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Sediment-rich meltwater plumes and ice-proximal fans at the margins of modern and ancient tidewater glaciers: Observations and modeling
Turbid meltwater plumes and ice-proximal fans occur where subglacial streams reach the grounded marine margins of modern and ancient tidewater glaciers. However, the spacing and temporal stability of these subglacial channels is poorly understood. This has significant implications for understanding the geometry and distribution of Quaternary and ancient ice-proximal fans that can form important aquifers and hydrocarbon reservoirs. Remote-sensing and numerical-modelling techniques are applied to the 200 km-long marine margin of a Svalbard ice cap, Austfonna, to quantify turbid meltwaterplume distribution and predict its temporal stability. Results are combined with observations from geophysical data close to the modern ice front to refine existing depositional models for ice-proximal fans. Plumes are spaced about 3 km apart and their distribution along the ice front is stable over decades. Numerical modelling also predicts the drainage pattern and meltwater discharge beneath the ice cap; modelled water-routing patterns are in reasonable agreement with satellite-mapped plume locations. However, glacial retreat of several kilometres over the past 40 years has limited build-up of significant ice-proximal fans. A single fan and moraine ridge is noted from marine-geophysical surveys. Closer to the ice front there are smaller recessional moraines and polygonal sediment lobes but no identifiable fans. Schematic models of ice-proximal deposits represent varying glacierterminus stability: (i) stable terminus where meltwater sedimentation produces an ice-proximal fan; (ii) quasi-stable terminus, where glacier readvance pushes or thrusts up ice-proximal deposits into a morainal bank; (iii) retreating terminus, with short still-stands, allowing only small sediment lobes to build up at melt-stream portals. These modern investigations are complemented with outcrop and subsurface observations and numerical modelling of an ancient, Ordovician glacial system. Thick turbidite successions and large fans in the Late Ordovician suggest either high-magnitude events or sustained high discharge, consistent with a relatively mild palaeo-glacial setting.We thank BP Algeria for sponsorship, Ed Jones and Liz Jolley of BP for supporting the project.This is the accepted manuscript. The final published version is available from Wiley at http://dx.doi.org/10.1111/sed.1219
Changes in Nicotinic Neurotransmission during Enteric Nervous System Development
Acetylcholine-activating pentameric nicotinic receptors (nAChRs) are an essential mode of neurotransmission in the enteric nervous system (ENS). In this study, we examined the functional development of specific nAChR subtypes in myenteric neurons using Wnt1-Cre;R26R-GCaMP3 mice, where all enteric neurons and glia express the genetically encoded calcium indicator, GCaMP3. Transcripts encoding α3, α4, α7, β2, and β4 nAChR subunits were already expressed at low levels in the E11.5 gut and by E14.5 and, thereafter, α3 and β4 transcripts were the most abundant. The effect of specific nAChR subtype antagonists on evoked calcium activity in enteric neurons was investigated at different ages. Blockade of the α3β4 receptors reduced electrically and chemically evoked calcium responses at E12.5, E14.5, and P0. In addition to the α3β4 antagonist, antagonists to α3β2 and α4β2 also significantly reduced responses by P10-11 and in adult preparations. Therefore, there is an increase in the diversity of functional nAChRs during postnatal development. However, an α7 nAChR antagonist had no effect at any age. Furthermore, at E12.5 we found evidence for unconventional receptors that were responsive to the nAChR agonists 1-dimethyl-4-phenylpiperazinium and nicotine, but were insensitive to the general nicotinic blocker, hexamethonium. Migration, differentiation, and neuritogenesis assays did not reveal a role for nAChRs in these processes during embryonic development. In conclusion, there are significant changes in the contribution of different nAChR subunits to synaptic transmission during ENS development, even after birth. This is the first study to investigate the development of cholinergic transmission in the ENS
Ion Channel Expression in the Developing Enteric Nervous System
The enteric nervous system arises from neural crest-derived cells (ENCCs) that migrate caudally along the embryonic gut. The expression of ion channels by ENCCs in embryonic mice was investigated using a PCR-based array, RT-PCR and immunohistochemistry. Many ion channels, including chloride, calcium, potassium and sodium channels were already expressed by ENCCs at E11.5. There was an increase in the expression of numerous ion channel genes between E11.5 and E14.5, which coincides with ENCC migration and the first extension of neurites by enteric neurons. Previous studies have shown that a variety of ion channels regulates neurite extension and migration of many cell types. Pharmacological inhibition of a range of chloride or calcium channels had no effect on ENCC migration in cultured explants or neuritogenesis in vitro. The non-selective potassium channel inhibitors, TEA and 4-AP, retarded ENCC migration and neuritogenesis, but only at concentrations that also resulted in cell death. In summary, a large range of ion channels is expressed while ENCCs are colonizing the gut, but we found no evidence that ENCC migration or neuritogenesis requires chloride, calcium or potassium channel activity. Many of the ion channels are likely to be involved in the development of electrical excitability of enteric neurons