Stable isotopic study of Oligocene-Miocene sediments from DSDP Site 354, Equatorial Atlantic

The oxygen- and carbon-isotope compositions of planktic and benthic foraminifera and calcareous nannofossils from Middle Oligocene-Early Miocene Equatorial Atlantic sediments (DSDP Site 354) indicate two important paleoceanographic changes, in the Late Oligocene (foraminiferal Zone P.21) and in the Early Miocene (foraminiferal Zone N.5). The first change, reflected by a δ¹⁸O increase of 1.45‰ inGlobigerina venezuelana, affected only intermediate pelagic and not surface, deep or bottom waters. The second change affected surface and intermediate waters, whereas deep and bottom waters showed only minor fluctuations. In the case of the former the isotope effect of the moderate ice accumulation on the Antarctic continent is amplified in the Equatorial Atlantic by changes in the circulation pattern. The latter paleoceanographic change, reflected by a significant increase in¹⁸O in both planktic and benthic forms (about 1.0‰ and 0.5‰, respectively), may have been caused by ice volume increase and temperature decrease. Both oxygen- and carbon-isotope compositions indicate a marked depth-habitat stratification for planktic foraminifera and calcareous nannofossils. Three different dwelling groups are recognized: shallowGlobigerinoides, Globoquadrina dehiscens, Globorotalia mayeri and nannofossils; intermediateGlobigerina venezuelana; and deepCatapsydrax dissimilis. The comparison of foraminifera and calcareous nannofossils suggests that the isotopic compositions of nannofossils are generally controlled by the same parameters which control the isotopic composition of shallow-dwelling foraminifera, but the former are more enriched in¹⁸O.     

Spatial distribution of Late Pliensbachian (Early Jurassic) calcareous nannofossils within the Lusitanian basin (Portugal)

Quantitative analyses of Pliensbachian calcareous nannofossils have been carried out on a proximal-distal transect in the Lusitanian Basin (Portugal). The studied sections Vale Venteiro near Tomar and Peniche represent proximal and distal environments with respect to the emerged land of the Iberian Meseta (to the East). The upper portion of the Vale das Fontes Fm (Davoei and Margaritatus ammonite Zones) is studied in both sections and correlated by means of ammonite and nannofossil integrated biostratigraphy. A careful analysis of the preservation state of nannofossils is performed. Preservation state is moderate to good in the two settings; changes in nannofossil assemblages are therefore considered as primary. Samples were analysed for nannofossil absolute and relative abundances, species diversity and wt%CaCO₃. The analysis of calcareous nannofossils (absolute abundance, percentage, average abundance) shows changes in the community structure in space (i.e., from proximal to distal), and vertically within the water column. This pattern suggests a partitioning of habitats within the photic zone, and with respect to emerged lands. Proximal environments within the Lusitanian Basin were probably more eutrophic, because of the proximity of emerged lands to the East (Iberian Meseta). This was the source area for nutrients delivered into the basin by river transport. Distal environments were likely characterized by a greater water depth and by a deeper light penetration leading to a relatively more expanded photic zone. The assemblages recorded in the proximal setting exhibit high mean relative abundance of placolith-bearing coccolithophorids (Lotharingius spp., Biscutum spp. and Similiscutum spp.) and of Schizosphaerella spp., while distal assemblages are dominated by Crepidolithus crassus and Schizospharella spp. Crepidolithus crassus is interpreted as a deep-dweller coccolithophorid, inhabiting preferentially distal and deeper settings in the Lusitanian Basin. Placolith-bearing coccolithophorids were more abundant in proximal settings with respect to emerged lands, under relatively elevated trophic conditions. The probable calcareous dinocyst Schizospharella spp. proliferated in surface waters of both proximal and distal environments.     

Response of calcareous nannofossil assemblages to paleoenvironmental changes through the mid-Pleistocene revolution at Site 1090 (Southern Ocean)

