Marine Micropaleontology最新文献

Benthic foraminifera are marine protists largely used as bioindicators and proxies of paleo- environments. Epifaunal species are supposed to live at or above the sediment surface and are therefore used as proxies for bottom water conditions, while infaunal inhabit the sediment column, thus tracing porewater chemistry. Traditional analytical methods based on core slicing, however, have a low resolution that does not allow to precisely characterise the preferential microhabitat(s) of indicator species.

In this study we performed microtomographic analyses on an experimental sediment core, to observe the life-position of living foraminifera of two surface-dwelling species Ammonia confertitesta and Haynesina germanica, reported both as epifaunal or shallow infaunal. The images we obtained offered for the first time the possibility to observe each individual in 3D space with a numerical resolution of 13 μm/voxel.

The results revealed that the two species are never located above or at the sediment surface and have their preferential microhabitats in a sub-superficial sediment layer constrained in the 0–500 μm interval below the surface. Rapid decrease of abundances below this layer suggests that their microhabitat could be even more specific than previously thought.

μCT-scan of sediment cores is also a valuable tool to obtain high-resolution information about foraminiferal ecology. The described method is useful to assess the effective microhabitat of all foraminiferal species that are usually used as proxies for paleorecords, to ensure that the information we can obtain from them is attributable to bottom water or to porewater conditions at a specific sediment depth.

The Late Lutetian Thermal Maximum (LLTM) was a transient and brief global warming event recorded in the middle Eocene, at 41.52 Ma. The biotic response to the LLTM has been documented at only a few marine sites so far. Here, we present the first record of deep-sea benthic foraminiferal assemblage changes during the LLTM in the southwest Pacific at International Ocean Discovery Program Hole U1508C (1609 m water depth) in the Tasman Sea. The LLTM coincides with a negative excursion in bulk sediment δ¹³C (0.47‰) and benthic foraminifera δ¹³C (0.36‰), with changes in the relative abundance of benthic foraminiferal species and in the deep-water organic geochemistry. The decrease in diversity of the assemblages indicates environmental stress during the event, potentially linked to oxygen deficiency, as evidenced by the occurrence of dysoxic taxa (e.g. Lenticulina spp., Turrillina brevispira). Although calcareous taxa dominate, the presence of corrosion-resistant species and poorly preserved foraminiferal tests suggest slightly CaCO₃-corrosive bottom waters, but no dissolution was evident. We suggest the shallowing of the thermocline and enhanced water column stratification at this site during the LLTM.

A new species of Crenulosepta, i.e., Crenulosepta perlisensis Vachard and Fassihi sp. nov., is described from a Cisuralian (Lower Permian) sandy limestones and calcareous sandstones in the Kubang Pasu Formation in northwestern Peninsular Malaysia, which forms part of the Peri-Gondwanan Sibumasu Block. Previous Malaysian authors reported the occurrence of Monodiexodina shiptoni and Monodiexodina sutschanica from the same area. The findings of this study provide a better overview of the Cisuralian fusulinid fauna found in this region. The presence of Crenulosepta confirms that the age of the uppermost part of the Kubang Pasu Formation is Artinskian, rather than extending into the Kungurian as previously recognized. The uppermost part of the Kubang Pasu Formation with its impoverished genus and species diversity is located just above the cold water brachiopods and diamictite beds. It indicates that during the Asselian (earliest Cisuralian), the northwestern Peninsular Malaysia was still part of the Gondwana continental shelf. During the late Early Permian, northwestern Peninsular Malaysia, as part of the eastern Cimmerian Continent, was located in a low latitude subtropical region of the paleo-equatorial tropical Tethyan Realm. The microfacies analysis of the Kubang Pasu Formation suggests a very high-energy, warm shallow marine environment of the inner ramp (e.g., sand shoals and banks).

