The analysis of the sea-ice diatom biomarker IP25 (a mono-unsaturated Highly Branched Isoprenoid (HBI) alkene) in Arctic marine sediments has previously been shown to provide a useful qualitative proxy measure for the past spring sea-ice occurrence. In the Southern Ocean the occurrence and variable abundance of a structurally similar di-unsaturated HBI (HBI diene II) has previously been proposed as a proxy measure of paleo sea-ice extent. However, the use of such biomarker proxies remains under development. In the current study, a number of additional palaeoceanographic developments of HBIs as sea-ice biomarkers in both polar regions has been undertaken. For the Arctic, an investigation into the combined analysis of IP25 and certain phytoplankton biomarkers has been conducted with the aim of providing more detailed and semi-quantitative descriptions of sea-ice conditions in the Barents Sea. In contrast, analysis of HBIs and other lipids within water column, surface sediment and sea-ice samples has been undertaken to provide further insights into the use of HBIs as proxies for Antarctic sea-ice. Analysis of surface sediments from across the Barents Sea has shown that the relative abundances of IP25 and a tri-unsaturated HBI lipid (HBI triene IIIa) are characteristic of the overlying surface oceanographic conditions, most notably, the location of the seasonal sea-ice edge. A semi-quantitative approach, in the form of the PIP25 index, showed a good positive linear relationship between PIP25 indices and spring sea-ice concentration, with a particularly strong relationship found when using HBI triene IIIa (PIIIaIP25) as the open-water counterpart to IP25. The quality of the linear fits were not especially dependent on the balance factor c, used in the PIP25 calculation, which may have important positive consequences for down-core sea-ice reconstruction, and when making comparisons between outcomes from different Arctic regions or climatic epochs. Further, a lower limit threshold for PIIIaIP25 (0.8) might represent a useful qualitative proxy for the past occurrence of summer sea-ice. The re-evaluation of biomarker data from three dated marine sequences in the Barents Sea suggests that the combined analysis of IP25 and HBI triene IIIa can provide information on temporal variations in the position of the maximum (winter) Arctic sea-ice extent, together with more quantitative sea-ice reconstructions. In the Southern Ocean, the distributions of di- and tri-unsaturated HBIs (HBI diene II and HBI trienes IIIa and IIIb) in surface waters were shown to be extremely sensitive to the local sea-ice conditions, consistent with significant environmental control over their biosynthesis by sea-ice diatoms and open water phytoplankton, respectively. Within the water column, the apparent alteration to HBI and other lipid abundances was evident between the photic and benthic parts of the water column, which, along with additional local factors (e.g. polynya formation), may have important implications for paleo sea-ice reconstructions. The sedimentary occurrence and distribution of HBI diene II (termed here as IPSO25) were consistent with the recent identification of the diatom Berkeleya adeliensis Medlin as a source of IPSO25. The tendency for B. adeliensis to flourish in platelet ice, the formation of which is strongly associated with super-cooled freshwater inflow, means that sedimentary IPSO25 may provide a potentially sensitive proxy indicator of landfast sea-ice influenced by meltwater discharge from nearby glaciers and ice shelves. Re-examination of some previous IPSO25 down-core records supports this suggestion, although further down-core analysis is required to confirm this hypothesis. The similar sedimentary distribution relationship between phytoplankton-derived HBI trienes and IPSO25, further indicates that the former may reflect production of these biomarkers by certain diatoms that flourish within the region of the retreating ice edge; however, the source identification of the HBI trienes is still needed to place this interpretation on a firmer footing.

Climate change disproportionately threatens alpine river ecosystems due to the strong connections between cryosphere, hydrology and physicochemical habitat. Our general understanding of how these systems will respond to warming is, however, based on conceptual models derived from studies undertaken at relatively small spatial scales. This research utilizes: (i) field data collected from five glacierized river basins in the French Pyrénées; (ii) field based experimentation; and (iii) climate/hydrological modelling, to improve understanding of alpine river ecosystem change. Despite a linear, harsh-begin, physicochemical habitat gradient running from high to low meltwater (snow and ice) contribution, observed benthic macroinvertebrate community level metrics were unimodal (i.e. mid-meltwater peak). Community assembly processes shifted from niche filtering/stochastic (trait convergence) at high meltwater sites, to limiting similarity/stochastic (trait divergence) at low meltwater sites. Benthic macroinvertebrate community structure, feeding interactions and body size spectra were altered when invertebrate predator range expansion was experimentally simulated. Empirical observation (space for time substitution) and statistical modelling both suggest an increase in reach scale diversity (alpha) is likely as glacier cover is lost. However, a reduction in habitat heterogeneity is likely to lead to biotic homogenization (reduced beta diversity) as a specialist high meltwater community is replaced by a more generalist community. The need to consolidate monitoring strategies is highlighted and functional trait profiles are suggested as useful bio-monitoring tools for detecting future change.

