Award: OCE-1041133

The deep ocean (below ~100 m water depth) stores approximately forty times more carbon than the atmosphere. Thus any change in deep ocean carbon chemistry could have profound implications for global climate, and understanding the deep ocean carbon cycle is a top priority for forecasting future climate change. However, direct measurements of deep ocean carbon chemistry only began in the 1970s, and available data are generally sparse in space and time. Additional indirect records of deep ocean carbon chemistry are necessary to better understand the dynamics of this system, particularly over the last two centuries of significant human influence on climate and the carbon cycle. This project sought to address this need by developing new records of deep ocean carbon chemistry using the boron and radiocarbon isotopic composition of deep-sea bamboo corals, and by improving understanding of the boron isotope proxy for ocean pH. Bamboo corals are globally distributed throughout the deep ocean, with their calcite skeletons a potential archive of ocean chemistry. Boron isotopes—a tool to reconstruct ocean pH—were measured in live-collected bamboo corals from the North Atlantic and North Pacific Oceans to test whether their skeletal composition tracks deep ocean pH. Results show that while coral boron isotopes do correlate with deep ocean pH, boron isotope variations within the coral skeleton are too large to be explained by changes in deep ocean pH over the coral lifespan. These variations most likely reflect the biology of the animal. In contrast, the radiocarbon composition of bamboo coral skeletons tightly correlates to the radiocarbon content of the deep ocean and gives coral lifespans of 50 to 300 years. This supports the use of radiocarbon for generating bamboo coral ages and growth rates, and for tracking the radiocarbon content of the deep ocean. Finally, the boron isotope proxy for ocean pH was advanced by addressing differences in isotope values between two measurement techniques. By measuring a set of samples via both techniques, it was shown that both techniques return fundamentally similar results, and the difference between techniques can be addressed through a constant correction factor. In summary, even though boron isotopes in bamboo corals cannot be employed as indirect records of deep ocean carbon chemistry, these results provide constraints on the biology of bamboo corals that allows them to grow in the otherwise inhospitable deep ocean. A better understanding of these adaptations is critical to projecting how bamboo corals and other marine calcifiers respond to present and future ocean acidification. The results of this project have fostered detailed investigation of these biological processes using advanced microscale analytical techniques. Last Modified: 09/12/2016 Submitted by: Baerbel Hoenisch