Quantitative study on calcareous nannofossil assemblages has been performed in high time resolution (2–3 kyr) at the Ocean Drilling Program Site 1090. The location of this site in the Southern Ocean is crucial for the comprehension of thermohaline circulation and frontal boundary dynamics, and for testing the employ of nannoflora as paleoceanographical tool. The chronologically well constrained investigated record spans between Marine Isotope Stage (MIS) 35 and 15, through an interval of global paleoclimate and paleoceanographical modification also known as mid-Pleistocene revolution (MPR). Measures of ecological (Shannon–Weaver diversity and paleoproductivity) and dissolution indices together with spectral and wavelet analyses carried out on the acquired time series provide valuable information for interpretation of data in terms of paleoecology and paleoceanography. Assemblages are mainly represented by dominant small Gephyrocapsa, common Calcidiscus leptoporus s.l., Coccolithus pelagicus s.l., Gephyrocapsa (4-5.5 μm), the extinct Pseudoemiliania lacunosa and Reticulofenestra spp. (R. asanoi and Reticulofenestra sp.). Morphotypes discriminated within Calcidiscus leptoporus s.l. and Coccolithus pelagicus s.l., reveal that they may have had different ecological preferences during Pleistocene with respect to the present. The composition and fluctuation in nannofossil assemblage and their comparison with the available Sea Surface Temperature (SST) and C-org curves suggest a primary ecological response to paleoenvironmental changes; relationships to different surface water features and boundary dynamics, as well as to different efficiencies and motions of the intermediate and deep water masses have been inferred. A more northward position of Subantarctic Front (SAF) during most of the Early Pleistocene record has been highlighted based on assemblage composition characterised by common Calcidiscus leptoporus s.l., Coccolithus pelagicus s.l., medium Gephyrocapsa (4–5.5 μm), and by the rarity or absence of Umbilicosphaera spp., Rhabdosphaera spp., Pontosphaera spp., Oolithotus fragilis. Exceptions are the more intense interglacials MIS 31, 17, and probably MIS 15, when a southward displacement of frontal system occurred, coincident with peaks in abundance of Helicosphaera spp. and Syracosphaera spp. Higher nutrient content and more dynamic conditions occurred between MIS 32 and MIS 25, in relation to shallower location of nutrient-rich Antarctic Intermediate Water (AAIW) core and to reduction of glacial–interglacial variability. A nannofossil barren interval is coincident with the known stagnation of South Atlantic deep water circulation during MIS 24, when North Atlantic Deep Water (NADW) was reduced or suppressed and an enhanced northward deep penetration of the more corrosive Circumpolar Deep Water (CPDW) took place. An event of strong instability in nutricline dynamics characterised the transition MIS 23–22 as suggested by sharp fluctuations in paleoproductivity proxies, linked to major changes in oceanographic circulation and to the first distinct increase of larger ice volumes at this time. From MIS 21 upward the nannofossil variations seem to be primarily controlled by glacial–interglacial cyclicity and temperature fluctuations. The cyclic fluctuation recognised in nannofossil abundance seems to be linked to orbitally-forced climatic variation, primarily to the obliquity periodicity recorded in the patterns of C. leptoporus intermediate (5–8 μm) and C. pelagicus pelagicus (6–10 μm); however no obvious and linear relations may be always observed between nannoflora fluctuation and Milankovitch parameters, suggesting more complex and unclear relationships between nannofossils and environmental change.     

Late Cretaceous (Cenomanian–Maastrichtian) calcareous nannofossils from Goban Spur (DSDP Sites 549, 551): Implications for the palaeoceanography of the proto North Atlantic

The early late Cretaceous (Cenomanian–early Turonian) is thought to have been one of the warmest periods of the Phanerozoic. This period was characterised by tropical sea surface temperatures of up to 36 °C and a pole-to-equator-gradient of less than 10 °C. The subsequent Turonian–Maastrichtian was characterised by a continuous climatic cooling, peaking in the Maastrichtian. This climatic cooling and the resulting palaeoceanographic changes had an impact on planktic primary producer communities including calcareous nannofossils. In order to gain a better understanding of these Cenomanian–Maastrichtian palaeoceanographic changes, calcareous nannofossils have been studied from the proto North Atlantic (Goban Spur, DSDP Sites 549, 551). In order to see potential differences between open oceanic and shelf dwelling nannofossils, the data from Goban Spur have been compared to findings from the European shelf (northern Germany).A total of 77 samples from Goban Spur were studied for calcareous nannofossils revealing abundant (mean 6.2 billion specimens/g sediment) and highly diverse (mean 63 species/sample) nannofossil assemblages. The dominant taxa are Watznaueria spp. (mean 30.7%), Prediscosphaera spp. (mean 18.3%), Zeugrhabdotus spp. (mean 8.3%), Retecapsa spp. (mean 7.2%) and Biscutum spp. (mean 6.6%). The Cenomanian assemblages of both Goban Spur (open ocean) and Wunstorf (shelf) are characterised by elevated abundances of high fertility taxa like Biscutum spp., Zeugrhabdotus spp. and Tranolithus orionatus. Early Turonian to Maastrichtian calcareous nannofossil assemblages of Goban Spur are, however, quite different to those described from European sections. Oceanic taxa like Watznaueria spp., Retecapsa spp. and Cribrosphearella ehrenbergii dominate in Goban Spur whereas the fertility indicators Biscutum spp. and T. orionatus are more abundant in the European shelf assemblages. This shift from a homogeneous distribution of calcareous nannofossils in the Cenomanian towards a heterogeneous one in the Turonian–Maastrichtian implies a change of the ocean circulation. The “eddy ocean” system of the Cenomanian was replaced by an oceanic circulation similar to the modern one in the Turonian–Maastrichtian, caused by the cooling. The increased pole-to-equator-gradients resulted in an oceanic circulation similar to the modern one.     

