Geomagnetic Polarity Time Scale Research Articles

Cretaceous magnetostratigraphy of the southern Simao Basin, SE Tibetan Plateau, and its paleogeographic implications

Precise stratigraphic chronology is the basis of many studies (e.g., tectonic, paleoclimate, and mineralization) in geoscience. Here, we carried out a detailed youngest detrital zircon U-Pb age-constrained magnetostratigraphic study on the middle-lower parts of a 2309-m-thick Upper Jurassic−Lower Cretaceous sequence from west Jiangcheng in the southern Simao Basin, southeastern Tibetan Plateau. A total of 2262 paleomagnetic samples were collected for magnetostratigraphy, and five siltstone/sandstone samples were collected for detrital zircon U-Pb analyses. Progressive thermal and alternating field demagnetizations isolated 1575 well-defined interpreted primary characteristic remanent magnetization directions, which yielded 25 normal polarity (N7−N31) and 25 reverse polarity (R8−R32) magnetozones. The interpreted maximum depositional ages calculated using the youngest detrital zircon U-Pb dates are 159.0 Ma, 154.0 Ma, 161.0 Ma, 149.3 Ma, and 139.5 Ma for the stratigraphic horizons at 2303 m, 1986 m, 1600 m, 1350 m, and 1110 m, respectively, in the section. Together with the previous magnetostratigraphic results of the upper part of the section, the observed polarity zones are best correlated with chrons C34n−M17r of the geomagnetic polarity timescale (GPTS2012), yielding magnetostratigraphic age estimates of ca. 143.5 Ma to ca. 65.0 Ma for the entire stratigraphic sequence. Paleogeographic analysis shows that the Simao Basin was in a low-latitude offshore environment during the Late Jurassic−earliest Cretaceous (before 142.3 Ma), a dominant fluvial environment during the early−middle Early Cretaceous (142.3−111.5 Ma), with widespread desertification at the end of this period, a saline lake sedimentary environment during the late Early Cretaceous−early Late Cretaceous (111.5−88.7 Ma), and a fluvial environment during the late Late Cretaceous (88.7−65.0 Ma). The changes in depositional environment are consistent with regional/global tectonic and paleoclimate changes, such as the dismission of Pangea, the termination of the megamonsoon, and the closure of the Meso-Tethys Ocean.

Chronostratigraphy and tectono‐sedimentary history of the Eastern South Pyrenean foreland basin (Ripoll Syncline, North‐East Spain)

AbstractThis paper contributes to an understanding of the tectono‐sedimentary evolution of the South Pyrenean foreland system by reviewing the chronostratigraphic framework of the basin infill in its eastern sector. Six sections are analysed and cross‐correlated to build a 6.5 km thick composite magnetostratigraphy that represents the complete record of the Cadí Nappe in the Ripoll Syncline. New and previous magnetostratigraphic sections are integrated with available biostratigraphy to provide a new age calibration of the sedimentary succession of the Cadí Nappe, encompassing from Palaeocene to Middle Lutetian age. The proposed correlation with the Geomagnetic Polarity Time Scale aims at best reconcile magnetostratigraphic data with the regional biochronology built on the marine Shallow Benthic Zonation (SBZ biozones), the continental mammalian biochronology (MP levels) and the newly collected charophyte data. A subsidence analysis was performed on the calibrated composite succession, resulting in two well‐defined intervals bounded by a hiatus. A Palaeocene to Early Eocene interval with low (11–21 cm/kyr) total subsidence rates, and an Early to Middle Eocene interval, characterised by high (70–75 cm/kyr) total subsidence rates. The detailed trends in both subsidence and sedimentation mark the development and evolution of the foreland depozones, from distal foreland depozones to foredeep and wedge‐top depozones, relative to the emplacement of the Pedraforca Nappe and Cadí Thrust Nappe. The most pronounced sedimentary shift in the Cadí Nappe occurred at 49 Ma, with the rapid drowning of the carbonate platforms and its transition into talus and deep basinal environments. Carbonate platforms collapsed and resedimented on the talus of the elongated trough, newly formed parallel to the orogenic front. This marked the onset of tectonic subsidence triggered by the submarine emplacement of the Lower Pedraforca Nappe. The emersion of the orogenic wedge drove the entry of siliciclastics, lagged by 1 Myr, into the Ripoll Trough. The foredeep filled rapidly (5.5 km thickness in less than 7 Myr) compared to other South Pyrenean regions, favoured by its semi‐enclosed palaeogeography. The emplacement of the Vallfogona Thrust as early as the Middle Lutetian (43 Ma) brought the Cadi Nappe into a wedge‐top setting. However, the Ripoll growth syncline continued acting as a temporary sink for alluvial sedimentation while a foredeep developed further south in the autochthonous Ebro Basin. The flexural response of the Iberian plate to the tectonic thickening of the Axial Zone counterbalanced for a period the local uplift of the Cadi Nappe, providing accommodation space for the top sediments filling the Ripoll Syncline.

