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The Roe Calcarenite is a 2–3-m-thick, mostly unlithified carbonate that accumulated in shallow water at the center of the Great Australian Bight on a marine erosional surface during the late Pliocene– early Pleistocene. The grainy deposits are profusely rich in whole mollusks and the large symbiont-bearing foraminifer Marginopora vertebralis. Articulated coralline algal rods, whole discorbid, rotaliid, and miliolid foraminifers, and innumerable fragments of M. vertebralis dominate sand-sized particles. A particularly conspicuous miliolid is the encrusting form Nubecularia sp. The unit is divided into two informal members. We interpret the lower member, an areally similar mollusk-rich facies, as a record of deposition during relative sea-level rise on shallow nearshore grass beds, probably dominated by an Amphibolis community living in an overall subtropical setting. The more areally diverse upper member comprises three facies, which we envisage as having accumulated during regression in a series of adjacent intertidal sand flat and beach or supratidal microbial-lacustrine environments. These Plio–Pleistocene deposits have many parallels with Holocene grass-bank facies in western and southern Australia and likely represent accumulation in a slightly warmer ocean than today wherein the depositional setting was heated by solar radiation. This unit is an important conceptual bridge into the older Cenozoic rock record.
Mudstones and siliceous concretions in the middle Cambrian Conasauga Formation, northwestern Georgia, contain body and trace fossils showing nonmineralized preservation and represent two temporally and spatially different marine environments. Identifiable, nonbiomineralized taxa include components of a Burgess Shale–type biota with red and green algae, primitive sponges, and the arachnomorph arthropod Naraoia compacta. Also exceptionally preserved are the filamentous appendages of a large ptychopariid trilobite and assemblages of oriented hyolithid tests we interpret as priapulid coprolites and cololites. Exceptional preservation in the Conasauga Formation has multiple causes. The Conasauga contains superabundant siliceous concretions, many with skeletal, trace, and some nonbiomineralized fossils. Shale specimens, especially sponges with preserved details, and whole-body trilobite preservations, often have iron (Fe) oxide halos that resulted from a biochemical cascade including bioimmuration, decomposition gas anoxia, Fe-sulfide crystallization, and Fe oxidation. Preservation of soft tissue is also partly attributable to the well-sorted clay matrix of inner shelf Conasauga shales, which allowed mechanical imprinting of body fossils. Several nonbiomineralized fossils show algal overgrowths, suggesting an additional form of bioimmuration. Exceptional preservation in the Conasauga Formation is relatively poor compared with such better-known Cambrian Lagerstätten as the Burgess and Wheeler Shales; nevertheless, it is significant for three reasons. The siliceous concretions are a rare vehicle for exceptional preservation and feature three-dimensional fossils rather than the more common compressed specimens. The older Conasauga biota occupied a shallow-shelf environment, a setting in which exceptional preservation is poorly understood. The Conasauga Formation extends the geographic range of a Burgess Shale–type biota to the extreme southeastern USA.
The graptolitic Early Ordovician succession of the Mount Hunneberg locality, southern Sweden, shows the response of graptolite faunas to sea-level changes. The exposed interval consists of intercalated carbonates and shales at the base, grading into pure black shales in its central and upper part. This facies trend records a deepening of the depositional environment due to an overall sea-level rise. The Mount Hunneberg graptolite fauna is dominated by nearshore, shallow-marine forms found in most layers throughout the succession. Deeper-water pandemic species occur only rarely together with shallow-water graptolites but are dominant at four distinct traceable levels at Mount Hunneberg. This change in graptolite faunal composition is interpreted to indicate sea-level fluctuations. With rising sea level, shallow-water endemics and pandemics migrated landward, and deeper-water pandemic graptolites became increasingly frequent on the shelf. During the peak of the transgression, deep-water forms dominated. With falling sea level, shallow-water forms again appeared and replaced the deep-water graptolites. The four levels with deep-water graptolites at Mount Hunneberg are, therefore, interpreted to represent maximum flooding surfaces. This study demonstrates that changes in the faunal composition of such planktic organisms as graptolites provide a promising tool for recognizing maximum flooding surfaces. As these faunal turnovers are also detectable in monotonous parts of the Mount Hunneberg succession, this biofacies-based approach enables the recognition of maximum flooding surfaces even when no lithologic changes are present, enhancing the applicability of sequence stratigraphic interpretations to monotonous outer shelf strata.
