Carbon and oxygen stable isotope composition of bulk sediment and abundance of Braarudospahaera in ODP Site 122-762 (Table 1)

Nannofossil assemblages enriched in Braarudosphaera occur in lower Oligocene to lower Miocene sediments at Ocean Drilling Program Sites 762 and 763 on the central Exmouth Plateau. Braarudosphaerids appear here rather abruptly in the lower Oligocene (in Zone NP21). They reach their greatest numbers in the lower Oligocene (in Zones NP22 and NP23), where they comprise up to 10% of some samples. Braarudosphaera bigelowii is the overwhelmingly dominant species, occurring together with rare specimens of B. discula and Micrantholithus pinguis. The holococcoliths Peritrachelina joidesa and Lantemithus minutus are also associated with the Braarudosphaera enrichment. There are two populations of B. bigelowii: one of normal size (10-14 µm) and one of large specimens (20-22 µm). The larger braarudosphaerids are more common than the smaller forms. Braarudosphaera-rich sediments are absent at Wombat Plateau sites during the same time interval. We attribute this to latitudinal control, because the Wombat sites are about 4° north of the central Exmouth Plateau sites. We believe that the occurrence of braarudosphaerids is related to an Oligocene to early Miocene oceanographic event on the Exmouth Plateau. We suspect that mid-ocean up welling of cool, low-salinity, nutrient-rich water along a divergent zone created the Braarudosphaera-nch sediments in the South Atlantic and Indian oceans.

Abundance of calcareous nannofossils in ODP Hole 122-761C (Table 1)

A biostratigraphically complete (all nannofossil biozones present) Cretaceous/Tertiary boundary was recovered at Site 761 on the Wombat Plateau, northwest Australian margin, during Ocean Drilling Program Leg 122. A quantitative study of calcareous nannofossils on closely spaced samples across the boundary reveals a rapid change in assemblages in a similar fashion to other Cretaceous/Tertiary boundary sites. Nannofossil species were placed into three categories: Tertiary, Cretaceous, and 'survivors'. The rapid sequential turnover in these assemblages is as follows: Cretaceous species are abruptly replaced at the boundary by opportunistic survivor species, which in turn are abruptly replaced by newly evolved Tertiary taxa. The uppermost Maestrichtian assemblage is distinctly mid-latitudinal with few Micula murus and rare to few Nephrolithus frequens. The nannofossil assemblage immediately above the boundary is dominated by an abundance bloom of Cyclagelosphaera spp. No Thoracosphaera or Braarudosphaera abundance blooms are present as at many other localities. The change from a survivor- to Tertiary-dominated assemblage is coincident with the CPla/CPlb nannofossil subzonal boundary, which is marked by the simultaneous first occurrence of several species including Cruciplacolithus tenuis and C. primus. The latter is found to first occur below C. tenuis in the most complete Cretaceous/Tertiary sections. A hiatus between Subzones CPla and CPlb is interpreted to explain this discrepancy.

Cenozoic radiolaria in the western North Atlantic

Eight Cenozoic radiolarian zones were recognized in samples from two holes at Site 603, drilled on the lower continental rise off North America during Leg 93 of the Deep Sea Drilling Project. Paleocene to early Eocene radiolarian zones (Bekoma bidartensis, Buryella clinata, and Phormocyrtis striata striata zones) and early to late Miocene radiolarian zones (Calocycletta costata, Dorcadospyris alata, Diartus petterssoni, and Didymocyrtis antepenultima zones) were recognized in sediments from Holes 603 and 603B. In addition, a new Paleocene Bekoma campechensis radiolarian Zone is defined by the interval between the first morphotypic appearance of B. campechensis and the B. campechensis-B. bidartensis evolutionary transition. This zone is immediately below the B. bidartensis Zone of Foreman (1973), and has previously been discussed as a Paleocene "unnamed zone" by other investigators. A hiatus between Neogene and Paleogene sequences was also recognized in the radiolarian faunas.

