An Isotope Perspective on the Precambrian-Cambrian Biosphere (R)evolution

Freitag, 08. August 2014 - 11:15 Uhr
GEO-Gebäude, Raum 1550 (Hörsaal)
Frank Ohnemüller

This dissertation deals with the reconstruction of ocean pH conditions and the quantification of weathering rates by boron and strontium isotope analyzes on Ediacaran and Early Cambrian marine carbonate rocks. In order to compare local signals and specific features with global patterns, a variety of sections on two different palaeo continents (South China, Kazakhstan) were examined.
The Earliest Ediacaran (635 Ma) represents the deglacial phase after a suspected Snowball Earth event. At this time, a global ocean acidification event occurred, which was apparently associated with highly enhanced atmospheric pCO2 levels during the ice age. The magnitude and duration of this acidification event not only varies considerably between different palaeo continents, but also within distinct facies environments of a single craton. Moreover, the entire Ediacaran is characterized by minor to major ocean pH fluctuations and a general increase in weathering rates and fluxes to the ocean. Southern Chinese and Kazakh boron isotope data corresponding to the Ediacaran-Cambrian transition indicate a temporary ocean acidification for this crucial time period, too. In addition, strontium isotope data from South China and Kazakhstan hint towards increasing weathering rates during the terminal Ediacaran. Both effects had a significant influence on ocean geochemistry and may have decisively contributed to the creation of new ecological niches, possibly enabling the appearance of multicellular animals and reinforcing their radiation.