Contusotruncana contusa abundances and morphotypes in the cretaceous mid-latitude South Atlantic

Spatial and temporal patterns in test size and shape (test conicity and spiral roundness) and absolute abundance (accumulation rate) of the planktonic foraminifer Contusotruncana contusa were studied in the South Atlantic Ocean (DSDP sites 356, 516, 525 and 527) during an interval corresponding to the last 800 kyr of the Cretaceous. The variation in absolute abundance of C. contusa was characterised by alternating periods of high and low abundance; some of these periods were traceable across the entire mid-latitude South Atlantic Ocean. While the mean spiral roundness did not show any interpretable patterns, a sudden increase of the mean test size and mean test conicity occurred between 65.3 and 65.2 Ma (based on linear interpolation within the Cretaceous part of Subchron C29R) at all sites studied, indicating a poleward migration followed by rapid withdrawal of the low-latitude C. contusa morphotypes from the mid-latitude South Atlantic Ocean. We suggest that this event was caused by a short period of surface-water warming in the southern mid-latitudes corresponding to the brief high-latitude warming event and associated faunal migrations in the Boreal and Austral realms.

Cretaceous radiolarians of the North Atlantic Ocean

Radiolarians form a remarkable part of the fossil plankton for Cretaceous sediments of the North Atlantic. Selected sites with long-term sedimentary successions of deep facies were studied (ODP Leg 103 and DSDP Site 398 off northwest Spain and DSDP Site 603 off the east coast of the United States). Preservation of the radiolarian faunas is generally poor, and the faunal abundance and diversity reflect the diagenetic history of the host sediment rather than the original faunal productivity. Several exceptions include abundant and some well-preserved radiolarian faunas from lower Campanian, Cenomanian/Turonian boundary, upper Albian, lower Albian, and Barremian sediments. These increases in radiolarian abundance and preservation coincide with well-established Cretaceous oceanic events in the North Atlantic. Typical faunal associations of these sections are described, and faunal associations from the Cenomanian/Turonian Boundary Event are documented for the first time in the North Atlantic. The relationship of the radiolarian blooms with coeval oceanic events in the North Atlantic is also discussed.

Cretaceous and Paleogene calcareous nannofossils from ODP Leg 123

Two of five holes drilled at two separate sites during Leg 123 of the Ocean Drilling Program intersected thick and relatively complete sections of Upper Cretaceous-Paleogene nannofossiliferous sediments. Although dominated by turbidite deposition in the upper part, Hole 765C contains a thick and relatively complete Albian-Oligocene section, including a particularly thick Aptian interval, with abundant and fairly well-preserved nannofossils. Several unconformities are confidently interpreted in this section that span much of the Santonian, late Campanian, Maestrichtian, late Eocene, and early Oligocene. Hole 766A contains a thick and relatively complete Albian-lower Eocene section having generally abundant and well-preserved nannofossils. Several unconformities also have been identified in this section that span much of the Coniacian, early Campanian, Maestrichtian, and late Eocene through early Pliocene. The chronostratigraphic position and length of all these unconformities may have considerable significance for reconstructing the sedimentary history and for interpreting the paleoceanography of this region. A particularly thick section of upper Paleocene-lower Eocene sediments, including a complete record across the Paleocene/Eocene boundary, also was cored in Hole 766A that contains abundant and diverse nannofossil assemblages. Although assemblages from this section were correlated successfully using a standard low-latitude zonation, difficulties were encountered that reduced biostratigraphic resolution. Several lines of evidence suggest a mid-latitude position for Site 766 during this time, including (1) high assemblage diversity characteristic of mid-latitude zones of upwelling and (2) absence of certain ecologically controlled markers found only in low latitudes.

Cretaceous stable isotopic record and sea surface temperture estimation for ODP Site 207-1259

The Turonian (93.5 to 89.3 million years ago) was one of the warmest periods of the Phanerozoic eon, with tropical sea surface temperatures over 35°C. High-amplitude sea-level changes and positive d18O excursions in marine limestones suggest that glaciation events may have punctuated this episode of extreme warmth. New d18O data from the tropical Atlantic show synchronous shifts ~91.2 million years ago for both the surface and deep ocean that are consistent with an approximately 200,000-year period of glaciation, with ice sheets of about half the size of the modern Antarctic ice cap. Even the prevailing supergreenhouse climate was not a barrier to the formation of large ice sheets, calling into question the common assumption that the poles were always ice-free during past periods of intense global warming.