A concentration of Pseudorca crassidens remains resulting from a mass stranding on the tidal flats of the Colorado River Delta, Baja California, Mexico, was analyzed to determine how bone and individual density and variation in taphonomic condition differs from a time-averaged assemblage of marine mammals. Five hundred and thirty seven skeletal elements, including 26 whole skulls, were found among 204 bone sites in a 13,000 m2 area. Skulls provide the best estimate of minimum number of individuals; all other skeletal elements are underrepresented. Twenty bones per individual, one bone per 26 m2, and one individual per 536 m2 characterize this the mass-stranding locality. Bone density and individual density are greater at this locality than in a previously studied time-averaged assemblage from the Colorado Delta. Although the lack of variation in taphonomic condition is sometimes used as one criterion for a mass-death assemblage, the condition of the remains in this mass stranding varies both within and among skeletal elements. Teeth tend to be in good condition, earbones in fair condition, and vertebrae in poor condition. The taphonomic differences are a result of variation in the density and size of the skeletal element, variation in associated sediment (sand or mud), and variation in exposure (surface or buried). Despite the fact that all the individuals died at the same time, the taphonomic condition of their skeletal elements varies greatly.
Recent studies have emphasized that faunal change is typically brief and most commonly occurs at sequence boundaries and major flooding surfaces. The Upper Ordovician of the Cincinnati, Ohio, region records a major biotic invasion in the Richmondian Stage, which offers an opportunity to test these generalizations and to understand how episodes of faunal change are reflected in the structure of ecological gradients. The early Cincinnatian (C1–early C4 depositional sequences) displays two relatively stable faunal gradients, with the primary gradient reflecting onshore-offshore setting and the secondary gradient reflecting substrate consistency. During the mid-C4 sequence, dominant taxa of the shallow subtidal are extirpated, while deep subtidal taxa expand into those habitats, leading to a loss of cross-shelf faunal differentiation. This faunal breakdown is accompanied into the mid-C5 by a series of ecological epiboles, indicating an ongoing flux in ecological associations. The onshore-offshore gradient is reestablished in the C5, albeit with new associations dominated by or containing immigrant taxa. Recognition of this gradient is hindered by widespread increased abundance of bryozoans and by the delayed appearance of at least seven common genera of brachiopods and corals. The Richmondian Invasion plays out over multiple sequences and is not confined to a brief interval at the beginning of a sequence. These faunal changes do not coincide with sequence boundaries or major flooding surfaces and therefore cannot be sequence stratigraphic artifacts, nor can they represent a geologically instantaneous faunal response to sea-level change.
The graphoglyptid trace fossil Paleodictyon, characterized by a stratiform hexagonal net, is a diagnostic member of the deep-water Nereites ichnofacies and usually found in deep-sea flysch successions. Scattered records of the trace fossil from shallower, lower-shelf-to-upper-slope environments, particularly since the mid-Mesozoic, have been substantially augmented by new data from Upper Triassic and Jurassic sedimentary basins of Iran. Paleodictyon has been found on the soles of mid-to-lower-shelf event beds produced by storm-induced currents and on the soles of prodelta turbidite deposits in the Upper Triassic Nayband Formation of east-central Iran, the lower Middle Jurassic Shemshak Formation of north-central Iran, the Middle Jurassic Kashafrud Formation of the Koppeh Dagh, and the Middle Jurassic Dalichai Formation of the Binalud Mountains. It appears that the bathymetric range of Paleodictyon throughout the Phanerozoic was considerably wider than generally assumed. The observed dominance of the trace fossil in deep-water flysch successions we interpret as being at least partially due to preservational effects. The conservation of the networks requires limited erosion, immediate casting and sealing of the exhumed negative epireliefs by sand, and limited subsequent bioturbation at deeper tiers. This restricts the trace fossil to event beds, either deep-sea turbidites or shallow-water storm beds in environments characterized by high-sedimentation rates. For bathymetric interpretations of paleoenvironments, whole ichnoassemblages should be used rather than single ichnotaxa to avoid erroneous conclusions.
