Geomagnetic Polarity Time Scale Research Articles

Magnetostratigraphically-calibrated dinoflagellate cyst bioevents for the uppermost Eocene to lowermost Miocene of the western North Atlantic (IODP Expedition 342, Paleogene Newfoundland sediment drifts)

The Oligocene epoch represents a somewhat neglected chapter in paleoclimate and paleoceanographic history, which is at least partially due to the scarcity of complete Oligocene sedimentary archives and poor biostratigraphic age control. Many of the biotic events registered in Oligocene microfossils are strongly diachronous across latitudes as a response to increased global cooling and enhanced meridional temperature gradients. To improve biostratigraphic age control for the Oligocene of the North Atlantic Ocean, we carried out a high-resolution study of dinoflagellate cysts from Integrated Ocean Drilling Program (IODP) Sites U1405, U1406 and U1411 off Newfoundland. Together the sites comprise an apparently complete uppermost Eocene (34.9Ma) to lowermost Miocene (21.7Ma) sequence with good magnetostratigraphic age control. This allows us to firmly tie identified dinoflagellate cyst bioevents to the geomagnetic polarity timescale. In the dinoflagellate cyst assemblages studied we have identified and magnetostratigraphically-calibrated ten first and 19 last appearance datums. Our magnetostratigraphically-calibrated dinocyst-based biostratigraphy, which is based on an average sample resolution of a sample every ~150kyrs, will contribute to an improved age framework for future paleoceanographical studies in the higher-latitude North Atlantic.

Preliminary magnetostratigraphic results from the late Miocene Maragheh Formation, NW Iran

Maragheh in northwestern of Iran is a world famous Miocene fossil-bearing area. The area has yielded classical late Miocene Turolian age fauna that has been collected and studied sporadically over the last 150 years. However, the precise correlation of these sediments to the Global Time Scale (GTS) has remained ambiguous. To address this, 115 levels along an approximately 27-m-thick interval were collected from the middle Maragheh Formation at Dareh Gorg (Gort Daresi) section. Characteristic remanent magnetization directions obtained by alternating field demagnetization produce a polarity pattern that is supported by thermal demagnetization on a set of sister specimens. Three polarity intervals were recognised, the middle part of the section at around 15–21 m showing reversed polarity, bounded by normal polarities above and below. Based on the palaeontological constraints and recent K-Ar age determinations from the Maragheh Fm, three correlations to the geomagnetic polarity time scale appear likely. According to these correlations, recently discovered hominoid locality is correlated to C3Br.1n, C4n.1n, or to C4n.2n. For a unique correlation, however, additional palaeomagnetic data is required from the upper and lower parts of the section.

Identification of the short-lived Santa Rosa geomagnetic excursion in lavas on Floreana Island (Galapagos) by40Ar/39Ar geochronology

A set of closely related basaltic lava flows (supersite GA-X) on Floreana Island in the Galapagos Archipelago has a published record of an excursional or transitional direction (virtual geomagnetic pole located at 153.1°E, 54.2°S with α 95 = 5.0°) and a geomagnetic field strength (1.1 × 10 22 Am 2 ) that is only ∼14% of the strength of the modern magnetic field (7.8 × 10 22 Am 2 ). The very large age uncertainty of previous dating of a lava flow (G43) from this set, however, has prevented placing this event in the geomagnetic polarity time scale. Here we report highly reproducible and precise 40 Ar/ 39 Ar ages on the lava flow that indicate that the distinct geomagnetic excursion is 925.7 ± 4.6 ka (2σ; n = 6; mean square of weighted deviates = 1.23). This shows that this dramatic weakening of the geomagnetic field is associated with the Santa Rosa Excursion instead of the Matuyama-Brunhes polarity reversal. Our high-precision 40 Ar/ 39 Ar ages for Floreana provide evidence for the global significance of the Santa Rosa Excursion.

