Macro‐scale bed roughness of the siple coast ice streams in West Antarctica

Subglacial bed conditions are key to understanding ice stream behaviour and evolution, with bed roughness re?ecting substrate composition and ?ow resistance. Here we present an analysis of bed roughness in the Siple Coast region of West Antarctica from airborne radio‐echo sounding data. The ice streams are associated generally with low bed roughness values, which decrease downstream. The bed of the slow‐?owing Ice Stream C (～10 000 km²) is also characterized by being smooth at all scales (wavelengths ranging from 5 km to in excess of 40 km). Furthermore, the bed is smooth either side of Ice Stream C. This suggests the location of the ice stream is controlled by internal ice sheet dynamics rather than by bed morphology. If the ice stream were encouraged to migrate laterally, when active, there would be little resistance offered by the subglacial morphology. Other inter‐ice stream regions are rough, however, indicating a subglacial topographic in?uence on ice stream position. Bed roughness increases up‐?ow of ice streams, which, unless the bed is modi?ed, may limit the inland migration of these systems. Copyright © 2004 John Wiley & Sons, Ltd.     

Late Weichselian relative sea-level changes and ice sheet history in southeast Greenland

Relative sea-level (RSL) observations from the margins of the Greenland Ice Sheet (GIS) provide information regarding the timing and rate of deglaciation and constraints on geophysical models of ice sheet evolution. In this paper we present the first RSL record for the southeast sector of the GIS based on field observations completed close to Ammassalik. The local marine limit is c. 69 m above sea-level (asl) and is dated to c. 11 k cal. yrs BP (thousand calibrated years before present) and is a minimum date for ice free conditions at the study site. RSL fell to c. 24 m asl by 9.5 k cal. yrs BP and continued to fall at a decreasing rate to reach close to present by 6.5 k cal. yrs BP. Our chronology agrees with radiocarbon dates from offshore cores that indicate ice free conditions on the adjacent mid-shelf by 15 k cal. yrs BP. We compare the new RSL data with predictions generated using two recently published glaciological models of the GIS that differ in the amount and timing of ice loading and unloading over our study area. These two GIS models are coupled to the same Earth viscosity model and background (global) ice model to aid in the data-model comparison. Neither model provides a close fit to the RSL observations. Based on a preliminary sensitivity study using a suite of Earth viscosity models, we conclude that the poor data-model fit is most likely due to an underestimate of the local ice unloading. An improved fit could be achieved by delaying the retreat of a thicker ice sheet across the continental shelf. A thick ice sheet extending well onto the continental shelf is in agreement with other recent observations elsewhere in east and south Greenland.     

An empirical function to estimate the depths of linear hot sources: Laboratory modeling and field measurements of lava tubes

Estimating depths of buried lava tubes is important for determining the thermal budgets and effusion rates of basaltic volcanic systems. This research used a laboratory experiment scaled to a lava tube system to measure the 3D temperature field surrounding a hot viscous fluid flowing through a buried glass tube while varying conditions such as flow rate and temperature. The depth of the glass tube was changed for different experimental runs. Numerical techniques were applied to model the laboratory experiment. The surface thermal distributions from 166 thermal traverses, constrained to a depth to width ratio of 0.6 to 1.6, were analyzed to empirically derive a depth estimation function using regression techniques. This “Linear Anomaly Surface Transect (LAST)” depth function is a scaleable depth estimation technique which can be solved with thermal imaging data alone. The minimum temperature, maximum temperature and width of a Lorentzian distribution fit to a surface thermal transect, are the only inputs required for the LAST function to estimate the depths of the hot source. The input parameters were then applied to non-laboratory situations including the Kuhio lava tube system in Hawai’i. The LAST function produced depth estimates of ∼ 0.3 m for the Kuhio lava tube in Hawai’i, which did not agree with observations on the ground. This is the result of the complex composition and geometry of an actual lava tube where heat transfer is controlled by more than a simple fluid filling a tube, but also by convection of gasses and fluids in a partially filled passage. Though not effective for lava tubes at this time, the model provides promising results for simple cases applied to engineering and underground coal fires.     

