Preservation Of Microfossils Research Articles

The Taphonomy of Plant and Livestock Dung Microfossils: An Ethnoarchaeological and Experimental Approach

ABSTRACT This study examines the contribution of ethnoarchaeological and experimental research to interdisciplinary approaches on the identification and taphonomy of livestock dung. Ethnographic and experimental records provide comparative reference models on a range of taphonomic issues that are still understudied, such as variation in the type and preservation of plant and faecal microfossils that are excreted with dung and the effects of heating. The focus in the present study is on the taphonomy of ingested phytoliths and calcitic spherulites that originate in the digestive tracts of cattle, sheep and goat found in fresh modern dung pellets. The reported records are from the modern farming village of Bestansur, Iraqi Kurdistan. The experimental results show significant changes in microfossil composition and phytolith and spherulite stability and integrity, with increased melting of phytoliths and variations in morphotype composition (e.g. grass short cells appear less stable) from heating at temperatures around 800°C, whereas the spherulite darkening occurs within a range between 500–700°C, with a maximum production at 650°C. This integrated approach illustrates the contribution of the much-needed ethnographic and experimental records of animal dung to understanding of taphonomic issues, which are fundamental for the interpretation of this valuable microfossil material in archaeology.

Microbiota of the Barakun Formation and Biostratigraphic Characteristics of the Dal’nyaya Taiga Group: Early Vendian of the Ura Uplift (Eastern Siberia)

The rich assemblage of Early Ediacaran (Early Vendian) acanthomorphic palynoflora (ECAP) was recognized in the middle part of the thick (2.2–2.7 km) clayey–carbonate sequence of the postglacial sediments of the Dal’nyaya Taiga Group (Ura Uplift). The degree of preservation of microfossils is determined by the instantaneous rate of their burial within the narrow facies zones of a deep-water basin, located at the base of large prograded carbonate bodies. The comparative analysis of microfossil remains from deposits of the Barakun Formation with the known Ura microfossil assemblage suggests that these assemblages belong to the unified complex palynozone within the Dal’nyaya Taiga Group. The comparative analysis of the Dal’nyaya Taiga assemblage with the Ediacaran palynofloras from other continents shows the high degree of diversification of microfossils, typical of the upper complex palynozones of the Lower Ediacaran of Australia and China, ascribed to the interval of 580–565 Ma. Thus, the greater part of the upper Dal’nyaya Taiga Group has a much younger age than was previously thought (<635 Ma). A noticeable reduction in the stratigraphic volume in most of postglacial deposits of the Dal’nyaya Taiga Group suggests the presence of significant stratigraphic hiatuses in the lower part of this sequence or the revision of the age of the Bol’shoy Patom glacial horizon.

Late Oligocene-Miocene proto-Antarctic Circumpolar Current dynamics off the Wilkes Land margin, East Antarctica

At present, the Southern Ocean plays an important role in the global climate system and in modern Antarctic ice sheet dynamics. Past Southern Ocean configurations are however poorly understood. This information is yet important as it may provide important insights into the climate system and past ice-sheet behavior under warmer than present day climates. Here we study Southern Ocean dynamics during the Oligocene and Miocene when reconstructed atmospheric CO 2 concentrations were similar to those expected during this century. We reconstruct snapshots of late Oligocene to earliest Miocene (~24.2–23 Ma) paleoceanographic conditions in the East Antarctic Wilkes Land abyssal plain. For this, we combine marine sedimentological, geochemical (X-ray fluorescence, TEX 86, ), palynological and isotopic (ε Nd ) records from ocean sediments recovered at Deep Sea Drilling Project (DSDP) Site 269. Overall, we find that sediments, delivered to the site by gravity flows and hemipelagic settling during glacial-interglacial cycles, were persistently reworked by a proto-Circumpolar Deep Water (CDW) with varying strengths that result from climatically controlled frontal system migrations. Just prior to 24 Ma, terrigenous input of predominantly fine-grained sediments deposited under weak proto-CDW intensities and poorly ventilated bottom conditions dominates. In comparison, 24 Ma marks the start of episodic events of enhanced proto-CDW current velocities, associated with coarse-grained deposits and better-ventilated bottom conditions. In particular, the dominance of P-cyst and low Calcium (Ca) in the sediments between ~ 24.2 Ma and 23.6 Ma indicate the presence of an active open ocean upwelling associated with high nutrient conditions. This is supported by TEX 86 -derived sea surface temperature (SST) data pointing to cool ocean conditions. From ~ 23.6 to 23.2 Ma, our records reveal an enrichment of Ca in the sediments related to increased calcareous microfossil preservation, high amounts of G-cysts and increasing TEX 86 -SSTs. This implies warmer water masses reaching the Antarctic margin as the polar front migrated southward. Together with the radiogenic Nd isotope data indicating modern-like CDW values, our records suggest a prominent poleward expansion of proto-CDW over our study site and reduced AABW formation during the latest Oligocene (i.e. ~23.2 Ma ago). Our findings support the notion of a fundamentally different Southern Ocean, with a weaker proto-ACC than present during the late Oligocene and the earliest Miocene. • Late Oligocene-early Miocene sediments at Site 269 record varying strengths of proto-CDW forced by frontal system migration • Enhanced proto-CDW current velocities at ~24 Ma is here related to ice sheet expansion across the continental shelf • A prominent southward migration of the Polar Front brought proto-CDW close to the margin between ~23.6 and 23.23 Ma • A weaker frontal system than today´s characterizes Late Oligocene-early Miocene ocean configuration offshore Wilkes Land

