Characterising amber from the Arctic and Antarctic circles: factors affecting the production and preservation of resin at high latitudes during the mid-Cretaceous

2017-05-15T07:26:44Z (GMT) by Quinney, Ann Edgerton
This thesis documents four discoveries of mid-Cretaceous amber from the high southern and northern latitudes of Australia and Canada, respectively. These deposits occur primarily in Turonian marginal marine/deltaic sediments and are important for their representation of a) high latitude forests, and b) terrestrial environments during a greenhouse period characterised by globally elevated sea levels, for which terrestrial outcrops are lacking. Whereas the Australian localities are particularly important for their representation of the oldest in situ deposits from Australia and the southern-most deposits from Gondwana, the fortuitous discovery of coeval deposits from northern Canada provides a unique opportunity to compare and contrast resin production and preservation from the high latitude forests during the mid-Cretaceous greenhouse. Like many amber deposits world-wide, the high latitude ambers are generally devoid of bioinclusions, but Otway amber contains abundant pseudoinclusion. All of the amber is cupressaceous-araucarian, permitting comparisons of stable C and H isotopic data. The results indicate that, like other ambers globally, values of δ13Cmean for the high latitude ambers are enriched in ¹³C by 3–4‰ relative to their modern equivalents due to a global decrease in pO2 during the mid-Cretaceous greenhouse. Notably, high latitude ambers are depleted in ¹³C by approximately 1.9‰ relative to coeval amber from the middle latitudes due to latitudinal changes in light regime. Further, because of the inverse relationship documented between light exposure and ²H fractionation in plant lipids, exposure to continuous (24 h) low intensity light during the Arctic and Antarctic summers is thought to reduce the amount of ²H fractionation during resin biosynthesis relative to the middle and low latitudes. Seasonal changes in light availability may also account for some of the variation in δD among high latitude amber samples. Additionally, the southern high latitude forest may have experienced seasonal aridity, forcing the trees to periodically utilise fog and/or evaporatively-enriched surface water as a water source. By contrast, the narrow range of δDamber values from Canada indicates that the trees likely relied on precipitation that was distributed evenly throughout the year. Differences in δDmean between the Canadian and Australian ambers are likely related to differences in the δD of coastal precipitation sourced from slightly depleted inland seas versus open oceans. The strong similarity in the stable carbon and hydrogen isotopic compositions reveal that the controls on resin production and preservation were consistent between the high latitude forests, with differences attributed to local environmental conditions and geographic position.