The Atapuerca sites and the Ibeas hominids

The Atapuerca railway Trench and Ibeas sites near Burgos, Spain, are cave fillings that include a series of deposits ranging from below the Matuyama/Bruhnes reversal up to the end of Middle Pleistocene. The lowest fossil-bearing bed in the Trench contains an assemblage of large and small Mammals including Mimomys savini, Pitymys gregaloides, Pliomys episcopalis, Crocuta crocuta, Dama sp. and Megacerini; the uppermost assemblage includes Canis lupus, Lynx spelaea, Panthera (Leo) fossilis, Felis sylvestris, Equus caballus steinheimensis, E.c. germanicus, Pitymys subtenaneus, Microtus arvalis agrestis, Pliomys lenki, and also Panthera toscana, Dicerorhinus bemitoechus, Bison schoetensacki, which are equally present in the lowest level. The biostratigraphic correlation and dates of the sites are briefly discussed, as are the paleoclimatic interpretation of the Trench sequences. Stone artifacts are found in several layers; the earliest occurrences correspond to the upper beds containing Mimomys savini. A set of preserved human occupation floors has been excavated in the top fossil-bearing beds. The stone-tool assemblages of the upper levels are of upper-medial Acheulean to Charentian tradition. The rich bone breccia SH, in the Cueva Mayor-Cueva del Silo, Ibeas de Juarros, is a derived deposit, due to a mud flow that dispersed and carried the skeletons of many carnivores and humans. The taxa represented are: Vrsus deningeri (largely dominant), Panthera (Leo) fossilis, Vulpes vulpes, Homo sapiens var. Several traits of both mandibular and cranial remains are summarized. Preliminary attempts at dating suggest that the Ibeas fossil man is older than the Last Interglacial, or oxygen-isotope stage 5

Milankovitch Cyclicity and Stable Isotope Calibration in the Paleogene (invited)

Significant progress has been made over the last decade in the extension of astronomically calibrated geological time scales for the Neogene (Hilgen et al., 1999, Shackleton et al., 1999). The validity of these time scales has been supported by comparison of data from different parts of the world’s oceans, through the improvement of astronomical calculations, and independent dating methods and intercalibrations (Renne et al., 1994). While evidence of astronomical forcing has been found for intervals from most parts of the Cenozoic, extending astronomically calibrated time scales into the Paleogene faces some fundamental problems that require a different approach than the sophisticated “pattern matching” that worked so well for the Neogene. These challenges are related to uncertainties and limits of astronomical calculations, sparser data coverage, and a climate system that behaved quite differently to today’s “ice-house” setting. This contribution reviews some of the challenges that will have to tackled, presents a new set of astronomically calibrated benthic isotope data from the late Eocene, and suggests a new approach to synthesise astronomically calibrated durations for magnetic reversals that arise from floating time scales, which are so far more common in the Paleogene.Challenges and limits of astronomical calculations for time scale use: One crucial challenge that is faced when tackling the astronomical calibration of the geological time scale is the fact that the Solar System is chaotic, limiting the age back to which one can compute astronomical solutions with confidence (Laskar, 1999). Thus, one has to constrain astronomical parameters from the rock record. Apart from tidal dissipation effects, which change the detailed interference of particularly obliquity and climatic precession cycles, a larger scale effect is more directly linked to the chaotic nature of the Solar System: amplitude variations of the obliquity and climatic precession cycles with periods of ~1.2 and ~2.4 million years can be affected by chaotic transitions in the planetary solutions. This contribution will review the effects these chaotic transitions can have on astronomical “target” curves, particularly during the Paleogene, and why astronomical time scale calibrations will have to take this into account.Astronomically calibrated stable benthic isotopes from the Eocene: High-resolution lithological proxy measurements from ODP 1052 in the western Atlantic (Pälike et al., 2001) have provided duration estimates for magnetochrons from the late middle Eocene. This contribution presents high-resolution benthic stable isotope measurements from the same location. The astronomically calibrated isotope measurements co-vary with the lithological measurements and the astronomy in the obliquity frequency band, documenting the interaction of astronomy and climate during this transition from the Paleogene “green-house” world to the Oligocene “ice-house” world (Figure 1), and significant events that were not recognised in previous, lower resolution studies.Integration of floating time scales with magnetostratigraphy: The geomagnetic polarity time scale (Cande & Kent, 1995) incorporates astronomically calibrated ages back to 5.23 Ma. Recent results have changed significantly the age of the Oligocene/ Miocene boundary (Shackleton et al. 1999, 2000). These changes, which we will show have now been corroborated by results from ODP 199, need to be incorporated into the astronomically calibrated polarity time scale. We present a new approach, using a combination of calibrated absolute ages, and constraints on sea-floor spreading rates obtained from astronomically calibrated magnetic reversals from floating time scales, to compute a consistent set of spline interpolated ages for sea-floor magnetic reversals. This approach allows us to incorporate durations of magnetic reversals that result from the floating time scales more common in the Paleogene so far. First results, constrained by results from ODP 1218 in the Oligocene, and ODP 1052 in the late middle Eocene, suggest that the Eocene/Oligocene boundary age could be slightly older than previously estimated. It is suggested that this approach might be a useful first step to integrate astronomically calibrated ages from the Cenozoic until a full coverage, with independent data from different ocean basins, becomes available

