Myr Cycle Research Articles

Milankovitch and sub‐Milankovitch forcing of the Oxfordian (Late Jurassic) Terres Noires Formation (SE France) and global implications

ABSTRACTHigh‐resolution analysis (2277 samples) of magnetic susceptibility (MS) was performed on ∼700‐m‐thick Early–Middle Oxfordian marine marls of the Terres Noires Formation, SE France. MS variations within these sediments record sub‐Milankovitch to Milankovitch frequencies with long‐term eccentricity (405 kyr and ∼2 Myr) being the most prominent. The 405 kyr cycle was used as a high‐resolution geochronometer for astronomical calibration of this poorly constrained interval of Late Jurassic time. The estimated duration of this Early–Middle Oxfordian interval concurs with the current International Geologic Time Scale GTS2004 (∼4 Myr), but the estimated durations of the corresponding ammonite zones are notably different. The calibration improves the resolution and accuracy of the M‐sequence magnetic anomaly block model that was previously used to establish the Oxfordian time scale. Additionally, the 405 kyr cyclicity is linked to third‐order sea‐level depositional sequences observed for Early–Middle Oxfordian time. Strong ∼2 Myr cycles are consistent with long‐term eccentricity modulation predicted for the Late Jurassic. These cycles do not match second‐order sequences that have been documented for European basins; this raises questions about the definition and hierarchy of depositional sequences in the Mesozoic eustatic chart. Our results require substantial revisions to the chart, which is frequently used as a reference for the correlation of widely separated palaeogeographic domains. Finally, a long‐term trend in the MS data reflects a progressive carbonate enrichment of the marls expressing an Early Oxfordian global cooling followed gradually by a warming in the Middle Oxfordian. This trend also records a major transgressive interval likely peaking at the Transversarium ammonite zone of the Middle Oxfordian.

Astronomical tuning of the La Vedova High Cliff section (Ancona, Italy)—Implications of the Middle Miocene Climate Transition for Mediterranean sapropel formation

Continuous marine successions covering the Middle Miocene Climate Transition (MMCT; ∼ 15–13.7 Ma) are scarce and the lack of a high-resolution magnetobiostratigraphic framework hampers the construction of astronomically tuned age models for this time interval. The La Vedova High Cliff section, exposed along the coast of the Cònero Riviera near Ancona (Italy), is one of the few Mediterranean sections covering the critical time interval of the MMCT. Starting from an initial magnetobiostratigraphic age model, a robust astronomical tuning was constructed for the interval between 14.2 and 13.5 Ma, using geochemical element data and time series analysis. A shift in δ 18O of bulk sediment towards heavier values occurs between ∼ 13.92 and 13.78 Ma and could be related to the Mi3b oxygen isotope event, which reflects the rapid expansion of the East Antarctic Ice Sheet in the middle Miocene. The onset of the CM6 carbon excursion is reflected in the bulk record by a rapid increase in δ 13C at 13.86 Ma. Our results confirm the proposition that these events coincide with a 405-kyr minimum in eccentricity and a node in obliquity related to the ∼ 1.2 Myr cycle. From 13.8 Ma onwards, distinct quadruplet cycles containing sapropelitic sediments were deposited. This may suggest a causal connection between the main middle Miocene cooling step and the onset of sapropel formation in the Mediterranean.

Do Extragalactic Cosmic Rays Induce Cycles in Fossil Diversity?

Recent work has revealed a 62 ± 3 Myr cycle in fossil diversity in the past 542 Myr; however, no plausible mechanism has been found. We propose that the cycle may be caused by modulation of cosmic-ray (CR) flux by the solar system vertical oscillation (64 Myr period) in the Galaxy, the Galactic north-south anisotropy of CR production in the Galactic halo/wind/termination shock (due to the Galactic motion toward the Virgo Cluster), and the shielding by Galactic magnetic fields. We revisit the mechanism of CR propagation and show that CR flux can vary by a factor of about 4.6 and reach a maximum at northernmost displacement of the Sun. The very high statistical significance of (1) the phase agreement between solar northward excursions and the diversity minima and (2) the correlation of the magnitude of diversity drops with CR amplitudes through all cycles provide solid support for our model. Various observational predictions which can be used to confirm or falsify our hypothesis are presented.

Periodic components in the diversity of calcareous plankton and geological events over the past 230 Myr