The warm interglacial Marine Isotope Stage 31: Evidences from the calcareous nannofossil assemblages at Site 1090 (Southern Ocean)

Calcareous nannofossil assemblages have been investigated at Ocean Drilling Program (ODP) Site 1090 located in the modern Subantarctic Zone, through the Pleistocene Marine Isotope Stages (MIS) 34–29, between 1150 and 1000 ka. A previously developed age model and new biostratigraphic constraints provide a reliable chronological framework for the studied section and allow correlation with other records. Two relevant biostratigraphic events have been identified: the First Common Occurrence of Reticulofenestra asanoi, distinctly correlated to MIS 31–32; the re-entry of medium Gephyrocapsa at MIS 29, unexpectedly similar to what was observed at low latitude sites.The composition of the calcareous nannofossil assemblage permits identification of three intervals (I–III). Intervals I and III, correlated to MIS 34–32 and MIS 30–29 respectively, are identified as characteristic of water masses located south of the Subtropical Front and reflecting the southern border of Subantarctic Zone, at the transition with the Polar Front Zone. This evidence is consistent with the hypothesis of a northward shift of the frontal system in the early Pleistocene with respect to the present position and therefore a northernmost location of the Subantarctic Front. During interval II, which is correlated to MIS 31, calcareous nannofossil assemblages display the most significant change, characterized by a distinct increase of Syracosphaera spp. and Helicosphaera carteri, lasting about 20 ky. An integrated analysis of calcareous nannofossil abundances and few mineralogical proxies suggests that during interval II, Site 1090 experienced the influence of subtropical waters, possibly related to a southward migration of the Subtropical Front, coupled with an expansion of the warmer Agulhas Current at the core location. This pronounced warming event is associated to a minimum in the austral summer insolation. The present results provide a broader framework on the Mid-Pleistocene dynamic of the ocean frontal system in the Atlantic sector of the Southern Ocean, as well as additional evidence on the variability of the Indian–Atlantic ocean exchange.     

Calcareous nannofossil biostratigraphic framework for middle Eocene sediments from ODP Hole 1260A, Demerara Rise

Ocean Drilling Program (ODP) Site 1260 recovered a near-continuous and expanded (187-m thick) middle Eocene carbonate sequence at mid-bathyal depths on Demerara Rise off Suriname, South America. A calcareous nannofossil biostratigraphic framework has been established for the sequence to aid multi-proxy cyclostratigraphic analyses. The diversity of calcareous nannofossils is reasonably high and preservation moderate throughout most of the section, and the proximity to the continent is indicated by the occurrence of braarudosphaerids in the lower three quarters of the section. The species richness of sphenoliths is particularly high, and our data confirm the occurrence of new sphenoliths during the middle Eocene as shown recently by Bown and Dunkley Jones (2006). In particular we corroborate the occurrences of Sphenolithus strigosus and S. runus that these authors found in their Zone NP16 Tanzania sediments, however we extend the ranges of these taxa down into Zone NP15 (CP13). Moreover we document the occurrence of Bramletteius serraculoides in Zone NP15 (CP13b) and the occurrence of Sphenolithus predistentus in NP16 (CP14a), extending their known ranges down column.     

Calcareous nannofossils and foraminifers herald the Messinian Salinity Crisis: The Pollenzo section (Alba, Cuneo; NW Italy)

During the Messinian, the Mediterranean area experienced fast and prominent paleoenvironmental changes, culminating in the so-called Messinian Salinity Crisis, with the deposition of the evaporitic series. This work investigates the micropaleontological assemblages in the pre-evaporitic sediments of the Sant’Agata Fossili Marls (SAF) of the Pollenzo section (Cuneo area, North Western Italy). A semiquantitative analysis is carried out on the upper part of the marly and pelitic sediments of the SAF underlying the first gypsum bed, ascribed to the Vena del Gesso Fm. (VDF). The studied interval belongs to the planktonic foraminifer Globorotalia conomiozea Zone and “non distinctive Zone” of Iaccarino and to the calcareous nannofossil MNN11b/c Zone of Raffi et al. (1998, 2003) ( [Raffi et al., 1998] and [Raffi et al., 2003]). Decrease of diversity and abundance of the foraminifer and calcareous nannofossil assemblages is recorded 12 m below the VDG and clearly reflects environmental stress. From bottom to top, six paleoecological events are recorded: (1) the first peak abundance of “small” Reticulofenestra and the last recovery (LR) of planktonic foraminifers; (2) the peak abundance of Pontosphaera japonica and the last recovery of warm water taxa Discoaster spp.; (3) the last recovery of benthic foraminifers; (4) the co-occurring peak abundances of Helicosphaera carteri and Sphenolithus abies, and the last recovery of warm water taxa Amaurolithus spp.; (5) the second peak of “small” Reticulofenestra; (6) the definitive disappearance of calcareous nannofossils. These paleoecological events describe a progressive isolation of the basin from the world ocean and increasingly stressed environment (LR planktonic foraminifers; LR Discoaster spp.), increasing dysoxic to anoxic conditions at the sea floor (LR benthic foraminifers), shallowing of the water column (peak of H. carteri), increasing salinity in surface waters (peak of S. abies), and enhanced nutrient concentration in surface waters (peak of “small” Reticulofenestra); these are related to paleoenvironmental changes predating gypsum deposition at Pollenzo and affecting the whole Mediterranean basin.     

