Dissolution Of Calcareous Tests Research Articles

Distribution of modern benthic foraminiferal assemblages across the Northeast Greenland continental shelf

Analysis of benthic foraminifera in surface samples from 23 sites on the Northeast Greenland continental shelf reveal key assemblage differences between sites. Cluster analysis creates two clear geographical faunal assemblage zones: the 1) inner shelf, and 2) mid and outer shelf sites. These assemblages differ significantly, with the inner shelf sites being characterised by a high percentage and concentration of calcareous species, whilst the mid and outer shelf sites are dominated by agglutinated taxa. At almost all sites, the calcareous assemblages are dominated by Cassidulina neoteretis and Cassidulina reniforme, suggesting that they thrive across the shelf. Stetsonia horvathi, Oridorsalis tener, as well as Glomulina oculus and other miliolid species are found to be key calcareous species at many sites in the inner shelf zone, but they are rare-to-absent on the mid and outer shelf. Canonical correspondence analysis shows that September sea-ice cover and bottom water oxygen content are positively correlated with benthic foraminiferal assemblages at inner shelf sites, whereas organic carbon content is correlated with those in the mid and outer shelf. The formation of seasonal sea-ice and the Northeast Water polynya rejects brine into surrounding waters and transports CO2 to the seafloor, creating a highly corrosive environment for calcium carbonate. These environments are also highly productive, as indicated by the high organic carbon content and low bottom water oxygen content. The oxidation of this organic material creates CO2. We propose that these processes are key drivers in the dissolution of calcareous tests. In contrast, extensive sea-ice, high bottom water oxygen content and low primary productivity in the glacier-proximal region facilitates carbonate preservation.

Comparison of alive and dead benthic foraminiferal fauna off the Changjiang Estuary: Understanding water-mass properties and taphonomic processes

Benthic foraminifera (BF) are utilized in palaeo-environmental reconstruction based on our understanding of how living individuals respond to environmental variations. However, there is still a lack of empirical insight into how non-environmental factors, such as taphonomic processes, influence the preservations of fossil BF in strata. In this study, we compare the spatial distribution and composition of alive and dead BF fauna in surface sediments to elucidate how well fossil foraminiferal fauna mirror quasi-contemporary alive BF groups indicative of different water masses off a mega-river (Changjiang) estuary, which is characterized by intense and complex river-sea interactions. On-site measurements of bottom water salinity, temperature, and dissolved oxygen were conducted in the summer to determine water mass properties. A same-site comparison of alive (Rose Bengal stained) and dead foraminiferal fauna in surface sediment samples over 73 stations was then carried out. Q-mode Hierarchical clustering analysis was used to differentiate foraminiferal assemblages based on the relative abundance of common species. Three distinct regions with different water-mass properties were identified. The distribution pattern of dead foraminiferal fauna is mainly inherited from alive fauna, while the density and diversity of the dead fauna were found to be higher than those of the alive one. Both alive and dead fauna were clustered into four assemblages. A few common alive species (small-agglutinated and thin-calcareous) were rarely found in dead fauna, and a few common dead species (preferring low temperature and indicating allochthonous sources) were rarely present in alive fauna. The alive foraminiferal abundance and diversity were mainly determined by food resources and environmental properties of salinity and temperature. Alive foraminiferal assemblages were separated by different water masses determined by river-sea interactions off the Changjiang Estuary. The “time-averaging” effect was found to be responsible for the higher density and diversity of the dead fauna. Disintegration of agglutinated tests, dissolution of calcareous tests and selective transportation were observed to contribute to the different species compositions between the alive and dead fauna. Nevertheless, indicative species-environment relations in alive and taphocoenose fauna were found to be almost homologous among most common species. This suggests that distinct benthic foraminiferal assemblages can be used to effectively differentiate between different water masses in the study coastal seas.

Distribution of Modern Salt-marsh Foraminifera from the Eastern Mississippi Sound, U.s.a.

