Award: OCE-1634250

Carbon is an important element when considering its role in controlling Earth's climate. As a gas in the atmosphere, it can absorb energy that would otherwise radiate to space. As such, scientists are interested to know the sources, fates and cycling of carbon through the ocean, the atmosphere and the terrestrial biosphere. The ocean contains a large amount of carbon, and much of this carbon exchanges with the atmosphere as CO2. There is also a large pool of organic carbon in the ocean, which is produced by ocean plants and consumed by ocean bacteria. Some of that organic carbon survives for long periods of time (thousands of years), so it is the controls on that surviving pool of carbon that we sought to understand in this work. More specifically, our attention was on the carbon that resides in the greatest depths of the ocean. How did it arrive there, how does it survive (avoid consumption by bacteria), and what is its fate? To better understand this carbon, we conducted measurements of organic carbon at several locations in the eastern North Pacific. A research vessel occupied the locations of interest 3 times each year; it was the variability in carbon concentrations as a function of seasons, as observable with these occupations, that informed us about the controls on that system. We found during spring and summer, when surface ocean plant growth was greatest, that deep organic carbon concentrations were enhanced. These enhancements were due to the sinking of that plant material from the surface to the deep ocean, where the plant carbon would then be found at great depth. This material was apparently consumed by deep microbes within months of arrival, given that the carbon concentrations were again low by the time of the following winter months. With this work, we were able to see upper ocean carbon be converted to plant biomass, which then sank to the deep ocean as dead organic carbon, and then be present in the deep ocean until ultimate removal by bacteria. This annual process of deep ocean organic carbon enrichment and removal had not been observed previously, so new knowledge has been developed. Last Modified: 12/02/2019 Submitted by: Dennis A Hansell