Magnetostratigraphy of the Punta Grohmann section (Dolomites, Italy): improving the chronology of the Ladinian/Carnian boundary

We provide the magnetostratigraphy of the Ladinian/Carnian Punta Grohmann stratigraphic section (Dolomites, Italy), calibrated with UPb ages from the literature (237.58 ± 0.04 Ma; 237.68 ± 0.05 Ma). The FO of ammonoid Zestoceras lorigae, the base of the vigens-densus palynomorphs Zone, and sequence stratigraphy suggest a Carnian age for the upper part of the section, which has been confirmed by the magnetostratigraphic correlation between Punta Grohmann and Prati di Stuores (Carnian GSSP). Additional magnetostratigraphic correlation between Punta Grohmann and key time-calibrated Ladinian–Carnian sections from the literature (e.g., Mayerling, Seceda, Rio Nigra) lead to a more precise age of the Carnian base (∼236.5 Ma) and to a better definition of the Middle–Late Triassic Geomagnetic Polarity Time Scale.

Magnetic and geochemical scanning reveals growth history of marine ferromanganese crust on Detroit Seamount, northwest Pacific since the early Miocene

The Hawaiian Chain-Emperor Seamounts is known to be one of the most active regions influenced by mantle hotspot. However, previous studies have provided limited information on the growth process and metal enrichment of marine ferromanganese crusts as indicators of post-eruption history. In this study, we conducted a geochronological and major elemental study of a hydrogenetic ferromanganese crust, from the Detroit Seamount using high-resolution magnetic and geochemical scanning. Our findings are as follows: (1) A total of 39 magnetozones are recognized in the studied section of the crust, which can be preliminarily correlated to the geomagnetic polarity timescale from chrons C1n to C5Dn. This correlation yields growth rates of 1.48–2.18 mm/Myr over the past ∼18 Myr. (2) Columnar and lamellar growth structures are identified with distinct metal enrichment patterns. A significant transition from lamellar to columnar growth occurred at ∼11.6 Ma. (3) The quantified elements can be clustered into three groups, (Mn, Cu–Co–Ni, and Al–Si), (Fe and Ti), and Ca. A strong correlation between metal enrichment in the crust and the global benthic foraminifera δ13C record is then observed. Based on these results, we suggest a predominant role of deep-sea redox conditions in the growth history of ferromanganese deposits on the Detroit Seamount since the Early Miocene.

Charophyte flora from the Oligocene fossil site of Suceag (Transylvanian Basin, Romania)

A charophyte assemblage composed of six species has been recovered from lower Oligocene deposits of the Dâncu Formation at the fossil site of Suceag (Transylvanian Basin, Romania). The assemblage is dominated by the two varieties of the species Lychnothamnus pinguis (L. pinguis forma pinguis and L. pinguis forma major). Accessory species include Lychnothamnus prealangeri, Nitellopsis (Tectochara) merianii, and few Sphaerochara sp. and Harrisichara sp. gyrogonites. L. pinguis is the index species of the homonymous Rupelian European biozone which was previously correlated in Western Europe with the Mammal Paleogene Reference Levels of MP21–MP23, and calibrated with the Geomagnetic Polarity Time Scale (GPTS), chrones C13N and C12R. From a paleobiogeographic viewpoint, this study confirms the presence of L. pinguis forma pinguis and L. praelangeri in Romania and in the whole Eastern Europe. Moreover, the charophyte assemblage from Suceag belongs to the Priabonian–Rupelian European charophyte bioprovince. While the vertebrate fossils reported in the same fossil locality indicate that the Dâncu Formation at Suceag sedimented in an estuarine environment, the charophyte assemblage suggests a more complex pattern, with presence of freshwater lakes too.

