Biological processes occur more quickly with 12C than 13C because 12C is slightly lighter. Thus, organic matter has more 12C than the surrounding ocean water. As more and more organic matter is produced, more and more 12C is removed from seawater and 13C builds up. As organic matter decays, 12C is released back into seawater. The d13C of carbonates varies, however, as biological productivity varies in the oceans and as the amount of organic matter that gets buried changes.
Sulfur is reduced and oxidized by bacteria in ocean sediments.
The sulfur isotopic ratio can be preserved in sulfate and sulfide minerals, respectively, and can reflect either regional or global changes in microbial processes that can be used to correlate stratigraphic sections and interpret the dynamics of ancient biospheres. Summary - Chemostratigraphy is one step more detailed than magnetostratigraphy because you have multiple values and can measure trends rather than north or south. However, lots of different processes can affect the values, including local variations. Careful data interpretation is always necessary. Sequence Stratigraphy - Sequence stratigraphy uses changes in relative sea level to correlate different depositional environments that were present at the same time.