查看更多>>摘要：? 2022The vast humid regions in southeast China are strongly influenced by the East Asian Monsoon (EAM); however, the origin and underlying mechanisms driving the EAM in this area remains uncertain. Here, we present Eocene palynological records from two sedimentary successions in the Qingjiang Basin of central Jiangxi Province, southeast China, which reveals a major vegetation transition from an early Eocene Ephedra-dominated shrub desert to late Eocene taxodioid Cupressaceae-dominated swamp forests. Using the Coexistence Approach and the Climatic Amplitude method, we quantitatively reconstructed climatic parameters, which indicate a climate shift from arid conditions with mean annual precipitation (MAP) lower than 200 mm to humid conditions with a MAP of 1479 ± 373 mm. We infer that this sharp increase in humidity is linked to the onset of a modern-style EAM, implying that the EAM has prevailed in southeast China since at least the late Eocene, much earlier than previously thought. We conclude that the establishment of the EAM in southeast China by the late Eocene was primarily controlled by paleogeographic changes, not changes in atmospheric CO2 levels. These results provide a new perspective on the early evolutionary history of the EAM, and challenges the prevailing view that the origin of humid southeast China was a Neogene phenomenon.
查看更多>>摘要：? 2022 The AuthorsOne of the keys to the success of modern and ancient reef ecosystems is the phenomenon of photosymbiosis. Photosymbiotic corals can thrive down to the limits of the photic zone, and the coral communities living in the lower part of the photic zone are known as mesophotic coral reef ecosystems (MCE). Recent studies have shown that MCEs were already present in the middle Palaeozoic. The reefs of that time reached the peak of their distribution during the Givetian, when they were present even as far as 50°S. One of the southernmost Givetian reefs is the Aferdou el Mrakib reef, exposed in the eastern Anti-Atlas (Morocco). At the base of this reef, numerous small carbonate bioherms are exposed. These structures formed low-relief banks that developed probably at moderate water depths of several tens of meters. In the present contribution, we analyse the paleoecology of these peculiar communities. The assemblage is dominated by platy and encrusting coenitid (Roseoporella) and alveolitid (Alveolites) tabulate corals, as well as abundant platy stromatoporoids and solitary cystyphillid rugose corals. The platy morphology of corals is widely considered as an adaptation related to photosymbiosis, and is especially common in MCEs. In combination with palaeobathymetric and sedimentological data, this characteristic suggests that the studied banks developed at the bottom of the euphotic zone. The MCEs of the Aferdou el Mrakib reef share major taxonomic and ecological similarities with the previously described MCEs from the southern shelf of Laurussia and a reef community from southern China, emphasizing a high degree of cosmopolitanism of the Middle Devonian mesophotic ecosystems. Communities studied here are also the most poleward MCEs described to date.
查看更多>>摘要：? 2022 Elsevier B.V.The end-Permian mass extinction led to the disappearance of >81% of marine species and the collapse of marine ecosystems. Despite the progress made in recent years, the severity of the impact of the mass extinction upon some groups still remains unclear. Horseshoe crabs are a typical example among these groups. Here we report a new fossil horseshoe crab, Guangyuanolimulus shangsiensis gen. et sp. nov. from the Permian-Triassic transitional beds in South China, representing the earliest putative fossil record of Limulidae. The occurrence of horseshoe crabs during the Permian-Triassic transition indicates the existence of a trophic web containing at least three levels immediately after the main pulse of the end-Permian mass extinction. Horseshoe crabs might have played an important role as predators in marine communities during the Permian-Triassic transition and the earliest stage of recovery.
查看更多>>摘要：? 2022Carbon cycle perturbations dramatically affected the ocean-climate system during the Aptian–Albian transition. However, the chronology of this interval is still intensively debated. Here we present a comprehensive astronomical calibration of planktonic foraminiferal and calcareous nannofossil bioevents along a complete marine sedimentary succession, comprising the latest Aptian to middle Albian interval at drilling Core SER-03, located in the Sergipe-Alagoas Basin. The SER-03 astrochronology is based on the tuning of a high-resolution gamma ray (GR) record to the long eccentricity (405 kyr) metronome, and is integrated with high-resolution paleomagnetic records. Our results span the interval between ~114.7 and 109.5 Ma, within magnetochron C34n, and our age estimates for biostratigraphic markers agree within one long eccentricity cycle with previous estimates. We also present geochemical records, such as stable carbon isotopes (δ13C), organic carbon and carbonate contents, as well as redox-sensitive elemental ratios, measured on bulk sediments at Core SER-03. Most of these proxy records depict marked changes across the Aptian–Albian boundary, likely characterizing the local expression of Oceanic Anoxic Event 1b (OAE 1b), filling the gap for low-latitude records of this event.
