Discriminating intra-parasequence stratigraphic units from two-dimensional quantitative parameters

ABSTRACT The intra-parasequence scale is still relatively unexplored territory in high-resolution sequence stratigraphy. The analysis of internal genetic units of parasequences has commonly been simplified to the definition of bedsets. Such simplification is insufficient to cover the complexity involved in the building of individual parasequences. Different types of intra-parasequence units have been previously identified and characterized in successive wave-dominated shoreface–shelf parasequences in the Lower Cretaceous Pilmatué Member of the Agrio Formation in central Neuquén Basin. Sedimentary and stratigraphic attributes such as the number of intra-parasequence units, their thickness, the proportions of facies associations in the regressive interval, the lateral extent of bounding surfaces, the degree of deepening recorded across these boundaries, and the type and lateral extent of associated transgressive deposits are quantitatively analyzed in this paper. Based on the analysis of these quantified attributes, three different scales of genetic units in parasequences are identified. 1) Bedset complexes are 10–40 m thick, basin to upper-shoreface successions, bounded by 5 to 16 km-long surfaces with a degree of deepening of one to three facies belts. These stratigraphic units represent the highest hierarchy of intra-parasequence stratigraphic units, and the vertical stacking of two or three of them typically forms an individual parasequence. 2) Bedsets are 2–20 m thick, offshore to upper-shoreface successions, bounded by up to 10 km long surfaces with a degree of deepening of zero to one facies belt. Two or three bedsets stack vertically build a bedset complex. 3) Sub-bedsets are 0.5–5 m thick, offshore transition to upper-shoreface successions, bounded by 0.5 to 2 km long surfaces with a degree of deepening of zero to one facies belt. Two or three sub-bedsets commonly stack to form bedsets. The proposed methodology indicates that the combination of thickness with the proportion of facies associations in the regressive interval of stratigraphic units can be used to discriminate between bedsets and sub-bedsets, whereas for higher ranks (bedsets and bedset complexes) the degree of deepening, lateral extent of bounding surfaces, and the characteristics of associated shell-bed deposits become more effective. Finally, the results for the Pilmatué Member are compared with other ancient and Holocene examples to improve understanding of the high-frequency evolution of wave-dominated shoreface–shelf systems.

Reconstructing the baseline population structure of the exploited bivalve Arca noae in the Northern Adriatic Sea

<p>The Northern Adriatic Sea is one of the most impacted ecosystems worldwide with a long history of anthropogenic impacts, ranging from overfishing and bottom trawling to eutrophication, deoxygenation and pollution. The impact of these multiple pressures on populations of economically important species is often difficult to evaluate due to paucity of long-term monitoring data. The edible bivalve Noah’s Ark shell (<em>Arca noae </em>L.) was intensively harvested in the eastern Adriatic Sea until 1949-1950 when it suffered a catastrophic population collapse due to unknown agents. The assessment of its subsequent recovery is hindered by the lack of data on the population size structure prior to that event. To reconstruct the natural baseline state of populations of <em>A. noae</em> before the onset of extensive harvesting, we studied fossil assemblages from two 1.5-m-long sediment cores collected in the southern Gulf of Trieste (off Piran, Slovenia), both recording the last ~9,500 years.</p><p>The abundance and shell length of <em>A. noae</em> remained low in the lower part of the cores but increased strongly within the oyster-<em>Arca</em> shell bed corresponding to maximum flooding and early highstand sea-level phases (6,500-1,000 years ago). In contrasts, the top 8 cm of the core (the late highstand phase), marked by high concentration of pollutants and organic enrichment, contained only few and small (< 10 mm) <em>A. noae</em> shells. Moreover, no living individuals were found in grab samples taken from the two stations suggesting that the dense populations of <em>A. noae, </em><span>persisting </span><span>there</span> <span>for</span><span> several thousand years, </span>were locally extirpated in the 20<sup>th</sup> century. To evaluate population recovery in other parts of the NE Adriatic, we compared the size distribution of<em> </em>fossil<em> </em><em>A. noae</em> from the shell bed interval to the previously published data on living populations of this species sampled<em> </em>along Istrian peninsula between 1966 and 1978. Both fossil and extant populations were characterized by similar <span>median </span><span>size, </span><span>modal size </span><span>class and</span><span> proportion of </span><span>specimens > 50 mm </span><span>(minimal legal landing size). </span><span>These results suggest that within few </span><span>decades</span><span> after the </span><span>1949-1950 </span><span>mass mortality event </span><span>the size structure of populations </span><span>of</span> <span><em>A. noae</em></span> <span>have largely returned to their earlier, natural state.</span><span> The recovery was </span><span>spatially variable, however, as attested by</span><span> the decline of</span> <span><em>A. noae</em></span> <span>populations </span><span>due to loss of </span><span>suitable shell-bed habitat</span><span>s</span><span> in</span><span> the two </span><span>studied</span><span> station</span><span>s</span><span> off Piran.</span></p>

