(Table 1) Benthic foraminifera at DSDP Site 72-516

DSDP Site 516 contains a complete middle Eocene to lower Miocene interval with a well-developed Oligocene sequence that is more than 300 m thick. In this paper, the most important and characteristic benthic foraminiferal species from this interval are described and illustrated, and their quantitative and biostratigraphic distribution is given. Middle Eocene benthic assemblages, derived from pelagic intercalations in a partly turbiditic sequence, are low in diversity. Benthic assemblages of fairly high diversity occur in limestones, chalks, and oozes of the upper Eocene to lower Miocene. The consistently high rate of new species appearances at Site 516 during late Eocene and Oligocene contrasted greatly with the very slow rate of change in abyssal faunas at that time; there were no significant faunal changes at the Eocene/Oligocene boundary. The assemblages are dominated by Cibicidoides (mostly C. ungerianus or C. kullenbergi) and Lenticulina. Buliminids were also important during the Eocene and early Oligocene. Faunal comparison with other Atlantic DSDP sites and drill holes in the Gulf of Mexico suggest an approximately mid-bathyal (500-1500 m) depth of deposition during late Eocene and Oligocene.

Abundance of benthic foraminifera in late Paleocene to eraly Pliocene sediments of DSDP Hole 74-525A on the Walvis Ridge, South Atlantic (Table 1)

Benthic forammifers in the size-fraction greater than 0.073 mm were studied in 88 Paleocene to Pleistocene samples from Deep Sea Drilling Project Site 525 (Hole 525A, Walvis Ridge, eastern south Atlantic). Clustering of the samples on the basis of the 86 most abundant foramimfers (in total, 331 taxa were identified) allowed separating two major assemblage zones: the Paleocene to Eocene interval, and the Oligocene to Pleistocene interval. Each of these, in turn, were subdivided into three minor subzones as follows: lower upper Paleocene (approx. 62.4 to 57 8 Ma); upper upper Paleocene (56.6 to 56 2 Ma), lower and middle Eocene (55.3 to 46 8 Ma); upper Oligocene to middle Miocene (25.3 to 16 Ma), middle Miocene to Pliocene (15.7 to 4.2 Ma), and lower Pleistocene (0.4 to 0.02 Ma), with only minor differences with the previous zone. Some very abundant taxa span most of the column studies (Bolivina huneri, Cassidulina subglobosa, Eponides bradyi, E. weddellensis, Gavelinella micra, Oridorsalis umbonatus, etc.). Several of the faunal breaks recorded coincide with conspicuous minima in the specific diversity curve, thus suggesting that the corresponding turnovers signal the final stages of periods of faunal impoverishment. At least one major bottomwater temperature drop (as derived from delta18O data) is synchronous with a decrease in the forammiferal specific diversity. On the other hand, a specific diversity maximum in the middle Miocene might be associated with a delta13C increase at approx 16 to 12 Ma. Highest foraminiferal abundances (up to 600-800 individuals per gram of dry sediment) occurred in the late Paleocene and in the early Pleistocene, in coincidence with the lowest diversity figures calculated. The magnitude of the most important faunal turnover recorded, between the middle Eocene and the late Oligocene, is magnified in our data set by the large hiatus which separates the middle Eocene from the upper Oligocene sediments. Considerably smaller overturns occurred within the late Paleocene (in coincidence with changes in the specific diversity, absolute abundance of forammiferal tests, and delta13C), and in the middle Miocene (in coincidence with a specific diversity maximum and a delta13C excursion). New reformation on the morphology and the stratigraphic ranges of several species is furnished. For all the taxa recorded the number of occurrences, total number of individuals identified and first and last appearances are listed.