Fossil, ichnofossil, or concretion: The enigma of “Dinocochlea ingens”

The misidentification of biomimetic or biomorphic structures (especially helical forms) as biogenic can lead to serious errors in the interpretation of depositional environments and stratigraphic relationships, and in the detection of primitive life. “Dinocochlea ingens” was originally described as a giant gastropod and recently has been interpreted as a diagenetically-modified ichnofossil, based on dextral- and sinistral-twist bodies reported from the Lower Cretaceous Wadhurst Clay Formation, near Hastings, Sussex, United Kingdom. Morphologically similar decimeter- to meter-scale helical concretions occur with spheroidal and macrobotryoidal concretions in the shale of the Ikalukrok unit in the upper part of the Mississippian Kuna Formation that hosts the Red Dog Mine in northwestern Alaska. The mineral assemblages of both the helical and spheroidal concretions at Ikalukrok Creek are identical. The calcite of these concretions consists of cm-scale radiating aggregates of elongate poikilotopic crystals; this texture is megaspherulitic. The calcite has a relatively narrow range of δ13CVDPB values between 1.9‰ to 3.0‰ and δ18OVPDB values from −5.9‰ to −8.0‰. Growth of the concretions is considered to be pre-compaction because there is no evidence of internal layering, or inclusions of the host shale, despite their preservation of numerous microfossils. There is no indication of a causative small-scale burrow or of large-scale burrows. Analogy with laboratory syntheses of biomimetic carbonates suggests that the megaspherulitic texture of the helical and spheroidal concretions probably resulted from crystallization under strong driving forces such as high supersaturation.

Using multiple palaeoecological indicators to guide biodiversity conservation in tropical dry islands: The case of São Nicolau, Cabo Verde

Tropical dry islands are currently facing major challenges derived from anthropogenic and climatic pressures. However, their trajectories of environmental change, which could provide relevant information applicable to biodiversity conservation, remain understudied. This is mainly due to poor micro-fossil preservation and irregular sediment deposition. Multi-proxy palaeoecological analyses spanning decades to 1000s of years can add perspective as to how vegetation, fungal communities, and the fauna responded to previous natural and anthropogenic disturbances. In São Nicolau, Cabo Verde, we used palaeoecological methods to analyse a highland soil profile (1000 m asl) dated to 5900 cal yr BP. We analysed how vegetation (abundances in pollen of native and introduced species, and leaf wax n-alkanes), ferns and fungal communities (abundance of non-pollen palynomorphs) varied over time in relation to fire (charcoal concentration) and erosion regimes (grain sizes and elemental composition). Between 5000 and 400 cal yr BP the highlands held native woody taxa such as Euphorbia tuckeyana, Dracaena draco subsp. caboverdeana, and Ficus, taxa that can be used for future reforestation programmes. From 400 cal yr BP to the present day, replacement of native taxa by introduced and cultivated taxa (Pinus, Eucalyptus, Asystasia, Opuntia) has occurred. Vegetation burning and grazing caused loss of vegetation and erosion, acting as conjoined drivers of scrubland degradation. This dataset helps to set historically contextualised restoration goals such as the re-introduction of native species, monitoring of recently introduced species and control of free grazing. This can serve as a model system for the conservation of tropical dry islands' biodiversity.

