Bruce H. Corliss

Microhabitats of benthic foraminifera within deep-sea sediments

Benthic foraminifera are protozoans found throughout the deep-sea environment, secreting a test of calcium carbonate or constructing a test of cemented sediment particles (agglutinated or arenaceous foraminifera). In typical deep-sea sediments, the organic cement of agglutinated taxa degrades upon burial in the sediment and, consequently, few specimens survive in the fossil record. In contrast, calcareous species are well preserved in most oceanic sediments, except at abyssal depths where most carbonate sediment is dissolved because of high levels of carbonate under-saturation of the bottom waters. Although benthic foraminifera have been widely used in studies of Cenozoic palaeoceanography, little is known about the ecology of deep-sea species. I present here an analysis of living (stained) benthic foraminifera within the upper 15 cm of deep-sea sediments, which reveals species-specific microhabitat preferences, with distinct morphological features found with epifaunal and infaunal species. The existence of infaunal habitats suggests that the distribution of certain foraminifera is not directly controlled by overlying bottom-water conditions, but by physicochemical conditions within the sediments. The microhabitat preferences may also explain interspecific carbon isotope differences, as existing data show that infaunal foraminifera generally have lower δ13C isotope values than epifaunal species.

Morphotype patterns of Norwegian Sea deep-sea benthic foraminifera and ecological implications

Deep-sea benthic foraminifera from Norwegian Sea surface sediments are classified into morphotypes on the basis of test shape and nature of test coiling and show distinct patterns with water depth. The morphotype data are used to determine microhabitat patterns of the foraminifera, which are suggested to be related to the organic-carbon content of the surficial deep-sea sediments.

Morphology and microhabitat preferences of benthic foraminifera from the northwest Atlantic Ocean.

The distribution of Rose Bengal stained calcareous benthic foraminifera was determined in six ☐ cores raised from water depths between 200 and 3000 m on the Nova Scotian continental margin and Gulf of Maine. The taxa can be separated into four microhabitats within the surficial sediments. Epifaunal taxa are generally found in the top cm, intermediate infaunal taxa are found from about 1 to 4 cm and deep infaunal taxa are found at > 4 cm sediment depth in at least one ☐ core. A fourth group, shallow infaunal taxa, is found in the top 2 cm and is inferred to be infaunal based on wall porosity characteristics and test shapes similar to infaunal taxa. The epifaunal, shallow infaunal and intermediate infaunal taxa maintain their positions within the sediments from core to core, whereas the deep infaunal taxa are found at progressively shallower sediment depths in cores within increasing organic carbon contents from shallower water depths. Each microhabitat category has distinct morphological characteristics. Epifaunal taxa have plano-covex or biconvex cross sections, trochospiral coiling and large pores absent or found on only one side. Shallow infaunal taxa have uniserial, triserial, or planispiral coiling, with surface ornamentation present on a number of taxa. The intermediate infaunal taxa have rounded peripheries, pores over the entire test and planispiral coiling, with the exception ofCibicidoides bradyi which has trochospiral coiling. The deep infaunal taxa have, in general, planispiral or triserial coiling with cylindrical or ovate shaped tests.

Distribution of rose bengal stained deep-sea benthic foraminifera from the Nova Scotian continental margin and Gulf of Maine

Abstract Analysis of Rose Bengal stained benthic foraminifera in six boxcores taken from the Nova Scotian margin and Gulf of Maine reveals that foraminifera are vertically stratified within surficial sediments raised from 200 to 3000 m water depth. The consistent presence of infaunal taxa within the sediments demonstrates that these tolerant of a range of low-oxygen conditions as determined by pore-water manganese profiles. The habitat depth of foraminiferal populations, defined as the depth within which 95% of the fauna is found in a subcore, varies between the stations. A shallow habitat depth of 3 cm exists in shallow water (a 202 m core) in the Gulf of Maine. The habitat depth gradually increases on the continental slope with increasing water depth, reaching 11–13 cm in cores at 2225 and 3000 m, and then shoals to 4 cm in a core taken from 4800 m water depth. We suggest that the habitat pattern is related to the flux of organic carbon to the seafloor. At shallow depths, relatively high organic carbon flux results in a shallow oxic layer. The inferred oxic layer gradually increases on the continental slope and rise with increasing water depth, due to decreased organic carbon flux to the sediment-water interface. Carbon fluxes in the deep ocean are so low that pore waters are oxic and the organic carbon content is low, creating a food-limiting environment best suited to epifaunal taxa and reflected in a shallow habitat depth.

