INDOMIX: Physical and geochemical transformations within the Indonesian throughflow

by TIM — last modified Nov 20, 2020 10:39 AM

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 to study vertical mixing in this region.

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/organization

	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 were compared to the estimates determined by the physical methods.

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 were compared to the estimates determined by the physical methods.

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, far from any lithogenic input. Sample volumes of 10 L 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 and perspectives

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 low 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; (Figure 1). These estimates can be compared with those deduced from the microstructure profile (Koch-Larrouy et al., 2015).
Fig. 1: Vertical profile of ²²⁸Ra activity at station S2 located in the Halmahera Sea (Koch-Larrouy et al., 2015).

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 low 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; (Figure 1). These estimates can be compared with those deduced from the microstructure profile (Koch-Larrouy et al., 2015).

Fig. 1: Vertical profile of ²²⁸Ra activity at station S2 located in the Halmahera Sea (Koch-Larrouy et al., 2015).

Neodymium concentrations measured at stations S0 (located at the entrance of the through flow) and S4 (located in the Banda Sea) reported in Fig.2 (top) show:

	An increase of the concentrations with depth, consistent with the nutrient-like behavior of Nd (surface scavenging, deep releasing) Concentrations becoming homogeneous and relatively low below 2000m in the Banda Sea, likely reflecting the vertical mixing. Neodymium Isotopic Composition (expressed as Nd, Fig2, bottom) show variable radiogenic profiles which reflect: In the upper layers, S4 thermocline is more negative than S0 one reflecting the different origins of these layers, S0 being clearly influenced by waters originating from the more radiogenic north Pacific. Below 2000 m, water in the deep Banda Sea has a constant signature (Nd, ~ -3) confirming that strong mixing and exchange with the margin are likely occurring. At S0, the water become more negative, reflecting the influence of mixing with Pacific Deep Waters, characterized by a signature of -6.
Fig. 2: Vertical profiles of Nd concentrations (top) and isotopic compositions (bottom) at stations S0 and S4 located at the entrance of the through flow and in the Banda Sea.

An increase of the concentrations with depth, consistent with the nutrient-like behavior of Nd (surface scavenging, deep releasing)

Concentrations becoming homogeneous and relatively low below 2000m in the Banda Sea, likely reflecting the vertical mixing.

Neodymium Isotopic Composition (expressed as Nd, Fig2, bottom) show variable radiogenic profiles which reflect:

In the upper layers, S4 thermocline is more negative than S0 one reflecting the different origins of these layers, S0 being clearly influenced by waters originating from the more radiogenic north Pacific.

Below 2000 m, water in the deep Banda Sea has a constant signature (Nd, ~ -3) confirming that strong mixing and exchange with the margin are likely occurring. At S0, the water become more negative, reflecting the influence of mixing with Pacific Deep Waters, characterized by a signature of -6.

Fig. 2: Vertical profiles of Nd concentrations (top) and isotopic compositions (bottom) at stations S0 and S4 located at the entrance of the through flow and in the Banda Sea.

Participants:

Pieter van Beek, Catherine Jeandel, François Lacan, Marc Souhaut, Marie Labatut, Nolwenn Lemaitre, Moustafa Belhadj, Ariane Koch-Larrouy

Collaborations:

LOCEAN, Paris

Fundings:

LEGOS, INSU

Peer review articles related to this project:

Koch-Larrouy A., Atmadipoera A., van Beek P., Madec G., Aucan J., Lyard F., Grelet J., Souhaut M., 2015. Estimate of tidal mixing in the Indonesian archipelago from multidisciplinary data, Deep-Sea Research I 106, 136–153.

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