Nannofossil Assemblage Composition Research Articles

Calcareous nannofossils across the Eocene-Oligocene transition: Preservation signals and biostratigraphic remarks from ODP Site 1209 (NW Pacific, Shatsky Rise) and IODP Hole U1411B (NW Atlantic Ocean, Newfoundland Ridge)

This work provides a detailed biostratigraphic correlation through the Eocene-Oligocene Transition (EOT), based on an integrated stratigraphic approach and the study of calcareous nannofossils, between two disparate sites, one in the NW Atlantic (IODP Hole U1411B) and one in the NW Pacific (ODP Site 1209).The precise site-to-site correlation provided by these data allows for a comparison of carbonate preservation across the EOT including identification of the main post-depositional processes that impact the calcareous nannofossil ooze at Site 1209. The main aim of this work is to understand the extent to which the bulk δ18O and δ13C records and their sources (mainly calcareous nannofossils) are altered by diagenesis. Our detailed SEM study highlights some differences before, during and after the EOT, suggesting local diagenetic dynamics.At Site 1209, a distinctive change, both in nannofossil assemblage composition and preservation state, is observed from the pre-EOT phase to the Late Eocene Event (LEE), with a shift in the dominant process from dissolution to recrystallisation. Surprisingly, despite the overall poor preservation, only the interval between 141 and 142.4 (adj. rmcd) was compromised in term of isotopic values and assemblage diversity and abundance. This interval, recorded in the upper Eocene, was characterized by severe dissolution, concomitant with deposition of secondary calcite on solution-resistant forms. Diagenetic processes have strongly biased the δ18O isotopic signal, resulting in a positive oxygen isotope anomaly through the upper Eocene that is difficult to reconcile with other published trends. For the remaining time intervals, diagenesis seems not to have altered the bulk δ18O profile, which closely resembles that of other sites across the world, and is particularly consistent with other data from the Pacific Ocean. In summary, the impact of diagenesis on nannofossil preservation even if clearly visible both in SEM and optical microscope observations does not always cause a pervasive alteration of the primary isotopic signal and can instead provide important clues on local depositional dynamics.

High‐resolution Pliocene–Pleistocene calcareous nannofossil distribution and palaeoenvironmental changes in the northwest Nile Delta, Egypt

The Pliocene–Pleistocene sequences in the Nile Delta are of immense economic significance due to their hydrocarbon potential. Hence, a better understanding of the time-calibrated basinal palaeoenvironmental changes is essential to improve the hydrocarbon recovery. A total of 182 sub-surface samples from the Sapphire-DC well in the offshore north-western part of the Nile Delta were analysed to reveal changes in calcareous nannofossil assemblage composition and palaeoenvironments during the late Miocene–Pleistocene (5.44–0.43 Ma). Sixteen marker bioevents and nine biozones are identified for the first time. The absence of Ceratolithus acutus suggests a hiatus at the Messinian–Zanclean (Miocene–Pliocene) boundary. The studied interval is marked by the dominance of five species; Pseudoemiliania lacunosa (24%), small Gephyrocapsa sp. (20%), Helicosphaera kamptneri (11%), Reticulofenestra minutula (10%) and Calcidiscus leptoporus (6%). Based on their distribution patterns, six intervals (Intervals 0 to 5) are identified. Throughout the study interval, cooler waters (except during Interval 0; Messinian Salinity Crisis, MSC, 5.44–5.26 Ma) and eutrophic conditions persisted within a shallow hemipelagic and near-continental setting, marked by moderate-to-high terrigenous input (more for Intervals 4 and 5; ˜2.73–0.42). Additionally, low species diversity and high species dominance are identified, suggesting a stressed environment. Species diversity shows a gradually declining trend, up section, whereas species dominance correspondingly increases. Interval 0 (MSC) is dominated by warm water species of Sphenolithus abies and Helicosphaera kamptneri. The cool water species, Reticulofenestra pseudoumbilica, and small Gephyrocapsa dominate in Interval 1; small Gephyrocapsa dominates in Interval 2. Interval 3 is marked by the deepening of the basin with a brief introduction of warm, somewhat oligotrophic and open-ocean waters. In the middle of this interval (at 3.73 Ma; the Early–Late Pliocene boundary), an abrupt decrease in species diversity with a corresponding increase in species dominance suggests deteriorating conditions. Interval 3 also encompasses the mid–late Pliocene warming period, which in the Nile Delta is marked by cooler waters. Intervals 3–5 are dominated by P. lacunosa, suggesting a continuation of cool waters and eutrophic conditions. Hence, throughout the study interval, cooler waters (except during the MSC) and eutrophic conditions within in a shallow hemipelagic and near-continental setting are inferred with moderate-to-high terrigenous input and low species diversity and higher species dominance, suggestive of stressed environmental conditions.