Planktic foraminifera across the Cretaceous-Tertiary transition in the Antarctic Ocean

Three Antarctic Ocean K/T boundary sequences from ODP Site 738C on the Kerguelen Plateau, ODP Site, 752B on Broken Ridge and ODP Site 690C on Maud Rise, Weddell Sea, have been analyzed for stratigraphic completeness and faunal turnover based on quantitative planktic foraminiferal studies. Results show that Site 738C, which has a laminated clay layer spanning the K/T boundary, is biostratigraphically complete with the earliest Tertiary Zones P0 and P1a present, but with short intrazonal hiatuses. Site 752B may be biostratigraphically complete and Site 690C has a hiatus at the K/T boundary with Zones P0 and P1a missing. Latest Cretaceous to earliest Tertiary planktic foraminiferal faunas from the Antarctic Ocean are cosmopolitan and similar to coeval faunas dominating in low, middle and northern high latitudes, although a few endemic species are present. This allows application of the current low and middle latitude zonation to Antarctic K/T boundary sequences. The most abundant endemic species is Chiloguembelina waiparaensis, which was believed to have evolved in the early Tertiary, but which apparently evolved as early as Chron 30N at Site 738C. Since this species is only rare in sediments of Site 690C in the Weddell Sea, this suggests that a watermass oceanographic barner may have existed between the Indian and Atlantic Antarctic Oceans. The cosmopolitan nature of the dominant fauna began during the last 200,000 to 300,000 years of the Cretaceous and continued at least 300,000 years into the Tertiary. This indicates a long-term environmental crisis that led to gradual elimination of specialized forms and takeover by generalists tolerant of wide ranging temperature, oxygen, salinity and nutrient conditions. A few thousand years before the K/T boundary these generalists gradually declined in abundance and species became generally dwarfed due to increased environmental stress. There is no evidence of a sudden mass killing of the Cretaceous fauna associated with a bolide impact at the K/T boundary. Instead, the already declining Cretaceous taxa gradually disappear in the early Danian and the opportunistic survivor taxa (Ch. waiparaensis and Guembelitria cretacea) increase in relative abundance coincident with the evolution of the first new Tertiary species.

Lower Cretaceous nannofossils of ODP Holes 113-682B and 113-693A

Organic-rich, moderately to sparsely nannofossiliferous Lower Cretaceous claystones ("black shales") were cored at two Ocean Drilling Program Leg 113 sites on the continental slope of East Antarctica off Dronning Maud Land. A 39 m section at Site 692 yielded a Neocomian assemblage of limited diversity with rare Cyclagelosphaera deflandrei, Diadorhombus rectus, and Cruciellipsis cuvillieri, and is probably Valanginian in age. A 70-m section at Site 693 is assigned to the Rhagodiscus angustus Zone (late Aptian-early Albian in age). The latter zone is represented at DSDP sites on the Falkland Plateau, but equivalents to the Neocomian section are absent there, probably due to a disconformity. Watznaueria barnesae is the dominant species at both ODP sites, but it shares dominance with Repagulum parvidentatum at Site 693, where they total 70%-90% of the assemblage; their dominance is attributed to a paleogeographic setting within a restricted basin rather than to postdepositional dissolution of other species. The evolutionary development of this restricted basin and its eventual ventilation in early Albian times is discussed in terms of the regional stratigraphy and the breakup and dispersal of southwestern Gondwanaland. One new species, Corollithion covingtonii, is described.

Late Cretaceous palynofloras of ODP Hole 120-748C

Pollen, spore, and dinoflagellate cyst floras of Late Cretaceous age were found at Sites 748 (120-748C-62R through -79R) and 750 (120-750B-11W) of Ocean Drilling Program Leg 120 to the Kerguelen Plateau area in the Southern Indian Ocean. The ranges of dinocyst and sporomorph species indicate ages between the Cenomanian and Coniacian (to possibly the early Santonian). The ratio of marine/terrestrial flora elements is extremely variable, showing a trend from highly terrestrial (up to -70%) in the late Cenomanian to highly marine (up to 90%) in the Coniacian/early Santonian. Low sedimentation rates of about 3-5 cm/1000 yr were calculated for the glauconitic sediments of Turonian and Coniacian age at Site 748 (lithologic Subunit IIIB).