Calcareous nannofossils of the Cenomanian/Turonian boundary interval from the Boreal Realm (Wunstorf,northwest Germany)

The Cenomanian/Turonian boundary interval (CTBI) is marked by an intense climatic warming presumably caused by large magmatic eruptions. This warming was characterised by one of the most prominent Mesozoic perturbations of the carbon cycle, the Oceanic Anoxic Event 2 (OAE2), which is marked by a well pronounced positive carbon isotope excursion (CIE). Sediments of the OAE2-interval often consist of organic rich black shales suggesting widespread bottom water anoxia during the CTBI. This study focuses on calcareous nannofossils from the CTBI of a European section (Wunstorf Core; northwest Germany). A total of 105 samples were examined for calcareous nannofossils using the settling technique. Eight bioevents (last occurrences: Corollithion kennedyi, Lithraphidites acutus; first occurrences: Rotelapillus biarcus, Corollithion exiguum, Eprolithus octopetalus, Eprolithus eptapetalus, Quadrum gartneri, Eiffellithus eximius) have been recognised throughout the middle Cenomanian to middle Turonian interval. With the exception of eleven samples preservation is moderate to good. Calcareous nannofossils are abundant (mean 2.0 billion specimens/g sediment) and highly diverse (mean 58 species/sample). Assemblages are dominated by Watznaueria spp. (32.3%), Prediscosphaera spp. (13.4%), Zeugrhabdotus spp. (11.2%) and Biscutum spp. (10.5%). Pre-OAE2 and also post-OAE2 nannofossil assemblages show high abundances of Biscutum spp. (～ 20%) indicative for stable mesotrophic conditions. The assemblages of the OAE2 itself are marked by high values for Watznaueria spp. and low frequencies of Biscutum spp. making oligotrophic conditions during the OAE2 likely. High absolute abundances of organic walled dinoflagellates and the occurrence of frequent stress tolerant nannofossil genera like Retecapsa spp. in the organic rich intervals suggest, however, a deposition of black shales enhanced by high primary productivity. Thus dinoflagellates and calcareous nannofossils are interpreted to reflect different seasonal signals during the time of black shale deposition. Short-term high fertile seasons allowed the blooming of the organic walled dinoflagellates whereas calcareous nannofossils dominated the longer oligotrophic seasons. The black shale deposition was supported by the formation of large amounts of organic matter during fertile seasons as well as by surface water stagnation during oligotrophic seasons.     

A new Oligocene-Miocene calcareous nannofossil species: Pontosphaera wallacei

A high-resolution study of sediments from DSDP Leg 72 Site 516 provided the opportunity to reconstruct a detailed biostratigraphy for the late Oligocene - early Miocene time interval. The detailed taxonomic investigation of calcareous nannofossils, performed by a quantitative analysis, highlighted the presence of a species of Pontosphaera that does not show the morphologic features of any recognized species of this genus. This species, which was previously classified as Pontosphaera sp., is here described as Pontosphaera wallacei nov. sp.     

Middle Miocene high-resolution calcareous plankton biostratigraphy at Site 926 (Leg 154, equatorial Atlantic Ocean): palaeoecological and palaeobiogeographical implications

High-resolution calcareous plankton (planktonic foraminifera and calcareous nannofossils) biostratigraphy is presented from the Middle to early Late Miocene interval (from 14.45 to 8.86 Ma) at Site 926 (ODP Leg 154, equatorial Atlantic Ocean). The main bioevents used in the low-latitude zonal schemes, and also auxiliary events revealing potential biostratigraphic value have been recognised. The investigated succession ranges from N.10 to N.16 Zones based on planktonic foraminifera, and from NN5 (CN4) to NN10 (CN8) Zones based on calcareous nannofossils. The evolution of the planktonic foraminiferal Globorotalia fohsi lineage appears to be environmentally controlled. The main diagnostic features of the species of this lineage are not always evident, rendering problematic the definition of the N.9/N.10, N.10/N.11 and N.11/N.12 zonal boundaries. Calcareous plankton events have been calibrated on the basis of the Astronomical Time Scale of Shackleton and Crowhurst 〚Shackleton, N.J., Crowhurst, S., 1997. Sediment fluxes based on an orbitally tuned time scale 5 Ma to 14 Ma, Site 926. In: Curry, W.B., Shackleton, N.J., Richter, C., Bralower, T.J. (Eds.), Proceedings of the Ocean Drilling Program, Scientific Results, College Station, TX (Ocean Drilling Program) 154, pp. 69–82〛. The astrobiochronology obtained at Site 926 has been compared with that of the Mediterranean astronomically calibrated deep marine successions, allowing the evaluation of the degree of synchroneity and diachroneity of bioevents. Some bioevents, such as the last occurrence of Globigerinoides subquadratus dated at 11.55 Ma, the last occurrence of Sphenolithus heteromorphus dated at 13.51 Ma and the last common occurrence of Cyclicargolithus floridanus calibrated at 13.32 Ma, are near-synchronous events between the equatorial Atlantic and the Mediterranean area indicating their high biostratigratigraphic value in global correlation. The diachroneity of the last occurrence of Paragloborotalia siakensis, the first occurrence of Neogloboquadrina acostaensis and the last occurrence of Globorotalia peripheroronda between equatorial Atlantic and the Mediterranean reflect a different spatial and temporal distribution of these marker species probably due to a sharp definition of surface plankton provinces related to the latitudinal thermal gradient.     

