Sarah Rosengard


Primary producers fix atmospheric CO2 to synthesize organic molecules.  In the ocean, this organic matter can either decompose back into CO2, or sink towards the deep sea as particles. The small fraction of particulate organic carbon (POC) that enters the deep sea resides there for hundreds to thousands of years, an important long-term carbon sink in the climate system.  With guidance from my advisors Valier Galy and Phoebe Lam, I explore different factors that preserve POC during transit towards the deep sea.  My investigation tracks the transport of POC from two globally significant carbon reservoirs into the ocean: the Amazon River, which exports primarily terrestrial organic matter to the Atlantic Ocean, and the Southern Ocean, which exports marine organic matter into the deep sea.
Since 2011, my advisors and I have collected samples of POC during several field trips to the Southern Ocean and the Amazon River Basin (to read more about the field work, visit my blog:  Back in lab, I characterize the composition of organic matter from these two sites using a range of techniques as broad as bulk mineral analysis and as specific as lipid identification.  These analyses shed light on the source of organic carbon in the particles and its stability/degradation state.
So far, our measurements from the Southern Ocean suggest that different primary producer communities are tied to different POC export regimes and may influence how POC degrades through the mesopelagic zone of the water column.  I am now beginning to investigate analogous questions with terrestrial-derived organic carbon in the Amazon River, as it moves towards the coastal Atlantic Ocean.  Ultimately, I hope that investigating the relationship between primary producer communities and organic carbon preservation may improve our understanding of how human-caused shifts in algae or terrestrial plant communities could, in turn, shift the transport of organic carbon in the ocean.

(profile photo credit: © Chris Linder)