Evidence for stratigraphy in molluscan death assemblages preserved in seagrass beds: St. Croix, U.S. Virgin Islands

Paleobiology ◽  
2018 ◽  
Vol 44 (1) ◽  
pp. 155-170 ◽  
Author(s):  
Kelsey M. Arkle ◽  
Arnold I. Miller
Keyword(s):  
Virgin Islands ◽  
Compositional Variation ◽  
Water Interface ◽  
Seagrass Beds ◽  
Shallow Marine ◽  
Stratigraphic Record ◽  
Death Assemblages ◽  
Compositional Changes ◽  
Relative Abundances ◽  
Mollusk Assemblages

AbstractDeath assemblages that occupy the upper tens of centimeters of sediment in shallow-marine settings are often subject to extensive mixing, thereby limiting their usefulness in assessing environmentally mediated compositional changes through time in the local biota. Here, we provide evidence that dense,Thalassia-rich seagrass beds preserve a stratigraphic record of biotic variation because their dense root–rhizome mats inhibit mixing. We sampled benthic mollusk assemblages at seven localities inThalassia-rich beds around St. Croix, USVI, collecting three separate sediment intervals of ~13 cm each to a total depth of ~40 cm below the sediment–water interface, and found evidence that sedimentary intervals preserved compositional stratigraphy. Further, some localities displayed systematic, directional changes down-core. An examination of interval-to-interval changes in composition revealed that compositional variation was unique from locality to locality rather than reflecting coordinated, island-wide transitions. In general, however, relative abundances of epifaunal gastropods and small lucinid bivalves tended to decrease with depth below the sediment–water interface. Quantitative comparisons of life-to-death assemblages from each successive sedimentary interval demonstrated that the shallowest death assemblages were typically more similar to the life assemblages than were deeper assemblages, suggesting that deeper intervals provide records of earlier community states.

A new molluscan assemblage from the pre-evaporitic Messinian of Crete (Greece)

2020 ◽  
Author(s):  
Efterpi Koskeridou ◽  
Danae Thivaiou
Keyword(s):  
Seagrass Beds ◽  
Shallow Marine ◽  
Environmental Consequences ◽  
Environmental Perturbation ◽  
New Material ◽  
Molluscan Fauna ◽  
First Time ◽  
Molluscan Assemblage ◽  
Hard Substrates ◽  
Normal Salinity

<p>The Messinian Salinity Crisis (MSC) was an environmental perturbation with dramatic environmental consequences that greatly affected marine organisms. Messinian deposits are found in several locations around the Mediterranean, but few offer marine faunas rich in molluscs. A section near Heraklion, central Crete, has provided new material that contains a well preserved and rich molluscan fauna that includes many micromorphic species. The section is of early Messinian age, belongs to Agios Miron Formation, and bears several layers of fossiliferous marly sands.</p><p>Molluscs from a fossiliferous bed of the section are presented here for the first time. Gastropods and bivalves are most common, but scaphopods and chitons are not infrequent. The assemblage seems to be composed of transported elements from nearby environments and the most frequent species are present in comparable abundances for gastropods and bivalves. The gastropod fauna is represented by <em>Bittium</em> sp. and <em>Gibbula </em>sp., accompanied by <em>Diodora</em> cf. <em>graeca</em>, <em>Turritella</em> sp., <em>Jujubinus</em> sp., species of Pyramidellidae and rarer <em>Homalopoma</em> sp. and Haliotis sp. The presence of <em>Bittium</em> sp. together with <em>Jujubinus</em> sp. suggests vegetated environments. Bivalves are represented by species dwelling mostly in sandy environments such as <em>Glycymeris</em> cf. <em>inflata</em> (also occurring in larger specimens), <em>Spisula</em> sp., <em>Timoclea</em> sp. and various cardiids. Exceptionally well-preserved chitons indicate the presence of hard substrates such as rocks, pebbles or roots of seagrass beds. This is confirmed by the presence of the gastropods <em>Diodora</em> cf. <em>graeca</em> and <em>Haliotis</em> sp.</p><p>The assemblage points towards normal salinity shallow marine conditions of sandy bottoms with patches of seagrass-type vegetation before the onset of the MSC.</p>

Preservation of spatial and environmental gradients by death assemblages

Paleobiology ◽  
2009 ◽  
Vol 35 (1) ◽  
pp. 119-145 ◽  
Author(s):  
Adam Tomašových ◽  
Susan M. Kidwell
Keyword(s):  
Community Structure ◽  
Community Composition ◽  
Environmental Gradients ◽  
Compositional Variation ◽  
Time Averaging ◽  
Data Sets ◽  
Square Root ◽  
Spatial Gradients ◽  
Death Assemblages ◽  
The Impact