Middle to late Miocene fluctuations in the incipient Benguela Upwelling System revealed by calcareous nannofossil assemblages (ODP Site 1085A)

Middle to late Miocene calcareous nannofossil data of ODP Site 1085 from the eastern South Atlantic off Namibia were analysed to document spatial and temporal changes in surface-ocean circulation, upwelling initiation, and associated productivity.

Our data show that calcareous nannofossils constitute a significant part of the carbonate fraction throughout the investigated interval from 12.5 to 7.7 million years (Ma). Highest numbers of calcareous nannofossils (up to 38,000 × 10⁶ nannofossils g^− 1 sediment) were observed during the intervals 9.9 to 9.7 and 8.7 to 8.0 Ma. These elevated numbers of calcareous nannofossils may generally be linked to the initiation of upwelling at about 10 Ma in the studied region. In contrast, diminished numbers of calcareous nannoplankton, as in the interval 9.6 to 9.0 Ma, probably characterise time intervals of weaker productivity resulting in a decrease of nannofossil carbonate contents in the sediments of Site 1085. This decrease in nannofossil production could be one possible explanation for the major CaCO₃ depression in between 9.6 and 9.0 Ma. Coccoliths of the genus Reticulofenestra are the most abundant taxa. Their occurrences are characterised by changes in the investigated time interval. In addition, Coccolithus pelagicus, Calcidiscus leptoporus and Umbilicosphaera spp. contribute a common part of the assemblage. Calcareous nannofossils account for more than half of the carbonate, with peak contribution up to 80% at 8.8 Ma.     

Pleistocene fluctuations in the Agulhas Current Retroflection based on the calcareous plankton record

Piston core (PS2487-6), recovered south of Cape Town, and sediment surface samples, recovered in the area of the Agulhas Current retroflection, were used to construct paleoceanographic scenarios for the late Quaternary in a region with an important role in global water mass transfer. Coccolithophore (calcareous nannofossil) and planktonic foraminifera assemblages and oxygen isotope data were collected. Stratigraphic control is based on calibration of the δ¹⁸O stratigraphic signals with calcareous nannofossil events that are thought to be synchronous over a broad range of latitudes. Study of the surface sediments permits the characterisation of the Agulhas Current, Subtropical Convergence and Subantarctic coccolithophore assemblages. The Agulhas Current assemblage has relatively high proportions of Florisphaera profunda, Gephyrocapsa oceanica and Umbilicosphaera spp. These species are absent or present in low proportions in subantarctic waters. The abundance of coccolithophores during isotope stages 1, 5 and 7 is characteristic of relatively warm, stratified surface waters, with a deep nutricline and chlorophyll maximum, which strongly suggests that the area was under the influence of the Agulhas Current retroflection. The incursion of Globigerinoides ruber, abundant today in the Agulhas Current, also supports this interpretation. Conversely, during glacial stages 2–4 and 6, a strong reduction in warm and stratified water indicators can be observed, together with an increase in cold-eutrophic species.The interval from isotope stages 8 to 12 displays an assemblage dominated by Gephyrocapsa caribbeanica, the highest values being seen in the so-called Mid-Brunhes event, accompanied by a clear reduction in subtropical Holocene species. During the glacial interval from isotope stages 10–12, G. caribbeanica dominated the assemblage. The ecological interpretation of this species is controversial due to a clear evolutionary overprint. Isotope stage 12 is here interpreted as having been the coldest one in the period studied. It is characterised by a strong increase in Neogloboquadrina pachyderma (dextral and sinistral) and a remarkable decrease in tropical and subtropical planktonic foraminifera. From isotope stage 13 to 18, a hiatus is interpreted. Below this hiatus the stratigraphic resolution is poorer, although the glacial–interglacial cyclicity is well defined in the Agulhas Current Retroflection area (Core PS2487-6) for the last 25 isotope stages. The glacial–interglacial cyclicity is thought to be due to a fluctuation in the Subtropical Convergence Zone, probably linked to the eastward and westward displacement of the Agulhas Current retroflection. In any case, the core studied was always in a subtropical environment, under the influence of the Agulhas Current, which was enhanced during interglacial periods. For the whole of the interval studied, increases in Calcidiscus leptoporus, Umbilicosphaera spp., and Syracosphaera spp. among the calcareous nannofossils, and in G. ruber and Globigerinoides sacculifer within the planktonic foraminifera, clearly respond to interglacial pulses, reaching maximum values during short periods close to the major deglaciations.     