Neogene magnetostratigraphy and rock magnetic study of the Kashi Depression, NW China: Implications to neotectonics in the SW Tianshan Mountains

AbstractThe southwest Tianshan Mountains of China are bordered by the Tarim foreland and comprise an actively deforming segment of the India‐Asia collisional system. We report a detailed magnetostratigraphic study of the Dashankou section in the Kashi Depression of the Tarim Basin to improve the understanding of the history of sedimentation, denudation, and mountain building in this region. The preferred correlation of the succession with the geomagnetic polarity timescale defines a depositional history between 12.4 and 3.0 Ma with a substantial increase in sedimentation rates identified at ~6.7 Ma corresponding to a pulse of rapid uplift in the southwest Tianshan Mountains. Although climatic changes may have modulated the record during Neogene times, they do not appear to have had an important influence on sediment accumulation rates between 7.0 and 2.6 Ma. Magnetic fabrics identify the influence of a regional stress field imparted by ongoing India‐Asia collision in the lower part of the succession contrasting with predominantly sedimentary fabrics in the higher part of the succession. A major clastic influx with a maximum age estimate of ~3.6 Ma comprises the Xiyu conglomerates, and integration with other magnetostratigraphic investigations around the Tianshan demonstrates unambiguously that depositional onset of this coarse clastic episode is diachronous. Hence, the Xiyu Formation cannot be considered as a chronostratigraphic marker related to any specific tectonic or climatic event.

New magnetochronology of Late Miocene mammal fauna, NE Tibetan Plateau, China: Mammal migration and paleoenvironments

Lanzhou Basin lies on the northeastern margin of the Tibetan Plateau in western China and is a rich source of Oligocene–Miocene mammalian fossils. Obtaining precise age determinations for these fossils is important to address key questions concerning mammalian and environmental evolution in Asia associated with stepwise Tibetan Plateau uplift. Here we report a new magnetostratigraphic record for the Xingjiawan fluvio-lacustrine section from the northwestern margin of Lanzhou Basin that can be correlated to the geomagnetic polarity timescale with two options. The Late Miocene Xingjiawan Fauna is located either at the boundary between reversed polarity chron C4r.1r and normal polarity chron C4n.2n or at the boundary between subchrons C5r.1r and C5n.2n, with an estimated age of at least ∼8 Ma or perhaps as early as ∼11 Ma. Both age estimations imply that the fossil Stegodon in the Lanzhou Basin is the oldest known record of Stegodon worldwide; it predates the formerly oldest Stegodon find from Africa by at least one million years and perhaps by as many as four million years. This provides new evidence for an Asian origin of Stegodon. Together with other faunal components, a mixed woodland/grassland setting existed in the Lanzhou Basin during the Late Miocene, in contrast to its modern arid environment.

Paleomagnetism of Triassic bedded chert from Japan for determining the age of an impact ejecta layer deposited on peri-equatorial latitudes of the paleo-Pacific Ocean: A preliminary analysis

Bedded chert samples from the Norian (Upper Triassic) Sakahogi section of the Mino Terrane in the Inuyama area, southwest Japan, have been collected for paleomagnetic study in order to determine the age of an impact ejecta layer interbedded within it and the paleolatitude of its deposition. Thermal demagnetization of the bedded chert samples revealed four distinct remanent magnetization components. The last demagnetized component with both polarities is interpreted to be the primary magnetization, which produced the first magnetostratigraphic data of the middle to upper Norian from Panthalassa, consistent with Tethyan magnetostratigraphic sections. Tie points were derived from paleomagnetic and paleontological data, from which the stratigraphic position of the ejecta layer was compared with the astronomically tuned geomagnetic polarity time scale (APTS). The age of ejecta layer is estimated to be about 212Ma. This magnetostratigraphic age is consistent with the radiometric age of the Manicouagan impact that formed the second largest known crater in the Phanerozoic at Quebec, Canada. The results of this study, as well as previous studies, suggest that the chert section, which includes the ejecta layer, was deposited within a paleolatitude range of 0.3–14.4°N. This indicates that the material ejected by the Manicouagan impact event reached near the equatorial region of the paleo-Pacific Ocean.

