Precambrian Ocean Chemistry
Oxygen Isotopes
In the Earth's ocean-atmosphere system, the stable isotopes 18O and 16O are most commonly fractionated by the physical processes of evaporation and condensation. A secondary fractionation, driven by temperature, occurs when carbonate minerals are precipitated from seawater. As a result, O-isotope signatures of carbonate rocks potentially reflect environment of deposition (open marine vs. evaporative marine), temperature of precipitation, and, perhaps most importantly, the effects of a rock's interaction with diagenetic fluids that are compositionally different from that of seawater (meteoric or burial fluids).
Because carbonate rocks are chemically reactive, there can be rapid O-isotope exchange between fluids and carbonate minerals during diagenesis. In many instances, a combination of O-isotope composition and trace element concentration (another method to diagnose diagenetic fluid interaction) provides the best indication of whether pristine geochemical signals are preserved in ancient carbonate rocks. These analyses, therefore, are integral to determining ancient seawater compositions.
 Common diagenetic pathways in carbonate rocks |
 Evaporation trends from the Society Cliffs Formation |
In rare cases, petrographic and trace element analysis suggests little diagenetic overprinting. In these cases, O-isotope compositions have the potential of highlighting environmental trends within a basin. For example, in the 1.2 Ga Society Cliffs Formation, northern Baffin Island, open marine facies preserve O-isotope compositions nearly 6% lighter than restricted peritidal facies (Kah 2001). An interpretation of increasing evaporation along an arid shoreline is supported by the presence of gypsum in peritidal deposits, and the 6% variation is similar to that observed in a Persian Gulf transect from marine settings to the sabkha flats.
Carbon Isotopes
During the photosynthetic production of organic matter (CO2 + H2O = CH2O + O2), organisms are able to fractionate C isotopes - favoring 12C for their metabolism and leaving marine waters isotopically enriched in 13C. When organic matter decomposes, isotopically light carbon is returned to the oceans, but when organic matter is preferentially removed from the ocean by burial, marine waters can become enriched in 13C over the long term.
The C-isotope system is therefore a powerful tool to examine the dynamics of biogeochemical cycling throughout Earth history. My colleagues and I have been concentrating on the Mesoproterozoic (1.6-1.0 Ga) in order to determine causes and consequences of global environmental change during this time. By analyzing carbonate rocks and associated organic matter from Canada, Russia, and the U.S. (as well as using published data from China, Australia, and elsewhere), we have constructed a global marine C-isotopic curve for this time period (Kah et al. 1999).
Mesoproterozoic to Neoproterozoic
marine C-isotope curve
Sulfur Isotopes
Increased oxygenation in the Mesoproterozoic should result in an increase in the concentration of sulfate in the global ocean. In turn, such oxygenation may potentially be recorded by the S-isotope composition of Mesoproterozoic sediments because S is fractionated during bacterial sulfate reduction (32S is preferentially incorporated into the reduced phase and preserved as pyrite). Broadly, when the sulfate reservoir is small, bacterial sulfate reducers frequently use all the available sulfate, resulting in an overall decrease in total fractionation between oxidized and reduced sulfur reservoirs (i.e. the S-isotope compositions of pyrite and gypsum converge). Also, with a small sulfate reservoir, even small amounts of bacterial sulfate reduction have the potential of resulting in large variations in the isotopic composition of marine sulfate.
Unfortunately, the scarcity of ancient gypsum deposits makes it difficult to determine the composition of marine sulfate in the Precambrian. My colleagues and I have therefore been concentrating on extracting S-isotope information from the small amount of sulfate ions that are structurally substituted in the carbonate crystal lattice (CAS, or carbonate-associated sulfate). Because only ~50-200 ppm sulfate are substituted into the carbonate, this can be a time-consuming process, but well worth the effort!
 Correlation of data from Mesoproterozoic Gypsum and Carbonate-associated sulfate |
 Silicified halite cube preserved in gypsum, Baffin Island |
Our recent work indicates that large shifts in the isotopic composition of marine sulfate are common in the Mesoproterozoic, suggesting that sulfate concentrations were significantly smaller than that of the modern ocean (Kah et al, 2001). Using a time-dependent equation for isotopic change we have calculated that sulfate concentrations in the Mesoproterozoic were ~2.0-2.8 mM, or 7-10% today's concentrations (Kah et al., 2004). At this concentration, marine waters would have to evaporate to ~3% of their original volume before gypsum saturation was reached, thereby helping to explain the rarity of gypsum in the geologic record and the apparent reversal of halite and gypsum in evaporation trends.
Linda Kah
Department of Earth and Planetary Sciences
1412 Circle Drive
Knoxville, TN 37996-1410
Phone: (865) 974-6399
Email: lckah@utk.edu