Milankovitch and sub-Milankovitch millennial-scale cyclicity in Middle Eocene deep-marine laminated sediments, Ainsa Basin, Spanish Pyrenees

Climate variability on the scale of millennia is conspicuous within Quaternary records but has, until recently, been less well documented in records from deeper geological time. Here we utilise multi-element x-ray fluorescence (XRF) data obtained from a Middle Eocene (Lutetian) sediment core (Ainsa basin, Spanish Pyrenees) to identify both Milankovitch and sub-Milankovitch millennial-scale cyclicity. Elemental data were collected from a 28 m interval of core, which consists of deep-marine fine-grained siliciclastic sediments (very thin-bedded/laminated turbiditic and hemi-pelagic deposits). These sediments accumulated during a time interval when coarse clastic supply to the submarine fans was switched off. Analysis of the stratigraphic time series, using the REDFIT program (Schulz and Mudelsee, 2002), reveals the presence of oscillations with wavelengths of 6.84 m and 0.61 m exceeding the 99% confidence level. Within similar surface sediments from the Ainsa basin, such oscillations are manifest as decimetre-scale colour and/or lithological variations. To determine their temporal duration, an age model was calculated from the identification of the three main Milankovitch orbital periods within both the subsurface and laterally adjacent and age-equivalent outcropping strata. These data provide a robust sediment accumulation rate (SAR) of 27.5 cm/kyr. Application of this SAR to the 6.84 m and 0.61 m oscillations yields temporal durations of ~23 kyr and ~2 kyr. The former corresponds to the average precession period, which is believed to have controlled local sediment supply, through changing terrestrial run-off, likely linked to changing precipitation (storminess). The ~2 kyr cyclicity is related to millennial-scale climate variability of unknown origin, but, nonetheless, adds to the growing evidence for the persistence of millennial-scale environmental change in deep geologic time. Schulz, M., and Mudelsee, M., 2002, REDFIT: estimating red-noise spectra directly from unevenly spaced paleoclimatic time series: Computers & Geosciences, v. 28, p. 421-426.