Simon H. Brocklehurst

Geomorphic limits to climate-induced increases in topographic relief

Recognition of the potential for strong dynamic coupling between atmospheric and tectonic processes has sparked intense cross-disciplinary investigation and debate on the question of whether tectonics have driven long-term climate change or vice versa. It has been proposed that climate change might have driven the uplift of mountain summits through an isostatic response to valley incision. Because isostasy acts to compensate mean elevations, the debate hinges on the question of whether climate change can significantly increase topographic relief or, more precisely, increase the volume of ‘missing mass’ between summits and ridges. Here we show that, in tectonically active mountain ranges, geomorphic constraints allow only a relatively small increase in topographic relief in response to climate change. Thus, although climate change may cause significant increases in denudation rates, potentially establishing an important feedback between surficial and crustal processes, neither fluvial nor glacial erosion is likely to induce significant isostatic peak uplift.

Glacial erosion and relief production in the Eastern Sierra Nevada, California

Abstract The proposal that climate change can drive the uplift of mountain summits hinges on the requirement that glacial erosion significantly enhances the relief of a previously fluvially sculpted mountain range. We have tested this hypothesis through a systematic investigation of neighbouring glaciated and nonglaciated drainage basins on the eastern side of the Sierra Nevada, CA. We present a simple, objective method for investigating the relief structure of a drainage basin, which shows noticeable differences in the spatial distribution of relief between nonglaciated and glaciated basins. Glaciated basins on the eastern side of the Sierra Nevada have only ∼80 m greater mean geophysical relief than nonglaciated basins. This “extra” relief, though, is attributable principally to the larger size of the glaciated basins, as geophysical relief generally increases with basin size. The glaciers on this side of the range were only responsible for relief production if they substantially increased headward erosion rates into low relief topography, such as an elevated plateau, and thus enlarged previously fluvial basins. We carried out a preliminary morphometric analysis to elucidate the importance of this effect and found that the glaciers of the eastern Sierra Nevada may have eroded headward at considerably faster rates than rivers, but only when they were not obstructed from doing so by either competing larger glaciers in adjacent valleys or transfluent ice at the head of the basin. Our results also suggest that, in temperate regions, alpine glaciers are capable of eroding downward at faster rates than rivers above the equilibrium line altitude (ELA). Although we can rule out significant peak uplift in response to local relief production, in the special case of the Sierra Nevada the concentration of mass removal above the ELA could have contributed to flexural uplift at the edge of a tilting block.

Buried iceberg scours reveal reduced North Atlantic Current during the stage 12 deglacial.

Reconstructing past ocean-climate environments and heat transport requires proxies from which these conditions can be quantified. This is particularly important for the evaluation of numerical palaeoclimate models. Here we present new evidence for a reduced North Atlantic Current (NAC) at the termination of the third last glacial, for which palaeocurrent information was previously unavailable. This is based on an exquisitely preserved set of buried iceberg scours seen in three-dimensional seismic reflection images from the mid-Norwegian slope. The scours were formed ∼430 ka during the transition from glacial to interglacial conditions. The spiral geometry of the scours suggests that they were carved by grounded icebergs influenced by tidal and geostrophic ocean currents. Using the ratio between the estimated tidal and geostrophic current velocities and comparing them with velocities from the Last Glacial Maximum and the present, we show that the stage 12 NAC velocities may have been ∼50% slower than the present.

Late Quaternary glacier sensitivity to temperature and precipitation distribution in the Southern Alps of New Zealand

