BONUS - GOODHOPE

by TIM — last modified Apr 03, 2015 07:18 PM

BONUS/GOODHOPE

Objectives:

Identify the processes and quantify the fluxes associated to land to ocean matter inputs
Identify the processes and quantify the fluxes associated to dissolved/particles interactions
Better constrain (processes and fluxes) the iron oceanic cycle, its sources and internal cycle

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.

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 Isotopy 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 schedule

At the 5 super stations, Nd isotopes have been measured in the dissolved phase. Rare Earth Element concentrations have been measured in both the dissolved and particulate phases (cf. Fig. 2). 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 re-mineralisation of REEs are partially related to the biogeochemical cycle of silicate [Garcia-Solsona et al., 2014].

Fig.2 Rare Earth Element patterns in the dissolved and particulate phases at 2 stations along the Bonus/GoodHope section.

At the 5 super stations, iron isotopes have been measured in both the dissolved and particulate phases. These data are the first iron isotope data ever measured in a HNLC area. Iron concentrations in both phases have also been measured, which is also quiet rare (for the particulate phase). One profile is shown in Fig. 3, at station S4.


Fig.3 Isotopic composition and conentration of dissloved (DFe) and particulate (PFe) Fe, and dissolved O₂ concentration at station S4.	The remarkable "light" iron (negative isotopic composition) signal observed in the core of the Upper Circumpolar Deep Water (UCDW) characterized by a vertical minimum in dissolved O₂ concentration suggests a relation between the iron isotope cycle and either the oxygen cycle or the remineralization of organic matter. Compared to the monotonously increasing Fe concentrations, the data clearly illustrate that Fe isotopes provide unique information, invisible from Fe concentrations data. This work is submitted [Abadie et al, Sub.].

Fig.3 Isotopic composition and conentration of dissloved (DFe) and particulate (PFe) Fe, and dissolved O₂ concentration at station S4.

The remarkable "light" iron (negative isotopic composition) signal observed in the core of the Upper Circumpolar Deep Water (UCDW) characterized by a vertical minimum in dissolved O₂ concentration suggests a relation between the iron isotope cycle and either the oxygen cycle or the remineralization of organic matter. Compared to the monotonously increasing Fe concentrations, the data clearly illustrate that Fe isotopes provide unique information, invisible from Fe concentrations data. This work is submitted [Abadie et al, Sub.].

The chemical composition of many elements have been measured in the suspended particles at the 5 super stations, 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; Abadie et al. Sub.]) other remain to be published.

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

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

Fundings: INSU, IPEV, ANR, IFREMER

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