Towards closing the Eocene Astronomical Time Scale Gap: Cyclostratigraphic Implications from IODP Expedition 342

Abstract:

Astronomical tuning using the 405-kyr eccentricity component as a prime target has been established as a standard technique for the Cenozoic timescale. There is an astronomically-tuned time scale across most of the Cenozoic. However, no definite astronomical tuning exists in the so-called “Eocene gap”. The main reason for the existence of this Eocene gap is the very shallow carbonate compensation depth (CCD) during this time (Pälike et al., 2012), preventing the deposition of cyclic carbonate-rich sediments. IODP Expedition 342 however drilled Eocene cyclic carbonate-rich sequences, which were deposited in sediment drifts offshore Newfoundland at a palaeodepth above the CCD while large parts of the ocean were starved of carbonate. As the variations in the studied sites are predominantly obliquity induced, we assess and use the 173-kyr obliquity amplitude modulation cycle as a tuning target. These 173-kyr cycles result from the resonance or “beat” between the (present-day) 41 and 53 kyr obliquity cycles, determined by the precession constant and the frequency of the ascending node precession of the Earth and Saturn (p+s3)-(p+s6). We use physical property measurements, as well as the ratio between the elemental intensities of calcium and iron (Ca/Fe) of the sediments from sites U1408, U1409 and U1410 to construct an astronomically-tuned chronology between (and including) magnetochrons C19r and C22n (~41 – 50 Ma), spanning the “Eocene gap”.