Characteristics of Striae and Clast Shape in Glacial and Non-Glacial Environments

<p>Linear abrasion features on rock surfaces are produced by interacting rock particles in relative motion. The most common examples are striae produced by temperate glaciers, and as a consequence, striae have long been used as a means of identifying the passage of past glaciers. However, there are many non-glacial processes that can produce striae. These have been sporadically documented in the geological literature but have failed to make a lasting impression on the wider Earth Sciences community. These non-glacial processes include tectonic deformation, meltwater flow, non-glacial ice, wind action, volcanic blasting, mass movements of rock debris, among many others. Many produce coarse-grained deposits similar in character to glacial tills and there are several instances where non-glacial deposits and striae have been misinterpreted as glacial in origin. This thesis examines linear abrasion features (mostly striae) from five different environments, three glacial (temperate, polythermal and cold) and two non-glacial environments (mass movement and tectonic) to characterise the striae from different origins. The aim was to assess if there are readily observable and measurable differences in striae character between environments and to develop field-based criteria that allow a sound judgement of their origin in the geological record. Over 760 measurements of individual striae were made (orientation and size) on around 20 representative clasts and characteristic features of about 50 striated clasts from the various environments are illustrated in an "Atlas of linear abrasion features". In addition clast shape and striae occurrence were measured on 1260 clasts from deposits and about 100 bedrock linear abrasions from a cold-based glacier were recorded. The results show that some striae are diagnostic of certain environments but a combination of clast shape and striae characteristics is the most reliable method of correctly interpreting coarse-grained deposits with striated clasts. Results also highlight the wide range of striae characteristics within each environment and the importance of lithology in striae generation. This is evident even within the well-known temperate glacial environment where there is a marked contrast between striae formed within a thick debris layer and those formed in thin debris-rich basal ice. There appears to be little difference in striae formed by temperate and polyhermal glaciers, but glacial striae are readily distinguishable from striae found in various mass movement deposits or tectonically deformed conglomerates. Glacial striae tend to be sub-parallel to the clast long axes and show a high density on individual surfaces, whereas those in non-glacial origin typically show a lower density of slightly shorter, wider striae and show either no preferred orientation or weak grouping. The survivability of glacial abrasion features of clasts once they have entered a fluvial system has been assessed in a small South Island glacier fed river. This has provided a basis for estimating the proximity of a glaciofluvial deposit to the glacier front. Striae are found to survive only 1 to 2 km and glacial facets are mostly removed within 6 km. The study has also documented previously undescribed linear abrasion features from a cold-based glacier in Antarctica. This discovery is a significant advance in understanding cold glacial processes, and has provided new criteria for recognising the passage of cold-based glaciers in polar areas or regions where cold-based ice may have existed in the past.</p>

Characteristics of Striae and Clast Shape in Glacial and Non-Glacial Environments

<p>Linear abrasion features on rock surfaces are produced by interacting rock particles in relative motion. The most common examples are striae produced by temperate glaciers, and as a consequence, striae have long been used as a means of identifying the passage of past glaciers. However, there are many non-glacial processes that can produce striae. These have been sporadically documented in the geological literature but have failed to make a lasting impression on the wider Earth Sciences community. These non-glacial processes include tectonic deformation, meltwater flow, non-glacial ice, wind action, volcanic blasting, mass movements of rock debris, among many others. Many produce coarse-grained deposits similar in character to glacial tills and there are several instances where non-glacial deposits and striae have been misinterpreted as glacial in origin. This thesis examines linear abrasion features (mostly striae) from five different environments, three glacial (temperate, polythermal and cold) and two non-glacial environments (mass movement and tectonic) to characterise the striae from different origins. The aim was to assess if there are readily observable and measurable differences in striae character between environments and to develop field-based criteria that allow a sound judgement of their origin in the geological record. Over 760 measurements of individual striae were made (orientation and size) on around 20 representative clasts and characteristic features of about 50 striated clasts from the various environments are illustrated in an "Atlas of linear abrasion features". In addition clast shape and striae occurrence were measured on 1260 clasts from deposits and about 100 bedrock linear abrasions from a cold-based glacier were recorded. The results show that some striae are diagnostic of certain environments but a combination of clast shape and striae characteristics is the most reliable method of correctly interpreting coarse-grained deposits with striated clasts. Results also highlight the wide range of striae characteristics within each environment and the importance of lithology in striae generation. This is evident even within the well-known temperate glacial environment where there is a marked contrast between striae formed within a thick debris layer and those formed in thin debris-rich basal ice. There appears to be little difference in striae formed by temperate and polyhermal glaciers, but glacial striae are readily distinguishable from striae found in various mass movement deposits or tectonically deformed conglomerates. Glacial striae tend to be sub-parallel to the clast long axes and show a high density on individual surfaces, whereas those in non-glacial origin typically show a lower density of slightly shorter, wider striae and show either no preferred orientation or weak grouping. The survivability of glacial abrasion features of clasts once they have entered a fluvial system has been assessed in a small South Island glacier fed river. This has provided a basis for estimating the proximity of a glaciofluvial deposit to the glacier front. Striae are found to survive only 1 to 2 km and glacial facets are mostly removed within 6 km. The study has also documented previously undescribed linear abrasion features from a cold-based glacier in Antarctica. This discovery is a significant advance in understanding cold glacial processes, and has provided new criteria for recognising the passage of cold-based glaciers in polar areas or regions where cold-based ice may have existed in the past.</p>

Meta-analysis of Cryogenian through modern quartz microtextures reveals sediment transport histories

