The Importance of Lithology and Throw Rate on Bedrock River Behaviour and Evolution in the Gediz (Alasehir) Graben, Turkey

The Gediz (Alasehir) Graben is located in the highly tectonically active and seismogenic region of Western Turkey, which has been experiencing high-angle normal faulting since ~ 2 Ma. Rivers upstream of the normal fault-bounded graben each contain a lithologic knickpoint related to the change in bedrock geology (from soft sediments to hard metamorphic rocks) and a non-lithologic knickpoint, caused by an increase in fault slip rate at ~ 0.8 Ma. Therefore, this system represents an ideal natural laboratory to investigate the relative roles of bedrock lithology / rock strength and rates of faulting on the behaviour and evolution of bedrock river systems. Our results show that metamorphic rocks in the catchments are 2-3 times harder than the sedimentary rocks. Stream power increases downstream reaching local maxima upstream of the fault within the metamorphic bedrock but declines rapidly once softer sedimentary rocks are encountered. We also demonstrate a positive correlation between throw rate and stream power in the metamorphic rocks characteristic of rivers obeying a detachment-limited model of erosion. In sedimentary rocks stream powers are invariant with throw rate but do scale with the river's sediment transport capacity. We also present new Be10 denudation rates that show correlations with calculated stream power and fault throw rates. This study demonstrates that the strength of underlying bedrock is a major influence on river evolution and that the nature of the underlying lithology profoundly influences the way in which the river behaves

Deglaciation of Fennoscandia

To provide a new reconstruction of the deglaciation of the Fennoscandian Ice Sheet, in the form of calendar-year time-slices, which are particularly useful for ice sheet modelling, we have compiled and synthesized published geomorphological data for eskers, ice-marginal formations, lineations, marginal meltwater channels, striae, ice-dammed lakes, and geochronological data from radiocarbon, varve, optically-stimulated luminescence, and cosmogenic nuclide dating. This 25 is summarized as a deglaciation map of the Fennoscandian Ice Sheet with isochrons marking every 1000 years between 22 and 13 cal kyr BP and every hundred years between 11.6 and final ice decay after 9.7 cal kyr BP. Deglaciation patterns vary across the Fennoscandian Ice Sheet domain, reflecting differences in climatic and geomorphic settings as well as ice sheet basal thermal conditions and terrestrial versus marine margins. For example, the ice sheet margin in the high-precipitation coastal setting of the western sector responded sensitively to climatic variations leaving a detailed record of prominent moraines and ice-marginal deposits in many fjords and coastal valleys. Retreat rates across the southern sector differed between slow retreat of the terrestrial margin in western and southern Sweden and rapid retreat of the calving ice margin in the Baltic Basin. Our reconstruction is consistent with much of the published research. However, the synthesis of a large amount of existing and new data support refined reconstructions in some areas. For example, we locate the LGM extent of the ice sheet in northwestern Russia further east than previously suggested and conclude that it occurred at a later time than the rest of the ice sheet, at around 17-15 cal kyr BP, and propose a slightly different chronology of moraine formation over southern Sweden based on improved correlations of moraine segments using new LiDAR data and tying the timing of moraine formation to Greenland ice core cold stages. Retreat rates vary by as much as an order of magnitude in different sectors of the ice sheet, with the lowest rates on the high-elevation and maritime Norwegian margin. Retreat rates compared to the climatic information provided by the Greenland ice core record show a general correspondence between retreat rate and climatic forcing, although a close match between retreat rate and climate is unlikely because of other controls, such as topography and marine versus terrestrial margins. Overall, the time slice reconstructions of Fennoscandian Ice Sheet deglaciation from 22 to 9.7 cal kyr BP provide an important dataset for understanding the contexts that underpin spatial and temporal patterns in retreat of the Fennoscandian Ice Sheet, and are an important resource for testing and refining ice sheet models

Weathering fluxes and sediment provenance on the SW Scottish shelf during the last deglaciation