Integrated stratigraphy and palaeoclimate history of the Carnian Pluvial Event in the Boreal realm; new data from the Upper Triassic Kapp Toscana Group in central Spitsbergen (Norway)

The Late Triassic climate is characterized by arid conditions interrupted by a humid phase known as the Carnian Pluvial Event (CPE). This wet phase is well documented in the Tethyan realm, but evidence from the Boreal realm is scarce. Here we present evidence from quantitative palynology for the CPE from the Kapp Toscana Group on central Spitsbergen integrated with organic carbon isotope data linked to the geomagnetic polarity time scale. Our data reveal an early to mid-Julian-1 age for the Tschermakfjellet Formation. The transition to the Julian-2 is located in the De Geerdalen Formation and the Isfjorden Member is confirmed as mostly Tuvalian-3 in age. The Aulisporites astigmosus pollen assemblage zone that marks the base of the CPE in the Tethys realm at the base of the Julian-2 is assigned to the Julian-1 in the Boreal region. Palaeoclimate proxy data inferred from principal component analysis indicate wetter conditions from the Julian-2 onwards, which is in agreement with the establishment of local swamp vegetation on top of a delta plain. The palaeotemperature curve indicates a period of cooler climate during the early Julian-1 followed by warming during the late Julian-1. Supplementary materials: A list of all identified morphotaxa of pollen, spores and aquatic palynomorphs is available at http://www.geolsoc.org.uk/SUP18879 .

Oligocene‐Miocene magnetostratigraphy and magnetic anisotropy of the Baxbulak section from the Pamir‐Tian Shan convergence zone

AbstractAs the northernmost part of the Indo‐Eurasian collision belt, the Pamir‐Tian Shan convergence zone (PTCZ) is a strategic location for understanding intracontinental deformation. Here we present a magnetostratigraphic investigation of a continuous section from the Baxbulak region, to better constrain regional tectonic history. Rock magnetic analyses indicate that hematite and magnetite are the main carriers of characteristic remanent magnetization. The resulting polarity sequence allows a distinct correlation to the geomagnetic polarity time scale, showing that the section spans the interval of 29.1–20.7 Ma. Rock magnetic results further suggest that paramagnetic and antiferromagnetic minerals dominantly contribute to anisotropy of magnetic susceptibility (AMS) of the sequence. Thus, the AMS would indicate the preferred orientations of the mineral grains that are sensitive to tectonic strain. At around 26 Ma, the grouped principal minimum perpendicular to the bedding diverts to a girdle distribution in a N‐S direction, demonstrating the overprint of tectonic fabric to previous weakly deformed sedimentary fabric. This would be interpreted as a marked increase in tectonic strain, consistent with various evidence from the Pamir and the neighboring basin that show the Pamir began to migrate northward. Moreover, the coincident changes in distribution of AMS principal axes, in both direction and magnitude, are comparable to the regional counterclockwise rotations observed from paleomagnetic data, likely related to orogenesis.

Magnetostratigraphic constraints on the age of the lower Beaufort Group, western Karoo basin, South Africa, and a critical analysis of existing U‐Pb geochronological data

AbstractWe studied three partially overlapping sections with a composite thickness of ∼600 m in the upper Permian fluvial siltstones and fine‐grained sandstones of the Abrahamskraal Formation, the basal unit of the Beaufort Group, in the Karoo Basin of Western Cape Province, South Africa. Paleomagnetic analysis reveals three components of Natural Remanent Magnetization (NRM). Heating to ∼180°C removes a remanent magnetization parallel to the present‐day field, which is interpreted as a viscous overprint. An intermediate unblocking temperature component is removed by heating to 450°C; this direction is always of normal polarity and is identical to a regional overprint imparted during the Early Jurassic emplacement of the Karoo Large Igneous Province. A high‐temperature component isolated above 450°C is of dual polarity and is interpreted as primary on the basis of a positive reversals test. The virtual geomagnetic pole position for the Abrahamskraal Formation computed from the average high‐temperature characteristic remanent magnetization direction is in agreement with the late Permian directions for stable Gondwana and with previous results from the lowermost Abrahamskraal Formation and Waterford Formation at the Ouberg Pass section. The predominantly normal polarity of this magnetization is in agreement with either a middle‐late Lopingian age (ca. 254–256 Ma) or a late Guadalupian age (ca. 262 Ma) according to the global geomagnetic polarity time scale. We integrate these new results with existing magnetostratigraphic, biostratigraphic, and geochronological results from the Karoo Basin, with particular emphasis on the controversy over zircon age data reported from the underlying Ecca Group.

