BOX 4.1 Determining Time in the Geological Record
One of the biggest challenges in using the deep-time record for understanding Earth systems is determining the rates of processes and dating when specific events occurred. Determining rates requires very precise time control, particularly if the processes being studied occur at an ecological timescale (1 year to several centuries). One method for such precise time control is by using annually layered sediments in ancient anoxic or hyper-saline basins, as long as a few age control points are present. Examples include laminated sediments from the Pleistocene Santa Barbara Basin (Figure 4.1a), Eocene sediments of the North Sea (Figure 4.1b) (Schiøler et al., 2007), black shale sequences in the Cretaceous North Atlantic (Figure 4.1c), and the Permian Castile Formation of West Texas (Anderson, 1982). Quite highly resolved relative timescales can also be achieved using cyclic sequences, with resolutions of a few centuries to several tens of millennia, based on the identification of distinct orbital periods (Figure 4.1d; Figure 4.2). Both annual layers and orbitally tuned records can reveal ecological dynamics, snapshots of natural variability at different parts of Earth history, and the duration of threshold shifts in ecosystems.
Orbital cycles can sometimes be tied to astronomically tuned timescales during the past 40 million years to provide excellent absolute timescales. One example is the tuning of orbital cycles in glacial events during the Oligocene using combined geochemical and sediment property cycles tied to an astronomically calibrated timescale (Pälike et al., 2006a,b; see Figure 4.2). Orbital cycles have been recognized far back in the Phanerozoic sedimentary record and, together with high-resolution U-Pb dating, offer the potential to reconstruct Earth system dynamics in great detail (Erwin 2006; Davydov et al., 2010).
et al., 1993; Olsen and Kent, 1996; Eriksson and Simpson, 2000; Loope et al., 2001, 2004; Ivany et al., 2004; Wagner et al., 2004; Elrick and Hinnov, 2007; Jahren and Sternberg, 2008; Kennedy et al., 2009). The ability to precisely and accurately quantify geological time has improved dramatically with recent advances in radiometric dating and interlaboratory cross-calibration (e.g., the EARTHTIME initiative) permitting unprecedented temporal resolution (e.g., ID-TIMS [isotope dilution-thermal ionization mass spectrometry] uranium-lead [U-Pb] ages with analytical uncertainties of ≤0.01 percent; Ramezani et al., 2007). Some recent radiometric calibrations of the sedimentary record integrate astrochronology, providing Milankovitch-scale resolution through long intervals of time