Project: OCE-PRF: A submicron scale investigation of foraminifera-bound organic matter: implications for preservation and the paleo-δ15N proxy

NSF abstract:
This project dives into the microscopic world of planktic foraminifera, tiny shelled zooplankton that inhabit oceans across the globe. As foraminifera build their shells, they incorporate tiny amounts or organic matter and, with it, information about their surroundings and ecology. When their shells sink and gather on the seafloor, they add to an ever-growing archive of Earth history. Exactly how well the organic matter and its original signals are preserved in shells during sinking and early burial is a key uncertainty for its use in reconstructing past climate. To address this, the PI will use ultra-high resolution crystal and chemical mapping to compare shells before and after this crucial transition to the fossil record. Understanding the feedbacks that have regulated Earth’s climate in the past is an essential part of our future response and preparedness as a nation to face the challenges of a warming planet, and is thus well aligned with NSF directives. Furthermore, by characterizing the biomineral structures of modern shells, this work will assist with monitoring ocean acidification, which can compromise fisheries, tourism and the natural buffering of coastal communities from storms (e.g., by coral reefs). As a female scientist and immigrant from a developing country, the PI is dedicated to encouraging other young women and underrepresented groups to pursue a career in science. By partnering with a local Alabama middle school that serves underrepresented students, she hopes to inspire those who might not otherwise consider science as a career path, to take STEM subjects in high school. In this way, students will be better equipped to take on the challenge of climate change, and be made aware of the need for their talents in building a more sustainable future. The nitrogen (N) isotope composition (δ15N) of organic matter within the shell walls of planktic foraminifera is emerging as a promising new tool for tracking the amount and partitioning of nitrogen, an essential nutrient for life, in the ocean. The number of foraminifera-bound δ15N records has grown rapidly in recent years, yielding tantalizing insights into the interplay between ocean fertility, oxygen concentrations and atmospheric greenhouse gas levels. Yet, fundamental questions remain about the mechanism of N incorporation into calcite, taxonomic differences in N uptake, and the preservation potential of the organic matter to which N is bound, particularly in the earliest stages of sinking and burial. The utility of bulk geochemical analyses (which require combining hundreds of shells) in answering these questions is limited by inherent issues of scale. Hence, the PI proposes to use electron backscatter diffraction (EBSD) and nanoscale secondary ion mass spectrometry (NanoSIMS) on individual shells from living (tow-caught) and dead (seafloor sediment) assemblages to evaluate changes in the biomineral structure, organic matter distribution and relative N content of foraminifera shells from life in the surface ocean to burial on the seafloor. The PI expects her investigation will also provide insight into the role of organic matter in the formation (i.e., biomineralization) and behavior (e.g., susceptibility to breakage/dissolution, mineral-fluid exchange) of biogenic calcite, a geologically important material of interest to a broad range of scientists. Thus while her specific motivation for this work is the N isotope paleo-proxy, the implications extend beyond N to other systems of organic matter and associated tracers (e.g., δ34S, I/Ca, Na/Ca), and to fundamental research areas like habitability and the co-evolution of life and planet. Already, the paleo-δ15N proxy is yielding high-impact results concerning the often-complex feedbacks between the marine biosphere and climate. Thus, by seeking to inform these interpretations, the proposed work has the potential to help predict the unintended consequences of a rapidly warming and acidifying ocean. 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.