The Late Miocene-Early Pliocene Biogenic Bloom (9.0–3.5 Ma) is a widespread paleoceanographic phenomenon marked by increased marine biological productivity and by high accumulations of biological components documented at multiple open ocean sites in the Indian, Pacific, and Atlantic oceans. We investigate the expression of the Biogenic Bloom at International Ocean Discovery Program (IODP) Site U1488 in the western equatorial Pacific Ocean. We generated an improved age model based on calcareous nannofossil biostratigraphy and a quantitative benthic foraminiferal record across the Late Miocene to the Early Pliocene. Increased carbonate mass accumulation rates suggest the Biogenic Bloom occurs between 8.1 and 4.0 Ma at Site U1488. We described four intervals with paleoenvironmental significance: Interval 1 (8.1–6.2 Ma), Interval 2 (6.2–5.5 Ma), Interval 3 (5.5–4.5 Ma), and Interval 4 (4.5–3.1 Ma), the Biogenic Bloom spans across Interval 1 and 3. Intervals 1, 3, and 4 are marked by high abundance of phytodetritus exploiting taxa, related to phases of El Niño-like conditions. The highest abundance of these species during Interval 1 has been related to a phase of higher seasonality. In contrast, intervals 3 and 4 show reduced seasonality and a steadier input of food to the seafloor, associated with increased dust supply through wind transport and/or increased continental weathering during the Pliocene. Interval 2 stands out as the sole interval encompassing La Niña-like conditions, marked by a shift in the nutrient composition reaching the seafloor, from labile phytodetritus to refractory organic matter, and possibly a decrease in seasonality.

The Lower Cretaceous sediments of the Sanfranciscana Basin, southeast Brazil, are considered to have been deposited under a Gondwanan continental context. However, different authors have occasionally observed marine elements in these same deposits. This study analyzes samples from five sections of the Sanfranciscana Basin, Quiricó and Três Barras formations, to characterize the paleoenvironment around these marine incursions. We recovered continental (non-marine ostracods and palynomorphs) and marine microfossils (e.g., foraminifera, radiolarians, and ascidian spicules). The alternance of quartz-rich biogenic chert and claystone might be indicative of the occurrence of radiolarites in the studied sessions. The presence of the radiolarian genus Turbocapsula sp., the radiolarian species Holocryptocapsa fallax, and the palynomorph Tucanopollis crisopolensis enabled us to date the interval as early Aptian. Based on the lithologies and microfossils recovered, we interpreted the studied interval as a continental setting that was episodically subjected to marine incursions. We propose three successive paleoenvironmental scenarios: (1) an initial lacustrine depositional setting with a non-marine biota; (2) a second stage, characterized by the presence of episodic marine influence in the sedimentary succession (as evidenced by radiolarians, foraminifera and ascidians), and (3) the return to continental settings dominated by aeolian dune fields. These environmental scenarios characterize the initial development of the Central Segment of the South Atlantic Ocean.

Calcareous microplankton increased in abundance during the latest Jurassic, coinciding with the increase in abundance of calcareous nannofossils and with the onset of deposition of pelagic calcareous oozes. However, the timing and causes of the shift from microgranular tests of the earliest microplankton (chitinoidellids) to hyaline tests of calpionellids are obscured because the ultrastructure of two-layered praecalpionellids that occur during the Tithonian is poorly documented. Here, we investigate the ultrastructure of chitinoidellids and praecalpionellids from Upper Tithonian deposits in the Western Carpathians. We show that (1) the chitinoidellid microgranular layer is formed by elongated, euhedral, densely-packed, nanometric needles rather than by fragments of calcareous nannofossils, (2) two-layered chitinoidellids (Semichitinoidella) are formed by an internal microgranular layer (identical to that of Chitinoidella) and by an external hyaline prismatic layer, and (3) two-layered Praetintinnopsella exhibits an internal hyaline layer (with densely-packed, equant microcrystals) and an external layer formed by a dark organic rim. The external layer in Praetintinnopsella thus does not have any relation to the microgranular layer in chitinoidellids and the external hyaline layer of Semichitinoidella is not equivalent in structure to the hyaline layer of Praetintinnopsella. As both single-layer and two-layered chitinoidellids appear prior to the first appearance of Praetintinnopsella but still co-occur with this genus in the lowermost Upper Tithonian deposits, the origin of two-layered Praetintinnopsella either reflects a major transformation in biomineralization towards larger and more packed crystals during their earlier divergence from the chitinoidellid lineage or an origination of two-layered tests with a hyaline layer from an independent non-chitinoidellid ancestor.

