Research

Coral Reef Paleoceanography in the Caribbean (and beyond)!

Coral skeletons are invaluable archives of Earth’s past oceanic and climatic conditions. As reef-building corals grow, they deposit a calcium carbonate (CaCO₃, primarily the mineral aragonite) skeleton built from ions dissolved in the surrounding seawater. However, seawater isn’t simply a pure mixture of calcium and carbonate ions, and thus the specific chemical makeup of the skeleton depends on (1) the chemical makeup of the ambient seawater and (2) environmental and biological controls governing how coral biology selects for/against the incorporation of skeletal “impurities”. In this way, variations in geochemical ratios (such as element-to-calcium and stable isotope composition) in coral skeletons are established proxies for past variability in temperature, salinity, and numerous other chemical parameters of ambient seawater. By analyzing skeletal samples of both living and fossilized coral colonies, we can combine these geochemical proxies to identify seasonal, annual, and decadal shifts in environmental conditions across various periods in Earth’s history. Our lab currently applies these techniques to better understand: (1) the interface of paleoclimate and marine archaeology before, during, and after the Danish Colonial Period in St. Croix, USVI (sponsored by a Level II Explorer award from National Geographic) and (2) the significance of El Niño Southern Oscillation (ENSO) and other natural climatic modes in driving trends in warmth and aridity in the Southern Caribbean throughout the Holocene (sponsored by a P4 Climate award by the National Science Foundation). We regularly collaborate with colleagues at UCLA, Vanderbilt University, Princeton University, and the Berkeley Geochronology Center to engage in interdisciplinary paleoclimate research – from historical and archaeological analyses of Colonial Caribbean bathymetry to co-generating complementary records of speleothem geochemistry for a terrestrial perspective of climate variability within our study locations.

The Biogeochemical “Heartbeat” of Modern Reef Ecosystems

Coral reefs both set and respond to the chemistry of the seawater around them. To build their massive carbonate skeletons most efficiently, corals require a surplus (supersaturation) of calcium (Ca²⁺) and carbonate ions (CO₃^2-), even though the act of building the skeleton (calcification) removes CO₃^2- from ambient seawater and increases acidity through production of carbon dioxide (CO₂). The symbiotic relationship between coral hosts and their photosynthetic algal symbionts (zooxanthellae) also thrives under warm and stable temperatures, minimal fluctuation in salinity, and plentiful sunlight (minimal sedimentation/light scattering). The optimization of these conditions regulates the rate and magnitude of oxygenation of the reef by corals through photosynthesis as well as the acidification of the reef by corals through respiration. These interactions between corals and the ambient seawater means that reef-scale biogeochemistry (i.e. carbonate chemistry, trace elements, stable isotopes, nutrient concentrations, etc.) can vary on timescales from daily to interannual and spatially across the reef as well. Our group is interested in quantifying the biogeochemical “heartbeat” of reef ecosystems around the world across these varied spatiotemporal to better understand: (1) the baselines of coral reef metabolism (calcification and productivity) that will be modified as stressors (i.e. warming and acidification) compound, (2) the extent to which the use of corals as paleoclimate archives may be further complicated by shifting biogeochemical baselines in ambient seawater, and (3) the role of accessible water quality monitoring efforts in improving reef management/adaptation strategies in a wetter, warmer, and more acidic ocean. Specifically, we combine discrete and continuous measurements of seawater composition in coral reef environments, mathematical modeling, and high-resolution mass spectrometry to construct a sophisticated and quantitative toolkit for detecting ecological transitions within these dynamic marine habitats. We are currently working in collaboration with scientists and local communities in many locations throughout the Caribbean (Curaçao, USVI, Puerto Rico, and Jamaica) on these efforts and have worked on similar questions with Pacific Island communities in French Polynesia and Micronesia in the past.

Emerging Methods of Geochemical Proxy Development via Mass Spectrometry

The biogeochemical cycling of elements within and between aqueous solutions and carbonates is central to the use of carbonates as archives of environmental change and spatiotemporal variability in seawater chemistry to track changes in metabolic rates in the modern ocean. As such, in many cases, the ability to investigate more nuanced questions in coral paleoceanography and marine biogeochemistry depends on the ability to detect and accurately quantify elemental and isotopic variability in these matrices at high precision. We are actively involved in efforts to develop novel and push the quantitation and precision limits of existing methods of mass spectrometry analyses of marine carbonates and seawater. This currently includes:

(1) The use of isotope dilution techniques applied to multicollector inductively coupled plasma mass spectrometry (Thermo Neoma MC-ICP-MS at the GT Materials Characterization Facility) to measure trace element concentrations in seawater at high precision…

(2) The use of collision cell and pre-mass filter technology combined with MC-ICP-MS (Thermo Neoma MC-ICP-MS at the GT Materials Characterization Facility) to make novel, robust, and interference-free characterizations of non-traditional stable isotope fractionation in natural waters, coral skeletons, and speleothems…

(3) The use of laser ablation (LA) ICP-MS to resolve vital and environmentally-driven variability in trace elements and traditional stable isotopes in marine carbonates at micron-scale spatial resolution (Teledyne Iridia Laser Ablation System coupled with Thermo Neoma MC-ICP-MS *or* Thermo iCAP RQ ICP-MS at the GT Materials Characterization Facility)…

But we’re ready and eager to explore more!