Strontium Isotope Stratigraphy for Oligocene-Miocene Carbonate Systems in Puerto Rico and the Dominican Republic: Implications for Caribbean Processes Affecting Depositional History
Ortega-Ariza, D., Franseen, E. K., Santos-Mercado, H., Ramírez-Martínez, W. R., and Core-Suárez, E. E.
Summarized by Andrea Gann, a graduate student pursuing a master’s degree in Environmental Science and Policy at The University of South Florida. Currently, she is in her second year. After graduation, she plans to work as an environmental science analyst for an environmental consulting firm in Tampa. When she’s not studying environmental science, she enjoys kayaking and swimming in the many springs located in Florida.
What was the hypothesis being tested? This paper aims to establish the ages of two fossil clam species, Kuphus incrassatus and Ostrea haitensis, by using absolute dating methods. Absolute dating is when scientists calculate the amount of radioactive decay in the isotopes of minerals found inside the fossil. Isotopes are known as elements that share the same number of protons while differing in their number of neutrons. This data was used to better understand under what conditions certain shallow marine systems in Puerto Rico and the Dominican Republic were deposited.
What data were used? The data collected in this study is called strontium (Sr) isotope data. Strontium is known as a common trace element, which is a chemical component present in many organisms that includes both essential and non-essential elements. For example, zinc is considered an essential element in most organisms, while other elements like aluminum or uranium are deemed non-essential. To study and determine the ages of certain fossils – in this case, bivalves – the authors are applying the 87Sr/86Sr ratio. Within strontium are four isotopes, and these two are the closest to each other in abundance. The ratio involves the decay of rubidium isotope 87Rb, a trace element, into 87Sr over geologic time. By studying the increase of 87Sr/86Sr, researchers can determine the absolute age and origin of certain fossils by calculating ratios.
Methods: The researchers began by collecting shell samples of both Kuphus incrassatus and Ostrea haitensis bivalves and drilling small amounts of their shell to chemically analyze. These shells are composed of low-magnesium calcite, which means that they tend to be fairly stable where other forms of minerals would change in the fossilization process. Scientists compared the ratio of the bivalve fossils to modern-day fossils to ensure no chemical changes would have affected the 87Sr/86Sr ratio. Here, a Thermo- Finnigan MAT 253 isotope ratio mass spectrometer was used to calculate the isotopic ratios of the bivalves, which were then juxtaposed with the ratios of modern mollusks. Concentration levels of other trace elements such as iron and manganese were also confirmed and compared with Sr values as another indicator of chemical alteration. Finally, the Sr isotope ratio produced three values – each with a corresponding minimum and maximum age. The 87Sr/86Sr ratio value reflects the mean age, while the minimum and maximum values exist as a range for any error or uncertainty.
Results: 117 samples were collected, and 41 of those samples were at values expected of a shallow-water environment. . The authors also discovered a depletion in carbon values that could have been caused by freshwater runoff or an insufficiency of open-ocean water interchange. The absolute age of the bivalves, used to determine the ages of the rocks in which they were found, were included in the results. For example, the San Sebastian Formation in Northern Puerto Rico was set in the middle-late Oligocene at a mean age range of 29.78 to 26.51 Ma. Another formation named the Yanigua-Los Haitises Formations were set in the Middle Miocene at a mean age range of 15.75 to 15.25 Ma.
Why is this study important? The framework created in this study provides insight into how the chronostratigraphy of bivalves is directly correlated with time and surrounding local processes and regional processes. The meaning behind chrono is time, while the meaning behind strat is ‘layer’. Thus, chronostratigraphy is the analysis of rock layers over time. By identifying the absolute age of these shells, the authors can then determine what global and local processes influenced its deposition.
Broader Implications beyond this study: This model will allow other stratigraphers and geologists to replicate this study with bivalves or other shells in their own regions globally. The authors describe methods using multiple pieces of scientific equipment (e.g., a Thermo- Finnigan MAT 253 isotope ratio mass spectrometer, microscope-mounted dental drill, transmitted light microscope petrography, plasma atomic emission spectroscope, and more). There is in-depth detail about the formulas utilized to calculate for chemical alteration that can help guide other geologists with their own chronostratigraphy and absolute dating analyses. Overall, absolute dating helps construct a structured timeline and establishes the depositional conditions and processes that were occurring.
Citation: Ortega-Ariza, D., Franseen, E. K., Santos-Mercado, H., Ramírez-Martínez, W. R., & Core-Suárez, E. E. (2015). Strontium Isotope Stratigraphy for Oligocene-Miocene Carbonate Systems in Puerto Rico and the Dominican Republic: Implications for Caribbean Processes Affecting Depositional History. The Journal of Geology, 123, 539–560. https://doi.org/10.1086/683335