I have been a leader in the development of Mg/Ca proxies for reconstructing seawater temperature. For decades, paleoceanographic reconstructions were confounded by the uncertainty of unequivocally interpreting down core isotopic records in terms of ice volume or temperature. The Rosenthal et al. (1997) paper was among the first to demonstrate the potential of using Mg/Ca in foraminifera for reconstructing ocean temperature. Now, almost 20 years later Mg/Ca measurements are a standard tool in paleoceanography as paired measurements of δ18O and Mg/Ca on the same foraminiferal shells offer a new way for assessing past variations in ocean temperature, salinity and Earth’s cryosphere. Over the past twenty years my lab has worked to improve the calibrations of Mg/Ca in benthic foraminifera, assess the importance of other environmental effects (e.g., dissolution and saturation effects) and expand the calibration to include new species (e.g., Katz et al., 2010, Lear et al., 2002, Rosenthal et al., 2006, 2011; Rosenthal and Linsley, 2007, 2013).
Our calibration studies are based on extensive core top benthic calibration program that I developed that included five cruises (Hawaii, 1998; Indonesia, 2003; Norwegian Sea, 2004; Cape Hatteras, 2004; New Zealand, 2005) in which surface sediments were collected along bathymetric transects covering different depths, bottom water temperatures and carbonate saturation states. These transects were used to refine the benthic foraminiferal Mg/Ca-temperature calibration and test the response of other trace element proxies (e.g., Cd/Ca, Sr/Ca, Li/Ca) to varying hydrographic conditions. The studies resulted in new proxies for reconstructing seawater temperature, including Mg/Ca and Sr/Ca, in the aragonoic foraminifer H. elegans (Rosenthal et al., 2006) and Mg/Ca in H. balthica (Rosenthal et al., 2011), which subsequently have been shown to be useful paleothermometers. However, we also demonstrated that at low saturation conditions, the carbonate ion content [∆CO3] exerts a significant effect on trace element (TE ) ratios in benthic foraminifera, which under certain conditions can compromise their accuracy as paleothermometers (Jordan MSc thesis, 2008). These studies highlight the need for a multi-proxy approach for improving the reliability of our reconstructions (e.g., Lear et al., 2006; 2010).
Our studies have also contributed to the understanding of the controls on Mg/Ca in planktonic foraminifera and its application for reconstructing sea surface temperature. Specifically, we have demonstrated the effects of foraminiferal shell dissolution on Mg/Ca and offered a way to account for its effect on temperature estimates (Rosenthal et al., 2000; Rosenthal and Lohmann, 2002; Sadekov et al., 2010). These papers highlight the potential of using size-normalized shell weight for correcting dissolution effects on planktonic foraminiferal Mg/Ca thermometry. We have also investigated the utility of other elemental proxies in planktonic foraminifera. Most recently, we have used data from culture experiments and sediment trap samples to improve our understanding of the environmental controls on planktonic foraminiferal B/Ca, and its potential for reconstructing seawater carbonate chemistry (Allen et al., 2012; Babila et al., 2014). Subsequently, combining Mg/Ca and B/Ca measurements in NJ cores we were able to evaluate the magnitude and relative timing of ocean carbonate chemistry and temperature changes at the initiation of the Paleocene-Eocene Thermal Maximum event (PETM) and showed, in contrast with previous studies, that the carbon released during the PETM and surface water acidification were apparently coincident with, rather than lagging after, the large warming at the PETM (Babila 2013 PhD thesis and in prep). This observation has significant implication for understanding the causes of this event.
We have also investigated the utility of TE proxies in other calcareous species including coccoliths (Stoll et al., 2001, 2002), gastropods (Rosenthal and Katz, 1989; Rosenthal et al., 1989; Sosdian et al., 2006, Gentry et al., 2006), belemnites (Bailey et al., 2003; van de Schootbrugge et al., 2005), ostracods (Elmore et al., 2012) and currently corals (Mass et al., 2012; Schaller et al., in prep). For example, we used O, C-isotopes and metal ratios (Mg/Ca and Sr/Ca) in belemnites to reconstruct the oceanographic conditions that occurred during the end of Triassic-early Jurassic interval, their possible causes (Bailey et al., 2003; van de Schootbrugge et al., 2005a) and relation to long term changes in marine phytoplankton communities (van de Schootbrugge et al., 2005b). The work sheds light on the response of marine photosynthetic phytoplankton to carbon cycle perturbations and changes in ocean redox state (Quigg et al., 2003) during this critical period in Earth’s history (it included the T/J mass extinction, major volcanism associated with the emplacement and degassing of the Central Atlantic Magmatic Province, and the Toarcian ocean anoxic event).
Interpretations of long-term oxygen isotope and Mg/Ca records of planktonic foraminifera in terms of ocean temperatures are somewhat ambiguous due to uncertainties about the history of oxygen isotopic and elemental composition of seawater, the effects of dissolution and diagenesis. As an independent test we also have explored the use of Sr/Ca-thermometry in tropical marine gastropods for reconstructing Paleogene tropical sea surface temperatures (SST). The thought was that mollusks thermometry might not only provide an independent assessment of changes in SSTs, but also add valuable information on seasonality, a key parameter of climate. Toward this, we examined factors controlling the seasonal and annual variations in Sr/Ca and C, O isotope ratios in the aragonitic gastropod, Conus sp., and established new paleotemperature calibrations (Sosdian et al., 2006; Gentry et al., 2006). When applied to fossil samples, however, the gastropod record suggests that the seawater Sr/Ca varied through the Cenozoic apparently reflecting the development of coral reefs in the late Cenozoic (Sosdian et al., 2012).
Using field calibrations and lab experiments to understand the mechanisms of trace elements (TE) uptake into biogenic carbonates has enable us, and others in the field, to reconstruct climate variability with very high precision. My lab has been a leader in developing analytical methods for the analysis of biogenic carbonates, which are now used by laboratories throughout the world (e.g., Rosenthal et al., 1999; Rosenthal et al., 2004; Andreasen et al., 2006). I conceived, organized and carried out an inter-laboratory comparison study of Mg/Ca and Sr/Ca measurements in planktonic foraminifera. I believe that these developments greatly advanced the field and helped in setting the standards for the entire community.