Isotopic Evidence for the Evolution of Subsurface Nitrate in the Western Equatorial Pacific

Abstract

Subsurface waters from both hemispheres converge in the Western Equatorial Pacific (WEP), some of which form the Equatorial Undercurrent (EUC) that influences equatorial Pacific productivity across the basin. Measurements of nitrogen (N) and oxygen (O) isotope ratios in nitrate (δ15NNO3 and δ18ONO3), the isotope ratios of dissolved inorganic carbon (δ13CDIC), and complementary biogeochemical tracers reveal that northern and southern WEP waters have distinct biogeochemical histories. Organic matter remineralization plays an important role in setting the nutrient characteristics on both sides of the WEP. However, remineralization in the northern WEP contributes a larger concentration of the nutrients, consistent with the older “age” of northern thermocline‐depth and intermediate‐depth waters. Remineralization introduces a relatively low δ15NNO3 to northern waters, suggesting the production of sinking organic matter by N2 fixation at the surface—consistent with the notion that N2 fixation is quantitatively important in the North Pacific. In contrast, remineralization contributes elevated δ15NNO3 to the southern WEP thermocline, which we hypothesize to derive from the vertical flux of high‐δ15N material at the southern edge of the equatorial upwelling. This signal potentially masks any imprint of N2 fixation from South Pacific waters. The observations further suggest that the intrusion of high δ15NNO3 and δ18ONO3 waters from the eastern margins is more prominent in the northern than southern WEP. Together, these north‐south differences enable the examination of the hemispheric inputs to the EUC, which appear to derive predominantly from southern hemisphere waters.

Distribution and rates of nitrogen fixation in the western tropical South Pacific Ocean constrained by nitrogen isotope budgets

“…Comparing the absolute magnitude of the δ 15 N-budgetbased N 2 fixation rates with previous measurements, we find that the 219 to 290 µmol N m −2 d −1 rate estimated for LD A represents a significant N 2 fixation rate relative to prior global measurements (Luo et al, 2012), in particular if it should be revised upwards to account for the under-collection of the export flux by the sediment trap. In contrast, the estimated rate range at LD B, 11 to 21 µmol N m −2 d −1 , is quite low, as is the range of 0 to 9 µmol N m −2 d −1 at LD C, and both of these rates are broadly similar to the rates previously measured in the ETSP (Knapp et al, 2016a;Raimbault and Garcia, 2008).…”

“…Historically, the highest rates of N 2 fixation in the global ocean have been associated with the tropical North Atlantic (Mahaffey et al, 2005;Sohm et al, 2011). The high 15 N 2 incubation-based N 2 fixation rates observed in the tropical Atlantic (Luo et al, 2012) are consistent with both the preference of diazotrophs for warm waters (Breitbarth et al, 2007;Stal, 2009) as well as the high atmospheric dust flux to the region (Mahowald et al, 2009;Prospero, 1996) that helps fulfil the high iron requirement of the enzyme, nitrogenase, carrying out N 2 fixation (Berman-Frank et al, 2001;Kustka et al, 2003). Additionally, the elevated ratio of nitrate (NO − 3 ) to phosphate (PO 3− 4 ) concentrations (Gruber and Sarmiento, 1997) and low δ 15 N-NO − 3 (Knapp et al, 2008) in the upper thermocline of the North Atlantic are attributed to high regional N 2 fixation rates and have supported the hypothesis that iron availability plays a key role in regulating the spatial distribution of N 2 fixation in the ocean (Moore et A. N. Knapp et al: Distribution and rates of nitrogen fixation in the South Pacific Ocean al., 2009;Moore and Doney, 2007) ("δ 15 N", where δ 15 N = { 15 N/ 14 N sample / 15 N/ 14 N reference − 1} × 1000, with atmospheric N 2 as the reference).…”

“…While the highest inputs of N to the ocean have traditionally been associated with the North Atlantic, it has also been argued that this association results from the significant sampling bias in favour of the tropical Atlantic (Sohm et al, 2011), with large regions of the South Pacific and Indian Ocean under-sampled with respect to direct N 2 fixation rate measurements (Luo et al, 2012). More recently, the eastern tropical South Pacific (ETSP) has seen increased sampling due to nutrient distribution-based modelling predictions that the highest global N 2 fixation rates would be found in surface waters above and adjacent to oxygen-deficient zones (ODZs), where significant phosphorus (P) would be available to support N 2 fixation (Deutsch et al, 2007).…”

Global trends in marine nitrate N isotopes from observations and a neural network-based climatology

“…1960), denitrification (Liu, 1979), and nitrification (Casciotti et al, 2013). For example, the consumption of nitrate by denitrification (red line in Fig.…”

Revised Figure

“…Historically, the highest rates of N 2 fixation in the global ocean have been associated with the tropical North Atlantic (Mahaffey et al, 2005;Sohm et al, 2011). The high 15 N 2 incubation-based N 2 fixation rates observed in the tropical Atlantic (Luo et al, 2012) are consistent with both the preference of diazotrophs for warm waters (Breitbarth et al, 2007;Stal, 2009) as well as the high atmospheric dust flux to the region (Mahowald et al, 2009;Prospero, 1996) that helps fulfil the high iron requirement of the enzyme, nitrogenase, carrying out N 2 fixation (Berman-Frank et al, 2001;Kustka et al, 2003). Additionally, the elevated ratio of nitrate (NO − 3 ) to phosphate (PO 3− 4 ) concentrations (Gruber and Sarmiento, 1997) and low δ 15 N-NO − 3 (Knapp et al, 2008) in the upper thermocline of the North Atlantic are attributed to high regional N 2 fixation rates and have supported the hypothesis that iron availability plays a key role in regulating the spatial distribution of N 2 fixation in the ocean (Moore et Published by Copernicus Publications on behalf of the European Geosciences Union.…”