadow to ∼2. mm/yr in the monsoon-drenched south. This strong modern spatial gradient in erosion rates mimics the precipitation gradient across the same area and broadly scales with specific discharge. In the wetter regions, nearly a meter of rain is required before significant sediment fluxes occur. After this initial meter of rain, the daily rainfall required to trigger sediment pulses (attributable to landsliding) gradually decreases during the remaining monsoon season from ∼40. mm to 10. mm. In the higher altitude rain shadow to the north, water discharge is more closely linked to temperature than to precipitation: a linkage suggesting that melting of snow and ice, rather than rainfall, modulates the runoff. The sediment flux in the rain shadow during the monsoon season displays a marked temporal hysteresis: fluxes broadly scale with discharge during the first two months of the monsoon, but decouple from discharge later in the monsoon. This behavior suggests that the sediment flux is supply limited. We interpret that much of the sediment is subglacially derived and that its transport into the river network is restricted either by limited bedrock erosion or subglacial hydrology. © 2012 Académie des sciences