The reconstruction of past ice sheet dynamics can shed a light on long-term ice stream activity, and in turn provide constraints on the response of modern ice sheets to climate change. The Hebrides Ice Stream (HIS) flowed across part of the western Scottish shelf to the shelf-break during the last glacial cycle and drained a large portion of the northern sector of the British Irish-Ice Sheet. To investigate the deglacial dynamics of the HIS following the Last Glacial Maximum, lead (Pb) isotope records were extracted from the FeMn oxyhydroxide and detrital fractions of recovered laminated glacimarine mud sequences to monitor the changing activity of HIS during its retreat. These provide timing and some source information of glacially weathered inputs to the marine environment. The FeMn oxyhydroxide fraction in the samples is dominated by allochthonous particles (pre-formed) and shows a marked decrease from radiogenic (≤ 20.05 206Pb/204Pb) at ~ 21 cal ka BP to less radiogenic Pb isotope compositions (~ 19.48) towards the Windermere Interstadial (15.4–13 ka). This decrease represents a reduction in the flux of subglacially-derived radiogenic Pb to the continental shelf is interpreted as being associated with the break-up of the ice-stream in western Scotland around that time. The Pb, Sr and Nd isotopic signatures of the detrital fraction indicate a preponderance of Moine-sourced fine sediments (originated from the NW Highlands) in the core locations from ~ 21 to 15 cal ka BP (Dimlington Stadial - Windermere Interstadial), most likely dictated by the orientation of ice flow, tidal and oceanic current directions and sediment delivery. In contrast, ice rafted debris in a ~ 21 cal ka BP old basal diamicton contains volcanic-derived material, suggesting different provenance for different grain sizes. The FeMn oxyhydroxide 208Pb/204Pb ratio shows an unusual inversion relative to the other Pb isotope ratios, and is attributed to the introduction of secondary weathering phases from a source with contrasting 208Pb/204Pb but similar 206Pb/204Pb and 207Pb/204Pb. In the detrital fraction, the inversions are constrained to periodic spikes, which may indicate an increased contribution from a high Th/U source, potentially the neighbouring Archaean amphibolitic Lewisian basement in the Outer Hebrides. This study demonstrates how geochemical investigation on continental shelves can be used to constrain the timing, activity and flow sources of palaeo-ice streams

Reply to comments by Bourgois et al. (2019) on: “Glacial lake evolution and Atlantic-Pacific drainage reversals during deglaciation of the Patagonia Ice Sheet”

No abstract available

Reply to comments by Bourgois et al. (2019) on: “Glacial lake evolution and Atlantic-Pacific drainage reversals during deglaciation of the Patagonia Ice Sheet”

We welcome the comments of Bourgois et al. (2019) and the opportunity to debate geomorphology, geochronology and palaeoclimate during the Late Glacial Interglacial Transition (LGIT, ~18.0-8.0 ka) in the region of the Río Baker, central Patagonia. Bourgois et al. (2019) conclude that we have propagated inconsistencies in our proposed reconstruction of palaeolake evolution due to geomorphic analytical bias. However, in our view the empirical geomorphological data we have compiled over many field seasons has resulted in a data-rich (though still incomplete) relative chronology that enables us to evaluate inconsistencies in landscape interpretations from previously published geochronological datasets. We would argue that a geochronological bias, over any geomorphological bias, has represented the main reason for multiple landscape interpretations in this region. Indeed, the conflicting palaeolake evolution models published for the Río Baker basin (Turner et al. 2005; Bell, 2008; Hein et al., 2010; Bourgois et al., 2016; Glasser et al., 2016; Martinod et al., 2016) was a major impetus for our paper. These contrasting models were in part a result of the coincident publication of two separate geochronological datasets in 2016, one focused on optically stimulated luminescence (OSL) dating of palaeolake landforms (Glasser et al., 2016), the other cosmogenic nuclide exposure ages (Bourgois et al., 2016). Both datasets provided updates on what we termed the Turner/Hein model in Thorndycraft et al. (2019), but as they did not have access to each other’s datasets they ended up with different landscape interpretations

Ice surface changes during recent glacial cycles along the Jutulstraumen and Penck Trough ice streams in western Dronning Maud Land, East Antarctica

Reconstructing past ice-sheet surface changes is key to testing and improving ice-sheet models. Data constraining the past behaviour of the East Antarctic Ice Sheet are sparse, limiting our understanding of its response to past, present and future climate change. Here, we report the first cosmogenic multi-nuclide (10Be, 26Al, 36Cl) data from bedrock and erratics on nunataks along the Jutulstraumen and Penck Trough ice streams in western Dronning Maud Land, East Antarctica. Spanning elevations between 741 and 2394 m above sea level, the samples have apparent exposure ages between 2 ka and 5 Ma. The highest-elevation bedrock sample indicates (near-) continuous minimum exposure since the Pliocene, with a low apparent erosion rate of 0.15 ± 0.03 m Ma−1, which is similar to results from eastern Dronning Maud Land. In contrast to studies in eastern Dronning Maud Land, however, our data show clear indications of a thicker-than-present ice sheet within the last glacial cycle, with a thinning of ∼35–120 m during the Holocene (∼2–11 ka). Difficulties in separating suitable amounts of quartz from the often quartz-poor rock-types in the area, and cosmogenic nuclides inherited from exposure prior to the last deglaciation, prevented robust thinning estimates from elevational profiles. Nevertheless, the results clearly demonstrate ice-surface fluctuations of several hundred meters between the current grounding line and the edge of the polar plateau for the last glacial cycle, a constraint that should be considered in future ice-sheet model simulations

