Geochemical and palynological results of IODP Hole 302-M0004A from Lomonosov Ridge, Arctic Ocean

Several episodes of abrupt and transient warming, each lasting between 50,000 and 200,000 years, punctuated the long-term warming during the Late Palaeocene and Early Eocene (58 to 51 Myr ago) epochs**1,2. These hyperthermal events, such as the Eocene Thermal Maximum 2 (ETM2) that took place about 53.5 Myr ago**2, are associated with rapid increases in atmospheric CO2 content. However, the impacts of most events are documented only locally**3,4. Here we show, on the basis of estimates from the TEX86' proxy, that sea surface temperatures rose by 3-5 °C in the Arctic Ocean during the ETM2. Dinoflagellate fossils demonstrate a concomitant freshening and eutrophication of surface waters, which resulted in euxinia in the photic zone. The presence of palm pollen implies**5 that coldest month mean temperatures over the Arctic land masses were no less than 8 °C, in contradiction of model simulations that suggest hyperthermal winter temperatures were below freezing**6. In light of our reconstructed temperature and hydrologic trends, we conclude that the temperature and hydrographic responses to abruptly increased atmospheric CO2 concentrations were similar for the ETM2 and the better-described Palaeocene-Eocene Thermal Maximum**7,8, 55.5 Myr ago.

A relationship between aerosol transport and compound-specific d13C land plant biomarker and pollen records

We examined near-surface, late Holocene deep-sea sediments at nine sites on a north-south transect from the Congo Fan (4°S) to the Cape Basin (30°S) along the Southwest African continental margin. Contents, distribution patterns and molecular stable carbon isotope signatures of long-chain n-alkanes (C27-C33) and n-alkanols (C22-C32) are indicators of land plant vegetation of different biosynthetic types, which can be correlated with concentrations and distributions of pollen taxa in the same sediments. Calculated clusters of wind trajectories and satellite Aerosol Index imagery afford information on the source areas for the lipids and pollen on land and their transport pathways to the ocean sites. This multidisciplinary approach on an almost continental scale provides clear evidence of latitudinal differences in lipid and pollen composition paralleling the major phytogeographic zonations on the adjacent continent. Dust and smoke aerosols are mainly derived from the western and central South African hinterland dominated by deserts, semi-deserts and savannah regions rich in C4 and CAM plants. The northern sites (Congo Fan area and northern Angola Basin), which get most of their terrestrial material from the Congo Basin and the Angolan highlands, may also receive some material from the Chad region. Very little aerosol from the African continent is transported to the most southerly sites in the Cape Basin. As can be expected from the present position of the phytogeographic zones, the carbon isotopic signatures of the n-alkanes and n-alkanols both become isotopically more enriched in 13C from north to south. The results of the study suggest that this combination of pollen data and compound-specific isotope geochemical proxies can be effectively applied in the reconstruction of past continental phytogeographic developments.

Age analysis, aragonite and high magnesium calcite content from different IODP Holes from Exp 310

The widespread occurrence of microbialites in the last deglacial reef frameworks (16-6 Ka BP) implies that the accurate study of their development patterns is of prime importance to unravel the evolution of reef architecture through time and to reconstruct the reef response to sea-level variations and environmental changes. The present study is based on the sedimentological and chronological analysis (14C AMS dating) of drill cores obtained during the IODP Expedition #310 "Tahiti Sea Level" on the successive terraces which typify the modern reef slopes from Tahiti. It provides a comprehensive data base to investigate the microbialite growth patterns (i.e. growth rates and habitats), to analyze their roles in reef frameworks and to reconstruct the evolution of the reef framework architecture during sea-level rise. The last deglacial reefs from Tahiti are composed of two distinctive biological communities: (1) the coralgal communities including seven assemblages characterized by various growth forms (branching, robust branching, massive, tabular and encrusting) that form the initial frameworks and (2) the microbial communities developed in the primary cavities of those frameworks, a few meters (1.5 to 6 m) below the living coral reef surface, where they heavily encrusted the coralgal assemblages to form microbialite crusts. The dating results demonstrate the occurrence of two distinctive generations of microbialites: the "reefal microbialites" which developed a few hundred years after coralgal communities in shallow-water environments, whereas the "slope microbialites" grew a few thousands of years later in significantly deeper water conditions after the demise of coralgal communities. The development of microbialites was controlled by the volume and the shape of the primary cavities of the initial reef frameworks determined by the morphology and the packing of coral colonies. The most widespread microbialite development occurred in frameworks dominated by branching, thin encrusting, tabular and robust branching coral colonies which built loose and open frameworks typified by a high porosity (> 50%). In contrast, their growth was minimal in compact coral frameworks formed by massive and thick encrusting corals where primary cavities yielded a low porosity (~ 30%) and could not host a significant microbialite expansion.