The Antarctic Peninsula (AP) has been one of the most rapidly warming regions on this planet. This warming has been accompanied by major glaciological changes such as tidewater glacier retreat, ice-shelf retreat and collapse alongside acceleration of outlet glaciers in response to ice-shelf removal. As faster owing glaciers deliver more ice from the ice sheet's interior to the margins, the AP has been identified as an important contributor to global sea-level rise (SLR). However, comprehensible SLR projections of the AP induced by ice dynamics over the next three centuries are still lacking. In this thesis, numerical ice-sheet models are utilised to present scenario-based ice dynamic SLR projections for the AP.

IP25 is a highly branched isoprenoid alkene derived from certain Arctic sea ice diatoms that, when detected in marine sediments, has been used as a proxy for past Arctic sea ice over the last decade. In the current study, the structure of this biomarker was determined following large-scale extraction from sediment material collected from the Canadian Arctic. After purification, the structure of IP25 was confirmed by NMR spectroscopy as being the same as that of a laboratory standard. The purified IP25 was subsequently used to obtain a quantitative (GC-MS) instrumental response factor that could be used to improve the future quantification of IP25 and would help to produce a robust database. IP25, other highly branched isoprenoid (HBI) lipids and some other phytoplanktonic lipids (sterols) were analysed to provide modern and past sediment-based sea ice reconstructions for the Barents Sea. First, a surface sediment study was conducted and biomarker distributions were compared to satellite sea ice records. The occurrence of IP25 was consistent with the presence/absence of seasonal sea ice but there was also evidence of lateral transport of IP25 and other biomarkers in sediments from the southern Barents Sea. In contrast to some previous studies, abundances of IP25, and of those combined with other biomarkers, including sterols, did not show strong quantitative relationships to sea ice concentration. The surface study was used to relate biomarker distributions to recent sea ice and oceanographic conditions and apply this information to long-term sediment records in the eastern and western Barents Sea covering ca. 2 kyr and 11 kyr (Holocene) respectively. IP25 concentrations for the former were found to be very variable and were used to identify the period with maximum sea ice cover occurring from ca. 900 - 400 cal. yr BP where the highest abundances of IP25 and IRD were observed. Similarly, biomarker results from the eastern Barents Sea provided evidence for a dynamic advance of the marginal sea ice zone potentially situated at ca. 78° N (maximum extent) during ca 9.4 – 5.9 cal. kyr BP, to late Holocene and modern day maximum MIZ advance ca. 75° N. Replicate analysis of various biomarkers in individual push-cores collected from a box core obtained from Rijpfjorden (north Svalbard) demonstrated some variability between cores. Variability in individual biomarker concentrations was lowest for HBI lipids and greatest for sterols. These data are consistent with a selective and relatively minor source of the former. In contrast, the somewhat more generic origins of sedimentary sterols likely explain the greater variability in their distributions between cores Finally, the strong abundance relationship between IP25 and a structurally related di-unsaturated HBI (C25:2) was confirmed in all sediments, similar to that found between two tri-unsaturated HBIs, consistent with co-production by certain marine phytoplankton. The progressive use of novel HBIs with two or three degrees of unsaturation (e.g. C25:2 and C25:3) could provide further valuable insights into environmental conditions.