A glacial chronology for sub-Antarctic Marion Island from MIS 2 and MIS 3

It is increasingly apparent that local and regional factors, including geographic location, topography and climatic variability, strongly influence the timing and extent of glaciations across the Southern Hemisphere. Glacial chronologies of sub-Antarctic islands can provide valuable insights into the nature of regional climatic variability and the localised response(s) of glacial systems during periods of climatic change. With new cosmogenic 36Cl exposure ages from Marion Island in the southern Indian Ocean, we provide the oldest dated terrestrial moraine sequences for the sub-Antarctic islands. Results confirm that a local Last Glacial Maximum was reached prior to ∼56 ka when ice retreated with localised stand still events at ∼43 ka and between ∼38 and 33 ka. Evidence of ice re-advances throughout MIS 2 are limited and particularly absent for the cooling periods at ∼32 and ∼21 ka, and retreat continued until ∼17 ka ago. Any MIS 1 readvances on the island would be confined to altitudes above 900 m a.s.l. but the Holocene exposure ages remains to be documented. We compare Marion Island's glacial chronology with other sub-Antarctic islands (e.g., the Kerguelen archipelago, Auckland and Campbell islands and South Georgia) and review the evidence for a Southern Hemisphere glacial maximum in late MIS 3 (∼41 ka). At a regional scale we recognize sea surface temperatures, sea ice extent and the latitudinal position of the Southern Westerly Wind belt as key controls on equilibrium-line altitudes and ice accumulation due to their influence on air temperature and precipitation regimes. At an island scale, geomorphological mapping shows that deglaciation of individual glacier lobes was a-synchronous due to local physiographical and topographical factors controlling the island's micro-climate. We suggest that variability in deglaciation chronologies at smaller scales (particularly at the sub-Antarctic Islands) are important to consider when untangling climatic drivers across the Southern Ocean

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

The deglacial history of 79N glacier and the Northeast Greenland Ice Stream

The Northeast Greenland Ice Stream (NEGIS) is the main artery for ice discharge from the northeast sector of the Greenland Ice Sheet (GrIS) to the North Atlantic. Understanding the past, present and future stability of the NEGIS with respect to atmospheric and oceanic forcing is of global importance as it drains around 17% of the GrIS and has a sea-level equivalent of 1.6 m. This paper reconstructs the deglacial and Holocene history of Nioghalvfjerdsbræ (or 79N Glacier); a major outlet of the NEGIS.At high elevation (>900 m asl) autochthonous blockfield, a lack of glacially moulded bedrock and pre LGM exposure ages point to a complex exposure/burial history extending back over half a million years. However, post Marine Isotope Stage 12, enhanced glacial erosion led to fjord incision and plateaux abandonment. Between 900 and 600 m asl the terrain is largely unmodified by glacial scour but post LGM erratics indicate the advection of cold-based ice through the fjord. In contrast, below ∼600 m asl Nioghalvfjerdsfjorden exhibits a geomorphological signal indicative of a warm-based ice stream operating during the last glacial cycle. Dated ice marginal landforms and terrain along the fjord walls show initial thinning rates were slow between ∼23 and 10 ka, but post-10 ka it is evident that Nioghalvfjerdsfjorden deglaciated extremely quickly with complete fjord deglaciation below ∼500 m asl between 10.0 and 8.5 ka.Both increasing air and ocean temperatures were pivotal in driving surface lowering and submarine melt during deglaciation, but the final withdrawal of ice through Nioghalvfjerdsfjorden was facilitated by the action of marine ice sheet instability. Our estimates show that thinning and retreat rates reached a maximum of 5.29 ma−1 and 613 ma−1, respectively, as the ice margin withdrew westwards. This would place the Early Holocene disintegration of this outlet of the NEGIS at the upper bounds of contemporary thinning and retreat rates seen both in Greenland and Antarctica. Combined with recent evidence of ice stream shutdown during the Holocene, as well as predictions of changing ice flow dynamics within downstream sections of the NEGIS catchment, this suggests that significant re-organisation of the terminal zone of the ice stream is imminent over the next century

The deglacial history of 79N glacier and the Northeast Greenland Ice Stream

Acknowledgements This work was funded by NERC Standard Grant NE/N011228/1. We thank the Alfred Wegner Institute, and particularly Hicham Rafiq and Daniel Steinhage, for their significant logistic support through the iGRIFF project. Additional support was provided from Station Nord (Jørgen Skafte), Nordland Air, Air Greenland, the Joint Arctic Command and the Department of Geography, Durham University. Naalakkersuisut, Government of Greenland, provided Scientific Survey (VU-00121) and Export (046/2017) licences for this work. We would also like to thank our Field Ranger Isak (Nanu-Travel) and dog Ooni for keeping us safe in the field. TCN Sample preparation was carried out at the National Environmental Isotope Facility, Scottish Universities Environmental Research Centre under grant allocation 9185.0814. Chris Orton in the Cartographic Unit, Geography, Durham University edited several figures. This paper is dedicated to Mr Arnold Jones – a true Quaternarist.Peer reviewe