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

Abstract

<p><strong>Abstract.</strong> Nitrate is a critical ingredient for life in the ocean because, as the most abundant form of fixed nitrogen in the ocean, it is an essential nutrient for primary production. The availability of marine nitrate is principally determined by biological processes, each having a distinct influence on the N isotopic composition of nitrate (nitrate <i>δ</i><sup>15</sup>N) – a property that informs much of our understanding of the marine N cycle as well as marine ecology, fisheries, and past ocean conditions. However, the sparse spatial distribution of nitrate <i>δ</i><sup>15</sup>N observations makes it difficult to apply this useful property in global studies, or to facilitate robust model-data comparisons. Here, we use a compilation of published nitrate <i>δ</i><sup>15</sup>N measurements (n = 12277) and climatological maps of physical and biogeochemical tracers to create a surface-to-seafloor, 1° resolution map of nitrate <i>δ</i><sup>15</sup>N using an Ensemble of Artificial Neural Networks (EANN). The strong correlation (R<sub>2</sub> > 0.87) and small mean difference (< 0.05 ‰) between EANN-estimated and observed nitrate <i>δ</i><sup>15</sup>N indicates that the EANN provides a good estimate of climatological nitrate <i>δ</i><sup>15</sup>N without a significant bias. The magnitude of observation-model residuals is consistent with the magnitude of seasonal-decadal changes in observed nitrate <i>δ</i><sup>15</sup>N that are not captured by our climatological model. As such, these observation-constrained results provide a globally-resolved map of mean nitrate <i>δ</i><sup>15</sup>N for observational and modeling studies of marine biogeochemistry, paleoceanography, and marine ecology.</p>

Modeling oceanic nitrate and nitrite concentrations and isotopes using a 3-D inverse N cycle model

“…This is partly related to the muted ODZ signal as mentioned above and its lessened impact on thermocline δ 15 N NO 3 across the basin. Peters et al (2018a) and Rafter et al (2013) also postulated that these elevated δ 15 N NO 3 values were in part driven by remineralization of organic matter with high δ 15 N. The δ 15 N of sinking PON in the model (6 ‰-10 ‰) was similar to those observed in the South Pacific (Raimbault et al, 2008), as well as those predicted from aforementioned N isotope studies Peters et al, 2018a). The model also shows slightly elevated δ 15 N NO 3 in the intermediate depths (500-1500 m), which is consistent with observations, again reflecting remineralization of PON with δ 15 N greater than mean ocean δ 15 N NO 3 .…”

Assessing Marine Nitrogen Cycle Rates and Process Sensitivities With a Global 3‐D Inverse Model

“…Remaining parameters were either experimentally manipulated in different model runs or calculated via an optimization procedure under a variety of model parameterizations; the optimized parameters are the rate constants for NO 3 − reduction (k NAR ), NO 2 − reduction (k NIR ), NO 2 − oxidation (k NXR ), and DON remineralization (k remin ; Table S1). The optimization procedure, as described by Martin et al (2019), compares the model output 14 N and 15 N concentrations to a large database of NO 3 − and NO 2 − concentration and isotope data (Martin et al, 2019;Rafter et al, 2019). The parameters were simultaneously modified over the course of the optimization to find the combination with which the 14 N and 15 N output best fit the observations and minimized a cost function consisting of a weighted least squares algorithm between modeled NO 3 − and NO 2 − concentrations and isotopes and their observed distributions interpolated to the model grid (Martin et al, 2019).…”

Assessment of C, N, and Si Isotopes as Tracers of Past Ocean Nutrient and Carbon Cycling

“…At these depths, NASTG δ 15 N nitrate is notably lower (<4‰), while ETSP and CTSP δ 15 N nitrate are higher (>8‰) than Southern Ocean δ 15 N nitrate (5‰–7‰). At depths below 3,000 m, δ 15 N nitrate from all major ocean basins converges on the mean deep ocean value of 5.0 ± 0.3‰ (Rafter et al., 2019; Sigman et al., 2000).…”

“…It is important to note that because denitrification reduces the NO 3 − inventory, in the context of an isotopic mass‐balance, increased denitrification actually diminishes the influence of this newly elevated δ 15 N nitrate on NO 3 − outside of the denitrification area (Deutsch et al., 2004). In the deep sea (below 3,000 m), small (∼0.5‰) inter‐basin differences in δ 15 N nitrate may reflect the regional regeneration of organic matter with different δ 15 N (Rafter et al., 2019).…”

“…To summarize, the isotopic fractionations associated with the following processes act to determine marine δ 15 N nitrate (Figure 1): (a) partial NO 3 − utilization, which elevates surface ocean δ 15 N nitrate as well as global thermocline δ 15 N nitrate via mode and intermediate waters (Marconi et al., 2015; Rafter et al. 2012, 2013), (b) N 2 fixation, which lowers marine δ 15 N nitrate via remineralization of organic matter, and (c) water column denitrification, which locally elevates the residual δ 15 N nitrate (e.g., Rafter et al., 2019; Sigman & Fripiat, 2019; Somes et al., 2010).…”