ALBERT

Description: The start of the Mesozoic Era is marked by roughly 5 m.y. of Earth system upheavals, including unstable biotic recovery, repeated global warming, ocean anoxia, and perturbations in the global carbon cycle. Intervals between crises were comparably hospitable to life. The causes of these upheavals are unknown, but are thought to be linked to recurrent Siberian volcanism. Here, two marine sedimentary successions at Chaohu and Daxiakou (South China) are evaluated for paleoclimate change from astronomical forcing. In these sections, gamma-ray variations indicative of terrestrial weathering reveal enhanced obliquity cycling over prolonged intervals, characterized by a 32.8 k.y. periodicity with strong 1.2 m.y. modulations. These suggest a 22 h length of day and 1.2 m.y. interaction between the orbital inclinations of Earth and Mars. Comparing the 1.2 m.y. obliquity modulation cycles in these sections with Early Triassic records of global sea level, temperature, redox, and biotic evolution suggests that long-term astronomical forcing was involved in the repeated climatic and biotic upheavals that took place throughout the Early Triassic.

Description: Astronomically tuned cyclic sedimentary successions provide unprecedented insight into the temporal evolution of depositional systems and major geologic events. However, placing astronomically calibrated records into an absolute time frame with confidence requires independent and precise geochronologic constraints. Astronomical tuning of the precessionally modulated sedimentary cycles of the Mediterranean Basin deposited during the Messinian Salinity Crisis (5.96–5.33 Ma) has indicated an ~90 k.y. "Messinian gap", corresponding to the evaporative drawdown of the Mediterranean following the closure of the Mediterranean-Atlantic gateway. In the Messinian deposits, a volcanic ash dated by 40 Ar/ 39 Ar geochronology was used to anchor the sedimentary cycles to the insolation curve. However, the uncertainty of the 40 Ar/ 39 Ar date introduces a potential two-cycle (~40 k.y.) uncertainty in the tuning. Using high-precision chemical abrasion–thermal ionization mass spectrometry (CA-TIMS) U-Pb geochronology on single zircon grains from two Messinian ash layers in Italy, we obtained dates of 5.5320 ± 0.0046 Ma and 5.5320 ± 0.0074 Ma with sub-precessional resolution. Combined with our astronomical tuning of the Messinian Lower Evaporites, the results refine the duration of the "Messinian gap" to at most 28 or 58 ± 9.6 k.y., which correlates with either the TG12 glacial interval alone, or both TG12 and TG14 glacial intervals, supporting the hypothesis of a glacio-eustatic contribution in fully isolating the Mediterranean from the Atlantic Ocean. Our new U-Pb dates also allow us to infer a precessionally modulated cyclicity for the post-evaporitic deposits, and hence enable us to tune those successions to the insolation curve.

Description: Over the past 25 yr, the science of stratigraphy has evolved to include time-correlative data from vastly disparate components of the Earth system. Not least of these is the global signal afforded by cyclostratigraphy, which has recorded the evolution of Earth’s astronomical ("Milankovitch") forcing of insolation and the paleoclimate system. Fossil astronomical signals are collected from cyclic sedimentary sequences by detailed sampling and study of facies, geochemistry, mineralogy, rock magnetism, color, etc. In step with the documentation of astronomically forced paleoclimate from ever-older older geologic times, innovations in computational science have provided ever-longer high-accuracy astronomical model "targets" that can be used for time scale calibration. The Earth’s orbit is affected by motions of other planets, notably the orbital perihelia of Venus and Jupiter, which impose a dominant 405 k.y. eccentricity cycle on Earth’s orbital evolution. The large mass of Jupiter stabilizes this cycle over hundreds of millions of years. The cyclostratigraphic record of 405 k.y. cycles is therefore often used to correct chronologies affected by variable sedimentation. Earth’s shape and rotation rate are influenced by tidal dissipation and climate friction; these effects affect Earth’s precession rate through time. Thus, a record of Earth-Moon evolution is also embedded in cyclostratigraphy. The geochronologic value of cyclostratigraphy has been affirmed through intercalibration with high-precision radioisotope dating, which today has the potential to define the ages of stratigraphic horizons with 2 uncertainties at the scale of a precession cycle. Precession index phasing relative to that of the obliquity elucidates the seasonal nature of astronomical forcing of the paleoclimate system. Cyclostratigraphy contributes to our knowledge of planetary dynamics for times prior to the current ca. 50 Ma limit of accurate astronomical solutions, and it will guide our future understanding of solar system evolution and the evidence for chaotic diffusion. Astronomical modeling is undergoing its own revolution with development of new numerical integrators to extend accuracy further back in time. Finally, space exploration has revealed prominent sedimentary bedding and ice stratigraphy on the surface of Mars, with patterns suggestive of astronomical forcing analogous to Earth.