Ice sheets are among the key controls on global climate and sea level. A detailed understanding of their dynamics is crucial to make accurate predictions of their future mass balance. Ice streams are the dominant negative component in this balance, accounting for up to 90% of the Antarctic ice flux into ice shelves and ultimately into the sea. Despite their importance, our understanding of ice-stream dynamics is far from complete. A range of observations associate ice streams with meltwater. Meltwater lubricates the ice at its bed, allowing it to slide with less internal deformation. It is believed that ice streams may appear due to a localisation feedback between ice flow, basal melting and water pressure in the underlying sediments. This thesis aims to address the instability of ice-stream formation by considering potential feedbacks between the basal boundary and ice flow. Chapter 2 considers ice-flow models, formulating a model that is capable of capturing the leading-order dynamics of both a slow-moving ice sheet and rapidly flowing ice streams. Chapter 3 investigates the consequences of applying different phenomenological sliding laws as the basal boundary condition in this ice-flow model. Chapter 4 presents a model of subglacial water flow below ice sheets, and particularly below ice streams. This provides a more physical representation of processes occurring at the bed. Chapter 5 then investigates the coupled behaviour of the water with the sediment, and Chapter 6 the coupled behaviour of the water with the ice flow. Under some conditions this coupled system gives rise to ice streams due to instability of the internal dynamics.

The presence of the sea ice diatom biomarker IP25 in Arctic marine sediments has been used in previous studies as a proxy for past spring sea ice occurrence and as an indicator of wider palaeoenvironmental conditions for different regions of the Arctic over various timescales. The current study describes a number of analytical and palaeoceanographic developments of the IP25 sea ice biomarker. First, IP25 was extracted and purified from Arctic marine sediments. This enabled the structure of IP25 to be confirmed and enabled instrumental (GC-MS) calibrations to be carried out so that quantitative measurements could be performed with greater accuracy. Second, palaeo sea ice reconstructions based on IP25 and other biomarkers were carried out for a suite of sub-Arctic areas within the Greenland, Norwegian and Barents Seas, each of which represent contrasting oceanographic and environmental settings. Further, an evaluation of some combined biomarker approaches (e.g. the PIP25 and DIP25 indices) for quantifying and/or refining definitions of sea ice conditions was carried out. Temporally, particular emphasis was placed on the characterisation of sea ice conditions during the Younger Dryas and the Holocene. Some comparisons with other proxies (e.g. foraminifera, IRD) were also made. A study of a sediment core from Andfjorden (69.16˚N, 16.25˚E), northern Norway, provided unequivocal evidence for the occurrence of seasonal sea ice conditions during the Younger Dryas. The onset (ca. 12.9 cal. kyr BP) and end (ca. 11.5 cal. kyr BP) of this stadial were especially clear in this location, while in a study from the Kveithola Trough (74.52˚N, 16.29˚E), western Barents Sea, these transitions were less apparent. This was attributed to the presence of colder surface waters and the occurrence of seasonal sea ice both before and after this stadial at higher latitudes. Some regional differences regarding the severity of the sea ice conditions were also observed, although an overall general picture was proposed, with more severe sea ice conditions during the early-mid Younger Dryas and less sea ice observed during the late Younger Dryas. A shift in the climate towards ice-free conditions was recorded in northern Norway during the early Holocene (ca. 11.5 – 7.2 cal. kyr BP). Milder conditions were also observed during the Holocene in the western Barents Sea, with three main climate periods observed. During the early Holocene (ca. 11.7 – 9.5 cal. kyr BP), the position of the spring ice edge was close to the study area which resulted in high productivity during summers. During the mid-late Holocene (ca. 9.5 – 1.6 cal. kyr BP), sea ice was mainly absent due to an increased influence of Atlantic waters and northward movement of the Polar Front. During the last ca. 1.6 cal. kyr BP, sea ice conditions were similar to those of the present day. In addition to the outcomes obtained from the Norwegian-Barents Sea region, comparison of biomarker and other proxy data from 3 short cores from Kangerdlugssuaq Trough (Denmark Strait/SE Greenland) with historical climate observations allowed the development of a model of sea ice conditions which was then tested for longer time-scales. It is suggested that the IP25 in sediments from this region is likely derived from drift ice carried from the Arctic Ocean via the East Greenland Current and that two main sea surface scenarios have existed over the last ca. 150 yr. From ca. AD 1850 – 1910, near perennial sea ice conditions resulted in very low primary productivity, while from ca. AD 1910 – 1986, local sea ice conditions were less severe with increased drift ice and enhanced primary productivity. This two-component model was subsequently developed to accommodate different sea surface conditions that existed during the retreat of the Greenland Ice Sheet during the deglaciation (ca. 16.3 – 10.9 cal. kyr BP).