Periodicities in the geomagnetic polarity timescale

The wavelet analysis of the geomagnetic polarity timescale is conducted for the past 250 Ma. The signs of cyclicity with a period of ~50 Ma are revealed.

Magnetic polarity stratigraphy of the Siwalik sequence in Nurpur area, NW Himalaya, India

Magnetic polarity stratigraphy studies were carried out on Jabbar Khad section of Nurpur area, Kangra District, Himachal Pradesh, India. Oriented block samples were collected at 202 sites with 3–5 samples per site from a 2350-m thick continuous sedimentary sequence consisting of sandstone–mudstone alternating beds. Specimens were subjected to thermal demagnetization (100–700∘C) after measuring their natural remanent magnetization (NRM). Palaeolatitudes of virtual geomagnetic pole (VGP) from each site were calculated from the acquired characteristic remanent magnetization (ChRM) directions. The observed remanence carrier could be haematite. Magnetic polarity scale (MPS) was erected. This MPS is correlated with the standard geomagnetic polarity time scale (GPTS) of Lourens et al. (2004). The MPS readily matched from chron C2An.1n to chron C4n.2n, therefore assigning an age 8.20–2.60 Ma to the middle and upper Siwalik sections at Nurpur. The rate of sedimentation in middle Siwalik is 29 mm/yr and upper Siwalik is 59 mm/yr.

Mantle superplumes induce geomagnetic superchrons

We use polarity reversal systematics from numerical dynamos to quantify the hypothesis that the modulation of geomagnetic reversal frequency, including geomagnetic superchrons, results from changes in core heat flux related to growth and collapse of lower mantle superplumes. We parameterize the reversal frequency sensitivity from numerical dynamos in terms of average core heat flux normalized by the difference between the present-day core heat flux and the core heat flux at geomagnetic superchron onset. A low-order polynomial fit to the 0-300 Ma Geomagnetic Polarity Time Scale (GPTS) reveals that a decrease in core heat flux relative to present-day of approximately 30% can account for the Cretaceous Normal Polarity and Kiaman Reverse Polarity Superchrons, whereas the hyper-reversing periods in the Jurassic require a core heat flux equal to or higher than present-day. Possible links between GPTS transitions, large igneous provinces (LIPs), and the two lower mantle superplumes are explored. Lower mantle superplume growth and collapse induce GPTS transitions by increasing and decreasing core heat flux, respectively. Age clusters of major LIPs postdate transitions from hyper-reversing to superchron geodynamo states by 30-60 Myr, suggesting that superchron onset may be contemporaneous with LIP-forming instabilities produced during collapses of lower mantle superplumes.

Magnetostratigraphy and its paleoclimatic significance of the PL02 borehole in the Yinchuan Basin

The basin sediment provides an important means of studying regional tectonic actives; however, regional climate factors, overlaying tectonic actives, affect the sedimentary process in the basin. Because of the lack of long-term scales data, the research on climate change in the arid and semi-arid region is limited. The PL02 core, located in the center area of deposition in the Yinchuan Basin, has a thick deposit with a higher resolution and sedimentation rate, making itself practical for the reconstruction of the paleoclimate and the coupling relationship between regional tectonic activities and climate change.The study of the 720-m borehole near Pingluo County is presented in this paper. The chronostratigraphic sequence of the core is established by palaeomagnetic dating. The magnetostratigraphic correlation to the geomagnetic polarity timescale shows that the PL02 core recorded the Brunhes, Matuyama, Gauss, Jaramillo, Olduvai, Kaena and Mammoth polarity chron or subchron. The sedimentation rate and the magnetostratigraphic of the core indicate that the onset of the PL02 core in this section occurred at ∼3.4Ma BP. The analysis of the core established a detailed chronological framework for regional geological research. At the stage of 3.22–3.04Ma, 2.581–1.95Ma and after 0.012Ma, the sedimentation rate increases gradually, which archive three regional tectonic subsidence events.The frequency-dependent susceptibility of the sediment is used to reconstruct the paleoclimatic change. The frequency-dependent susceptibility is more effective than the magnetic susceptibility on reflecting regional climate change in the basin. From 3.4 to 0.467Ma, the gradual decrease of frequency-dependent susceptibility reflects the regional climate change from warm and humid to cold and dry. Since 0.467Ma, the gradual increase of the frequency-dependent susceptibility corresponds to the climate becoming warm and humid again.