Calcareous nannofossil productivity and carbonate production across the Middle-Late Jurassic transition in the French Subalpine Basin

The distribution pattern of calcareous nannofossils was analysed across the Middle-Late Jurassic transition in the French Subalpine Basin (south-eastern France). This basin is characterized in the hemipelagic-pelagic domain by a continuous sedimentary succession, allowing a good biostratigraphic resolution for this time interval. The nannofossil assemblages are consistently dominated by Watznaueria britannica. However, major changes in trophic and paleoenvironmental conditions are recorded across the Middle-Late Jurassic transition. An increase in marine primary productivity and cooling of surface waters is recorded across the Callovian-Oxfordian boundary, as already shown in the higher latitude setting of the eastern Paris Basin. Increased precipitation and runoff under contrasting seasonal climatic conditions (monsoon-type) has led to eutrophication of marine surface waters in the French Subalpine Basin at this period. Then, decreased runoff and associated nutrients certainly linked to drier climatic conditions lead to a decrease in calcareous nannofossil productivity during the middle part of the Early Oxfordian (mariae-cordatum ammonite Zone transition). At the Early-Middle Oxfordian transition, more favourable conditions for the nannofossil community (warmer and mesotrophic surface waters) prevailed. The pelagic (nannofossil) carbonate contribution is limited, and the carbonate fraction is predominantly of nektonic/benthic origin at the Callovian-Oxfordian transition and of allochthonous origin from carbonate platforms at the Early Oxfordian-Middle Oxfordian transition.     

Evidence of increasing surface water oligotrophy during the Campanian–Maastrichtian boundary interval: Calcareous nannofossils from DSDP Hole 390A (Blake Nose)

The latest Cretaceous (Campanian–Maastrichtian) is characterized by several global cooling and intermittent warming events. These climatic changes influenced the palaeoceanography substantially, including changes of the deep water sources and surface water currents. One of the most prominent episodes of climatic cooling occurred during the Campanian–Maastrichtian transition. This study focuses on the palaeoclimate and palaeoceanography of the Campanian–Maastrichtian transition by analysing the calcareous nannofossils of DSDP Hole 390A (139.92–126.15 mbsf; Blake Nose). For the examination of calcareous nannofossils sixty samples were processed using the settling technique. Biostratigraphical index taxa (Broinsonia parca constricta, Uniplanarius trifidus, and Tranolithus orionatus) suggest a late Campanian age for the major part of the studied section. The calcareous nannofossils are well preserved, highly abundant (6.80 billion specimens/gram sediment) and diverse (80 species/sample). The assemblages are dominated by Prediscosphaera spp. (20.5%), Watznaueria spp. (20.3%) and Retecapsa spp. (9.8%). Cool water taxa (Ahmuellerella octoradiata, Gartnerago segmentatum, and Kamptnerius magnificus), however, appear less frequently and do not exceed more than 1%. Due to their rarity these cool water taxa do not support the existence of an intense cooling phase during the Campanian–Maastrichtian transition at DSDP Hole 390A. Around 133 mbsf several nannofossil taxa, however, show a distinctive turnover. Mesotrophic species like Discorhabdus ignotus, Zeugrhabdotus bicrescenticus and Zygodiscus exmouthiensis are abundant below 133 mbsf, whereas oligotrophic taxa like Watznaueria spp., Eiffellithus spp. and Staurolithites flavus become common above this level. These changes imply a decrease in the input of nutrients, perhaps caused by a reorganization of ocean currents (Palaeo Gulf Stream) and reduced upwelling.     

Pliensbachian (Early Jurassic) calcareous nannofossils from the Peniche section (Lusitanian Basin,Portugal): A clue for palaeoenvironmental reconstructions

