For the last few decades, the concept of global change has been associated most commonly with anthropogenic climate change and its impacts on the atmosphere, hydrosphere, and biosphere. The burning of fossil fuels has caused atmospheric CO2 concentrations to rise from a pre-industrial level of ~280 parts per million (ppm) to 403 ppm today (CO2now.org). This rise in atmospheric pCO2 has been accompanied, since the late 19th century, by a 0.85 °C increase in global temperature, a 26% increase in the acidity of the surface ocean, a ~0.2 m increase in global sea level, and increased extinction rates for organisms living both on land and in the oceans (IPCC, 2014).
Since Earth’s formation, its surface environment has also sustained significant natural changes that beg fundamental questions about the world around us: What are the geologic processes responsible for driving observed climatic changes on Earth and how are they regulated? How do these climatic changes relate to other major changes in Earth’s surface environment, such as changes in tectonics or the chemical composition of the oceans? How does Earth’s biosphere respond to variations in environmental boundary conditions?
My research aims to address parts of the above mentioned questions. In particular, my thesis work focuses on the last ~200 million years of Earth history, during which climate has naturally transitioned between times of relative warmth (a ‘greenhouse climate’) and relative cold (an ‘icehouse climate’) (see Fig. 1). The mechanisms driving the shifts between ‘greenhouse’ and ‘icehouse’ are not completely understood. However, the fact that fluctuations between greenhouse and icehouse have occurred every ~100 million years or so since the beginning of the Phanerozoic (542 million years ago), points to the existence of geological forces that are able to both perturb and strongly stabilize the climate system. Importantly, in line with the questions above, the shifts between greenhouse and icehouse climate coincide with other notable environmental changes: (1) variations in global sea levels, (2) changes in the motion and configuration of Earth’s continents, and (3) variations in the chemical composition of the oceans.
Figure 1. Summary of changes in climate, sea level, ocean chemistry, and the configuration of the continents for the last 542 million years. Timing of changes in ocean chemistry coincide with changes in global climate and sea level. Data in the ocean chemistry panel come from Lowenstein et al. (2001; 2003; 2005), Horita et al. (2002), Timofeeff et al. (2006), Dickson et al. (2002; 2004), Coggon et al. (2010), Rausch et al. (2010), and Gothmann et al. (2015).
For my thesis work I have developed a new fossil coral archive, consisting of ~60 extremely well preserved samples, that can be used to reconstruct properties of seawater chemistry for the past ~200 My. I have also applied this fossil coral archive to reconstruct the chemistry of seawater over the Mesozoic and Cenozoic. To date, I have reconstructed records of past seawater Mg/Ca, Sr/Ca, U/Ca and δ26Mg. I have also measured the Ca isotope compositions of fossil corals and we are currently working to reconstruct a coral-based record of seawater δ7Li.
Through these studies, I have become very interested in diagenetic alteration. If unrecognized, diagenesis may lead to wrongful conclusions about past environmental change. Thus, it is essential to carefully characterize diagenetic alteration in samples used for paleoenvironmental work. I have employed a range of tools in my research to characterize diagenetic alteration in fossil corals including: petrographic microscopy, SEM imaging, X-Ray diffractometry, cathodoluminescence microscopy, micro-raman spectroscopy, carbonate clumped isotope paleothermometry, 87Sr/86Sr measurements, [4He] measurements and He/U age calculations, and measurements of Mn/Ca using Secondary Ion Mass Spectrometry. Fig. 2 below shows examples of petrographic microscopy and SEM images of well preserved and diagenetically altered scleractinian fossil corals.
Figure 2. (presented as Fig. 3 in Gothmann et al. 2015) (A) Petrographic thin section image (crossed-polars) of Upper Cretaceous coral. Crystals within the primary aragonite coral septum exhibit an acicular habit, typical of coralline aragonite. The space around the coral septum is infilled with calcite cement. (B) Petrographic thin section image (crossed-polars) of Middle Miocene coral. Acicular habit exhibited both within the primary coral skeleton, and by secondary aragonite outgrowths. Spaces are visible between needles of secondary aragonite, distinguishing them from primary material. Black regions show pore space. (C) Thin section image (transmitted light) of Upper Jurassic coral, showing both well preserved aragonite and dissolution and alteration of the COCs as evident by discoloration and texture. COCs are avoidable by using microanalysis techniques to sample. (D) Petrographic thin section image (crossed-polars) of Cretaceous coral, exhibiting significant recrystallization of the skeleton. (E) SEM image of recrystallized coral showing blocky euhedral textures. (F) SEM image poorly preserved fossil coral showing dissolution and ‘fibrous micropore’ textures.