Late Miocene to Present Paleoclimatic and Paleoenvironmental Evolution of the South China Sea Recorded in the Magneto‐Cyclostratigraphy of IODP Site U1505

AbstractThe continuous sedimentary cores recovered at the International Ocean Discovery Program (IODP) Site U1505, Expedition 368, provide an opportunity for paleoceanography and paleoclimate reconstruction in the continental margin of the northern South China Sea (SCS). In this study, we conducted detailed rock‐ and paleomagnetic studies on 420 discrete samples from the top ∼200 m of the synthetic records of Holes U1505C and U1505D. Rock magnetic analyses indicate that low‐coercivity pseudosingle domain magnetite dominates as the primary ferromagnetic mineral of Site U1505. The magnetostratigraphic age model was constructed by correlating the interpreted polarity sequence with the Geomagnetic Polarity Time Scale 2020 with the constraints of the biostratigraphic data and the distribution probability of the age of each polarity zone provided by a Dynamic Time Warping algorithm. The Milankovitch cycles of the short eccentricity, obliquity, and precession cycles were identified in the magnetic susceptibility (MS) and natural gamma radiation (NGR) series based on paleomagnetic results. We established an ∼9 Myr high‐resolution astronomical time scale by tuning the MS and NGR records to the global oxygen isotope curves, the obliquity, and the eccentricity curves of the La2004 astronomical solution. Our new age model reveals detailed sedimentation rate variations and a ∼500 kyr hiatus across the Brunhes‐Matuyama boundary related to local tectonic activity. These results lay the foundation for understanding the paleoceanography and paleoclimate evolution of the SCS.

First magnetostratigraphic results in the Aïn Beni Mathar-Guefaït Basin, Northern High Plateaus (Morocco): The Pliocene-Pleistocene Dhar Iroumyane composite section

The Aïn Beni Mathar-Guefaït Basin, in the High Plateau Region (Morocco), is dissected by the Za River, the main eastern tributary of the Moulouya, which incises more than 150 m into Plio-Pleistocene sediments. The main goal of the present study is to provide an initial geochronologic framework for such basin infill based on a combination of magnetostratigraphy and electron spin resonance (ESR). The combined results have implications on the age of the paleontological record, the evolution of the Moulouya River, and the activity of the faults that delimit the basin. We have studied sedimentary rocks that are essentially flat-lying and of an alluvial and lacustrine/palustrine origin. An approximately 140 m-thick section has been sampled at an average of 2.5 m per site, allowing to build a local magnetic polarity stratigraphy that includes nine geomagnetic reversals. Although no fold test is available, the presence of dual polarities and rockmagnetic analysis give us confidence that magnetization directions are primary. We then anchored the obtained magnetozones to the Geomagnetic Polarity Timescale (GPTS) using the biostratigraphic data as well as local geological observations. Our proposed magnetostratigraphy-based chronology reveals a Plio-Pleistocene sedimentary infill spanning from Gauss to Olduvai Chrons. The lower, detrital formations mostly fall within the normal Gauss Chron, whereas the upper lacustrine and palustrine carbonates, which are almost widespread to the top of the sedimentary fill, are Olduvai in age. These results provide the first chronological constraints for the basin fill in one of the largest intermontane basins of the High Plateaus. The new magnetostratigraphy also reveals that the major environmental change that triggered a switch from alluvial to lacustrine-palustrine conditions in Northern Maghreb occurred near the Gauss-Matuyama reversal, ca. 2.6 Ma. In addition, it shows that the age of the paleontological site Guefaït-4 is approximately 1 myr older than the ESR ages obtained from the quartz grains. The possible sources for this underestimation are discussed. However, we also acknowledge and discuss other possible chronostratigraphic interpretations of the current data, although less likely.

Magnetostratigraphy of the Oligocene and Miocene of the Linxia Basin, northwestern China

The Cenozoic biochronology framework for northern China has been based on few vertebrate fossil localities with unverified age constraints. The Linxia Basin, located on the northeastern margin of the Qinghai-Tibetan Plateau, northwestern China, preserves thick, successive terrestrial sequences with an enormous quantity of vertebrate fossils of ages spanning the Late Oligocene to Early Pleistocene. This record has bearing on the definition of Cenozoic mammalian biochronology of North China, and its better dating. However, our understanding of detailed mammalian evolution and infilling history of the Linxia Basin has been hampered by poor age constraints and disagreement with new paleontological findings. To clarify the age range of the fluvial-lacustrine deposits and their faunas in the lower part of the succession, we present here a revised magnetostratigraphy and its chronology obtained from three well-correlated and overlapping sections in the eastern Linxia Basin. Magnetite and hematite were identified as the main ChRM (characteristic remanent magnetization) carriers in the sediments. Reliable specimens from 1142 sampled layers representing a total thickness of 484.4 m from the three sections yielded 28 normal and 23 reversed polarities. With the aid of updated biochronological constraints, the fluvial-lacustrine sequence of five stratigraphic units (Tala, Jiaozigou, Shangzhuang, Dongxiang, and Liushu/Hujialiang formations), was successfully correlated to chrons of the GPTS (geomagnetic polarity timescale). These represent the top of Chron C13r (ca. 33.8 Ma) to lower Chron C5n.2n (ca. 10.8 Ma). The recalibrated basin-infilling history shows response to episodic growth of topographic highs during these time intervals. Moreover, this new age model yields an estimated age of ca. 27 Ma (within Chron C9n of early Late Oligocene) for the paracerathere fauna from the eastern Linxia Basin, which is in accordance with other biochronological findings. The new age estimate for the paracerathere fauna supports a proposal for relocating the basal boundary of the Chinese Tabenbulukian Land Mammal Stage/Age to Chron C9r.

