A quasi-annual record of time-transgressive esker formation: implications for ice sheet reconstruction and subglacial hydrology

We identify and map chains of esker beads (series of aligned mounds) up to 15 m high and on average ~ 65 m wide across central Nunavut, Canada from the high-resolution (2 m) ArcticDEM. Based on the close one-to-one association with regularly spaced, sharp crested ridges interpreted as De Geer moraines, we interpret the esker beads to be quasi-annual ice-marginal deposits formed time-transgressively at the mouth of subglacial conduits during deglaciation. Esker beads therefore preserve a high-resolution record of ice-margin retreat and subglacial hydrology. The well-organised beaded esker network implies that subglacial channelised drainage was relatively fixed in space and through time. Downstream esker bead spacing constrains the typical pace of deglaciation in central Nunavut between 7.2 and 6 ka 14C BP to 165–370 m yr−1, although with short periods of more rapid retreat (> 400 m yr−1). Under our time-transgressive interpretation, the lateral spacing of the observed eskers provides a true measure of subglacial conduit spacing for testing mathematical models of subglacial hydrology. Esker beads also record the volume of sediment deposited in each melt season, thus providing a minimum bound on annual sediment fluxes, which is in the range of 103–104 m3 yr−1 in each 6–10 km wide subglacial conduit catchment. We suggest the prevalence of esker beads across this predominantly marine terminating sector of the former Laurentide Ice Sheet is a result of sediment fluxes that were unable to backfill conduits at a rate faster than ice-margin retreat. Esker ridges, conversely, are hypothesised to form when sediment backfilling of the subglacial conduit outpaced retreat resulting in headward esker growth close to but behind the margin. The implication, in accordance with recent modelling results, is that eskers in general record a composite signature of ice-marginal drainage rather than a temporal snapshot of ice-sheet wide subglacial drainage

Conceptual model for the formation of bedforms along subglacial meltwater corridors (SMCs) by variable ice‐water‐bed interactions

Subglacial meltwater landforms found on palaeo-ice sheet beds allow the properties of meltwater drainage to be reconstructed, informing our understanding of modern-day subglacial hydrological processes. In northern Canada and Fennoscandia, subglacial meltwater landforms are largely organized into continental-scale networks of subglacial meltwater corridors (SMCs), interpreted as the relics of subglacial drainage systems undergoing variations in meltwater input, effective pressure and drainage efficiency. We review the current state of knowledge of bedforms (hummocks, ridges, murtoos, ribbed bedforms) and associated landforms (channels, eskers) described along SMCs and use selected high-resolution DEMs in Canada and Fennoscandia to complete the bedform catalogue and categorize their characteristics, patterning and spatial distributions. We synthesize the diversity of bedform and formation processes occurring along subglacial drainage routes in a conceptual model invoking spatiotemporal changes in hydraulic connectivity, basal meltwater pressure and ice-bed coupling, which influences the evolution of subglacial processes (bed deformation, erosion, deposition) along subglacial drainage systems. When the hydraulic capacity of the subglacial drainage system is overwhelmed glaciofluvial erosion and deposition will dominate in the SMC, resulting in tracts of hummocks and ridges arising from both fragmentation of underlying pre-existing bedforms and downstream deposition of sediments in basal cavities and crevasses. Re-coupling of ice with the bed, when meltwater supply decreases, facilitates deformation, transforming existing and producing new bedforms concomitant with the wider subglacial bedform imprint. We finally establish a range of future research perspectives to improve understanding of subglacial hydrology, geomorphic processes and bedform diversity along SMCs. These perspectives include the new acquisition of remote-sensing and field-based sedimentological and geomorphological data, a better connection between the interpreted subglacial drainage configurations down corridors and the mathematical treatments studying their stability, and the quantification of the scaling, distribution and evolution of the hydraulically connected drainage system beneath present-day ice masses to test our bedform-related conceptual model

Modulation of Macrophage Activation State Protects Tissue from Necrosis during Critical Limb Ischemia in Thrombospondin-1-Deficient Mice

