Nutrient cycling in the Atlantic basin: The evolution of nitrate isotope signatures in water masses

Abstract

We document diazotrophy-producing high N:P particle ratios (18-21:1) for remineralization, which is further confirmed by a decrease in δ 15 N NO3 through the subtropical Atlantic. These modifications influence the isotopic signatures of the North Atlantic Deep Water (NADW) which is subsequently exported from the Atlantic to the Southern Ocean. This study reveals the dominance of recycling processes and diazotrophy on nitrate cycling in the Atlantic. These processes provide a source of low δ 15 N NO3 to the Southern Ocean via the NADW, to counteract enrichment in δ 15 N NO3 from water column denitrification in the Indo/Pacific basins.We hence identify the Southern Ocean as a key hub through which denitrification and N 2 fixation communicate in the ocean through deepwater masses. Therefore, the balancing of the oceanic N budget and isotopic signatures require time scales of oceanic mixing.

One size fits all? – Calibrating an ocean biogeochemistry model for different circulations

“…Finally, the combined outcrop area of SAMW and AAIW determines the optimal maximum rate of nitrogen fixation. This may be explained with the role of the Southern Ocean as "mixer" for waters of different origin and nitrogen deficit (Tuerena et al, 2015). The extent and properties of waters originating from this region (and their fixed nitrogen 20 https://doi.org/10.5194/bg-2020-9 Preprint.…”

“…Outcrop area of mode and intermediate water masses: On centennial timescales the Antarctic Intermediate Water (AAIW) and Subantarctic Mode Water (SAMW) formed in the Southern Ocean determine nutrient concentrations in subtropical areas. Their nitrate deficit and isotopic composition carries signatures of denitrification and nitrogen fixation (Rafter et al, 2013;Tuerena et al, 2015). Given that the models differ so strongly with respect to the surface density in the Southern Ocean (Figure 1), we evaluated the outcrop area of waters defined by a density of 26.5  ✓ < 27.5 and 27.5  ✓ in both the Southern Ocean and the northern North Atlantic (defined as above).…”

“…These two regions in the Atlantic and Pacific Ocean are connected through large scale circulation, which transports N ⇤ on centennial to millennial time scales from areas of fixed nitrogen loss to areas of fixed nitrogen gain. When passing the CDW of the Southern Ocean, these waters can act as a "mixer of deep waters with distinct isotopic signatures and nutrient stoichiometry" (Tuerena et al, 2015); the resulting mixed properties provide the source of AAIW and SAMW. The subsequent transport via AAIW and SAMW then can trigger nitrogen fixation, e.g., in the Atlantic, and balance the nitrate deficit arising mainly in the Pacific Ocean.…”

Data Constraints on Glacial Atlantic Water Mass Geometry and Properties

“…In the modern ocean, northward flowing AAIW contributes substantially to the return flow that compensates for southward NADW export at depth (Schmitz & McCartney, ). AAIW is also a significant source of nutrients to the Atlantic basin (Sarmiento et al, ; Tuerena et al, ).…”

“…In the modern ocean, northward flowing AAIW contributes substantially to the return flow that compensates for southward NADW export at depth (Schmitz & McCartney, 1993). AAIW is also a significant source of nutrients to the Atlantic basin (Sarmiento et al, 2004;Tuerena et al, 2015). δ 13 C DIC and Cd concentration are negatively (Kroopnick, 1985) and positively (Boyle, 1988a) correlated to phosphate concentration, respectively, although δ 13 C DIC is also influenced by the air-sea gas exchange that occurs before surface source waters are transported to depth (e.g., Lynch-Stieglitz et al, 1995).…”