The Late Cretaceous alkaline magmatism in the SE Brazilian coast: new paleomagnetic data and age constraints

Alkaline dikes from the Santos-Rio de Janeiro coast, SE Brazil, mainly from São Sebastião and Búzios islands, yielded a Late Cretaceous paleomagnetic pole (SR) located at 319.7°E 81.2°S (N = 44, A95 = 3.0°, k = 44). This pole includes some sites of alkaline stocks from São Sebastião Island of the same age and supersedes the previous pole for this island. To match the available radiometric ages and the prevailing normal polarity remanence of the rocks with the geomagnetic polarity time scale, the SR pole is placed at 84 Myr. Another group of alkaline rocks, dikes located mainly in Rio de Janeiro, was assigned an age of less than 70 Myr. The ages of the Poços de Caldas, Itatiaia, and Passa Quatro paleomagnetic poles are also discussed based on available radiometric data. Assuming a rigid plate, the SR pole indicates the southward movement of about 7° with virtually no rotation between 100 and 84 Myr. From approximately 84 to 70 Myr, a clockwise rotation of 8° is postulated, with slight variation in latitude.

Eocene to Miocene Charophytes from the Qaidam Basin on the northern Tibetan Plateau and its calibration to the Geomagnetic Polarity Time Scale

Charophyte assemblages from the Qaidam Basin on the northern edge of the Tibetan Plateau were studied. The taxonomy of charophytes is based on a study of intraspecific gyrogonite polymorphism and biometric data. A total of 6 species belonging to 5 genera were identified, including Sphaerochara inconspicua Feist-Castel, 1977, Psilochara sp., Chara media Grambast, 1958, Lychnothamnus barbatus var. antiquus Soulié-Märsche, 1989, Lychnothamnus pinguis forma major (Grambast and Paul, 1965) Soulié-Märsche, 1989, Nitellopsis (Tectochara) merianii (Braun ex Unger, 1852) Grambast and Soulié-Märsche, 1972. Lamprothamnium ganchaigouensis (Tang and Di, 1991) Li et al., 2020 and Lychnothamnus vectensis (Groves et al., 1926) Soulié-Märsche, 1989 have already been reported from the Xiaganchaigou Formation by Li et al. (2020). These Eurasian species allow direct correlation between Chinese and European basins. The distribution of these species in the Qaidam Basin is correlated to the high-resolution magnetostratigraphy of the Paleogene–Neogene sequence, ranging from >52 Ma to ∼7 Ma at Dahonggou in the northern Qaidam Basin. N. (T.) merianii, with living representatives Nitellopsis obtusa, was reported to be expanded eastwards to Asia during the late Oligocene and Miocene. On the contrary, L. barbatus var. antiquus was found in the Oligocene (Chattian) of the Tibetan Plateau, representing the oldest record of this still living species. This may indicate that L. barbatus originated in Asia and expanded to other parts of Eurasia during the Miocene.

Timing of Tectonic and Magmatic Events in the Philippine Sea Plate Since 50 Ma From High‐Resolution Magnetostratigraphy of IODP Site U1438