Carbon isotope compositions of sedimentary organic matter (average −24.1‰) from an Albian marine siliciclastic succession in Hokkaido, Japan, exhibit a distinct anomaly by ∼ 1.2‰ with a trifurcate shape across the Albian–Cenomanian boundary and two relatively small shifts (< 1‰) in the middle and upper Albian, respectively. The organic matter consist predominantly of woody materials with an insignificant degree of thermal alteration, judged from the visual and elemental characteristics of kerogen; the stratigraphic δ13Corg fluctuations are independent of lithological or total organic carbon variations. Thus, the Hokkaido δ13Corg profile is interpreted as representing the temporal δ13C changes in whole C3 plant vegetation in the provenance of East Asia during Albian time. The patterns and amplitudes in δ13Cwood values and their relationship with planktonic foraminiferal zones are conformable with coeval Tethyan δ13C records of pelagic carbonates. This observation reinforces the view that δ13C compositions of marine and terrestrial carbon reservoirs fluctuated simultaneously by the same amplitude within the ocean-atmosphere-biosphere system regardless of changes in such paleoenvironmental parameters as pCO2. From a chemostratigraphic viewpoint, time-equivalent levels of Oceanic Anoxic Events and stage boundaries are constrained for the Hokkaido sections, allowing for the proposal of a detailed chronostratigraphic framework for future advanced paleoceanographic research in the mid-Cretaceous northwestern Pacific region.
Seasonally sampled cores of burrowed sediment containing chironomid larvae were collected from Cooking Lake, Alberta, and analyzed to (1) assess and establish the typical burrowing behavior and burrow architecture of chironomid larvae; (2) record micrometer-scale geochemical profiles of O2, H2S, and pH in the uppermost sedimentary layers throughout a seasonal cycle; and (3) link changing geochemical conditions to changing burrowing behaviors. We observed that the larvae lived in soft, water-saturated sediment, maintained by open burrows accreted by the animal's mucous. Chironomid-larvae burrows were small and Y-shaped (e.g., Polykladichnus-like) or Y-shaped with basal branches (Thalassinoides-like) and were 20 cm deep. The larvae moved up and down from the oxygenated zone (“sounding” behavior) to exploit food in suboxic and anoxic sediment. Geochemical analyses showed that H2S was present in the pore waters to within 1.5 mm of the sediment-water interface during the summer, when lake-bottom algae and cyanobacteria generated sufficient O2 to drive the oxic-anoxic redoxcline into the sediment. In the winter, the H2S front extended upward into the water column owing to the cessation of algal and cyanobacterial activity. The prevalence of H2S results from a combination of high-dissolved-sulfate concentrations in the lake and the abundance of microbial biomass that fuels an active subsurface population of sulfate-reducing bacteria. Interestingly, burrowing behavior was not linked to seasonal changes in the sediment chemistry. This is in part due to the ability of chironomid larvae to exploit oxygen islands in the sediment: in the winter, the chironomid larvae harvest their oxygen from the uppermost photosynthetic layer in otherwise O2-impoverished sediments.
The fossil remains of eurypterid cuticles in this study yield long-chain (<C9 to C22) aliphatic components similar to type II kerogen during pyrolysis–gas chromatography/mass spectrometry, in contrast to the chitin and protein that constitute the bulk of modern analogs. Structural analysis (thermochemolysis) of eurypterid cuticles reveals fatty acyl moieties (derived from lipids) of chain lengths C7 to C18, with C16 and C18 components being the most abundant. The residue is immune to base hydrolysis, indicating a highly recalcitrant nature and suggesting that if ester linkages are present in the macromolecule, they are sterically protected. Some samples yield phenols and polyaromatic compounds, indicating a greater degree of aromatization, which correlates with higher thermal maturity as demonstrated by Raman spectroscopy. Analysis (including thermochemolysis) of the cuticle of modern scorpions and horseshoe crabs, living relatives of the eurypterids, shows that C16 and C18 fatty acyl moieties likewise dominate. If we assume that the original composition of the eurypterid cuticle is similar to that of living chelicerates, fossilization likely involves the incorporation of such lipids into an aliphatic polymer. Such a process of in situ polymerization accounts for the fossil record of eurypterids.
The soft-bodied Chengjiang fossils show great potential for assessing the forces operating during the burial process, particularly subtle gravitational effects that are often overlooked. Evidence from gravitational sorting is consistent with the assumption that most Chengjiang organisms underwent local transport. Based on an examination of 1295 specimens and orientation frequency data, we found that the Chengjiang animals followed hydrostatic rules and obtained equilibrium during the burial process, resting regularly on bedding planes rather than randomly, as seen in the Burgess Shale. Specific gravity could be one of crucial factors determining the burial orientation of these organisms, even before normal compaction occurs during early diagenesis.