Maastrichtian climate changes – the calcareous nannofossil record from flysch deposits of the Outer Carpathians

The Outer Carpathians are known for a few sections, where transitions from the Upper Cretaceous to the Palaeogene, including the K-Pg boundary interval, were described. One of them, the Bąkowiec section in the Skole Nappe, was examined with reference to the record of biostratigraphy and palaeoenvironmental changes, mainly based on the analysis of calcareous nannofossil assemblages. This study shows no evidence for the K-Pg boundary; however, the presence of the nannofossil species Micula prinsii , marking the topmost Maastrichtian UC20dTP Zone, was noted. The appearance of this low-latitude taxon and relevant changes in the composition of nannofossil assemblages indicate an influx of Tethyan warm water into the northern Carpathian basins during the latest Maastrichtian, shortly before the K-Pg boundary event. Therefore, the authors infer that the upper part of the Bąkowiec section recorded the latest Maastrichtian warming of climate, probably triggered by Deccan volcanic activity.

Absolute nannofossil abundance estimates: Quantifying the pros and cons of different techniques

A quick and inexpensive method to determine absolute nannofossil abundance in deep sea sediments – the “drop” technique (modified dilution method) – was compared to two other available methods – the filtration and random settling techniques. All techniques rely on the same basic principle, under which a volume of known concentration (bulk sediment weight/mL) is distributed evenly over a known total area (glass slide or filter) to then count particles within a set of (randomly) selected fields of view. The three preparation techniques were also calibrated by spiking the samples with microbeads to approach the “real values” as closely as possible. Significant offsets in abundance estimates between methods mainly reflect bias due to the uneven distribution and/or loss of particles. We show that the drop technique is most consistent and accurate in estimating “real values” and offers similar or better reproducibility than the other techniques. The drop method also allows detection of the same trends with or without calibration with microbeads. The filtration method holds the risk to drastically underestimate absolute abundances, while the settling technique is demanding in terms of time and may suffer from advection processes. The composition of nannofossil assemblages can be reliably determined by any of the three different techniques.

Spatial distribution patterns of calcareous nannofossils across the Callovian–Oxfordian transition in the French Subalpine Basin

Callovian–Oxfordian calcareous nannofossils are documented from four sections distributed along a proximal–distal (slope to central basin) transect in the French Subalpine Basin (south-eastern France). The sections were dated and correlated using integrated ammonite, dinoflagellate and nannofossil biostratigraphies. The nannofossil assemblages are constantly dominated by Watznaueria britannica. However, significant spatial changes in nannofossil assemblage composition are recorded. Nannofossil assemblages recovered in slope and slope-to-basin transition sections are characterized by high abundances, lowest diversities, highest percentages of smallest-sized morphotypes of W. britannica, subordinated W. britannica/manivitiae (a morphotype described in this study), and are indicative of turbulent, unstable and/or eutrophic conditions. Towards the central part of the basin, as the nutrient content and turbulence declined and paleoenvironmental stability increased, the nannofossil assemblages become more diverse and characterized by lower relative abundances of W. britannica but higher contribution of larger morphotypes and higher relative abundances of W. barnesiae/fossacincta, Biscutum dubium and Zeugrhabdotus erectus. These latter small coccoliths, markers of high surface-waters fertility for the Cretaceous, were probably not competitive with respect to smaller-sized W. britannica in unstable and eutrophic surface waters for the Jurassic. An increase in the surface water productivity is recorded both in proximal and distal sections at the Callovian–Oxfordian transition, and is demonstrated both by an increase in nannofossil total abundances and in the relative abundances of taxa adapted to high-trophic levels. Thermal minimum in surface waters, as indicated by an increase in the percentages of the cool-water nannofossil taxa, is associated to increased productivity. Further studies will be necessary to demonstrate whether surface-water productivity changes, already recorded in the eastern Paris Basin, were global.