Planktonic foraminifera of Oligocene sediments from DSDP Hole 77-538A in the Gulf of Mexico

A relatively extended Oligocene pelagic sequence with good to medium recovery, drilled during DSDP Leg 77 in the Gulf of Mexico, yielded rich and well diversified planktonic foraminiferal faunas. Planktonic foraminifera recorded in Hole 538A span the interval from Zone P19 through P22. Evolutionary lineages were observed among the globoquadrinids, the globigerinitids, and the "Globigerina" ciperoensis and Globigerinoides primordius groups. Quantitative analysis of planktonic foraminiferal assemblages shows that faunas fluctuate in abundance and species diversity throughout the sequence. A few of these fluctuations that could be related to selective dissolution are mainly confined to the early-mid Oligocene. A climatic curve was constructed using as warmer indicators, Turborotalia pseudoampliapertura, Globoquadrina tripartita, Dentoglobigerina globularis, Dentoglobigerina baroemoenensis,. "Globigerina" ciperoensis and Globigerinoides groups, and Cassigerinella chipolensis; and as coller indicators, Catapsydrax spp., Globorotaloides spp., Subbotina angiporoides group, Globigerina s. str., and the tenuitellides. Three major intervals are identifiable in the climatic curve: Interval 1 (lower) up to Zone P20 predominantly cooler: Interval 2 (intermediate) up to the upper part of Zone P21a with warm and cool fluctuations: and lnterval 3 (upper), warmer, with a large positive peak, due to abundant "G." angulisuturalis, at the beginning of Zone P21b with recooling midway in Zone P22. In Intervals 1 and 2 planktonic foraminiferal faunas are dominated by temperate forms. Interpretation of planktonic foraminiferal data suggests that cooler water conditions characterize the early-mid Oligocene: during the mid Oligocene (most of Zone P21a) water masses exhibit peculiar characteristics transitional to the warmer waters prevailing during the late Oligocene. Warmer conditions were not definitely settled in Zone P22, however, as indicated by the cooler episode following the warmest peak. These climatic trends are inconsistent with those inferred from oxygen isotopes except at small scale. In fact, oxygen isotope values for Oligocene Atlantic Ocean are too heavy (thus too cool) in comparison with the high abundance and diversity of warm taxa, expecially in Zone P22. When values are lighter (warmer), as in Zone P19 abundance and diversity of warm indices are too low. To explain such a cool isotope values in presence of highly diversified and abundant warm planktonic foraminifera, we suggest (1) that the oxygen isotope ratio used for estimating Oligocene paleotemperatures might be 1? heavier than Eocene values and further increased for the late Oligocene. This hypothesis implies the presence of a relatively extended ice cap in Antarctica in the early and mid Oligocene, and probably an increase in ice volume during the late Oligocenc: (2) heavier isotope values might be related to an increase in salinity, or (3) by a combination of both ice cap and increase in salinity.

Geochemistry of pore water and sediments from Exmouth Plateau

During Leg 122, a transect was drilled across the Wombat Plateau, a marginal spur of the Exmouth Plateau, complemented by two sites on the Exmouth Plateau proper. In this report, pore-water analyses for major seawater constituents, alkalinity, Ba, Fe, Mn, Li, Sr, Rb, and silica are presented. Large gradients in the pore-water profiles provide evidence of complex sediment/pore-water interactions associated with carbonate and silica diagenesis and the formation of authigenic minerals. Diffusion affects pore-water profiles but differs considerably from site to site. Advection of freshwater, probably of continental origin, helps maintain negative Cl and salinity gradients deep within the sediments of the Exmouth Plateau.

Lithology and geochemistry of ODP Leg 198 sites, Shatsky Rise

Samples of chert, porcellanite, and chalk/limestone from Cretaceous chert-bearing sections recovered during Leg 198 were studied to elucidate the nature and origin of chert color zonations with depth/age. Sedimentary structures, trace fossils, compactional features, sediment composition, texture, geochemistry, and diagenetic history were compared among lithologies. Trends in major and minor element composition were determined. Whereas geochemical analyses demonstrate systematic elemental differences among the different lithologies, there are less distinct patterns in composition for the colored cherts. The color of the chert appears to be related primarily to the amount of silica and secondarily to the proportion of other components. Red cherts are almost pure silica with only minor impurities. This may allow pigmentation from fine Fe oxides to dominate the color. These red cherts are from places where geophysical logs indicate that chert is the dominant rock type of the section. These red chert intervals cannot be unequivocally distinguished from surrounding chert-bearing lithologies in terms of sedimentary structures.