Although only a few studies have explicitly evaluated live-dead agreement of species and community responses to environmental and spatial gradients, paleoecological analyses implicitly assume that death assemblages capture these gradients accurately. We use nine data sets from modern, relatively undisturbed coastal study areas to evaluate how the response of living molluscan assemblages to environmental gradients (water depth and seafloor type; “environmental component” of a gradient) and geographic separation (“spatial component”) is captured by their death assemblages. We find that:1. Living assemblages vary in composition either in response to environmental gradients alone (consistent with a species-sorting model) or in response to a combination of environmental and spatial gradients (mass-effect model). None of the living assemblages support the neutral model (or the patch-dynamic model), in which variation in species abundance is related to the spatial configuration of stations alone. These findings also support assumptions that mollusk species consistently differ in responses to environmental gradients, and suggest that in the absence of postmortem bias, environmental gradients might be accurately captured by variation in species composition among death assemblages. Death assemblages do in fact respond uniquely to environmental gradients, and show a stronger response when abundances are square-root transformed to downplay the impact of numerically abundant species and increase the effect of rare species.2. Species' niche positions (position of maximum abundance) along bathymetric and sedimentary gradients in death assemblages show significantly positive rank correlations to species positions in living assemblages in seven of nine data sets (both square-root-transformed and presence-absence data).3. The proportion of compositional variation explained by environmental gradients in death assemblages is similar to that of counterpart living assemblages. Death assemblages thus show the same ability to capture environmental gradients as do living assemblages. In some instances compositional dissimilarities in death assemblages show higher rank correlation with spatial distances than with environmental gradients, but spatial structure in community composition is mainly driven by spatially structured environmental gradients.4. Death assemblages correctly identify the dominance of niche metacommunity models in mollusk communities, as revealed by counterpart living assemblages. This analysis of the environmental resolution of death assemblages thus supports fine-scale niche and paleoenvironmental analyses using molluscan fossil records. In spite of taphonomic processes and time-averaging effects that modify community composition, death assemblages largely capture the response of living communities to environmental gradients, partly because of redundancy in community structure that is inherently associated with multispecies assemblages. The molluscan data sets show some degree of redundancy as evidenced by the presence of at least two mutually exclusive subsets of species that replicate the community structure, and simple simulations show that between-sample relationships can be preserved and remain significant even when a large proportion of species is randomly removed from data sets.

scholarly journals Organism motility in an oxygenated shallow-marine environment 2.1 billion years ago

2019 ◽  
Vol 116 (9) ◽  
pp. 3431-3436 ◽  
Author(s):  
Abderrazak El Albani ◽  
M. Gabriela Mangano ◽  
Luis A. Buatois ◽  
Stefan Bengtson ◽  
Armelle Riboulleau ◽  
...  
Keyword(s):  
Marine Environment ◽  
Early Diagenesis ◽  
Water Interface ◽  
Slime Molds ◽  
Shallow Marine ◽  
Shallow Marine Environment ◽  
Great Oxidation Event ◽  
Cellular Slime ◽  
Amoeboid Cells ◽  
Colonial Organisms

Evidence for macroscopic life in the Paleoproterozoic Era comes from 1.8 billion-year-old (Ga) compression fossils [Han TM, Runnegar B (1992) Science 257:232–235; Knoll et al. (2006) Philos Trans R Soc Lond B 361:1023–1038], Stirling biota [Bengtson S et al. (2007) Paleobiology 33:351–381], and large colonial organisms exhibiting signs of coordinated growth from the 2.1-Ga Francevillian series, Gabon. Here we report on pyritized string-shaped structures from the Francevillian Basin. Combined microscopic, microtomographic, geochemical, and sedimentologic analyses provide evidence for biogenicity, and syngenicity and suggest that the structures underwent fossilization during early diagenesis close to the sediment–water interface. The string-shaped structures are up to 6 mm across and extend up to 170 mm through the strata. Morphological and 3D tomographic reconstructions suggest that the producer may have been a multicellular or syncytial organism able to migrate laterally and vertically to reach food resources. A possible modern analog is the aggregation of amoeboid cells into a migratory slug phase in cellular slime molds at times of starvation. This unique ecologic window established in an oxygenated, shallow-marine environment represents an exceptional record of the biosphere following the crucial changes that occurred in the atmosphere and ocean in the aftermath of the great oxidation event (GOE).

scholarly journals Current synthesis of the penultimate icehouse and its imprint on the Upper Devonian through Permian stratigraphic record

2021 ◽  
pp. SP512-2021-124
Author(s):  
Isabel Patricia Montañez
Keyword(s):  
Ice Sheets ◽  
Late Paleozoic ◽  
Earth System ◽  
Low Latitude ◽  
New Techniques ◽  
Shallow Marine ◽  
Rigorous Testing ◽  
System Components ◽  
Stratigraphic Record ◽  
Age Constraints