The influence of fluids on the species composition and preservation of microfossils in biogenic carbonate sediments by hydrothermal cluster “Pobeda” (Mid-Atlantic ridge)

The species composition, distribution and preservation of carbonate shells of nanno and microfossils of biogenic bottom sediments within the hydrothermal cluster Pobeda (1707.451708.7 n) were studied on the basis of materials obtained in flight No. 37 of NIS Professor Logachev in the Russian exploration area (Mid-Atlantic ridge). Physical-chemical parameters and the total carbonate content of sediments were measured, their changes in column sections were revealed. For complexes of coccoliths studied precipitation is attributed to the top (acme) biostratigraphic zone of Emiliania huxleyi. It is established that within the zone of hydrothermal activity (ore cluster Pobeda) the total number of microorganisms decreases and their species diversity decreases, as a rule, in the lower part of the columns. Down the column section also decreases the Eh, pH and carbonate precipitation. These changes are explained by the influence of diffuse hydrothermal fluids.

Influence of Fluids on the Species Composition and Preservation of Microfossils in Biogenic Carbonate Sediments in the Pobeda Hydrothermal Cluster (Mid-Atlantic Ridge)

Based on materials obtained during Cruise 37 of the R/V Professor Logatchev in the Russian exploration area, the species composition, distribution, and preservation of nanno- and microfossil carbonate shells in biogenic bottom sediments within the Pobeda hydrothermal cluster (17°07.45′–17°08.7′ N MAR) were studied. Physicochemical parameters and total carbonate content were measured and their changes along column sections were revealed. Based on the coccolith assemblages, the studied sediments are assigned to the upper part (acme) of the biostratigraphic zone Emiliania huxleyi. It has been established that the hydrothermal activity zone (Pobeda hydrothermal cluster) is marked usually by a decrease of both total population and species diversity of microrganisms in the lower part of columns. The column is also marked by downsection decrease of Eh, pH, and carbonate content. These changes are attributed to the influence of diffuse hydrothermal fluids.

Imaging coprolite taphonomy and preservation

The impact of coprolite taphonomy on parasite remains and aDNA recovery has been recognized. In general, coprolites from sites protected by geologic features such as caves and rock shelters exhibit the best preservation. In contrast, coprolites from open sites can be badly affected by taphonomic processes as shown by analyses of parasite eggs. For eggs, the impact of mites and free-living nematodes has been quantified. Mites are associated with poor pinworm egg preservation. In other studies, percolation of water through sediments has a negative impact on egg recovery. We note that dietary remains can also decompose at open sites. Through scanning electron microscopy (SEM), we present examples of screened but chemically untreated microscopic remains. “Panorama” SEM images provide an excellent visual overview of the taphonomy of dietary remains. For this study, our focus is on Southwestern coprolites as a demonstration of diversity within a single region. Examples from caves and rock shelters were examined first to describe the taphonomic challenges for protected sites. Then, attention was turned to coprolites from open sites. In general, the challenges noted for parasite preservation are seen for other microfossils. However, the preservation of lignin, sporopollenin, calcium oxalate, and siliceous microfossils is generally better than cellulose structures. These observations are relevant to the selection process of samples for aDNA analysis and immunological study. This is especially relevant for the gut microbiome since decomposer fungi and bacteria molecular signals could be recovered in metagenomic analysis.

Cyclostratigraphic age constraining for Quaternary sediments in the Makarov Basin of the western Arctic Ocean using manganese variability

