Oxygen and carbon isotopes from benthic and planktonic foraminifers at DSDP Leg 74 Holes

Oxygen and carbon isotope measurements have been made in picked planktonic and benthonic foraminifers from the five sites drilled on Leg 74, covering the whole Cenozoic. For the Neogene, the coverage gives good information on the development of the vertical temperature structure of Atlantic deep water. For the Paleogene, vertical gradients were weak and it is possible to combine data from different sites to obtain a very detailed record of both the temperature and carbon isotope history of Atlantic deep waters.

Eocene to Quaternary benthic foraminifers from the Izu-Bonin Arc

The benthic foraminifer fauna at Sumisu Rift Sites 790 and 791 indicates that a deep open-ocean (>2300 m) or a basin with open-ocean access existed between 1.1 and 0.7 Ma at the time of the initiation of rifting. The appearance of a low- to medium-oxygen fauna (1600-2300 m) between 0.7 and 0.5 Ma suggests that the open-ocean access may have been terminated at this time because of the development of volcanoes and rift flank uplifts around the basin. The occurrence of low-oxygen faunas at 0.03 Ma suggests a secondary closing of the basin. The lower bathyal benthic faunas from lower Pliocene sediments of rift margin Site 788 suggest about 0.6-1.6 km of total basement uplift. This uplift may have led to the formation of the major hiatus between 2.3 and <0.3 Ma. The faunal changes of benthic foraminifers at Sites 792 and 793 in the forearc basin document a shallowing water depth from below the carbonate compensation depth (CCD) (about 3.5 km) in the late early Oligocene to the present depths of 1800 and 2975 m, respectively. These data suggest about 1 km of total basement uplift in the inner part of the forearc basin (Site 792) and about 0.6 km total basement subsidence in the central part of the forearc basin (Site 793) since about 31 Ma. The former uplift led to a thinner sediment accumulation (800 m) and the latter subsidence to a thicker sediment accumulation (1400 m) at these sites. Faunal changes of benthic foraminifers observed in Sites 782 and 786 sequences drilled at the outer-arc high document a deepening water depth from 1.3 to 2.1 km in late Eocene to the present depth of about 3 km. These data suggest about 1.1-1.9 and 1.3-2.1 km of total basement subsidence at Sites 786 and 782, respectively. These results indicate total basement uplift in the inner part of the Bonin arc-trench system since late Oligocene and total basement subsidence in the outer part of the system since late Eocene. The last occurrence (LO) of Stilostomella spp. and Pleurostomella spp. and the first occurrence (F0) of Bulimina aculeata d'Orbigny occurred consistently at 0.7 Ma at all three arc proximal sites (790,791, and 792). This fact is taken to suggest a change of water mass, from one originating from the central part of the ocean to that originating from ocean-margin areas at that time.

Foraminifer, epifauna and infauna abundance of ODP Hole 183-1138A and core ELT54.007-PC, Kerguelen Plateau

Late Campanian and Maastrichtian benthic foraminifers are recorded from 12 samples from Ocean Drilling Program (ODP) Leg 183, Cores 183-1138A-52R through 63R (487.3-602.4 meters below seafloor), Kerguelen Plateau, Indian Ocean, and Danian benthics from one sample in the same section. The entire late Maastrichtian foraminifer fauna is noted from a dredge sample 220 km to the north. The structure of the fauna is compared with the Cenomanian-Turonian of the nearby Eltanin core E54-7. Faunas are reviewed in terms of planktonic percentage, composition, epifaunal/infaunal ratios, and dominance/diversity indices. The region was in the cool Austral Faunal Province through the Campanian-Maastrichtian and was probably warmer in the Cenomanian-Turonian. The ODP section is now 1600 meters below sea level and has subsided several hundred meters since deposition. Its fauna is dominated by epifaunal species suggesting little influence of upwelling. The dredge location has subsided little. Its fauna has a high infaunal content consistent with significant influence of upwelling near the plateau edge. The dominant benthic species remain constant through the ODP Cretaceous section, but subdominance changes, and the section is divided into three informal zones based on dominance/subdominance characteristics of the benthic fauna. Brief taxonomic comments are made on several species and some are figured.

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.

