Physical Processes Leading to Export of Fixed Carbon Out of the Surface Ocean

• Main Author: Erickson, Zachary K.
• Format: Thesis
• Language: English
• Published: California Institute of Technology 2019
• Subjects: carbon cycle; Environmental Science and Engineering; oceanography; Drake Passage
• Online Access: https://dx.doi.org/10.7907/xdx7-8j36; https://resolver.caltech.edu/CaltechTHESIS:06092019-160257514

The ocean sequesters carbon on long time scales by depositing it deep in the ocean, where it is no longer in contact with the atmosphere. This sequestration is also termed "carbon export", and is accomplished via a vertical flux of carbon into the interior of the ocean. Marine photosynthesis by phytoplankton, which consume carbon dioxide dissolved in the surface ocean and are transported to depth to be eventually remineralized or form sediments at the ocean surface, is a key component of this flux (the biological pump). This mechanism is primarily thought to occur via sinking of particulates. However, research over the past few decades has highlighted the role of instabilities at the "submesoscale", or 0.1--20 km, to induce large, O(100 m day-1) vertical velocities in the ocean. These vertical velocities can potentially subduct carbon from the surface ocean into the interior, where it would contribute to export. Observations of the ocean are, however, rarely made at scales which would detect these submesoscale instabilities. In this thesis, I use in situ observations from autonomous underwater vehicles, Seagliders, which make measurements in the upper 1000 m of the water column at horizontal scales of 1-3 km, to understand when and where submesoscale instabilities are present, and the extent to which they act to transport biologically fixed carbon out of the surface ocean. Three different types of instabilities are active in the surface mixed layer: baroclinic, gravitational, and symmetric. Each of these has potential to subduct material below the mixed layer; however, these instabilities are generally strongest during the winter, when biological production is at its minimum. An interesting exception is in southern Drake Passage, where interactions between the intense frontal system and the continental shelf result in subduction of water masses off the continental shelf during summer, when phytoplankton are photosynthesizing. In general, however, carbon export via submesoscale instabilities is expected to be largest during spring, when phytoplankton become more productive but conditions can still be ripe for submesoscale subduction. Scaling up these observations to the global ocean system is difficult because in situ observations at submesoscales are sparse. This thesis explores the ability of surface flux measurements, from reanalysis products and remote sensing measurements, to accurately depict carbon export via subduction processes by modeling the water profile in a one-dimensional model following Lagrangian floats in the ocean. This approach holds promise to advance the ultimate goal of determining the global effect of submesoscale-driven carbon export.