A Geochemical Study of Permian Marine Cements: Investigating Changes in Ancient Seawater Chemistry

Elizabeth Smith
University of Michigan Geological Sciences Department
Ann Arbor, Michigan
lizsmith@umich.edu

The composition of various mineral phases precipitated from seawater has varied over time, dividing the Phanerozoic into intervals of “aragonite seas” and “calcite seas.” Changing Mg/Ca and Sr/Ca ratios of seawater may be the primary control on abiotic carbonate mineralogy. This change in ocean chemistry likely reflects variation in hydrothermal activity because mid-ocean ridges extract magnesium from seawater. However, knowledge of the evolution of seawater compositions based on elemental ratios in marine carbonate phases remains incomplete since relatively few time intervals have been examined. One critical time period is the end-Paleozoic. The Permian Reef Complex of New Mexico and West Texas contains well-preserved marine calcite cements, precipitated under “aragonite sea” conditions, which may preserve the original seawater chemistry at the time of their precipitation. Paired oxygen and carbon isotopic compositions and elemental analyses of these calcite cements will lend valuable information to resolve fluctuations of ancient seawater chemistry. Although prior work has shown that original carbon and oxygen isotopic values are preserved, few elemental data exist by which to derive ancient marine elemental chemistry. An estimate of original seawater chemistry from the Permian would not only add to the information gained from studies of other Paleozoic carbonates, but could be compared to modern seawater chemistry, in which aragonite is the dominant abiotic carbonate phase. Information from this study could also be used to constrain estimates of the relative importance of global processes such as hydrothermal activity, continental weathering, and riverine influxes, which ultimately control the isotopic and elemental composition of seawater.