Abstract The Quaternary paleoenvironmental history of the Arctic Ocean remains uncertain, mainly due to the limited chronological constraints, especially beyond the 14C dating limits of accelerator mass spectrometry (AMS). The difficulty in establishing reliable chronostratigraphies is mainly attributed to low sedimentation rates and diagenetic sediment changes, resulting in very poor preservation of microfossils and altered paleomagnetic records. In the absence of independent chronostratigraphic data, the age model of Pleistocene sediments from the Arctic Ocean is mainly based on cyclostratigraphy, which relates lithologic changes to climatic variability on orbital time scales. In this study, we used the Mn/Al record measured from the sediment core ARA03B-41GC retrieved from the Makarov Basin in the western Arctic Ocean. The Mn/Al variation was tuned to the global benthic oxygen isotope stack (LR04) curve under different assumptions for computational correlation. Regardless of assumptions, our computational approach led to similar ages of about 600–1,000 ka for the bottom part of the core. These age models were up to about 200 ka older than those derived from lithostratigraphic approaches. Interestingly, our new age models show that the Ca/Al peak, a proxy for a detrital input from the Laurentide Ice Sheet, first occurred about 150 ka earlier than those previously proposed. Therefore, our results suggest that the glaciers in northern North America developed more extensively at about 810 ka than in earlier glacial periods, and influenced the sedimentary and paleoceanographic environments of the Arctic Ocean much earlier than previously thought. In order to establish a more comprehensive age model, more work is needed to validate our findings with different sediment cores recovered from the western Arctic Ocean.

Organic-rich microfossils produced by oil infiltration of hollow silicified bacteria: Evidence from the ca. 340 Ma Red Dog Zn-Pb deposit, Alaska

Abstract The remarkable preservation of carbonaceous microfossils in Precambrian chert has been attributed to rapid silica precipitation enclosing original cellular carbon. However, while silicification of bacteria can preserve cellular morphology, in modern silicifying environments, organic molecules are rapidly destroyed after death, raising questions about the origin of carbon in ancient microfossils. Here, we show that carbonaceous matter in filamentous microfossils in chert from the Carboniferous Red Dog Zn-Pb deposit, northern Alaska, represents relicts of migrated oil. Black, carbonaceous microfossils are associated with chert stained brown by solidified oil surrounding hairline fractures. The black filaments contain thin carbon films (&lt;50 nm) lining minute quartz crystals in intracellular cavities. Our observations indicate that silica nucleation on bacterial cell walls was followed by the decomposition of cellular carbon, producing silica-encased, bacterial molds that were later infiltrated by oil. This process represents a new mode of fossilization that raises questions about the origin of carbonaceous matter in some of Earth’s oldest microfossils.

Out of rock: A new look at the morphological and geochemical preservation of microfossils from the 3.46 Gyr-old Strelley Pool Formation

The biogenicity of most of the putative Archean organic-walled microfossils discussed in the literature remains debated. Here, we report morphological and geochemical characterizations of an assemblage of microfossils isolated from the 3.46 Gyr-old Strelley Pool Formation (Western Australia), thereby providing a new set of data on the oldest authentic microfossils on Earth. Isolated microstructures/microfossils were studied by combining scanning electron microscopy imaging and elemental analyses, Raman spectroscopy and nanoscale secondary ion mass spectrometry (NanoSIMS). We identified four morphological types, namely filaments, films, spheroids and lenses. A minority of spheroids, films and lenses exhibits a continuous wall cell surface and high N concentrations and substantial P contents (as suggested by 31P−/12C2− ionic ratios), i.e. exceptionally high morphological and geochemical preservation levels. In addition to the detection of P within microfossil walls, P was also found within micrometric patches in lenses exhibiting a fusiform shape. These patches may be remains of polyphosphate granules possibly formed during sporulation, suggesting, in turn, that cellular organization appeared as soon as 3.46 Gyr ago.

1 billion-year-old cell contents preserved in monazite and xenotime

Exceptional microfossil preservation, whereby sub-cellular details of an organism are conserved, remains extremely rare in the Precambrian rock record. We here report the first occurrence of exceptional cellular preservation by the rare earth element (REE) phosphates monazite and xenotime. This occurs in ~1 billion-year-old lake sediments where REEs were likely concentrated by local erosion and drainage into a closed lacustrine basin. Monazite and xenotime preferentially occur inside planktonic cells where they preserve spheroidal masses of plasmolyzed cell contents, and occasionally also membranous fragments. They have not been observed associated with cell walls or sheaths, which are instead preserved by clay minerals or francolite. REE phosphates are interpreted to be the earliest minerals precipitated in these cells after death, with their loci controlled by the micro-scale availability of inorganic phosphate (Pi) and REEs, probably sourced from polyphosphate granules within the cells. The strong affinity of REEs for phosphate and the insolubility of these minerals once formed means that REE phosphates have the potential for rapid preservation of cellular morphology after death and durability in the rock record. Hence, authigenic REE phosphates provide a promising new target in the search for the preservation of intra-cellular components of fossilised microorganisms.

