Benthic foraminiferal Mn / Ca ratios reflect microhabitat preferences

The Mn / Ca of calcium carbonate tests of living (rose-Bengal-stained) benthic foraminifera (Elphidium batialis, Uvigerina spp., Bolivina spissa, Nonionellina labradorica and Chilostomellina fimbriata) were determined in relation to pore water manganese (Mn) concentrations for the first time along a bottom water oxygen gradient across the continental slope along the NE Japan margin (western Pacific). The local bottom water oxygen (BWO) gradient differs from previous field study sites focusing on foraminiferal Mn / Ca and redox chemistry, therefore allowing further resolution of previously observed trends. The Mn / Ca ratios were analysed using laser ablation inductively coupled plasma-mass spectrometer (ICP-MS), allowing single-chamber determination of Mn / Ca. The incorporation of Mn into the carbonate tests reflects environmental conditions and is not influenced by ontogeny. The inter-species variability in Mn / Ca reflected foraminiferal in-sediment habitat preferences and associated pore water chemistry but also showed large interspecific differences in Mn partitioning. At each station, Mn / Ca ratios were always lower in the shallow infaunal E. batialis, occupying relatively oxygenated sediments, compared to intermediate infaunal species, Uvigerina spp. and B. spissa, which were typically found at greater depth, under more reducing conditions. The highest Mn / Ca was always recorded by the deep infaunal species N. labradorica and C. fimbriata. Our results suggest that although partitioning differs, Mn / Ca ratios in the intermediate infaunal taxa are promising tools for palaeoceanographic reconstructions as their microhabitat exposes them to higher variability in pore water Mn, thereby making them relatively sensitive recorders of redox conditions and/or bottom water oxygenation.Peer reviewe

Changes in ultrastructural features of the foraminifera Ammonia spp. in response to anoxic conditions: Field and laboratory observations

The ultrastructure of the living foraminiferan, Ammonia sp. (phylotype unknown), collected from surficial and deeper, subsurface (anoxic) sediments from the Dutch Wadden Sea, was examined to provide information on the physiology of the foraminiferal cell and its adaptive strategies to low‑oxygen conditions. The observed changes in cell ultrastructure under anoxia were further compared with the cell ultrastructure of Ammonia sp. (phylotype T6), from oxic and anoxic incubation experiments. The ultrastructural evidence indicates that under low‑oxygen conditions Ammonia spp. may accumulate lipid droplets. In addition, the size of the lipid droplets may increase with the duration of anoxic conditions, becoming over 5 μm in size, while the remaining cytosol of the foraminiferan become less electron dense. In some specimens, lipid droplets were also found in the space between the plasma membrane and the organic lining. We expect that the apparent increase in the number and size of the lipid droplets is indicative of a stress response of the foraminifera to the adverse anoxic conditions. Other ultrastructural changes in response to anoxia include the presence of intact bacteria and electron dense opaque bodies within the foraminiferal cytosol, and a possible thickening of the organic lining. The role of the bacteria remains enigmatic but they may be linked to foraminiferal dormancy in anoxia

The dynamic balance between food abundance and habitat instability: benthic foraminifera of Portuguese margin canyons. Geologica Ultraiectina (286)

