Temporal shifts in terrestrial uptake of atmospheric CO2

The Sarmiento Group’s plan is to continue to work with statistical approaches in order to understand the 1988/1989 shift in the net land carbon uptake and also to search for other abrupt shifts by applying change point detection to other key data sets and model outputs. The improved understanding of this shift will help for the prediction of future shifts in the carbon cycle.


Constraining ocean uptake of anthropogenic CO2

The uncertainty in estimates of anthropogenic carbon uptake is often large compared to any yearto-year trend. In the coming year, one goal of the Sarmiento Group will be to apply inverse methods to the problem of ocean carbon uptake in an effort to understand those uncertainties. These methods will combine information from a recently available surface pCO2 database with estimates of the carbon system from ocean general circulation and biogeochemistry models to provide a new estimate of the annual carbon flux over the last few decades as well as a quantitative measure of the uncertainty in those fluxes. Such a method could then be extended to include interior data, further our understanding of the ocean carbon problem ,and provide greater understanding of carbon uptake predictions under climate change.


Impacts of ocean acidification

A focus of the Morel Group’s continuing work is the CO2 concentrating mechanism of marine phytoplankton, including an estimation of the energetic cost of the process and the variability of the underlying cellular mechanisms among taxa. In addition, the researchers plan to undertake a study of the effect of ocean acidification on nitrogen fixation by marine cyanobacteria. This work will be done with Trichodesmium, the dominant N2 fixer in the oceans, which fixes N2 and CO2 simultaneously. Because N2 fixation requires large quantities of iron, its response to ocean acidification is made complicated by the dual (and likely opposite) effects of low pH on iron availability and high CO2 on photosynthesis.

As ocean acidification is expected to make the seawater less hospitable to calcium carbonate building organisms, it is likely to feed back on the ocean’s ability to absorb anthropogenic carbon dioxide. The ability to predict future interactions between the ocean’s carbonate pump and acidification highly depends on better mechanistic understanding of the present-day carbonate pump. To this end, the Sarmiento Group will constrain model formulations and parameterizations that control calcium carbonate cycling through the water column and sediments using a wide range of observations. The team will also assess the uncertainty associated with the constrained model formulations and parameterizations to help increase our ability to predict the future carbon cycle.


Impacts of CO2 on ocean oxygen distribution

In 2011, the Sarmiento Group plans to expand research on carbon remineralization and oxygen in the ocean, and address the following research questions using a combination of a 1-D biophysical model and 3-D general circulation models. A 1-D model of bacterial extracellular enzymes is in development and will explore the depth and timing of carbon remineralization and oxygen utilization. In addition, the Group plans to assess the performance of a set of fully coupled biogeochemical atmosphere-ocean general circulation models in representing present-day oxygen minimum zones, identify the impact of model resolution on oxygen minimum zones, and provide a projection of O2 changes in oxygen minimum zones over this century using global warming simulations. Biological studies using 3-D models will include an analysis of the depths of the upper and lower oxyclines, which are boundaries restricting the movement of organisms that can not survive in low oxygen conditions.


Southern Ocean biogeochemistry

The Bender Group will continue its studies of Southern Ocean biogeochemistry, focusing on measuring rates of net community production and organic carbon export as part of a broader collaborative effort on the South African icebreaker, Agulhas. They will continue to analyze the results to understand controls on the fertility of ocean ecosystems. They will also deploy the new high-precision instrument for measurements of dissolved inorganic carbon, beginning continuous observations of the baseline DIC concentration of Southern Ocean surface waters.


Ice age reconstructions of polar ocean conditions

A new opportunity has arisen for the reconstruction of changes in polar ocean conditions over the most recent ice age cycles. Recent graduate student Abby Ren in the Sigman Group has developed a method to measure nitrogen isotopes of the minute amount of organic matter trapped within the calcium carbonate walls of foraminifera, ameboid zooplankton whose shells are an important paleoclimate tool. Ren has validated the utility of this tool for reconstructing biogeochemical conditions in the tropical and subtropical ocean and has generated records from two sediment cores, one in the Atlantic and one in the Pacific, that clarify the operation of the ocean nitrogen budget over ice age cycles.

Based on Ren’s work, the Sigman Group is now ready to progress into polar ocean regions with this new tool, with the central goal of reconstructing surface nutrient concentrations in each of the major polar regions during the last ice age: the Southern Ocean, the North Pacific, and the North Atlantic. The foraminiferal data would represent a constraint that is largely independent from that of the diatom microfossil-based methods that Sigman’s team has used previously. Given the results from the diatom work, it is possible that the foraminiferal data will be the last piece needed to confirm our hypothesis of polar ocean stratification and nutrient drawdown during cold climates, or to preclude it. This would be a major step forward in the effort to (1) understand the cause and effect of CO2 changes in the ice age cycles and (2) use the past as a predictor of how the polar ocean will respond to ongoing climate change. This work fits into the larger CMI science theme of the Southern Ocean as a critical unknown in the future trajectory of the carbon cycle and climate.


Ice core studies of greenhouse gas concentrations prior to 800,000 years ago

Michael Bender’s research in paleoclimatology for the coming year will focus on the argon isotope dating of new ice cores, collected in recent months, from the Allan Hills, Antarctica. If ages greater than 1 million years are confirmed, they will begin analyzing greenhouse gas concentrations in the trapped air of these samples, thereby extending back in time the record of the relationship between greenhouse gas concentrations and global climate. Whatever the age, these samples represent old ice at the surface that can be sampled in great detail, and the team will measure gas properties in addition to argon isotopes that clarify the conditions under which the ice formed, and allow the age to be determined more accurately than is possible based on Ar isotopes alone.