1,721,103 research outputs found
Constraints on tidal dissipation from the rock record (abstract of paper presented at: 25th EGS General Assembly, Nice, France, 26-30 Apr 2000)
No full textMilankovitch 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
Milankovitch Cyclicity and Stable Isotope Calibration in the Paleogene (invited)
No full textSignificant 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 astronomical parameters from the geological record for the last 25 My
No full textWe 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
Astronomical forcing in late Eocene marine sediments (abstract of paper presented at EUG XI, Strasbourg, France, 8-12 April 2001)
No full textRecently 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
Erratum to 'Astronomical forcing in late Eocene marine sediments' [Earth Planet. Sci. Lett. 193 (2001) 589-602]
No full textThe reference list of this article was not reproduced correctly during the production process (issue numbers were used as volume numbers). The correct list is given below [1-35]. The publishers would like to apologize for any inconvenience caused by this error
Geologic constraints on the chaotic diffusion of the Solar System
No full textThe correlation of Earth's orbital parameters with climatic variations has been used to generate astronomically calibrated geologic time scales of high accuracy. However, because of the chaotic behavior of the solar system, two initially close calculations of Earth's orbit diverge exponentially and have a large uncertainty beyond several million years in the past. This chaotic behavior is related to a combination of angles in the precession motion of the orbits of Earth and Mars, θ, which currently is in resonance. How long θ stays in libration critically depends on the dynamical model and initial conditions for the solar system. Here we show that geologic data can differentiate between astronomical solutions that do and do not exhibit a transition in θ since 40 Ma and that sediments can thus provide a history for the evolution of θ. We find that the chaotic transition of θ from libration to circulation did not occur after ca. 30 Ma. We can thus constrain the chaotic diffusion of the solar system in the past, and our results provide new and challenging constraints for astronomical models
Astronomical calibration of the Late Eocene timescale
No full textRecently 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
Orbitally forced climate signals in mid-Pliocene nannofossil assemblages
No full textDowncore cyclic variation in high-resolution nannofossil abundance records from mid-Pliocene equatorial Atlantic ODP Sites 662 and 926 demonstrate the direct response by several Pliocene taxa (notably Discoaster, Sphenolithus and Florisphaera profunda) to orbitally forced climatic variation. In particular, these records display strong obliquity and precessional signals reflecting primarily high latitude, Southern hemisphere changes influencing upwelling intensity and local low-latitude, insolation-driven climatic changes (via the productivity and/or turbidity influence of Amazon-sourced terrigenous material) at Sites 622 and 926 respectively.In seasonal studies of coccolithophorid assemblages, only part of the variation observed can be explained by abiotic processes, so it is perhaps not surprising that in this study few Pliocene nannofossil taxa demonstrate significant correlations with each other or with physical environmental parameters. Only some variance in nannofossil abundances can be explained by the primary controls of temperature and productivity. The rest is attributed to nonlinear responses to climatic changes; biotic processes such as grazing, predation, viral infection and competition, and/or, abiotic factors for which there is no readily available proxy (e.g. salinity). The lack of strong, consistent intra- and inter-relationships of the nannoflora and the environment reflects an ecologically complex, differentiated original community producing a complex integrated signal transmitted into the fossil record
Astronomical calibration of the late Oligocene through early Miocene geomagnetic polarity time scale (abstract of paper presented at AGU Fall Meeting, San Francisco, 8-12 Dec 2003)
No full textAt Ocean Drilling Program Site 1090 (subantarctic South Atlantic) benthic foraminiferal stable isotope data (from Cibicidoides and Oridorsalis) span the late Oligocene through the early Miocene (24-16 Ma) at a temporal resolution of 5 kyr. In the same time interval, a magnetic polarity stratigraphy can be unequivocally correlated to the geomagnetic polarity timescale (GPTS), thereby providing direct correlation of the isotope record to the GPTS. In an initial age model we use the newly derived age of the Oligocene/Miocene boundary of 23.0 Ma (Shackleton et al., 2000) revised to the new astronomical calculation of Laskar (2001) to recalculate the spline ages of Cande and Kent (1995). We then tune the site 1090 oxygen isotope record to obliquity, assuming a 7.2 kyr phase lag, using the new astronomic solution of Laskar (2001). In this manner we are able to refine the ages of polarity chrons C7n through C5Cn.1n. The new age model is consistent, within one obliquity cycle, with previously tuned ages for polarity chrons C7n to C6Bn from Shackleton et al. (2000), rescaled to the new astronomical solution of Laskar (2001). For early Miocene polarity chrons C6AAr through C5Cn, our obliquity-scale age model is the first to allow a direct calibration to the GPTS. The new ages are also close to, within one obliquity cycle, to those obtained by rescaling the Cande and Kent (1995) interpolation using the new age of the O/M boundary (23.0 Ma), and the same middle Miocene control point (14.8 Ma) used by Cande and Kent (1992). Thus we have confidence in the orbitally tuned age model and the refined GPTS calibration for the late Oligocene through early Miocene
The climatic consequences of a rare orbital anomaly at the Oligocene/Miocene boundary (23Ma)
No full textThe late Oligocene to early Miocene (20-26Ma) is characterized by a complex climate history that includes a stepped transition toward a cooler climate, intermittent partial glaciations of Antarctica, and a transient glaciation, Mi-1, at the Oligocene/Miocene (O/M) boundary. The Mi-1 event is characterized by an anomalous positive oxygen isotope excursion, the magnitude of which suggests the brief appearance of a full-scale ice-sheet on east Antarctica coupled with a few degrees of deep sea cooling. A recent breakthrough in extending the astronomical calibration back to ~30 Ma has provided a unique opportunity to compare the climatic events of the O/M transition relative to Earth’s orbital variations. Here, we present an uninterrupted 5.5 My long high-fidelity chronology of late Oligocene-early Miocene climate and ocean carbon chemistry that is based on a composite in the western equatorial Atlantic. This unique isotope record provides a rare window into how the climate system responded to orbital forcing uncer boundary conditions significantly different from those of the recent past. Time-series analyses reveal climate variance concentrated at all Milankovitch frequencies, but with unusually strong power at the primary eccentricity band periods of 406, 125, and 95-ky. These cycles, which represent in part glacial advances and retreats of Antarctic ice-sheets, show significantly enhanced variability over a 1.6 my period (21.4-23.0 Ma) of suspected low greenhouse gas levels as inferred from the carbon isotope record. Perhaps the most unexpected finding is that of a rare orbital congruence between eccentricity and obliquity that precisely corresponds with the Mi-1 glaciation. This orbital anomaly involves ~four consecutive cycles of low amplitude variance in obliquity (a node) during a period of low eccentricity. The net result is an extended period (~200ky) of low seasonality orbits, which allows for a step-like expansion of an Antarctic ice-sheet
- …