Paleoenvironmental controls on the morphology and abundance of the coccolith Watznaueria britannica (Late Jurassic, southern Germany)

The coccolithophore species Watznaueria britannica is dominant in Middle-Upper Jurassic calcareous nannofossil assemblages and presents morphological variation, including different coccolith size, shape and length of the central area and of the bridge. Six morphotypes can be recognized in the polarizing light microscope. The aim of this work is to better understand the morphological variability of W. britannica and determine if this variability is controlled by paleoecological factors. In order to investigate the potential paleoecological controls on W. britannica morphology and abundance, we carried out a biometric study on a restricted temporal interval: the Late Oxfordian, in the Swabian Alb (southern Germany), characterized by increasing carbonate production linked to climatic changes. The Balingen–Tieringen section, where previous works on sedimentology, nannofossil assemblage composition, and δ 18O and δ 13C analyses were performed, was selected for this study. The variations in morphology and abundances of W. britannica were studied on 40 samples of the Balingen–Tieringen section, presenting variable lithologies and calcium carbonate contents. For each level, seven biometric parameters (coccolith length, width and ellipticity, central area length, width and ellipticity and central area proportion with respect to the coccolith) were measured or calculated on digitally captured images of the first 100 W. britannica coccoliths observed in the light microscope. The relationships between the different biometric variables were described using bivariate and Principal Component Analyses. Biometric parameters and Principal Component factors extracted from nannofossil assemblages as well as other paleoenvironmental proxies, were investigated using regression, and their stratigraphic trends were compared. Principal component analysis of the six biometric variables (3938 measurements) on W. britannica coccoliths shows a reduced morphological variability compared to a significant size gradient. An allometric trend recognized on the total placolith and on the central area within the W. britannica assemblages suggests that the different morphotypes may represent intra-specific variability rather than different species. The general trend through Late Oxfordian shows an increase in size of W. britannica coccoliths, mainly driven by an increase in the contribution of the large morphotypes. Increasing placolith size is associated with drier and warmer climatic conditions during the latest Oxfordian.

Palaeoenvironmental control on sponge-microbialite reefs and contemporaneous deep-shelf marl-limestone deposition (Late Oxfordian, southern Germany)

The Late Oxfordian (Bimammatum Zone) deep-shelf deposits of southern Germany are characterised by abundant reefs that laterally pass into marl-limestone alternations. The bioconstructions are made from three main components, sponges, microbialites and allochthonous sediment. A detailed morphological analysis of sponge-microbialite reefs in the Plettenberg section (Swabian Alb, southern Germany) is combined with investigation of calcareous nannofossil assemblages in laterally correspondent marl-limestone alternations in order to trace the evolution of palaeoenvironmental conditions during reef development. Fluctuations of CaCO3 content in deep-shelf deposits provides evidence of a succession of small-scale lithological units, generally formed by five to six marl-limestone couplets. Within a small-scale lithological unit, a close link is observed between growth patterns of sponge-microbialite reefs and the changes in the nannofossil assemblage composition. Phases of expansion of the build-ups correspond to carbonate deposits, characterised by a relative dominance of large, oligotrophic nannoplankton species (large Watznaueria britannica, Watznaueria manivitiae and Schizosphaerella spp.). Marly intervals reveal a high relative abundance of small-sized taxa (mainly Lotharingius hauffii, small W. britannica and subordinated Biscutum dorsetensis), and bioconstructions show a net reduction in size, or a total demise. This suggests that the same palaeoenvironmental parameters controlled, at the same time, marl versus limestone deposition, reef growth-phases, and calcareous nannofossil assemblage composition. Successive reef growth-phases recognized within a bioherm are correlated to the limestone hemi-couplets. Each reef growth-phase is composed of several stacked elementary sequences, comprising the following succession: (i) sponge, (ii) microbialites and (iii) carbonate mud. In turn, the microbialitic crust is formed of three successive microbial layers: (1) a fine crust of dense to clotted micrite; (2) a centimetre-scale to multicentimetric columnar layer of clotted to peloidal micrite; (3) a centimetre-scale stromatolitic crust. The passage from an initial dense microbial layer, with numerous microencrusters, to a final stromatolitic crust is interpreted as resulting from an increase of the microbial growth-rate, mainly driven by changes in the accumulation rate. Deep-shelf deposits resulted from changes in platform carbonate production and export basinwards controlled both by sea-level and climate changes. Climate controlled the intensity of carbonate production in shallow-platform environments and export towards the deep-shelf, as well as the trophic conditions of the deep-shelf marine-waters. Low trophic conditions favoured platform carbonate production and export basinwards, and a large development of the sponge-microbialite reefs in the deep-shelf. However, a high carbonate accumulation rate in the deep-shelf was also commonly responsible for the demise of sponge-microbialite reefs. During unfavourable and higher trophic conditions (during marl deposition), reduced development and frequent demise of the sponge-microbialite reefs is observed.

