Project Summary

February 15, 2007  (OCB DMO)

Project Summary

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 particulate organic carbon (POC) is returned to inorganic form and redistributed in the water column. This redistribution determines the surface concentration of dissolved CO2, and hence the rate at which the ocean can absorb CO2 from the atmosphere. The ability to predict quantitatively the depth profile of remineralization is therefore critical to predicting the response of the global carbon cycle to environmental change.

We hypothesize that minerals produced by organisms, or introduced into the surface ocean by winds, critically influence carbon export to the deep ocean and sediments. Minerals typically constitute more than half the mass of sinking particles, and are important for making less dense organic matter sink. Minerals may also protect organic matter from degradation, allowing it to penetrate deeper into the ocean. We recently demonstrated (a) that ratios of particulate organic carbon to mineral ballast converge to a nearly constant value (~6 wt% OC) at depths >1800 m, and (b) that decreases in flux of over two orders of magnitude are attended by minimal changes in bulk organic composition. Because these patterns are the hallmark of physical protection, we hypothesize that a substantial fraction of particulate organic matter raining through marine water columns is protectively associated with mineral grains. Thus, the types and amounts of mineral ballast introduced to the surface ocean may be critical, although largely overlooked, determinants of the ocean’s ability to take up and store bioactive elements

We propose here a multi-tracer approach to explicitly consider different ballast types, along with the associated organic matter and radioisotopes. Hypothesis 1 is that ballast minerals physically protect a fraction of their associated total organic matter, which persists to predominate over the unprotected fraction in the lower (>1000 m) part of the water column. Understanding the mechanistic basis of such processes will require an understanding of organic-mineral interaction at the compound-specific level. Hypothesis 2 is that the ratio of organic carbon to ballast is key to predicting variability in the export fluxes and sinking velocities of organic carbon as estimated using radiotracers.

Our overall goal is to develop a seamless description of carbon fluxes and associated mineral ballast fluxes throughout the water column. To achieve this goal, we propose to measure simultaneously a suite of properties that are thought to be indicative of fluxes. We will synthesize these measurements from the top of the water column to the sediments using a variety of modeling and statistical techniques. Our strategy is to unite the power of several disciplines: (i) organic geochemistry for characterizing organic matter in protected and unprotected forms and determining its degradation state; (ii) radiochemistry for assessing processes and time-scales involved in particle dynamics and transport; (iii) zooplankton ecology for assessing radioisotope partitioning and organic biomarker alteration; and (iv) microbiology for its role in organic matter decomposition, and (v) modeling and statistical analyses to provide a process-based model of flux out of the euphotic zone to the sea floor.

funding is provided by the U.S. National Science Foundation