Welcome to MedFlux

Sinking particulate matter is the major vehicle for exporting carbon from the sea surface to the ocean interior. During its transit towards the sea floor, most (>90%) particulate organic carbon (POC) is returned to inorganic form and redistributed in the water column. This redistribution determines the depth profile of dissolved CO₂, which in turn determines the concentration of CO2 in the surface mixed layer, and hence the rate at which the ocean can absorb CO₂ from the atmosphere. It also determines the depth profile of nutrient regeneration, which determines the time scale of return of mineral nutrients to the photic zone. The ability to predict quantitatively and mechanistically the depth profile of remineralization is therefore critical to predicting the response of the global carbon cycle to environmental change.

Minerals typically constitute more than half the mass of particles sinking out of the ocean surface, and this fraction increases dramatically with depth. Marine plankton contribute biominerals, e.g., opal by diatoms and radiolarians, and CaCO₃ by coccolithophorids and foraminifera. Detrital minerals (largely quartz and aluminosilicates) introduced from land by rivers and wind also can become associated with marine plankton (or their remains) through sorption and aggregation processes. Minerals are important for making less dense organic matter (OM) sink, and may also protect OM from degradation, allowing it to penetrate deeper into the ocean.

Recently, we demonstrated that ratios of particulate organic carbon to mineral ballast converge to a nearly constant value (~3-7 wt% POC) at depths >1800 m (Armstrong et al. 2002), and Klaas & Archer (2002) demonstrated that the variability in the data can largely be explained by the chemical composition of the ballast (opal vs. carbonate vs. dust). The focus of MedFlux is to develop a better mechanistic understanding of this “ballast hypothesis”. In particular, given the many processes that could cause large deviations from this ratio, a fundamental goal is to understand why POC:mass ratios seem to be well-delimited, and to use this understanding to create, as fully as possible, a new mathematical description of remineralization to replace those currently in use. This last goal is of utmost significance if, for example, lowered pH causes carbonate minerals to dissolve preferentially, affecting both ballasting and the average remineralization depth of POC in the ocean.

MedFlux is a collaborative research project that includes investigators from the U.S. and Europe.

We currently have two major objectives:

• To assess the extent to which settling velocity separation techniques accurately and reliably measure in-situ settling velocities and to devise mechanical improvements and/or statistical correction procedures to overcome any deficiencies.
  
• To develop perspectives and protocols that take advantage of the different sampling characteristics of in-situ pumps, sediment traps, and optical instruments, combined with radiochemical analysis, to assess the dependence of settling velocity and remineralization on particle size and the organic and inorganic composition of particles.

For more information on the project, see the proposal. For more information on our novel particle sampling devices and research results, see a description of our IRS sediment traps and information listed on the Publications and Presentations page.

The U.S. portion of this project is supported by the National Science Foundation's Ocean Sciences Division. MedFlux is part of the U.S. NSF Ocean Carbon Biogeochemistry (OCB) mid-size program consortium. Further information and hydrographic data from MedFlux cruises can be found at http://ocb.whoi.edu.

Monaco Harbor (by Lars Sebastian)