Marine Carbon Cycle

The growth of phytoplankton in the surface ocean, and their eventual death and sinking into the deep, are a key component of the global carbon cycle. This "biological pump" transfers carbon and nutrients into the deep ocean and sediments, where it can remain sequestered from the atmosphere anywhere from years to millennia depending on where and how quickly it is degraded by microbes. If plankton degrade in the near-surface ocean, that carbon will quickly return to the surface and atmosphere. Particles that make it into the deep ocean before being remineralized can sequester CO2 for hundreds of years; get those particles into the sediments and it can easily be thousands of years or longer. There are thus many outstanding questions about the formation, sinking, and preservation of marine organic matter: where does this material come from exactly, who degrades it (and how quickly), and how does the physical and chemical environment affect the cycling of this organic matter? Stable isotopes are useful 'fingerprints' in these pursuits, allowing us to infer both the source(s) of the organic matter, and the processes affecting it.

Much of our past work has been on marine particulate organic matter (POM), bits of organic debris larger than ~0.7 µm. We collect this POM using submersible pumps like the one shown at left, deployed off of large research vessels to great depths. The filters that collect the POM are frozen at sea and brought back to the lab for us to study. Some of our early work was using hydrogen isotope measurements of lipids from surface particles to understand the isotopic composition of relatively fresh phytoplankton and bacteria. We found that the phytoplankton (mostly single-celled algae) are quite similar to plants in their D/H ratios, but the bacteria around them had significantly more D in their lipids. We went on to show that lipids in deep particles are 'heavier' (i.e. have higher D/H ratios) than their shallow counterparts, likely because the algal contributions to POM are being replaced by bacterial lipids as those particles sink. Very recently we have begun applying a new measurement that we developed – hydrogen isotope analysis of amino acids – to study these same particles. The patterns revealed by amino acids are much more complex than those of lipids, and promise to provide more detailed information about the processing of these particles.

Recently we have also turned our focus to the dissolved organic matter (DOM) in the oceans. This is, in essence, everything that won't go through our POM filters. It is low concentration but because the oceans are so big there is roughly an atmosphere's worth of carbon in marine DOM. One particularly intriguing aspect of DOM is that it is very long-lived, apparently 1000's of years based on radiocarbon dating. Why doesn't it degrade more quickly? We've been using our new sulfur-isotope methods to test the hypothesis that it is quite recalcitrant in part because it has become sulfurized by reactions with porewater H2S. In a recent paper in PNAS, we showed that the S isotope composition is not consistent with porewater as a major source of marine DOS.