Changes In Benthic Foraminiferal Assemblages Research Articles

Early Maastrichtian benthic foraminiferal assemblages from the western North Atlantic (Blake Nose) and their relation to paleoenvironmental changes

Benthic foraminiferal faunas from the Lower Maastrichtian ( Globotruncana falsostuarti– Gansserina gansseri Planktic Foraminiferal Zone) of DSDP Site 390 (Blake Nose, western North Atlantic) were investigated in order to characterize changes in organic matter flux and bottom-water oxygenation and their relation to paleoceanographic changes. Benthic foraminiferal faunas in the lower part of the studied succession show high abundances of Praebulimina reussi and Nuttallides truempyi and are proposed to reflect fluctuations in organic matter flux to the seafloor (meso- to eutrophic) under oxygenated bottom-water conditions. The middle interval is characterized by very low numbers of benthic foraminiferal specimens but a dominance of thick-walled species (e.g., lenticulinids, laevidentalinids), may reflect carbonate dissolution. In contrast to the lower part, benthic foraminifera from the upper part of the succession show well-oxygenated bottom waters. The most dominant species during this interval are Gavelinella beccariiformis, Reussella szajnochae, and N. truempyi. Parallel to this change in the benthic foraminiferal assemblages the number of inoceramid shells decreases, reflecting a significant increase in bottom-water oxygenation. Based on these data we speculate, that the observed changes in benthic foraminiferal assemblages towards a well-oxygenated environment within the Early Maastrichtian of DSDP Site 390 could reflect the onset of a shift from low-latitude to high-latitude deep-water sources. This speculation will predate the major reorganization of the oceanic circulation resulting in a circulation mode similar to today at the Early/Late Maastrichtian boundary by ∼ 1 Ma.

Past and recent sedimentation rates in the Lagoon of Venice (Northern Italy)

A chronology for the accumulation of subtidal and intertidal sediments in the Lagoon of Venice has been obtained from past and recent biofacies, with the aim of obtaining the sedimentation rates in various palaeo-environments and over various time-spans. In the sedimentary sequences, changes in benthic foraminiferal assemblages, i.e. ecological transitions, reflect changes in environmental conditions. Quantitative analyses of the foraminiferal assemblages of 89 samples from 26 cores, each about 1 m long, reveal a number of these ecological transitions. Statistical analysis based on the percentage of each foraminiferal species in the samples identified older lagoonal morphologies. In addition, 57 radiometric age determinations on organogenic material in sediments yielded the conventional BP ages of major events and the various sedimentation rates occurring in different environments in the course of time. Littoral deposits, identified at a depth of 90 cm, are dated for the first time, giving an age of 3180 ± 50 y BP. Various buried salt marshes gave ages ranging from 820 ± 40 to 1887 ± 40 y BP; in the inner part of the Lagoon, the cores reach continental deposits, and indicate that the earliest lagoonal deposits go back to historic or proto-historic times, with ages ranging from 820 ± 40 to 2403 ± 37 y BP. The oldest age (3437 ± 38 y BP) was obtained from shells of lagoonal organisms found at a depth of 90 cm in core 2, entirely within lagoonal deposits. The long-term sedimentation rates, inclusive of subsidence and eustasy, are about 1.1 mm/yr −1 between 2500 and 1500 y BP and about 0.5 mm/yr −1 from 1500 y BP to the present. The highest accumulation rates were recorded in the southern part of the Lagoon and the lowest rates around the city of Venice. In view of the decreased sedimentation rates, the recent resumption of the Sea Level Rise after a phase of quiescence may produce the deepening of the lagoonal basin.

Benthic foraminiferal evidence for the existence of an early Miocene oxygen-depleted oceanic water mass?