Palaeoenvironmental control on sponge-microbialite reefs and contemporaneous deep-shelf marl-limestone deposition (Late Oxfordian, southern Germany)

The Late Oxfordian (Bimammatum Zone) deep-shelf deposits of southern Germany are characterised by abundant reefs that laterally pass into marl-limestone alternations. The bioconstructions are made from three main components, sponges, microbialites and allochthonous sediment. A detailed morphological analysis of sponge-microbialite reefs in the Plettenberg section (Swabian Alb, southern Germany) is combined with investigation of calcareous nannofossil assemblages in laterally correspondent marl-limestone alternations in order to trace the evolution of palaeoenvironmental conditions during reef development. Fluctuations of CaCO 3 content in deep-shelf deposits provides evidence of a succession of small-scale lithological units, generally formed by five to six marl-limestone couplets. Within a small-scale lithological unit, a close link is observed between growth patterns of sponge-microbialite reefs and the changes in the nannofossil assemblage composition. Phases of expansion of the build-ups correspond to carbonate deposits, characterised by a relative dominance of large, oligotrophic nannoplankton species (large Watznaueria britannica, Watznaueria manivitiae and Schizosphaerella spp.). Marly intervals reveal a high relative abundance of small-sized taxa (mainly Lotharingius hauffii, small W. britannica and subordinated Biscutum dorsetensis), and bioconstructions show a net reduction in size, or a total demise. This suggests that the same palaeoenvironmental parameters controlled, at the same time, marl versus limestone deposition, reef growth-phases, and calcareous nannofossil assemblage composition. Successive reef growth-phases recognized within a bioherm are correlated to the limestone hemi-couplets. Each reef growth-phase is composed of several stacked elementary sequences, comprising the following succession: (i) sponge, (ii) microbialites and (iii) carbonate mud. In turn, the microbialitic crust is formed of three successive microbial layers: (1) a fine crust of dense to clotted micrite; (2) a centimetre-scale to multicentimetric columnar layer of clotted to peloidal micrite; (3) a centimetre-scale stromatolitic crust. The passage from an initial dense microbial layer, with numerous microencrusters, to a final stromatolitic crust is interpreted as resulting from an increase of the microbial growth-rate, mainly driven by changes in the accumulation rate. Deep-shelf deposits resulted from changes in platform carbonate production and export basinwards controlled both by sea-level and climate changes. Climate controlled the intensity of carbonate production in shallow-platform environments and export towards the deep-shelf, as well as the trophic conditions of the deep-shelf marine-waters. Low trophic conditions favoured platform carbonate production and export basinwards, and a large development of the sponge-microbialite reefs in the deep-shelf. However, a high carbonate accumulation rate in the deep-shelf was also commonly responsible for the demise of sponge-microbialite reefs. During unfavourable and higher trophic conditions (during marl deposition), reduced development and frequent demise of the sponge-microbialite reefs is observed.

