Award: OCE-1060947

In order for us to understand how climate and the environment may change into the future, observing changes from year to year will simply not provide enough information. Year-to-year variations are typically minor and affected by random chance. If we seek to understand how climate and the environment behave on a meaningful time scale, we must observe them over at least many decades, probably centuries, as these are the shortest times over which conditions change significantly. The problem is that we only recently starting measuring the global environment, and we will not accumulate enough data if we just collect it one year after the next. Instead, scientists have to find ways to reconstruct changes before the time that measurements were being made. This usually involves a natural property that can be measured in material from a natural record such as a sediment or glacial ice core. We often refer to this measurable natural property as an environmental or climate "proxy," signifying that it provides a replacement (albeit an imperfect one) for direct instrumental measurements of the environment. The purpose of this project was to provide background information on a new "proxy" that the principle investigatorÆs research group recently developed for reconstructing nutrient conditions in the ocean. Nutrients are chemicals required for the growth of plankton, and their availability often controls the biological productivity of the ocean. This biological productivity can affect natural resources such as fisheries and can also affect climate by changing the concentration of the greenhouse gases (most importantly, carbon dioxide) in the atmosphere. The proxy being studied is the ratio of stable nitrogen isotopes of the organic matter trapped in the fossil shells of single-celled protist zooplankton called "foraminifera." It took decades to develop the ability to make this measurement, in part because there is so little organic matter in the fossil shells. With the ability to make the measurement in hand, the NSF-funded project produced data to test the proxyÆs ability to reconstruct nutrient conditions by studying the proxy in the modern ocean and in the fossils of recently deposited sediments that should reflect modern conditions. Most of the study was conducted in the North Atlantic near Bermuda, taking advantage of the Bermuda Atlantic Time-series Study (BATS), a site in the open ocean visited by a ship every month to monitor conditions. Living foraminifera were collected with a net towed behind the ship, and the resulting samples were preserved and sent back to the lab for isotopic analysis. Many other types of dissolved and solid samples were collected at the same time and also analyzed. Analysis of these samples along with the information routinely collected by BATS provided information on the nutrient conditions that the foraminifera are believed to record. Foraminifera were also recovered from deep "sediment traps" that collect particles sinking out of the surface ocean as well as from the seafloor sediments where the fossils naturally accumulate. Comparison of the foraminifera measurements with the environmental data provided a test of the foraminifera nitrogen isotope proxy. The results indicate that the foraminifera shells reflect the nitrogen of the organismÆs biomass, a critical requirement if this tool is to be reliable. The isotopic differences among foraminifera species were shown to be relatively constant through time and similar in the surface ocean, sinking material, and the sediments. The nitrogen isotope ratio of individual foraminifera species showed signs of tracking the very subtle seasonal variation in the phytoplankton nitrogen isotopes at BATS, providing unexpected evidence that the proxy can be very precise. Moreover, there was no significant change in the foraminifera shell nitrogen isotopes from the surface ocean to the sediments, providing a partial check on whether this proxy will be well preserved ...