Award: OCE-1602810

To address these questions, the research team undertook a study of the climate-sensitivity of N2 fixation in the southeast Pacific gyre over the last glacial cycle as well as its plausible "master controls". This oligotrophic region experiences little modern N2 fixation despite proximity to a large supply of excess phosphate from the adjacent Peru-Chile oxygen minimum zone. This is consistent with modern iron limitation due to low aeolian supply that would have been relieved during past dusty conditions. The research team had intended to use the natural experiment of the last full glacial cycle, captured in the foraminiferal-bound N isotopes of gyre sites as well as sites at its southern margin, to probe controls on the marine N cycle exerted by variable dust inputs and changes in N-loss in the adjacent oxygen minimum zone, and relate these to known changes in greenhouse forcing of climate. Specific objectives of the project were: 1) Identification of suitable sediment cores for achieving the goals of this project and obtain core material for foram d15N analysis as well as the other supportive measurements. This goal was fully realized 2) Create a detailed methodological protocol with quality control metrics for foram d15N analysis. Specifically, the method needed to be optimized to minimize the amount of foram material necessary for isotope analysis and ensure compatibility between all the preparative steps prior to isotope analysis. This is critical, as foram picking time (to achieve single species samples) is the chief limitation on the size of the data set we can achieve. This goal was only partially realized. 3). Create a dust flux record using Th normalization techniques for the last 30 ky for the study region. This goal was realized. 4) To obtain foram d15N records spanning the last glacial/interglacial period at sites dispersed throughout the study region. This goal was not realized due a combination of difficulties which included loss of research time due to the COVID pandemic. Significant outcomes of the project were; 1) Through radiocarbon dating, many archived cores for the SE Pacific were found to have core tops older than the last glacial maximum (LGM) and were not suitable for this project. However, a sufficient number of cores were identified with Holocene (<10,000 years old) core tops and sufficient sediment accumulation rates (>1 cm/kyr) for creating paleo-records for the last 30,000 yr. 2) We found that dust flux in our target core was higher during the LGM, partially supporting the hypothesis that increased delivery of Fe at this time stimulated N2 fixation in oligotrophic waters of the SE Pacific. Last Modified: 01/19/2023 Submitted by: Mark A Altabet