Science Goals of Cruise:
The primary scientific goal of the SNOWBIRDS project is to improve reconstructions of the surface nitrogen (N) and silicon (Si) nutrient contents, based on N and Si isotope proxies, across the Southern Ocean.
Because N and Si are essential nutrients for diatoms, the most important primary producer in the Southern Ocean, their isotope records provide key constraints on the role for ocean biology and ocean circulation in regulating atmospheric carbon dioxide in the past.
We aim to improve the N and Si isotope proxy reconstructions with three parallel lines of inquiry:
1). quantification of in situ isotopic relationships across the entire Southern Ocean;
2). community-scale culture experiments from key regions to determine characteristic isotope effects for a community (rather than an individual);
3). mono-specific laboratory growth experiments with organisms collected at sea to determine individual species related isotope effects;
4). alteration of sedimentary N and diatom N and Si isotope signals recorded during early burial.
At sea, we will go to 13 sites along 170°W (see map) to collect materials for analysis back in our labs at URI and UCSB. We will collect water, particles, and surface sediments and package them up for return home. We will also use some of the water we collect for growth experiments and measurements while on the ship. At sea, we will measure nitrate, silicic acid, and chlorophyll contents of waters from the near surface, as well as from the growth experiments. We will also look at the diatom communities under the microscope. These diatom identifications and nutrient data will help us to monitor the conditions we are encountering along our trip.
The main outcomes of the project will be:
1). a catalog of species-specific culture-based estimates of the degree of Si and N fractionation when diatoms make their frustules;
2). assemblage-based estimates of these fractionations for the important zones of the Southern Ocean;
3). a greater understanding of how the culture results relate to in situ observations and sedimentary reconstructions.
The Southern Ocean and Atmospheric CO2
Atmospheric CO2 concentration is rising as a result of fossil fuel use by humans. Ice core records of atmospheric CO2 concentrations show us that it has varied significantly in the past as well. With this work, we are seeking explanations for natural CO2 variation.
In particular, we are interested in the roles of:
1. ocean circulation and
2. ocean biology in regulating atmospheric CO2.
We concentrate our efforts on the Southern Ocean, around Antarctica, because this region is in direct contact with the deep ocean where C can be stored away from the atmosphere for hundreds to thousands of years.
In addition, Southern Ocean biology has the capacity to take up more CO2, via the biological pump. At present, biology does not consume all of the major nutrients supplied to the surface ocean. If it did, more CO2 would be drawn out of the atmosphere and potentially delivered to the deep sea.
1. The full record of atmospheric CO2 measurements made by Scripps University at Mauna Loa, Hawaii (1957-2016).
2. Vostok ice core records of atmospheric CO2 (top), dD (middle- a record of atmospheric air temperature in Antarctica), and insoluble dust (bottom). These data show the natural variation of atmospheric CO2, between 180 and 280 ppmv. Atmospheric CO2 was at a minimum during peak glacial periods and maximum during interglacials.
3. Generalized cross section of vertical and lateral circulation in the Southern Ocean and the role of Southern Ocean in setting nutrient content of the low latitude thermocline (from Sarmniento et al., 2004).
4. A schematic of the biological pump, where phytoplankton (microscopic, floating plants) take up nutrients and CO2 in the presence of sunlight to produce energy and build biomass. When these organisms die, this biomass sinks out and some fraction is delivered to the deep ocean. This “pump” moves carbon from the surface ocean into the deep. It has hypothesized that the Southern Ocean biological pump is enhanced by the addition of iron, and that the dustiness of the glacial periods may have stimulated the drawdown of atmospheric CO2.
Rebecca Robinson at work
Credit: Jens Gruetzner