Exploring controls of the early and stepped deglaciation on the western margin of the British Irish Ice Sheet

New optically stimulated luminescence dating and Bayesian models integrating all legacy and BRITICE-CHRONO geochronology facilitated exploration of the controls on the deglaciation of two former sectors of the British–Irish Ice Sheet, the Donegal Bay (DBIS) and Malin Sea ice-streams (MSIS). Shelf-edge glaciation occurred ~27 ka, before the global Last Glacial Maximum, and shelf-wide retreat began 26–26.5 ka at a rate of ~18.7–20.7 m a^–1. MSIS grounding zone wedges and DBIS recessional moraines show episodic retreat punctuated by prolonged still-stands. By ~23–22 ka the outer shelf (~25 000 km²) was free of grounded ice. After this time, MSIS retreat was faster (~20 m a^–1 vs. ~2–6 m a^–1 of DBIS). Separation of Irish and Scottish ice sources occurred ~20–19.5 ka, leaving an autonomous Donegal ice dome. Inner Malin shelf deglaciation followed the submarine troughs reaching the Hebridean coast ~19 ka. DBIS retreat formed the extensive complex of moraines in outer Donegal Bay at 20.5–19 ka. DBIS retreated on land by ~17–16 ka. Isolated ice caps in Scotland and Ireland persisted until ~14.5 ka. Early retreat of this marine-terminating margin is best explained by local ice loading increasing water depths and promoting calving ice losses rather than by changes in global temperatures. Topographical controls governed the differences between the ice-stream retreat from mid-shelf to the coast.     

Pattern,style and timing of British–Irish Ice Sheet retreat: Shetland and northern North Sea sector

The offshore sector around Shetland remains one of the least well-studied parts of the former British–Irish Ice Sheet with several long-standing scientific issues unresolved. These key issues include (i) the dominance of a locally sourced ‘Shetland ice cap’ vs an invasive Fennoscandian Ice Sheet; (ii) the flow configuration and style of glaciation at the Last Glacial Maximum (i.e. terrestrial vs marine glaciation); (iii) the nature of confluence between the British–Irish and Fennoscandian Ice Sheets; (iv) the cause, style and rate of ice sheet separation; and (v) the wider implications of ice sheet uncoupling on the tempo of subsequent deglaciation. As part of the Britice-Chrono project, we present new geological (seabed cores), geomorphological, marine geophysical and geochronological data from the northernmost sector of the last British–Irish Ice Sheet (north of 59.5°N) to address these questions. The study area covers ca. 95 000 km², an area approximately the size of Ireland, and includes the islands of Shetland and the surrounding continental shelf, some of the continental slope, and the western margin of the Norwegian Channel. We collect and analyse data from onshore in Shetland and along key transects offshore, to establish the most coherent picture, so far, of former ice-sheet deglaciation in this important sector. Alongside new seabed mapping and Quaternary sediment analysis, we use a multi-proxy suite of new isotopic age assessments, including 32 cosmogenic-nuclide exposure ages from glacially transported boulders and 35 radiocarbon dates from deglacial marine sediments, to develop a synoptic sector-wide reconstruction combining strong onshore and offshore geological evidence with Bayesian chronosequence modelling. The results show widespread and significant spatial fluctuations in size, shape and flow configuration of an ice sheet/ice cap centred on, or to the east of, the Orkney–Shetland Platform, between ~30 and ~15 ka BP. At its maximum extent ca. 26–25 ka BP , this ice sheet was coalescent with the Fennoscandian Ice Sheet to the east. Between ~25 and 23 ka BP the ice sheet in this sector underwent a significant size reduction from ca. 85 000 to <50 000 km², accompanied by several ice-margin oscillations. Soon after, connection was lost with the Fennoscandian Ice Sheet and a marine corridor opened to the east of Shetland. This triggered initial (and unstable) re-growth of a glaciologically independent Shetland Ice Cap ca. 21–20 ka BP with a strong east–west asymmetry with respect to topography. Ice mass growth was followed by rapid collapse, from an area of ca. 45 000 km² to ca. 15 000 km² between 19 and 18 ka BP , stabilizing at ca. 2000 km² by ~17 ka BP. Final deglaciation of Shetland occurred ca. 17–15 ka BP , and may have involved one or more subsidiary ice centres on now-submerged parts of the continental shelf. We suggest that the unusually dynamic behaviour of the northernmost sector of the British–Irish Ice Sheet between 21 and 18 ka BP – characterized by numerous extensive ice sheet/ice mass readvances, rapid loss and flow redistributions – was driven by significant changes in ice mass geometry, ice divide location and calving flux as the glaciologically independent ice cap adjusted to new boundary conditions. We propose that this dynamism was forced to a large degree by internal (glaciological) factors specific to the strongly marine-influenced Shetland Ice Cap.     

