The Agulhas Current Transports Signals of Local and Remote Indian Ocean Nitrogen Cycling

Abstract

The role that western boundary currents (WBCs) play in Earth's climate system by transporting heat and salt from low-to higher latitudes is well-known (

Glacial Southern Ocean Expansion Recorded in Foraminifera‐Bound Nitrogen Isotopes From the Agulhas Plateau During the Mid‐Pleistocene Transition

“…A decrease in the δ 15 N of the Subtropically sourced nitrogen can be explained by increases in nitrogen fixation derived nitrogen in the Agulhas Current region. Today N fixation, which brings in N with a δ 15 N value of ∼−1‰, occurs in the source regions to and along the flowpath of the Agulhas Current (Dupuy et al., 2016; Kolasinski et al., 2012; T. A. Marshall et al., 2023). In addition, throughout interglacial periods of the last 800 Kyr, increased denitrification has been documented in the Arabian Sea (Kim et al., 2018) thus it is probable that during this “super interglacial” we may see an exceptional response in N fixation to balance N losses from denitrification.…”

“…The need to understand the endmember δ 15 N values is highlighted by the fact that the δ 15 N FB values observed during MIS 31–33 are exceptionally low relative to the rest of the record, and also relative to expectations based on to modern southern Indian Ocean values (Harms et al., 2019; T. A. Marshall et al., 2023). These low δ 15 N FB values require not only an increase in contributions of Subtropical nitrogen but also a decrease in the δ 15 N of the nitrogen sourced to the STFZ.…”

“…The need to understand the endmember δ 15 N values is highlighted by the fact that the δ 15 N FB values observed during MIS 31-33 are exceptionally low relative to the rest of the record, and also relative to expectations based on to modern southern Indian Ocean values (Harms et al, 2019;T. A. Marshall et al, 2023).…”

“…A decrease in the δ 15 N of the Subtropically sourced nitrogen can be explained by increases in nitrogen fixation derived nitrogen in the Agulhas Current region. Today N fixation, which brings in N with a δ 15 N value of ∼−1‰, occurs in the source regions to and along the flowpath of the Agulhas Current (Dupuy et al, 2016;Kolasinski et al, 2012;T. A. Marshall et al, 2023).…”

Instabilities Across the Agulhas Current Enhance Upward Nitrate Supply in the Southwest Subtropical Indian Ocean

“…The vertical nutrient flux from WBC subsurface nutrient streams to overlying oligotrophic surface waters has received less attention (cf., Liao et al., 2022; Nagai et al., 2019). Numerous characteristics of WBCs favor upward nutrient supply, including (a) strong cross‐stream isopycnal shoaling of nutrient‐rich thermocline waters (Guo et al., 2012; Marshall et al., 2023; Pelegrí & Csanady, 1991), (b) the occurrence of upwelling‐favorable winds and wind stress curl (Imawaki et al., 2013; Leber et al., 2017), (c) high (sub)mesoscale eddy and frontal activity driven by strong density and velocity gradients (Imawaki et al., 2013; Lazaneo et al., 2020; Lévy et al., 2018; Tedesco et al., 2019), and (d) steep bathymetry and boundary effects (Liao et al., 2022; Schaeffer et al., 2013).…”