A new record of late Ediacaran acritarchs from La providencia group (Tandilia System, Argentina) and its biostratigraphical significance

A well preserved assemblage of organic-walled microfossils is described from the Alicia and Cerro Negro formations of the La Providencia Group. The microbiota is dominated by Leiosphaeridia species (L. minutissima, L. tenuissima, L. crassa and L. jacutica) and comprises colonial sphaeroids (Synsphaeridium sp.) and filamentous microfossils of the genus Siphonophycus. The assemblage is assigned to the Late Ediacaran Leiosphere Palynoflora (LELP), which is consistent with the occurrence of the discoidal fossil Aspidella (<565 Ma) in the overlying unit. The distribution of microfossils within the Alicia Formation is mainly controlled by the shallowing upward trend and thus, environmental factors. This is indicated by the abundance and size of filaments, which tend to occur near to the paleoshore in shallow water settings. Furthermore, large leiospheres also occur in shallower facies together with filaments, in accordance with existing models for acritarch ecology and distribution in the Proterozoic marine successions. The absence of acanthomorphic acritarchs is thus interpreted as a real evolutionary trend, and not as a result of preservational or environmental biases. Regarding the preservation of microfossils, a higher Thermal Alteration Index for the Alicia Formation, compared with older and younger units, is interpreted as a result of hydrothermal overprint and/or contact metamorphism by basic dikes.

Terraced Iron Formations: Biogeochemical Processes Contributing to Microbial Biomineralization and Microfossil Preservation

Terraced iron formations (TIFs) are laminated structures that cover square meter-size areas on the surface of weathered bench faces and tailings piles at the Mount Morgan mine, which is a non-operational open pit mine located in Queensland, Australia. Sampled TIFs were analyzed using molecular and microanalytical techniques to assess the bacterial communities that likely contributed to the development of these structures. The bacterial community from the TIFs was more diverse compared to the tailings on which the TIFs had formed. The detection of both chemolithotrophic iron-oxidizing bacteria, i.e., Acidithiobacillus ferrooxidans and Mariprofundus ferrooxydans, and iron-reducing bacteria, i.e., Acidobacterium capsulatum, suggests that iron oxidation/reduction are continuous processes occurring within the TIFs. Acidophilic, iron-oxidizing bacteria were enriched from the TIFs. High-resolution electron microscopy was used to characterize iron biomineralization, i.e., the association of cells with iron oxyhydroxide mineral precipitates, which served as an analog for identifying the structural microfossils of individual cells as well as biofilms within iron oxyhydroxide laminations—i.e., alternating layers containing schwertmannite (Fe16O16(OH)12(SO4)2) and goethite (FeO(OH)). Kinetic modeling estimated that it would take between 0.25–2.28 years to form approximately one gram of schwertmannite as a lamination over a one-m2 surface, thereby contributing to TIF development. This length of time could correspond with seasonable rainfall or greater than average annual rainfall. In either case, the presence of water is critical for sustaining microbial activity, and subsequently iron oxyhydroxide mineral precipitation. The TIFs from the Mount Morgan mine also contain laminations of gypsum (CaSO·2H2O) alternating with iron oxyhydroxide laminations. These gypsum laminations likely represented drier periods of the year, in which millimeter-size gypsum crystals presumably precipitated as water gradually evaporated. Interestingly, gypsum acted as a substrate for the attachment of cells and the growth of biofilms that eventually became mineralized within schwertmannite and goethite. The dissolution and reprecipitation of gypsum suggest that microenvironments with circumneutral pH conditions could exist within TIFs, thereby supporting iron oxidation under circumneutral pH conditions. In conclusion, this study highlights the relationship between microbes for the development of TIFs and also provides interpretations of biogeochemical processes contributing to the preservation of bacterial cells and entire biofilms under acidic conditions.

