INDOMIX project
INDOMIX project
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Objectives: The general scientific objectives of the INDOMIX project are to study the internal tides, the associated turbulent mixing and the chemical and physical transformations of the water masses when transiting through the Eastern Indonesian Seas. A collaborative project (PI: Ariane Koch-Larrouy) was conducted off the Indonesian archipelago and put together physicists and geochemists. |
Scientific Rationale
The Indonesian archipelago is a region of strong water mass transformation. This region is characterized by intense vertical mixing because strong internal tides remain trapped in semi-enclosed seas, with the result that a large amount of tidal energy remains available for vertical mixing. To study the internal tides, the associated turbulent mixing and the chemical and physical transformations of the water masses of the Eastern Indonesian Seas, a collaborative project was conducted in that area. To achieve our goals, we combined geochemical tools (Rare Earth concentrations, Nd isotopic composition, radium isotopes, 227Ac, and trace element concentrations on the particulate samples) and physical observations (hydrophysical (CTD)-current profile (LADCP) and Microstructure (VMP 600) measurements).
Strategy and sampling
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The INDOMIX cruise was conducted in July 2010 on board RV. Marion Dufresne (PI : Ariane Koch-Larrouy). Radium isotopes: Large volume water samples were collected in order to analyze radium isotopes (223Ra, 224Ra, 226Ra, 228Ra) and actinium-227 (227Ac) activities in order to build vertical profiles. Both 228Ra and 227Ac diffuse out of deep-sea sediments and are mixed upward because of the vertical mixing. The vertical profiles of 228Ra and 227Ac can thus be used to estimate vertical eddy diffusivity coefficients (Kz). Our estimates will be compared to these values determined by the physical methods. |
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Rare Earth (REE) concentrations and Nd isotopes: These parameters are good tracers of processes occurring at the land-ocean interface. They are also tagging the different water masses with different signatures, which allows following water mass trajectories and mixing. Sample volumes of 10l where collected, filtered on board. An aliquote of 500 ml was dedicated to the measurement of the Rare Earth concentrations. Filtered particles were stored for the analysis of the solid REE and the lithogenic tracers.
Results
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During the INDOMIX cruise, we benefited from the repeated CTD casts designed for physical oceanography. Between 24 and 48 Niskin bottles (i.e. one to two full CTD rosettes) were gathered to provide one single sample. We could thus get between 250 and 500 L for a single sample. Such large volumes of seawater are needed to analyse all 4 radium isotopes (226Ra, T1/2=1600 y; 228Ra, T1/2=5.75 y; 223Ra, T1/2=11.4 d; 224Ra, T1/2=3.66 d) and 227Ac (T1/2=21.77 y) that are at very concentration in the ocean. Applying a simple one-dimensional advection-diffusion model to the vertical profiles of Ra and Ac allows us to estimate vertical eddy diffusivity coefficients (Kz). An estimate of 4.25 x 10- 4 m2 s-1 is thus obtained at station S2 in the Halmahera Sea (Figure 1). These estimates can be compared with those deduced from the microstructure profile. |
fig. 1: Vertical profile of 228Ra activity at station S2 located in the Halmahera Sea. |
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Fig. 2: Vertical profile of 228Ra activity at station S0 and S4 |
The neodymium concentrations of the stations S0 (located at the entrance of the through flow) and S4 (located in the Banda Sea) reported in Fig.2 show |
Participants: P. van Beek, C. Jeandel, F. Lacan, M. Souhaut, M. Labatut, A. Koch-Larrouy
Partners:
LEGOS : Laboratoire d'Etudes en Géophysique et Océanographie Spatiales (Toulouse)
Fundings: LEGOS, INSU
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