The Philippine Sea Plate in the West Pacific is a unique natural laboratory to study subduction dynamics and the evolution of upper plate magmatism following subduction initiation. To investigate these processes, International Ocean Discovery Program Expedition 351 recovered at Site U1438, located in a rear-arc position, a complete sedimentary sequence from recent age to the early Eocene and the top of the underlying volcanic basement. The recovered cores offered the opportunity to study for the first time and in unprecedented detail the styles, products, and timing of the volcanic events that marked the emplacement, growth, and demise of the Kyushu-Palau volcanic arc following the inception of the Izu-Bonin-Mariana subduction. Here, we report a magnetostratigraphy for Site U1438 based on ∼60,000 remanence directions isolated from 1,063 archive half core sections and 429 discrete specimens. We identified 142 magnetic reversals and correlated 115 of them with the geomagnetic polarity timescale. When combined with additional biostratigraphic and geochronological constraints, our magnetostratigraphy allowed construction of a high-resolution age model for Site U1438 and the determination of changes in sedimentation rates. We show that following subduction initiation at 52–50 Ma and the emplacement of basalts in the rear-arc at 48.7 Ma, a diffuse volcanism in the rear-arc (48.4–45.6 Ma) preceded the true emplacement of the Kyushu-Palau arc at 40.2 Ma, which then grew through four compositionally distinct eruptive phases until 28.8 Ma. Subsequent rollback of the Pacific slab triggered rifting of the arc (28.8–24.3 Ma) and ultimately back-arc spreading in the Shikoku and Parece Vela basins.

The chronology and rotational kinematics in the South-Eastern Jaca Basin (Southern Pyrenees): Las Bellostas section

Despite the large number of magnetostratigraphic studies in the South Pyrenean Basin aiming to calibrate the basin chronostratigraphy and the biostratigraphic scales, the South Eastern Jaca Basin remains unexplored from this perspective, and its relation with the Ainsa Basin is not fully understood. In this work we contribute with new magnetostratigraphic data from the 950m thick Las Bellostas section, located in the northern hinge of the Balzes anticline. Well-proven primary signal (positive fold test and two pseudo-antiparallel polarities) supported by numerous primary data in the surroundings allow us building a reliable local polarity sequence of eight magnetozones (from R1 to N4). Additionally, seven new biostratigraphic samples (Nummulites and Assilina) in the lower part of the section (marine environment) allows tightening the section to the Geomagnetic Polarity Time Scale (GPTS) and proposing a refined age model for the southeastern Jaca Basin. The section starts in the Boltaña Fm., of Cuisian age (Shallow Benthic Zone, SBZ11), is followed by a sedimentary gap from C22n to C20n as witnessed by biostratigraphic data (SBZ11 underneath the hiatus and SBZ16 just atop). The deltaic Sobrarbe Fm. can be tracked until the C19n (Late Lutetian). From this point (200m) until the top of the section (950m), at least, the entire C18n chron can be recognized within the molassic Campodarbe Fm. (C18n.2n-C18n.1r and C18n.1n) (Bartonian) equivalent to the West to the prodeltaic Arguis marls Fm. The Middle Cusian (SBZ11) to Middle Lutetian (SBZ15) stratigraphic hiatus is, in part, enhanced by the structural position at the hinge of the Balzes anticline. These new chronostratigraphic constraints help refining the W-E and N-S stratigraphic relationships in the eastern Jaca Basin and in the Ainsa Basin. This section also allows us to accurately refine the kinematics of the rotational activity in the eastern External Sierras. The important difference in magnetic declination along the section and neighboring paleomagnetic data from the Balzes anticline (from ≈70º clockwise at the base of the stratigraphic section to non-significant at the top) together with the new age model for the Eastern Jaca Basin help characterizing the rotational activity of the Balzes thrust sheet. The rotation took place between chrons C20r (Middle Lutetian; 45Ma) and C17 (Lower Priabonian 37-38Ma) in agreement to nearby structures (Boltaña, Picodel Aguila anticlines) but clearly diachronic to western ones (Santo Domingo anticline). Besides, the rotational activity seems to follow a linear and continuous pattern (velocity 9-11º/Ma, R: 0.83-0.96) in contrast to closer structures that show two distinct rotational velocities (i.e. Boltaña). These new data still let open the debate on the rotational kinematics along the South Pyrenean basal thrust.

New chronological constraints for the lowermost stratigraphic unit of Atapuerca Gran Dolina (Burgos, N Spain)

