Development and paleoceanographic application of planktonic foraminifera-bound nitrogen isotopes

Abstract

Biologically available nitrogen (“fixed N”) is the proximately limiting nutrient in much of today's ocean. The input/output budget has been argued to control the fertility of the ocean and the ocean's role in setting atmospheric CO2. The two stable isotopes of N (¹⁴N and ¹⁵N) and their fractionation by natural processes provide a mechanism for studying the budget of oceanic fixed N in the past. However, outside of sedimentary environments characterized by high organic matter preservation, bulk sedimentary N is subjected to diagenetic alteration and contamination with allochthonous N and thus cannot be used reliably to reconstruct past N isotope changes.

In the search for reliable archives, this dissertation describes the first method for isotopic analysis of planktonic foraminiferal shell-bound N, which has its basis in the very high sensitivity of the “denitrifier method” for N isotope analysis. Isotopic measurements of foraminifera biomass from Sargasso Sea net tow material and of shell-bound N from Cariaco Basin sediment traps and from shallow sediments collected in different open ocean regions show a strong correlation between foraminifera-bound δ ¹⁵N (FB-δ¹⁵N) and changes in the subsurface nitrate δ ¹⁵N, which is the dominant source of new N to the euphotic zone. The isotopic difference among multiple species of foraminifera with different depth habitats and behaviors offers insight into the biological controls on FB-δ¹⁵N and also speaks to the potential richness of this paleoceanographic tool.

This dissertation has resulted in the first evidence for reduced N fixation in the oligotrophic open oceans during the last glacial maximum (LGM). Down core studies from the Caribbean Sea and the South China Sea both show higher FB-δ¹⁵N during the last ice age than the current interglacial. This is best explained by a reduction in N fixation rate during the last ice age in both ocean basins, with or without assumption of constant mean ocean nitrate δ¹⁵N. These data, when compared with reconstructions of ice age denitrification, provides perhaps the strongest direct evidence to date for the longstanding hypothesis of a feedback between denitrification and N fixation across the global ocean that stabilizes the size of the ocean N reservoir.