Glaciers respond to climate variations and leave geomorphic evidence that represents an important terrestrial paleoclimate record. However, the accuracy of paleoclimate reconstructions from glacial geology is limited by the challenge of representing mountain meteorology in numerical models. Precipitation is usually treated in a simple manner and yet represents difficult-to-characterize variables such as amount, distribution, and phase. Furthermore, precipitation distributions during a glacial probably differed from present-day interglacial patterns. We applied two models to investigate glacier sensitivity to temperature and precipitation in the eastern Southern Alps of New Zealand. A 2-D model was used to quantify variations in the length of the reconstructed glaciers resulting from plausible precipitation distributions compared to variations in length resulting from change in mean annual air temperature and precipitation amount. A 1-D model was used to quantify variations in length resulting from interannual climate variability. Assuming that present-day interglacial values represent precipitation distributions during the last glacial, a range of plausible present-day precipitation distributions resulted in uncertainty in the Last Glacial Maximum length of the Pukaki Glacier of 17.1 km (24%) and the Rakaia Glacier of 9.3 km (25%), corresponding to a 0.5°C difference in temperature. Smaller changes in glacier length resulted from a 50% decrease in precipitation amount from present-day values (−14% and −18%) and from a 50% increase in precipitation amount (5% and 9%). Our results demonstrate that precipitation distribution can produce considerable variation in simulated glacier extents and that reconstructions of paleoglaciers should include this uncertainty.

Drainage capture and discharge variations driven by glaciation in the Southern Alps, New Zealand

Sediment flux in proglacial fluvial settings is primarily controlled by discharge, which usually varies predictably over a glacial–interglacial cycle. However, glaciers can flow against the topographic gradient to cross drainage divides, reshaping fluvial drainage networks and dramatically altering discharge. In turn, these variations in discharge will be recorded by proglacial stratigraphy. Glacial-drainage capture often occurs in alpine environments where ice caps straddle range divides, and more subtly where shallow drainage divides cross valley floors. We investigate discharge variations resulting from glacial-drainage capture over the past 40 k.y. for the adjacent Ashburton, Rangitata, and Rakaia basins in the Southern Alps, New Zealand. Although glacial-drainage capture has previously been inferred in the range, our numerical glacier model provides the first quantitative demonstration that this process drives larger variations in discharge for a longer duration than those that occur due to climate change alone. During the Last Glacial Maximum, the effective drainage area of the Ashburton catchment increased to 160% of the interglacial value with drainage capture, driving an increase in discharge exceeding that resulting from glacier recession. Glacial-drainage capture is distinct from traditional (base level–driven) drainage capture and is often unrecognized in proglacial deposits, complicating interpretation of the sedimentary record of climate change.

Glacial geomorphological mapping of Coire Mhic Fhearchair, NW Scotland: The contribution of a high-resolution ground based LiDAR survey

Abstract Please click here to download the map associated with this article. Terrestrial Light Detection and Ranging (tLiDAR) surveys are valuable supplements to existing geomorphological mapping techniques, providing information which is of considerable interest to palaeoclimatologists and glaciologists. Fieldwork observations, and the interpretation of aerial photographs and digital elevation models, have been augmented by the study of 3D digital models produced from tLiDAR data, and have led to the production of a detailed geomorphological map at a scale of 1:10,000. A 2 km2 study was carried out in Coire Mhic Fhearchair (NW Scotland), a cirque landform which was covered by an ice-sheet at the Last Glacial Maximum, and experienced localised ice ow during subsequent deglaciation and readvances. The combined map includes glacial (moraines, striae, and depositional lineations) and paraglacial features (debris fans) which have been observed using some or all of the above methods. In addition to this, tLiDAR has been used in conjunction with colour photography to provide a ‘virtual reality’ observational tool at resolutions of up to 50 mm, with great potential for the detailed study of glacial geomorphology on the sub-km scale.

The early Quaternary North Sea Basin

The onset of the Quaternary (2.58 Ma) corresponds to significant paleo-environmental events, such as the intensification and southward extension of Northern Hemisphere glaciation. In the North Sea Basin a significant late Cenozoic succession has been identified as a high-resolution archive of paleo-environmental changes during the Pliocene and Pleistocene. However, the identification of the base of the Quaternary has been a long-standing issue owing to lack of stratigraphic calibration. This study incorporates continuous, regional 3D seismic data with high-quality chronostratigraphic markers to map the base-Quaternary surface at high resolution across the entire North Sea. Depth conversion, backstripping, seismic geomorphology and sedimentation rate calculations are integrated to analyse the paleogeographical evolution of the North Sea Basin and its infill of c. 83 × 103 km3 of northward prograding marine to deltaic sediments. The basin is 600 km long from SSE to NNW and largely localized above residual topography of the Mesozoic graben system. During the earliest Quaternary (2.58 – 2.35 Ma) paleo-water depths were c. 300 ± 50 m and solid sedimentation rates (calculated from 0% porosity) c. 32 km3 ka−1. The base-Quaternary provides an important marker for further studies of the changing environment of the Quaternary of NW Europe as well as resource and shallow geohazard analysis. Supplementary material: A base Quaternary two-way travel time structure map is available at https://doi.org/10.6084/m9.figshare.c.3900343

