Devising quality assurance procedures for assessment of legacy geochronological data relating to deglaciation of the last British-Irish Ice Sheet

This contribution documents the process of assessing the quality of data within a compilation of legacy geochronological data relating to the last British-Irish Ice Sheet, a task undertaken as part of a larger community-based project (BRITICE-CHRONO) that aims to improve understanding of the ice sheet's deglacial evolution. As accurate reconstructions depend on the quality of the available data, some form of assessment is needed of the reliability and suitability of each given age(s) in our dataset. We outline the background considerations that informed the quality assurance procedures devised given our specific research question. We describe criteria that have been used to make an objective assessment of the likelihood that an age is influenced by the technique specific sources of geological uncertainty. When these criteria were applied to an existing database of all geochronological data relating to the last British-Irish Ice Sheet they resulted in a significant reduction in data considered suitable for synthesis. The assessed data set was used to test a Bayesian approach to age modelling ice stream retreat and we outline our procedure that allows us to minimise the influence of potentially erroneous data and maximise the accuracy of the resultant age models

Организация деятельности педагога по совершенствованию умений диалогической речи у детей старшего дошкольного возраста.

В работе рассмотрены основы психического и речевого развития детей старшего дошкольного возраста, охарактеризованы лингво-методические основы развития диалогической речи у старшего дошкольного возраста, раскрыто содержание деятельности педагога по совершенствованию умений диалогической речи у детей старшего дошкольного возраста, разработан комплекс педагогических мероприятий по теме исследования

Extent and retreat history of the Barra Fan Ice Stream offshore western Scotland and northern Ireland during the last glaciation

During the Last Glacial Maximum (LGM) the marine-terminating Barra Fan Ice Stream (BFIS), a major conduit of the British Irish Ice Sheet (BIIS), drained much of western Scotland and northwest Ireland with ice streaming onto the continental shelf of the Malin Sea. The extent and retreat history of this ice stream across the shelf, until now, is not well known. In particular, geochronological constraints on the history of this ice stream have thus far been restricted to deep-sea cores or terrestrial cosmogenic nuclide dating onshore, with ages across the shelf absent. To understand the possible external forcing factors acting on this marine terminating ice stream during retreat, improved geochronological constraint on its deglaciation is necessary. Here, we present new geophysical data, marine sediment cores and over forty radiocarbon dates to provide important constraints on maximum extent of the BFIS, as well as the timing and pattern of retreat back across the Malin Shelf. Dated moraines and grounding-zone wedges (GZW) seen in seafloor sub-bottom profiles provide evidence that the BFIS reached the Malin Shelf edge during the LGM and was at its maximum extent around 26.7 ka BP. The presence of two sets of GZWs suggests that the style of retreat was episodic. The new radiocarbon chronology shows that retreat from the shelf edge was underway by 25.9 ka BP, with the majority of the continental shelf ice free by 23.2 ka BP, and that glacimarine conditions were present in the Sea of Hebrides by 20.2 ka BP at the latest. Collectively, these results indicate that the majority of the Malin Shelf was free of grounded ice by ∼21.5–20 ka BP, which is up to 4000 years earlier than previously reconstructed. We attribute this early deglaciation to high relative sea level caused by glacial isostatic depression when the BIIS reached its maximum extent promoting ice shelf and grounding line instability. Two deep troughs, forming reverse bed slopes, aided the continued retreat of the BFIS. This suggests that local ice loading and bed morphology can be significant controls on the destabilisation of a marine-terminating ice stream and can override the influence of ocean and atmospheric temperatures

Timing and pace of ice-sheet withdrawal across the marine-terrestrial transition west of Ireland during the last glaciation

