Barun K. Sen Gupta

Benthic foraminifera in oxygen-poor habitats

Abstract Numerous eurytolerant species of calcareous and agglutinated benthic foraminifera, belonging to diverse suprageneric taxa, survive under modern dysoxia and su☐ia (2-0 ml/l O2), as well as under higher oxygen levels. Under the usual combination of oxygen depletion and organic-matter enrichment of the substrate, the populations of some of these species expand significantly. Thus, generally, a relatively small number of opportunistic species become conspicously dominant in bathyal oxygen minimum zones; endobenthic adaptations are common within this group. Such high-dominance, low-diversity, foraminiferal biotas are present even in environments where the oxygen minimum stays near 3 ml/l. Various stenobathic but geographically widespread species are known from inshore waters under seasonal or long-term oxygen stress. Some of the species may be facultative anaerobes. Some ecophenotypes with larger pores may be associated with oxygen-poor waters, and small, thin-walled species are common (but not exclusively present) in oxygen minimum zones. Present data from modern environments do not support the hypothesis that most dysoxic-su☐ic benthic foraminifers, regardless of their phylogeny, are characterized by a particular test morphology.

Seasonal oxygen depletion in continental-shelf waters of Louisiana: Historical record of benthic foraminifers

A strong spring and summer oxygen depletion is induced in nearshore bottom waters of the Louisiana continental shelf by density stratification and by the carbon flux from phytoplankton production, which, in turn, is related to the nutrient load of the Mississippi and Atchafalaya rivers. In an attempt to read the historical record of this shelf hypoxia during the past two centuries, we compared the stratigraphic signals of benthic foraminifera (as reflected in a relative-dominance index for two common species of Ammonia and Elphidium ) in 210 Pb-dated cores, and we found evidence of an overall rise in oxygen stress (in intensity or duration), especially in the past 100 yr. This implies a progressive increase in the influence of river-borne nutrients, particularly anthropogenically influenced nitrates. Judging by our results, foraminiferal indices based on appropriate species ratios should prove useful in testing hypotheses about long-term environmental stresses, including eutrophication and paleohypoxia, on other marine shelves.

SEDIMENTS TELL THE HISTORY OF EUTROPHICATION AND HYPOXIA IN THE NORTHERN GULF OF MEXICO

We examined a suite of paleoindicators in 210Pb-dated sediment cores to determine the historical course of primary production, eutrophication, and oxygen stress in the coastal ocean adjacent to the plumes of the Mississippi and Atchafalaya rivers. The assumption that hypoxia is a natural feature of the coastal ecosystem in the northern Gulf of Mexico influenced by the discharge of the Mississippi River system is not supported by paleoindicators in accumulated sediments. There is a propensity for the ecosystem to develop hypoxia because of the high discharge of the Mississippi River and physical dynamics on the continental shelf that support stratification. However, paleoindicators of eutrophication and oxygen conditions record recent anthropogenic influences. The evidence for increased carbon production and accumulation comes from diatoms and their remnants, marine-origin carbon in the sediments, and phytoplankton pigments. Surrogates for oxygen condition, including mineral, isotopic, microfossil, and phytoplankton pigment indicators, indicate worsening oxygen stress. The changes are more apparent in areas of present chronic hypoxia and are coincident with the increasing nitrogen loads from the Mississippi River system beginning in the 1950s. Longer-term shifts in offshore ecology parallel landscape changes within the watershed in the last two centuries. The temporal shifts in this coastal ecosystem parallel the time sequence of similarly eutrophied coastal waters globally and coincide nicely with sediment analyses from other locations.

Benthic foraminifera of bathyal hydrocarbon vents of the Gulf of Mexico: Initial report on communities and stable isotopes

Substrates associated with active hydrocarbon vents in bathyal Gulf of Mexico support numerous foraminiferal species, with a few of them showing unusually high relative abundances. In the 584- to 695-m-depth range,Bolivina ordinaria, Gavelinopsis translucens, andCassidulina neocarinata strongly dominate the vent community, whereasBolivina subaenariensis andUvigerina laevis play this role around a vent at 216 m water depth. The bathymetric imprint on the foraminiferal record is also seen in theδ18O compositions of some species, includingUvigerina peregrina. The adaptation of foraminiferal communities to bacterial (Beggiatoa) mats, in which the redox boundary is very close to the sediment—water interface, and anomalous depletions of13C inU. peregrina (relative to the same species from nonventing sites) indicate that several species are probably facultative anaerobes and tolerant of H2S toxicity.

Association of bathyal foraminifera with water masses in the northwestern Gulf of Mexico

Abstract Q-mode cluster analysis and R-mode factor analysis of benthic foraminiferal data from 120 core tops from the northwestern Gulf of Mexico (87–1361 m; 26.4–29.0°N; 88.5–96.4°W) distinguish separate thanatotopes and faunal assemblages related to the five water masses present within the depth range of the samples. These water masses are the Surface Mixed Layer (SML), Gulf Water (GW), Oxygen Minimum Water (OMW), Subantarctic Intermediate Water (SAIW) and Caribbean Midwater (CMW). Reussella atlantica, Bigenerina irregularis and Angulogenerina bella are particularly associated with SML, Bulimina spicata, Planulina foveolata and Cibicidoides umbonatus with GW, Sphaeroidina bulloides, Bulimina mexicana and Siphonina bradyana with OMW, Epistominella exigua with SAIW and Gyroidinoides laevis, Cibicides wuellerstorfi, Pullenia subsphaerica, Bulimina aculeata, Gyroidinoides polius and Laticarinina pauperata with CMW. Cluster analysis also indicates the presence of three faunal groups related to water depth and bottom-water oxygen content within the OMW assemblage. An additional assemblage recognized in the Mississippi River Delta area may be related to low-oxygen conditions above the normal depth for the oxygen minimum. The close correspondence between the results of cluster and factor analyses suggests that similar numerical analyses of fossil data should be useful in deciphering ancient bathyal assemblages influenced by water-mass properties.