Glacial landscape evolution in the Uummannaq region,West Greenland

The Uummannaq region is a mosaic of glacial landsystems, consistent with hypothesized landscape distribution resulting from variations in subglacial thermal regime. The region is dominated by selective linear erosion that has spatially and altitudinally partitioned the landscape. Low altitude areas are dominated by glacial scour and higher elevations are dominated by plateaux or mountain valley and cirque glaciers. The appearance and nature of each landscape type varies locally with altitude and latitude, as a function of bedrock geology and average glacial conditions. Selective linear erosion has been a primary control on landscape distribution throughout Uummannaq, leading to plateau formation and the growth of a coalescent fjord system in the Uummannaq region. This has allowed the development of the Uummannaq ice stream's (UIS) onset zone during glacial periods. Fjord development has been enhanced by a downstream change in geology to less‐resistant lithologies, increasing erosional efficiency and allowing a single glacial channel to develop, encouraging glacier convergence and the initiation of ice streaming. The landscape has been affected by several periods of regional uplift from 33 Ma to present, and has been subject to subsequent fluvial and glacial erosion. Uplift has removed surfaces from the impact of widespread warm‐based glaciation, leaving them as relict landsurfaces. The result of this is a regional altitude‐dependent continuum of glacial modification, with extreme differences in erosion between high and low elevation surfaces. This study indicates that processes of long‐term uplift, glacial erosion/protection and spatial variability in erosion intensity have produced a highly partitioned landscape.     

The environment of early Mars and the missing carbonates

Abstract– A model is presented in which the aqueous conditions needed to generate phyllosilicate minerals in the absence of carbonates found in the ancient Noachian crust are maintained by an early CO₂‐rich atmosphere, that, together with iron (II) oxidation, would prevent carbonate formation at the surface. After cessation of the internal magnetic dynamo, a CO₂‐rich primordial atmosphere was stripped by interactions with the solar wind and surface conditions evolved from humid to arid, with ground waters partially dissolving subsurface carbonate and sulfide minerals to produce acid‐sulfate evaporitic deposits in areas with upwelling ground water. In a subsequent geochemical state (Late Noachian to Hesperian), surface and subsurface acidic solutions were neutralized in the subsurface through interaction with basaltic crust, allowing the precipitation of secondary carbonates. This model suggests that, in the early Noachian, the surface waters of Mars maintained acidity because of a drop in temperature. This would have favored increased dissolution of CO₂ and a reduction in atmospheric pressure. In this scenario, physicochemical conditions precluded the formation of surface carbonates, but induced the precipitation of carbonates in the subsurface.     

Insights into the morphology of the Serra da Cangalha impact structure from geophysical modeling

Forward modeling is commonly applied to gravity field data of impact structures to determine the main gravity anomaly sources. In this context, we have developed 2.5‐D gravity models of the Serra da Cangalha impact structure for the purpose of investigating geological bodies/structures underneath the crater. Interpretation of the models was supported by ground magnetic data acquired along profiles, as well as by high resolution aeromagnetic data. Ground magnetic data reveal the presence of short‐wavelength anomalies probably related to shallow magnetic sources that could have been emplaced during the cratering process. Aeromagnetic data show that the basement underneath the crater occurs at an average depth of about 1.9 km, whereas in the region beneath the central uplift it is raised to 0.5–1 km below the current surface. These depths are also supported by 2.5‐D gravity models showing a gentle relief for the basement beneath the central uplift area. Geophysical data were used to provide further constraints for numeral modeling of crater formation that provided important information on the structural modification that affected the rocks underneath the crater, as well as on shock‐induced modifications of target rocks. The results showed that the morphology is consistent with the current observations of the crater and that Serra da Cangalha was formed by a meteorite of approximately 1.4 km diameter striking at 12 km s^?1.     

Karst subsidence as a control on the accumulation and preservation of aeolian deposits: A Pleistocene example from a proglacial outwash setting,Ebro Basin,Spain

Meltwater flows emanating from the Pyrenees during the Pleistocene constructed a braided outwash plain in the Ebro Basin and led to the karstification of the Neogene gypsum bedrock. Synsedimentary evaporite dissolution locally increased subsidence rates and generated dolines and collapses that enabled the accumulation and preservation of outwash gravels and associated windblown deposits that were protected from erosion by later meltwater flows. In these localized depocentres, maximum rates of wind deceleration resulted from airflow expansion, enabling the accumulation of cross‐stratified sets of aeolian strata climbing at steep angles and thereby preserving up to 5 m thick sets. The outwash plain was characterized by longitudinal and transverse fluvial gravel bars, channels and windblown facies organized into aeolian sand sheets, transverse and complex aeolian dunes, and loess accumulations. Flat‐lying aeolian deposits merge laterally to partly deformed aeolian deposits encased in dolines and collapses. Synsedimentary evaporite dissolution caused gravels and aeolian sand deposits to subside, such that formerly near‐horizontal strata became inclined and generated multiple internal angular unconformities. During episodes when the wind was undersaturated with respect to its potential sand transporting capacity, deflation occurred over the outwash plain and coarse‐grained lags with ventifacts developed. Subsequent high‐energy flows episodically reached the aeolian dune field, leading to dune destruction and the generation of hyperconcentrated flow deposits composed in part of reworked aeolian sands. Lacustrine deposits in the distal part of the outwash plain preserve rhythmically laminated lutites and associated Gilbert‐type gravel deltas, which developed when fluvial streams reached proglacial lakes. This study documents the first evidence of an extensive Pleistocene proglacial aeolian dune field located in the Ebro Basin (41˙50° N), south of what has hitherto been considered to be the southern boundary of Pleistocene aeolian deposits in Europe. A non‐conventional mechanism (evaporite karst‐related subsidence) for the preservation of aeolian sands in the stratigraphic record is proposed.