365 Days of Climate Awareness
11 - The Paleocarbon Record
Tracking the atmospheric concentration of CO2 through the earth's existence is nearly as important as tracking global mean temperature, in order to reconstruct overall conditions and processes. The discipline of geology is founded on the idea of uniformitarianism: that the processes active today are the same ones active throughout the earth's past. So we look for evidence of carbon dioxide's role in earth's changing climate of past millennia in order to better estimate what will happen in the future.
Paleo-carbon dioxide data can be found directly for the last 800,000 years in the air bubbles trapped in Antarctic ice cores, and via proxy data. The main source for paleo-CO2 proxy data is through the ocean floor drilling program, where cores of unconsolidated (i.e. still loose, not lithified) ocean sediment are drilled, recovered and analyzed. Data from the last 65 million years--corresponding to the end of the Cretaceous, when the Chicxulub meteor struck the Gulf of Mexico and ended the era of the dinosaurs--is analyzed for proxy chemical signatures to reconstruct the ancient history of atmospheric CO2.
The main type of proxy data is the ratio between two isotopes of boron (B), 10B (typical) and 11B (rare), known as d11B. Boron is found in the carbonate shells (tests) of microorganisms called foraminifera (forams). The ratio between boron isotopes (d11B) depends on the acidity of seawater, and the acidity of seawater is directly affected by the CO2 concentration in the atmosphere. A higher atmospheric CO2 concentration means a lower oceanic pH (tomorrow's post), which means a lower value for d11B in the microscopic fossils.
Another type of proxy data is the d13C record, the ratio of 13C to 12C isotope, in the remains of ancient algae. However, the d13C measure is sensitive to a wide range of influences beyond temperature and pH, so that record remains a work in progress.
Tomorrow: carbon dioxide and ocean acidity.
Be well!
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