Reduced seasonality of Holocene climate and pervasive mixing of Holocene marine section: Northeastern Gulf of Mexico shelf

The large porcelaneous foraminifers Cyclorbiculina compressa, Parasorites orbitolitoides , and Peneroplis proteus are conspicuous in death assemblages from Holocene marine sediments of the Alabama and Florida panhandle shelf. The species inhabited the northeastern Gulf of Mexico in the Holocene (ca. 6.4–1.9 ka) but do not live in the region today. These foraminifers require warm, clear waters, and thus are important paleoclimatic and paleoenvironmental indicators. They apparently were derived from sublittoral seagrass habitats and indicate reduced seasonality in the region during the middle to late Holocene. In addition, a mixed foraminiferal fauna, a hydrodynamically reworked macrofaunal assemblage, and stratigraphic disorder in foraminiferal 14 C dates indicate extensive reworking of the entire Holocene marine transgressive package. Evidence that mollusks are indigenous but large foraminifers are transported supports the generalization that out-of-habitat transport of macrofauna is negligible in most marine settings.

Bathymetric reconstructions of the Miocene-age “calcari aLucina” (northern Apennines, Italy) from oxygen isotopes and benthic Foraminifera

Two independent methods of paleobathymetry, applicable to hydrocarbon-derived carbonates, are explored in this study. The oxygen isotope method exploits the temperature decline with depth that leaves a measurable imprint on theδ18O composition of pristine products of venting comprised of aragonites and dolomites. The other method makes use of the bathymetric preferences of benthic foraminiferal taxa entrapped in the carbonate buildups. These two methods were tested on the “calcari aLucina” limestones hosted in turbidites and mudstones infilling the Miocene-age Marnoso-arenacea basin. The limestone blocks, rich in fossils of chemosymbiotic-like fauna, preserve a 10-Ma record of hydrocarbon venting from Langhian to lower Messinian times. Our results indicate that carbonate accretion and lithification occurred at upper bathyal depths in waters not shallower than 200–250 m. Theδ18O of venting products contains the imprints of profound hydrographic changes that occurred in the ancient Mediterranean basin from the lower Serravallian to the lower Messinian.

Foraminifera as indicators of marine pollutant contamination on the inner continental shelf of southern Brazil

Analyses of living foraminiferal and environmental parameters near an outfall at Mar Grosso Beach (Laguna, SC, Brazil) demonstrate its usefulness as indicators of domestic sewage pollution. The low species diversity may be due to sand accumulation in the central part. Higher diversity was noted closer to the mouth of Laguna estuarine system where reduced salinity and higher temperatures indicate freshwater influence, suggesting a relationship between increased diversity and greater availability of terrestrial food. On the basis of foraminiferal diversity and average coliform count the higher values are closer to the mouth of the estuarine system and under the influence of the outfall. Due to the effect of local hydrodynamics, the particulate organic waste derived from the outfall does not settle down locally, and thus, do not accumulate nearby. Our hypothesis is that the fine material derived from the outfall is accumulating on the southwestern and northwestern parts of the beach.

Late Quaternary benthic foraminifera of the Grenada Basin: Stratigraphy and paleoceanography

Abstract The record of benthic foraminifera in the Grenada Basin of the Caribbean Sea for the past 127,000 years, based on three cores raised from depths near 2000 m, indicates subtle changes but no drastic faunal turnovers at the boundaries of planktonic biostratigraphic or isotopic zones. In two cores, Bulimina aculeata is clearly the dominant species, with no apparent relationship between the peaks and valleys of its frequency curve and the zone boundaries. It is associated with six other species or species-groups in the first recurrent group of benthic foraminifera in one core; the occurrence value of this group is high throughout the core. The most noticeable differences between the fauna of the Ericson Y zone and those of the X and Z zones are in the severe depletion of Nuttallides umbonifera, a non-dominant constituent of the assemblage, and the near-absence of the second recurrent group (which includes N. umbonifera) in the Y zone. In terms of the relative abundances of species and patterns of dominance, the successive late Quaternary benthic foraminiferal faunas of the Grenada Basin are significantly different from those associated with major water masses outside the Antillean Arc. The influence of the well-oxygenated core of the North Atlantic Deep Water on the assemblage was not much greater in the past than it is today. Frequency variations of Uvigerina peregrina, however, suggest that the contribution from NADW to the Grenada Basin bottom-water has been increasing in the past 7000 years. A similar increase, ending at 92–95,000 yrs. B.P., is also indicated for the beginning of the last interglacial period.

Holocene benthonic foraminifera in leeward bays of Saint Lucia, West Indies

Two distinct assemblages recognized, related to subtrate: sandy with coral growth and sandy-mud with no coral