Stable oxygen and carbon isotope composition of benthic foraminifera from the western Mediterranean Sea

The influence of microhabitat, organic matter flux, and metabolism on the stable oxygen and carbon isotope composition of live (Rose Bengal stained) and dead (empty tests) deep-sea benthic foraminifera from the Gulf of Lions (western Mediterranean Sea) have been studied. The total range of observed foraminiferal isotope values exceeds 1.0 per mil for d18O and 2.2 per mil for d13C demonstrating a wide range of coexisting disequilibria relative to d18O of equilibrium calcite (d18OEQ) and d13C of bottom water dissolved inorganic carbon (d13CDIC). The mean d18O values reveal strongest disequilibria for the studied epifaunal to shallow infaunal species (Cibicidoides pachydermus, Uvigerina mediterranea, Uvigerina peregrina) while values approach equilibrium in deep infaunal species (Globobulimina affinis, Globobulimina pseudospinescens). The mean d13C values decrease with increasing average living depths of the different species, thus reflecting a dominant microhabitat (pore water) signal. At the axis of the Lacaze-Duthier Canyon a minimum d13CDIC pore water gradient of approximately -2.1 per mil is assessed for the upper 6 cm of the surface sediment. Although live individuals of U. mediterranea were found in different depth intervals their mean d13C values are consistent with calcification at an average living depth around 1 cm. The deep infaunal occurrence of U. mediterranea specimens suggests association with macrofaunal burrows creating a microenvironment with geochemical characteristics similar to the topmost centimeter. This also explains the excellent agreement between stable isotope signals of live and dead individuals. The ontogenetic enrichment in both d18O and d13C values of U. mediterranea suggests a slow-down of metabolic rates during test growth similar to that previously observed in planktic foraminifera. Enhanced organic carbon fluxes and higher proportion of resuspended terrestrial organic material at the canyon axis are reflected by d13C values of U. mediterranea on average 0.58 per mil lower than those from the open slope. These results demonstrate the general applicability of the d13C signal of this species for the reconstruction of past organic matter fluxes in the Mediterranean Sea. Further studies on live specimens are needed for a more quantitative paleoceanographic approach.

Benthic Foraminifera in Upper Pleistocene - Holocene bottom sediments from the southeastern Sea of Okhotsk

A comparative analysis of benthic foraminiferal assemblages in the last glacial sediments obtained by gravity cores from the southern Kamchatka slope (Vulk-34-98) and from the eastern slope of the Akademii Nauk Rise in the central Sea of Okhotsk (Vulk-34-90) revealed, along with their undoubted similarity, substantial differences caused by hydrological regime in these areas during the considered period. It is shown that during the last glacial period bottom waters near the northern Kuril Islands were warmer and less aerated than those in the Akademii Nauk Rise area. As is evident from low-amplitude variations in proportions of dominant species, hydrological parameters in the bottom layer of the latter area at that time were relatively more stable than in the former area.

Plio-Pleistocene benthic foraminifera of the Tyrrhenian Sea

Benthic foraminifers from Site 652, Site 653 (Hole 653A), and Site 654 of Leg 107 (Tyrrhenian Sea, Western Mediterranean), which penetrated with more or less good recovery the Plio-Pleistocene stratigraphic interval, were studied in a total of 699 close-spaced samples. A total number of 269 species have been classified and their quantitative distribution in each sample is reported. The benthic foraminifers assemblage is more diversified in Site 654, less diversified in Site 652. Less than a half of the benthic foraminifers species listed from Plio-Pleistocene Italian land sections are present in the coeval deep-sea Tyrrhenian record, in which shallow water species are missing and Nodosarids are poorly represented. A very few species have comparable stratigraphic distribution in the three deep-sea sequences and in Italian land sections when compared against calcareous plankton biostratigraphy. In the same three sites, the first appearance levels of several species are younger and younger, and last appearance levels are earlier and earlier from Site 654 to Site 653 and Site 652. Five biostratigraphic events, biochronologically evaluated and occurring at the same level in the deepsea Tyrrhenian record and in several land sections, have been selected as zonal boundaries of the proposed benthic foraminifers biostratigraphic scheme. The Plio-Pleistocene interval has been subdivided into four biozones and one subzone, recognizable both in the deep-sea and land-based sequences. The Cibicidoides (?) italicus assemblage zone stretches from the base of the Pliocene to the extinction level of the zonal marker, biochronologically evaluated at 2.9 Ma. The Cibicidoides robertsonianus interval zone stretches from the Cibicidoides (?) italicus extinction level to the Pliocene Mediterranean FO of Gyroidinoides altiformis, evaluated at 2.4 Ma. The Gyroidinoides altiformis interval zone stretches from the Mediterranean Pliocene FO of the zonal marker to the appearance level of Articulina tubulosa, evaluated at 1.62 Ma. The Articulina tubulosa assemblage zone stretches from the appearance level of the zonal marker to the Recent. In the Articulina tubulosa biozone, the Hyalinea baltica subzone is proposed. The appearance level of Hyalinea baltica is evaluated at 1.35 Ma, well above the Plio-Pleistocene boundary as defined in the Vrica stratotype section.

