Tuesday, November 16, 2010
Sharpening the tools in the carbon dioxide garden shed
This weeks Smith Lecturer is Barbel Honisch, a tenure-track geochemist at Lamont-Doherty Earth Observatory (Columbia University). Lamont is center for research on Earth and ocean processes with a strong emphasis on climate. Like last weeks speaker Honisch is both German (which is irrelevant) and interested in reconstructing past ocean pH and atmospheric CO2 levels. Honisch was initially trained as a marine biologist, so she approaches ocean history questions with biological in mind. She is specifically interested in using boron isotopes as an indicator of past seawater pH. To check if calcium carbonate organisms are recording the pH of the seawater they float in correctly, Honisch has grown these organisms (foraminifera) in the lab.
Boron isotopes in marine carbonates have the potential to provide us with information about past ocean carbonate chemistry, as the boron isotopic composition of marine carbonates is primarily controlled by the pH of seawater. The boron isotope paleoacidity indicator is that the uncharged species B(OH)3 is enriched in 11B by 20% over the charged species B(OH)4-. As the fraction of boron changes with pH, so must the isotope.
11B(OH)3 + 10B(OH)4- double arrow 10B(OH)3 + 11B(OH)4-
Boron isotopes alone provide us with only one parameter (i.e. pH) of the marine carbonate system. For accurately translating boron isotope data into pH values and subsequently for calculating other parameters of the carbonate system such as aqueous PCO2, we need additional information on temperature, salinity and a second carbonate parameter such as carbonate ion concentration or alkalinity (the amount of carbonate ion).
Logic diagram shows how seawater pH (on the left) influences the ratio of stable isotopes of boron in corals that grew in that water (on the right). From Chacko (2009)
Application of the boron isotope pH proxy to the late Pleistocene ice ages has led to a convincing estimation of surface ocean pH that can be compared to atmospheric pCO2 as recorded in ice cores. Now Honisch is in the process of extending the pH and pCO2 reconstructions beyond the reach of ice records, into the Cenozoic era. The first application beyond the reach of ice core CO2 measurements focused on the mid-Pleistocene transition when the ice ages became significantly longer in length and duration. Her results suggest that although CO2 and climate were tightly coupled, the climate transition was not driven by an overall decrease in atmospheric CO2 that could have cooled the planet.
Video about growing 'pets' that will tell you about climate change through time.