International audienceBACKGROUND: Macrophages, key regulators of healing/regeneration processes, strongly infiltrate ischemic tissues from patients suffering from critical limb ischemia (CLI). However pro-inflammatory markers correlate with disease progression and risk of amputation, suggesting that modulating macrophage activation state might be beneficial. We previously reported that thrombospondin-1 (TSP-1) is highly expressed in ischemic tissues during CLI in humans. TSP-1 is a matricellular protein that displays well-known angiostatic properties in cancer, and regulates inflammation in vivo and macrophages properties in vitro. We therefore sought to investigate its function in a mouse model of CLI. METHODS AND FINDINGS: Using a genetic model of tsp-1(-/-) mice subjected to femoral artery excision, we report that tsp-1(-/-) mice were clinically and histologically protected from necrosis compared to controls. Tissue protection was associated with increased postischemic angiogenesis and muscle regeneration. We next showed that macrophages present in ischemic tissues exhibited distinct phenotypes in tsp-1(-/-) and wt mice. A strong reduction of necrotic myofibers phagocytosis was observed in tsp-1(-/-) mice. We next demonstrated that phagocytosis of muscle cell debris is a potent pro-inflammatory signal for macrophages in vitro. Consistently with these findings, macrophages that infiltrated ischemic tissues exhibited a reduced postischemic pro-inflammatory activation state in tsp-1(-/-) mice, characterized by a reduced Ly-6C expression and a less pro-inflammatory cytokine expression profile. Finally, we showed that monocyte depletion reversed clinical and histological protection from necrosis observed in tsp-1(-/-) mice, thereby demonstrating that macrophages mediated tissue protection in these mice. CONCLUSION: This study defines targeting postischemic macrophage activation state as a new potential therapeutic approach to protect tissues from necrosis and promote tissue repair during CLI. Furthermore, our data suggest that phagocytosis plays a crucial role in promoting a deleterious intra-tissular pro-inflammatory macrophage activation state during critical injuries. Finally, our results describe TSP-1 as a new relevant physiological target during critical leg ischemia

Subglacial Drainage Networks of the Fennoscandian Ice Sheet

Ice sheet dynamics are modulated by subglacial meltwater, which can promote basal sliding and affect ice flow velocity. However, logistical challenges of measuring subglacial processes beneath contemporary ice sheets hinder our understanding of their spatio-temporal evolution. This thesis uses the extensive landform record of subglacial meltwater landforms in Fennoscandia to forward our understanding of the evolution of subglacial drainage networks and their relation to ice sheet retreat. Using high-resolution (2-5 m) digital elevation models, integrated networks of subglacial meltwater landforms – herein called subglacial meltwater routes – are mapped at an ice sheet- scale (~1.4 million km^2). Subglacial meltwater routes comprise eskers, tunnel valleys, subglacial meltwater channels and subglacial meltwater corridors. The analyses of >34,000 features show that subglacial meltwater routes preferentially occur in thick drift and form integrated networks. Elongated network geometries are interpreted to reflect the control of ice-surface gradients on subglacial drainage. Abrupt and laterally traceable line density changes are interpreted to record periods of greater cumulative geomorphic work during pauses in retreat. Esker enlargements – significant ridge- widenings along esker ridges – are interpreted to form due to collapse of subglacial conduits which is a potentially underestimated process during deglaciation. The orientation of subglacial meltwater routes is used to build a geometrical reconstruction of the retreat of the Fennoscandian Ice Sheet using formlines – subglacial hydrologic equipotential lines constructed perpendicular to the prevalent subglacial meltwater route direction. Cross-cutting relationships between formlines are used to disentangle the relative drainage chronology in Fennoscandia. Finally, these observations are combined with theoretical considerations to derive a conceptual model of the time-transgressive development of subglacial drainage networks during deglaciation. The model invokes progressive headward growth of subglacial meltwater routes, such that already established routes grow at the expense of other routes that become abandoned in order to sustain an efficient thread network

Distribution, characteristics and formation of esker enlargements

Eskers are primarily ridges of glaciofluvial sediment deposited in subglacial, englacial and supraglacial conduits. They are typically straight to sinuous features, however, their planform morphology can be highly diverse. Esker enlargements are spatially confined ridge sections that are significantly wider than the trunk ridge (typically 250–400 m) and that reconverge downflow. The enlargements include complex ridge networks or coherent sediment bodies. We mapped >1400 esker enlargements across Fennoscandia and Keewatin, Canada, to investigate their distribution and morphological characteristics. Esker enlargements are less abundant below the marine limit, and tend to become more abundant in areas of faster ice retreat. They form local clusters along particular ridges, and can occasionally be traced laterally between adjacent esker systems. Based on morphological observations, we link their formation to roof collapses in subglacial conduits. The distribution of esker enlargements indicates that subglacial conduit collapse became an increasingly significant process during the final stages of deglaciation of both the Scandinavian and Laurentide ice sheets, and may have exerted a positive feedback on ice sheet retreat at land-terminating ice margins