The presence of the sea ice biomarker IP25 in Arctic marine sediments has previously been used as a proxy measure of past sea ice conditions in the Arctic. Although the sea ice diatom origin of IP25 was established previously, the nature of its production within sea ice, along with its transport through the water column to underlying sediments and its short-term preservation therein, had not been investigated in any significant detail. Variations in the concentration of the sea ice diatom biomarker IP25, were measured in sea ice collected from the eastern Beaufort Sea and Amundsen Gulf from January to June 2008. Temporal and vertical changes in IP25 concentrations were compared against other established indicators of sea ice algal production to determine, for the first time, that approximately 90% of the total sea ice IP25 accumulation occurred coincident with the ice algal bloom period. It was further established that IP25 biosynthesis was restricted, by sea ice porosity, to within the lower few centimetres of the sea ice and specifically to where brine volume fractions were >5%. Concentration differences of IP25 between sea ice and filtered seawater samples were also compared with those of established lipid indicators of algal production to estimate the dispersion of these lipids following seasonal sea ice melt. The largest concentration differences between sea ice and seawater samples were observed for IP25 and some other HBIs, consistent with a sea ice origin, while concentrations of fatty acids and sterols suggested contributions from both sea ice and phytoplankton. A novel analysis of a range of macrofaunal species revealed the presence of IP25 and other HBIs, with distributions somewhat resembling those observed in sea ice but more closely reflecting distributions of HBIs measured in sediments. As such, it is hypothesised that IP25 and HBI distributions in macrofaunal species reflect those of the sediments in which they live. The presence of IP25 and HBIs in macrofaunal species revealed, for the first time, a significant potential for biological cycling and storage of IP25 and other HBIs in the Arctic resulting from exposure during transport of the biomarker between sea ice and sediment. The observed presence of IP25 in 75% of the specimens investigated has presented important evidence for the potential of IP25 to act as a tracer of Arctic sea ice diet in the marine food web. Measurement of the downcore profiles of IP25 in shallow marine sediments alongside other biogeochemical parameters provided new evidence for the early diagenesis of this biomarker. Statistical correlations between some IP25 and Mn/Ti profiles (Station 405b; r = 0.89), that aid determination of the oxygen penetration depth, provided novel evidence for the partial degradation of IP25 (and other HBIs) in the upper sediment sections considered to be oxic. As such, it is suggested here, for the first time, that reactions under oxic conditions could be responsible for degradation of HBIs in some Arctic marine sediments, with the supply of organic carbon influential on the depth of oxygen penetration. The observations recorded in this thesis have therefore offered a much greater understanding of the concentration and distribution of IP25 and related lipids in a wide range of Arctic environments including sea ice, seawater, macrofauna and sediments, than was previously known. Since in most cases these observations represent the first of their kind, it is anticipated that the work carried out here will play an important role, forming the foundation of many important future studies.

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.

The recent, rapid decline in Arctic summer sea ice extent has prompted questions as to the rates and magnitude of previous sea ice decline and the affect of this physical change on icerelated ecosystems. However, satellite data of sea ice only extends back to 1978, and mapped observations of sea ice prior to the 1970s are sparse at best. Inventories of boreal ecosystems are likewise hampered by a paucity of investigations spanning more than the past few decades. Paleoclimate records of sea ice and related primary productivity are thus integral to understanding how sea ice responds to a changing climate. Here I examine modern sedimentation, decadal-scale climate change in the recent past, and centennial- to millennial-scale changes of the past 400 ka using both qualitative and quantitative diatom data in concert with sedimentology and organic geochemistry. Diatom taxonomy and corresponding ecological affinities are compiled in this study and updated for the Bering Sea region and then used as recorders of past climate changes. In recent decades, the Pacific Decadal Oscillation and the strength of the Aleutian Low are reflected by subtle changes in sediment diatom assemblages at the Bering Sea shelf-slope break. Farther back in time, the super-interglacial, marine isotope stage (MIS) 11 (428-390ka), began in Beringia with extreme productivity due to flooding of the Bering Land Bridge. A moisture-driven advance of Beringian glaciers occurred while eustatic sea level was high, and insolation and seasonality both decreased at the global peak of MIS 11. Atlantic/Pacific teleconnections during MIS 11 include a reversal in Bering Strait throughflow at 410 ka and a relationship between North Atlantic Deep Water Formation and Bering Sea productivity. Finally, concentrations of the biomarker-based sea ice proxy, IP25, are compared to sea ice concentration across the Bering and Chukchi seas. Changes in the concentration of IP25 in the sediments may be driven by the length of time that the epontic diatom bloom lasts. When combined with a sediment-based proxy for sea surface temperatures, IP25 can be used to reconstruct spring ice concentration.

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).