Constraints on tidal dissipation from the rock record (abstract of paper presented at: 25th EGS General Assembly, Nice, France, 26-30 Apr 2000)

Milankovitch band variations of past climate, inferred from the rock record, have been used to astronomically tune large parts of the Neogene (Shackleton et al. 1999) and are being extended to older times. Laskar (1993) showed that the position of insolation peaks in time depends on the parameters chosen for the dynamical ellipticity and tidal dissipation (the Earth model). Hence it is crucial to study the temporal evolution of these parameters to use astronomical solutions as a template for further time scale calibration (Lourens, 1996). Here we present a method to extract the evolution of the slowdown of the Earth due to tidal dissipation from geological data for most of the last 25Ma (ODP 154, Shackleton et al. 1997), using a new interference pattern method. Results indicate that the best fitting parameters are close to present day values. This is surprising because a varying average ice-volume would suggest a change in dynamical ellipticity and in general a decreased slowdown rate of the Earth is expected due to the effects of mantle convection

Astronomical calibration of the Late Eocene timescale

Recently the astronomically calibrated geological timescale has been extended to the base of the Oligocene (Shackleton et al, 1999). Here we present a new relative age calibration of sediments of late-Middle Eocene (39.5Ma) to Late Eocene age (35Ma) that were obtained from deep-marine sediment cores during ODP Leg 171B from Site 1052.We analyse elemental ratios of Ca and Fe as a proxy for calcium carbonate content, obtained by using the X-ray Fluorescent core-scanner (XRF) in Bremen. Our data match very well with other proxy data (magnetic susceptibility and colour reflectance) but show a significantly higher signal-to-noise ratio and a more consistent hole-to hole agreement. The data obtained hence allow the construction of a more accurate composite depth scale.The data display a strong orbital signal that shows variability at all major Milankovitch frequencies as well as long term amplitude modulation patterns. We use the eccentricity driven amplitude modulation of precession to put our record onto a relative timescale, assuming that the 400kyr eccentricity cycle has been stable at that time (Laskar, 1999). The exact nature of the orbital signal might be subject to revision pending further calculations, but the consistent relationship between the different orbital frequencies present in the data suggests new ages for Magnetochrons C16, C17, and C18 that will refine the magneto-stratigraphic timescale created by Cande and Kent (1995).Our astronomical calibration suggests that the relative durations of these magnetochrons has not changed significantly, although the absolute ages might be ~200ky younger than on the Cande and Kent timescale (1995). Our study should allow a better time control for high-resolution studies over the Late Eocene time interval

Astronomical forcing in late Eocene marine sediments (abstract of paper presented at EUG XI, Strasbourg, France, 8-12 April 2001)

