A High Resolution Pennsylvanian-Early Permian Seawater ⁸⁷Sr/⁸⁶Sr Curve

Publication Date

4-2007

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Geology

Department

Geosciences

Supervisory Committee Chair

Mark Schmitz

Abstract

The 87Sr/86Sr seawater curve is an important tool used to understand dynamic periods in Earth history, such as the Carboniferous and Permian periods. The 87Sr/86Sr seawater curve is derived from plotting measured 87Sr/86Sr of marine carbonates or phosphates versus their respective estimated absolute ages within geologic time. The result is a graphical history of seawater 87Sr/86Sr evolution as a useful chemostratigraphic correlation tool. The challenges in using the seawater 87Sr/86Sr curve for global correlation are the requirements of well preserved and unaltered samples that have retained the original seawater 87Sr/86Sr signal and highly precise biostratigraphy for accurate age assignments. Highly precise regional biostratigraphy is possible, but global biostratigraphy is more difficult and less precise. The current Carboniferous-Permian 87Sr/86Sr curve is constructed from carbonate samples collected from a global distribution of localities (the southern mid-continent U.S.A., Canada, Germany, Great Britain, and China), possibly contributing large error within the time axis of the curve and compromising the use of the curve as a precise global correlation tool. Additionally, the effects of diagenesis on marine rocks may alter the original seawater 87Sr/86Sr signal and introduce significant error into the 87Sr/86Sr axis.

Stratigraphic sections within the Southern Urals offer a unique opportunity to create a temporally accurate Sr curve. The marine sections within the Southern Urals are geographically adjacent to one another, contain abundant and well-preserved fauna for precise biostratigraphic correlation, and are recognized as type sections and principal reference sections for the Carboniferous-Permian. These sections also contain common interstratified volcanic ash beds, allowing for precise absolute time (U-Pb) calibration.

With new U-Pb age constraints combined with the biostratigraphy, the sections of the Southern Urals provide an accurate age model for the Late Carboniferous-Early Permian. Using the new time model, a temporally refined 87Sr/86Sr curve was constructed from interstratified conodont elements. Conodont elements are composed of biogenic apatite. Sr substitutes for Ca in apatite, so if well preserved and unaltered, conodont elements retain original seawater 87Sr/86Sr compositions. To ensure analysis of original seawater 87Sr/86Sr and not surficial contaminant Sr, a pre-treatment method that removed the outer layers of the conodont elements was used in this study. The exceptional preservation of the conodont elements as well as the pre-treatment regimen resulted in a very small degree of 87Sr/86Sr data scatter.

The Sr seawater curve follows a generally decreasing trend with an apparent increase at around 292 Ma. This temporally refined Sr curve may prove to be a useful tool for chemostratigraphic correlation between Late Carboniferous-Early Permian marine sections worldwide, with estimated age errors ±0.5 Ma. This 87Sr/86Sr seawater curve also provides constraints on the processes that caused fluctuations in the seawater 87Sr/86Sr composition, with a 20-40% change in hydrothermal or riverine Sr flux necessary to produce the constrained Late Carboniferous-Early Permian (lower Gzhelian to middle Sakmarian) decline in seawater 87Sr/86Sr.

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