Geomorphology of the western Ionian Sea between Sicily and Calabria, Italy

In the westernmost Ionian Sea lies a steep, tectonically active marine basin influenced by turbidity currents generated by terrigenous river input from the adjacent mountains and strong tidal currents propagating through the Strait of Messina. Like many young marine rifts, the basin is lined by steep streams draining the uplifting coasts and supplying sediment across narrow shelves. However, unlike many rifts, this basin is semi-enclosed. The present study explores the seabed morphology and sediment structures in this complex environmental setting, based on multibeam sonar, chirp profiler and seismic reflection data collected in 2006. Offshore channels include many that can be directly linked to onshore streams, suggesting that hyperpycnal flows are important for their formation. Near the Strait of Messina in depths shallower than 400 m, the channels are subdued, plausibly explained as an effect of strong tidal currents. The Messina Channel is characterised by abundant mass-wasting features along its outer bends, particularly on the Calabrian side. Coincidence of the channel course with faults suggests that the channel is structurally controlled in places. The chirp profiles generally show only shallow penetration, the evidence for coarse texture being consistent with the steep gradient of the basin that inhibits deposition from turbidity currents. By contrast, some locally discontinuous mounds exhibiting layered sub-bottom reflectors in the chirp profiles are interpreted as modern levee deposits formed from channelised turbidity current overspill. Overall, this semi-enclosed basin shows little evidence of substantial accumulations associated with modern turbidity current activity, any contemporaneous sediment supply evidently bypassing the area to be deposited in the Ionian Trench; as a consequence, this trench should be an archive of local slope failure and flood events.

Extensive marine-terminating ice sheets in Europe from 2.5 million years ago

Ice sheets repeatedly advanced into the central North Sea, south of ~60°N, from 2.53 Ma ago onward moving over slippery beds with low slung profiles. Geometries of Early Pleistocene [2.58 to 0.78 million years (Ma) ago] ice sheets in northwest Europe are poorly constrained but are required to improve our understanding of past ocean-atmosphere-cryosphere coupling. Ice sheets are believed to have changed in their response to orbital forcing, becoming, from about 1.2 Ma ago, volumetrically larger and longer-lived. We present a multiproxy data set for the North Sea, extending to over a kilometer below the present-day seafloor, which demonstrates spatially extensive glaciation of the basin from the earliest Pleistocene. Ice sheets repeatedly entered the North Sea, south of 60°N, in water depths of up to ~250 m from 2.53 Ma ago and subsequently grounded in the center of the basin, in deeper water, from 1.87 Ma ago. Despite lower global ice volumes, these ice sheets were near comparable in spatial extent to those of the Middle and Late Pleistocene but possibly thinner and moving over slippery (low basal resistance) beds.

Earth science: How glaciers grow

A state-of-the-art numerical model shows that the advance of glaciers in a cooling climate depends strongly on the pre-existing landscape, and that glacial erosion paves the way for greater glacial extent in the future. See Letter p.206 Large parts of the Earths surface were glaciated during the Quaternary cold periods of the past 2.5 million years, and these events have left their mark on the mountain topography we see today. Post-glacial landscapes tend to feature large areas concentrated at the same elevation, but the effect of this on subsequent glaciations was unknown. Vivi Pedersen and David Egholm use numerical simulation of glaciation and case studies of alpine topography of the Sierra Nevada, Spain, where there was little glacial activity in the Quaternary, and the Bitterroot Range in Idaho, which was significantly modified by glaciers during the Quaternary. The results show that prior glaciations transform a system from one in which climate exerts a near-linear influence on glacial extent to one in which a small change in climate can result in massive glacial expansion. This helps to explain the long-term patterns of erosion in the Quaternary period.