Holocene floristic diversity and richness in northeast Norway revealed by sedimentary ancient DNA (sedaDNA) and pollen

We present a Holocene record of floristic diversity and environmental change for the central Varanger Peninsula, Finnmark, based on ancient DNA extracted from the sediments of a small lake (sedaDNA). The record covers the period c. 10 700 to 3300 cal. a BP and is complemented by pollen data. Measures of species richness, sample evenness and beta diversity were calculated based on sedaDNA sampling intervals and 1000‐year time windows. We identified 101 vascular plant and 17 bryophyte taxa, a high proportion (86%) of which are still growing within the region today. The high species richness (>60 taxa) observed in the Early Holocene, including representatives from all important plant functional groups, shows that modern shrub‐tundra communities, and much of their species complement, were in place as early as c. 10 700 cal. a BP. We infer that postglacial colonization of the area occurred prior to the full Holocene, during the Pleistocene‐Holocene transition, Younger Dryas stadial or earlier. Abundant DNA of the extra‐limital aquatic plant Callitriche hermaphroditica suggests it expanded its range northward between c. 10 200 and 9600 cal. a BP, when summers were warmer than present. High values of Pinus DNA occur throughout the record, but we cannot say with certainty if they represent prior local presence; however, pollen influx values >500 grains cm^?2 a^?1 between c. 8000 and 7300 cal. a BP strongly suggest the presence of pine woodland during this period. As the site lies beyond the modern tree limit of pine, it is likely that this expansion also reflects a response to warmer Early Holocene summers.     

The geomorphology of the Anthropocene: emergence,status and implications

The Anthropocene is proposed as a new interval of geological time in which human influence on Earth and its geological record dominates over natural processes. A major challenge in demarcating the Anthropocene is that the balance between human‐influenced and natural processes varies over spatial and temporal scales owing to the inherent variability of both human activities (as associated with culture and modes of development) and natural drivers (e.g. tectonic activity and sea level variation). Against this backdrop, we consider how geomorphology might contribute towards the Anthropocene debate by focusing on human impact on aeolian, fluvial, cryospheric and coastal process domains, and how evidence of this impact is preserved in landforms and sedimentary records. We also consider the evidence for an explicitly anthropogenic geomorphology that includes artificial slopes and other human‐created landforms. This provides the basis for discussing the theoretical and practical contributions that geomorphology can make to defining an Anthropocene stratigraphy. It is clear that the relevance of the Anthropocene concept varies considerably amongst different branches of geomorphology, depending on the history of human actions in different process domains. For example, evidence of human dominance is more widespread in fluvial and coastal records than in aeolian and cryospheric records, so geomorphologically the Anthropocene would inevitably comprise a highly diachronous lower boundary. Even to identify this lower boundary, research would need to focus on the disambiguation of human effects on geomorphological and sedimentological signatures. This would require robust data, derived from a combination of modelling and new empirical work rather than an arbitrary ‘war of possible boundaries' associated with convenient, but disputed, ‘golden’ spikes. Rather than being drawn into stratigraphical debates, the primary concern of geomorphology should be with the investigation of processes and landform development, so providing the underpinning science for the study of this time of critical geological transition. Copyright © 2016 John Wiley & Sons, Ltd.     

Optimal tuning of a GCM using modern and glacial constraints

In climate models, many parameters used to resolve subgrid scale processes can be adjusted through a tuning exercise to fit the model??s output to target climatologies. We present an objective tuning of a low resolution Atmosphere?COcean General Circulation Model (GCM) called FAMOUS where ten model parameters are varied together using a Latin hypercube sampling method to create an ensemble of 100 models. The target of the tuning consists of a wide range of modern climate diagnostics and also includes glacial tropical sea surface temperature. The ensemble of models created is compared to the target using an Arcsin Mielke score. We investigate how the tuning method used and the addition of glacial constraints impact on the present day and glacial climates of the chosen models. Rather than selecting a single configuration which optimises the metric in all the diagnostics, we obtain a subset of nine ??good?? models which display great differences in their climate but which, in some sense, are all better than the original configuration. In those simulations, the global temperature response to last glacial maximum forcings is enhanced compared to the control simulation and the glacial Atlantic Ocean circulation is more in agreement with observations. Our study demonstrates that selecting a single ??optimal?? configuration, relying only on present day constraints may lead to misrepresenting climates different to that of today.     