Taphonomy of a Panopea Ménard de la Groye, 1807 shell bed from the Pisco Formation (Miocene, Peru)

Invertebrate taphonomy can provide significant information about the post-mortem processes that affected the fossil record. In the East Pisco Basin of southern Peru, a Panopea Ménard de la Groye, 1807 shell bed was found in the upper Miocene strata of the Pisco Formation, hinting at a peculiar biostratinomic and diagenetic history. This bed contains abundant invertebrate fossil molds cemented by dolomite. The specimens of the deep infaunal bivalve, Panopea sp., occur together with bivalves representative of shallow infaunal species (Trachycardium sp. and Dosinia ponderosa [Gray, 1838]) and balanid barnacles, which are sessile encrusters. The Panopea specimens host compound molds evidencing an abundant encrusting fauna, including serpulids, ?foraminifera, bryozoans, and barnacles that colonized the inner surfaces of the valves before their final burial. We hypothesize that short-term, storm-related processes exhumed the living bivalves, resulting in a sedimentological concentration of relatively well-preserved shells. After the death of the exhumed bivalves, the inner surfaces of the articulated Panopea shells, representing hard-substratal, sheltered environments on an otherwise unstable sandy seafloor (i.e., “benthic islands”), were colonized by different encrusting organisms. Following the final burial, dolomite precipitated, cementing the sediment infill of the valves. Lastly, a decrease of pH occurred at the sulfate reduction-methanogenesis boundary, inducing the dissolution of the shell carbonate.

Fossil decapods from the Upper Quaternary in Shinjima Island in Kagoshima, Kyushu, Japan, and description of a new species of ghost shrimp (Axiidea, Eucalliacidae)

Five species of decapod crustaceans, including Calliax nishiki sp. nov. (Axiidea, Eucalliacidae) and Carcinoplax longimana (de Haan), are described from the upper part of the Moeshima Silt Bed (latest Pleistocene to Holocene: ca 13,000 to 8,000 cal BP). Ten decapods, including Laticallichirus grandis (Karasawa & Goda) and Ebalia tuberculosa (A. Milne-Edwards) are described from the Moeshima Shell Bed (Holocene: ca 8,000 to 2,000 cal BP). Calliax nishiki is abundant and Carcinoplax longimana is common in the decapod assemblage of Moeshima Silt Bed. The new species of Calliax seems to have dispersed around the deep marine setting with a reducing environment. The decapod assemblage of the Moeshima Shell Bed is characterized by a predominance of L. grandis. The present record suggests that L. grandis is abundant under sandy and gravelly bottoms mixed with shells of the lower sublittoral to subtidal zones during the Quaternary.

Time averaging and stratigraphic unmixing: reconstructing ecological decline in molluscan production (Holocene, Brijuni, NE Adriatic)

<p>The northeastern Adriatic seafloor is formed by warm-temperate bioclastic carbonates with coralline algae, bryozoans and mollusks. These sediments represent a mixture of past and present-day production owing to low sedimentation rates and bioturbation. Although low sedimentation rates do not allow resolution of ecological history at centennial or even millennial scales on the basis of raw stratigraphic data, age unmixing based on radiocarbon-calibrated amino acid racemization shows that one of the major molluscan sediment producers – the infaunal suspension-feeder Timoclea ovata  – markedly peaked in production ~5,000 years during the maximum flooding and earliest highstand phase and significantly diminished in abundance during the late highstand phase at Brijuni, with a large proportion of dead shells now present in surface sediments representing shells that are several centuries old. This species still occurs in living assemblages but our analyses indicate that its former production was by several orders of magnitude higher. In contrast, stratigraphic trends in absolute and proportional abundance of this species in ~1.5 m-thick sediment cores show a gradual or a very mild upcore decline, indicating that raw stratigraphic data do not efficiently detect millennial-scale ecological dynamic. The temporal decline in production of Timoclea ovata is associated with an increase in water depth and an increase in sediment-accumulation rate, and led to a transition from molluscan oyster-scallop shell bed to late highstand bryomol sediments.</p>