This study presents new data derived from changes in benthic foraminiferal assemblages between 24 and 8 Ma in the Atlantic and Indian oceans which are integrated with published stable isotope and fauna1 records. We have identified an intermediate to deep water mass which was lower in oxygen and was restricted to the NW Indian, Atlantic and the Tethys (Mediterranean) oceans. Our interpretations are based on the coeval occurrences (19.5–16.5 Ma) of high abundances of smooth-walled bolivinids in these oceans. Our observations agree with the interpretations of previous workers for the occurrence of Tethys-derived intermediate and deep water in the northern Indian Ocean between 24 and 14 Ma and for Northern Component Water production in the Atlantic Ocean between 20 and 16 Ma. The water mass defined by these bolivinids formed a southward flowing Northern Component Water in the Atlantic Ocean and contributed, via Tethys, to the intermediate and deep water input into the Indian Ocean.

Late Miocene‐Early Pliocene manganese redirection in the central Indian Ocean: Expansion of the Intermediate Water oxygen minimum zone

Decomposition of organic matter combined with density stratification generate a pronounced intermediate water oxygen minimum zone (OMZ) in the northwest Indian Ocean. This zone currently lies between water depths of 200 and 2000 m and extends approximately 5000 km southeast from the Arabian coast. Based upon benthic foraminiferal assemblage changes, it has been suggested that this OMZ was even more extensive during the late Miocene‐early Pliocene (6.5–3.0 Ma), with a maximum volume and/or intensity at approximately 5.0 Ma. While this inference may contribute to an understanding of the history of northwest Indian Ocean upwelling, corroborating geochemical evidence for this interpretation has heretofore been lacking. Ocean Drilling Program (ODP) sites 752, 754, and 757 on Broken and Ninetyeast ridges are located within central Indian Ocean intermediate water depths (1086–1650 m) but outside the present lateral dimensions of the Indian Ocean OMZ. High‐resolution chemical analyses of sediment from these sites indicate significant reductions in the flux of Mn and normalized Mn concentrations between 6.5 and 3.0 Ma that are most pronounced at approximately 5.0 Ma. Because late Miocene‐Pliocene paleodepths for these sites were essentially the same as at present and because extremely low sedimentation rates (0.3–1.3 cm/ky) most likely precluded sedimentary metal oxide diagenesis, we suggest that the observed Mn depletions reflect diminished deposition of reducible Mn oxyhydroxide phases within O2 deficient intermediate waters and that this effect was most intense at approximately 5.0 Ma. This interpretation implies that waters with less than 2.0 mL/L O2 extended at least 1500 km beyond their present limits and is consistent with changes in benthic foraminifera assemblages. We further suggest this expanded Indian Ocean OMZ is related to regionally and/or globally increased biological productivity.

Latest Paleocene lithologic and biotic events in neritic deposits of southwestern New Jersey

In the southwestern New Jersey Coastal Plain, four drill holes contain continuous neritic sedimentation across the Paleocene/Eocene boundary (calcareous nannofossil Zone NP 9/NP 10 boundary). Significant lithologic and biotic changes occur in these strata near the top of the Paleocene. Global warming, increased precipitation, and other oceanographic and climatic events that have been recognized in high‐latitude, deep‐oceanic deposits of the latest Paleocene also influenced mid‐latitude, shallow‐marine, and terrestrial environments of the western North Atlantic. The diverse, well‐preserved calcareous nannofossil flora that is present throughout the entire New Jersey boundary section accurately places these events within the uppermost part of the upper Paleocene Zone NP 9. Several rapid but gradational changes occur within a 1.1‐m interval near the top of Zone NP 9. The changes include (1) a change in lithology from glauconitic quartz sand to clay, (2) a change in clay mineral suites from illite/smectite‐dominated to kaolinite‐dominated, (3) a change in benthic foraminiferal assemblages to a lower diversity fauna suggestive of low‐oxygen environments, (4) a significant increase in planktonic foraminiferal abundance, and (5) an increased species turnover rate in marine calcareous nannofossils. Pollen was sparse in the New Jersey drill holes, but terrestrial sporomorph species in Virginia exhibit increased turnover rates at a correlative level. Foraminiferal assemblages and lithology indicate that relative sea level rose in New Jersey at the same time as these late Paleocene events occurred in late Biochron NP 9. The higher sea levels influenced sediment type and absolute abundance of planktonic foraminifers in the deposits. Above the initial increase of kaolinite in the upper part of Zone NP 9, the kaolinite percentage continues to increase, and the maximum kaolinite value occurs in the uppermost part of Zone NP 9. There are few changes in either the sediments or the biota precisely at the Zone NP 9/NP 10 boundary in New Jersey. The clay‐rich deposits with a high kaolinite clay mineral suite, the lowered diversity benthic foraminiferal assemblages, the abundant planktonic foraminiferal specimens, and the calcareous nannofossil assemblages continued essentially unchanged into the earliest Eocene Zone NP 10. Within the lower part of Zone NP 10, the kaolinite percentage decreased to very low values.