The impact of calcareous nannofossils on the pelagic carbonate accumulation across the Jurassic–Cretaceous boundary

Calcareous nannofossils were important producers of pelagic carbonates in Mesozoic oceans. In order to better understand the origin of Mesozoic pelagic carbonates we studied upper Jurassic to lower Cretaceous sediments in the Central Atlantic Ocean (DSDP Sites 105, 367, 534A) with respect to their content of calcareous nannofossils. The interval under investigation is characterized by a significant increase in the deposition of carbonate-rich sediments, a trend that goes along with the rapid radiation of calcareous nannofossils. The assemblage composition and the size variation of common taxa were analyzed, and subsequently the contribution of this phytoplankton group to the pelagic carbonate accumulation was calculated. Results reveal two long-term changes of the nannofossil assemblage composition. (1) The early Tithonian coccolith assemblages are of low diversity ( Watznaueria spp., Cyclagelosphaera spp., Zeugrhabdotus spp.), whilst the mid- to late Tithonian assemblage is dominated by nannoliths ( Conusphaera mexicana, Polycostella beckmannii, Nannoconus spp.) and large-sized Watznaueria. (2) The early Berriasian is characterized by a shift from the late Tithonian nannolith-rich assemblage to a highly diverse coccolith assemblage. Morphometric studies of the placolith genus Watznaueria show for all three DSDP sites large-sized forms of this genus in the mid- and late Tithonian, followed by a decrease of up to 2 μm for the mean size in the earliest Berriasian. At DSDP Site 105 the studied nannolith taxa show an increase in size during the mid-Tithonian to Berriasian interval. The records of both nannofossil carbonate estimates and the measured bulk-rock carbonate reveal two periods of increase in the nannofossil carbonate record of DSDP Site 105. A first significant increase of the carbonate accumulation is observed in the mid- and late Tithonian, probably caused by mass occurrences of strongly calcified taxa ( C. mexicana, P. beckmannii, Nannoconus spp., Watznaueria cf. manivitae). This interval is here named ‘Nannofossil Calcification Event’ (NCE). The second carbonate maximum in the late Berriasian is related to a rise in absolute abundances of nannofossils. This peak is amplified by an overall increase of the sedimentation rate. The calculated accumulation rates of nannofossils, nannofossil carbonate and bulk-rock carbonate for the late Berriasian are on the same scale as values from recent ocean surface sediments. A comparison of nannofossil carbonate values with the bulk-rock carbonate content shows that on average only 27% of the total carbonate can be explained by our nannofossil carbonate estimates. This discrepancy is most likely caused by the high amount of unidentifiable micrite and fragments of calcareous nannofossils. Other factors contributing to the error are possible inaccuracies in the determination of absolute abundances and nannofossil volume calculations. The NCE occurs during a long-term sea-level fall, dry climate and presumed low pCO 2 levels. A decline in abundance of strongly calcified nannofossils coincides with the opening of the Pacific–Atlantic seaway via Central America, which may have had a substantial impact on the palaeoceanographic situation in the Central Atlantic Ocean. These changes are here considered to have at least partly caused the shifts in abundance, size and assemblage composition of calcareous nannofossils observed across the Jurassic–Cretaceous boundary interval.

Shallow-platform palaeoenvironmental conditions recorded in deep-shelf sediments: C and O stable isotopes in Upper Jurassic sections of southern Germany (Oxfordian–Kimmeridgian)

A high-resolution carbon and oxygen isotope analysis of Late Oxfordian–Early Kimmeridgian deep-shelf sediments of southern Germany is combined with investigation of nannofossil assemblage composition and sedimentological interpretations in order to evaluate the impact of regional palaeoenvironmental conditions on isotopic composition of carbonates. This study suggests that carbonate mud was essentially derived from the Jura shallow platform environments and also that the isotopic signature of carbonates deposited in the Swabian Alb deep shelf indirectly expresses the palaeoenvironmental evolution of the platform. Short-term fluctuations in δ 13C and δ 18O are probably controlled by changes in salinity (fresh-water input versus evaporation) in platform environments. Long-term fluctuations in carbon and oxygen isotope record throughout the Late Oxfordian–Early Kimmeridgian result from the interplay of increasing temperature and decreasing humidity, which both control the trophic level. Changes from mesotrophic to oligotrophic conditions in platform environments and in the deep-shelf surface waters are inferred. During the Late Oxfordian (Bimammatum Subzone to Planula Zone), the δ 13C curve displays a positive shift of about 1‰, which is comparable in intensity to global perturbations of the carbon cycle. This evident isotopic shift has not been documented yet in other basinal settings. It can be reasonably explained by local palaeoenvironmental changes on the Jura platform (salinity, temperature, and nutrient availability) that controlled platform carbonate production, and the geochemistry of overlying waters. However, increasing carbonate production on the Jura platform and related positive δ 13C shifts recorded in the Swabian Alb deep shelf are the regional signatures of climatic changes affecting other palaeogeographical domains of Europe in which the carbonate production increased throughout the Late Oxfordian.

