Project: US GEOTRACES GP17-OCE: Evaluating Southern Ocean Control of Global Marine Si Isotope Distribution

NSF Award Abstract:
The Southern Ocean around Antarctica is connected to the rest of the global ocean through the meridional overturning circulation. That circulation moves water from the Southern Ocean through the tropics and all the way into the northern hemisphere. Those waters carry nutrient chemicals that fuel the growth of phytoplankton throughout the global ocean whose photosynthetic carbon fixation is a major factor in setting how much atmospheric carbon dioxide is removed by ocean biology. Diatoms are a key group of phytoplankton involved in this process of carbon uptake and they are especially abundant in the Southern Ocean. Diatoms are unique among the phytoplankton because they need the element silicon to grow. They take the silicon (Si) that is dissolved in seawater and use it to produce a shell made of opal called a frustule. In this project we are interested in how the availability of dissolved Si, as set by the Southern Ocean, controls the distribution and abundance of diatoms in the sea and their contribution to ocean biogeochemistry such as the uptake and removal of carbon. We will use a relatively new tool to trace diatom dynamics – silicon isotopes. Variations in silicon isotopes in the dissolved Si in seawater and in diatom frustules can inform the level of diatom growth that has occurred in ocean waters and how diatom growth is coupled to ocean circulation and nutrient transport. The distribution of isotopes of silicon between Tahiti in the tropics to the Southern Ocean will allow us to better understand how the physics, chemistry and biology of the Southern Ocean controls diatom activity at global scales. The research will involve an early career scientist and undergraduate students at UCSB. The PIs will also reach out to regional K-12 schools through UCSB’s Research and Education Experience Facility (REEF) by developing curricula on Southern Ocean biogeochemistry and the GEOTRACES program.

Silicon isotopes will be measured in full ocean depth profiles along US GEOTRACES GP17OCE from Tahiti, French Polynesia to Punta Arenas, Chile. It is now clear that successful application of the Si isotope proxy in the modern ocean and for paleoclimate reconstructions requires a mechanistic understanding of how the silicon isotopic composition of ventilating waters masses varies in time and space. Our goal is to test hypotheses related to predictions from models that processes in the Southern Ocean dictate silicon isotopes distributions throughout the global ocean. That prediction arises because the Southern Ocean is the central hub of the meridional overturning circulation. The interplay between silica production in Southern Ocean surface waters, opal export and the Southern Ocean counter current meridional circulation traps silicic acid in the Southern Ocean while partitioning and redistributing Si isotopes between mode and deep waters. Heavy Si isotopes are distilled out of the Southern Ocean in relatively shallow mode waters while light isotopes ride the northward flow of deep waters. Individual aspects of these predictions have been examined previously; however, measurements of Si isotopes in GP17OCE will provide the first section that samples all relevant water masses synoptically. Those distributions together with other data collected on GP17OCE will allow a more comprehensive evaluation of these mechanisms partitioning Si isotopes in the Southern Ocean and their redistribution in the overturning circulation more globally.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.