Late Cretaceous dinoflagellate cysts of ODP Hole 120-748C

At Ocean Drilling Program Hole 748C in the Southern Indian Ocean, a total of 171 Late Cretaceous dinoflagellate taxa were encountered in 38 productive samples from Cores 120-748C-27R through 120-748C-62R (407-740 mbsf). Four provisional dinoflagellate assemblage zones and five subzones were recognized based on the character of the dinoflagellate flora and the first/last occurrences of some key species. Isabelidinium korojonense and Nelsoniella aceras occur in Zone A together with Oligosphaeridium pulcherrimum and Trithyrodinium suspect urn. Zone B was delineated by the total range of Odontochitina cribropoda. Zone C was separated from Zone B by the presence of Satyrodinium haumuriense, and Zone D is dominated by new taxa. The dinocyst assemblages bear a strong affinity to Australian assemblages. Paleoenvironmental interpretations based mainly on dinocysts suggest that during the ?Santonian-Campanian to the Maestrichtian this portion of the Kerguelen Plateau was a shallow submerged plateau, similar to nearshore to offshore to upper slope environments with water depths of tens to hundreds of meters, but isolated from the major continents of the Southern Hemisphere. Starting perhaps in the late Cenomanian (Mohr and Gee, 1992, doi:10.2973/odp.proc.sr.120.196.1992), the Late Cretaceous transgression over the plateau reached its maximum during the late Campanian. The plateau may have been exposed above sea level and subjected to weathering during the latest Maestrichtian. The studied dinocyst assemblages characterized by species of Amphidiadema, Nelsoniella, Satyrodinium, and Xenikoon together with abundant Chatangiella (the large-size species) and Isabelidinium suggest that a South Indian Province (tentatively named the Helby suite) may have existed during the Campanian-Maestrichtian in comparison with the other four provinces of Lentin and Williams. One new genus, three new species, and two new subspecies of dinocysts are described.

Cretaceous planktonic foraminifera of the Kerguelen Plateau

An almost complete Upper Cretaceous sedimentary sequence recently recovered on the Kerguelen Plateau (southern Indian Ocean) during ODP Leg 183 was analysed for planktonic foraminifera in order to refine and integrate the zonal schemes previously proposed for the Southern Ocean area. Detailed biostratigraphic analysis carried out on holes 1135A, 1136A and 1138A (poleward of 50°S palaeolatitude during Late Cretaceous time) has allowed recognition of low and mid-high latitude bioevents, useful for correlation across latitudes, in addition to known Austral bioevents. The low latitude biozonation can be applied to Turonian sediments, because of the occurrence of Helvetoglobotruncana helvetica, which marks the boundary between Whiteinella archaeocretacea and Helvetoglobotruncana helvetica zones. The base of the Whiteinella archeocretacea Zone falls within the uppermost Cenomanian-Turonian black shale level in Hole 1138A. The stratigraphic interval from upper Turonian to uppermost Santonian can be resolved using bioevents recognized in the mid-high latitude sections. They are, in stratigraphic order: the last occurrence of Falsotruncana maslakovae in the Coniacian, the first occurrence of Heterohelix papula at the Coniacian/Santonian boundary, the extinction of the marginotruncanids in the late Santonian, and the first occurrence of Globigerinelloides impensus in the latest (?) Santonian. The remainder of the Late Cretaceous fits rather well in the Austral zonal scheme, except that Globigerinelloides impensus exhibits a stratigraphic range in agreement with its record at the mid-high latitude sections and extends further downwards than previously recorded at southern sites. Therefore, despite the poor recovery in certain intervals and the presence of several hiatuses of local and regional importance as revealed by correlation among holes, a more detailed zonal scheme has been obtained (mainly for the less resolved Turonian-Santonian interval). Remarks on some species often overlooked in literature are also provided.

