One of the most significant threats for marine organisms is acidification as a result of unabated anthropogenic CO2 emission. On a global scale, acidification has the potential to impact biota that make their shells out of the minerals aragonite and calcite, that have a hard time forming shells in low pH waters. One such group, the calcareous nannoplankton (haptophyte algae including coccolithophores that build shells of the mineral calcite), are a vital part of the open-ocean food chain. Laboratory experiments suggest that coccolithophores can adapt to more acidic conditions, but whether the group can adapt in the natural environment is uncertain. The geological record contains a series of natural experiments that allow us to address the biological response of nannoplankton to greenhouse gas perturbations. The Paleocene-Eocene thermal maximum (PETM), a ~200,000 year long transient warming event 56 million years ago, is likely the best ancient model for the impact of massive greenhouse gas on marine ecosystems. Studies of nannoplankton from deep-sea sites show new taxa during the PETM including an increase in malformed morphotypes that could signify adaptation to lower calcite saturation. Moreover, geochemical evidence suggests a similar magnitude pH decline during the initial stages of the event as projected to occur in the 21st century. The insight gained from this study will complement modern experimental and ecological studies, allowing mapping of the impact of progressive carbonate undersaturation on the ancestors of modern nannoplankton through space and time. In addition to better understanding the long-term consequences of ocean acidification, broader impacts include training of students at the graduate and undergraduate level, support of a postdoctoral researcher, and outreach to K-12 classrooms.
Much of the information on the PETM derives from the study of cores drilled in the deep sea. However, as the deep ocean also acidified during the event, the key early stage of the PETM and likely when the maximum surface ocean acidification took place, calcium carbonate has been dissolved in most deep-sea locations. Thus to study the full impact of ocean acidification on nannoplankton, this research focuses on nannoplankton contained within sedimentary sections deposited at continental shelf water depths. A superb opportunity to explore the impact of ocean acidification on PETM plankton exists in cores from the shelf on the Atlantic Coastal Plain where some of the shallowest and most expanded records of the event are found. These core sections offer high temporal resolution and preserve a high-fidelity record of the early part of the PETM. This project will use new and existing cores from Maryland and New Jersey to study the development and impact of acidification on the coastal ocean over millennial time scales. The goal is to address the following hypotheses: (1) Surface ocean carbonate saturation reached a minimum within the first twenty thousand years of the PETM then slowly recovered; (2) The response of nannoplankton to surface acidification was systematic with existing species tolerant of low saturation increasing in abundance followed by the evolution of new species and morphotypes; and (3) Eutrophication in the coastal ocean amplified the impact of acidification locally and played a critical role in drawing down CO2 during the early stages of the PETM. These hypotheses will be addressed using closely integrated: (a) nannoplankton assemblage studies to determine how species composition and morphology changed; (b) a suite of inorganic and organic proxies to elucidate the nature of environmental changes and particularly to unravel acidification from temperature signals; and (c) models to simulate the temporal variability of pH, saturation state and eutrophication on the shelf.