In addition, I am interested in biomineralization and 'vital effects' because they also have implications for the interpretation of paleoenvironmental reconstructions from biominerals. ‘Vital effects’ can be broadly defined as deviations in the chemical composition of biologically precipitated minerals away from the composition expected for the inorganic mineral form (e.g,. aragonite precipitated by coral vs. aragonite cements precipitated from seawater).
Historically, there have been two main approaches taken to deal with the presence of vital effects in biologically precipitated minerals: (1) avoid using taxa for which vital effects are present as paleoenvironmental archives, or (2) calibrate offsets in vital effects for modern species, and assume that they stay constant through time. It is not clear that the assumption in point (2), however, is always valid. For example, results of measurements of Ca isotopes that we've conducted in fossil corals may indicate that Ca isotope discrimination in corals is sensitive to past changes in seawater Ca concentrations or changes in carbonate chemistry (see Fig. 3 below). It is clear that vital effects have the potential to significantly limit the development of robust paleorecords. However, ongoing studies of the biomineralization mechanisms by which organisms form mineralized skeletons or shells can help provide new insight into the origin of vital effects.
Figure 3. Apparent offsets between Ca isotope measured in fossil corals (red circles) and modern cultured corals (black circles) for varying seawater [Ca]. The magnitude of apparent Ca isotope discrimination is inversely related to seawater [Ca]. For fossil corals, seawater [Ca] is estimated based on reconstructions from fluid inclusions in halite (e.g., Lowenstein et al. 2003). Results plotted are from Gothmann et al. (in prep).
Coggon, R.M., Teagle, D.A.H., Smith-Duque, C.E., Alt, J.C., Cooper, M.J., 2010. Reconstructing past seawater Mg/Ca and Sr/Ca from mid-ocean ridge flank calcium carbonate veins. Science 327, 1114-1117.
Dickson, J.A.D., 2002. Fossil echonoderms as monitor of the Mg/Ca ratio of Phaneorzoic Oceans. Science 298, 1222-1224.
Dickson, J.A.D., 2004. Echinoderm skeletal preservation: calcite-aragonite seas and the Mg/Ca ratio of Phaneorozoic oceans. Journal of Sedimentary Research 74, 355-365.
Gothmann, A.M., Stolarski, J., Adkins, J.F., Dennis, K.J., Schrag, D.P., Schoene, B., Bender, M.L., 2015. Fossil corals as an archive of secular variations in seawater chemistry. Geochimica et Cosmochimica Acta 160, 188-208.
Horita, J., Zimmermann, H., Holland, H.D., 2002. Chemical evolution of seawter during the Phanerozoic: implications from the record of marine evaporites. Geochimica et Cosmochimica Acta 66, 3733-3756.
IPCC, 2014. Climate Change 2014: Synthesis Report, in: Pachauri, R.K., Meyer, L.A. (Eds.), Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. IPCC, Geneva, Switzerland, p. 151.
Lowenstein, T.K., Hardie, L.A., Brennan, S.T., Hardie, L.A., Demicco, R.V., 2001. Oscillations in Phanerozoic seawater chemistry: Evidence from fluid incluclusions. Science 294, 1086-1088.
Lowenstein, T.K., Hardie, L.A., Timofeeff, M.N., Demicco, R.V., 2003. Secular variation in seawater chemistry and the origin of calcium chloride basinal brines. Geology 31, 857-860.
Lowenstein, T.K., Timofeeff, M.N., Kovalevych, V.M., Horita, J., 2005. The major-ion composition of Permian seawater. Geochimica et Cosmochimica Acta 69, 1701-1719.
Rausch, S., Bohm, F., Bach, W., Klugel, A., Eisenhauer, A., 2013. Calcium carbonate veins in ocean crust record a threefold increase of seawater Mg/Ca in the past 30 million years. Earth and Planetary Science Letters 362, 215-224.
Timofeeff, M.N., Lowenstein, T.K., Martins da Silva, M.A., Harris, N.B., 2006. Secular variation in the major-ion chemistry of seawater: Evidence from fluid inclusions in Cretaceous halites. Geochimica et Cosmochimica Acta 70, 1977-1994.