Orbital eccentricity and inclination metronomes in Middle Miocene lacustrine mudstones of Jiuxi Basin, Tibet: Closing an astrochronology time gap and calibrating global cooling events
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Keywords:
Aridification
Milankovitch cycles
Cyclostratigraphy
Paleoclimatology
Magnetostratigraphy
Orbital forcing
We have studied an expanded succession of coastal marine,
estuarine and lacustrine sediments of Late Eocene-Early
Oligocene age in the Isle of Wight southern England. In this
succession, a strong Milankovitch signal (406, 100, 40 and
weaker 20Ka) is recorded from the relative abundance of
neoformed illite and illite-smectite, which formed in soils
by seasonal wetting and drying. The orbital timescale is
calibrated using magnetostratigraphic, and to a lesser
extent, biostratigraphic data. Combined orbital calibration
and sequence stratigraphic analysis allows us to identify the
major control on sea-level as the 406Ka long eccentricity
cycle, which caused sea-level to fluctuate by 10-15 m.
These values have been determined from the amount of
incision at observed at sequence boundaries on a regional
scale. Minor sea-level changes of 1-3 m were controlled by
obliquity. The position of the Early Oligocene heavy ?18O
event can be inferred in the Isle of Wight from its magnetostratigraphic
proxy (base of chron 13n). We have determined
the sea-level fall at this level to be approximately
12 m, close in magnitude to drops associated with the
preceding 3 Late Eocene 406 Ka sequences. This evidence
does not support recent estimates of a 50-90 m sea-level fall
within the Early Oligocene based on the calculation that a
significant part of the oxygen isotope event was caused by
rapid Antarctic ice buildup. Rather, orbitally driven sealevel
changes throughout the Late Eocene-Early Oligocene,
although probably glacioeustatic in origin, remained of
similar magnitude.
Milankovitch cycles
Cyclostratigraphy
Sequence Stratigraphy
Orbital forcing
Magnetostratigraphy
δ18O
Sequence (biology)
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Aridification
Milankovitch cycles
Cyclostratigraphy
Paleoclimatology
Magnetostratigraphy
Orbital forcing
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Magnetostratigraphy
Marl
Cyclostratigraphy
Milankovitch cycles
Cyclothems
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Magnetostratigraphy
Cyclostratigraphy
Milankovitch cycles
Stratotype
Outcrop
Eccentricity (behavior)
Chronostratigraphy
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Aridification
Global cooling
Paleoclimatology
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Abstract. The Eocene-Oligocene Transition (EOT) marks the onset of the Antarctic glaciation and the switch from greenhouse to icehouse climates. However, the driving mechanisms and the precise timing of the EOT remain controversial mostly due to the lack of well-dated stratigraphic records, especially in continental environments. Here we present a cyclo-magnetostratigraphic and sedimentological study of a ∼7.6 Myr-long lacustrine record spanning the late Eocene to the earliest Oligocene, from a drill-core in the Rennes Basin (France). Time-series analysis of natural gamma-ray (NGR) log data shows evidence of Milankovitch cycle bands. In particular, the 405 kyr stable eccentricity is expressed with strong amplitudes. Astronomical calibration to this 405 kyr periodicity yields duration estimates of Chrons C12r through C16n.1n, providing additional constraints on the middle–early Eocene timescale. Correlations between the orbital eccentricity curve and the 405 kyr tuned NGR time series and assumptions on their phase relationships, enable to test previously proposed ages for the EO boundary, indicating that 33.71 and 34.10 Ma are the most likely. Additionally, the 405 kyr tuning calibrates the most pronounced NGR cyclicity to a period of ∼1 Myr matching the g1-g5 eccentricity term. Such cyclicity has been recorded in other continental records, pointing to its significant expression in continental depositional environments. The record of g1-g5 and sometimes g2-g5 eccentricity terms in previously acquired sedimentary facies proxies in CDB1 core led us to hypothesize that the paleolake level may have behaved as a lowpass filter for orbital forcing. Two prominent changes in the sedimentary facies were detected across the EOT, which are temporally equivalent to the two main climatic steps, EOT-1 and Oi-1. Combined with previously acquired geochemical (δ15Norg, TOC), mineralogical (Quartz, clays) and pollen assemblage proxies from CDB1, we suggest that these two facies changes reflect the two major Antarctic cooling/glacial phases via the hydrological cycle, as significant shifts to drier and cooler climate conditions, thus supporting the stepwise nature of the EOT. Remarkably, a strongly dominant obliquity expressed in the latest Eocene corresponds in time to the interval from the EOT precursor glacial event till the EOT-1. We interpret the obliquity dominance as reflecting preconditioning phases for the onset of the major Antarctic glaciation, either from its direct impact on the formation/(in)stability of the incipient Antactic Ice Sheet (AIS), or through its modulation of the North Atlantic Deep Water production given the North Atlantic coastal location of the CDB1 site.
