Graptolitic mudrocks and their implications for the taphonomy of organic compression fossils

This thesis addresses graptolitic mudrocks in an Earth system context and examines issues concerning the preservation of organic carbon. It investigates the palaeoenvironmental and paleoecological aspects of these mudrocks. It relates their deposition to climate modulation in an hitherto-unrecognised Early Palaeozoic Icehouse, and reassesses the fossils of the problematic genus Dawsonia Nicholson. This work identifies seven glacial maxima in the Late Ordovician and Early Silurian recognised by the occurrence of well-dated glacial deposits coincident with stable isotope excursions and eustatic regressions. Comparison of these data with the occurrence of graptolitic shales reveals that deglacial transgressions led to increased stratification of the water column and the onset of widespread marine anoxia. The burial of organic carbon in these deglacial anoxic events may have served as a negative feedback mechanism by drawing down sufficient atmospheric carbon dioxide to prevent runaway warming and stabilise this long-lived Early Palaeozoic Icehouse. This study suggests that graptolites are best viewed as a mixed-layer zooplankton and that their occurrence in anoxic mudrocks should be regarded as representing the conditions they were preserved in rather than those in which they lived. Comparison of published data on graptolite diversity with the oxic-anoxic stratigraphy for the EPI shows there is no strong link between graptolite diversity and marine anoxia. Meanwhile, the general absence of graptolites from bioturbated facies may reflect enhanced decay and scavenging related to well-oxygenated conditions; whilst the documentation of rare occurrences of graptolites in oxic facies and those from above the storm wave base shows that they could live in well-oxygenated waters. Though anoxia alone is insufficient to explain the fossilisation of organic-walled fossils and conversion of subfossil cuticle to a more stable biopolymer may have inhibited degradation, it is clear that fossilisation does not render such fossils entirely inert or homogeneous. Analysis of multifaunal assemblages, such as those preserving Dawsonia Nicholson (shown to be brachiopods, crustacean tailpieces and an organic-walled problematicum), where graptolites co-occur with shelly fossils and diagenetic pyrite, shows that graptolites acted as a key site for phyllosilicate authigenesis in very low-grade metamorphism. Petrographic evidence and comparison with white mica crystallinity data suggests that the expulsion of volatiles in maturation may have catalysed the formation of phyllosilicates on these fossils. This mode of phyllosilicate formation may also account for the formation of phyllosilicate films on Burgess Shale fossils.