Quantitative analysis of Early Jurassic calcareous nannofossil assemblages from the Peniche section in Portugal have been performed in order to interpret palaeoenvironmental changes occurring in the Lusitanian Basin during the Late Pliensbachian Davoei, Margaritatus and Spinatum Ammonite Zones. Nannofossil data are compared to already published carbon and oxygen stable isotope data, organic matter content (wt.%Total Organic Carbon, TOC), and biomarker analysis. A significant change in calcareous nannofossil assemblages and species diversity at the transition between the Margaritatus and Spinatum Ammonite Zones matches with the pattern shown by geochemical data. This suggests that a profound change in environmental conditions occurred at that time. In the Davoei and Margaritatus Ammonite Zones, in a context of general sea-level rise, the Lusitanian Basin was characterized by water column stratification that favoured the sedimentation and preservation of organic matter. Biomarkers and oxygen isotope trends suggest that stratification of water masses occurred because of low salinity in surface waters. The shallowest part of the water column, characterized by oligotrophic conditions, was inhabited by the (probable) calcareous dinocyst Schizosphaerella spp., while the deep-dweller Crepidolithus crassus flourished in the lower photic-zone layers. In the Spinatum Ammonite Zone, a regressive trend occurred and a salinity increase is inferred on the basis of oxygen isotope values. Water masses were probably less stratified at that time. The upper photic-zone nannofossil assemblages were still dominated by Schizosphaerella spp. whilst, in the deep photic-zone, Mitrolithus jansae (a Mediterranean taxon) replaced C. crassus (a taxon with NW-European affinity). This pattern may indicate a change in palaeoceanographic conditions related to surface current circulation. The sea-level fall occurring during the Spinatum Ammonite Zone may have resulted in the partial isolation of the Lusitanian Basin from the NW-Europe basins because of the creation of a threshold. Alternatively, tectonic drowning of North African carbonate platforms in the Late Pliensbachian may have promoted better exchanges of nannoplankton between the Lusitanian Basin and the Mediterranean Tethys via the creation of new seaway connections.     

Fertilization of the northwestern Tethys (Vocontian basin,SE France) during the Valanginian carbon isotope perturbation: Evidence from calcareous nannofossils and trace element data

A high resolution calcareous nannofossil study associated with a geochemical analysis (major, trace elements, and carbon and oxygen isotope stratigraphies) was carried out in the Angles section (hemipelagic setting of the Vocontian basin, SE France) during the Valanginian positive carbon isotope excursion. The behaviour of calcareous nannofossil taxa in relation to fertility conditions was studied to elaborate new nutrient indices in this environment: a high nutrient index based on Biscutum spp., Discorhabdus rotatorius, Zeugrhabdotus fissus, (high fertility indicators) and Watznaueria barnesae (low fertility indicator); and a medium nutrient index based on Lithraphidites carniolensis (medium fertility indicator) and W. barnesae (low fertility indicator). These two indices show a major fertilization from the Stephanophorus ammonite Zone to the Trinodosum ammonite Zone, with a maximum during the positive carbon isotope excursion.Since high values of the nutrient indices are in phase with high values of chemical elements related to terrigenous material and low values of the coccolith total abundance, it is proposed that pulses of detrital inputs into the basin triggered the nutrification which, in turn, caused a biocalcification crisis of the calcareous nannofossils. Nutrification is also responsible for the reef demise in the surrounded platforms, as indicated by the increased Sr/Ca seawater ratio at that time.The intensification of the Paranà–Etendeka volcanic activity, triggering CO₂ excess in the atmosphere, is probably responsible for an acceleration of the hydrological cycle, the increased weathering, and the subsequent higher terrigenous and nutrient transfer from continents to the Vocontian basin. In such a scenario, nutrification is a dominant factor controlling neritic and hemipelagic biocalcification. However, one cannot exclude that the global increase of atmospheric CO₂ could generate chemical changes of the sea-surface waters, acting with the nutrification, to modify the biocalcification of the carbonate producers.     

Malacofauna Cuaternaria del litoral norte de Santa Cruz (Patagonia, Argentina)

The molluscan content from three “marine terraces” (TM) preserved along 250 km of the Santa Cruz province provided 49 molluscan taxa (33 Gastropoda and 16 Bivalvia) and the first Quaternary records for Ataxocerithium pullum, Crepidula protea, C. aculeata, Adelomelon ferrusacii, Brachidontes rodriguezi, Aequipecten tehuelchus, Ostrea tehuelcha, and Solen tehuelchus. The mollusca assemblages suggest typical marine nearshore conditions similar to the adjacent modern littoral (Southwestern Atlantic), with mostly hard substrates, shallow and cold waters. The TMVI (+ 5-15 m a.s.l., ca. 8-1.2 ka A.P.) is mainly characterized by dominance of Brachidontes spp., Nacella spp. and Mytilus edulis, associated with Siphonaria lessoni. No evidence has been found for the TMVI of clear biotic responses or strong palaeobiogeographic effects during the time span of the Climatic Optimum (“Hypsithermal”, mid-Holocene) which has been documented for other coastal areas northward in Argentina, down to ca. 44-45° S. Scarce differences distinguish the TMV (+ 15-30 m a.s.l., ca. 108-137 ka A.P., MIS5?) and TMIV (+ 20-30 m a.s.l., ca. 242 ka A.P.) regarding the Holocene and the modern littoral. Within the TMV, Clausinella, Eurhomalea, Solen tehuelchus and Brachidontes cf. darwinianus (typically in warmer waters) are exclusive; in the TMIV, Trophon, Buccinanops, Brachidontes and Pectinidae are characteristic. Predominance of Trophon spp. (typically in cold Magallanic waters) in modern samples vs. scarcity or absence in the TM suggest slightly lower modern SST. Several taxonomic similitudes with the Pacific Sudamerican coast (e.g., Fissurella,Nacella, Tegula, Crepidula, Trochita, and Acanthina) could respond to cold currents helping cold-water immigrants to join southern Southamerican Atlantic after the opening of the Drake Passage.     

