Dr. Grossman applies stable isotope geochemistry to understand environmental and paleoenvironmental change and its causes. Ongoing projects of current students include (1) the Neogene closing of the Central American Isthmus and its relation to climate change, circulation change, and the Caribbean extinction event (Kai Tao, Ph.D. candidate, and Aaron O'Dea, Smithsonian Tropical Research Institute); (2) circulation changes in the epicontinental seas of Carboniferous North America during formation of Pangea and its role in Late Paleozoic Ice Age (funded by NSF; Ryan Flake, M.S. candidate, and Debbie Thomas, Brent Miller, Tom Olszewski, Anne Raymond, Tom Yancey); (3) the causes of hypoxia off the Texas Coast through stable isotopic analyses of waters, mollusks, and fossil foraminifera (Josiah Strauss, Ph.D. candidate; funded by Texas' Hackerman Advanced Research Program).
Figure 1 (at left): Comparison of 13C/12C ratios (as δ13C) for (1) brachiopod shells from U.S. Craton and the Russian Platform, (2) micritic limestone from Arrow Canyon, Nevada (Saltzman, 2003), and (3) terrestrial organic matter (Peters-Kottig et al., 2006), and glacial events. Timing of Glacial events I, II, and III is from Isbell et al. (2003a) as modified by Montañez et al. (2007). The red curve represents the running average of all brachiopod shells (4 Ma window, 2 Ma steps) ±2 standard errors of the mean. Note the major increase in δ13C at the Mississippian boundary suggesting increased burial of organic carbon coincident with glaciation (from Grossman et al., 2008).
Examples of Isotopic Research:
By studying the soil we can determine where plants get their water from through the stable isotopes 18O and 2H.
Where Do Plants Get Their Water From?
Figure 2 (at left): Mean isotopic composition of water extracted from woody, green stems (young), woody stems with well-developed bark (old), leaves and soil from five potted individuals of P. velutina. Two-tailed paired t-tests were used to determine the statistical significance of mean differences between the isotopic composition of water from stems and soil samples. 2H and 18O values of plant organs that were significantly different from that of the soil are marked with an asterisk (*P < 0.01, **P < 0.001, ***P < 0.0001). Mean 18O values of water extracted from old stems were slightly more positive than that of soil water (P = 0.07).
By resolving nitrogen baselines from nitrogen fixation vs. NH4+ or NO3- uptake in forest ecosystems through the stable isotope 15N.
Nitrogen Fixation: refers to the natural process of nitrogen converting to ammonia (NH3) within the atmosphere. This process is vital to life as is a basic building block of life.
By establishing a base of a food chain through isotopes 13C and 34S and by creating the length of a food chain through isotope 15N.
Figure 3 (at the left): The ratio of 15N/14N presents a characteristic distinction between herbivores and carnivores, as the movement up along the food chain tends to concentrate the 15N isotope, by 3-4‰ with each step of the food chain (terrestrial plants have an isotopic ratio of 2-6‰ of N).
The tissues and hairs of vegans contain a significantly lower percentage of 15N than those who eat mostly meat.
By using Glacial Ice Cores, we can determine past climate changes from 13C, 2H, and 18O isotopes.
Figure 4 (at the left): Through stable isotopes found within ice cores like this one from Vostok, Antarctica, scientists can plot the carbon dioxide and methane levels in reference to a specific age in time.