Award: OCE-1131816

The bioavailability of nutrients plays a critical role in ocean biological productivity, the ocean's carbon cycle, and climate change. Nitrogen (N) is perhaps most important, controlling most primary production, and representing the foundation for all ocean life, including ocean fisheries. Critical bio-available N is controlled by a complex balance of sources and removal mechanisms, mostly mediated by microorganisms, which varying widely across ocean regions. One of the greatest challenges in our current era is to understand how changing climate may change these complex cycles, since any changes will ripple through all levels of the ocean's food webs. Since we cannot see into the future, the only way to really tackle this problem is to look into the past: to reconstruct how N cycles in past ocean's have varied with major changes in the earth's climate. This task, however, is extremely difficult. To go very far back in time, we must rely on sedimentary records. The stable isotope ratios organic nitrogen (d15N) preserved in ocean sediment cores represents one of the most powerful geochemical approaches to reconstructing past N cycles, because all processes involved d15N values in unique and traceable ways. Most paleoceanographic studies have used the d15N of total (bulk) sediment for such studies. However, we now know that bulk d15N also has very significant drawbacks: these values can be impacted by factors, which are often almost impossible to untangle, making interpretations about past N cycle changes difficult, or somtimes impossible. The overarching goal of this project has been to develop a unique new tracer solve such problems, using measurement of d15N values of individual amino acids (AA), extracted from ancient proteins preserved in sediment cores. This technique is called compound-specific analysis of amino acids (CSIAA), and we hypothesized it could become a revolutionary tool to reconstruct paleo-ocean N and C cycles at unprecedented precision. Isotope signatures locked in these molecules are directly tied to the evolution and biochemistry of the organisms which produced them, and with new technology these patterns can potentially be read in sediments like a book, unlocking detailed specifics of past N cycling, such as the original N source used by ancient algae, and the trophic structure of past plankton food webs. These approaches have been developed in ocean ecology, but no one has before attempted to apply them to sediments. In this proposal, we hypothesized that if this was possible, it would represent a fundamentally new area for both paleoceanography, and for wider study of the N cycle in the oceans. However, applying CSI-AA to sediments presents multiple new challenges. This is because sediments are a very complex mixtures, often extensively degraded by bacteria. To explore how CSIAA data can used in sediments, we measured patterns in multiple contrasted sediment cores, representing very different ocean regions and conditions. We compared results from areas with excellent organic matter preservation regions wehre sediments were extensively degraded. In addition, we conducted complimentary laboratory experiments, measuring CSIAA patterns in controlled degradations, designed to mimic natural conditions. Finally, we also expanded our project, testing if CSIAA approach can also be used in ancient shells from archaeological deposits, bringing our new approach out of the deep sea, and possibly allowing us to examine changes in past coastal conditions. The bulk of our findings are quite technical, related to details of 15N isotope patterns in extracted amino acids and proteins. However, the most important take-home result is simple: it worked. Specifically: 1. We found CSIAA patterns were well preserved, or could be resconstructed, with all potential new paleo-Information "readable" in almost all sediments we studied. 2. Where degradation was most extreme, we were able to develop new CSIAA-based proxies to quantify bacterial changes, and essentially predict in what sediments our new approach can work. 3. In surprising results, our data also suggested that much of the organic N in sediments may not be in the chemical form geochemists have long assumed, indicating exciting new research directions which may alter our understanding of the N cycle. 4. Our experiments in archaeological shell samples also worked, and we demonstrated a new tool for recreating coastal ocean production and biological samples, and potentially tying these results to archaeological studies of native cultures. We believe that our results are highly significant, both within the core disciplines of paleoceanography and organic geochemistry, but also in many related fields. Our project has pioneered the use of CSIAA in sediments and ancient shells, for reconstructing new, highly detailed information about past N cycles, and together provides a road-map for future work. A detailed understanding of how the earth?s biogeochemical systems will change in a shifting climate is one of the most compelling and societally important issues of our time, and this research has contributed in a major way toward this goal. Last Modified: 01/25/2017 Submitted by: Matthew D Mccarthy