Determining the Maturity of Bivalves in Puerto Rico and the Dominican Republic to Determine Historical Processes that Affected Deposition

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.

A four panel figure labeled A to D. A- shelly layer of fossils with a sharpie for scale. B- shelly layer of fossils with a pointer finger indicating to a zigzag break in the structure. C-horizontally striped microstructures that appear similar to a ripple effect. D. layered fossilized tissue with round edges and smooth overlapping structure that resembles a palmetto leaf.
The images above display the samples of Kuphus incrassatus and Ostrea haitensis bivalves tested in the study. Images (A) and (C) are Kuphus incrassatus which show that the interior shell texture is still intact, as well as the outer layers which have recrystallized. Images (B) and (D) display Ostrea haitensis bivalves with consecutive layers of preserved tissue and partially recrystallized shell texture. The condition of these bivalves demonstrates the lack of chemical alteration found in the study.

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.

Andrea’s Academic Background and Career Goals

In 2017, I began my academic journey majoring in Political Science and International Affairs at Florida State University. Throughout this time, I learned how ingrained politics is in every aspect of our society and how important it is to get involved in civic duties such as voting and researching legislation on major social and fiscal issues. A significant part of politics involves the country’s history as well, which was required curriculum taught by several courses including American History, Protests in America, European History, and International Affairs. These courses taught critical material about the oppression and discrimination that has shaped the legislation still in existence today targeting all minority groups living in the U.S. These courses helped to dismantle unconscious biases and stereotypes that help us become more educated voters in the future. My courses also focused on the processes of the U.S. government system, as well as how the U.S. interacts with other countries and global entities. This is especially important when it comes to global issues where it is crucial for all states, countries, and territories to work together. In this day and age, one of the most pressing and time-sensitive issues of all is climate change.

Alt text: Woman in jean jacket and blue hat stands in front of Zion National Park canyons by river.
In Zion National Park.

In 2021, I decided to take my political science background and apply it to a master’s degree in Environmental Science and Policy at the University of South Florida. Above all, I realized that my passion has always been to protect and conserve the planet’s biodiversity and natural ecosystems. I knew how much policy determined either the protection or destruction of the environment, and I made it my goal to use my background to be on the side of preservation and restoration. Since then, I have begun my third semester of graduate school and have learned  about environmental policy, conservation in urban environments, geology, remote sensing, and environmental ethics and philosophy. 

This summer, I also had the opportunity to spend two months working with the Student Conservation Association in Yellowstone National Park. During my time there, I volunteered alongside a National Park Service conservation crew replacing a bridge with sustainable materials. The purpose of this is to ensure that people can appreciate nature in a safe way for both themselves and the wildlife and minimize impacts to areas outside of the trails. I plan to continue pursuing these opportunities that expand my knowledge on best practices for environmental policy and learning first-hand from the most experienced people in the field. These experiences have only augmented my appreciation for this field, and I hope to build a career in conservation in Florida upon graduation next May.