2009 Annual Science Report
Massachusetts Institute of Technology Reporting | JUL 2008 – AUG 2009
Timscales of Events in the Evolution and Maintenance of Complex Life
This project involves high precision dating of major events in earth history. Using the decay of uranium (U) to lead (Pb) in the mineral zircon we are able to date 600 million year old rocks to ± less than 1 millon years. Such precision allows us to investigate rates of change in the ancient past from climate to evolution.
Our work so far has concentrated on three major projects: 1) using detrital zircons to constrain the depositional age of Neoproterozoic sedimentary rocks that contain one of the largest perturbations to the carbon cycle in earth history 2) a Sr isotopic investigation of the end-Permian extinction interval; and 3) work on the timescale for the end-Permian extinction in a terrestrial setting, the Karoo Basin of South Africa.
1) Our work on the first project has been concentrated on clastic rocks interlayered with carbonates from Oman below and within the Shuram anomaly-the large perturbation to the carbon cycle. We have analyzed using high-precision IDTIMS approximately 20 zircon grains from rocks well below the anomaly that yield dates as young as 600 Ma and as old as 850 Ma. The young dates confirm earlier speculation that the perturbation began after 600 Ma not before as previous temporal constraints were consistent with.
2) The cause of the end Permian extinction is of great interest to our group. Previously we have established a high-resolution record of the extinction and have documented a very rapid extinction pulse (< 50 thousand years in duration) as well as a rapid negative shift in the isotopic composition of both the carbonate carbon and organic carbon reservoirs. In this study we have attempted an unprecedented Sr isotopic study of the extinction interval using both conodonts and whole rock carbonates. We have acquired Sr isotopic data for 300 single conodont elements and about 150 whole rocks carbonate samples from 300 cm (!) of section straddling the P/Tr boundary at Meishan, China. The biostratigraphically-defined boundary is about 30 cm from the top of the section we have studied, and the bulk of our analyses are in bed 24, 25 to 100 cm below the P – T boundary, in which the carbon isotopic data indicate that the Permian extinction event actually occurred.
Conodont data show a gradual increase up section, from ^87^Sr/^86^Sr of about 0.70708 to about 0.70715, with no excursions. The conodont data thus show no discontinuity across the P/Tr boundary, and contrast with literature data that suggest a very rapid increase of Sr isotopic compositions in the earliest Triassic.
Whole-rock carbonate data follow the conodont trend below bed 24. In bed 24, there is a rapid increase in the 87Sr/86Sr of whole rock carbonates, from 0.7071 to about 0.7073. About 10 cm below the top of bed 24, there is a dramatic drop in whole rock Sr isotopic values, from 0.70730 to about 0.70712, followed by an equally rapid increase, over the next 10 cm, back to values above 0.70730. Above bed 24, the whole rock carbonate data show variable but consistently high values. This remarkable decoupling is not well understood and is the subject of continued study.
3) New work on the Karoo has allowed us to date five ash beds from outcrop and drill core that range in age from 260 Ma to 254 Ma and will allow new constraints on the distribution and extinction of terrestrial animals.
PROJECT INVESTIGATORS:Samuel Bowring
PROJECT MEMBERS:Douglas Erwin
RELATED OBJECTIVES:Objective 4.1
Earth's early biosphere.
Effects of environmental changes on microbial ecosystems