ABSTRACT Quantitative analysis of quartz microtextures by means of scanning electron microscopy (SEM) can reveal the transport histories of modern and ancient sediments. However, because workers identify and count microtextures differently, it is difficult to directly compare quantitative microtextural data analyzed by different workers. As a result, the defining microtextures of certain transport modes and their probabilities of occurrence are not well constrained. We used principal-component analysis (PCA) to directly compare modern and ancient aeolian, fluvial, and glacial samples from the literature with nine new samples from active aeolian and glacial environments. Our results demonstrate that PCA can group microtextural samples by transport mode and differentiate between aeolian transport and fluvial and glacial transport across studies. The PCA ordination indicates that aeolian samples are distinct from fluvial and glacial samples, which are in turn difficult to disambiguate from each other. Ancient and modern sediments are also shown to have quantitatively similar microtextural relationships. Therefore, PCA may be a useful tool to constrain the ambiguous transport histories of some ancient sediment grains. As a case study, we analyzed two samples with ambiguous transport histories from the Cryogenian Bråvika Member (Svalbard). Integrating PCA with field observations, we find evidence that the Bråvika Member facies investigated here includes aeolian deposition and may be analogous to syn-glacial Marinoan aeolian units including the Bakoye Formation in Mali and the Whyalla Sandstone in South Australia.

Microscale evidence of the role of water during the emplacement of mass flows in glacial environments

Microscale analysis of unlithified glacial soils can provide far greater detail regarding their depositional and deformation histories than can be obtained from macroscale studies alone. This paper presents the results of three detailed case studies which examine the processes occurring during overriding and emplacement of mass flows in glacial environments: (i) laminated soils deposited in a proglacial lake setting at Heinabergsjökull, Iceland; (ii) a channelised, ice-marginal to submarginal mass flow at Whitburn (County Durham), England; and (iii) a mass flow exposed at Carstairs, Central Scotland which was emplaced in a glaciofluvial to glaciolacustrine setting. Microscale evidence from all three sites is combined to develop a conceptual model of the role played by water during mass flow; from the initial soil disruption under and/or in front of an advancing mass flow, to the formation of a basal shear zone facilitating mass flow transport and emplacement, through to the decoupling of the flow from the underlying substrate as a result of the injection of fluidised soil along its base. The development of these detachments during mass flow has the potential to increase the velocity of the flow and its run out distance, increasing the potential impact of these geohazards.Thematic collection: This article is part of the Role of water in destabilizing slopes collection available at: https://www.lyellcollection.org/cc/Role-of-water-in-destabilizing-slopes

Ge/Si and Ge Isotope Fractionation During Glacial and Non-glacial Weathering: Field and Experimental Data From West Greenland

Glacial environments offer the opportunity to study the incipient stages of chemical weathering due to the high availability of finely ground sediments, low water temperatures, and typically short rock-water interaction times. In this study we focused on the geochemical behavior of germanium (Ge) in west Greenland, both during subglacial weathering by investigating glacier-fed streams, as well as during a batch reactor experiment by allowing water-sediment interaction for up to 2 years in the laboratory. Sampled in late August 2014, glacial stream Ge and Si concentrations were low, ranging between 12–55 pmol/L and 7–33 µmol/L, respectively (Ge/Si = 0.9–2.2 µmol/mol, similar to parent rock). As reported previously, the dissolved stable Ge isotope ratio (δ74Ge) of the Watson River was 0.86 ± 0.24‰, the lowest among global rivers and streams measured to date. This value was only slightly heavier than the suspended load (0.48 ± 0.23‰), which is likely representative of the bulk parent rock composition. Despite limited Ge/Si and δ74GeGe fractionation, both Ge and Si appear depleted relative to Na during subglacial weathering, which we interpret as the relatively congruent uptake of both phases by amorphous silica (aSi). Continued sediment-water interaction over 470–785 days in the lab produced a large increase in dissolved Si concentrations (up to 130–230 µmol/L), a much smaller increase in dissolved Ge (up to ∼70 pmol/L), resulting in a Ge/Si decrease (to 0.4–0.5 µmol/mol) and a significant increase in δ74Ge (to 1.9–2.2‰). We argue that during the experiment, both Si and Ge are released by the dissolution of previously subglacially formed aSi, and Ge is then incorporated into secondary phases (likely adsorbed to Fe oxyhydroxides), with an associated Δ74Gesecondary−dissolved fractionation factor of −2.15 ± 0.46‰. In summary, we directly demonstrate Ge isotope fractionation during the dissolution-precipitation weathering reactions of natural sediments in the absence of biological Ge and Si uptake, and highlight the significant differences in Ge behavior during subglacial and non-glacial weathering.

Volcanic Tremor and its Relation to Volcano- and Glacier-related Processes

&lt;p&gt;Volcanic eruptions can affect the climate system, the environment and society. On ice covered volcanoes this threat intensifies due to the increasing explosivity in contact with water. Monitoring and early-warning of such eruptions is closely linked to real-time, multidisciplinary data analysis. This builds on a good understanding and location of the recorded signals.&lt;/p&gt;&lt;p&gt;I will summarize my work on understanding and modelling volcanic tremor, a long-lasting seismic signal with emergent onset. This tremor accompanies various volcano- and glacier-related processes and has to be reliably detected and distinguished from other sources. My examples range from modelling pre-eruptive subglacial tremor and silent magma flow, to monitoring eruptive tremor, to early warning of subglacial flooding, to hydrothermal explosions and boiling and other sources such as helicopters. These results are based on array analysis, amplitude location techniques and single-station arrays but I will also risk a look into the future embracing the emerging field of rotational seismology which might solve some challenges we face in volcanic and glacial environments and advance our understanding and modelling of volcanic signals at remote sites.&lt;/p&gt;