We present the results of a combined Electron Spin Resonance (ESR) and Luminescence dating study for the lowermost stratigraphic unit (TD1) of the palaeoanthropological site of Atapuerca Gran Dolina, Spain. Ten samples collected through the Main Section were dated using either the Multiple Centre (MC) ESR approach or the single-grain thermally transferred optically stimulated luminescence (SG TT-OSL) technique, both of which were applied to quartz grains. The two methods yield consistent ages, providing a robust chronostratigraphic framework for the TD1 deposits and enabling improved correlation of the magnetostratigraphic succession (comprising three intervals of normal polarity; N1 to N3, from bottom to top) with the Geomagnetic Polarity Time Scale. Specifically, the results provide a late Early Pleistocene chronology for TD1, and clearly position the deposits in the post-Olduvai time range (<1.77 Ma). The final age estimates range from 1.30 ± 0.14 Ma (TT-OSL) close to the bottom of the section to a mean ESR age of 1.05 ± 0.12 Ma (1 s.d.) for three samples from the N3 interval towards the top. Consequently, the N3 interval may confidently be correlated to the Jaramillo Subchron, while we propose a correlation of N1 and N2 with the Cobb Mountain subchron and Punaruu excursion, respectively; although we cannot exclude that these two normal intervals may correspond to other, unknown, geomagnetic events given the existing uncertainties associated with the ESR and luminescence ages. From a methodological point of view, the ESR MC approach shows that the Titanium signal provides the most reliable burial ages for TD1. In contrast, the Aluminium centre yields systematically overestimated age estimates, as a result of incomplete resetting of the signal before sediment deposition. The SG TT-OSL ages presented here are amongst the oldest published so far, and highlight the potential of the high temperature SG TT-OSL 290 approach for obtaining finite ages exceeding 1 Ma at Atapuerca. Finally, these new dating results provide key insights into the rates and nature of sedimentary processes at Gran Dolina, modifying long-standing interpretations of the cave formation and infilling dynamics. An age of at least 1.4 Ma may be inferred for the base of the sedimentary infill at the Main Section, providing minimum age constraint for the formation of the cavity and, by extension, for the intermediate level of the Atapuerca karst. The opening of the cave, which pre-dates human occupation, most likely occurred close to or slightly after 0.9 Ma. This suggests that both TD4 and TD6 archaeological levels were deposited within a relatively short time range of <100 kyr. The so-called autochthonous fluvial deposits from TD1 unit, or at least those dated in the present study, were also clearly primarily derived from outside the karst system, and were not reworked from older deposits within the karst. They experienced sufficient daylight exposure and minimum transport times within the karst system to enable consistency between the ESR and TT-OSL burial ages and the independent magnetostratigraphic evidence. Our results highlight the need to reconsider the terminology traditionally employed to differentiate the two main phases of Gran Dolina's sedimentary infill. New numerical age results for stratigraphic unit TD1 of Atapuerca Gran Dolina, Spain The presence of Jaramillo magnetic Subchron (1.07-0.99 Ma) is confirmed The base of the sedimentary infill post-dates Oduvai Subchron (<1.77 Ma) Cave opening occurred ca. 0.9 Ma ago, suggesting that TD4-6 units deposited in < 100 kyr TD1 fluvial deposits do not originate from within the karst

New 40Ar/39Ar ages from the Grande Ronde and Wanapum Basalt, Columbia River Basalt Group (CRBG): Compilation of all ages and relationship to the geomagnetic polarity time scale for ~17–15 Ma

New 40Ar/39Ar ages from the Grande Ronde and Wanapum Basalt, Columbia River Basalt Group (CRBG): Compilation of all ages and relationship to the geomagnetic polarity time scale for ~17–15 Ma

Geochronological Evidence Inferring Carbonate Compensation Depth Shoaling in the Philippine Sea after the Mid-Brunhes Event

Carbonate compensation depth (CCD) is an important factor in the global deep ocean and in global carbon cycling; however, its variabilities have not been well documented in previous studies. In this study, we investigate two deep-sea cores collected from the Philippine Sea in terms of geochronology and geochemical properties over the past ~900 kyr. The principle results are as follows: (1) Two magnetozones are determined from the sediment’s magnetic records, which can be correlated with the Brunhes and Matuyama chrons in the geomagnetic polarity timescale. (2) The age models can be refined by tuning the Ba and Sm intensities of the two studied cores to the global ice volume, and the estimated sediment accumulation rate is ~4 mm/kyr. (3) Chalky mud and the bulk carbon δ13C record vary abruptly at ~430 ka and imply 200 m shoaling of the CCD. Based on these results, a close link is inferred between marine productivity, aeolian dust, and CCD changes, which can be correlated with a major change that occurred during the Mid-Brunhes Event. Therefore, we propose that the sedimentary processes in the Philippine Sea are evidence of global climate change, providing a unique window to observe interactions between various environmental systems.