Raman microspectroscopy, bitumen reflectance and illite crystallinity scale: comparison of different geothermometry methods on fossiliferous Proterozoic sedimentary basins (DR Congo, Mauritania and Australia)

Sedimentary rocks containing microfossils are crucial archives to reconstitute early life evolution on Earth. However, the preservation of microfossils within rocks depends on several physico-chemical factors. Among these factors, the thermal evolution of the host rocks can be decisive. Here, we investigated carbonaceous shale samples containing exquisitely preserved organic-walled microfossils assemblages from three Proterozoic shallow marine sedimentary sequences: the Mbuji-Mayi Supergroup (Democratic Republic of Congo, Congo Basin), the Atar/El Mreïti Group (Mauritania, Taoudeni Basin) and the Kanpa Formation (Australia, Officer Basin). Thermal maturity of these rock samples is evaluated with Raman geothermometry, Raman reflectance, solid bitumen reflectance, illite crystallinity and Thermal Alteration Index. The comparison of results coming from these different techniques validates the use of Raman reflectance on Proterozoic carbonaceous material and especially for poorly-ordered carbonaceous material. We show that extracted kerogen (microfossils and amorphous organic material) is more accurate to estimate the thermal maturity of low-grade temperature Proterozoic sequences than kerogen in thin section. All techniques provide consistent range of temperatures except for Raman geothermometry, giving slightly higher estimates. Raman reflectance appears to be a fast, robust and non-destructive tool to evaluate the thermal maturity of poorly-organized carbonaceous material from Proterozoic rocks.

Mineralisation of filamentous cyanobacteria in Lake Thetis stromatolites, Western Australia.

Stromatolites are cited as some of the earliest evidence for life on Earth, but problems remain in reconciling the paucity of microfossils in ancient carbonate examples with the abundance of microbes that help construct modern analogues. Here, we trace the mineralisation pathway of filamentous cyanobacteria within stromatolites from Lake Thetis, Western Australia, providing new insights into microfossil preservation in carbonate stromatolites. Lake Thetis cyanobacteria exhibit a spectrum of mineralisation processes that include early precipitation of Mg-silicates, largely controlled by the morphochemical features of the cyanobacteria, followed by aragonite formation that is inferred to be driven by heterotrophic activity. Fossilised cyanobacteria with high-quality morphological preservation are characterised by a significant volume of authigenic Mg-silicates, which have preferentially nucleated in/on extracellular organic material and on cell walls, and now replicate the region once occupied by the cyanobacterial sheath. In such specimens, aragonite is restricted to the outer sheath margin and parts of the cell interior. Cyanobacteria that display more significant degradation appear to possess a higher ratio of aragonite to Mg-silicate. In these specimens, aragonite forms micronodules in the sheath zone and is spatially associated with the inferred remains of heterotrophic bacteria. Aragonite also occurs as an advancing front from the outer margin of the sheath where it is commonly intergrown with Mg-silicates. Where there is no evidence of Mg-silicates within filaments, the fidelity of microfossil preservation is poor. In these cases, individual filaments may no longer be visible under light microscopy, and little organic material remains, but filament traces remain detectable using electron microscopy due to variations in aragonite texture. These data provide further evidence that authigenic silicate minerals play a crucial role in the fossilisation of micro-organisms; in their absence, carbonate crystal growth potentially mediated by heterotrophic microbial decay may largely obliterate morphological evidence for life within stromatolites, although mineralogical traces may still be detectable using electron microscopy.

Mineralization and Preservation of an extremotolerant Bacterium Isolated from an Early Mars Analog Environment

The artificial mineralization of a polyresistant bacterial strain isolated from an acidic, oligotrophic lake was carried out to better understand microbial (i) early mineralization and (ii) potential for further fossilisation. Mineralization was conducted in mineral matrixes commonly found on Mars and Early-Earth, silica and gypsum, for 6 months. Samples were analyzed using microbiological (survival rates), morphological (electron microscopy), biochemical (GC-MS, Microarray immunoassay, Rock-Eval) and spectroscopic (EDX, FTIR, RAMAN spectroscopy) methods. We also investigated the impact of physiological status on mineralization and long-term fossilisation by exposing cells or not to Mars-related stresses (desiccation and radiation). Bacterial populations remained viable after 6 months although the kinetics of mineralization and cell-mineral interactions depended on the nature of minerals. Detection of biosignatures strongly depended on analytical methods, successful with FTIR and EDX but not with RAMAN and immunoassays. Neither influence of stress exposure, nor qualitative and quantitative changes of detected molecules were observed as a function of mineralization time and matrix. Rock-Eval analysis suggests that potential for preservation on geological times may be possible only with moderate diagenetic and metamorphic conditions. The implications of our results for microfossil preservation in the geological record of Earth as well as on Mars are discussed.