Submarine canyons are dynamic sedimentary environments influenced by sediment transport, erosion and deposition. Gravity flows can scour and erode the canyon floor, thus redistributing sediment to distal locations. In addition, submarine canyons can act as sedimentary traps where sediment transported laterally across the continental shelf and slope is advected into. Hence, elevated sedimentation rates and high organic carbon content are often found in submarine canyons in comparison with the adjacent open slope. Such dynamic processes must affect the ecosystems inhabiting these exceptional environments. To investigate and quantify this relationship, living benthic foraminifera were sampled from Nazaré and Lisbon-Setbal submarine canyons located on the Portuguese continental margin. The foraminiferal abundances and species distribution were correlated with a host of geochemical (e.g. organic carbon, phytopigment content, redox chemistry) and physical/sedimentological parameters (e.g. current speed, sedimentation rate, frequency of gravity flows). Eventually this information was used to reconstruct temporal variations in sedimentation processes and associated changes in foraminiferal community structure in two piston cores derived from the lower canyon. The results of these studies highlight the importance of habitat instability and food abundance in structuring the foraminiferal communities in canyons. Food, quantified in terms of the sedimentary phytopigment content, was readily available in the studied canyons, declining in abundance with increasing water depth. Food abundance was also reflected in the pore water chemistry (nitrate penetration depth used as a redox indicator), higher pigment content coinciding with shallower nitrate penetration depth in sediment. The standing stocks of foraminifera and community structure changed with these parameters, and a positive correlation was observed between foraminiferal numbers and the sedimentary pigment content and a negative with the nitrate penetration depth. Stations recording the highest pigment loads (> 15g/cm3) and the shallowest redox zones (nitrate penetration depth <1cm) were inhabited by infaunal taxa e.g. Chilostomella oolina, Melonis barleeanum, Bigenerina cylindrica and Globobulimina spp. At the deepest sites, where the pigment concentrations were very low (? 2 g/ cm3) and the redox zonation deep (nitrate penetration depth exceeding 5 cm depth in sediment) communities were dominated by agglutinated taxa and only few calcareous species were present, including Nuttallides umbonifera. Despite the high pigment concentrations however, foraminiferal numbers were low in the upper and middle Nazaré Canyon axis. At these sites the development of stable ecosystems appeared to be hindered by sedimentary disturbance. Distinct agglutinated foraminifera, Technitella spp., were found to inhabit the unstable environment. This species can be regarded as a highly opportunistic recoloniser of newly exposed habitats, and/or may be able to tolerate high sedimentation rates and frequent sedimentary disturbance (i.e. gravity flows). In the paleostudy, a postglacial increase in the sediment transport was clearly reflected in the foraminiferal record as an increase in the relative abundance of shallow water taxa. Further, a shift in faunal composition was observed as a response to changing sedimentary regime; higher sediment and organic carbon supply from the canyon leading to a development of an infaunal Pullenia spp. assemblage and decreasing activity to the hemipelagic Nutallides umbonifera assemblage

或る労務屋の回想

The Mn/Ca of carbonate tests of living deep-sea foraminifera (Hoeglundina elegans, Bulimina aculeata, Uvigerina peregrina and Melonis barleeanus) were determined together with pore water manganese along a bottom water oxygen gradient across the lower boundary of the Arabian Sea oxygen minimum zone. Although Mn has long been considered an indicator for contamination, new cleaning protocols and high-resolution laser ablation ICP-MS now allow the reliable analyses of test-associated Mn. Within locations, Mn incorporation between species varies as a function of their in-sediment depth preferences and associated pore water chemistry. Under well-oxygenated bottom water conditions, shallow infaunal species incorporate little Mn in their test, whereas the species collected from deeper habitats show elevated Mn concentrations. With decreasing oxygen contents pore water Mn concentrations and benthic foraminiferal in-sediment distribution change. Whereas Mn/Ca in shallow infaunal species responds moderately to bottom water oxygenation, Mn/Ca of the infaunal species M. barleeanus correlates well to oxygenation. Although high productivity results in a shallower redox cline within the sediment, pore water Mn is retained as long as the bottom water remains oxygenated. Under reduced bottom water oxygen conditions, Mn escapes to the overlying water column and test-associated Mn/Ca decreases also in the infaunal species. By combining pore water chemistry of Mn, calcitic Mn/Ca and foraminiferal ecology, a new conceptual model is presented (TROXCHEM3) that provides a framework for deconvolving past organic matter input and bottom water oxygenation

Vertical migration, nitrate uptake and denitrification: survival mechanisms of foraminifers (Globobulimina turgida) in low oxygen conditions

15NO3− isotope labelling experiments were performed to investigate foraminiferal nitrate uptake strategies and the role of pseudopodial networks in nitrate uptake. Globobulimina turgida were placed below the nitrate penetration depth in homogenized sediment cores incubated in artificial seawater containing 15NO3−. A nylon net prevented the vertical migration of foraminifera to strata containing nitrate and oxygen, but allowed potential access to such strata by extension of pseudopods. No 15NO3− was found in G. turgida in these cores, suggesting that foraminifera cannot extend their pseudopods for nitrate uptake through several millimetres of sediment, but must physically migrate upwards closer to nitrate-containing strata. However, foraminiferal migration patterns in control cores with no nylon net were erratic, suggesting that individuals move in random orientations until they find favourable conditions (i.e. free nitrate or oxygen). A second experiment showed that foraminifera actively collect nitrate both in the presence and in the absence of oxygen, although uptake was initiated faster if oxygen was absent from the environment. However, no systematic influence of the size of the intracellular nitrate pool on nitrate uptake was observed, as specimens containing a large range of intracellular nitrate (636–19 992 pmol per cell) were measured to take up 15NO3− at comparable rates

