nai

Project Progress

In the past year we have concentrated on our goals of providing precise temporal constraints on major events in the history of life as well as the use of geochemistry and Sr isotopes to understand the chemical signals of seawater during the end-Permian extinction. Central to this work is the evaluation of whether Large Igneous Provinces (LIPS) have played a role in the extinctions. Our work on the end-Ediacaran and early Cambrian continues with work on existing samples from the Duoshantuo Formation from southern China and new work in conjunction with A. J. Kauffman of the University of Maryland on the early Cambrian of Siberia.

One of the major issues in understanding the end-Permian extinction is the nature of an earlier extinction first recognized in China less than twenty years ago, the so-called Guadalupian-Lopingian boundary and its role in making ecosystems more susceptible to the end Permian extinction ca 5 Ma later. We have just completed a study on the middle to late Permian of the Karoo Supergroup in South Africa. Our new temporal framework allows correlation to marine zonations and improves understanding of rates of faunal evolution. We see no vertebrate extinctions in the Karoo Supergroup that correlate with the marine end-Guadalupian mass extinction and raise the question of whether there is any record of a terrestrial extinction.

In addition to our Sr isotopic and trace-element work on samples straddling the Permian-Triassic boundary at the GSSP at Meishan, China (over 400 Sr analyses), we are now working on 150 conodont samples from P-Tr boundary sections at Liangfengya in Chongqing Province, and Daijiagou in Sichuan Province, in China, and from Abadeh, Iran, and additional whole rock carbonate samples from Meishan and Liangfengya.

We have completed almost 100 additional Sr isotopic analyses on bulk rock carbonates from Meishan in an effort to understand why the bulk rock carbonate isotopic compositions differ from those of conodonts from the same stratigraphic units. The bulk chemical data show that there is a notable change in carbonate chemistry at the top of bed 24, with increased dolomitization in the beds above 24, as well as an increase in the clastic silicate component. The Meishan bulk carbonate Sr data are elevated with respect to the conodont Sr data because of admixture of radiogenic Sr from a sudden influx of clastic detritus. Cleaning and selective carbonate dissolution are not sufficiently selective to remove the radiogenic Sr signature completely.

Bulk chemical analyses of Meishan carbonates show anomalies in several redox-sensitive element abundances near and above the extinction horizon (between beds 24c and 24f). These anomalies are partly coincident with anomalies in other transition metals (Ni, Pb, Zn) that are not typically redox-sensitive. These anomalies are stratigraphically restricted, and can be identified only in detailed, cm-scale sampling.

In conjunction with Francis McDonald we have also completed Sr isotopic work on the Neoproterozoic section in the Mackenzie Mountains, Northwest Territories, Canada. Additional samples from the glacial cap-carbonate in the Twitya Formation, immediately overlying the Sturtian Rapitan Group, capture Sr isotopic variation in marine waters following the end of glaciation.