The Cenozoic Evolution of Atmospheric Carbon Dioxide

Cenozoic climates have varied across a variety of timescales, including slow, unidirectional change over tens of millions of years, as well as severe, geologically abrupt shifts in Earth's climatic state. Establishing the evolution of atmospheric carbon dioxide is critical in prioritizing the factors responsible for past climatic events, and integral in positioning future climate change within a geological context.  One approach in this pursuit utilizes the stable carbon isotopic composition of marine organic molecules known as alkenones.  Alkenones are organic molecules exclusively derived from specific marine algae, and have the potential to act as isotopic recorders of ambient surface-water conditions.

Paleogene pCO₂

Permanent and significant changes in Earth's climate occurred during the Eocene and Oligocene.  Whether or not pCO₂ was the primary forcing mechanism during these intervals of time is still a fundamental question in paleoclimate research. Boron-based pH reconstructions for the Cenozoic suggest that high carbon dioxide conditions (6-8x modern pCO₂) came to a gradual or an abrupt end sometime in the late Eocene (ca. 37 Ma).  However, pCO₂ records derived from stomata densities of terrestrial plant leaf remains suggest that pCO₂ during the Eocene was not substantially higher than modern concentrations. In either case, it would appear that changes in post-middle Eocene climates were driven by factors other than pCO₂, such as changes in continental elevations, oceanic circulation, and possibly sea level.

One approach toward assessing the role of pCO₂ in forcing climate change is to evaluate records of isotopic fractionation that occurred during marine photosynthetic carbon fixation.  The isotopic composition of photosynthetic marine organic carbon is primarily a function of [CO_2aq], growth rate, and cell geometry of the organism.  By sampling sedimentary alkenones from oligotrophic-type settings, the effect of growth rate and cell geometry is presumably minimized, thereby leaving [CO_2aq] as the major control on alkenone isotopic compositions. 

Our results show that pCO₂ ranged between 1000 to 1500 ppmv in the middle to late Eocene, and then decreased in several steps during the Oligocene, and reached modern levels by the latest Oligocene.  The fall in pCO₂ likely allowed for a critical expansion of ice sheets on Antarctica, and promoted conditions that forced the onset of terrestrial C₄ photosynthesis. 

Early to Middle Miocene pCO₂

The late early Miocene brackets an interval of global warming that was immediately followed by rapid high latitude cooling and expansion of the East Antarctic ice sheet in the middle Miocene. This middle Miocene climate transition is considered a major change in Earth's climatic state that was potentially driven by critically low levels of pCO₂ and/or large-scale changes in ocean circulation.

The results of this effort provide no evidence for either high pCO₂ during the late early Miocene climatic optimum or a sharp pCO₂ decrease associated with the expansion of the East Antarctic ice sheet. Instead, it is possible that tectonic and physical oceanographic factors, including the development of unimpeded flow of the Antarctic Circumpolar Current through the Drake Passage and Scotia Sea, and the restriction/cessation of flow across the Indian Ocean and eastern Tethys, exerted dominant control over the development of the early Miocene climatic optimum and expansion of the East Antarctic ice sheet.  As ice sheets expanded, the resulting increase in albedo enhanced regional and global cooling.  The lack of an appreciable ice sheet in the early Miocene with a correspondingly lower global albedo, may have promoted equable latitudinal temperature gradients to develop under low pCO₂.  However, low pCO₂ acted to prime the climate system to respond dramatically to changes in oceanography and ice albedo.

Late Miocene pCO₂ and Terrestrial Plants

Three photosynthetic pathways have evolved that allow higher plants to assimilate inorganic carbon.  The two dominant modes of carbon fixation are commonly known as the C₃ and C₄ photosynthetic pathways.  C₃-type photosynthesis is the most primitive and dominant pathway, representing the majority (~85%) of plant species (Sage et al., 1999). 

During C₃ photosynthesis a diffusive flux of carbon dioxide is directly fixed by the enzyme Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) via the Calvin cycle.  Although Rubisco has a high specificity for CO₂, it will also react with diatomic oxygen resulting in the release of CO₂.  This reaction, known as photorespiration, effectively reduces net photosynthesis. In contrast, the initial enzyme that catalyzes the photosynthetic reaction in C₄ plants  (PEP carboxylase) has a greater affinity for CO₂ than the enzyme Rubisco and is insensitive to O₂, with the result of concentrating CO₂ within their vascular bundle sheath cells where carbon fixation proceeds via the Calvin cycle.  As a consequence, CO₂ concentrations at the site of Rubisco reactions in C₄ plants are an order of magnitude higher than those found in C₃ plants (Ehleringer and Monsoon, 1993).

In summary, the C₄ pathway can be viewed as a CO₂-concentrating mechanism that enhances rates of photosynthesis by eliminating the effects of photorespiration. In addition, a variety of specific environmental conditions amplify photorespiration, providing an advantage for C₄ over C₃ plants.  Such conditions include an atmospheric chemistry with a low CO₂/O₂ ratio, high minimum temperatures, and water stressed conditions.  Presently, the distribution of C₄ grasses is strongly correlated to regions with high minimum temperatures, strong seasonal precipitation, and a wet growing season.

Given the above discussion, the Cenozoic evolution and expansion of C₄ flora potentially reflects a record of environmental and climatic change.  Positive shift in the carbon isotope composition of fossil mammal tooth enamel and paleosol carbonates has been used to infer an abrupt increase in C₄ grasses across low latitudes between 8 to 4 million years ago (Cerling et al., 1997).

My results indicate that following the expansion of ice on East Antarctica (ca. 14.5-12 Ma), pCO₂ steadily increased until about 9 Ma and stabilized at pre-industrial values (ca. 290 ppmv; generally below the threshold level required by the pCO₂-C₄ hypothesis).pCO₂ remained relatively constant throughout the late Miocene and therefore provide no evidence that changes in pCO₂ forced ecological change during this time.  More likely, the development of regional aridity on a global scale or a low-latitude change in seasonal precipitation, rather than changes in pCO₂, led to the well-documented C₄ grass expansion between 8-4 Ma.