Middle Miocene-Pleistocene Magneto-Cyclostratigraphy from IODP Site U1501 in the Northern South China Sea

Site U1501 of International Ocean Discovery Program (IODP) Expedition 368 locates on a broad regional basement high in the northern margin of the South China Sea (SCS). This study refines the chronostratigraphy of the upper 160 m sedimentary succession from Hole U1501C using paleomagnetic measurements and cyclostratigraphic analysis on the Natural Gamma Radiation (NGR) data. Rock magnetic analysis displays that the magnetic signal of the sediments is mainly carried by single-domain (SD) and multi-domain (MD) magnetite. A total of 12 geomagnetic reversals are identified and correlated to the geomagnetic polarity time scale (GPTS) in Geologic Time Scale 2020 (GTS 2020), combining biostratigraphic data and planktonic foraminiferal oxygen records. The Milankovitch cycles of 405-kyr long orbital eccentricity, ∼100-kyr short orbital eccentricity, and obliquity cycles are identified in the NGR profile. A 15.54 Myr astronomical time scale is constructed by tuning the short eccentricity cycles filtered from the NGR profiles to the La2010 astronomical solution with the constraints of the magnetostratigraphic results, biostratigraphic age datum and planktonic foraminiferal oxygen records. This new high-resolution age model provides a new temporal constraint on the tectonic and paleoenvironmental evolution in the South China Sea.

Magnetostratigraphy of the Upper Cretaceous Nenjiang Formation in the Songliao Basin, northeast China: Implications for age constraints on terminating the Cretaceous Normal Superchron

We developed an integrated chronology for the non-marine Upper Cretaceous Nenjiang Formation based on high-resolution magnetostratigraphic results and previously published secondary ion mass spectrometry (SIMS) U–Pb zircon analyses of the eastern borehole of the Cretaceous Continental Scientific Drilling (CCSD-SK-II) borehole and two outcrop sections located in different structural provinces of the Songliao Basin, China. Detailed rock magnetic results demonstrated that pseudo-single-domain magnetite and single-domain greigite coexisted in the lacustrine black shales of the Nenjiang Formation, with the latter dominating remanence carriers. The reliable primary remanent magnetizations were isolated, which passed a class A positive reversal test and positive bootstrap reversal test, after correction for inclination shallowing, yielding a high-quality paleopole of 79.6°N/208.4°E, A95 = 2.3°. The borehole sequence and outcrop sections were stratigraphically correlated by combining lithostratigraphy, biostratigraphy, and SIMS U–Pb zircon geochronology. The correlation of the recognized magnetic polarity sequences to the geomagnetic polarity timescale suggests that the Nenjiang Formation from the CCSD-SK-II borehole sequence and the two outcrop sections span from very late chron C34n to very early chron C33r. Furthermore, the age of the Cretaceous Normal Superchron (CNS) termination can be constrained to 82.7 ± 0.6 Ma based on magnetostratigraphy, radiometric dating, and the perfect/typical averaged sediment accumulation rate for Member 1 of the Nenjiang Formation of the borehole sequence. The estimated age obtained in this study accurately represents the age at CNS termination.

Paleomagnetism, magnetostratigraphy, provenance, and tectonic setting of the Lower Cretaceous of nuclear southern Mexico

We studied the sedimentology, provenance, and paleomagnetism of Lower Cretaceous strata in nuclear southern Mexico deposited in continental, marginal and marine settings in order to better understand their tectonic setting and paleographic correlations. The studied units have been assigned to the Zicapa, Atzompa, Caltepec, Xonamanca, and Jaltepetongo formations, which are interpreted as an eastward-deepening paleogeography (in present day coordinates) deposited between Valanginian and Aptian time in a system of extensional basins contemporaneous with arc magmatism. Provenance interpreted from sandstone petrology, conglomerate clast counts, and detrital zircon U–Pb geochronology indicates that these successions consist of polymictic conglomerate, quartz-lithic metamorphoclastic and feldspathic-quartz lithic volcaniclastic sandstone, and mudstone. These were sourced from the underlying Precambrian and Paleozoic basement rocks, as well as from proximal suites of mafic to acid Jurassic (∼182-145 Ma), and Cretaceous volcanic rocks (∼140-123 Ma). The remanent magnetization in the Atzompa and Caltepec formations is multivectorial, and the characteristic remanent magnetization resides in hematite. For the Atzompa the grand mean is Dec = 335.3° and Inc = 25.4° (k = 15.4, α95 = 12.7°; N = 10 accepted sites). For the Caltepec the grand mean is Dec = 354.3°, Inc = 27.9° (k = 29.15, α95 = 6.1°, N = 20 accepted sites). Magnetizations in both units predate folding and pass reversal tests. For the uppermost Zicapa the magnetization is nearly univectorial and resides in a soft magnetic phase; the grand mean is D = 275.2°, I = 36.0° (k = 34.4, α95 = 10.1°, N = 7 accepted sites). For the Caltepec and Atzompa formations, I/E corrected directions indicate a paleolatitude of ∼20° to 23°N which is indistinguishable from that expected assuming stability with respect to the North America craton (∼24°), implying only minor tectonic motions since Early Cretaceous deposition, and demonstrating that the present distribution of units studied resembles their paleographic relations. The magnetostratigraphy of these units can be correlated with some confidence to the M10-M0r sequence of the geomagnetic polarity time scale, suggesting that the formations record late Valanginian and early Aptian marine transgressions that might correlate with global eustatic curves. The temporal and paleogeographic distribution of magmatism in southern Mexico is consistent with the presence of a continental arc formed by eastward subduction under the North American plate continuously from Early Jurassic to Aptian time, and extension in a back-arc setting during the Hauterivian-Barremian driven by slab rollback. We assign the system of extensional basins to the “southlands rift”, a short-lived event that predates amalgamation of oceanic terranes in the west and Albian carbonate platform development.