Magnetostratigraphy of the Miocene Las Arcas Formation, Santa María Valley, northwestern Argentina

The first magnetostratigraphic study of the Las Arcas Formation (Late Miocene) was carried out in Las Totoritas creek (26º12′S; 65º47′W, NW Argentina), a key place in between of two geological provinces: Northwestern Pampean Ranges and Eastern Cordillera, in northwestern Argentina. This was accompanied by isotopic dating (9.01 ± 0.12 Ma, 40Ar–39Ar in amphibole) of the unit, obtained from a 3.4 m thick tuff intercalated at ∼45 m above the base. The Las Arcas Formation is 810 m thick at the sampling locality and it is mainly composed of tabular reddish conglomerates, sandstones and siltstones in both coarsening- and thickening-upward arrangements. The exposed section was sampled at 48 sites, 26 of which are interpreted as carrying primary magnetization. The new magnetostratigraphic column was correlated with the Geomagnetic Polarity Time Scale (GPTS), and suggests that deposition of the Las Arcas Formation strata started at around 9.1 Ma and ended around 6.8 Ma. The paleomagnetic pole obtained for this unit (Dec = 8.7° Inc = −43.9° dp = 14.9 dm 9.3) indicates that this area underwent non-significant rotation (11.0° ± 13.6°) since the Late Miocene.

Reversals in the large-scale αΩ-dynamo with memory

Abstract. Inversion of the magnetic field in a model of large-scale αΩ-dynamo with α-effect with stochastic memory is under investigation. The model allows us to reproduce the main features of the geomagnetic field reversals. It was established that the polarity intervals in the model are distributed according to the power law. Model magnetic polarity timescale is fractal. Its dimension is consistent with the dimension of the real geomagnetic polarity timescale.

Magnetostratigraphy of the Xiaolongtan Formation bearing Lufengpithecus keiyuanensis in Yunnan, southwestern China: Constraint on the initiation time of the southern segment of the Xianshuihe–Xiaojiang fault

The late Cenozoic extensional basins in Yunnan Province (southwestern China), which are kinematically linked with the regional strike-slip faults, can provide meaningful constraints on the fault activity history and tectonic evolution of the southeast margin of the Tibetan Plateau (SEMTP), and further on the geodynamic evolution of the Tibetan Plateau. However, this has been severely impeded by the lack of precise age constraints on the timing of fault activity. To better constrain the timing of fault activity and the tectonic rotation of SEMTP, we undertook a high-resolution magnetostratigraphic study on the Xiaolongtan Formation in the Xiaolongtan Basin, which is located at the southern tip of the Xianshuihe–Xiaojiang fault and is well-known by the presence of hominoid Lufengpithecus keiyuanensis. Rock magnetic experiments indicate that magnetite is the main remanence carrier. Correlation to the geomagnetic polarity timescale was achieved by combining magnetostratigraphic and biostratigraphic data. Our correlation suggests that the Xiaolongtan Formation sedimentary sequence spans from Chron C5Ar.1r to Chron C5n.2n, which indicates that the age of the Xiaolongtan Formation ranges from ~10Ma to 12.7Ma, and that the ages of the two sedimentary layers possibly bearing the hominoid L. keiyuanensis are ~11.6Ma or ~12.5Ma. The basal age of the sediments is 12.7Ma, which indicates that the activation of the southern Xianshuihe–Xiaojiang fault was initiated at this time. The overall mean paleomagnetic direction (D=353.2°, I=34.2°, α95=2.1°, n=166) documents a counter-clockwise vertical axis rotation of −8±3° with respect to Eurasia, which is the response to the activity of the left-lateral Xianshuihe–Xiaojiang Fault.