Recently the astronomically calibrated geological timescale has been extended to the base of the Oligocene (Shackleton et al, 1999). Here we present a new relative age calibration of sediments of late-Middle Eocene (39.5 Ma) to late Eocene age (35 Ma) that were obtained from deep-marine sediment cores during ODP Leg 171B from Site 1052. We analyse elemental ratios of Fe and Ca as a proxy for calcium carbonate content, obtained by using an X-ray Fluorescent Scanner (XRF). Our data match very well with other proxy data (magnetic susceptibility and colour reflectance) but show a significantly higher signal-to-noise ratio and a more consistent hole-to hole agreement. The data obtained hence allow the construction of a more accurate composite depth scale. The data display a strong orbital signal that shows variability at all major Milankovitch frequencies. We use the eccentricity driven amplitude modulation of precession to put our record onto a relative timescale, assuming that the 400 kyr eccentricity cycle has been stable at that time (Laskar, 1999). The exact nature of the orbital signal might be subject to revision pending further calculations, but the consistent relationship between the different orbital frequencies present in the data suggests new ages for Magnetochrons C16, C17, and C18 that will refine the magneto-stratigraphic timescale created by Cande and Kent (1995). Our astronomical calibration suggests that the relative durations of these magnetochrons has not changed significantly, although the absolute ages might be ~200 ky younger than on the Cande and Kent timescale. Our study should allow a better time control for high-resolution studies over the late Eocene time interval

Implications from a new continuous astronomically calibrated geological time scale back to ~42 Myrs (abstract of invited talk presented at AGU Fall Meeting, San Francisco, 8-12 Dec 2003)

Precise, orbitally calibrated geological time scales form a pre-requisite to further our understanding of phase relationships between orbitally driven climatic processes, and to decipher the detailed mechanisms that interact to encode orbitally forced (Milankovitch) processes in the geological record. One of the great successes of ODP Leg 199 was the recovery of a high-resolution ($\sim$1-2 cm/ky) biogenic sediment record, together with an uninterrupted set of geomagnetic chrons, as well as a detailed sequence of calcareous and siliceous biostratigraphic datum points. In addition, lithological measurements revealed clearly recognisable cycles that can be attributed to climatic change, driven by Milankovitch style orbital variations of the Earth. By integrating lithological, geochemical, and stable isotope data sets, we have now derived a long, astronomically calibrated, time scale from the Miocene into the latest Eocene from ODP Leg 199. Using additional data from ODP Legs 177 and 171B, we have generated a detailed continuous time scale back to $\sim$ 42 Myrs. We can contrast the encoding of astronomical forcing terms in sedimentary records from different ocean basins, latitudes, water-depths, and water masses. Our results show that the dominantly recorded orbital parameters vary as a function of the carbonate system response, with a very strong eccentricity component in the record from the deep equatorial Pacific, and a stronger obliquity component in the equatorial Atlantic. In addition, we investigate the phase relationship between astronomical forcing terms and carbonate preservation, with a potentially different response during "green-house" and "ice-house" conditions, separating the Oligocene and Eocene

Constraints on astronomical parameters from the geological record for the last 25 My

We develop a new method, based on interference patterns between the precession and obliquity components of geological data from Ocean Drilling Program (ODP) Leg 154 and astronomical solutions, to extract small changes in the precession constant p due to tidal dissipation over the last 25 million years and to put numerical constraints on the parameters for tidal dissipation and the dynamical ellipticity of the Earth. We show that these parameters have remained close to the present day values over the last 25 million years. The best fitting astronomical solution we obtained gives rise to a value of 0.9999 times the current day dynamical ellipticity, and 1.004 times the current day tidal dissipation value as used in the algorithm of Laskar (1993). Our range of uncertainty is 0.9996–1.0001 for the dynamical ellipticity of the Earth and 0.945–1.025 for the tidal dissipation. Our model does not require changes in these parameters over the last 25 Ma and we show that using the solution of Laskar (1993) with present day values for dynamical ellipticity and tidal dissipation as a tuning target does not introduce large errors during astronomical tuning. Our results indicate that the Earth has not crossed into resonance with Saturn and Jupiter during the last few million years. Our conclusions depend on the assumption of a correct initial tuning of the ODP Leg 154 data

The heartbeat of the Oligocene climate system

A 13-million-year continuous record of Oligocene climate from the equatorial Pacific reveals a pronounced “heartbeat” in the global carbon cycle and periodicity of glaciations. This heartbeat consists of 405,000-, 127,000-, and 96,000-year eccentricity cycles and 1.2-million-year obliquity cycles in periodically recurring glacial and carbon cycle events. That climate system response to intricate orbital variations suggests a fundamental interaction of the carbon cycle, solar forcing, and glacial events. Box modeling shows that the interaction of the carbon cycle and solar forcing modulates deep ocean acidity as well as the production and burial of global biomass. The pronounced 405,000-year eccentricity cycle is amplified by the long residence time of carbon in the oceans