Tests Of Benthic Foraminifera Research Articles

Characterization of Depositional Environments in Isolated Carbonate Platforms Based on Benthic Foraminifera, Belize, Central America

Abstract Tests of benthic foraminifera were analyzed quantitatively in 43 surface sediment samples collected along E–W traverses across the modern isolated carbonate platforms of Glovers Reef (N=14), Lighthouse Reef (N=14), and Turneffe Islands (N=15), offshore Belize, Central America. Water depths of sample stations range from 0 m (beach) to 40 m (fore reef). Each sample was sieved in the size classes 2–1, 1–0.5, 0.5–0.25, and 0.25–0.125 mm (ϕ=1–0, 0–1, 1–2, and 2–3, respectively) and 300 foraminiferal identifications were attempted for each sieve fraction (i.e., 1,200 individuals per sample, totaling 46,003 specimens). Cluster analyses of samples characterized platform environments and distinguished four benthic foraminifera associations. Fore reef samples were characterized by abundant Amphistegina gibbosa and the occurrence of Asterigerina carinata, both rotaliids. In high-energy, marginal-reef areas the encrusting rotaliid Homotrema rubrum predominated. Platform-interior environments were marked by a...

Holocene shelf sea evolution offshore northeast England

Spatial and temporal changes of tidally mixed and stratified water conditions during the Holocene are poorly understood and constrained within the North Sea region. This study investigates the changing tidal conditions at a core site from offshore northeast England through the Holocene. A continuous Holocene sedimentary record in British Geological Survey vibrocore 54/-02/97 (54°53.05′N, 1°3.88′W; water depth 55 m) collected from a natural trough in the North Sea has been investigated. A multiproxy approach, including the interpretation of magnetic susceptibility values, bulk sedimentological characteristics, benthic foraminifera faunal and isotopic data is adopted. The chronology of this record is constrained by a total of ten accelerator mass spectrometry 14C dates obtained from molluscs and mixed benthic foraminiferal samples. The age derived from benthic foraminifera and molluscs obtained at the same core depth are statistically the same. These results suggest that benthic foraminifera tests do not undergo significant post-mortem reworking at this shallow site and that the foraminifera-based proxy records are reliable. Sedimentary characteristics indicate that tidally mixed waters between 11 150 and 5869 calendar (cal) yr BP affected the core site. The core site was bathed by tidally mixed water conditions indicated by the presence of Ammonia batavus Hofker with stratified conditions and associated frontal systems developing at or close to the core site between 5869 and 2032 cal yr BP, based on the presence of Stainforthia fusiformis (Williamson). Finally, species that are indicative of fully stratified water such as Bulimina marginata d’Orbigny, Hyalinea balthica (Schröter) and S. fusiformis record the transition from mixed-frontal to stratified conditions at 1158 cal yr BP. This site records a faunal change in the last 500 years that may be a consequence of fully stratified conditions and enhanced phytodetritus delivery to the sea floor. Alternatively, supported to some extent by heavy metal concentrations, the fauna may be responding to eutrophication of coastal waters through anthropogenic influences.

Microfabric analysis of Mn-carbonate laminae deposition and Mn-sulfide formation in the Gotland Deep, Baltic Sea

The manganese carbonate deposits of the anoxic Littorina sediments of the Gotland Deep have been commonly related to the periodic renewal of deep water by inflowing saline water from the North Sea. The use of scanning electron microscopy-based techniques allows identification of small-scale sedimentary and geochemical features associated with Mn-carbonate laminae, which has significant implications for models of Mn-carbonate formation. Varves occurring in the Littorina sequence contain up to four laminae that may be placed in a seasonal cycle, and kutnahorite laminae occur within varves only as a winter-early spring deposit. This kutnahorite laminae seasonality is in agreement with the seasonal distribution of major Baltic inflow events recorded in historical records, and a direct causal link between inflows and kutnahorite deposition is implied. Benthic foraminifera tests are found to be heavily encrusted in kutnahorite, implying that benthic recolonization during oxidation events occurs concurrently with kutnahorite formation. The relatively common occurrence of small (50 to 100 μm) hexagonal γ-Mn-sulfide pseudomorphs, associated with 13% of kutnahorite laminae studied, is reported in Gotland Deep sediments for the first time. Although Mn-sulfide crystals are not usually preserved in the sediment, the discovery of Mn-sulfide pseudomorphs suggests that initial formation of Mn-sulfide in the Gotland Deep may occur much more commonly during the process of kutnahorite formation than previous reports of Mn-sulfide occurrence have implied.

