THE EARLY PHANEROZOIC 'TAPHONOMIC WINDOW'

It has long been recognized that the Cambrian is enriched in fossil lagerstätten, which are critical to understanding the Cambrian explosion. This early Phanerozoic “taphonomic window” is defined by the prevalence of Burgess Shale-type (BST) preservation, an unusual mode of fossilization that is rare after the middle Cambrian. Geochemical and macrostratigraphic data were used to construct a model that accounts for the secular distribution of BST fossil assemblages and for slight but important variations in their taphonomic expression. The widespread occurrence of this type of exceptional fossilization was controlled by changes in seawater chemistry and in the distribution of marine environments over the continents. Low oxidant availability (O₂, SO₄^2-) in the oceans was critical to BST fossilization but this condition is not unique to this interval of geologic time. Preservation of non-biomineralized taxa also required early sealing of the sediment-water interface by seafloor cements to isolate the burial environment from overlying seawater; this was the result of an elevated weathering-derived flux of alkalinity to the oceans during the Cambrian and early Ordovician. Macrostratigraphic data show that the sharp decline of BST preservation after the middle Cambrian coincides with a sharp restriction of favorable outer shelf environments, which were progressively displaced from continental crust by the expansion of carbonate platforms. Recent findings of BST-like fossil assemblages in late Cambrian and early Ordovician strata confirm that exceptional preservation persisted where favorable facies developed. Variation in preservational style is correlated with a global shift in carbonate chemistry. δ³⁴S data suggest that the efficacy of seafloor cements as a sealing agent increased with the transition from high-Mg to low-Mg calcite seas in the middle Cambrian. The steep decline in seawater Mg/Ca reduced kinetic barriers to carbonate precipitation, facilitating more rapid precipitation at the seafloor and the preservation of pristine organic fossils in the Burgess Shale and elsewhere. Carbonate sealing was important but less efficient in the early Cambrian, when microbial activity proceeded further, resulting in pyritization of limited aspects of some carbonaceous fossils in the Chengjiang biota.