Berriasian–Aptian black shales of the Maiolica formation (Lombardian Basin, Southern Alps, Northern Italy): local to global events

The Lower Cretaceous Maiolica Formation represents the classic pelagic succession of the Apulian margin of the Tethys Ocean, in the Southern Alps. It consists of white calcilutites with minor marly layers and black shales. A multidisciplinary study including lithostratigraphy, nannofossil assemblage composition and organic geochemistry was performed on two composite sections in order to characterise the marl–shale layers that record production and storage of organic matter (OM) and terrigenous input in the Berriasian–Aptian interval. Such layers are associated with turbidite deposition and bottom current activity in the Late Berriasian–earliest Valanginian, Late Valanginian and Early Aptian. The uppermost Hauterivian–Barremian black shales alternate with pelagic calcilutites, forming an interval barren of redeposited layers. The quantity of organic carbon increases from the Berriasian to the Barremian; the background composition of the kerogen is continental-dominated, with varying proportions of marine-derived OM. Preservation of OM did not necessarily occur under anoxic conditions; on the contrary, storage of OM probably resulted from enhanced terrestrial input of nutrients, siliciclastics, woody and herbaceous fragments, inducing mesotrophic conditions and increased primary productivity. Higher fertility of surface waters during the Late Valanginian and Early Aptian is reflected by distinctive decreases in abundance of oligotrophic nannoconids and increase in abundance of mesotrophic Diazomatholithus in the Valanginian. During the Barremian, the rhythmic establishment of eutrophic conditions, documented by the highest amounts of marine OM, determined high-frequency fluctuations in nannoconid abundance. The Early Aptian nannoconid crisis is interpreted as the result of a major climate change and enhanced primary productivity. Variations of calcareous nannofloras, as well as changes of lithology and OM, which we describe in the Lower Cretaceous Maiolica Formation of the Lombardian Basin, can also be recognised in coeval units from other sites of the Apulian side of the Tethys. Similar episodes of siliciclastic and terrestrial OM accumulation and major changes in calcareous nannofossils are also recorded in the Upper Valanginian and Lower Aptian pelagic sediments of the Tethys, Atlantic and, partly, Pacific oceans. Such nannofossil variations are coeval with positive excursions of the δ 13C curve and suggest perturbations of the bio-geochemical cycles at a global scale. Additional minor changes in siliciclastic and terrestrial OM accumulation as well as in nannofloras are probably the result of local events, strictly related to the physiography and evolution of the Tethyan margin.

The carbonate signal and calcareous nannofossil distribution in an Upper Jurassic section (Balingen-Tieringen, Late Oxfordian, southern Germany)

Total abundance, assemblage composition and the stratigraphic distribution of the six most abundant species of calcareous nannofossil were analysed in the Upper Oxfordian succession of Balingen-Tieringen (western Swabian Alb, south Germany). This locality represents a deep-shelf (less than 200 m) environment that is strongly influenced by its proximity to the Jura shallow platform. Nannofossil abundance negatively correlates with carbonate content as well as with accumulation rate. Calcareous nannofossils thus did not produce the bulk of carbonates, and their abundance was essentially controlled by the import of allochthonous carbonate mud. Gradual increase in carbonate content in the Balingen-Tieringen succession can be related to an intensified carbonate mud export from the Jura shallow platform to the Swabian Alb deep shelf during the Late Oxfordian. Changes in the nannofossil assemblage composition along the profile are interpreted as primary although a partial control by diagenesis on the assemblage can not be completely excluded. The coccoliths Lotharingius hauffii, Watznaueria britannica, Cyclagelosphaera margereli, Watznaueria barnesae, Watznaueria manivitae and the incertae sedis Schizosphaerella spp. display their highest relative abundance one after the other, in a stratigraphic order. This nannofossil succession parallels the gradual increase in allochthonous carbonate mud accumulation on the deep shelf as well as the gradual decrease in the intensity of siliciclastic input recorded on the Jura shallow platform. This suggests a climatic control by humidity on shallow-platform carbonate production and export basinwards, and on nutrient availability in deep-shelf surface waters that itself controlled the nannofossil assemblage composition. Therefore, the succession of highest relative abundance of L. hauffii, W. britannica, C. margereli, W. barnesae, W. manivitae and Schizosphaerella spp. can represent a trophic preference continuum from more eutrophic to oligotrophic conditions.

