Proxy Records in
Scientic Ocean Drilling
Over the past 40 years, a wide range of climate proxies that measure different components of marine sediments have been developed for use as tracers of past changes in climate and ocean circulation. These proxies include variations in plant and animal species abundances, which track past changes in environmental conditions at a specific location, as well as evolutionary distributions of fossils, which provide important age control and stratigraphic markers for correlation. Isotopic and geochemical measurements of fossil shell material provide information on past oceanographic conditions (e.g., temperature, salinity), past ocean chemistry (e.g., pH, carbonate ion concentration, deepwater mass circulation), and the concentration of paleo-atmospheric CO2 (boron isotopes and boron-calcium ratios). More recently, chemical measurements of fossil organic compounds have been developed to reconstruct sea surface temperatures (e.g., alkenone saturation ratios, long-chain tetraethers), partial pressure of CO2 (pCO2; e.g., alkenone isotopic chemistry), and hydrology (e.g., leaf wax biomarkers such as compound specific deuterium measurements on alkanes).
A number of proxies are now well established and routinely applied (e.g., stable oxygen and carbon isotopic ratios of foraminiferal calcite), while others are still in a more developmental state. All proxies must deal with various levels of associated uncertainty due to a lack of knowledge regarding precise relationships between the proxy and the environmental characteristic being measured (e.g., some pCO2 and sea surface temperature proxies), particularly when applied to older sediments. Nevertheless, as laboratory studies continue and calibrations improve, there has been significant convergence between different proxies used to estimate temperature and pCO2 (e.g., Beerling and Royer, 2011). This explosive growth in the number, type, and utility of proxies has led to a significantly better understanding of past global environmental conditions. The combination of physical measurements of past temperatures with chemical measurements indicating past atmospheric CO2 concentrations has been particularly valuable for understanding the sensitivity of the climate svstem to CO2 forcing.
The reconstruction of Cenozoic surface temperature distributions and their relationship to changes in atmospheric CO2 concentrations have been among the most important contributions of scientific ocean drilling to paleoclimate studies. Climates prior to 3 myr (particularly the past 65 myr) were generally warmer than today, and were associated with higher pCO2 levels. Reconstructions show a long-term decrease in global average temperatures, from a maximum of about 26 °C in the early Eocene Epoch (~ 50 Ma) to a pre-industrial Holocene average value of 14 °C. This pattern of global cooling is associated with declining pCO2 levels, from 2,000-4,000 ppm range in the Paleocene and Eocene to less than 400 ppm by ~24 myr (Pearson and Palmer, 2000; Pagani et al., 2005b; Beerling and Royer, 2011). Studying warm climate extremes recorded in ocean sediments enable new insights into Earth system responses to elevated greenhouse gas levels.
Scientific Accomplishments and Significance
Scientific ocean drilling has significantly contributed to the recognition and quantification of latitudinal differences in temperature in response to pCO2 (Figure 4.1) and other high-latitude Earth system feedbacks (e.g., sea ice albedo) that can lead to polar temperature amplification (e.g., Dowsett, 2007; Huber, 2008; Bijl, 2009). Sea surface temperatures reconstructed from globally distributed drill cores have demonstrated that the early Eocene (55-48 Ma) had the warmest climates of the past 65 myr, depicting a world that was ~10-12 °C warmer and with greatly reduced latitudinal temperature gradients compared with the present day (Bijl et al., 2009; see Figure 4.1).
Cores recovered from the Arctic (ODP Legs 151, 163; IODP Leg 302; Figure 4.2a) and the Antarctic (ODP Legs 113, 119, 120, 188, 189; IODP Leg 318; Figure 4.2b) indicate that polar regions of the greenhouse world could support only small terrestrial ice sheets, or had limited perennial sea ice (Moran et al., 2006; Stickley et al., 2009). This finding implies global sea levels more than 60 m higher than the present, when atmospheric CO2 levels may have been as high as 2,000-4,000 ppm (Pagani et al., 2005b; also discussed in the following section).
Observations of past warm extremes are important for evaluating the performance of climate models in response to higher levels of pCO2 (Huber and Caballero, 2011, and references therein). Although amplification of polar warming during the past warm periods appears to be underestimated by the current generation of climate models, the sensitivity of past tropical temperatures is generally overestimated relative to proxy-based temperatures from scientific ocean drilling. Comparisons between models and paleoenvironmental observations from drill cores play an important role in evaluating the performance of Intergovernmental Panel on Climate Change (IPCC) climate models that simulate warmer global climates.
A key discovery of the Paleocene-Eocene greenhouse world (55-50 myr) was the potential of the climate system