Sedimentological and paleoclimatological investigation of two sediment cores off Cape Barbas, North-West Africa

Two box cores taken off Cape Barbas (North-West Africa) have been studied using three methods. The analyses of the coarse fraction, of biogenic opal and of planktonic foraminifera revealed : 1. Core GIK12310-4 penetrates Z, Y, X and upper part of W zone, whereas core GIK12379-1 penetrates Z and upper part of Y zone. 2. Holocene sedimentation rates are 2.5 cm/1000 y for core GIK12310-4 and 6.0 cm/1000 y for core GIK12379-1. During the Y zone 5 cm/l000 y were sedimented incore GIK12310-4 and > 10-20 cm/1000 y in core GIK12379-1. 3. Paleoclimatohgical results are: arid climate and relatively warm water temperatures during the Holocene (Z zone) and during X zone; humid climate and relatively cool water temperatures within the Wuerm (Y zone) (with a non-dated more arid interval found in the middle part of the Y zone) and in the upper part of the W zone. 4. Increased contents of benthos and radiolaria in the Y zone indicate upwelling. Upwelling, characterized by high content of biogenic opal and low water temperatures, was found in core GIK12310-4 at 250 to 350 cm in the lower part of the Y zone. The plankton/benthos ratio of foraminifera, the benthos/radiolaria ratio and water temperatures derived from planktonic foraminifera, differ in both cores in the Holocene, and are nearly identical during the Wuerm.

Radiolarian faunal turnover across the Oligocene/Miocene boundary in the equatorial Pacific Ocean

The global warming trend of the latest Oligocene was interrupted by several cooling events associated with Antarctic glaciations. These cooling events affected surface water productivity and plankton assemblages. Well-preserved radiolarians were obtained from upper Oligocene to lower Miocene sediments at Ocean Drilling Program (ODP) Leg 199 Sites 1218 and 1219 in the equatorial Pacific, and 110 radiolarian species were identified. Four episodes of significant radiolarian faunal changes were identified: middle late Oligocene (27.5 to 27.3 Ma), latest Oligocene (24.4 Ma), earliest Miocene (23.3 Ma), and middle early Miocene (21.6 Ma). These four episodes approximately coincide with increases and decreases of biogenic silica accumulation rates and increases in delta18O values coded as "Oi" and "Mi" events. These data indicate that Antarctic glaciations were associated with change of siliceous sedimentation patterns and faunal changes in the equatorial Pacific. Radiolarian fauna was divided into three assemblages based on variations in radiolarian productivity, species richness and the composition of dominant species: a late Oligocene assemblage (27.6 to 24.4 Ma), a transitional assemblage (24.4 to 23.3 Ma) and an early Miocene assemblage (23.3 to 21.2 Ma). The late Oligocene assemblage is characterized by relatively high productivity, low species richness and four dominant species of Tholospyris anthophora, Stichocorys subligata, Lophocyrtis nomas and Lithelius spp. The transitional assemblage represents relatively low values of productivity and species richness, and consists of three dominant species of T. anthophora, S. subligata and L. nomas. The characteristics of the early Miocene assemblage are relatively low productivity, but high species richness. The two dominant species present in this assemblage are T. anthophora and Cyrtocapsella tetrapera. The most significant faunal turnover of radiolarians is marked at the boundary between the transitional/early Miocene assemblages.

Multi-proxy analyses of sediment cores from the Pakistan continental margin

Late Holocene laminated sediments from a core transect centred in the oxygen minimum zone (OMZ) impinging at the continental slope off Pakistan indicate stable oxygen minimum conditions for the past 7000 calendar years. High SW-monsoon-controlled biological productivity and enhanced organic matter preservation during this period is reflected in high contents of total organic carbon (TOC) and redox-sensitive elements (Ni, V), as well as by a low-diversity, high-abundance benthic foraminiferal Buliminacea association and high abundance of the planktonic species Globigerina bulloides indicative of upwelling conditions. Surface-water productivity was strongest during SW monsoon maxima. Stable OMZ conditions (reflected by laminated sediments) were found also during warm interstadial events (Preboreal, Bølling-Allerød, and Dansgaard-Oeschger events), as well as during peak glacial times (17-22.5 ka, all ages in calendar years). Sediment mass accumulation rates were at a maximum during the Preboreal and Younger Dryas periods due to strong riverine input and mobilisation of fine-grained sediment coinciding with rapid deglacial sea-level rise, whereas eolian input generally decreased from glacial to interglacial times. In contrast, the occurrence of bioturbated intervals from 7 to 10.5 ka (early Holocene), in the Younger Dryas (11.7-13 ka), from 15 to 17 ka (Heinrich event 1) and from 22.5 to 25 ka (Heinrich event 2) suggests completely different conditions of oxygen-rich bottom waters, extremely low mass and organic carbon accumulation rates, a high-diversity benthic fauna, all indicating lowered surface-water productivity. During these intervals the OMZ was very poorly developed or absent and a sharp fall of the aragonite compensation depth favoured the preservation of pteropods. The abundance of lithogenic proxies suggests aridity and wind transport by northwesterly or northeasterly winds during these periods coinciding with the North Atlantic Heinrich events and dust peaks in the Tibetan Loess records. The correlation of the monsoon-driven OMZ variability in the Arabian Sea with the rapid climatic fluctuations in the high northern latitudes suggests a close coupling between the climates of the high and low latitudes at a global scale.

