Abstract:
Knowledge of the evolution of atmospheric carbon dioxide concentrations
throughout the Earth's history is important for a reconstruction of the links
between climate and radiative forcing of the Earth's surface temperatures.
Although atmospheric carbon dioxide concentrations in the early Cenozoic era
(about 60Myr ago) are widely believed to have been higher than at present,
there is
... disagreement regarding the exact carbon dioxide levels, the timing of
the decline and the mechanisms that are most important for the control of CO2
concentrations over geological timescales. Here we use the boron-isotope
ratios of ancient planktonic foraminifer shells to estimate the pH of
surface-layer sea water throughout the past 60 million years, which can be
used to reconstruct atmospheric CO2 concentrations. We estimate CO2
concentrations of more than 2,000 p.p.m. for the late Palaeocene and earliest
Eocene periods (from about 60 to 52 Myr ago), and find an erratic decline
between 55 and 40 Myr ago that may have been caused by reduced CO2 outgassing
from ocean ridges, volcanoes and metamorphic belts and increased carbon
burial. Since the early Miocene (about 24Myr ago), atmospheric CO2
concentrations appear to have remained below 500 p.p.m. and were more stable
than before, although transient intervals of CO2 reduction may have occurred
during periods of rapid cooling approximately 15 and 3 Myr ago. 