Carbon and oxygen isotopic disequilibria of recent deep-sea benthic foraminifera

Abstract The carbon and oxygen isotopic compositions of 149 samples of benthic foraminifera from deep-sea core tops indicate that none of the nine species studied secrete calcium carbonate in isotopic equilibrium with ambient bottom water. Uvigerina, Pyrgo murrhina, and Oridorsalis tener are the closest to 18O equilibrium (with average deviations about −0.4‰), while Planulina wuellerstorfi and P. murrhina are the closest to 13C equilibrium (with average deviations about −1‰). P. wuellerstorfi shows the most systematic relationship between δ 13C and bottom water apparent oxygen utilization. The intraspecific variabilities in δ 18O and δ 13C suggest that estimates of bottom water paleotemperatures can be made to a precision of ± 0.7°C, while estimates of past apparent oxygen utilization (AOU) can be made to ± 35 μmol/kg. Based on intraspecific comparisons of the Recent samples with fossils, no temporal changes in the degree of either 18O or 13C disequilibrium have been detected for Planulina wuellerstorfi, Uvigerina, Oridorsalis tener and Globocassidulina subglobosa.

Vertical distributions and stable isotopic compositions of live (stained) benthic foraminifera from the North Carolina and California continental margins

The vertical distributions of live (Rose Bengal stained) benthic foraminifera were determined in Soutar box cores from six sites on the North Carolina continental margin (337–1477 m) and three sites on the California continental margin (786–3705 m). Stained specimens of the most abundant taxa were analyzed for their carbon and oxygen isotopic compositions. Bottom water δ13C values and pore water δ13C profiles were determined at seven of the sites to aid in interpretation of the live benthic foraminiferal δ13C data. The abundance profiles of most benthic foraminiferal species show consistent patterns within the sediments at each site. These patterns enable us to characterize taxa as epifaunal and shallow, intermediate, and deep infaunal. The vertical range of the living assemblage is small (2–4 cm) in several of the cores, presumably as a consequence of the relatively high organic carbon fluxes and correspondingly small oxygen penetration depths in these continental margin environments. At each site, species with deeper within-sediment microhabitats have lower average δ13C values than do shallow-dwelling species. The δ13C offset from bottom water for each species is larger at high bottom water oxygen sites (North Carolina) than at low bottom water oxygen sites (California). Both of these observations are consistent with a pore water influence on benthic foraminiferal δ13C. The Atlantic-Pacific differences rule out a constant species-specific fractionation as the explanation for the δ13C values of these foraminifera. Despite substantial downcore changes in pore water δ13C, foraminiferal δ13C values for most species change very little over the entire depth range of stained individuals within each core. This lack of vertical δ13C variation may imply that calcification takes place in a relatively small sub-zone of the microhabitat range suggested by the distribution data.

Deposit feeding in selected deep-sea and shallow-water benthic foraminifera

Ultrastructural evidence for deposit feeding in two deep-sea foraminifera, Globobulimina pacifica and Uvigerina peregrina, is presented and compared with results on Ammonia beccarii, a common nearshore dweller. In all three taxa, food vacuoles are common in the last chamber and contain numerous aggregates of sediment and organic detritus. Within aggregates, bacteria are often found surrounded by a sheath of sediment particles generally bound by bacterial exopolymers. In actively-feeding individuals of A. beccarii, food vacuoles along the distal edge of the cytoplast contain live bacteria associated with sediment aggregates as well. Bacteria do not occur in the cells interior, although hollow sheaths of sediment and detritus do persist. This indicates that the digestion of bacteria may occur very near the distal margin of the cytoplast in this species. Likewise, sediment aggregates both with and without bacteria occur in food vacuoles of the deep-sea species examined. All three species ingest relatively large volumes of organic detritus associated with sediments, although the role of this material in the diet of foraminifera is uncertain. These results suggest that the deep-sea and shallow-water species examined feed on bacteria by deposit feeding and ingest bacterial cells, in addition to relatively large volumes of associated sediment and organic detritus.

Comparisons of the ecology and stable isotopic compositions of living (stained) benthic foraminifera from the Sulu and South China Seas