Upper Maestrichtian to Eocene benthic foraminifera in ODP Leg 121 holes

Late Maestrichtian to late Eocene bathyal benthic foraminiferal faunas at Sites 752,753, and 754 on Broken Ridge in the eastern Indian Ocean were analyzed as to their stratigraphic distribution of species to clarify the relation between faunal turnovers and paleoceanographic changes. Based on Q-mode factor analysis, eight varimax assemblages were distinguished: the Stensioina beccariiformis assemblage in the upper Maestrichtian to upper Paleocene; the Cibicidoides hyphalus assemblage in the upper Maestrichtian; the Cibicidoides cf. pseudoperlucidus assemblage in the upper Paleocene; the Anomalinoides capitatusldanicus assemblage in the uppermost Paleocene to lower Eocene; the Cibicidoides subspiratus assemblage in the lower Eocene; the Nuttallides truempyi assemblage in the lower and middle Eocene; the Osangularia sp. 1 - Hanzawaia ammophila assemblage in the upper Eocene; and the Lenticulina spp. assemblage in the uppermost Eocene, Oligocene, and lower Miocene. The presence of the Osangularia sp. 1 - Hanzawaia ammophila assemblage is related to the shallowing episode on Broken Ridge (upper bathyal), as a result of the rifting event that occurred in the middle Eocene. The most distinct faunal change (the disappearance of about 37% of the species) occurred between the S. beccariiformis assemblage and the A. capitatusldanicus assemblage, at the end of the upper Paleocene. A. capitatusldanicus, Lenticulina spp., and varied forms of Cibicidoides replaced the Velasco-type fauna at this time. The timing of this event is well correlated with the known age at South Atlantic sites (Thomas, 1990 doi:10.2973/odp.proc.sr.113.123.1990; Kennett and Stott, 1990 doi:10.2973/odp.proc.sr.113.188.1990; Katz and Miller, 1990 doi:10.2973/odp.proc.sr.114.147.1991). The primary cause of the extinction of the Stensioina beccariiformis assemblage is elusive, but may have resulted from the cessation of deep-water formation in the Antarctic (Katz and Miller, 1990), and subsequent arrival of warm saline deep water (Thomas, 1990; Kennett and Stott, 1990). Another possibility may be a weakened influence of high-salinity water formed at the low latitudes such as the Tethys Sea. The extinction event corresponds to the change from higher delta13C values in benthic foraminifers to lower ones. An interpretation of delta13C values is that the eastern Indian deep water, characterized by young and nutrient-depleted water, became old water which was devoid of a supply of new water during the latest Paleocene to early Eocene. Prior to this benthic event, signals of related faunal change were detected in the following short periods: early and late Paleocene, near the boundary of nannofossil Zone CP4, and Zone CP5 of the late Paleocene at Site 752. Among common taxa in the upper Maestrichtian, only seven species disappeared or became extinct at the Cretaceous/ Tertiary boundary at Site 752. The benthic foraminiferal population did not change for up to 2 m above the boundary, in contrast to the rapid decrease of the plankt onic foraminiferal population at the boundary. A decrease in the number of benthic foraminifers occurs after that level, corresponding to an interval of decreased numbers of planktonic foraminifers and higher abundance of volcanic ash. Reduced species diversity (H') suggests a secondary effect attributable to the dissolution of foraminiferal tests. The different responses of planktonic and benthic foraminifers to the event just above the boundary suggest that the Cretaceous/Tertiary event was a surface event as also suggested by Thomas (1990). In addition, a positive shift of delta13C in benthic foraminifers after the event indicates nutrient-depleted bottom water at Site 752.

Benthic foraminifers in Mesozoic and Cenozoic sediments of the southwestern Atlantic

Benthic foraminiferal assemblages in Mesozoic and Cenozoic sediments were studied at Sites 511, 512, 513, and 514 drilled during Leg 71 in the southwestern Atlantic on the Maurice Ewing Bank and in the Argentine Basin. Benthic foraminifers in almost all stratigraphic subdivisions of Sites 511 and 512 reflect the gradual subsidence of the Falkland Plateau from shelf depths in the Barremian-Albian, when a semiclosed basin with restricted circulation of water masses and anaerobic conditions existed, to lower bathyal depths in the Late Cretaceous and Cenozoic, with an abrupt acceleration at the boundary of Lower and Upper Cretaceous. The composition, distribution, and preservation of Late Cretaceous assemblages of benthic foraminifers suggest considerable fluctuations of the foraminiferal lysocline and the CCD. This is evidenced by dissolution facies and foraminiferal assemblages in which agglutinated and resistant calcareous forms predominated during high stands of the CCD and by calcareous facies in which rich assemblages of calcareous species predominated during low stands. The highest position of the CCD on the Plateau (less than 1500-2000 m) was in the late Cenomanian, Turonian, and Coniacian. In the Santonian and Campanian the CCD was at depths below 1500-2000 meters. At the end of the Campanian the CCD shifted again to depths comparable with those of Cenomanian and Turonian time. In the latest Campanian and the Maestrichtian the CCD was low and nanno-foraminiferal oozes with a rich assemblage of benthic foraminifers accumulated. Foraminiferal assemblages at Sites 513 and 514 in the Argentine Basin also testify to oceanic subsidence from lower bathyal depths in the Oligocene to abyssal ones at present. This process was complicated by the influence of geographical migrations of the Polar Front caused by extensions of the ice sheet in the Antarctic after the opening of the Drake Passage during the Oligocene. In Mesozoic and Cenozoic deposits of the Falkland Plateau and the Argentine Basin seven assemblages of benthic foraminifers were distinguished by age: early-middle Albian, middle-late Albian, Late Cretaceous (including four groups), middle Eocene, late Eocene-early Miocene, middle-late Miocene, and Pliocene-Quaternary. The Albian assemblages contain many species common to the foraminiferal fauna of the Austral Biogeographical Province. The Late Cretaceous assemblage contains, along with Austral species, species common to foraminifers of North America, Western Europe, the Russian platform, and the south of the U.S.S.R. Deep-sea cosmopolitan species prevail in Cenozoic assemblages.