AbstractThis study documented surface distributions of live and dead foraminiferal assemblages in the low-gradient tidal marshes of the barrier island and estuarine complex of the eastern Mississippi Sound (Grand Bay, Pascagoula River, Fowl River, Dauphin Island). A total of 71,833 specimens representing 38 species were identified from a gradient of different elevation zones across the study area. We identified five live assemblages and nine biofacies for the dead assemblages from estuarine, low marsh, middle marsh, high marsh, and upland transition environments. Although dissolution of calcareous tests was observed in the dead assemblages, characteristic species and abundance patterns dependent on elevation in the intertidal zone were similar between living assemblages and dead biofacies. The assemblages from the eastern Mississippi Sound estuaries were dominated by Ammonia tepida, Cribroelphidium poeyanum, C. excavatum, and Paratrochammina simplissima. The low marshes were dominated by Ammotium salsum, Ammobaculites exiguus, and Miliammina fusca. The dominant species in the middle marshes was Arenoparrella mexicana. The most abundant species in the high marshes was Entzia macrescens. The upland–marsh transition zones were dominated by Trochamminita irregularis and Pseudothurammina limnetis. Canonical correspondence analysis was applied to assess the relationship between a priori defined biofacies and measured environmental data (elevation, grain size, organic matter, and salinity) to test the hypothesis that distribution of foraminiferal assemblages is driven by elevation and hence flooding frequency. Salinity was the second most important explanatory variable of dead assemblages. Riverine freshwater from the Pascagoula River markedly influenced the live and dead assemblages in the Pascagoula River marsh, which was represented by low diversity and densities and dominance by Ammoastuta inepta. The relationship between the measured environmental variables and assemblage distributions can be used in future Mississippi Sound paleo-environmental studies.

Foraminiferal biofacies across mangrove-mudflat environments at Cocoa Creek, north Queensland, Australia

Contemporary and relict (subsurface) intertidal foraminiferal dead assemblages were collected from a site near to Cocoa Creek, north Queensland, Australia, and analysed for the purpose of understanding fossil assemblage development and preservation. A marked dichotomy was identified between dead foraminiferal assemblages within mangrove sediments and those within adjacent mudflat sediments. Mangrove assemblages were almost exclusively represented by agglutinated species (mainly Arenoparrella mexicana, Haplophragmoides wilberti, Milliammina fusca, M. obliqua, Trochammina inflata) while calcareous species dominated the mudflat (mainly Ammonia aoteana, Pararotalia venusta, Parrellina hispidula). Surface and subsurface test abundances were also an order of magnitude greater within mudflat sediments (1000–3000 tests per cm 3) than in the sediments beneath the mangroves (100–300 tests per cm 3). Statistically significant differences in the abundance of calcareous and agglutinated tests between wet and dry seasons within the mangrove suggest that tests are systematically lost from this part of the system on an annual basis. A model of assemblage formation under shoreface progradation is presented which makes a direct link between surface assemblages, post-depositional processes and the composition and environmental resolution of the fossil assemblages entering the stratigraphic record. Assemblages within the mangrove undergo rapid taphonomic loss and are then overprinted during burial by the deep infaunal communities associated with the prograding higher-elevation environments. As such, much of the eventual mangrove sequence is represented entirely by assemblages produced relatively recently within an upper mangrove environment, and therefore cannot be differentiated according to elevation at the time of deposition. In addition, organic enrichment of upper mudflat deposits by mangrove roots, which occurs during progradation, results in the dissolution of calcareous tests from an ~ 40 cm thick stratigraphic interval. It is argued that in order to reliably recognise the changes to foraminiferal assemblages which occur during burial, assemblages must be compared with respect to stratigraphy, rather than reflexively downcore.

Living (stained) and dead foraminifera from the newly ice-free Larsen Ice Shelf, Weddell Sea, Antarctica: ecology and taphonomy

Within the past 7 years, the northern Larsen Ice Shelf has broken up so it is now possible to sample the sea floor that formerly lay beneath it. Box cores have yielded surface sediment samples (0–1 cm) that give information on living and dead foraminiferal assemblages. The living assemblages are of moderate diversity and four have >50% calcareous tests while five have >50% agglutinated tests. This is an area of high primary production and the standing crops of the benthic foraminiferal assemblages are high. All the dead assemblages are much enriched in agglutinated tests, often >90%. They give a time-averaged record of the past 7 ice-free years and several decades of ice cover. The loss of permanent ice cover (there is still seasonal ice cover) may have caused some response from the fauna, but it is likely that it was mainly changes in relative/absolute abundance of the existing fauna. The differences between the live and dead assemblages in the surface 1 cm are attributed mainly to taphonomic effects: dissolution of calcareous tests and loss of fragile agglutinated tests. Subsurface samples down to 5 cm show that dissolution of calcareous tests is widespread and there may be some loss of fragile agglutinated forms such as Reophax subdentaliniformis. For these reasons, in this area, it may be best to make palaeoecological interpretations on the agglutinated component of the fossil assemblages.

