Global Biogeochemical Cycling of C, N, & O

Observations of physical/biological/ecological coupling on local to basin-sized spatial scales give insight into how these processes contribute to global biogeochemical cycles. In recent decades, major advancements in estimates of phytoplankton biological production (NPP) from satellite measurements of ocean color on a global scale. However, since there is still no method to estimate rates of organic matter remineralization from space, we must rely on ecosystem models and empirical algorithms to predict carbon export rates. So far, these estimates have not been very reliable.

There are three main reasons for this: 1. Submesoscale ‘patchiness’ in biological production and export, 2. Difficulty imaging the subsurface euphotic zone, and 3. Export efficiency varies in time and space. Much of my current and future work is/will be aimed at reconciling these issues. The figure below is an illustration of current approaches (left) and the possible contribution of future advancements in satellite imagery and autonomous technology (right) that could help solve these fundamental challenges.


One contribution I have made resulted in a paper published in GRL. Using measurements of wind speed and density in the oligotrophic South Pacific Gyre, my colleagues and I derived a practical method to estimate vertical turbulent mixing rates of O2/Ar and the three isotopes of oxygen (16O, 17O, 18O) across the base of the mixed layer. By having this transport term, we demonstrated that biological production and export are decoupled vertically in this region, indicating that most production that is available for export does not occur in the surface mixed layer, but instead in the deep euphotic zone, a region that is difficult to image by satellites. This finding, if applicable to large oligotrophic regions of the world’s oceans, highlights the need for further consideration of deep euphotic zone production and surface ocean ecology in algorithms used for satellite-derived export estimates.

Profiling biogeochemical floats are another tool we use to study the vertical structure of biogeochemical parameters in the water column in remote parts of the ocean. They are especially useful because of their long endurance (~ 5 years) and ability to transmit data remotely via satellite. Below is a map of six profiling floats that have been in deployed in the North Pacific near Station Papa (yellow diamond) since 2009. In 2018-2019, I collaborated with Andrea Fassbender at MBARI to use measurements of O2, NO3-, and pH from these floats to study the rates of carbon export over seasonal to annual timescales in collaboration with the NASA Exports campaign. NEPacMap.png

Collaborators: Andrea Fassbender (MBARI), Maria Prokopenko (Pomona), Angela Knapp (FSU), Nicholas Nidzieko (UCSB), Doug Hammond (USC), Will Berelson (USC), Naomi Levine (USC)