Late Holocene sediment in Nara Inlet, central Great Barrier Reef platform, Australia: sediment accumulation on the middle shelf of a tropical mixed clastic/carbonate system

Nara Inlet in the Whitsunday Islands is a natural depocenter for clastic and carbonate sediment on the middle shelf of the central Great Barrier Reef (GBR) platform. Three sediment cores were collected from the inlet to understand sediment accumulation in this tropical mixed clastic/carbonate system over time. Surface and subsurface sediment is comprised of two main components: terrigenous clay and shallow marine carbonate. Similar to other regions on the middle shelf of the GBR platform, the carbonate component, 25–80% by weight and dominated by Sr-poor foraminifera tests and mollusc shells, generally increases towards the surface. Radiocarbon ages derived from tests of benthic foraminifera indicate that the top 3 m of sediment accumulated within the last 3000 years, but that the rate has slowed toward present-day. The change in clastic and carbonate abundance therefore indicates a system where both clastic and carbonate accumulation has decreased over time, but where clastic accumulation has decreased faster. Reduced clastic accumulation may reflect progressive impedance of terrigenous input by a laterally growing fringing reef at the mouth of Nara Inlet. Likewise, reduced carbonate accumulation may reflect preferential reef growth and carbonate deposition outside of the inlet. Even though clastic accumulation has decreased significantly over time, the late Holocene mass, composition, and distribution of clastic material in Nara Inlet implies a terrigenous source previously unrecognized in models for the evolution of the GBR platform. Large amounts of terrigenous sediment probably were stored around topographic highs on the middle shelf during lowstand and early transgression. This clastic material was then removed over time, mixed with autochthonous carbonate, and deposited in embayments on the middle shelf. The accumulation of sediment on the middle shelf of a tropical mixed clastic/carbonate margin is not straightforward.

Origin of double and multiple tests in benthic foraminifera: observations in laboratory cultures

Many juvenile and adult double and multiple tests of benthic foraminifera were observed on specimens grown in laboratory cultures or collected in various natural environments. Our observations bring to light three possible causes for such abnormalities, each one referring to characteristic morphological features. Double tests may result: (1) from an anomaly in the development of a single juvenile, building two or three second chambers or two third chambers, each one possibly developing in an individual whorl; (2) from the early fusion of two juveniles, which both develop after their fusion; (3) from the attachment of a juvenile on a parental test after the schizogony followed by the young's development.

A conservative tracer for glacial ocean circulation from carbon isotope and palaeo-nutrient measurements in benthic foraminifera

THE ratio of cadmium to calcium (Cd/Ca) and the carbon isotope ratio (δ13C) in the calcite tests of benthic foraminifera both record nutrient distributions in the ocean1,2. Strict interpretation of both δ13C and Cd as nutrient tracers has led to conflicting views of glacial ocean circulation3–5. Here we show that, when one takes into account the fact that δ13C reflects air–sea exchange as well as nutrient distributions, these two proxies can provide complemen-tary information about the movement of deep water in the glacial ocean. We use the Cd concentration (assumed to be controlled primarily by biological cycling) to infer the age history of glacial deep water, and deduce the sources of deep water from the carbon isotope air–sea exchange signature, a conservative tracer that we construct using both Cd and δ13C measurements. Our analysis suggests that there were at least two sources of glacial deep water: a less dense component originating in the North Atlantic Ocean, and a more dense component which may have originated in the Pacific Ocean. As well as demonstrating the potential of this approach, our findings provide further support for a Pacific glacial deep water source, evidence for which has until now been both scarce and conflicting3,5–12.

