The mass extinction event at the Cretaceous-Paleogene (K-Pg) boundary was one of the largest such events during the Phanerozoic and was following by a period of drastic changes in ecological and biogeochemical conditions. This research is the first to investigate changes in the nitrogen cycle across the extinction event. The PI applies a previously established a relationship between nitrogen isotopes and the oxidation state of the environment based on the predominant nitrogen reactions and their associated isotopic fractionation. They will measure the bulk, kerogen, and porphyrin d15N values across the K-Pg boundary for several cores to determine nitrogen cycling and paleoredox reactions before, during, and after the mass extinction event. The PI will test the hypothesis that the oxygen concentration in the deep waters decreased following the impact event, and that cores located in different ocean basins will record different d15N profiles as dictated by the deep water redox state at each location. The results will provide important information about the processes of global mass extinction and the subsequent recovery of biogeochemical systems. Funding supports a new female PI and a graduate student. Further broader impacts include collaboration with the COSEE-NOW (Center for Ocean Sciences Education Excellence Networked Ocean World) to develop educational products aimed at improving ocean literacy. Funding support is provided by the Marine Geology and Geophysics Program of NSF's Ocean Science Division and by the Experimental Program to Stimulate Competitive Research (EPSCoR).
The goal of the project was to analyze the nitrogen isotope (δ15N) record and other geochemical parameters in sediment samples from across the Cretaceous-Paleogene (K-Pg) mass extinction in order to evaluate changes in the paleoenvironmental conditions through the mass extinction and subsequent recovery. We have previously linked δ15N values in sediment cores to the oxygen content of the water column during sediment deposition using a conceptual model based on nitrogen cycle reactions and systematics. Analysis of the water column oxygen concentration will allow for characterization of paleoenvironmental conditions and patterns through the K-Pg mass extinction and recovery. Five deep-sea sediment cores (Blake Nose, Demerara Rise, Bass River, Shatsky Rise, and Maud Rise) were selected from different ocean basins and at varying distances from the Chixulub impact crater, the location of the bolide impact that initiated the mass extinction at the K-Pg boundary. Nitrogen isotope profiles from the Blake Nose, Demerara Rise, and Bass River cores show a slight enrichment after the K-Pg boundary, which indicates a relative increase in the amount of water column denitrification through this period. Interpretation of this data with respect to our model of δ15N change with deep water oxygen level suggests that post-K-Pg water column had decreased oxygen levels. This interpretation is supported by total organic carbon (TOC) and trace metal concentrations. The post-K-Pg positive nitrogen isotope excursion is coincident with a negative carbon isotope excursion; both excursions revert to previous baseline at the same time. This indicates that the alteration of the nitrogen cycle (as driven by the increasing suboxia) occurred at the same time as the disruption of the carbon cycle, which is considered a global event. The specific timing and intensity of these post-K-Pg excursions for carbon and nitrogen isotopes appears to differ slightly between sites. These differences may be due to relative location of the site to the impact location, variations in circulation, paleodepth differences or other local water column effects, as well as dating uncertainties in the age models. Given the location of these three sites, this would imply that the global ocean was also likely more suboxic or contained more extensive suboxic zones at this time. Analysis of the Maud Rise and Shatsky Rise samples is still ongoing. To date, data from this project have been incorporated into one submitted paper, two presentations at American Geophysical Union (AGU) Fall Meetings in 2011 and 2012, and eight invited seminars; additional manuscripts are in preparation. Investigations to address the broader topic of correlation between the nitrogen cycle and water column oxygen concentrations, particularly the use of δ15N values as paleoredox proxies, were also performed; data from this research has been published in one paper and included in three presentations, seven invited seminars, and an additional submitted manuscript. Five Master’s students and three undergraduate students were trained on a range of geochemical techniques, particularly isotope ratio mass spectrometry. Students were in charge of analyzing the majority of the samples and collecting the majority of the data to date. In addition to training on instrumentation, the students were educated in the systematics of nitrogen and carbon cycling, the relationship between water column redox state and nitrogen isotopes, and the dynamics of mass extinctions, particularly the K-Pg event. Students were also guided in data analysis protocols, the creation of appropriate figures, and written and oral communication skills. As part of this grant, we worked with educational consultant Dr. Edward Cohen to develop a lesson plan for secondary school students, titled "How Do Scientists Conduct Research on Ancient Environments?" This activity introduced students to the IODP deep sea drilling program and the scientific method, detailing how deep sea cores are obtained, and how scientists use them to answer research questions. The activity can be found in the Deep Earth Academy Educator Resources section of the International Ocean Discovery Program Joides Resolution webpage at http://joidesresolution.org/node/3495. This collaboration also generated two AGU Fall Meeting presentations in 2012, describing the partnership and detailing the development and implementation of the lesson plan. Dr. Quan was also interviewed in 2012 by the Centers for Ocean Sciences Education Excellence-Networked Ocean World (COSEE-NOW), and discussed the role of broader impacts in NSF proposals, the planned lesson activities, and the collaboration with Dr. Cohen. This interview can be found at http://coseenow.net/scientist-resources/broader-impact/case-studies/getting-to-the-core-of-ocean-science/.