查看更多>>摘要：? 2022 Elsevier B.V.The study presents a detailed record of foraminiferal faunas from CK2 core, a 928.75 m long core drilled in the center of Xisha Islands, South China Sea, for biostratigraphy and palaeoenvironmental reconstruction of the study area. Coupling foraminiferal biostratigraphy and bulk-sediment strontium-isotope chronologic age revealed that carbonate deposition occurred from Early Miocene (~19.6 Ma) to present. Examination of 281 petrologic thin sections and 246 unconsolidated sediment samples revealed 141 foraminiferal taxa, among which the foraminiferal abundance and simple diversity peaked in the Middle Miocene and Pliocene to early Pleistocene strata. Nine foraminiferal assemblages (FAs) were distinguished, documenting eight distinct facies that indicated five intervals in the development of Xisha Islands. At the initial interval (19.6–17 Ma), a coral, coralline algal and shallow-water larger foraminifers (FA1 and FA2) association colonized the shallow platform facies, forming an aggrading sequence in response to tectonic subsidence and sea-level rise. Platform development slowed from 17 to 10.19 Ma, dominated by the miliolids and small benthic foraminifers as well as larger benthic foraminifers (FA3 and FA4) representing lagoonal to reef-flat facies with agradation to progradation (backreef infilling) associated with sea-level fall. At 10.19–4.35 Ma, a deepening sequence was indicated by the change from shallow-water benthic foraminifers (FA5) upward to deeper dwelling species (FA6), as a result of climatic cooling and abrupt sea-level fluctuation. As sea level rose, the platform was drowned from 4.35 to 2.18 Ma, as indicated by abundant planktic foraminifers and large, flat benthic foraminifers (FA7 and FA8). At 2.18–0.089 Ma, the platform aggraded rapidly with sea level fall indicated by the appearance of thick benthic foraminifers (FA9) and increased coral. The close association between the biofacies and regional paleobathymetry, which was in phase with the long-term global sea-level change, suggest that foraminiferal morphology and assemblage composition are ideal parameters for paleobathymetric and paleo-depositional environmental interpretations.
查看更多>>摘要：? 2022 Elsevier B.V.Lateglacial and Holocene summer sea-surface temperature (SST) variability in the northern North Atlantic was reconstructed based on diatom records from two sediment cores from the North Icelandic shelf. The temperature changes are discussed in relation to palaeoceanographic circulation patterns and past climatic changes. Modern diatom data from surface sediments from around Iceland and Southeast and West Greenland, with known modern environmental variables, were used as the basis for the quantitative reconstruction of summer SST. The results show that summer SSTs varied during both the Lateglacial and the Holocene, but the amplitude of SST variations during the Lateglacial was greater than that during the Holocene. No pronounced warm or cool events were recorded on the North Icelandic shelf during the GI-1e to GI-1a events (B?lling-Aller?d) and the GS-1 event (Younger Dryas). The changes in SST were possibly caused by major changes in the ocean circulation pattern around Iceland, and by minor variations in the interaction between the cold and warm currents in the region. Comparison of the reconstructed summer SSTs with other marine records from the North Atlantic suggests an antiphase relationship between the oceanic climatic responses off North Iceland and in the eastern North Atlantic during the Lateglacial and the earliest part of the Holocene (the Preboreal).
查看更多>>摘要：? 2022 Elsevier B.V.The indirect detection of cryptotephra in black shales has become a key aspect of stratigraphic age, stratigraphic correlation, environmental variation, climate change, and organic matter enrichment in relation to volcanic eruptions. Common proxies for volcanic ash in sediments include the ratios of Hg to total organic carbon (TOC) and total sulfur (TS). Recent studies have shown that volcanic eruptions can lead to an increase in organic matter, but whether the relationship between elements found in volcanic ash and TOC can be used as an additional proxy to detect cryptotephra within lacustrine sediments has yet to be tested. In this study, black shales from the Chang 73 Member of the Triassic Yanchang Formation in the Ordos Basin of North China were collected for petrological and geochemical analysis. Petrological analysis revealed small amounts of zircon, cryptotephra, and fibrous apatite in the black shales. Geochemical analysis revealed that Zr in the shales originated mainly from zircon deposited directly from volcanic eruptions and that the Zr/Al ratio is positively correlated with TOC. Thus, the positive correlation between Zr/Al and TOC is a reliable proxy for cryptotephra in black shales.-
查看更多>>摘要：? 2022 Elsevier B.V.The evolution of the monsoonal climate in the Indian Himalayan region remains poorly understood due to the sparse coverage of paleoclimatic datasets. In the present paper, elemental concentrations and grain size parameters are measured for a radiocarbon-dated sediment core retrieved from Shilloi Lake (Nagaland, India) to understand climate variability over north-eastern India during the past two thousand years. The end-member modeling of grain-size distribution along with the elemental composition and ratios (Mg/Al, Sr/Al, Ti/Al) are used to identify changes in clastic input and authigenic carbonate precipitation as well as lake level changes. In addition, the REE distribution pattern and elemental ratios (U/Th, Europium anomaly – Eu*) are evaluated to understand provenance and redox conditions. We identify two main hydrological events: (i) wet conditions during ~1900 to ~1000 cal yrs. BP characterised by high clastic sediment input (as indicated by increased Mg, Al, Mg/Al) and dominant contribution of fine-grained sediments as represented by end member (EM) 1 in zone 1, and (ii) an increase in aridity as signified by decreasing detrital content and increased coarse-grained sediments (EM3) in zone 2 (~1000 cal yrs. BP to present). The regional comparison of paleoclimatic records from NE Himalaya and central India shows an opposing trend in precipitation patterns. However, a synthesis of hydrological proxy records from the tropical Pacific region does not point to an active role of ENSO in the study region. Comparative analysis of modern precipitation data and paleo-records shows that the opposing behaviour resembles an “active-break” pattern that produces an east-west precipitation dipole in central and northeast India.