Temperature reconstruction of sediment cores from the northeastern Arabian Sea

In order to reconstruct the monsoonal variability during the late Holocene we investigated a complete, annually laminated sediment record from the oxygen minimum zone (OMZ) off Pakistan for oxygen isotopes of planktic foraminifera and alkenone-derived sea surface temperatures (SST). Significant SST changes of up to 3°C which cannot be explained by changes in the alkenone-producing coccolithophorid species (inferred from the Gephyrocapsa oceanica / Emiliania huxleyi ratio) suggest that SST changes are driven by changes in the monsoon strength. Our high-(decadal)-resolution data indicate that the late Holocene in the northeastern Arabian Sea was not characterized by a stable uniform climate, as inferred from the Greenland ice cores, but by variations in the dominance of the SW monsoon conditions with significant effects on temperatures. Highest SST fluctuations of up to 3.0°C and 2.5°C were observed for the time interval from 4600 to 3300 years B.P. and during the past 500 years. The significant, short-term SST changes during the past 500 years might be related to climatic instabilities known from the northern latitudes ("Little Ice Age") and confirm global effects. Surface salinity values, reconstructed from delta18O records after correction for temperature-related oxygen isotope fractionation, suggest that in general, the past 5000 years were characterized by higher-than-recent evaporation and more intense SW monsoon conditions. However, between 4600 and 3700 years B.P., evaporation dropped, SW monsoon weakened, and NE monsoon conditions were comparatively enhanced. For the past 1500 years we infer strongly fluctuating monsoon conditions. Comparisons of reconstructed salinity records with ice accumulation data from published Tibetan ice core and Tibetan tree ring width data reveal that during the past 2000 years, enhanced evaporation in the northeastern Arabian Sea correlates with periods of increased ice accumulation in Tibet, and vice versa. This suggests a strong climatic relationship between both monsoon-controlled areas.

Benthic foraminifera in Lower Cretaceous sediments from various DSDP samples

From the DSDP Legs 1, 11, 13, 17, 25, 27, 32, 36, 41, 43, 44, 50, and 62 the Lower Cretaceous foraminifers have been investigated for biostratigraphical, taxonomical, and palaeoecological purposes. An overview of the cored Lower Cretaceous sections of Leg 1-80 is given. In the Northern Atlantic Ocean characteristic foraminiferal faunas are missing from the Upper Tithonian to the Valanginian due to a marked regression which caused hiatuses. In areas without black shale conditions Valanginian to Barremian medium rich to poor microfaunas with Praedorothia ouachensis (Sigal) of the Praedorothia ouachensis Zone (Valanginian-Hauterivian). The Hauterivian-Aptian interval is characterized by zones of Gavelinella barrerniana, Gaudryina dividens, and Conorotalites aptiensis. During the Albian a world-wide fauna consisting of agglutinated and calcareous foraminifers of the Pseudoclavulina gaultina Zone is established in areas lacking the wide-spread black-shale conditions. The Upper Albian and the Cenomanian are represented by the Gavelinella eenomanica Zone. Some ornamented species of the nodosariids (Citharina, Lenticulina), Gavelinella, Conorotatites, Pleurostomella, Vatvulineria, and Osangularia are of some importance for the biostratigraphy of the Berriasian-Albian interval. The Berriasian to Albian zones introduced for the Tethys and the DSDP by Moullade (1984) could only be of some local importance due to the long stratigraphical range of the foraminiferal species used. In the Indian Ocean an exact stratigraphical age cannot be assigned to the few Neocomian foraminiferal faunas of a cooler sea water (Site 261). These faunas mainly contain primitive agglutinated foraminifers, because in most cases the calcareous tests are dissolved or redeposited. In the Pacific Ocean most of the Berriasian to Aptian microfaunas are of minor biostratigraphical and palaeoecological importance for reasons of poor core recoveries, contaminations or original foraminiferal poverty (black shales). Since the Albian there are somewhat higher-diverse faunas of calcareous and agglutinated foraminifers with index species of the Pseudoclavulina gaultina Zone. As a rule, the boundary Albian/Cenomanian is set by means of planktonic foraminifers because no other foraminifer has its first appearance datum during this interval, except Gavelinella cenornanica. During the Albian very uniform, world-wide foraminiferal faunas without a marked provincialism are obvious.