Late Pleistocene–Holocene river dynamics at the Trent‐Soar confluence,England, UK

Although river confluences have received geomorphic attention in recent years it is difficult to upscale these studies, so confluence‐dominated reaches are commonly presumed to be either: (1) braided; or (2) meandering and characterized by laterally migrating channels. If the geomorphology of a confluence zone is to be considered over longer timescales, changes in river style need to be taken into account. This paper uses a combination of remote sensing techniques (LiDAR, GPR, ER), borehole survey and chronometric dating to test this differentiation in the confluence‐zone of a medium‐sized, mixed‐load, temperate river system (Trent, UK), which on the basis of planform evidence appears to conform to the meandering model. However, the analysis of ‘confluence sediment body stratigraphy’ demonstrates that the confluence does not correspond with a simple meander migration model and chronostratigraphic data suggests it has undergone two major transformations. Firstly, from a high‐energy braid‐plain confluence in the Lateglacial (25–13 K yrs cal BP), to a lower‐energy braided confluence in the early to middle Holocene (early Holocene‐2.4 kyr BP), which created a compound terrace. Second, incision into this terrace, creating a single‐channel confluence (2.4–0.5 kyr cal BP) with a high sinuosity south bank tributary (the River Soar). The confluence sediment‐body stratigraphy is characterized by a basal suite of Late Pleistocene gravels bisected by younger channel fills, which grade into the intervening levee and overbank sediments. The best explanation for the confluence sediment body stratigraphy encountered is that frequent switching (soft‐avulsions sensu Edmonds et al., 2011) of the tributary are responsible for the downstream movement of the channel confluence (at an average rate of approximately 0.5 m per year) dissecting and reworking older braid‐plain sediments. The late Holocene evolution of the confluence can be seen as a variant of the incisional‐frequent channel reorganization (avulsion) model with sequential downstream migration of the reattachment point. Copyright © 2012 John Wiley & Sons, Ltd.     

Marine Terraces and Quaternary Tectonism,Northwest Baja California,Mexico

Deformed marine terraces provide excellent measures of Quaternary tectonism along 400 km of the Pacific coast of Baja California between Ensenada and Punta Blanca. The nature and magnitude of deformation are well expressed by the present elevation of the marine limit, of Upper Pliocene or Lower Pleistocene age, which has been upwarped to 345m, 357m, and at least 195m in three separate localities, while occurring at lower levels elsewhere. Multiple terraces are found below this marine limit where appreciable uplift has occurred. Where relative subsidence has prevailed, fewer broader terraces or thick alluvial and eolian deposits occur. Terraces are further dislocated by NW-SE strike-slip faulting and by normal faults that trend NE-SW across the coast. Folding and faulting are less significant on the younger terraces than on the older features, and negligible for the Sangamonian terrace, except close to major faults. Crustal movements revealed by these terraces, and by similar features in southern California, Baja California Sur, and around the Gulf of California, are interpreted in terms of plate tectonics. Over the past 5 million years, the Peninsular Ranges have been carried northwestwards away from mainland Mexico, at an average rate of 6cm/yr, on the dis located limb of a spreading sea floor associated with the East Pacific Rise now found beneath the Gulf of California. The Pacific coast of the Peninsular Ranges comprises several discrete crustal slivers which, on the evidence of the marine terraces, have been warped and dislocated in response to tec tonic forces associated with rafting and subsequent crowding against transverse structures to the north. Uplift rates range as high as 1.0m/1000 yrs locally but are generally less, and are much less than rates found in the Transverse Ranges farther north. As in the latter province, basins typified by net sub sidence during Quaternary time are also found.     

THE NATURE AND RATE OF ALLUVIAL FAN AGGRADATION IN A HUMID TEMPERATE ENVIRONMENT,NORTHWEST WASHINGTON

Comparatively little is known about net aggradation on alluvial fans, despite fan construction wherever sediment-delivery rates from uplands exceed sediment-removal rates from receiving basins. In January 1983, 20 alluvial fans in the forested Cascade foothills, northwest Washington, experienced net aggradation in response to debris torrents and stream floods triggered by intense warm rains falling on antecedent snow. Five trenches were excavated to 5 m depth on the Mills Creek fan to place the 1983 event in temporal perspective. The deposits reflect normal streamflow, hyperconcentrated streamflow and debris torrent (flow) conditions. One trench revealed residues of 7 events since 1720 BP. Net rates of Holocene aggradation, based on sediments overlying late Pleistocene fluvioglacial deposits, average 0.42 m ka^-1. Net rates for later Holocene time range from 2.17 m ka^-1 since 1720 BP to 2.36 m ka^-1 since 430 BP. These recent rates exceed the local value for the entire Holocene and rates for humid temperate fans elsewhere. This suggests that accelerated aggradation may characterize later Holocene times, a view that contrasts with the widespread belief that the later Holocene has been dominated by fan-head incision and net degradation. More comparative data are needed to test this observation. Locally, large debris-producing events in 1917 and 1983 are consistent with later Holocene recurrence intervals and thus appear to be independent of timber harvesting. [Key words: alluvial fan, debris torrent, debris flow, streamflow, Washington.]