Live (Rose Bengal stained) foraminiferal faunas from the northern Arabian Sea: Faunal succession within and below the OMZ

Live (Rose Bengal stained) benthic foraminifera from the Murray Ridge, within and below the northern Arabian Sea oxygen minimum zone (OMZ), were studied in order to determine the relationship between faunal composition, bottom water oxygenation (BWO), pore water chemistry and organic matter (organic carbon and phytopigment) distribution. A series of multicores were recovered from a ten-station oxygen (BWO: 2–78 μM) and bathymetric (885–3010 m depth) transect during the winter monsoon in January 2009. Foraminifera were investigated from three different size fractions (63–125 μm, 125–150 μm and >150 μm). The larger foraminifera (>125 μm) were strongly dominated by agglutinated species (e.g. Reophax spp.). In contrast, in the 63–125 μm fraction, calcareous taxa were more abundant, especially in the core of the OMZ. On the basis of a principal components analysis, three foraminiferal groups were identified and correlated to the environmental parameters by canonical correspondence analysis. The faunas from the shallowest stations, in the core of the OMZ (BWO: 2 μM), were composed of "low oxygen" species, typical of the Arabian Sea OMZ (e.g. Rotaliatinopsis semiinvoluta, Praeglobobulimina sp., Bulimina exilis, Uvigerina peregrina type parva). These taxa are adapted to the very low BWO conditions and to high phytodetritus supplies. The transitional group, typical for the lower part of the OMZ (BWO: 5–16 μM), is composed of species that are tolerant as well to low-oxygen concentrations, but may be less critical with respect to organic supplies (e.g. Globocassidulina subglobosa, Ehrenbergina trigona). Below the OMZ (BWO: 26–78 μM), where food availability is more limited and becomes increasingly restricted to surficial sediments, cosmopolitan calcareous taxa were present, such as Bulimina aculeata, Melonis barleeanus, Uvigerina peregrina and Epistominella exigua. Miliolids were uniquely observed in this last zone, reflecting the higher BWO and/or lower organic input. At these deeper sites, the faunas exhibit a clear succession of superficial, intermediate and deep infaunal microhabitats, which can be linked to the deeper oxygen and nitrate penetration into the sediment

Cold-water coral habitats of Rockall and Porcupine Bank, NE Atlantic Ocean: Sedimentary facies and benthic foraminiferal assemblages

The extent of the cold-water coral mounds in the modern ocean basins has been recently revealed by new state-of-the-art equipment. However, not much is known about their geological extent or development through time. In the facies model presented here seven different types of seabed substrate are distinguished, which may be used for reconstruction of fossil coral habitats. The studied substrates include: off-mound settings, (foram) sands, hardgrounds, dead coral debris, and substrates characterized by a variable density of living coral framework. Whereas sediment characteristics only provide a basis for distinguishing on- and off-mound habitats and the loci of most prolific coral growth, benthic foraminiferal assemblages are the key to identifying different mound substrates in more detail. Specific foraminiferal assemblages are distinguished that are characteristic of these specific environments. Assemblages from off-mound settings are dominated by (attached) epifaunal species such as Cibicides refulgens and Cibicides variabilis. The attached epibenthic species Discanomalina coronata is also common in off-mound sediments, but it is most abundant where hardgrounds have formed. In contrast, the settings with coral debris or living corals attract shallow infaunal species that are associated with more fine-grained soft sediments. The typical 'living coral assemblage' is composed of Cassidulina obtusa, Bulimina marginata, and Cassidulina laevigata. The abundance of these species shows an almost linear increase with the density of the living coral cover. The benthic foraminifera encountered from off-mound to top-mound settings appear to represent a gradient of decreasing current intensity and availability of suspended food particles, and increasing availability of organic matter associated with fine-grained sediment trapped in between coral framework