Calcareous dinoflagellate cycst in the mid-Cenomanian Boreal realm

A transect from the bathyal to proximal shelf facies of the Boreal Realm was investigated to compare spatial and temporal distribution changes of calcareous dinoflagellate cysts (c-dinocysts) throughout the mid-Cenomanian in order to gain information on the ecology of these organisms. Pithonelloideae dominated the cyst assemblages to more than 95% on the shelf, a prevalence that can be observed throughout most of the Upper Cretaceous. The affinity of this group with the dinoflagellates, which is still controversially discussed, can be confirmed, based on evidence from morphological features and distribution patterns. The consistent prevalence of Pithonella sphaerica and P. ovalis in c-dinocyst assemblages throughout the Upper Cretaceous indicates that they were produced more frequently than cysts of the other species and might, therefore, represent a vegetative dinoflagellate life stage. P. sphaerica and P. ovalis are interpreted as eutrophic species. P. sphaerica is the main species in a marginal-shelf upwelling area, offshore Fennoscandia. Here, sedimentary cyclicity appears to have been reduced to the strongest light/dark changes, while in the outer shelf sediments, light/dark cycles are well-developed and show pronounced temporal assemblage changes. Cyclic fluctuations in the P. sphaerica / P. ovalis ratio reflect shifts of the preferred facies zones and indicate changes in surface mixing patterns. During periods of enhanced surface mixing most parts of the shelf were well-ventilated, and nutrient-enriched surface waters led to high productivity and dominance of the Pithonelloideae. These conditions on the shelf contrasted with those in the open ocean, where more oligotrophic and probably stratified waters prevailed, and an assemblage with very few Pithonelloideae and dominance of Cubodinellum renei and Orthopithonella ? gustafsonii was characteristic. While orbitally-forced light/dark sedimentary cyclicity of the shelf sections was mainly related to surface-water carbonate productivity changes, no cyclic modulation of productivity was observed in the oceanic profile. Therefore, dark layer formation in the open ocean was predominantly controlled by the cyclic establishment of anoxic bottom water conditions. Orbitally-forced interruptions in mixing on the shelf resulted in cyclic periods of stratification and oligotrophy in the surface waters, an expansion of oceanic species to the outer shelf, and a shelfward shift of pithonelloid-facies zones, which were probably related to shelfward directed oceanic ingressions.

Relative abundance and ranges of selected diatoms from Pliocene-Pleistocene sections of ODP Leg 177, Atlantic sector of the Southern Ocean

During Ocean Drilling Program (ODP) Leg 177, seven sites were drilled aligned on a transect across the Antarctic Circumpolar Current in the Atlantic sector of the Southern Ocean. The primary scientific objective of Leg 177 was the study of the Cenozoic paleoceanographic and paleoclimatic history of the southern high latitudes and its relationship with the Antarctic cryosphere development. Of special emphasis was the recovery of Pliocene-Pleistocene sections, allowing paleoceanographic studies at millennial or higher time resolution, and the establishment of refined biostratigraphic zonations tied to the geomagnetic polarity record and stable isotope records. At most sites, multiple holes were drilled to ensure complete recovery of the section. A description of the recovered sections and the construction of a multihole splice for the establishment of a continuous composite is presented in the Leg 177 Initial Reports volume for each of the sites (Gersonde, Hodell, Blum, et al., 1999). Here we present the relative abundance pattern and the stratigraphic ranges of diatom taxa encountered from shore-based light microscope studies completed on the Pliocene-Pleistocene sequences from six of the drilled sites (Sites 1089-1094). No shore-based diatom studies have been conducted on the Pliocene-Pleistocene sediments obtained at Site 1088, located on the northern crest of the Agulhas Ridge, because of the scattered occurrence and poor preservation of diatoms in these sections (Shipboard Scientific Party, 1999b). The data included in our report present the baseline of a diatom biostratigraphic study of Zielinski and Gersonde (2002), which (1) includes a refinement of the southern high-latitude Pliocene-Pleistocene diatom zonation, in particular for the middle and late Pleistocene, and (2) presents a biostratigraphic framework for the establishment of age models of the recovered sediment sections. Zielinski and Gersonde (2002) correlated the diatom ranges with the geomagnetic polarity record established shipboard (Sites 1090 and 1092) (Shipboard Scientific Party, 1999c, 1999d) and on shore (Sites 1089, 1091, 1093, and 1094) by Channell and Stoner (2002). The Pliocene-Pleistocene diatom zonation proposed by Zielinski and Gersonde (2002) relies on a diatom zonation from Gersonde and Bárcena (1998) for the northern belt of the Southern Ocean. Because of latitudinal differentiation of sea-surface temperature, nutrients, and salinity between Antarctic and Subantarctic/subtropical water masses, the Pliocene-Pleistocene stratigraphic marker diatoms are not uniformly distributed in the Southern Ocean (Fenner, 1991; Gersonde and Bárcena, 1998). As a consequence, Zielinski and Gersonde (2002) propose two diatom zonations for application in the Antarctic Zone south of the Polar Front (Southern Zonation, Sites 1094 and 1093) and the area encompassing the Polar Front Zone (PFZ) and the Subantarctic Zone (Northern Zonation, Sites 1089-1092). This accounts especially for the Pleistocene zonation where Hemidiscus karstenii, whose first abundant occurrence datum and last occurrence datum defines the subzonation of the northern Thalassiosira lentiginosa Zone, occurs only sporadically in the cold-water realm south of the PFZ and thus is not applicable in sections from this area. However, newly established marker species assigned to the genus Rouxia (Rouxia leventerae and Rouxia constricta) are more related to cold-water environments and allow a refinement of the Pleistocene stratigraphic zonation for the southern cold areas. A study relying on quantitative counts of both Rouxia species confirms the utility of these stratigraphic markers for the identification of sequences attributed to marine isotope Stages 6 and 8 in the southern Southern Ocean (Zielinski et al., 2002).