Paleoceanographic significance of Neogene benthic foraminiferal changes in a southwest Pacific bathyal depth transect

Temporal changes in benthic foraminiferal assemblages were quantitatively examined (> 63 μm fraction) in four southwest Pacific deep-sea Neogene sequences in a depth transect between approximately 1300 and 3200 m to assist in evaluating paleoceanographic history. The most conspicuous changes in benthic foraminiferal assemblages occurred in association with paleoclimatic changes defined at least in part by oxygen isotopic changes. The largest, centered at ∼ 15 Ma (early Middle Miocene), is represented by an increase in the relative frequencies of Epistominella exigua, which underwent a major upward depth migration at that time. This was contemporaneous with the well-known positive oxygen isotopic shift in the early Middle Miocene. In Sites 588 and 590, most of the increase in relative abundances of E. exigua occurred during the middle to later part of the δ 18O shift, following major growth of the east Antarctic ice sheet. Later assemblage changes occurred at 8.5 and 6.5 Ma. These associations indicate that the benthic foraminiferal assemblages in this depth transect largely adjusted to changes in deep waters related to Antarctic cryospheric evolution. In general, the Neogene benthic foraminiferal assemblages in this region underwent little change during the last 23 million years. This faunal conservatism suggests that deep-sea environments underwent relatively little change in the southwest Pacific during much of the Neogene. Although paleoceanographic changes did occur, partly in response to high-latitude cryospheric evolution, these were not of sufficient magnitude to create major deep-sea faunal changes in this part of the ocean. The benthic foraminiferal assemblages are dominated by individuals smaller than 150 μm. Most taxonomic turnover occurred in the larger (> 150 μm) size fractions.

Response of Northern Indian Ocean deep-sea benthic foraminifera to global climates during Pliocene-Pleistocene

The Northern Indian Ocean (DSDP Site 214) benthic foraminifera have been examined quantitatively to understand their response to the global climatic events and to determine the history of bottom-water circulation in the Northern Indian Ocean during Pliocene-Pleistocene. Uvigerina spp., Cibicides kullenbergi, C. wuellerstorfi, Globocassidulina pacifica, G. subglobosa, Oridorsalis umbonatus and Bulimina alazanensis are the dominant members of the benthic assemblage and reveal significant changes in distribution patterns. Based on benthic foraminiferal frequency changes, four intervals of major faunal and climatic turnover have been recognized at 5.2–5.1 Ma, 3.9–3.2 Ma, 3.2–3.1 Ma and 3.1–0.6 Ma. The interval between 5.2–5.1 Ma is marked by widespread Antarctic glaciation, deep-sea hiatuses, bottom-water cooling, increased upwelling, high sedimentation rates, highest Uvigerina abundances and intensified bottom-water circulation. During 3.9–3.2 Ma the bottom waters were warmer and bottom-water activity waned, with a sharp decline in Uvigerina abundance and increase in Cibicides. The interval between 3.2–3.1 Ma was a time of bottom-water cooling and coincides with a global benthic positive oxygen isotopic shift, Antarctic ice volume increase and possible initiation of Northern Hemisphere glaciation. The Late Pliocene and Pleistocene interval (3.1-Recent) is punctuated by major changes in benthic foraminiferal assemblages, warm/cold cycles, major glacial/interglacial intervals, increase in polar ice volume and intensification of upwelling. On the basis of cumulative percentages of Uvigerina and Cibicides, nine intervals of major bottom-water cooling have been inferred in the Pliocene and two in the Pleistocene at Site 214 reflecting the major changes in bottom-water circulation.

