Little Ice Age climate in the Western Tropical Atlantic inferred from coral geochemical proxies

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2016Paleoclimate archives place the short instrumental record of climate variability in a longer temporal context and allow better understanding of the rate, nature and extent by which anthropogenic warming will impact natural and human systems. The ocean is a key component of the climate system and records of past ocean variability are thus essential for characterizing natural variability and quantifying climate sensitivity to radiative forcing. Coral skeletons are high-resolution archives of tropical sea surface temperatures (SSTs), but inconsistencies call the accuracy of existing coral proxy records into question. In this thesis, I first quantify the errors associated with the traditional coral thermometer, Sr/Ca, by comparing in situ logged SST with Sr/Ca-derived SST in four corals on the same reef. I show that intercolony disparities in mean Sr/Ca, amplitude of variability, and trend are not due to differences in water temperature, but rather to “vital effects” that result in a ± 2 ̊C uncertainty on reconstructed SST. I then expand, refine, and test a new paleothermometer, Sr-U, across multiple coral species and through time. I show that Sr-U captures spatial SST variability with an uncertainty of ± 0.6 ̊C. When applied to two corals outside of the calibration, Sr-U accurately captures the mean SST and the 20th century trend in the Western Tropical Atlantic. Finally, I apply Sr-U to a coral from the Little Ice Age (LIA) to address uncertainties in the magnitude of western tropical Atlantic cooling during a 95-year period spanning 1465-1560. Results suggest the region was 1.1 ̊C±0.6°C cooler than the 1958-1988 mean, but within error of early 20th century SST at this site. Critically, several periods of warmth, equivalent to the 1958-1988 mean, occurred during a solar minimum that is widely believed to have been a cool period of the LIA. My results indicate that Sr/Ca exaggerates the actual cooling by almost 3 °C. My record demonstrates the value of Sr-U and highlights the need for continuous accurate SST records to better constrain the amplitude, drivers, and mechanisms of LIA tropical climate change.would like to acknowledge the National Science Foundation’s Graduate Research Fellowship Program for funding three years of my education. NSF grants OCE-0926986 to Anne Cohen and Delia Oppo, OCE-1031971to Anne Cohen, OCE-1338320 to Glenn Gaetani and Anne Cohen supported portions of my research, fieldwork, and stipend. WHOI’s Ocean Exploration Institute funded the last year of my stipend. WHOI’s Access to the Sea Program and the Bermuda Biological Station for Research Fund funded fieldwork. WHOI’s Ocean Ventures Fund provided me the experience of writing a scientific proposal and funds for sample analysis. am grateful to research grants as well as travel grants from WHOI’s Academic Programs Office and the MIT Graduate Student Council for funding my conference travel expenses