AbstractIcehouses are the less common climate state on Earth, and thus it is notable that the longest lived (∼370 to 260 Ma) and possibly most extensive and intense of icehouse periods spanned the Carboniferous Period. Mid- to high-latitude glaciogenic deposits reveal a dynamic glaciation-deglaciation history with ice waxing and waning from multiple ice centers and possible transcontinental ice sheets during the apex of glaciation. New high-precision U-Pb ages confirm a hypothesized west-to-east progression of glaciation through the icehouse, but reveal that its demise occurred as a series of synchronous and widespread deglaciations. The dynamic glaciation history, along with repeated perturbations to Earth System components, are archived in the low-latitude stratigraphic record revealing similarities to the Cenozoic icehouse. Further assessing the phasing between climate, oceanographic, and biotic changes during the icehouse requires additional chronostratigraphic constraints. Astrochronology permits the deciphering of time, at high resolution, in the late Paleozoic record as has been demonstrated in deep- and quit-water deposits. Rigorous testing for astronomical forcing in low-latitude cyclothemic successions, which have a direct link to higher latitude glaciogenic records through inferred glacioeustasy, however, will require a comprehensive approach that integrates new techniques with further optimization and additional independent age constraints given challenges associated with shallow-marine to terrestrial records.

scholarly journals Eocene Sediments in the South China Sea, precursor deposits of the Oligocene expansion?

2021 ◽  
Vol 47 (2) ◽  
pp. 1-24
Author(s):  
Franz L. Kessler ◽  
John Jong ◽  
Mazlan Madon
Keyword(s):  
South China Sea ◽  
South China ◽  
The South China Sea ◽  
Petroleum Exploration ◽  
The South ◽  
Shallow Marine ◽  
China Sea ◽  
Marine Deposits ◽  
Stratigraphic Record ◽  
Well Data

The stratigraphic record of Eocene in the Malaysian waters of the South China Sea is scarce; the few deep petroleum exploration wells and outcrops are located on the fringes of the SCS. Yet, despite the paucity of data we observe a variety of sediments that cover the range from fluviatile to (at least) neritic marine deposits. Whilst fluvial deposits dominate the Western Rim (Penyu, Malay basins), the Southern Rim (Sarawak) is characterized by deposits of a narrow and rapidly deepening shelf, with fluviatile, shallow marine clastics and carbonates passing seawards to outer shelf and neritic deposits. The proven record of the Eocene to-date is located in relatively small and deep sub-basins.  Possibly, the Eocene underlies additional areas of the SCS, but there is to-date no sufficient well data to offer further confirmation. The presence of Eocene strata on the margins of Sundaland is associated with an early phase of extensional and/or transpressional tectonism, probably acting as precursor movements related to the onset of rifting of the crust underlying the SCS.

scholarly journals Diversity and compositional changes in the gut microbiota of wild and captive vertebrates: a meta-analysis

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Antton Alberdi ◽  
Garazi Martin Bideguren ◽  
Ostaizka Aizpurua
Keyword(s):  
Gut Microbiota ◽  
Phylogenetic Signal ◽  
Meta Analysis ◽  
Compositional Variation ◽  
Vertebrate Species ◽  
Normal Functioning ◽  
Environmental Pressures ◽  
Heterogeneous Landscape ◽  
Captive Populations ◽  
Compositional Changes

AbstractThe gut microbiota is recognised as an essential asset for the normal functioning of animal biology. When wild animals are moved into captivity, the modified environmental pressures are expected to rewire the gut microbiota, yet whether this transition follows similar patterns across vertebrates is still unresolved due to the absence of systematic multi-species analyses. We performed a meta-analysis of gut microbiota profiles of 322 captive and 322 wild specimens from 24 vertebrate species. Our analyses yielded no overall pattern of diversity and compositional variation between wild and captive vertebrates, but a heterogeneous landscape of responses, which differed depending on the components of diversity considered. Captive populations showed enrichment patterns of human-associated microorganisms, and the minimal host phylogenetic signal suggests that changes between wild and captive populations are mainly driven by case-specific captivity conditions. Finally, we show that microbiota differences between wild and captive populations can impact evolutionary and ecological inferences that rely on hierarchical clustering-based comparative analyses of gut microbial communities across species.

High dielectric constant and small temperature coefficient bismuth-based dielectric compositions

1990 ◽  
Vol 5 (8) ◽  
pp. 1752-1762 ◽  
Author(s):  
H. C. Ling ◽  
M. F. Yan ◽  
W. W. Rhodes
Keyword(s):  
Temperature Coefficient ◽  
Room Temperature ◽  
Dissipation Factor ◽  
Dielectric Constants ◽  
Compositional Variation ◽  
Multilayer Ceramic Capacitors ◽  
Temperature Coefficients ◽  
Compositional Changes ◽  
High Dielectric ◽  
High Silver

We have studied the crystal structure and the dielectric properties of a scries of Bi-based ceramic compositions as a function of compositional variation and sintering temperature. These dielectrics have dielectric constants hetween 70 and 165 and their temperature coefficients are within ±500 × 10−6/°C. The precise temperature coefficient can be controlled via compositional changes such that dielectrics with temperature coefficients within ±50 × 10−6/°C are easily obtainable. The room temperature dissipation factor is smaller than 0.001 or equivalently, the Q value is greater than 1000. The electrical resistivity is greater than 1014 ohm-cm. Furthermore, these dielectrics are sinterable below 960 °C, rendering it possible to use silver or high silver metallization as the internal electrode in making the multilayer ceramic capacitors.

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