Stable oxygen isotope record and benthic foraminiferal abundances in Cretaceous sediments of Demerara Rise

During the mid-Cretaceous period, the global subsurface oceans were relatively warm, but the origins of the high temperatures are debated. One hypothesis suggests that high sea levels and the continental configuration allowed high-salinity waters in low-latitude epicontinental shelf seas to sink and form deep-water masses (Brass et al., 1982, doi:10.1038/296620a0; Arthur and Natland, 1979; Chamberlin, 1906). In another scenario, surface waters in high-latitude regions, the modern area of deep-water formation, were warmed through greenhouse forcing (Bice and Marotzke, 2001, doi:10.1029/2000JC000561), which then propagated through deep-water circulation. Here, we use oxygen isotopes and Mg/Ca ratios from benthic foraminifera to reconstruct intermediate-water conditions in the tropical proto-Atlantic Ocean from 97 to 92 Myr ago. According to our reconstruction, intermediate-water temperatures ranged between 20 and 25 °C, the warmest ever documented for depths of 500-1,000 m. Our record also reveals intervals of high-salinity conditions, which we suggest reflect an influx of saline water derived from epicontinental seas around the tropical proto-North Atlantic Ocean. Although derived from only one site, our data indicate the existence of warm, saline intermediate waters in this silled basin. This combination of warm saline intermediate waters and restricted palaeogeography probably acted as preconditioning factors for the prolonged period of anoxia and black-shale formation in the equatorial proto-North Atlantic Ocean during the Cretaceous period.

Distribution of Cretaceous redeposited benthic foraminifers at DSDP Site 89-585

Cretaceous benthic foraminifers from Site 585 in the East Mariana Basin, western Pacific Ocean, provide an environmental and tectonic history of the Basin and the surrounding seamounts. Age diagnostic species (from a fauna of 155 benthic species identified) range from late Aptian to Maestrichtian in age. Displaced species in sediments derived from the tops and flanks of nearby seamounts were deposited sporadically on the Basin floor well below the carbonate compensation depth (CCD) at abyssal depths of 5000 to 6000 m. These depths, characterized by an indigenous assemblage of benthic foraminifers, recrystallized radiolarians, fish debris, and sponge spicules, existed in the Mariana Basin from late Aptian to the present. Early Albian and older edifice-building volcanism had reached the photic zone with associated shallow-water bank or reef environments. By middle Albian, the dominant source areas subsided to outer-neritic to upper-bathyal depths. Major volcanic activity ceased and fine-grained sediments were deposited by distal turbidites, although intermittent volcanism and the influx of rare neritic material continued until the late Albian. By the Cenomanian to Turonian, upper- to middle-bathyal depths were reached by the dominant source areas, and the sediments recovered from this interval include organic carbon-rich layers. Rare benthic foraminifers from the Coniacian-Santonian interval indicate a continuation of dominantly middle-bathyal source areas. A change in sedimentation during the Campanian-Maestrichtian from older zeolitic claystone to abundant chert in the Campanian, and nannofossil chalk and claystone in the Maestrichtian resulted from migration of the site beneath the equatorial productive zone due to northwestward plate motion. The appearance of rare middle-neritic and upper-bathyal species in the Maestrichtian interval associated with volcanogenic debris gives evidence of the remobilization and downslope transport of pelagic deposits due to thermally induced uplift. Episodic redeposition of shallow-water material during the Aptian-Albian was produced by edifice-building volcanism perhaps combined with eustatic lowering of sea level. The Cenomanian-Turonian pulse coincided with a low global sea-level stand as does the transported material during the Coniacian-Santonian. The Maestrichtian pulse was caused by renewed midplate volcanism that extended over a large area of the central Pacific.

Late Cretaceous benthic foraminiferal assemblages from Demerara Rise, western tropical Atlantic

