Title: Paleoceanographic-Proxy Development in Scleractinia (Stony Corals) Throughout the Pacific Ocean
Description: Scleractinia (stony corals) are powerful tools in the field of paleoceanography, allowing researchers to reconstruct past ocean conditions based on variations in coral geochemistry. As corals regularly accrete their aragonite skeletons they preserve a history of climate on regional to global scales. Often able to provide centuries long continuous records of climate, an individual coral colony can provide insight into significant environmental perturbations. If preservation permits, fossil corals can be used to evaluate climate thousands of years in the past. Researchers use paleoclimate proxies, which are indirect geochemical fingerprints of environmental conditions, to create paleoclimate time series. Paleoclimate proxies are prevalent throught the literature and while many are well constrained by years to decades of use, individual conditions unique to study sites and timescale prevent the use of blanket assumptions regarding their interpretation. In this dissertation I illustrate the varied ways that the same or similar coral-based climate proxies can be used to reconstruct past ocean conditions.

Part I (Ch. 2, 3) presents two studies based along of the Pacific Coast of Panamá to examine the influence of the seasonal migration of the Intertropical Convergence Zone (ITCZ). I analyzed a long-term coral δ18O time series from a living massive Porites colony to address low-frequency variation overprinted by the wet-dry seasonality. The coral record uncovered a clear decadal (~11 year) cycle in coral δ18O-inferred precipitation. I propose this mode is related to basin-wide processes, specifically a component of the Pacific Decadal Oscillation, which describes large-scale patterns in sea surface temperature (SST) and precipitation influencing marine ecosystems. In Chapter 2, I supplement the coral δ18O record with a coral Ba/Ca time series from a different coral colony. Coral Ba/Ca can be used as a proxy for river discharge (Q), although this practice is relatively new. Our coral record outlined seasonal variation in river Q and can also be used to identify past El Niño events and prolonged periods of drought.

Uncovering a geochemical indicator of El Niño in this region is particularly powerful since conditions become warm and dry, which negate each other in coral δ18O rendering the proxy unable to consistently identify these climate events. This chapter furthers the community’s understanding of the many ways that trace metals can be used in paleoceanographic research, specifically to constrain local hydroclimate. In Part II (Ch. 4, 5) I present two studies in the Great Barrier Reef (GBR) based on coral δ18O and Sr/Ca records from modern and fossil Isopora, a coral species that is nearly completely absent from the paleoceanographic literature. Although this suite of climate proxies is similar to those used in Part I, in Part II the GBR corals provide a history of sea surface temperature rather than hydroclimate, which is due to prevailing local environmental conditions over a given timescale. In Chapter 4 I developed the first modern Sr/Ca- and δ18O-Sea Surface Temprature (SST) calibration using Isopora, which approaches those calculated for the commonly used Porites corals.

Using Isopora in Pacific-based paleoceanographic research allows us to analyze coral records from reefs that might not be dominated by Porites. In Chapter 5 I applied the new Isopora Sr/Ca- and δ18O-SST calibrations to fossil corals recovered during Integrated Ocean Discovery Program (IODP) 25. The fossil corals date beyond the Last Glacial Maximum (LGM, ~20 kyr BP) to 25 kyr BP. In the Pacific SST change since the LGM is better constrained for more equatorial locations so our fossil samples from the GBR extend the geographic network of LGM-aged coral-based climate proxies. I measured ~5-7°C of cooling in the GBR at the LGM compared to today. The SST change through the LGM deglaciation provides valuable understanding of reef resilience and future risk of or adaptability to climate change. Each chapter in this dissertation uses similar strategies but provides a unique perspective on past climate change in the tropical Pacific. This dissertation identifies the many ways that coral proxies can be utilized with specific examples of the ways in which interpretation can vary.

It is necessary to consider the environmental specifics of a given region before blindly interpreting paleo-proxy data. Furthermore, coral-based proxy records are supremely powerful tools in exploring and uncovering past climate histories of a given region. Coral-records can supplement and extend the limited instrumental record with centuries to millenia long information on SST and hydroclimate. These data can improve climate models, further our knowledge of coral reef growth, and deepen our understanding of regional hydroclimate, which are all vital to our understanding of global climate.