Significant differences are observed between living (Rose Bengal stained) deep-sea benthic foraminifera found in 14 box cores (510–4515 m) from the thermospheric (> 10°C) environments of the Sulu Sea and the psychrospheric ( 2‰ range and are similar to those presented by previous workers, but have no consistent relationship with microhabitat preferences. Vertical distribution patterns and carbon isotope compositions of species, however, reflect microhabitat preferences and are consistent with previous observations from other regions. Epifaunal species (0–1 cm interval) such as Cibicidoides pachyderma, Cibicidoides wuellerstorfi, Hoeglundina elegans and Anomalinoides colligera, have higher δ13C values than taxa which have the ability to live deeper within the sediments. Infaunal taxa that live in the upper 2–3 cm, including Uvigerina peregrina, Uvigerina proboscidea, and Bulimina mexicana, have lower δ13C values than epifaunal species, and the deep infaunal species, Chilostomella oolina, has the lowest δ13C. Cibicidoides bradyi and Oridorsalis umbonatus are found between 0 and ∼ 4 cm and have lower carbon isotope values (by > 1.4‰ in some cores) than epifaunal Cibicidoides species. Exceptions to this pattern include the aragonitic species, Gavelinopsis lobatulus, (0–4 cm) which produces significantly lower δ13C values than deep infaunal taxa, and the shallow infaunal species, Ceratobulimina pacifica (also aragonitic) and Bolivinopsis cubensis (deep infaunal), which yield higher carbon isotopic values than epifaunal taxa. These exceptions are found primarily in only one core, and additional samples are needed to confirm the relationship between their distribution patterns and isotopic compositions. Each of the species examined has a relatively consistent δ13C value throughout its distribution within the sediments that may result from heterogeneity of microhabitats within the intervals sampled. Intrageneric differences in δ13C of Cibicidoides, and possibly Uvigerina and Bulimina, are evident. The isotopic differences between C. bradyi and many other Cibicidoides species are related to differences in microhabitat preferences between species. The δ13C results confirm the influence of microhabitat preferences on the carbon isotopic composition of deep-sea benthic foraminifera and reaffirm the importance of assessing the microhabitat preferences of species used for isotopic analyses.

Late Quaternary deep-ocean circulation

A review of paleoceanographic studies dealing with late Quaternary deep-water circulation in the oceans is presented. These studies, which are based on the analysis of deep-sea benthic foraminiferal faunal and geochemical data and sedimentological data, are discussed in light of physical oceanographic conditions in the modern ocean. We include in this review a re-evaluation of benthic foraminiferal data from North Atlantic and Southern Ocean piston cores and suggest that the dominance of Uvigerina during glacial intervals reflects increased amounts of organic carbon at the sea floor compared to modern values. A number of geochemical studies have suggested that the production or characteristics of North Atlantic Deep Water (NADW) changed during glacial times. Although these changes have been thought to result from a cessation of overflow water from the Norwegian-Greenland Seas, it is suggested here that seasonal sea-ice cover was possible over southern portions of the Norwegian Sea during glacial intervals. The presence of seasonally open water would have allowed Norwegian Sea Overflow Water to have been produced, although perhaps at lower volumes and with different hydrographic properties than at present. The record of Antarctic Bottom Water (AABW) circulation does not show a simple relationship with paleoclimatic oscillations, indicating that changes in oceanographic conditions in the Southern Ocean had little effect on AABW formation. The AABW record contrasts with the glacial-interglacial cycles of NADW, suggesting no direct link between AABW and NADW circulation. A variety of data suggests that changes in Pacific Deep Water circulation occurred as a result of glacial production of North Pacific deep water or from an increased flux of Southern Ocean water.

Stable isotopes in late middle Eocene to Oligocene foraminifera

Oxygen and carbon isotope ratios in Eocene and Oligocene planktonic and benthic foraminifera have been investigated from Atlantic, Indian, and Pacific Ocean locations. The major changes in Eocene-Oligocene benthic foraminiferal oxygen isotopes were enrichment of up to l‰ in 18O associated with the middle/late Eocene boundary and the Eocene/Oligocene boundary at locations which range from 1- to 4-km paleodepth. Although the synchronous Eocene-Oligocene 18O enrichment began in the latest Eocene, most of the change occurred in the earliest Oligocene. The earliest Oligocene enrichment in 18O is always larger in benthic foraminifera than in surface-dwelling planktonic foraminifera, a condition that indicates a combination of deep-water cooling and increased ice volume. Planktonic foraminiferal δ18O does not increase across the middle/late Eocene boundary at our one site with the most complete record (Deep Sea Drilling Project Site 363, Walvis Ridge). This pattern suggests that benthic foraminiferal δ18O increased 40 m.y. ago because of increased density of deep waters, probably as a result of cooling, although glaciation cannot be ruled out without more data. Stable isotope data are averaged for late Eocene and earliest Oligocene time intervals to evaluate paleoceanographic change. Average δ18O of benthic foraminifera increased by 0.64‰ from the late Eocene to the early Oligocene δ18O maximum, whereas the average increase for planktonic foraminifera was 0.52‰. This similarity suggests that the Eocene/Oligocene boundary δ18O increase was caused primarily by increased continental glaciation, coupled with deep sea cooling by as much as 2 °C at some sites. Average δ18O of surface-dwelling planktonic foraminifera from 14 upper Eocene and 17 lower Oligocene locations, when plotted versus paleo-latitude, reveals no change in the latitudinal δ18O gradient. The Oligocene data are offset by ∼0.45‰, also believed to reflect increased continental glaciation. At present, there are too few deep sea sequences from high latitude locations to resolve an increase in the oceanic temperature gradient from Eocene to Oligocene time using oxygen isotopes.