Magnetostratigraphic record of the early evolution of the southwestern Tian Shan foreland basin (Ulugqat area), interactions with Pamir indentation and India–Asia collision

The Tian Shan range is an inherited intracontinental structure reactivated by the far-field effects of the India–Asia collision. A growing body of thermochronology and magnetostratigraphy datasets shows that the range grew through several tectonic pulses since ~25Ma, however the early Cenozoic history remains poorly constrained. The time-lag between the Eocene India–Asia collision and the Miocene onset of Tian Shan exhumation is particularly enigmatic. This peculiar period is potentially recorded along the southwestern Tian Shan piedmont. There, late Eocene marine deposits of the proto-Paratethys epicontinental sea transition to continental foreland basin sediments of unknown age were recently dated. We provide magnetostratigraphic dating of these continental sediments from the 1700-m-thick Mine section integrated with previously published detrital apatite fission track and U/Pb zircon ages. The most likely correlation to the geomagnetic polarity time scale indicates an age span from 20.8 to 13.3Ma with a marked increase in accumulation rates at 19–18Ma. This implies that the entire Oligocene period is missing between the last marine and first continental sediments, as suggested by previous southwestern Tian Shan results. This differs from the southwestern Tarim basin where Eocene marine deposits are continuously overlain by late Eocene–Oligocene continental sediments. This supports a simple evolution model of the western Tarim basin with Eocene–Oligocene foreland basin activation to the south related to northward thrusting of the Kunlun Shan, followed by early Miocene activation of northern foreland basin related to overthrusting of the south Tian Shan. Our data also support southward propagation of the Tian Shan piedmont from 20 to 18Ma that may relate to motion on the Talas Fergana Fault. The coeval activation of a major right-lateral strike-slip system allowing indentation of the Pamir Salient into the Tarim basin, suggests far-field deformation from the India–Asia collision zone affected the Tian Shan and the Talas Fergana fault by early Miocene.

Long duration (>4 Ma) and steady-state volcanic activity in the early Cretaceous Paraná–Etendeka Large Igneous Province: New palaeomagnetic data from Namibia

There is long-standing correlation between Large Igneous Provinces (LIPs) and major mass extinction events in the Geological Record, postulated to be due to the emission of large quantities of volcanic gases over a geologically short period of time causing major climatic perturbations within the Earth system. The ∼135 Ma Paraná–Etendeka volcanic province of Brazil and Namibia represents something of an enigma amongst LIPs. Despite an erupted volume (>1 Mkm3) comparable to other LIPs associated with mass extinctions, such as the Siberian or Deccan traps, it is not linked to a known mass extinction event. This suggests that the Paraná–Etendeka volcanic province was emplaced over longer timescales than other LIPs, and/or emitted a lower concentration of volatiles, directly or indirectly during its emplacement.We present a new, detailed magnetostratigraphy for the Etendeka portion of the province that suggests emplacement took place over longer timescales (>4 Ma) than those associated with other LIPs. Palaeomagnetic analysis of 893 specimens from 99 sites, in sections that encompass nearly the complete Etendeka stratigraphy, yielded high-quality data from 70 sites (612 specimens). These record 16 individual polarity intervals, which can be correlated with Chrons 15 to 11 of the geomagnetic polarity time scale (GPTS) while also providing two new, high quality palaeopoles for South Africa at 130–135 Ma. Our magnetostratigraphy reveals a minimum period of volcanic activity in excess of 4 Myrs and, importantly, we find no evidence for major changes in the rates of volcanic activity through that time period, in contrast to other LIPs where volcanism seems to be concentrated in major pulses. This suggests that the anomalously feeble environmental impact of Paraná–Etendeka volcanism may be due to lower effusion rates reducing the atmospheric loading due to volcanogenic volatiles.

