USE OF THE NITRATE ISOTOPES IN THE OCEAN INTERIOR TO EXPLORE THE ISOTOPIC COMPOSITION OF SINKING NITROGEN AND ITS IMPLICATIONS FOR MARINE BIOGEOCHEMICAL CYCLES

Abstract

This thesis uses measurements and models of the 15N/14N and 18O/16O (δ15N and δ18O) of nitrate (NO3-) to gain new constraints on the δ15N of sinking nitrogen that derives from the photosynthetically active surface layer and is remineralized in the ocean interior. The δ15N of sinking nitrogen, in turn, reflects important biogeochemical processes in the ocean. For example, it can reveal (1) the spatial distribution of N2 fixation, the dominant source of biologically available nitrogen (“fixed N”) to the ocean and (2) the transport of nitrate from high to low latitude regions and its subsequent processing. Moreover, better information regarding the δ15N of sinking N would significantly improve our ability to simulate denitrification, the dominant loss mechanism for oceanic fixed N.

The fixed N budget is central in the debate regarding the degree to which interactions among ocean organisms stabilize the ocean’s biogeochemical cycles, with implications for the variability of global biological fertility and of CO2 and O2 concentrations in the atmosphere. The system of nitrogen isotopes can constrain the fixed N budget, but this requires a robust estimate for the isotope effect of water column denitrification (εwcd). N isotope budgeting using an εwcd of ~25‰ (inferred from treating observations from ocean suboxic zones with the Rayleigh model) result in a global denitrification rate that is much higher than the global rate of N2 fixation and thus an ocean N budget far in deficit. Recent culture experiments indicate that εwcd may be much lower (13‰). In the context of the isotope budgeting, this value would imply a lower global rate of denitrification and a roughly balanced fixed N budget. Chapter 2 explores the case for a lower εwcd using a multi-box model framework under various assumptions regarding the routes of nitrate-bearing water transport from the high latitude regions to the denitrification zones and the δ15N of the sinking N that adds nitrate to this water during its transport to these zones. The analysis shows that, assuming a plausible δ15N range for sinking N in the low latitudes, denitrification zone observations can be reproduced with a low isotope effect of ~13‰. Further, the estimates supporting the canonical εwcd of ~25‰ may be an artifact of the inaccurate assumption that the regeneration of sinking N is a minor influence on the nitrate isotope distributions near denitrification zones.

In the Atlantic, the lack of substantial water column denitrification implies that the δ15N of sinking N is exclusively sensitive to (1) the δ15N of the nitrate supply to the euphotic zone, (2) the degree of nitrate consumption in surface waters, and (3) the input of N to the surface by N2 fixation. In a large fraction of the low- to mid-latitudes of the Atlantic, surface nitrate consumption is complete, further reducing the number of potential drivers of sinking N δ15N change. The implication is that meridional gradients in the δ15N of sinking N in the Atlantic, mostly reflect the input of fixed N by N2 fixation. Given enough information on the δ15N of the nitrate supply to the euphotic zone, this link can be used to investigate the distribution of N2 fixation in the North Atlantic.

Chapter 3 investigates the impact of the subduction of high latitude surface waters (SAMW) with high-δ15N (~6.2‰) and high-δ18O (~3.5‰) nitrate on the δ15N of nitrate supply to North Atlantic euphotic zone (from ~20°N to ~40°N). Nitrate isotope measurements along the zonal US GTUS transect indicate that the nitrate δ15N of SAMW- derived waters is up to ~0.6‰ higher than the δ15N of deep nitrate (~4.8‰) while the δ18O is not similarly elevated. Its low δ18O reflects that nitrate at shallow and mid-depths is mostly regenerated, masking the high δ18O of preformed nitrate from the Southern Ocean. The input of fixed N by N2 fixation to the Atlantic is reflected by (a) the isotopic difference between the nitrate δ15N of SAMW- derived waters from the US GTUS transect (~5.4‰) and the nitrate δ15N of SAMW in the Southern Ocean (~6.2‰) and (b) the low nitrate δ15N measured for southward flowing NADW in the face of the substantial contribution of SAMW-derived water (with its high nitrate δ15N) to the formation of this water mass. From an isotopic mass balance between southward- and northward- flowing waters, it is shown that most of the input of newly fixed N is accomplished south of ~40°N.

Chapter 4 reports and interprets nitrate isotope data along the CLIVAR A16N, a hydrographic section from south of Iceland to 6°S in the tropical Atlantic that provides a meridional view that is closely related to the largely north/south subsurface circulation of the Atlantic. Three approaches are used to calculate the δ15N of regenerated nitrate and of sinking N. At 6°S, the nitrate δ15N of SAMW- derived waters is as high as the nitrate δ15N of SAMW in the Southern Ocean (~6.2‰), and the δ15N of sinking N is similarly elevated or higher. This result indicates that (a) nitrate assimilation is complete at the low latitudes of the Atlantic such that δ15N of sinking N is as high as the δ15N of SAMW and (b) only a small amount of newly fixed N is added to the nitrate pool in the South Atlantic. The regeneration of this high δ15N sinking N smears down high δ15N nitrate to levels deeper than northward flowing SAMW, raising the nitrate δ15N of southward flowing NADW. North of the equator, the δ15N of sinking N declines northward, with most of this decline being accomplished well south of the subtropical gyre. This suggests that the highest rates of N2 fixation for the North Atlantic occur south of 10°N, a result with implications for what controls the N2 fixation process in the basin.

Chapter 5 uses nitrate δ15N measurements along different transects of the Atlantic (including A16N and US GTUS) to pursue the question of the controls on the rate of N2 fixation in the Atlantic basin. A novel strategy is developed for calculating the rate and spatial distribution of Atlantic N2 fixation, based on isotopic differences between northward- and southward-transported nitrate. It is found that 90% of the total rate of Atlantic N2 fixation (30.5 Tg N/yr north of 30°S) is controlled by the supply of excess P. This link has important consequences for the stability of the fixed N budget, implying that Atlantic N2 fixation can stabilize the ocean N-to-P ratio over the relatively short time scale of ocean overturning (~2000 yrs). However, the Atlantic currently accounts for only ~25% of global ocean N2 fixation (130 Tg N/yr), requiring that N loss in the Pacific and Indian basins is largely compensated by N2 fixation within those basins.