The isotope effect of nitrate assimilation in the Antarctic Zone: Improved estimates and paleoceanographic implications

Abstract

Both the nitrogen (N) isotopic composition (d 15 N) of the nitrate source and the magnitude of isotope discrimination associated with nitrate assimilation are required to estimate the degree of past nitrate consumption from the d 15 N of organic matter in Southern Ocean sediments (e.g., preserved within diatom microfossils). It has been suggested that the amplitude of isotope discrimination (i.e. the isotope effect) correlates with mixed layer depth, driven by a physiological response of phytoplankton to light availability, which introduces complexity to the interpretation of sedimentary records. However, most of the isotope effect estimates that underpin this hypothesis derive from acid-preserved water samples, from which nitrite would have been volatilized and lost during storage. Nitrite d 15 N in Antarctic Zone surface waters is extremely low (-61 ± 20‰), consistent with the expression of an equilibrium isotope effect associated with nitrate-nitrite interconversion. Its loss from the combined nitrate+nitrite pool would act to raise the d 15 N of nitrate, potentially yielding overestimation of the isotope effect. Here, we revisit the nitrate assimilation isotope effect in the Antarctic Zone with measurements of the d 15 N and concentration of nitrate with and without nitrite, using frozen sea water samples from 5 different cruises that collectively cover all sectors of the Southern Ocean. The N isotope effect estimated using nitrate+nitrite d 15 N is relatively constant (5.5 ± 0.6‰) across the Antarctic Zone, shows no relationship with mixed layer depth, and is in agreement with sediment trap d 15 N measurements. Estimates of the N isotope effect derived from nitrate-only d 15 N are higher and more variable (7.9 ± 1.5‰), consistent with an artifact from nitrate-nitrite isotope exchange. In the case of the Southern Ocean, we conclude that the d 15 N of nitrate +nitrite better reflects the isotope effect of nitrate assimilation. The stability of this isotope effect across the Antarctic Zone simplifies the effort to reconstruct past degree of nitrate consumption.

The Seasonal Cycle of Nitrogen Uptake and Nitrification in the Atlantic Sector of the Southern Ocean

“…The NO 2 − concentrations over the upper~200 m of the water column show no trend with depth and are similar in summer and winter (~0.2 μmol L −1 ) despite the large seasonal differences in NH 4 + ox and NO 2 − ox (Text S3). This has been observed previously in high-latitude surface waters (Fripiat et al, 2019;Zakem et al, 2018) and is in contrast to the oxygenated oligotrophic ocean where NO 2 − builds up at the base of the euphotic zone as a primary NO 2 − maximum (Lomas & Lipschultz, 2006;Ward, 2008). The accumulation of NO 2 − throughout the upper water column poleward of 45°has been explained as deriving from the upward mixing in winter of NO 3 − + NO 2 − that cannot be depleted by phytoplankton due to iron and/or light limitation and that is then available to nitrifying organisms (Zakem et al, 2018).…”

“…In the summer by contrast, light inhibition and rapidly growing phytoplankton allow for rates of surface nitrification that are insignificant relative to both wintertime nitrification and summertime autotrophic NO 3 − consumption. This seasonal pattern is also evident in the available Southern Ocean NO 3 − isotope data that show assimilation to be the major biological process acting on the surface NO 3 − pool in summer (DiFiore et al 2009; Fripiat et al, 2019; Kemeny et al, 2016) with nitrification dominating in winter (Smart et al, 2015).…”

A Test of the Diatom‐Bound Paleoproxy: Tracing the Isotopic Composition of Nutrient‐Nitrogen Into Southern Ocean Particles and Sediments

“…While the trends suggest that the primary signal in the δ 15 N DB values is related to surface nutrient uptake, the differences between the observed and predicted absolute values of δ 15 N changes indicate that the processes controlling the δ 15 N DB are not strictly limited to the isotopic fractionation associated with nitrate uptake and assimilation. Sedimentary δ 15 N DB values are generally elevated relative to expectations for total biomass produced with a typical isotope effect of assimilation (∼4–7‰; Fripiat et al, 2019) and the latitudinal gradients in δ 15 N DB are steeper than simple predictions; δ 15 N DB values are higher than predicted at the far southern and northern end of the latitudinal transects (Robinson & Sigman, 2008).…”

Delivering Sustained, Coordinated, and Integrated Observations of the Southern Ocean for Global Impact

“…To constrain the Southern Ocean's biogeochemical cycles, observations of oxygen, nutrients, dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), transient tracers (e.g., CFCs), particulate organic carbon, and ocean-color measurements are a priority. Stable isotope measurements of nitrate, as well as DIC and silicic acid, are becoming powerful tools for constraining past and present biogeochemical processes and ocean circulation, and substantial datasets are accumulating across the Southern Ocean over the annual cycle (e.g., Smart et al, 2015;Henley et al, 2018;Kemeny et al, 2018;Fripiat et al, 2019). Carbon isotopes and elements such as neodymium are also becoming increasingly important for paleo-circulation reconstructions (Basak et al, 2018), but modern measurements are scarce, such that there is a need to include these parameters in future observing efforts.…”