Milankovitch cycles
Orbital forcing
Cyclostratigraphy
Eccentricity (behavior)
Magnetostratigraphy
Paleogene
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Paleoclimate proxy records from marine pelagic sediments show that a link exists between long-period orbital cyclicity and the pattern of high latitude glaciations. Thus, a sound possibility exist that transgressive-regressive third-order sequences from shallow marine environments reflect long-period orbital (glacioeustatic) forcing, as suggested from a variety of shallow marine settings of different ages, from Mesozoic to Paleogene. In this study we aim at testing the role of the 400 kyr eccentricity cycle in the sequential organization of the Late Eocene deltaic sequences of the Belsue-Atares Formation, in the Jaca-Pamplona Basin. The overall record spans from latest Lutetian to early Priabonian and consists of nearly 1000 meters of siliciclastic deltaic to mixed platform sequences of various scales. Very notorious lateral changes in both stratigraphic thickness and sedimentary facies witness the synkinematic character of these sediments, deposited simultaneously to intrabasinal fold growth. A magnetostratigraphy based chronostratigraphic framework is used, first, to determine the age and duration of the sequences and, second, to establish a robust correlation with other deltaic sequences within the south-pyrenean foreland. The long-distance correlation exercise is used to discriminate between local (tectonic) and global (climatic) forcing factors, under the assumption that climate signature is synchronous, while tectonic forcing is prone to yield diachronic units at basin scale. Astronomical tuning with the 400-kyr cycle of the eccentricity solution of the Earth orbit is attempted on the basis of derived magnetostratigraphic age constrains. Our results suggest that transgressive (regressive) trends correlate with maxima (minima) of eccentricity cycle, a phase-relationship which is compatible with a base-level (accommodation) driven forcing.
Milankovitch cycles
Orbital forcing
Magnetostratigraphy
Siliciclastic
Paleogene
Sequence Stratigraphy
Transgressive
Cyclostratigraphy
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Abstract A cyclostratigraphic interpretation of peritidal to shallow-marine ramp deposits of the early Middle Triassic (Anisian) Opletnya Member exposed in outcrops along the Iskar River gorge, NW Bulgaria, is presented. Based on facies trends and bounding surfaces, depositional sequences of several orders can be identified. New biostratigraphic data provide a time frame of the studied succession with placement of the boundaries of the Anisian substages and show that the Aegean (early Anisian) substage lasted about 1.6 Myr. In the corresponding interval in the two studied sections, 80 elementary sequences are counted. Five elementary sequences compose a small-scale sequence. The prominent cyclic pattern of the Opletnya Member can thus be interpreted in terms of Milankovitch cyclicity: elementary sequences represent the precession (20-kyr) cycle and small-scale sequences the short eccentricity (100-kyr) cycle in the Milankovitch frequency band. Medium-scale sequences are defined based on lithology but only in two cases can be attributed to the long eccentricity cycle of 405 kyr. The transgressive-regressive facies trends within the sequences of all scales imply that they were controlled by sea-level changes, and that these were in tune with the climate changes induced by the orbital cycles. However, the complexity of facies and sedimentary structures seen in the Opletnya Member also implies that additional factors such as lateral migration of sediment bodies across the ramp were active. In addition, three major sequence boundaries have been identified in the studied sections, which can be correlated with the boundaries Ol4, An1, and An2 of the Tethyan realm.
Cyclostratigraphy
Milankovitch cycles
Outcrop
Transgressive
Sequence (biology)
Lithology
Magnetostratigraphy
Precession
Ladinian
Eccentricity (behavior)
Chronostratigraphy
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Milankovitch cycles
Conodont
Magnetostratigraphy
Cyclostratigraphy
Paleoclimatology
Chronostratigraphy
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The concept of orbital forcing of climate has been invoked for a century to explain rhythmic patterns of sedimentation. Since the calculation of variations of insolation with fluctuating orbital parameters by the eponymous Milankovitch, orbitally forced sedimentary sequences have been interpreted from almost all sedimentary environments, including lacustrine, evaporite, fluvial, desert, marginal marine, carbonate ramp, and pelagic. Recognition of orbital cycles in the sedimentary record is based on matching sedimentary-cycle periods with the calculated frequencies of the eccentricity, obliquity, and precession cycles, typically by the techniques of spectral analysis. Controversy surrounding the interpretation of orbitally forced cyclicity results when the data for the interpretations lack sufficient rigor or alternative mechanisms for producing cyclicity can be invoked. Despite some shortcomings, this theory illustrates the power of allocyclic controls on sedimentation and should be included in the curriculum of sedimentary geology at a level beyond that of current texts.
Milankovitch cycles
Cyclostratigraphy
Orbital forcing
Cyclothems
Geologic record
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