Understanding the pace and drivers of marine-based ice-sheet retreat relies upon the integration of numerical ice-sheet models with observations from contemporary polar ice sheets and well-constrained palaeo-glaciological reconstructions. This paper provides a reconstruction of the retreat of the last British–Irish Ice Sheet (BIIS) from the Atlantic shelf west of Ireland during and following the Last Glacial Maximum (LGM). It uses marine-geophysical data and sediment cores dated by radiocarbon, combined with terrestrial cosmogenic nuclide and optically stimulated luminescence dating of onshore ice-marginal landforms, to reconstruct the timing and rate of ice-sheet retreat from the continental shelf and across the adjoining coastline of Ireland, thus including the switch from a marine- to a terrestrially-based ice-sheet margin. Seafloor bathymetric data in the form of moraines and grounding-zone wedges on the continental shelf record an extensive ice sheet west of Ireland during the LGM which advanced to the outer shelf. This interpretation is supported by the presence of dated subglacial tills and overridden glacimarine sediments from across the Porcupine Bank, a westwards extension of the Irish continental shelf. The ice sheet was grounded on the outer shelf at ~26.8 ka cal bp with initial retreat underway by 25.9 ka cal bp. Retreat was not a continuous process but was punctuated by marginal oscillations until ~24.3 ka cal bp. The ice sheet thereafter retreated to the mid-shelf where it formed a large grounding-zone complex at ~23.7 ka cal bp. This retreat occurred in a glacimarine environment. The Aran Islands on the inner continental shelf were ice-free by ~19.5 ka bp and the ice sheet had become largely terrestrially based by 17.3 ka bp. This suggests that the Aran Islands acted to stabilize and slow overall ice-sheet retreat once the BIIS margin had reached the inner shelf. Our results constrain the timing of initial retreat of the BIIS from the outer shelf west of Ireland to the period of minimum global eustatic sea level. Initial retreat was driven, at least in part, by glacio-isostatically induced, high relative sea level. Net rates of ice-sheet retreat across the shelf were slow (62–19 m a−1) and reduced (8 m a−1) as the ice sheet vacated the inner shelf and moved onshore. A picture therefore emerges of an extensive BIIS on the Atlantic shelf west of Ireland, in which early, oscillatory retreat was followed by slow episodic retreat which decelerated further as the ice margin became terrestrially based. More broadly, this demonstrates the importance of localized controls, in particular bed topography, on modulating the retreat of marine-based sectors of ice sheets

Catchment hydro-geomorphological responses to environmental change in the southern uplands of Scotland

Lake sediment and geomorphic evidence from the Loch of the Lowes/St Mary's Loch basin in the central Southern Uplands of Scotland provide a multiproxy reconstruction of changing sediment availability and transmission through the catchment. Interrogation of magnetic, geochemical and grain size parameters for lake and catchment materials suggests it is possible to identify independent proxies that reflect both supply (availability) and discharge (capacity) controls on the sediment signal. Chronological control for the lake sediment record is proposed by linking a 210Pb/137Cs chronology for the last c. 120 yr to an age/depth profile based on proposed temporal correlations between cyclic HIRM/χLF variability and the North Atlantic Oscillation. Geochronological studies on debris cones and alluvial fans yield evidence for episodic hillslope gullying ~2000—0 BC with more extensive region-wide slope instability AD 700—900, 1100—1300 and after AD 1450—1550 and gully stabilization over the last 150 years. The latter two episodes coincide with the lake sediment evidence for increased sediment supply from ~AD 1600 declining after ~AD 1870. The capacity-related lake proxies appear to identify phases of increased flooding ~AD 1625—1650, 1680—1700, 1730—1760, 1800—1815, 1850—1880, 1910—1930, 1960—1970 and possibly the 1990s. Close correspondence between the sediment `flood' archive and historical records of flooding in Scotland suggests that lake-catchment systems of this type have the potential to yield valuable information on past hydrological response

Testing a cellular modelling approach to simulating late-Holocene sediment and water transfer from catchment to lake in the French Alps since 1826