Relationship of Benthic Foraminiferal Biofacies to Lithofacies in Phosphatic Miocene Sediments, Mid-Atlantic Continental Shelf: ABSTRACT

Changes in benthic foraminiferal assemblages accompany changes in total sediment texture and mineralogy (primarily percent phosphatic grains) throughout the Pungo River Formation in Onslow Bay, North Carolina. Only Burdigalian (late-early Miocene) deposits have been cored in southern Onslow Bay. Basal phosphorite sands (30% phosphate) are overlain by phosphatic (8%) muds and slightly phosphatic (4%) quartz sands. Elongate buliminaceans (Bolivina, Bulimina, Buliminella, Uvigerina) comprise over 50% of the benthic assemblage in phosphorites. They also predominate (43%) in phosphatic muds where Siphogenerina and Florilus become conspicuous faunal elements. Diverse trochospirally coiled forms (mainly Hanzawaia, also Valvulineria and Cibicides) become predominant in quartz san s; buliminaceans decline to 30% of the fauna. Pungo River deposits in northern Onslow Bay are Burdigalian, Langhian (early-middle Miocene), and Serravallian (middle Miocene) in age. Burdigalian deposits are nonphosphatic, muddy quartz sands in which Hanzawaia predominates and buliminaceans comprise only 22% of the fauna; Florilus accounts for 5%. Hanzawaia remains the dominant genus in the slightly phosphatic (4%) quartz sands of the Langhian and the phosphatic (10%) sands of the Serravallian; buliminaceans increase to 29% of the fauna, but Florilus nearly disappears. Both vertically and laterally through the Miocene of Onslow Bay, nutrient-loving buliminaceans thrive where phosphate content increases. Florilus and Siphogenerina are associated with the influx of fine-grained terrigenous ediments. The Hanzawaia-dominated assemblage thrives in clean, End_Page 529------------------------------ coarser-grained substrates. Regional trends in the distributional patterns of these taxa may aid in locating additional phosphate deposits. End_of_Article - Last_Page 530------------

Eocene -Oligocene paleoceanography of the deep Bay of Biscay: Benthic foraminiferal evidence

A major change in benthic foraminiferal assemblages occurred in the deep Bay of Biscay (> 3 km water; DSDP Sites 119, and Site 400A) between early middle Eocene and earliest Oligocene. Predominant Eocene deep-sea taxa ( Nuttallides truempyi, Clinapertina spp., Abyssamina spp.) and associated rarer species became extinct in this interval. These extinctions were followed by an increase in abundance of bathymetrically wide-ranging and stratigraphically long-ranging taxa: Globocassidulina subglobosa, Oridorsalis spp., Gyroidinoides spp., and the Cibicidoides ungerianus plexus. The extinctions cannot be dated precisely from the stratigraphic record recovered to date in the Bay of Biscay; however, the replacement of the N. truempyi-dominated assemblage has been noted previously in the deep South Atlantic/Caribbean as occurring near the middle/late Eocene boundary. Other than the decrease in abundance and extinction of N. truempyi, no major abundance changes are noted within the Eocene at the shallower Site 401 (∼ 2 km water) in the Bay of Biscay. During the Oligocene, Nuttallides umbonifera replaced the Eocene species N. truempyi as the predominant deep-sea benthic foraminifera, reaching peak abundance in the middle Oligocene at Sites 119 and Site 400A. In the modern oceans, the abundance ot N. umbonifera is positively correlated with increased corrosiveness of bottom water, while at Site 119 the abundance of Nuttallides spp. is negatively correlated with δ 13C values in benthic foraminifera. As lower δ 13C values are often associated with older water masses, large numbers of Nuttallides spp. are thought to reflect older, and more corrosive bottom water. The faunal data and oxygen and carbon isotopic data are compared with a circulation model derived from North Atlantic seismic stratigraphic studies to show that old, warm, corrosive, and sluggish Eocene bottom water was replaced by younger, colder, less corrosive, more vigorously circulating bottom water of northern origin by the early Oligocene. Faunal and isotopic data suggest that bottom water became older and more corrosive again in the middle Oligocene, reflecting a reduction in circulation that can also be inferred from the seismic record in the nearby Rockall Plateau region.