查看更多>>摘要：? 2022 Elsevier B.V.Fire and its controls span several spatial and temporal scales in the Earth System and sedimentary paleofire archives are the primary means of inferring how fire varies on timescales exceeding observational records. However, our understanding of the biases affecting paleofire records remains limited. We address this gap by assembling a dataset of Holocene paleofire records to test whether preservation biases interfere with paleofire interpretations. The dataset contains 40 records composed of a total of 17,225 charcoal accumulation rate (CHAR) samples. We find that the “Sadler effect,” which is the observation that sedimentation rates decrease systematically when measured over longer timescales due to the incorporation of sedimentary hiatuses, is pervasive in these paleofire records. In the compiled dataset, the age ranges of measurement share a negative power law relationship with both accumulation rate (AR; AR = 0.4018*[sample age range]-1.09) and CHAR (CHAR = 1.118*[sample age range]-0.6655), indicating that longer time spans of measurement are more likely to incorporate longer period hiatuses into sediment records. This biases AR measurements, which subsequently bias CHAR values. Indeed, more than half of the paleofire records (n = 21) are composed of CHAR values which share a statistically significant negative relationship with the sample age range of their measurement. To our knowledge, our results are the first to identify this sedimentary bias in Holocene paleofire records. As a solution, we therefore provide an interpretative framework which outlines necessary steps to identify preservation bias in paleofire records and intervals. Lastly, we explore the implications of these findings for paleofire research.
查看更多>>摘要：? 2022 Elsevier B.V.We present new, high resolution bio- and chemostratigraphic data (calcareous nannofossils, ammonites, δ13Corg) of the lower Toarcian from an outcrop (Hondelage) in northern Germany. The three data sets, which cover lithostratigraphically the uppermost Amaltheenton-Formation (upper Pliensbachian) and the lower part of the Posidonienschiefer-Formation (lower Toarcian), allow a calibration of the different stratigraphic schemes in ultra high resolution. They further shed light on the ecologic conditions controlling the assemblage composition and evolutionary patterns of calcareous nannofossils, ammonites and gastropods. The distinctive negative δ13Corg excursion (negative CIE, Jenkyns Event), which is part of the early Toarcian Oceanic Anoxic Event (T-OAE), falls lithostratigraphically into the lower part of the laminated black shales of the Posidonienschiefer facies. The lower part of the Jenkyns Event is marked by the first occurrences of the calcareous nannofossil species Carinolithus superbus crassus and Diductius constans. These two events are biostratigraphically assigned to the Tiltoniceras antiquum ammonite Biohorizon (uppermost Dactylioceras tenuicostatum ammonite Zone). The top of the Jenkyns Event has been assigned to the Cleviceras exaratum ammonite Biohorizon (Harpoceras serpentinum ammonite Zone) of the lower Toarcian. The last occurrence of the nannofossil species Biscutum grande lies in the middle part of the Jenkyns Event. Calcareous nannofossils, ammonite faunas and gastropods are discussed with respect to the environmental and climatic changes, which occurred across the Pliensbachian/Toarcian boundary interval and throughout the Jenkyns Event. The extreme impoverished nannofossil and ammonite assemblages of the uppermost Pliensbachian are interpreted in the context of a cooling, primarily affecting the calcareous shelled primary producers. The onset of the Jenkyns Event in the lower Toarcian post-dates this cooling and is related to a subsequent warming. This climatic shift caused a bloom of diverse opportunistic nannofossil and ammonite taxa in the lower Toarcian. Nannofossil abundance patterns were controlled by an increased nutrient availability. The rapid evolution of the ammonite genera Tiltoniceras, Eleganticeras and Cleviceras (family Hildoceratidae), which started in the lower part of the Jenkyns Event, is potentially related to their adaptation to a shallow water dwelling habitat. The mass occurrence of phylogenetically first holoplanktonic gastropods in the lower part of the Jenkyns Event is interpreted in the context of impoverished ocean water oxygenation, which in turn triggered the evolution of a new ecological strategy.
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