Paleogene benthic foraminifera from the Kerguelen Plateau

Benthic foraminifers were studied from lower Paleocene through upper Oligocene sections from Sites 747 and 748. The composition of the benthic foraminifer species suggests a middle to lower bathyal (600-2000 m) paleodepth during the Neogene and a probable upper abyssal (2000-3000 m) paleodepth during the Paleocene at Site 747. Site 748 is thought to have remained at middle to lower bathyal paleodepths throughout the Cenozoic. Principal component analysis distinguished four major benthic foraminifer assemblages: (1) a Paleocene Stensioina beccariiformis assemblage at Sites 747 and 748, (2) an early Eocene Nuttallides truempyi assemblage at lower bathyal Site 747, (3) an early through middle Eocene Stilostomella-Lenticulina assemblage at middle bathyal Site 748, and (4) a latest Eocene through Oligocene Cibicidoides-Astrononion pusillum assemblage at both sites. Major benthic foraminifer changes, as indicated by the principal components and first and last appearances, occurred at or close to the Paleocene/Eocene boundary, and in the late Eocene close to the middle/late Eocene boundary.

Oliogocene to Pleistocene benthic foraminifera in ODP Leg 121 sites

Oligocene to Pleistocene bathyal benthic foraminifers at Broken Ridge (Site 754) and Ninetyeast Ridge (Site 756), eastern Indian Ocean, were investigated for then- stratigraphic distribution and their response to paleoceanographic changes. Q-mode factor analysis was applied to relative abundance data of the most abundant benthic foraminifers. At Site 754, seven varimax assemblages were recognized from the upper Oligocene to the Pleistocene: the Gyroidina orbicularis-Rectuvigerina striata Assemblage in the uppermost Oligocene; the Lenticulina spp. Assemblage in the upper Oligocene to lower Miocene, and in lower Miocene to lowermost middle Miocene; the Burseolina cf. pacifica-Cibicidoides mundulus Assemblage in the lower Miocene; the Planulina wuellerstorfi Assemblage in the upper middle Miocene; the Globocassidulina spp. Assemblage in the upper Miocene; the Gavelinopsis lobatulus-Uvigerina proboscidea Assemblage in the Pliocene; and the Ehrenbergina spp. Assemblage in the Pleistocene. The major faunal changes are complex, but exist between the Lenticulina spp. Assemblage and the P. wuellerstorfi Assemblage at ~13.8 Ma, and between the Ehrenbergina spp. Assemblage and the G. lobatulus Assemblage at ~5 Ma. The development of the P. wuellerstorfi and Globocassidulina spp. Assemblages after 13.8 Ma is correlated with the decrease in temperature of the intermediate waters of the ocean, in turn related to Antarctic glacial expansion. The faunal changes at ~5 Ma are related to the development of low oxygen intermediate water, formed in the presence of a strong thermocline. At Site 756, six varimax assemblages are distributed as follows: the Cibicidoides cf. mundulus-Oridorsalis umbonatus Assemblage in the lower Oligocene; the Epistominella umbonifera-Cibicidoides mundulus Assemblage from the upper Oligocene to the lower Miocene; the Cibicidoides mundulus-Burseolinapacifica Assemblage from lower Miocene to the lower middle Miocene; the Globocassidulina spp. Assemblage from the upper lower Miocene to the Pliocene; the Uvigerina proboscidea Assemblage in the upper Miocene and the Pliocene; and the Globocassidulina sp. D Assemblage in the Pliocene. The main faunal change at this site is between the E. umbonifera Assemblage and the Globocassidulina spp. Assemblage, at ~17.1 Ma. The timing of this faunal change is coeval with faunal changes in the North Atlantic and the Pacific. The change is related to a change in bottom water characteristics caused by an increased influence of carbonate corrosive water from the Antarctic source region, and a change in surface productivity. A low oxygen event at Site 756, which started at about 7.3 Ma, occurred about 2.3 m.y. before that at Site 754. The different response to global paleoceanographic changes is not yet explained, but may be due to the difference of marine topography and the degree of upwelling