Patterns of planktonic foraminiferal abundance and diversity in surface sediments of the western North Atlantic

Seventy-two core tops and grab samples from the western North Atlantic were analyzed to determine what aspects of planktonic foraminiferal abundance and diversity are most closely related to ocean circulation. Some species appear to be reliable indicators of the Gulf Stream, a warm surface current. Both Globorotalia menardii and Globigerinoides sacculifer have their highest abundances under the main trend of the Gulf Stream. Globorotalia inflata reaches high abundances in the cold slope water north of the Gulf Stream but its distribution is not as continuous as the Gulf Stream indicators.Contoured values of species diversity, the Shannon diversity index, and species equitability also reflect surface circulation. A plot of species diversity (number of species) shows a poorly defined region of high diversity beneath the major trend of the Gulf Stream. Use of the Shannon diversity index enhances and clarifies this region of high diversity. A map of species equitability shows a broad belt of low species dominance (high equitability) beneath the Gulf Stream. North of the Gulf Stream, a tongue of high dominance (low equitability) corresponds to the increased relative abundance of Globorotalia inflata.High diversity of planktonic foraminifera in bottom sediments characterizes the warm shifting surface currents of the Gulf Stream; low diversity is typical of slope and Sargasso Sea waters. Low equitability (high species dominance) indicates either cold currents or gyre center waters. Maps of foraminiferal diversity and equitability for other intervals of geologic time may be useful in tracing the evolution of ocean circulation.     

Early Cretaceous (Valanginian - Hauterivian) calcareous nannofossils and isotopes of the northern hemisphere: proxies for the understanding of Cretaceous climate

From three boreholes (DSDP Site 535; ODP Site 638; BGS borehole 81/43) of the Central Atlantic and the North Sea Basin 379 samples of early Cretaceous age (Valanginian-Hauterivian) were examined. The localities cover a S-N transect of approximately 3000 km stretching from 17°N to 40°N palaeolatitude. The distribution of calcareous nannofossils and fluctuations of the stable isotopes (δ¹³C, δ¹⁸O) have been recorded and were compared with results of recent studies. We differentiate between high nutrient indicators and oligotrophic taxa and propose a four step scheme to characterize the trophic level of the surface water. (1) High abundances of the fertility group (Biscutum constans/Zeugrhabdotus spp.) combined with a high dominance of B. constans and low abundances of Watznaueria barnesae/W. fossacincta represent a high nutrient environment (eutrophic setting). (2) High abundances of the fertility group combined with a high dominance of Zeugrhabdotus spp. and low abundances of W. barnesae/W. fossacincta reflect enhanced nutrient contents of the surface water (mesotrophic setting). (3) Enhanced abundances of the fertility group combined with high abundances of W. barnesae/W. fossacincta indicate slightly increased nutrient contents of the surface water (meso- to oligotrophic setting). (4) Low abundances of the fertility group and high abundances of W. barnesae/W. fossacincta are of low nutrient affinities (oligotrophic setting). Our estimations of seawater palaeotemperatures in combination with literature data show a distinctive trend for the Valanginian to Hauterivian interval. A general decrease of water temperature from the Valanginian to the early Hauterivian is obvious. This decrease of temperature coincides with the southward migration of the high latitudinal cold water species Crucibiscutum salebrosum to lower latitudes. Our findings shed new light on the evolution of the earliest Cretaceous climate, which may be characterized as a warm greenhouse world with interludes of short cooling.     

Albian calcareous nannofossils from ODP Site 1258, Demerara Rise

Site 1258, drilled during Ocean Drilling Program (ODP) Leg 207 on Demerara Rise off the northern coast of South America, recovered organic-rich Albian sediments that yield abundant, moderately to well preserved calcareous nannofossils. Biostratigraphic analysis shows the section primarily spans Roth’s (Initial Reports DSDP 44. US Government Printing Office, Washington (1978) 731) middle to late Albian Zone NC9. A disconformity separates these sediments from overlying uppermost Albian laminated shales from Zone NC10. The presence of Seribiscutum primitivum within the Albian section represents the first known occurrences of this species at such low latitudes, as Demerara Rise was located within 15° of the equator during the mid-Cretaceous. This species exhibits a bipolar distribution and is considered a cool-water, high-latitude species. Its presence on Demerara Rise indicates cooler water incursions either through changes in surface circulation or upwelling conditions during the opening of the Equatorial Atlantic.