ver the past decade major advances in U-Pb zircon geochronology have allowed us to evaluate the distribution of time in the rock record and rates of geological processes with unprecedented precision. There is considerable promise for a highly calibrated time-scale from Neoproterozoic to Holocene that will permit increasingly more sophisticated questions to be addressed using the rock record. It has been long appreciated that there are dramatic events in the diversification and extinction of life such as the Cambrian radiation, the end-Permian extinctions, and the end-Cretaceous extinction. However, important questions remain regarding the tempo and causes of evolutionary radiations and extinction. For example, what are the durations of mass extinctions?
How long does ecological recovery take following a major extinction? Do evolutionary radiations correlate with changes in chemistry and temperature of ocean-atmosphere system and global climate? What are the relationships between evolution and the aggregation and dispersal of supercontinents? Are apparently abrupt isotopic excursions globally synchronous and of the same duration? Although, historically, the biostratigraphic record was used to address these questions, we are now moving into an era where the final arbiter of correlation and tempo will and must be high-precision geochronology of volcanic rocks interlayered with fossil-bearing rocks. This means moving beyond simple calibration of the time-scale to understanding the detailed distribution of time in the rock record.
Our laboratory is involved in three major time-scale projects:
- terminal Neoproterozoic history and the rise of Metazoans;
- the Cambrian-Precambrian boundary and the Cambrian explosion;
- the end-Permian mass extinction and subsequent Triassic recovery.
In additon, we are developing methods for increasing the resolving power of the U-Pb technique to better constrain these events.
or most of the nearly 4 billion years that life has existed on Earth, evolution produced little beyond bacteria, plankton, and algae; but, beginning about 600 million years ago, the fossil record speaks of more rapid change and increasing diversity. First, there was the development of the enigmatic Ediacaran fauna, named for the fossil site in Australia where they were first discovered. Some of these animals may have belonged to groups that survive today, but others don’t seem at all related to animals we know.T
hen, about 540 million years ago, the fossil record changed dramatically. First, fossils of Ediacaran fauna disappear from the rock record and “shelly” fossils, representing most of the classes and orders of animals we see today, appear. Coincident with this faunal change is a global short-lived 5 per cent drop in the carbon isotopic signature of seawater. These observations led some workers to suggest an extinction event followed by a major reorganization of Cambrian ecosystems. This stunning and unique period is often termed the “Cambrian explosion”, The precise timing of this evolutionary radiation is one of the foci of our research group.T
he past decade has seen dramatic new discoveries regarding the paleontology and chronostratigraphy of the transition between the late Neoproterozoic and the Cambrian. This transition corresponds to a time of major tectonic and climatic change including the assembly and dispersal of a supercontinent, glaciation, and large fluctuations in the chemistry of the oceans and atmosphere. Chemostratigraphic and biostratigraphic studies are now focusing on evaluating the connections between tectonics, climate change and the evolution of life and the age, duration, and global synchroneity of dramatic isotopic excursions such as the one that coincides with the Cambrian-Precambrian boundary. Further resolution of these questions will depend on abundant high-precision geochronology of ash-beds integrated with the paleontological and chemostratigraphic records.
A major extinction event occurred approximately 250 million years ago and marks the boundary between the Permian Period and the Triassic Period as well as the transition between the Paleozoic and Mesozoic Eras. At the end of the Permian more than 85% of all species in the oceans, approximately 70% of land vertebrates, and significant numbers of plants and insects vanished. The Permian extinction caused the most fundamental reorganization of ecosystems and animal diversity in the past 500 million years. The marine communities of today are largely a result of the recovery following this extinction. In addition, dinosaurs and mammals arose in the aftermath of the extinction.
A better understanding of how long the end-Permian extinction and its recovery took will allow for new insights and better understanding into the role of mass extinctions in evolution. Mass extinctions are marked in the fossil record by the abrupt disappearance of taxa, sometimes associated with a discrete “boundary bed”; in the case of the end Cretaceous extinction this bed is a layer rich in impact ejecta with distinctive chemical signatures. A critical question is how abrupt such extinctions really were. A satisfactory answer must involve statistical analysis of the stratigraphic and fossil record. Differences in sediment accumulation rate and the preservation potential of organisms can lead to an artificially abrupt and/or drawn out extinction signal, especially if the extinction is of short duration, say, less than 1 million years. Because sedimentation rates vary, stratigraphic thickness does not convert to time directly. Therefore, understanding an extinction requires constraining its tempo by combining high-precision geochronology with paleontological studies.