Modern Benthic Foraminifera off Novaya Zemlya Tidewater Glaciers, Russian Arctic

Living and dead foraminifera have been investigated along three transects away from tidewater glaciers of northern Novaya Zemlya. There are three glacier-proximal dominant foraminiferal taxa: Allogromiina (unidentified species), Elphidium excavatum f. clavata, and Cassidulina reniforme. In this environment the sediment consists of ice-front diamicton (0.17 km from the terminus) and massive glaciomarine clay (0.2 to 7 km). Allogromiids dominate the low-diverse fauna closest to the glacier outlet, followed by E. e. clavata. Diagenetic decay of the allogromiid tests will result in the preservation of an E. e. clavata dominated fossil assemblage. The occurrence of C. reniforme increases with nutrient enrichment reflected in phytoplankton abundance and sediment Corg content. The C. reniforme/E. e. clavatum-ratio exceeds 50% in locations where glacial impact is moderate as marked by the absence of glacial-cooled surface water and low concentrations of suspended sediment. The presence of Quinqueloculina stalkeri is specific to the glaciomarine clay.Islandiella norcrossi is abundant in distal stations (7 to 22 km from the glacier termini) in distal glaciomarine sediments enriched with ice rafted debris and in marine mud. A Nonionellina labradorica peak is also characteristic of glacial-distal habitats. The frequency of Cibicides lobatulus and other attached forms correlates with the abundance of clasts and shows no relation to water depth as the hard substrate is primarily ice rafted. Larger foraminifera (Saccorhiza ramosa and others) occur only in the marine mud. Numbers of living foraminifera and macrofauna biomass vary likewise, and remain unexplained by the measured physical parameters. Dissolution of calcareous tests, ubiquitous on the adjacent shelf, occurs only in distal, oxidized sediments.

High benthic fertility and taphonomy of foraminifera: a case study of the Skagerrak, North Sea

The Skagerrak basin is a deep water extension of the North Sea. It is of particular interest as an analogue for past epicontintal shelf basins because it presents environmental contrasts with the adjacent shelf seas. In this study the distribution patterns of benthic foraminifera have been used to infer taphonomic and oceanographic processes. Only by separating living from dead assemblages is it possible to interpret taphonomic changes. The transport of foraminiferal tests to the Danish slope is inferred from the presence there of dead exotic tests whose provenance is considered to be from the south. The abundance of detrital organic matter on the Danish slope is likewise inferred to be sourced from the same direction. Thus, the Danish slope is interpreted to be a depositional sink. Apart from transport, another taphonomic process is the dissolution of calcareous tests. This is clearly demonstrated both by the fragility of some tests as viewed under the microscope and by the disparity between the composition of the living and dead factor associations. In the deep basin in particular, the two predominantly calcareous living associations are replaced by a single predominantly agglutinated dead association due to carbonate dissolution. The Danish slope of the Skagerrak Basin is demonstrated to be an area of high benthic fertility. This is based on the high density of living (stained) benthic foraminifera (comparable with that of the Mississippi delta), particularly the abundance of Stainforthia fusiformis, an opportunistic species, and tubular agglutinated forms. The fertility is linked with the high abundance of particulate organic matter here. Previous regional studies have focused on total (living plus dead) distributions of > 100 or > 125 μm sized foraminifera. These factor assemblages are distinct from the >63 μm living and dead associations described here.

Syndepositional dissolution of calcareous foraminifera in modern shallow-water sediments

Dissolution of calcareous tests in waters undersaturated with respect to calcium carbonate causes postmortem changes to assemblages of modern benthic foraminifera. Comparison of the percentage of agglutinated tests in the living and dead assemblages from the same sample enables the amount of dissolution to be quantified. Re-examination of data from the continental shelf off New York and the estuarine Chezzetcook Inlet, Canada, reveals major dissolution losses which lead to dead assemblages rich in agglutinated tests. It is probable that partial dissolution of the calcareous component of modern assemblages is more widespread than has been recognised.