Carbonates – archive for carbon cycle history and for chemostratigraphy
Helmut Weissert
ETH Zuerich
Helmut Weissert is a paleoceanographer and paleoclimatologist, his roots are in sedimentology, isotope geochemistry and stratigraphy as well as in field geology. Improved measuring of geological time marks the beginning of any investigation related to earth history. His official research and teaching career at ETH ended in October 2014. Now he can devote more time to trace global carbon cycle through earth history and to learn more about the evolution of the biosphere through geological time.
Abstract
The discovery of isotope geochemistry as a powerful tool in oceanography and paleoclimatology dates back into middle of the 20th century. Oxygen isotope records stored in carbonate of planktic and benthic foraminifera were... [ view full abstract ]
The discovery of isotope geochemistry as a powerful tool in oceanography and paleoclimatology dates back into middle of the 20th century. Oxygen isotope records stored in carbonate of planktic and benthic foraminifera were recognized by Emiliani as archives of the “orbital pulse”, or in other words, of Milankovich cyclicity. Oxygen isotope geochemistry resulted in a new understanding of ice age history and it developed into a powerful instrument in stratigraphy. Even if fundamentals of C-isotope geochemistry also were established in the early years of isotope geochemistry, the importance of C-isotope composition of marine carbonates for geology and oceanography was appreciated only in the 1970ties. Carbonate carbon isotope values were used as a proxy of changes in the marine carbon pool and, in a next step, of variations the global carbon cycle. Changes in the open marine carbon pool are regarded as synchronous on a global scale due to short mixing time of the oceans in the order of thousand years. Based on this, fluctuations in the C-isotope record of marine carbonates can be used as accurate stratigraphic markers. Pelagic carbonates continue to serve as reference archives for C-isotope stratigraphy. Orbital variations (long eccentricity), greenhouse climate conditions triggered by volcanic degassing or by sudden methane release are identified as cause of change in marine C-isotope records. However, also system intrinsic variations in carbonate and organic carbon burial rates can results in C-isotope excursions of global scale as described by Louis-Schmid et al. in 2007. Bulk carbonate records provide reproducible C-isotope signatures far back into earth history. However, poor understanding of source and origin of pelagic micrites in pre-Jurassic oceans complicates interpretations of C-isotope records. Projection of pelagic C-isotope curves into hemipelagic or neritic environments asks for a thorough understanding of environmental conditions during formation of these sediments and of diagenesis affecting the isotopic composition of hemipelagic and neritic sediments.
Session
KN5 » Keynote Lecture (08:30 - Thursday, 25th June, Pangea)