The influence of microhabitats on the carbon isotopic composition of deep‐sea benthic foraminifera

We report δ13C and δ18O values for Rose Bengal‐stained benthic foraminifera from a set of box cores from continental margin environments in the Atlantic and Pacific oceans. These isotopic results are compared with foraminiferal distribution data and pore water δ13C profiles to evaluate the importance of environmental (microhabitat) effects on the carbon isotopic composition of benthic foraminifera. The δ13C values of infaunal taxa are consistently lower than those of epifaunal taxa, suggesting that microhabitat effects on test composition do exist. The δ13C differences between foraminiferal carbonate and bottom water dissolved inorganic carbon are not correlated with the δ18O differences between benthic foraminifera tests and equilibrium calcite, but they do correlate with variations in the chemistry of sediment pore waters. However, interspecific δ18O differences as well as δ13C differences between species with similar vertical distributions in the sediments indicate that taxon‐specific “vital” effects also influence test composition.

Quantitative Paleobathymetry Using Oxygen Isotopes and Shape Changes in Benthic Foraminifera: ABSTRACT

Accurate estimates of paleodepth are of critical importance to oil exploration in determining environment of deposition and geologic history. Models based on the test shape and the /sup 18/O//sup 16/O ratio in benthic foraminifera from the northwestern Gulf of Mexico indicate that a resolution of +/- 75 ft can be achieved in paleobathymetric reconstructions. The proportion of /sup 18/O and /sup 16/O incorporated into the tests of benthic foraminifera varies with bottom water temperature in a predictable manner. This depth/temperature relationship is the result of the temperature dependence of oxygen isotopic fractionation between sea water and calcium carbonate, and it allows the tests of benthic foraminifera to be used as indicators of paleotemperature. Since subbottom water temperatures on the outer shelf and slope decrease systematically with increasing water depth, these paleotemperatures can be used to reconstruct paleobathymetric trends. Paleobathymetric interpretations can also be independently inferred from Fourier shape analysis of benthic foraminiferal species. Combining the oxygen isotope and shape relationships relative to water depth increases the resolution of paleobathymetric reconstructions and provides an independent check on interpretations based on faunal assemblages and sedimentological data. These paleodepth models should allow extinct taxa to be used for paleobathymetric reconstructions as well.

Stable carbon isotopes in marine porewaters

We present centimeter-scale porewater δ 13C profiles from four sites in the Central Equatorial Pacific Ocean, which reveal steep porewater δ 13C gradients (up to −1.0‰ cm −1) at the deep-sea sediment—water interface. We describe the processes controlling porewater carbon isotopic composition and incorporate them into a model of the deep-sea benthic carbon cycle. Our measured δ 13C profiles are then used to estimate carbon rain rates at two of these sites, MANOP sites C and S. The δ 13C data yield organic carbon rain rates of 10 μmol C cm −2 yr −1 at site C and 3 μmol C cm −2 yr −1 at site S. These carbon rain rates agree with recent carbon rain estimates based on sediment trap data and porewater nitrate profiles, but are a factor of 2 lower than estimates based on millimeter-scale porewater oxygen profiles. Our model results indicate that changes of the same magnitude in organic carbon rain rate and bottom water composition (O 2, ΣCO 2, and δ 13C) have comparable effects on the carbon isotopic composition of surficial porewaters. This suggests the need to consider the effects of benthic carbon cycling on the stable carbon isotope record preserved in deep-sea benthic foraminifera tests.

Stable isotope fractionation in benthic foraminifera from the Peruvian continental margin

Stable isotope ratios of the carbonate tests of benthic foraminifera are now widely used to reconstruct past changes in ocean circulation and chemistry. Such studies depend on the accuracy with which changing environmental conditions are recorded and preserved in the tests of foraminifera found in deep-sea deposits. Assessment of this aspect of the method is best accomplished through rigorous study of modern core-tops collected from a wide range of depths and bottom water temperatures. Stable isotope analyses of eighteen taxa of benthic foraminifera from surface sediments of the Peru continental margin indicate that most foraminifera are slightly depleted in 18O relative to the benthic foram Uvigerina peregrina. For at least five taxa we observed subparallel δ 18 O-temperature relationships with slopes nearly equal to that predicted by Shackleton (1974) for Uvigerina. For seven taxa, isotopic analysis of six size fractions (in the range 210–>590 mm) reveals that, with the exception of U. peregrina, variation between different size classes for each species is small, <0.35‰) for δ 18 O and <0.5‰) for δ 13 C . A slight inverse correlation between foraminiferal 18O and 13C content implies a complex interplay of biological and environmental factors which control isotopic composition.