The crystallographic orientation and its relationship to the morphology of coccoliths were investigated for the Neogene-Quaternary calcareous nannoplankton Umbilicosphaera lineage using scanning electron microscopy (SEM) and electron back-scattered diffraction (EBSD). The c-axis of the calcite forming the distal shield elements was inclined upward at 66–68° (U. sibogae), 65–68° (U. foliosa), 57–58° (U. rotula), and 55–57° (U. patera) from the coccolith plane. Accordingly, the outward dip angle of one of the {$10\overline{1}4$} faces forming the surface of the distal shield of U. patera coccolith was shallower than those of U. sibogae and U. foliosa, explaining the nearly flat distal shield and the steep inner slope, formed by another equivalent {$10\overline{1}4$} face, around the central opening of U. patera. Our results showed that the evolution from U. rotula to U. patera during the Late Miocene was not accompanied by a change in crystallographic orientation. In contrast, the evolution from U. patera to U. sibogae and U. foliosa during the Pliocene was accompanied by a rotation of the orientation. The crystallographic orientation of calcite nuclei on the baseplate with a combination of other factors would have resulted in species-specific differences in shield shape and suture lines within the same phylogeny, consequently producing morphological diversity in the coccolith throughout geological time.

The shallow-water Capo Mortola succession (Liguria, NW Italy) yields diverse assemblages of smaller benthic and planktic foraminifera, larger benthic foraminifera (LBF), and calcareous nannofossils. With the aim of improving the understanding of the Middle Eocene Climatic Optimum (MECO) impact on the shallow-water marine biotic communities due to global warming, we provide biostratigraphic and stable isotope data to achieve a reliable stratigraphic constraint of the MECO. The correlation of the stable isotope oxygen data with datasets of similar age from other regions suggests that only the onset of the MECO interval is recorded in the Capo Mortola section. Quantitative analyses of smaller benthic foraminiferal assemblages indicate that the shallow-water setting of Capo Mortola was not particularly affected by the onset of the MECO perturbance because no variation in nutrient supply or oxygen level were detected. A different scenario is recorded by the LBF genera Operculina and Discocyclina, which increased in abundance across the MECO onset, probably due to a rise in temperature and adapting to the increase in nutrient supply. In the upper water column, the variations in calcareous plankton communities appear to be controlled by both the MECO warming and a moderate increase in eutrophic conditions related to the enhanced hydrological cycle. Nutrients, mostly consumed in the upper water column, reached the seafloor in a limited amount, as benthic foraminifera record a meso-oligotrophic environment across the studied MECO interval.

Over the past decades, fossil assemblages of benthic foraminifera have been used to reconstruct the variability of oxygen-depleted areas, including oxygen minimum zones. These areas currently represent almost a tenth of the global oceans' surface area, and further expansion is expected due to global warming; with major impacts on marine ecosystems, biodiversity, and fisheries. To predict their future evolution, accurate estimates and quantification of past oceanic oxygenation are needed, and thus consistent calibration of the $\left(O_2\right)$ estimation transfer functions is required.

Here, we combine the BENFEP database that compiles all the benthic foraminiferal census data published for the East Pacific Ocean, with dissolved oxygen data interpolated from the WOA18 and GLODAPV2.2022 databases, to describe the oxygen affinities of the 1526 benthic foraminiferal taxa from the BENFEP database among 1691 samples. The affinities of the most common 202 species of the database are detailed here. For each of these taxa, the range of oxygen concentration, average $\left(O_2\right)$ value where the species is usually found, and $\left(O_2\right)$ value associated with its peak in relative abundance are listed and used to assign each taxon to the oxygen categories anoxic, dysoxic, suboxic, low oxic, and high oxic.

Finally, using the relative abundance of each of these five oxygen assemblages and their associated taxa, transfer function indices of dissolved oxygen estimation were refined. The new ${\mathrm{BFA}}_{\mathrm{ex}}$ extends the range of applicability of the formerly published BFA index, here updated using all the available samples $\left({\mathrm{BFA}}_{\mathrm{up}}\right)$ and extended from 0.04 to 2.58 mL L⁻¹ to 0.02–6.62 mL L⁻¹.

Paleontological observations of ancient flora and fauna provide powerful insights into past diversity and relationship dynamics between organisms and their environments. Diatoms are globally distributed protists that influence major biogeochemical cycles and sustain oceanic food webs. The fossil diatom record extends 120 million years back to the Early Cretaceous where rare deposits were discovered worldwide and are occasionally represented by diverse communities. However scarce, the taxonomic richness and geographical spread of these diatom communities suggest prior evolutionary events and therefore earlier deposits. To complement the existing fossil information and to discover diatom deposits predating 120 Ma, we examined 33 study sites from cores and outcrops across oceans and continents. These efforts did not generate new fossil discoveries, however. Our assessment suggests biogenic silica that comprises the cell wall of diatoms was likely dissolved from Mesozoic sediments through diagenetic processes. Altogether, the search for the oldest diatoms must continue but should target sediments that experienced shallow burial and concretions.