Marine Micropaleontology最新文献

The Ludlow epoch of the Silurian period was a time of significant geobiological perturbations, the most significant being the mid-Ludfordian Lau Event and associated huge positive carbon isotopic excursion. On the other hand, the impact of the Lau Event on ecosystems is far from understood, with the majority of studies being concentrated on conodonts, graptolites, and brachiopods. Therefore, here we present the high-resolution Gorstian to mid-Ludfordian Baubliai-2 core section record of the phytoplankton (acritarch and green algae) change from the deep shelfal facies belt of the Silurian Baltic Sedimentary Basin. Seven distinct phytoplankton assemblages are distinguished. The general feature of the assemblage change is an overarching trend toward higher relative abundances of the green algae disaster taxon Leiosphaeridia spp., which peaked during the Lau Event. The absolute abundance variability of the various components of the plankton including the hyper-abundant Leiosphaeridia spp. and Tasmanites spp. revealed the presence of consistent 0.93 Ma cyclicity. The diversity of phytoplankton is directly correlated with graptolite species diversity, which suggests common driving mechanisms of evolution for different components of the plankton community. The recurrence and joint recurrence plots and recurrence quantification analysis revealed a coordinated evolution of phytoplankton and graptolite diversities. The Lau Event interval stood out as a distinct coordinated low diversity state in both phytoplankton and in graptolites. Similar anomalous community states were previously detected in benthic communities (brachiopods), and in communities of pelagic predators (conodonts). Therefore, the current evidence suggests that the Lau Event had a significant impact across the whole range of communities and ecosystems.

Bioerosion is rare in large benthic foraminifera (LBF) in the Eocene of Ziarat Formation, Iran. Three bioerosion ichnotaxa have been identified, including more common Trypanites isp. and Entobia isp., and rare Meandropolydora sulcans. The rare bioerosional traces in LBF tests support the scenario of an oligotrophic nutrient regime during the studied interval. The sedimentation rate may have also been among the controls on the bioerosion of LBF in the Middle Eocene of Iran, but the oligotrophic nutrient regime likely had a stronger effect. The bioerosion of LBF tests during the Middle Eocene Climatic Optimum (MECO) was rather low or at maximum moderate, suggesting that an oligotrophic nutrient regime could compensate for the increase in bioerosion caused by ocean acidification and warming.

A detailed study of calcareous nannofossils was conducted on a well-exposed succession encompassing the Cretaceous-Paleogene (K-Pg) boundary interval at Wadi Nukhul, Sinai, Egypt, as a part of the southern Tethyan margin. The nannofossil biostratigraphic data show that the Nukhul succession sustains a complete K-Pg boundary interval. The nannofossil assemblage indicates a shift from cool/oligo-mesotrophic conditions in the late Maastrichtian Micula murus Zone to warm/oligotrophic in the latest Maastrichtian M. prinsii Zone prior to the K-Pg boundary. At the K-Pg boundary, diversity, and total abundance of the calcareous nannofossils remarkably decreased signifying unstable marine conditions. The post-impact assemblage (lowermost part of NP1 Zone) was marked by acme of Cervisiella operculata indicating oligotrophic conditions in the photic zone related to the diminished of the marine primary producers in the lowermost Danian. At the upper portion of the NP1 Zone, the assemblage is still dominated by C. operculata, besides the successive occurrence of the small-sized incoming Paleocene taxa Biantholithus sparsus and Neobiscutum romeinii. This pattern was accompanied by increased frequencies of Zeugrhabdotus sigmoides indicating stressful cool/meso-eutrophic surface water conditions signify the initial recovery interval. Upward in the early Danian NP2-NP3 zones, the nannofossil species diversity and total abundance progressively increased. Also, the nannofossil assemblage characterized by the existence of the new Paleocene oligotrophic indicators like Cruciplacolithus primus, Coccolithus pelagicus, and Ericsonia subpertusa. This probably indicates a tendency toward rebuilding of the biological pump efficiency and, therefore, a full recovery phase was resumed.

We used the samples from International Ocean Discovery Program (IODP) Hole U1505C in the northern South China Sea (SCS) to reconstruct the upper ocean profiles and discussed the East Asian Monsoon (EAM) variations during 15.0–12.5 Ma. 15 genera and 41 species were identified, and 7 planktonic foraminifer datums were recognized, then a reliable chronostratigraphic framework was updated. A principal component analysis (PCA) was used to identify the suitable assemblages for reconstructing upper water profiles, based on planktonic foraminifera relative abundance and isotope records. The sea surface temperature (SST), paleo-productivity, and the depth of thermocline (DOT) were reconstructed by the content of warm water species, high productivity species, and the ratio of mixed layer to thermocline species, respectively. Three main phases were identified: (1) during 15.0–14.7 Ma, the SST and paleo-productivity were high, while the thermocline was shallow, indicating a warm and rainy climate influenced by East Asian Summer Monsoon (EASM); (2) during 14.7–13.8 Ma, the SST was lower, the paleo-productivity increased significantly, and the thermocline deepened, indicating an enhanced East Asian Winter Monsoon (EAWM); (3) during 13.8–12.5 Ma, the SST increased slightly, the paleo-productivity deceased, while the DOT showed neglectable change, indicating a stable EAWM and a waning EASM. The results of the Morlet wavelet spectrum revealed that EAM and the upper ocean profiles in the northern SCS were predominantly regulated by the eccentricity cycles, emphasizing the significant impact of low-latitude processes on climate variations.

