Production and preservation of the Arctic sea ice diatom biomarker IP25
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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.
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