Quantitative analysis of calcareous microfossils across the Albian–Cenomanian boundary oceanic anoxic event at DSDP Site 547 (North Atlantic)

Nannofossil and planktonic and benthic foraminiferal assemblages were studied quantitatively in an expanded Albian to Cenomanian section at DSDP Site 547 in the eastern North Atlantic. The section spans a 0.5‰ positive δ13C excursion across the Albian–Cenomanian boundary, representing one of the mid-Cretaceous Oceanic Anoxic Events (OAE1d). The base of the carbon excursion at Site 547 coincides with a major faunal turnover in planktonic foraminifera, including extinctions and first appearances of several species as well as geographic entries. In contrast, nannofossil assemblage composition, which reflects conditions in the photic zone, shows little variation related to the carbon excursion, arguing against a noticeable change in surface water productivity during the event. Benthic foraminifera, which represent a paleowaterdepth of around 1km, show that there was no change in environmental conditions at that water depth, and offer no support for increased deep water anoxia. Combined with published data, microfossils at Site 547 suggest that improved preservation of organic matter due to a decrease in oxygen content in a shallow oxygen minimum zone is a more likely cause for the Albian–Cenomanian boundary anoxic event than an increase in productivity.

Stable isotopic and carbonate cyclicity in Lower Cretaceous deep-sea sediments: Dominance of diagenetic effects

Oxygen and carbon isotopic variability of the dominant (<38 μm) carbonate fraction within bedded, organic-carbon rich Lower Cretaceous sediment intervals from various DSDP sites are closely correlated with preservational changes in the carbonates. Isotopic fluctuations are absent where carbonate contents vary little and where the carbonate fraction is dominated by biogenic phytoplankton remains. Within each of the studied intervals oxygen and carbon isotopic ratios become increasingly more negative in samples with carbonate contents higher than about 60% in which the proportion of diagenetic microcarbonate increases rapidly. Carbon isotopic ratios show a trend towards positive values in samples with carbonate contents of less than 40% and strong signs of dissolution. The taxonomic composition of nannofossil assemblages varies little within single intervals, despite significant differential diagenesis among individual beds; this points towards ecological stability of oceanic surface waters during the deposition of alternating beds. Bedding is, however, closely related to changing bioturbation intensity, indicating repeated fluctuations of the deep-water renewal rates and oxygen supply. Various microbial decomposition processes of organic matter leading to bed-specific differential carbonate diagenesis resulted in an amplification of primary bedding features and are considered responsible for most of the observed fluctuations in the stable isotopic ratios and carbonate contents.

The abundance and species composition of nannofossil assemblages in sediments from continental shelf to offshore basin, western Tasman Sea

Abundance and species composition of recent nannofossil assemblages are described from continental shelf, slope, rise and offshore basin sediments (36 to 37°S Lat.). Shelf and upper slope assemblages contain extremely few nannofossil specimens and species. Numbers of specimens and species diversity increases from slope to basin areas. The overall nannofossil content of the sediment is divided into three groups: all very small species; Gephyrocapsa oceanica; the remaining large species. It is the abundance of the large species which changes most noticeably between shelf, slope, rise and basin areas. Some individual species have specific distribution patterns. Some decrease and others increase in proportion from shelf to basin. Comparing shelf, slope and rise assemblages with basin assemblages indicates that shelf and slope assemblages are not indicative of offshore oceanic hydrological regimes. These differences may be attributed to a variety of hydrological factors which distinguish coastal shelf waters from oceanic waters. Rise assemblages are the shallowest water assemblages which give an indication of offshore oceanic hydrological conditions.