New Constraints on Assemblage‐Driven Variation in the Relationship Amongst Diatom‐Bound, Biomass, and Nitrate Nitrogen Isotope Values

Abstract

Nitrate is a crucial nutrient that supports and limits marine primary productivity, now and through the past (Falkowski, 1997;Moore et al., 2013). The biological pump, through which carbon and nutrients are exported from the surface to the deep ocean by biological processes, is closely linked to atmospheric CO 2 concentrations and global climate. The Southern Ocean has the world's largest pool of underutilized surface nitrate, which, if it were fully consumed, would reduce atmospheric pCO 2 by more than 40 ppm (Sarmiento & Orr, 1991). Greater relative nitrate utilization in the glacial Southern Ocean due to either a decrease in supply or an increase in demand likely contributed to the observed lowering of CO 2 during glacial intervals (Ai et al., 2020;Kohfield et al., 2013;Robinson et al., 2009;Sigman et al., 2010). Changes to the efficiency of nitrate use in the glacial Southern Ocean are monitored using sedimentary nitrogen isotope measurements as a proxy for the relative consumption of nitrate (

Estimates of the Isotope Effect for Nitrate Assimilation in the Indian Sector of the Southern Ocean

“…While an 15 ε of 6.5‰ has been used to interpret the degree of nutrient utilization in the Subantarctic during the LGM (Martínez‐Garcia et al., 2014), culture‐based estimates suggest that the 15 ε can range from ∼1 to 20‰ in response to light and Fe variability, as well as among phytoplankton species (Granger et al., 2004; Needoba & Harrison, 2004; Needoba et al., 2003, 2004; Pennock et al., 1996; Waser, Harrison, et al., 1998, Waser, Yin, et al., 1998), all of which may have differed between the LGM and present conditions. Additionally, field‐based 15 ε estimates from the modern Southern Ocean range from ∼4 to 10‰ during the spring and summer growing season (Altabet & Francois, 2001; DiFiore et al., 2009, 2010; Fripiat et al., 2019; Jones et al., 2022; Sigman, Altabet, McCorkle, et al., 1999; Trull et al., 2008). We report a similar range of 15 ε org estimates from the mid‐to‐late summer Southern Ocean, from ∼4.9‰ at STN 6 in the SAZ near the SAF and 5.1‰ at STN 4 in the AZ to ∼9.3‰ at STN 7 in the northern SAZ; we discuss the relatively low 15 ε org of 4.1‰ estimated for STN 3 in Section 5.2 below.…”

“…broadly applicable. For example, field-based studies from the Southern Ocean have yielded 15 ε org estimates ranging from ∼1 to 11‰ (Altabet & Francois, 2001;DiFiore et al, 2009DiFiore et al, , 2010Jones et al, 2022;Karsh et al, 2003;Lourey et al, 2003;Smart et al, 2015;Trull et al, 2001), with values <5‰ thought to reflect active surface-layer NO 3 production (i.e., nitrification) superimposed on NO 3 supplied from the subsurface, thus introducing an additional, low-δ 15 N NO 3 source to surface waters (Smart et al, 2015). While 15 ε org estimates from Southern Ocean waters poleward of the Subantarctic Front (i.e., the Polar Frontal and Antarctic Zones) typically fall between 4 and 6‰ (DiFiore et al, 2009;Fripiat et al, 2019;Karsh et al, 2003;Trull et al, 2008), estimates of 15 ε org for the Subantarctic (i.e., north of the Subantarctic Front) are higher, with values from the modern ocean typically falling between 6 and 11‰ (DiFiore et al, 2006(DiFiore et al, , 2010Lourey et al, 2003;.…”

“…While previous studies have typically applied an 15 ε org of 5–7‰ for both the modern and paleo ocean (Martínez‐Garcia et al., 2014; Robinson & Sigman, 2008; Smart et al., 2020), field and culture studies have documented values of 15 ε ranging from 1 to 20‰, raising questions as to whether an 15 ε org of 5–7‰ for NO 3 − assimilation is broadly applicable. For example, field‐based studies from the Southern Ocean have yielded 15 ε org estimates ranging from ∼1 to 11‰ (Altabet & Francois, 2001; DiFiore et al., 2009, 2010; Jones et al., 2022; Karsh et al., 2003; Lourey et al., 2003; Sigman, Altabet, McCorkle, et al., 1999; Smart et al., 2015; Trull et al., 2001), with values <5‰ thought to reflect active surface‐layer NO 3 − production (i.e., nitrification) superimposed on NO 3 − supplied from the subsurface, thus introducing an additional, low‐δ 15 N NO 3 − source to surface waters (Smart et al., 2015). While 15 ε org estimates from Southern Ocean waters poleward of the Subantarctic Front (i.e., the Polar Frontal and Antarctic Zones) typically fall between 4 and 6‰ (DiFiore et al., 2009; Fripiat et al., 2019; Karsh et al., 2003; Sigman, Altabet, McCorkle, et al., 1999; Trull et al., 2008), estimates of 15 ε org for the Subantarctic (i.e., north of the Subantarctic Front) are higher, with values from the modern ocean typically falling between 6 and 11‰ (DiFiore et al., 2006, 2010; Lourey et al., 2003; Sigman, Altabet, McCorkle, et al., 1999).…”