Removal of Chromium from Aqueous Solution using Multilayer-Mixed (Reduction/Sorption) Bed from Iron Scrap and Nutshells

In this research removal of chromium from an aqueous solution by multilayer-mixed (reduction/sorption) bed via scrap iron as zero valent iron and charred nutshell (walnut shell) as adsorber were studied. Both batch and continuous system were conducted. The batch system reveals that the adsorption process was of favorable type. Three models were used to describe the adsorption process. Freundlich model is matching well to the data with (R2=0.975). In the continuous system , two sets of experimental were considered, the first set (multilayer bed) conducted by fixing the characteristics of the scrap iron bed (bed height 0.2 cm, particle size 500 µm) and varying the walnut shell parameters, the optimum value of this set recorded as (bed height 2.5 cm, particle size250µm). While for the second set of experimental after fixing the optimum values of the walnut shell bed and varying the scrap iron bed characteristics, the optimum values were (height 1cm, particle size 500µm as filling). Also, the Mixed bed configuration were studied by conducting the optimum conditions for the previous experiment. it was found that at any certain time the mixed layer offers the maximum efficiency of removal rather than the multi-layer bed.

The Moyjil site, south-west Victoria, Australia: chronology

An unusual shell deposit at Moyjil (Point Ritchie), Warrnambool, in western Victoria, has previously been dated at 67±10 ka and has features suggesting a human origin. If human, the site would be one of Australia’s oldest, justifying a redetermination of age using amino acid racemisation (AAR) dating of Lunella undulata (syn. Turbo undulatus) opercula (the dominant shellfish present) and optically stimulated luminescence (OSL) of the host calcarenite. AAR dating of the shell bed and four Last Interglacial (LIG) beach deposits at Moyjil and Goose Lagoon, 30 km to the west, confirmed a LIG age. OSL analysis of the host sand revealed a complex mixing history, with a significant fraction (47%) of grains giving an early LIG age (120–125 ka) using a three-component mixing model. Shell deposition following the LIG sea-level maximum at 120–125 ka is consistent with stratigraphic evidence. A sand layer immediately below the shell deposit gave an age of ~240 ka (i.e. MIS 7) and appears to have been a source of older sand incorporated into the shell deposit. Younger ages (~60–80 ka) are due to bioturbation before calcrete finally sealed the deposit. Uranium/thorium methods were not applicable to L. undulata opercula or an otolith of the fish Argyrosomus hololepidotus because they failed to act as closed systems. A U–Th age of 103 ka for a calcrete sheet within the 240 ka sand indicates a later period of carbonate deposition. Calcium carbonate dripstone from a LIG wave-cut notch gave a U–Th age of 11–14 ka suggesting sediment cover created a cave-like environment at the notch at this time. The three dating techniques have collectively built a chronology spanning the periods before and after deposition of the shell bed, which occurred just after the LIG sea-level maximum (120–125 ka).

Age structure, carbonate production and shell loss rate in an Early Miocene reef of the giant oyster <i>Crassostrea gryphoides</i>

We present the first analysis of population structure and cohort distribution in a fossil oyster shell bed based on 1121 shells of the giant oyster Crassostrea gryphoides (von Schlotheim, 1813). Data derive from terrestrial laser scanning of a Lower Miocene shell bed covering 459 m2. Within two transects, individual shells were manually outlined on a digital surface model and cross-checked based on high-resolution orthophotos, resulting in accurate information on center line length and area of exposed shell surface. A growth model was calculated, revealing this species as the fastest growing and largest Crassostrea known so far. Non-normal distribution of size, area and age data hints at the presence of at least four distinct recruitment cohorts. The rapid decline of frequency amplitudes with age is interpreted to be a function of mortality and shell loss. The calculated shell half-lives range around a few years, indicating that oyster reefs were geologically short-lived structures, which could have been fully degraded on a decadal scale. Crassostrea gryphoides reefs were widespread and common along the Miocene circum-Tethyan coasts. Given its enormous growth performance of ∼ 150 g carbonate per year this species has been an important carbonate producer in estuarine settings. Yet, the rapid shell loss impeded the formation of stable structures comparable to coral reefs.