Early to middle Eocene magneto-biochronology of the southwest Pacific Ocean and climate influence on sedimentation: Insights from the Mead Stream section, New Zealand

The Mead Stream section (South Island, New Zealand) consists of a 650-m-thick series of continuous, well-exposed strata deposited on a South Pacific continental slope from the Late Cretaceous to the middle Eocene. We examined the uppermost Paleocene–middle Eocene part of the section, which consists of ∼360 m of limestone and marl, for detailed magnetic polarity stratigraphy and calcareous nannofossil and foraminifera biostratigraphy. Magneto-biostratigraphic data indicate that the section straddles magnetic polarity chrons from C24r to C18n, calcareous nannofossil zones from NP9a to NP17 (CNP11–CNE15, following a recently revised Paleogene zonation), and from the Waipawan to the Bortonian New Zealand stages (i.e., from the base of the Ypresian to the Bartonian international stages). The Mead Stream section thus encompasses 17 m.y. (56–39 Ma) of southwest Pacific Ocean history. The ages of calcareous nannofossil biohorizons are consistent with low- to midlatitude data from the literature, indicating that during the early–middle Eocene, the low- to midlatitude calcareous nannofossil domain extended at least to ∼50°S–55°S in the South Pacific. Correlation of the magnetic polarity stratigraphy from the Mead Stream section with the geomagnetic polarity time scale allows us to derive sediment accumulation rates (SAR), which range between 8 and 44 m/m.y. Comparing the SAR with paleotemperature proxy records, we found that two intervals of increased SAR occurred during the early Eocene climatic optimum (52–50 Ma) and during the transient warming event peaking with the middle Eocene climatic optimum (40.5 Ma). This correlation indicates that, at Mead Stream, the climate evolution of the early–middle Eocene is recorded in a sedimentation pattern whereby, on a million-year time scale, warmer climate promoted continental weathering, transportation, and accumulation of terrigenous sediments.

Cyclostratigraphy and orbital tuning of the terrestrial upper Santonian–Lower Danian in Songliao Basin, northeastern China

The Songke-1 north (SK-1n) borehole recovered a continuous, 1541.66 m Late Santonian–Early Danian terrestrial succession in Songliao Basin (SB), northeastern China. It provides a unique record for improving our understanding of continental paleoclimate and ecological system in Cretaceous greenhouse world. Here we use thorium (Th) logging data as a paleoenvironmental and paleoclimatic proxy to conduct a detailed cyclostratigraphic study on the SK-1n core. Power spectra, evolutionary fast Fourier transformation and wavelet analysis all reveal significant decameter- to meter-scale sedimentary cycles in the Nenjiang (K2n), Sifangtai (K2s) and Mingshui (K2m) formations. The ratios of cycle wavelengths in these stratigraphic units are ∼20:5:2:1, and are interpreted as Milankovitch cycles of 405 kyr and 100 kyr eccentricity, 38.4 kyr obliquity and 20 kyr precession cycles, respectively. An astronomical time scale (ATS) is established by tuning filtered 405 kyr eccentricity cycles to a target curve of the astronomical solution La2010d based on the magnetostratigraphic time framework of the SK-1n borehole. This ATS provides precise numerical ages for stratigraphic boundaries, biozones, geological and geophysical events, and serves as a basis for correlation of strata and events between marine and terrestrial systems. The Cretaceous/Paleogene (K/Pg), Campanian/Maastrichtian, Santonian/Campanian boundaries are estimated at core depths of 318 m, 752.8 m and 1751.1 m, respectively. A ∼3.8 myr-long hiatus between the Nenjiang (K2n) and Sifangtai (K2s) formations occurs from 76.1 to 79.9 million years ago. The ages and durations of magnetochrons C33r to C30n are precisely estimated and provide new constraints on the Late Cretaceous Geomagnetic Polarity Time Scale (GPTS) and South Atlantic sea-floor spreading rates.