Astrochronology of the Cenozoic Era: A Critical Review
The global stratigraphic record of Milankovitch cyclicity within oceanic sediments has been compiled for almost 100% of the Cenozoic Era. This cycle stratigraphy record provides a continuous “Astronomical Time Scale (ATS)” for the Era that includes integration with magnetic stratigraphy, biostratigraphy and geochronology from formations in the western Equatorial Atlantic Ocean and Mediterranean Sea. This ATS has been correlated to a high-precision model of the Earth’s astronomical parameters, the nominal La2004 astronomical model (Laskar et al., 2004), thereby placing numerical ages on each major cycle and calibrated event. The building of this ATS took place over many years, including the landmark “Astronomically Tuned Neogene Time Scale (ATNTS)” with a resolving power finer than ~20,000 years through the majority of the Neogene Period that was implemented in the Geologic Time Scale 2004 (GTS2004) (Lourens et al., 2004). Since 2004, additional astrochronologic scales developed from extended cyclic sedimentary sequences drilled by the Ocean Drilling Program have closed gaps in the early Neogene and extended the scaling to the base of the Paleogene Period. The only significant remaining gap is an 11 million year interval spanning the Early Eocene Climatic Optimum (42 to 53 Ma), which is the focus of IODP Expeditions 320-321 in the equatorial Pacific (during the time of this AAPG meeting). Despite this gap, the early Paleogene ATS is anchored in absolute time by using recalibrated Ar-Ar ages for the base-Cenozoic that are tied to longer-term 405-kyr astronomical cycles. In early 2008, the pan-European Earthtime-EU Project initiated activities to further improve the Cenozoic ATS. This involves, among other things, fine-scale ATS inter-calibration with high-precision radioisotope dating, and advanced modeling of geodynamic variables (e.g., tidal dissipation) that are used in the astronomical solution. The community goal of a robust ATS with calibrated ages for all biologic, magnetic and geochemical events through the entire Cenozoic is nearing fruition. Selected references: Lourens, L, Hilgen, F, Shackleton, NJ, Laskar, J, Wilson, D, 2004, Chapter 21, in Gradstein, F, Ogg, J, Smith, AG, eds., Cambridge Univ. Press, 409-440; Laskar, J, Robutel, P., Joutel, F, Gastineau, M, Correia, ACM, Levard, B, 2004, Astron. Astroph., 428, 261-285.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009