This paper is based on Santonian-Campanian sediments of Ocean Drilling Program Sites 1257 (2951 mbsl) and 1259 (2353 mbsl) from Demerara Rise (Leg 207, western tropical Atlantic, off Surinam). According to its position, Demerara Rise should have been influenced by the early opening of the Equatorial Atlantic Gateway and the establishment of a bottom-water connection between the central and South Atlantic Oceans during the Late Cretaceous. The investigated benthic foraminiferal faunas demonstrate strong fluctuations in bottom-water oxygenation and organic-matter flux to the sea-floor. The Santonian-earliest Campanian interval is characterised by laminated black shales without benthic foraminifera in the lowermost part, followed by an increasing number of benthic foraminifera. These are indicative of anoxic to dysoxic bottom waters, high organic-matter fluxes and a position within the oxygen minimum zone. At the shallower Site 1259, benthic foraminifera occurred earlier (Santonian) than at the deeper Site 1257 (Early Campanian). This suggests that the shallower site was characterised by fluctuations in the oxygen minimum zone and that a re-oxygenation of the sea-floor started considerably earlier at shallower water-depths. We speculate that this re-oxygenation was related to the ongoing opening of the Equatorial Atlantic Gateway. A condensed glauconitic chalk interval of Early Campanian age (Nannofossil Zone CC18 of Sissingh) overlies the laminated shales at both sites. This interval contains benthic foraminiferal faunas reflecting increasing bottom-water oxygenation and reduced organic-matter flux. This glauconitic chalk is strongly condensed and contains most of the Lower and mid-Campanian. Benthic foraminiferal species indicative of well-oxygenated and more oligotrophic environments characterise the overlying mid- to Upper Campanian nannofossil chalk. During deposition of the nannofossil chalk, a permanent deep-water connection between the central and South Atlantic Oceans is proposed, leading to ventilated and well-oxygenated bottom waters. If this speculation is true, the establishment of a permanent deep-water connection between the central and South Atlantic Oceans terminated Oceanic Anoxic Event 3 "black shale" formation in the central and South Atlantic marginal basins during the Early Campanian (Nannofossil Zone CC18) and led to well-oxygenated bottom waters in the entire Atlantic Ocean during the Late Campanian (at least from Nannofossil Zone CC22 onwards).

Benthic foraminifera across the Cretaceous/Paleogene boundary in the Southern Ocean

The impact of an asteroid at the Cretaceous/Paleogene (K/Pg) boundary triggered dramatic biotic, biogeochemical and sedimentological changes in the oceans that have been intensively studied. Paleo-biogeographical differences in the biotic response to the impact and its environmental consequences, however, have been less well documented. We present a high-resolution analysis of benthic foraminiferal assemblages at Southern Ocean ODP Site 690 (Maud Rise, Weddell Sea, Antarctica). At this high latitude site, late Maastrichtian environmental variability was high, but benthic foraminiferal assemblages were not less diverse than at lower latitudes, in contrast to those of planktic calcifiers. Also in contrast to planktic calcifiers, benthic foraminifera did not suffer significant extinction at the K/Pg boundary, but show transient assemblage changes and decreased diversity. At Site 690, the extinction rate was even lower (~3%) than at other sites. The benthic foraminiferal accumulation rate varied little across the K/Pg boundary, indicating that food supply to the sea floor was affected to a lesser extent than at lower latitude sites. Compared to Maastrichtian assemblages, Danian assemblages have a lower diversity and greater relative abundance of heavily calcified taxa such as Stensioeina beccariiformis and Paralabamina lunata. This change in benthic foraminiferal assemblages could reflect post-extinction proliferation of different photosynthesizers (thus food for the benthos) than those dominant during the Late Cretaceous, therefore changes in the nature rather than in the amount of the organic matter supplied to the seafloor. However, severe extinction of pelagic calcifiers caused carbonate supersaturation in the oceans, thus might have given competitive advantage to species with large, heavily calcified tests. This indirect effect of the K/Pg impact thus may have influenced the deep-sea dwellers, documenting the complexity of the effects of major environmental disturbance.

Uppermost Jurassic to lower Cretaceous benthic foraminifera from ODP Hole 123-765C

Benthic foraminifers were studied in 99 samples collected from the lower 200 m of Hole 765C. The studied section ranges from the Tithonian to Aptian, and benthic foraminifers can be subdivided into five assemblages on the basis of faunal diversity and stratigraphic ranges of distinctive species. Compared with deep-water assemblages from Atlantic DSDP sites and Poland, assemblages from the Argo Abyssal Plain display a higher diversity of agglutinated forms, which comprise the autochthonous assemblages. Assemblages at the base of Hole 765C are wholly composed of agglutinated forms, reflecting deposition beneath the carbonate compensation depth (CCD). Most calcareous benthic species are found in turbidite layers, and the presence of an upper Valanginian Praedorothia praehauteriviana Assemblage may indicate deposition at or just below the CCD. The P. praehauteriviana Assemblage from Hole 765C is the temporal equivalent of similar assemblages from DSDP Holes 534A, 416A, 370, 105, and 101 in the Atlantic Ocean and Hole 306 in the Pacific Ocean. Stratigraphic ranges of cosmopolitan agglutinated species at Site 765 generally overlap with their reported ranges in the Atlantic and in the bathyal flysch sequences of the Carpathians; however, several species from Hole 765C have not been previously reported from Uppermost Jurassic to Lower Cretaceous abyssal sediments.