General Research Statement

My research focuses on understanding climatic and oceanographic change over the past century, the impact of those changes on living corals, and the implications of global change on coral reef resilience. My research questions typically revolve around the issue of carbon: carbon as biogenic carbonate proxy records, carbon in biological systems, and biological and inorganic carbon cycling. I approach this by studying paleoceanographic proxy records in corals and sclerosponges, by studying modern biological and physiological processes in living corals, and by studying how the overlying biological processes influence the skeletal proxy records. In addition, the carbon work is complemented by major and trace metal analysis of coral skeletons and seawater, as these measurements can often shed additional light on the paleoceanographic, biogeochemical, and biological questions being asked. My research is interdisciplinary in nature and combines both in situ observations and field experiments, with laboratory analyses. My analytical tools include stable isotopes, radiocarbon, trace and major elements, and organic chemistry. Summaries of my three major areas of research are given below.

1. Carbon in Coral and Sclerosponge Skeletal Records

I have focused primarily on using the stable carbon (d13C) isotopes in corals and sclerosponges to understand the environmental and biological variables that drive skeletal d13C variability, and to reconstruct both local and regional scale oceanographic and environmental change over the past century. In corals, my studies have revealed that both changes in light intensity (which drives photosynthesis) and changes in plankton availability (which coral eat) significantly affect coral skeletal d13C. Studies of the shift in the proportionate contribution of photosynthesis and feeding with depth increasingly show that the d13C record of healthy shallow corals from non-upwelling regions is a reliable recorder of cloud cover.

Despite the large influence of metabolic effects on coral d13C on seasonal timescales, ongoing research shows that long term trends in the skeletal d13C and D14C of corals growing near the mouths of rivers strongly suggest that change in land use (i.e., from highly agricultural to forested) can be detected in the coral proxy records, and could provide a means for quantifying the history of carbon flux from land to the coastal ocean in the tropics.

In Palau and Saipan sclerosponges, ongoing research confirms that the sclerosponge skeletal d13C records the d13C of seawater dissolved inorganic carbon (d13C-DIC) on decadal timescales. In addition, sclerosponge d18O is deposited in isotopic equilibrium with seawater and is inversely correlated with the Southern Oscillation Index on interannual timescales indicating that the Western Pacific Warm Pool is cooler/saltier (warmer/fresher) during El Nino (La Nina), which is consistent with model and available instrumental data for the Western Pacific Warm Pool. The ultimate results of the ongoing research will make a significant contribution to the field by extending our records of seawater d13C-DIC and d18O by at least 70 years for this region. Thus, these sclerosponge proxy records will shed light on the temporal and spatial variability in the rate of anthropogenic CO2 flux in the tropical Pacific Ocean, and the influence of ENSO variability on that flux.

Complimentary work on the d13C and d15N proxy records in deep-water corals (i.e., gorgonians and antipatharians) from the same site in Palau will shedlight on ENSO-timescales changes in thermocline depth that are thought to be connected to the ENSO-timescales changes in oceanographic conditions detected by the sclerosponges.

Throughout this body of work, radiocarbon (D14C) has been used to date the sclerosponges and gorgonians using the bomb-carbon signature in their skeletons. In addition, research on a coral D14C record from Fanning Island and showed that a significant overall shift in the source waters contributing to the surface waters at Fanning in the mid-1940s that coincides with a phase switch in the Pacific Decadal Oscillation (PDO). This finding indicated that the extent and frequency of PDO can vary spatially in the Pacific, and that the character of PDO can vary over time.

Work in this area has been funded by the National Science Foundation Chemical Oceanograophy program, the Andrew Mellon Foundation, the Petroleum Research Foundation, the NOAA Global Programs, and others.

2. Coral Bleaching Biogeochemistry

From a more biological perspective, the effect of bleaching on the acquisition, allocation, and utilization of carbon in bleached and recovering corals, and the implications for coral reef resilience over the coming decades is of particular interest. To date, we have learned that bleached corals either utilize their fixed carbon energy reserves to sustain themselves, or can dramatically increase their heterotrophic feeding rates to maintain their reserves. These findings have been confirmed using several lines of investigation including organic chemistry and stable isotope biogeochemistry techniques. Closer examination revealed that: 1- the classes of lipids consumed during bleaching and recovery can vary dramatically between species, suggesting that metabolic pathways may vary among corals, 2- that corals all seem to feed on zooplankton of the same size class, 3- that heterotrophically acquired carbon is the main source of carbon for long-term energy stores but that photosynthetic carbon is utilized for daily metabolic needs (this item is unpublished), and that 4- heterotrophic plasticity may facilitate resilience in some species of corals when bleached while other species depend on stored energy reserves when bleached. This line of research has implications for defining resilience of corals and projecting coral diversity given that coral bleaching is expected to become an annual event in the coming decades. Newly funded work will examine the effect of repeated bleaching on coral physiology and biogeochemistry of Caribbean corals. We are also investigating the potential of a bleaching signature in the isotopic and trace metal skeletal records of bleached corals.

Work in this area is currently funded by two National Science Foundation grants in Biological Oceanography.

3. Trace Metals in Corals

The ratio of cadmium to calcium (Cd/Ca) chronicled in coral skeletal material is a promising proxy tracer of coastal upwelling. Our field studies revealed that skeletal Cd varies dramatically among species and depths, and that Pavonid corals are more sensitive to changes in Cd concentrations than Porites corals making Pavonids the preferred corals for paleo-upwelling reconstruction studies. Thus, published coral Cd records of upwelling to date are misleading and need to be reconsidered in light of this new work. The boron work revealed that d11B in coral skeletons reflects the pH of the surrounding seawater, uncomplicated by metabolic activities, and with a species-specific offset. Ongoing work on the barium to calcium (Ba/Ca) ratios archived in Puerto Rican corals growing near the mouths of rivers indicate that Ba/Ca is a strong recorder of seasonal river discharge events in that region and can be effectively used to constrain the coral chronology, as well as enhance the interpretation of river discharge events.

Additional ongoing work on Ba/Ca, Mg/Ca, and Cu/Ca in bleached corals is being investigated as a multi-proxy method for identifying bleaching events in coral skeletal records. Estimates to date suggest that coral bleaching is a recent phenomenon that only started to occur on large scales since the 1980s. However, without a proxy method for identifying bleaching events in coral records, this hypothesis has remained untested. We propose that a combination of stable isotopes and trace metal signatures could be used to identify bleaching events in skeletal coral records and to determine the appearance and rate of bleaching events prior to 1980 where there is incredibly a lack of reliable observational and instrumental data.

Work in this area is currently funded by the National Science Foundation Biological Oceanography program, the Andrew Mellon Foundation, and the Petroleum Research Foundation.