This paper describes the application of a hydrogeomorphological numerical model (CAESAR) to simulate, at hourly time steps, changes in the hydrological and sediment regime of the Petit lac d’Annecy catchment. The outputs of the model were validated in three ways. In the short term (~5 years), water discharge outputs were compared against observed instrumental data. Over the longer term, modelled sediment discharge (AD 1825–2005) was compared with proxies for detrital sediment influx (environmental magnetism) and accumulation rates discerned from a 210Pb chronology for the lake sediments.Finally, spatial validation of the modelled erosion and deposition of sediment was undertaken by comparison with a field and remotely sensed survey of catchment geomorphology. The results suggest that while minor perturbations in forest cover during the last 180 years have partially conditioned the response of the sediment system, the bulk of modelled sediment discharge and particularly the peaks in sediment discharge were controlled by flood duration and magnitude, which in turn is driven by precipitation (storms/floods) and snowmelt. Basin geometry and geomorphology of each sub-catchment (Ire and Tamie) were also important in producing differences in the modelled sediment discharge. In essence, these differences were a function of sediment accommodation space and the ability of each system to store and release sediments. Modelled sediment discharge and χpara (lake sediments) display similar histories, and thus are both interpreted as reflecting variations in detrital sediment supply. Intriguingly the style of modelled sediment discharge from the Ire, a confined mountain torrent, displays a greater similarity to and perhaps dominates the lake sediment record. These results provide partial validation of the CAESAR model and indicate that perhaps in the future it may be used as an exploratory and predictive tool in determining the impact of changes in climate, meteorology and land use on lake-catchment systems

Fluvial development and the sediment regime of the lower Calder, Ribble catchment, northwest England

In the lower reaches of the Calder (Lancashire) tributary of the Ribble catchment (northwest England), five river terraces set into the surrounding glacial terrain have been mapped and their underlying sediments radiocarbon dated. The earliest terrace (T1) reflects sandur-style deposition during deglaciation and it aggraded in a reach cut into glacial diamict and glaciolacustrine muds. Incision below T1 spanned the start of the Holocene, driven by a combination of reduced sediment supply under a stabilising landscape and lower base-levels. T2 had aggraded and was being abandoned by 4000 BC; its formation latterly involving a period of comparative stability in terms of channel migration, perhaps a function of extensive forest cover and sea levels reaching above present day levels. The last 3000 years were marked by substantial changes in the sedimentary regime, with increased and varying sediment supply driving cycles of cut-and-fill and greater lateral channel migration. These changes coincided with the first widespread and substantial reductions in forest cover and a progression towards an agricultural landscape. Between the aggradation of terraces T3 and T4 there was a substantial increase in sediment supply, which is broadly in keeping with the erosion history in the surrounding uplands. This sediment slug appears to have induced a switch from highly sinuous meandering to lower sinuosity channels characterised by lateral scroll-style migration. Late Holocene vegetation changes, particularly woodland removal, driven by anthropogenic land-pressure appear to have rendered the floodplain prone to channel migration and the landscape more susceptible to erosion

Compilation of non-annually resolved Holocene proxy climate records: stacked Holocene peatland palaeo-water table reconstructions from northern Britain

The number and range of Holocene palaeoclimate reconstructions from various regions of the world have increased dramatically over the last decade. The data density for many regions and proxies now offers the potential of robust regional-scale reconstructions that avoid the problems of records from individual sites, and improve communication between palaeoclimate subdisciplines and climate modellers. However, there are problems with chronological uncertainties and quantification of proxies, which make compilation of multiple records difficult. Here we explore a �stacking� and �tuning� approach to the derivation of regional records from peatland climate proxies to test its applicability to non-annually resolved terrestrial records. Twelve individual records from northern Britain based on water table reconstructions from testate amoebae analysis were divided into four regions. Records were detrended, normalised and compared within regions to identify clear correlative events. The original chronologies of the records were tuned using both these events and independent age markers. The stacked record for northern Britain indicates pronounced changes to wet conditions at 3600, 2760 and 1600 cal yr BP with more minor changes at 3060, 2050, 1260, 860, 550 and 260 cal yr BP. The main wet phases are highly correlated with mid-European lake highstands, wider North Atlantic climate change inferred from ocean and ice core records, and solar variability. Tuning and stacking of non-annual terrestrial palaeoclimate records is a new approach to the compilation and reconciliation of individual records within coherent climatic regions and provides a tool for upscaling of palaeoclimate records for climate model-data comparisons