Nonlamellar dendrolites, a type of microbial carbonate, exhibit dendritic fabric on the macroscale, which is different from stromatolites, thrombolites, and leiolites. This work reports the sedimentary environment and sedimentary characteristics of dendrolites dominated by calcified microbes in the Zhangxia Formation in the Dongping section of Shandong Province, China. Thick oolitic limestone affected by the rise in sea level is developed in the middle section of the Zhangxia Formation in the Dongping section, and a set of thick massive dendrolites are developed above the oolitic limestone. It exhibits the special sedimentary phenomenon of microbes seeking stability in a high-energy environment. Microscopically, the dendrolites are characterized by the rich development of calcified cyanobacteria fossils, such as the epiphyton group and Girvanella, demonstrating the diversity and complexity of the microbes that dominate the dendrolite formation in the Dongping section. The epiphyton group can be classified by morphological characteristics, namely, Epiphyton, Korilophyton, and Hedstroemia. The latter can be further categorized into typical Hedstroemia and Cayeuxia. Therefore, this study enhances our understanding of dendrolite sedimentary environments and provides a reference example for exploring diverse fossil records in ancient strata.

The Paleocene hemipelagic deposits of the Dakhla Formation in Sinai offer an ideal setting to investigate paleoenvironmental fluctuations using benthic foraminifera. This study examined a Danian-Selandian sedimentary succession at Themed, Sinai using quantitative foraminiferal analysis within a sequence stratigraphic framework. Quantitative multivariate analyses, including R-mode clustering and non-metric multidimensional scaling (NMDS) ordination of biofacies trends and diversity parameters, indicate considerable paleoenvironmental and paleobathymetric changes throughout the studied succession. Five transgressive-regressive (T-R) sequences were identified based primarily on benthic foraminiferal biofacies trends. The sequence boundaries correlated well with major global sea-level lowstands. Benthic foraminiferal assemblages elucidated paleobathymetry fluctuations between upper bathyal and middle-outer neritic settings across the sequences. The identified sequences strongly correlate to global eustatic cycles, with subtle disparities suggesting a subordinate role for regional tectonics overprinting the dominant sea level control. The Latest Danian Event (LDE) interval coincides with the regressive phase of the fourth T-R sequence, as revealed from the biostratigraphic-constrained isotopic signature. It showed pronounced peaks in the relative abundance of the eutrophic buliminids, likely reflecting enhanced export productivity. Overall, benthic foraminifera provided vital paleoenvironmental insights, enabling sequence stratigraphic interpretation in the monotonous hemipelagites and highlighting possible coupled dynamics of both the carbon cycle and sea level.

During the Devonian, the Gondwana supercontinent was in a paleogeographic position in high-latitudes, close to the South Pole. The arrangement of continents, tectonic activity, climate fluctuations, and the evolution of marine life played an important role in the marine ecosystems during this period. Conodont occurrences in high-latitudes are scarce. In the Upper Devonian (Frasnian/Famenian) of western Gondwana, conodonts have been found in anoxic black-shale deposits associated with maximum flooding surface (MFS) of a distal platform, in the Barreirinha Formation of the Amazonas Basin and Jandiatuba Formation of the Solimões Basin, in northern Brazil. The fauna is characterized by low diversity and well-preserved platform and ramiform elements of Mehlina gradata, Ozarkodina sp. aff. O. sannemanni, Cryptotaxis sp. and Polygnathus sp. in the Amazonas Basin and Cryptotaxis sp., Parapolygnathus sp., Pandorinellina sp., and Hibbardella sp. in the Solimões Basin. The conodont faunal association and sedimentary succession suggest a marine ingression in cool water conditions to the Devonian sea of the Amazonian region.

Assessing the distribution of species in natural environments is essential for their use in environmental surveys. Here, we investigate the distribution of three pseudo-cryptic species formerly lumped in the morphospecies Ammonia tepida (Cushman, 1926), commonly found on estuarine mudflats along the European coasts: Ammonia veneta Schultze, 1854 (T1), Ammonia aberdoveyensis Haynes, 1973 (T2) and Ammonia confertitesta Zheng, 1978 (T6). We studied their distribution at 51 sites located in seven estuaries of the French North Atlantic coast (Elorn, Aulne, Odet, Crac'h, Auray, Vilaine, Vie), using both morphological and molecular identification methods. Ammonia veneta was detected by both approaches at most of the stations. While A. aberdoveyensis was frequently identified by the morphological method but not detected with metabarcoding, the presence of A. confertitesta in the eDNA data often contrasted with its absence in the morphological analysis. The absence of A. aberdoveyensis in eDNA of sites where it was identified morphologically could be the consequence of its relative scarcity, and eventually a patchy distribution. Concerning A. confertitesta, we hypothesise that these contradictory results can be explained by the supposedly invasive character of this species. Despite the widespread presence of A. confertitesta genetic material (including adults, juveniles and propagules), a mature population has not yet fully developed everywhere. The seven investigated estuaries seem to represent different stages of replacement of the autochthonous species A. veneta and A. aberdoveyensis by A. confertitesta. Our study demonstrates that the combination of visual observations and molecular approaches is ideal for monitoring the progressive spreading of exotic species.