Stable isotope analysis and temperature reconstruction data from DSDP Hole 94-609B and ODP Hole 162-984B

The Greenland ice sheet is accepted as a key factor controlling the Quaternary glacial scenario. However, the origin and mechanisms of major Arctic glaciation starting at 3.15 Ma and culminating at 2.74 Ma are still controversial. For this phase of intense cooling Ravelo et al. proposed a complex gradual forcing mechanism. In contrast, our new submillennial-scale paleoceanographic records from the Pliocene North Atlantic suggest a far more precise timing and forcing for the initiation of northern hemisphere glaciation (NHG), since it was linked to a 2-3 °C surface water warming during warm stages from 2.95 to 2.82 Ma. These records support previous models, claiming that the final closure of the Panama Isthmus (3.0- ~2.5 Ma induced an increased poleward salt and heat transport. Associated strengthening of North Atlantic Thermohaline Circulation and in turn, an intensified moisture supply to northern high latitudes resulted in the build-up of NHG, finally culminating in the great, irreversible climate crash at marine isotope stage G6 (2.74 Ma). In summary, there was a two-step threshold mechanism that marked the onset of NHG with glacial-to-interglacial cycles quasi-persistent until today.

Neogene planktonic foraminifera of ODP Site 130-806

The Neogene biostratigraphy presented here is based on the study of 230 samples through 737 m of pelagic sediment in Hole 806B. Sediment accumulation is interrupted only once in the uppermost lower Miocene (Zone N6), apparently coincident with a widespread deep-sea hiatus. Preservation of planktonic foraminifers through the section ranges from good to moderately poor. One hundred and ten species of planktonic foraminifers were identified; taxonomic notes on most species are included. All of the standard low-latitude Neogene foraminiferal zones are delineated, with the exceptions of Zones N8 and N9 because of a high first occurrence of Orbulina, and Zones N18 and N19 because of a high first occurrence of Sphaeroidinella dehiscens. Good agreement exists between the published account of the variation in planktonic foraminiferal species richness and the rates of diversification and turnover, and measurements of these evolutionary indexes in the record of Hole 806B. The global pattern of change in tropical/transitional species richness is paralleled in Hole 806B, with departures caused by either ecological conditions peculiar to the western equatorial Pacific or by inexactness in the estimation of million-year intervals in Hole 806B. Temporal changes in the relative abundance of taxa in the sediment assemblages, considered in light of their depth habitats, reveal a detailed picture of historical change in the structure of the upper water column over the Ontong Java Plateau. The dominance of surface dwellers (Paragloborotalia kugleri, P. mayeri, Dentoglobigerina altispira, Globigerinita glutinata, and Globigerinoides spp.) throughout the lower and middle Miocene is replaced by a more equitable distribution of surface (D. altispira and Globigerinoides spp.), intermediate (Globorotalia menardii plexus), and deep (Streptochilus spp.) dwellers in the late Miocene, following the closing of the Indo-Pacific Seaway and the initiation of large-scale glaciation in the Antarctic. The shoaling of the thermocline along the equator engendered by these climatic and tectonic events persisted through the Pliocene, when initial increases in the abundance of a new set of shallow, intermediate, and deep dwelling species of planktonic foraminifers coincide with the closing of the Panamanian Seaway.

Carbon and oxygen isotopic compositions of Cibicidoides spp. and fine fraction separates (<10 µm) from ODP Holes 113-689B, 119-738B, and 120-748B

A prominent middle Eocene warming event is identified in Southern Ocean deep-sea cores, indicating that long-term cooling through the middle and late Eocene was not monotonic. At sites on Maud Rise and the Kerguelen Plateau, a distinct negative shift in d18O values (~1.0 per mil) is observed ca. 41.5 Ma. This excursion is interpreted as primarily a temperature signal, with a transient warming of 4°C over 600 k.y. affecting both surface and middle-bathyal deep waters in the Indian-Atlantic region of the Southern Ocean. This isotopic event is designated as the middle Eocene climatic optimum, and is interpreted to represent a significant climatic reversal in the midst of middle to late Eocene deep-sea cooling. The lack of a significant negative carbon isotope excursion, as observed during the Paleocene-Eocene thermal maximum, and the gradual rate of high-latitude warming suggest that this event was not triggered by methane hydrate dissociation. Rather, a transient rise in pCO2 levels is suspected, possibly as a result of metamorphic decarbonation in the Himalayan orogen or increased ridge/arc volcanism during the late middle Eocene.