Collaborative Research: A revised Plio-Pleistocene view of the effect of climate on North Pacific oxygenation from foraminifera-bound nitrogen isotopes

University Of Massachusetts Boston, Dorchester MA

Investigators

Abstract

Like most life on land, life in the ocean such as zooplankton, fish, and marine mammals require oxygen to break down their food and generate energy. However, in three different regions of the ocean, there are depths (from roughly 200 to 500 m) where the oxygen has been exhausted due to bacteria. These are known as the oxygen-deficient zones (ODZs), with the largest of these occurring in the eastern tropical North Pacific. The ODZs restrict the habitats and movements of ocean life, including fisheries on which humans rely. They also affect the availability and cycling of important nutrients in the ocean. The impact on these nutrients can affect the ocean’s biological productivity and the prevalence of different types of phytoplankton. Finally, the ODZs may affect the ocean’s exchange of greenhouse gases (both carbon dioxide and nitrous oxide) with the atmosphere. These gases can alter Earth’s climate. Studies have suggested that the ODZs have expanded over the last century, possibly due to ongoing global warming. However, climate model predictions of the ODZ extent in the future yield an uncertain picture. Reconstructing ODZ changes in the past will clarify the main controls on ODZ extent, improving the ability to predict future changes. In this project, the chemical composition of nitrogen trapped in the wall of fossil plankton will be measured in a deep-sea sediment core to reconstruct past changes in the ODZ of the eastern tropical North Pacific. The data will be compared with climate and ocean properties in the past, indicating which of these properties most strongly controls the ODZs. At Princeton, the team pursuing this research will lead a workshop on climate and the ocean for middle and high school STEM teachers. This will be one of multiple opportunities to communicate the results to the public. The research team at University of Massachusetts Boston will support paid research opportunities for undergraduate students. University of Massachusetts Boston is a minority-serving institution with a high proportion of both first-generation and Pell grant recipient students. These students will learn and apply cutting-edge geochemical analysis techniques in close collaboration with the research team at Princeton University. The ocean’s ODZs and their changes over time deserve our attention in multiple capacities: as monitors of ocean and atmospheric circulation; as hotspots of biogeochemical fluxes; as major ocean sub-environments and constraints on ocean species; and as reflections of the ocean’s biological carbon pump. Sedimentary nitrogen (N) isotopes have been used to reconstruct past changes in the ODZs through their sensitivity to water column denitrification, which increases the 15N/14N (δ15N) of the nitrate supplied to surface waters. The δ15N of bulk sedimentary N (δ15Nbulk) has been applied extensively in this effort, leading to the general conclusion that the Pleistocene ice ages have been associated with ODZ contraction. However, δ15Nbulk may be overprinted by variations in seafloor diagenesis and exogenous N inputs. To avoid these potential biases, it is now possible to measure the δ15N of the small quantity of organic matter bound within and protected by planktonic foraminifera shells (foraminifera-bound δ15N, or FB-δ15N). Recent FB-δ15N data from the eastern equatorial Pacific and new pilot data from the eastern tropical North Pacific (ETNP) argue against previous δ15Nbulk-based inferences of a strong ice age/interglacial contrast in ODZ extent. To test prior inferences and provide new insight into past variations in ETNP denitrification and ODZ extent, a FB-δ15N record will be generated at Ocean Drilling Program Site 1241 in the ETNP. Four time-windows of 300-500 kyr will be analyzed over the last 3.3 Ma, each at suborbital (~3 kyr) resolution. The reconstruction of the history of the ETNP ODZ will add a key biogeochemical component to the paleoclimate data sets from this key region, yielding insights into (1) the controls on tropical Pacific upper ocean circulation, which include the wind and temperature fields, (2) the biogeochemical responses to climate-driven changes in these physical parameters, and (3) the consequences for the biological conditions of the ocean and the distribution of biologically-stored excess CO2 in the ocean interior. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.