Benthic foraminifera and stable isotope composition in Paleocene-Eocene sediments

In the late Paleocene to early Eocene, deep sea benthic foraminifera suffered their only global extinction of the last 75 million years and diversity decreased worldwide by 30-50% in a few thousand years. At Maud Rise (Weddell Sea, Antarctica; Sites 689 and 690, palaeodepths 1100 m and 1900 m) and Walvis Ridge (Southeastern Atlantic, Sites 525 and 527, palaeodepths 1600 m and 3400 m) post-extinction faunas were low-diversity and high-dominance, but the dominant species differed by geographical location. At Maud Rise, post-extinction faunas were dominated by small, biserial and triserial species, while the large, thick-walled, long-lived deep sea species Nuttallides truempyi was absent. At Walvis Ridge, by contrast, they were dominated by long-lived species such as N. truempyi, with common to abundant small abyssaminid species. The faunal dominance patterns at the two locations thus suggest different post-extinction seafloor environments: increased flux of organic matter and possibly decreased oxygen levels at Maud Rise, decreased flux at Walvis Ridge. The species-richness remained very low for about 50 000 years, then gradually increased. The extinction was synchronous with a large, negative, short-term excursion of carbon and oxygen isotopes in planktonic and benthic foraminifera and bulk carbonate. The isotope excursions reached peak negative values in a few thousand years and values returned to pre-excursion levels in about 50 000 years. The carbon isotope excursion was about -2 per mil for benthic foraminifera at Walvis Ridge and Maud Rise, and about -4 per mil for planktonic foraminifera at Maud Rise. At the latter sites vertical gradients thus decreased, possibly at least partially as a result of upwelling. The oxygen isotope excursion was about -1.5 per mil for benthic foraminifera at Walvis Ridge and Maud Rise, -1 per mil for planktonic foraminifera at Maud Rise. The rapid oxygen isotope excursion at a time when polar ice-sheets were absent or insignificant can be explained by an increase in temperature by 4-6°C of high latitude surface waters and deep waters world wide. The deep ocean temperature increase could have been caused by warming of surface waters at high latitudes and continued formation of the deep waters at these locations, or by a switch from dominant formation of deep waters at high latitudes to formation at lower latitudes. Benthic foraminiferal post-extinction biogeographical patterns favour the latter explanation. The short-term carbon isotope excursion occurred in deep and surface waters, and in soil concretions and mammal teeth in the continental record. It is associated with increased CaC03-dissolution over a wide depth range in the oceans, suggesting that a rapid transfer of isotopically light carbon from lithosphere or biosphere into the ocean-atmosphere system may have been involved. The rapidity of the initiation of the excursion (a few thousand years) and its short duration (50 000 years) suggest that such a transfer was probably not caused by changes in the ratio of organic carbon to carbonate deposition or erosion. Transfer of carbon from the terrestrial biosphere was probably not the cause, because it would require a much larger biosphere destruction than at the end of the Cretaceous, in conflict with the fossil record. It is difficult to explain the large shift by rapid emission into the atmosphere of volcanogenic CO2, although huge subaerial plateau basalt eruptions occurred at the time in the northern Atlantic. Probably a complex combination of processes and feedback was involved, including volcanogenic emission of CO2, changing circulation patterns, changing productivity in the oceans and possibly on land, and changes in the relative size of the oceanic and atmospheric carbon reservoirs.