Nitrogen and carbon cycling in the South Atlantic Ocean: a stable isotope study along a 40°S transect (UK GEOTRACES)

Abstract

Fixed N (nitrate, nitrite, and ammonium) is a limiting nutrient for photosynthesis in the surface ocean. The rates and relative importance of N cycling processes, however, are temporally and spatially complex, which hamper their direct measurement and quantification. The South Atlantic subtropical front separates the Atlantic Ocean and the subantarctic, an area which can elucidate information about water masses both entering and leaving the basin. Through the GEOTRACES programme, an oceanographic section across 40°S in the South Atlantic is used to investigate biogeochemical cycling of nitrogen and carbon in this region. Hydrographic data, in combination with the isotopic composition of nitrate (NO3-), particulate organic carbon and particulate nitrogen (δ15NNO3, δ18ONO3, δ13CPOC, δ15NPN), is used to provide integrative measurements for temporally and spatially variable processes of the marine N-cycle and C-cycle.

A thorough examination of the stable isotope cycling of particulate and dissolved N across the subtropical front is used to quantify the supply of fixed N to the mixed layer. The relative importance of nitrate from the subsurface, N2 fixation, terrestrial input and atmospheric deposition in supplying production is determined. Typically, 30-50% of the export flux in the subtropical water masses is sourced from N2 fixers and up to 75% within the Brazil Current. This finding suggests that diazotrophs may be abundant in the South West Atlantic providing a source of new N to this region.

To assess the basin scale N-cycling processes, the deep water masses were analysed to reveal the origin and history of NO3-. Intermediate waters formed in the subantarctic are enriched in δ15NNO3 and δ18ONO3 from partial utilisation by algae.

This enrichment in δ15NNO3 is not present in the subtropical North Atlantic or the return flow of the North Atlantic Deep Water (NADW), which decreases from ~5.9‰ in the newly formed intermediate waters to ~4.8‰ in the NADW at 40°S. The modification of isotopic signatures through the subtropical Atlantic can be calculated as an incorporation of 26-36 Tg N yr-1 of newly fixed N from an isotopic source of -1‰ (N2 fixation). The extent of N addition is higher than estimated rates of N loss within the Atlantic and surpasses the amount of N deficit supplied to the basin.

Fixed N inputs and losses through the global ocean are investigated by the assessment of remineralised nitrate added to the ocean interior. A lower δ15N is observed in Atlantic remineralised nitrate in comparison to the Pacific. The relative importance of N2 fixation and pelagic denitrification within each ocean basin is quantified and through this approach, N2 fixation rates are estimated at 92-116 Tg N yr-1 in the Pacific and 24-32 Tg N yr-1 in the Indian Ocean. Combining Atlantic N2 fixation of ~32 Tg N yr-1 with Indo-Pacific, global N2 fixation rates can be estimated at 142-184 Tg N yr-1. The high inputs in the Pacific suggest that excess P is the dominant control on the success of N2 fixers. However, estimates of new N addition to the Atlantic indicate other mechanisms such as the recycling efficiency of P and supply of Fe to the surface ocean increase N2 fixation rates above this threshold.

The organic matter supplied to sediments is principally derived from phytoplankton across the subtropical front. High organic content is associated with the productive Brazil-Malvinas Confluence region where a diverse supply of nutrients sustains elevated biomass. The Rio Plata outflow is characterised with high δ15NNO3 and δ15NPN, suggesting denitrification processes occur in the estuary. A low δ13C source associated with high Al concentrations is identified on the western slope, indicating a supply of terrestrial derived C to the deep ocean. The fractionation of C uptake by phytoplankton is assessed in subtropical and subantarctic waters. In the subantarctic, CO2[aq] and growth rates determine the extent of C isotope fractionation. In this region, low species diversity and a small range in cell size enable the fractionation from CO2[aq] and growth rate to be expressed in phytoplankton. In subtropical water masses a larger range of cell size is the principal determinant of C fractionation.

Increased surface area to volume is the main mechanism for increasing C uptake, arguing against the use of δ13CPOC as a palaeoproxy. The low δ13CPOC and δ15NPN observed in the subtropics (from C fractionation and N2 fixation) contrast the heavier signatures in the subantarctic. These observations are propagated to the sediments, wherein organic matter shifts are determined by changes in the subtropical front over time. The results of this study have greatly improved knowledge of N and C cycling within the South Atlantic, providing new insight into the cycling of these two important elements in the surface and deep ocean, on a regional and global scale.