Astronomical calibration of late middle Eocene radiolarian bioevents from ODP Site 1260 (equatorial Atlantic, Leg 207) and refinement of the global tropical radiolarian biozonation

Abstract. The middle Eocene sedimentary sequence drilled at Ocean Drilling Program Site 1260 (Leg 207), Demerara Rise, western equatorial Atlantic, yielded a rich and diverse radiolarian fauna. The exceptionally expanded and complete sedimentary record of this site, as well as the existence of an orbital chronological framework, allowed us to study a number of radiolarian bioevents with a very fine temporal resolution. We compiled a well-resolved succession of 71 radiolarian bioevents (first occurrences, last occurrences, and evolutionary transitions) and provided calibrations to the geomagnetic polarity timescale and astronomical timescale. Comparison of the radiolarian successions at ODP Site 1260A with the northwestern Atlantic IODP Site U1403 and the IODP Sites U1331, U1332, and 1333, situated in eastern equatorial Pacific, allowed the demonstration of the synchroneity of primary radiolarian bioevents that underpin the middle Eocene zonal scheme. Several secondary bioevents were also found to be synchronous between the two oceans and were therefore used to define seven new subzones for the low-latitudinal middle Eocene sequences: Siphocampe ? amygdala interval subzone (RP13a), Coccolarnacium periphaenoides interval subzone (RP13b), Artostrobus quadriporus interval subzone (RP14a), Sethochytris triconiscus interval subzone (RP14b), Podocyrtis (Podocyrtopsis) apeza interval subzone (RP14c), Artobotrys biaurita interval subzone (RP15a), and Thyrsocyrtis (Pentalocorys) krooni interval subzone (RP15b). This refined radiolarian biozonation has significantly improved stratigraphic resolution and age control for the late middle Eocene interval (an average of two subzones per 1.5 million years). A substantial diachronism was also found in 20 secondary radiolarian bioevents between the two oceans. The majority of radiolarian species appear to have evolved first in the equatorial Atlantic Ocean and subsequently in the equatorial Pacific. However, the reasons for this pattern of diachroneity currently remain uncertain and would require a greater sampling coverage to be elucidated.

Geochemical and Mineral Properties of Quaternary Deep-Sea Sediments in the Central-Tropical Pacific and Its Response to the Mid-Pleistocene Transition

Global climate and oceanic water masses have undergone profound changes during the middle Pleistocene transition; however, due to a lack of foraminiferal fossils, the nonfossiliferous pelagic deposits were less detected in previous reports. In this work, a gravity core from the Kamehameha Basin in the Central Pacific was studied in terms of magnetostratigraphy, clay mineral and geochemical elements. The main results are: (1) nine magnetozones are recognized in the core, which can be correlated to the geomagnetic polarity timescale from chrons C2n to C1n; (2) smectite is the dominant clay mineral, and the others are illite, chlorite and kaolinite; and (3) the sediments are mainly composed of Al2O3, Fe2O3, MnO, Na2O and TiO2. Based on these results, a geochronological framework for the study area was established, and the depositional rates are estimated as 3–7 m/Myr in the Quaternary, showing an increase during the middle Pleistocene transition. By comparing the findings to various paleoenvironmental processes, it is inferred that the increased sedimentation in the Kamehameha Basin may have resulted from the induced weathering processes and the strengthened aeolian inputs from inner Asia. Moreover, regional circulation related to bottom water evolution has experienced a rapid reorganization across the middle Pleistocene transition. All these findings illustrate the potential of deep-sea sediments in the central tropical Pacific in revealing some key features in paleoclimatology and paleoceanography, which are worthy of further investigation in the future.