Rich shallow-water benthic ecosystem in Late Miocene East Antarctica (Fisher Bench Fm, Prince Charles Mountains)

Abstract A single shell of a pectenoid bivalve was found in the lower part of the Fisher Bench Formation (Fisher Massif, Prince Charles Mountains). In contrast to glacigenic sediments that form extensive outcrops in this area and are almost barren of foraminifera, small pieces of muddy sand attached to this shell contained a rich assemblage of calcareous and siliceous microfossils. The diatom assemblage associated with this shell was dominated by marine benthic forms with abundant Saeptifera, associated with subordinate marine neritic taxa, and Chaetoceros resting spores. Benthic foraminifera are very abundant but of low diversity, strongly dominated by Rosalina globularis with uniform chambers. It is the first Miocene foraminiferal assemblage reported from East Antarctica excluding the Ross Sea area. These new results confirm the early Late Miocene age of the shell that was dated by Sr isotope values in an earlier study, and suggest an initial phase of deposition in a shallow-water (possibly no > 10 m) marine, cold, and possibly oligotrophic environment. After initial burial in microfossil-bearing sediment that was weakly lithified, the shell was eroded and transported to be incorporated in fossil-poor glacigenic sediments. Since transport from a nearby location could have been facilitated by glacial and/or down-slope movements in this fjordal setting, the shell is considered para-autochthonous. Based on the Antarctic foraminiferal fossil records, it appears that through most of the Cenozoic, those parts of the Antarctic continental shelf that were deeper than a few tens of meters, with the exception of the Ross Sea area, were mostly unfavorable for development and/or preservation of calcareous microfossils.

Remarkable Preservation of Microfossils and Biofilms in Mesoproterozoic Silicified Bitumen Concretions from Northern China

Prokaryotes, often generally referred to as “bacteria,” are the original and thus oldest life on Earth. They have shaped the chemical environment of the Earth, but they are difficult to find as ancient fossils due to their subtle structure. Here we report well-preserved fossilized microbial communities in silicified bitumen concretions from unit 3 of the Xiamaling Formation (1.39 Ga) in northern China. The numerous silicified bitumen concretions are in a variety of forms including ellipsoidal, spindle, and pancake ones, with diameters of 1~16 cm and thicknesses of 0.5~3 cm. The principal planes of the concretions are at low angle or directly parallel to the depositional plane level, showing obvious depositional characteristics. The concretions are silicified with abundant bitumen inside. Many different kinds of microbial fossils are found in the bitumen, including spherical forms, rods, and filaments, and some of the microbes are aggregated together in the forms of multicellular structures. These concretions preserve a delicate Mesoproterozoic biotic community.

Southeastern Tanzania depositional environments, marine and terrestrial links, and exceptional microfossil preservation in the warm Turonian

Sediment cores from southeastern Tanzania contain exceptionally well-preserved calcareous and organic-walled microfossils in numerous samples spanning from the Aptian to the Miocene. The unusually high quality of preservation is commonly attributed to shallow burial and high clay content of the host sediments. However, such attributes apply to many deposits that are not characterized by exceptional preservation, and, thus, invoking only grain size and burial depth is clearly an incomplete explanation for the exceptional microfossil preservation. In an attempt to better characterize additional factors that were important in creating the Tanzanian microfossil lagerstatte, we integrated a wide range of paleontological, geochemical, and sedimentological observations to constrain the paleoenvironmental and early diagenetic conditions of a Turonian interval where exceptional preservation is common. Planktic microfossil assemblages suggest that open-ocean surface-water conditions prevailed at the site, but, despite excellent organic matter preservation, marine biomarkers are rare. The sediments are dominantly composed of terrigenous silts and clays and terrestrially derived organic matter. Paleontological and geochemical observations record a remarkably stable interval dominated by excellent preservation through the Lower−Middle Turonian. During the Middle−Upper Turonian, though, the preservation quality declines, and this is associated with notable shifts in foraminiferal assemblages, palynological species diversity, carbon and sulfur isotope compositions, and biomarker distributions. The integrated data suggest that an increase in subsurface microbial activity and associated changes in pore-water chemistry were important proximate variables that led to the decline in the quality of microfossil preservation up section within these shallowly buried, Turonian silty claystones from Tanzania.