BONUS - GOODHOPE

Published by François Lacan, November 2019 

Scientific Rationale

The oceanic area located South of South Africa is important in various respects:
- It is a critical crossroad for the global thermohaline circulation as it provides an inter-ocean communication route for heat and freshwater anomalies (Sloyan & Rintoul, 2001), and for chemical elements, notably through Mesoscale transports.
- As a part of the Southern Ocean, it is also a key area for the global carbon cycle. The outcropping of deep water masses allows for the exchange of gases such as CO2 between the deep sea and the atmosphere, while the incomplete utilization of nutrients by marine phytoplankton allows the concentration of CO2 in the atmosphere to be substantially greater than would be the case if these nutrients were used efficiently.
- Transport processes in this area could transfer significant amount of terrigeneous matter form the African area southwards into the Antarctic Circumpolar Current.
The scarcity of direct observations has greatly hampered our understanding of this physical, biological and chemical environment.

Strategy/organization

Bonus/GoodHope is a multidisciplinary project, coupling physics, chemistry and biology. It is mainly based on a cruise, carried out on board the R/V Marion Dufresne in February March 2008 (Fig. 1). The project, led by LPO and LEMAR (Brest) involves a large number of investigators, from 16 laboratories, from 6 countries. It also includes a modelling task.

	In this context, the Toulouse Marine Geochemistry Group was in charge of the analyses of Nd isotopes and Rare Earth Element concentrations, of Fe isotopes and of suspended particle trace element concentrations. These tracers were expected to help constraining continental/oceanic matter fluxes, dissolved/particulate interactions, and the iron cycle. Fig. 1 Map of the Bonus/GoodHope cruise, indicating the locations of the "super stations", where we samples were taken for our analyses. Colors indicate surface temperature (from satellite).
Results and perspectives Fig. 2 shows the dissolved iron concentrations and isotopic compositions along the transect. These is the first Fe isotope transect ever measured in a HNLC area. These sections illustrate that Fe isotopes provide unique information, invisible to Fe concentration data. In contrast to the common but oversimplified view, according to which organic matter remineralization is the major pathway releasing dissolved iron below the surface layers, these data reveal other dominant processes at depth, likely abiotic de-sorption/dissolution from lithogenic particles (Abadie et al 2017).
	Fig. 2 Dissolved iron concentrations and isotopic compositions along the Bonus/GoodHope cruise (Abadie et al. 2017)
Nd isotopes have been measured in the dissolved phase and Rare Earth Element concentrations have been measured in both the dissolved and particulate phases (Fig. 3). All together these data allow evidencing lithogenic inputs released from the African margin sediments into the water masses flowing along that margin, but also a transfer of this lithogenic matter southwards, at least as far as station S2 (flat REE pattern in the particles at S2). The data also indicate that surface removal and deep remineralization of REEs are partially related to the biogeochemical cycle of silicate (Garcia-Solsona et al., 2014).
	Fig. 3 Rare Earth Element patterns in the dissolved and particulate phases at 2 stations along the Bonus/GoodHope section

The chemical composition of many elements have been measured in the suspended particles notably, Al, Ca, Mn, Fe, Co, Ni, Cu, Sr, Cd, Ba, REE, Th. These kind of data are very rare for most of these elements. They provide diverse information (micronuient cyclings, dissolved particles interactions, land to ocean fluxes etc…). Some have been published (Fe, Co, REE, Ba, Th [Bown et al., 2011; Garcia-Solsona et al., 2014]) other remain to be published. A work on 230Th is under progress in collaboration with LSCE.

Participants:

F. Lacan, C. Jeandel, C. Pradoux, C. Abadie, E. Garcia-Solsona, M. Labatut, A. Radic

Collaborations:

LEGOS; LEMAR; LPO; VUB (Belgium), LSCE

Funding:

INSU, IPEV, ANR, IFREMER

Peer review articles related to this project:

Lacan, F., Radic, A., Jeandel, C., Poitrasson, F., Sarthou, G., Pradoux, C., Freydier, R., 2008. Measurement of the isotopic composition of dissolved iron in the open ocean. Geophysical Research Letters 35. https://doi.org/10.1029/2008GL035841
Bown, J., Boye, M., Baker, A., Duvieilbourg, E., Lacan, F., Le Moigne, F., Planchon, F., Speich, S., Nelson, D.M., 2011. The biogeochemical cycle of dissolved cobalt in the Atlantic and the Southern Ocean south off the coast of South Africa. Marine Chemistry 126, 193–206. https://doi.org/10.1016/j.marchem.2011.03.008
Garcia-Solsona, E., Jeandel, C., Labatut, M., Lacan, F., Vance, D., Chavagnac, V., Pradoux, C., 2014. Rare earth elements and Nd isotopes tracing water mass mixing and particle-seawater interactions in the SE Atlantic. Geochimica et Cosmochimica Acta 125, 351–372. https://doi.org/10.1016/j.gca.2013.10.009
Conway, T.M., John, S.G., Lacan, F., 2016. Intercomparison of dissolved iron isotope profiles from reoccupation of three GEOTRACES stations in the Atlantic Ocean. Marine Chemistry 183, 50–61. https://doi.org/10.1016/j.marchem.2016.04.007
Abadie, C., Lacan, F., Radic, A., Pradoux, C., Poitrasson, F., 2017. Iron isotopes reveal distinct dissolved iron sources and pathways in the intermediate versus deep Southern Ocean. PNAS 114, 858–863. https://doi.org/10.1073/pnas.1603107114