We performed statistical analysis on high-resolution records of the diversity of calcareous plankton (planktonic foraminifera and calcareous nannoplankton), and on records of global sea level, marine isotopes ( δ 18O, δ 13C, 87Sr/ 86Sr), large igneous province (LIP) eruptions, and dated impact craters over the last 230 Myr. Results of Continuous Wavelet Analysis (CWT), Fast Fourier Transform (FFT) spectral analysis and cross-spectral analysis indicate that the records of diversity of calcareous nannoplankton (CN) and planktonic foraminifera (PF) and all of the geologic time series tested show similar dominant 25–33 Myr cycles. Based on the statistical results, best-fit stationary-periodic models for PF and CN evolutionary records can be constructed (with t=time in Myr): CN diversity=9.5 sin(2 π( t−7.5 Myr)/29 Myr)+4.8 sin(2 π( t−4 Myr)/15.3 Myr), and PF diversity=7.6 sin(2 π( t−12 Myr)/26 Myr)+3 sin(2 πt/9.2 Myr). These periodic models describe major patterns in the diversity history, such as: (1) a sawtooth-shaped 29-Myr cycle for calcareous nannoplankton diversity, with gradual increases and abrupt decreases in diversity; (2) plateau-shaped ∼26-Myr cycles in the planktonic foraminiferal record, with abrupt diversity increases and decreases; and (3) the presence of ∼15.3- and 9.2-Myr periodic components that modify the shapes of the 26- to 29-Myr cycles. Except for the synchronous decreases in diversity of PF and CN at 65 and ∼34 Ma, the diversity extrema for the two planktonic groups have been on average ∼2 to 3 Myr out of phase. In the ∼30-Myr-cycle band, CN diversity increased with sea-level rise and increased paleotemperatures, whereas PF diversity was, in general, greater during times of lower sea levels. Diversity of PF and CN was reduced at times of major LIP eruptions and large impacts that follow ∼15 or 30 Myr periodicities. These results suggest that the diversity of calcareous plankton since their appearance in the early Mesozoic has been modulated by long-term cyclical changes in global environmental conditions and by periodic large volcanic and impact events. The pacemaker, or pacemakers, for these cycles may be astrophysical, geophysical or some combination of the two.

Foraminiferal response to mid-Cenomanian (Upper Cretaceous) palaeoceanographic events in the Anglo-Paris Basin (Northwest Europe)

We have analysed the sedimentology, macrofossil event horizons (including pulse faunas) and foraminiferal succession (80 samples) from an 8 m succession in the upper Lower and lower Middle Cenomanian (Upper Cretaceous) at Folkestone, Southeast England. Comparative sedimentological analysis of successions in southern and northeastern England indicates the presence of three hierarchies of cycle: 20 kyr productivity (chalk-marl couplets) cycles attributed to the Milankovitch precession cycle; 100 kyr sea-level cycles attributed to the Milankovitch eccentricity cycle; and l Myr cycles attributed to longer-term sea-level cycles. Previously recognised macrofossil pulse faunas occur preferentially at the bases of the 100 kyr eccentricity cycles. We analysed the foraminiferal fauna across the Lower Middle Cenomanian boundary at Abbott's Cliff, Folkestone, in great detail (80 samples in 8 m). The foraminiferal succession is also intricately related to the 1 Myr sea-level, and 100 kyr eccentricity cycles. Changes related to the long-term cycles include variations in the abundance of the benthic foraminifera Tritaxia, Gyroidinoides. Lingulogavelinella and Dorothia, and an increase in benthic diversity in the transgressive deposits. Changes related to the 100 kyr eccentricity cycles include: the presence of Rotalipora ex gr. reicheli in the lower part (and rarely upper part) of cycles; the abundance of planktics (both absolute and relative) in the middle to upper part of cycles; an increase in the proportion of Gavelinella berthelini and G. reussi in the upper part of cycles. We present a stratified water column model to explain the changes related to the 100 kyr scale cycle. © 1998 Elsevier Science B.V.

Long-periodic variations in the earth's obliquity and their relation to third-order eustatic cycles and late Neogene glaciations

Abstract Evolutive spectra of four high-resolution climatic proxy records reveal that the spectral power in the obliquity frequency band increases during five well-defined intervals in the last 5.3 million years. These intervals correspond to periods of high-amplitude variations in the tilt (obliquity) of the Earth's axis, connected with a 1.2 myr cycle, and to sea-level lowstands of third-order eustatic cycles in the Haq et al. (1987) curve. This implies that the development of major Plio-Pleistocene glaciations proceeded episodically and that third-order cycles are glacio-eustatically controlled. In (at least) two of these intervals sinistrally-coiled neogloboquadrinids entered the Mediterranean at times of glacial periods. One interval starts around 2.8 Ma and is characterized by the first occurrence of N. atlantica (s). Another interval begins at the Plio-Pleistocene boundary around 1.8 Ma and is characterized by the common occurrence of sinistrally-coiled N. pachyderma. Also during the late Miocene third-order cycles correlate well with the 1.2 myr cycle of obliquity. A significant drop in the third-order eustatic curve starting at the Tortonian-Messinian boundary, for instance, coincides with a period of high-amplitude variations in the obliquity time series connected with the 1.2 myr cycle.

The Galactic Theory of Mass Extinctions: an Update

Astronomical and geological evidence is consistent with the hypothesis that mass extinctions of life on Earth are related to impacts of comets whose flux is partly modulated by the dynamics of the Milky Way Galaxy. Geologic evidence for impact (ejecta and large impact craters) has been found at times of mass extinction events, and the record of large dated craters shows a significant correlation with extinctions. Statistical analyses suggest that mass extinction events exhibit a periodic component of about 30 Myr, and periodicities of 30 ± 0.5 Myr and 35 ± 2 Myr have been extracted from sets of well-dated large impact craters. These results suggest periodic or quasi-periodic showers of impactors, probably Oort Cloud comets, with an approximately 30 or 36 Myr cycle. The best explanation for these proposed quasi-periodic comet showers involves the Sun's vertical oscillation through the galactic disk, which may have a similar cycle time between crossings of the galactic plane. Further refinement of the model will depend on the identification and quantification of the dark matter component in the galactic disk, and discovery and accurate dating of additional impact craters.