An examination of the marine nitrogen cycle: insights from novel stable nitrogen isotopic approaches
Abstract
Paleoceanographic studies typically employ stable nitrogen isotopic measurements of total combustible N (δ15NTN) from marine sediments to infer temporal changes in surface ocean marine N dynamics. However, δ15NTN is a highly non-specific measurement, susceptible to confounding issues including non-marine N sources, and offers little, if any, information regarding degradation and/or alteration of primary δ15N signals. Recent development of novel biomarker and microfossil based proxies provide new analytical capabilities to circumvent issues with δ15NTN.
Here, I explore the utility of compound specific nitrogen isotopic analysis of individual amino acids measurements (δ15NAA) of proteinaceous material isolated from marine sediments and deep-sea coral. With these data I infer past changes in nitrate δ15N fueling primary production at the base of the food web, ecosystem structure, and the extent of heterotrophic resynthesis influencing sedimentary and coral-skeletal δ15NTN records. Within sediments and corals, I demonstrate that δ15NAA based parameters reflect that of sinking particles and surface collected planktonic biomass and thus serve as a robust, albeit complex proxy to infer ecosystem and nutrient dynamic changes for at least the last few hundred years on the California Margin. In sediments I find an unexpected δ15N offset in major operational fractions and molecular compound classes. This result (1) challenges a major assumption organic nitrogen in sediments is dominated by amide functionality (i.e., proteinaceous N) and (2) highlights possible issues with standard protein hydrolysis techniques.
Lastly, I analyze the δ15N composition of foraminifera-bound N (FB-δ15N) to assess the Plio-Pleistocene history of whole ocean nitrogen dynamics from the oligotrophic North Pacific for the last 5 million years. This region contains calcareous nannofossil rich sediments, ideal for preserved FB-δ15N records. These new FB-δ15N records support an expansion of suboxic regions, consistent with previous evidence, but also indicate that the whole ocean δ15N budget was stable throughout the Plio-Pleistocene. This latter result implies that kinetic isotopic fractionations of marine N transformations and the ratio of pelagic to benthic denitrification, must not have changed during the Plio-Pleistocene transition, despite drastic global scale changes in sea level, shelf area, and volume of the major oxygen deficient zones at this time.