Changes In Nannofossil Assemblages Research Articles

Calcareous nannoplankton response to early Eocene warmth, Southwest Pacific Ocean

Episodes of pronounced climatic warming in the early Eocene (56–48 Ma) provide insights into how biological systems might respond to future climate changes. Eocene climate reconstructions for the Southwest (SW) Pacific have proved challenging due to some disparities between geochemical proxy-based estimates for sea temperatures and estimates derived from climate models. Changes in marine phytoplankton populations through the early Eocene provide a means to evaluate model and proxy-based approaches and also reveal how climatic changes affected the ocean ecosystem. This study documents early to middle Eocene calcareous nannofossil assemblages from legacy Deep Sea Drilling Project (DSDP) sites in the SW Pacific. We integrate nannofossil assemblage changes with foraminiferal-based stable isotopes and other microfossil data to provide updated age models for DSDP Sites 207 (Lord Howe Rise) and 277 (Campbell Plateau), and to infer past environmental change through the early to mid-Eocene in the SW Pacific. Although these sites record a short hiatus in the earliest Eocene, deposition resumed by ~54 Ma, prior to the onset of the Early Eocene Climate Optimum (EECO). Both sites show an increase in warm-water taxa from the onset of the EECO until approximately 50.5 Ma. Abundance of warm-water taxa was higher (~23%) at the EECO onset at the more northerly Site 207 and increased only slightly to ~27%, whereas at Site 277 warm-water taxa comprised only ~3% at the EECO onset but increased to ~31% by 50.5 Ma. Cool-water taxa at both sites were present in very sparse numbers (<3%) through much of the EECO and only began to increase in abundance after 50 Ma, concomitant with a decrease in warm-water taxa. These results are similar to other mid-latitude sites in the SW Pacific and Indian Ocean, including mid-Waipara River (Canterbury Basin, New Zealand) and ODP Site 762 (Exmouth Plateau), and indicate assemblages during the EECO are best described as warm temperate. Benthic δ18O records from both sites show good agreement with nannofossil trends and, despite the possibility of recrystallization at Site 277, sea surface temperatures (SSTs) indicated by planktic foraminifera also imply warm temperate conditions. This suggests that a warm proto-East Australian Current extended to at least 55°S during the EECO, displacing the proto-Tasman Front southward, which migrated northward after termination of the EECO possibly due to expansion of a proto-Ross Sea Gyre.

Integrated stratigraphy of the Priabonian (upper Eocene) Urtsadzor section, Armenia

The transition from the Bartonian to the Priabonian, as traditionally understood, has long been associated with a series of extinctions and originations in several microfossil groups. The planktonic foraminifer genus Morozovelloides and large species of Acarinina suffered a rapid global extinction, as did many radiolarians. Calcareous nannofossils show several assemblage changes including the acme beginning of Cribrocentrum erbae and the lowest and highest occurrences of Chiasmolithus oamaruensis and C. grandis respectively. In shallow water environments, larger foraminifera also show an extinction among large species of Nummulites, as well as the first occurrences of the stratigraphically important genus Spiroclypeus. However, the correlation between shallow and deep water records remains uncertain, as do the mechanisms driving these biotic events. Here we present the results of a new integrated stratigraphical study (calcareous nannofossils, planktonic foraminifera, larger benthic foraminifera, and low-resolution magnetostratigraphy) of the Urtsadzor section in south-western Armenia which appears to be continuous through this interval. The Urtsadzor section consists of calcareous siltstones rich in micro- and nannofossils, with interbedded limestones containing abundant larger benthic foraminifera. Our new data enable us to correlate larger foraminiferal events with global plankton biostratigraphy, in a section outside of southwest Europe where most previous correlations have been based. At Urtsadzor, the large Nummulites species of N. millecaput-group are present throughout the whole section but decrease in abundance toward the top. The first occurrence of Spiroclypeus, also occurs in the upper part of the section, marking the SBZ 18/19 boundary. These events are associated with the phylogenetic development of the Nummulites fabianii and Heterostegina reticulata lineages. However, the calcareous plankton biostratigraphy indicates the section is well within the Priabonian; within planktonic foraminiferal Zones E14 and E15 and calcareous nannoplankton Zones CNE 18 and CNE 19. These results indicate larger foraminiferal events occur well above the planktonic foraminiferal extinction level and nannofossil assemblage changes indicating the events are not synchronous across groups, with implications for biostratigraphy and recognition of the basal Priabonian in different depositional settings and regions.

Calcareous nannofossil paleoecology of the mid-Cretaceous Western Interior Seaway and evidence of oligotrophic surface waters during OAE2

Analysis of calcareous nannofossils, total organic carbon(TOC) values, and carbon isotope records from outcrop and drill cores in the central (Colorado and Kansas) and southern (Texas) Western Interior Basin (WIB) indicates predominantly oligotrophic surface water conditions during the late Cenomanian–Turonian Oceanic Anoxic Event 2(OAE2). High-resolution bio-chemostratigraphy of this interval allows for detailed correlation of changes in nannofossil assemblages and TOC across the WIB and aids comparison with other global records. Prior to and at the onset of OAE2, the A interval in the δ13C positive excursion identified by Pratt and Threlkeld (1984), mesotrophic to eutrophic nannofossil taxa Biscutum and Zeugrhabdotus are relatively high in abundance and foraminiferal assemblages record a high diversity “benthonic zone”. TOC sharply declines at the start of OAE2 despite evidence of high surface productivity and benthic diversity, implying that the seafloor was briefly oxygenated during a period of enhanced mixing in the expanding Western Interior Seaway. During the latter two thirds of OAE2 (intervals B and C in the excursion) TOC periodically increases, suggesting that anoxic conditions returned at the substrate, despite persistently low abundances of Biscutum and Zeugrhabdotus, implying oligotrophic surface water conditions. The divergence of these two proxies suggests a highly stratified water column in the WIB, not higher productivity, which resulted in anoxic conditions during OAE2.If productivity was not the primary cause of increased carbon burial in the WIB it is probable that increased terrestrial runoff and incursion of warm, saline Tethyan waters from the south led to salinity stratification. In the absence of mixing, warm and saline bottom waters would have been isolated from freshened Boreal currents and become stagnant, allowing for preservation of greater amounts of organic material in the substrate. Nannofossil records from Cuba, KS are an exception, and reveal relatively high productivity throughout, suggesting that occasional mixing of nutrients to the surface may have continued along the eastern margin of the central WIB.

Eocene calcareous nannofossil biostratigraphy and community structure from Exmouth Plateau, Eastern Indian Ocean (ODPSite 762)

A relatively complete section of Eocene (∼33.9-55.8 Ma) pelagic chalk from offshore northwestern Australia was used to analyze range and abundance data of ∼250 Eocene species to test the efficacy of the existing CP (Okada and Bukry 1980) and NP (Martini 1971) biostratigraphic zonation schemes. Changes in nannofossil diversity, abundance, and community structure were monitored through several Eocene paleoenvironmental events, as identified by changes in δ 13C and δ 18O data, to examine variations in surface water conditions. Major changes in nannofossil assemblages, as indicated by dominance crossovers, correspond to paleoenvironmental shifts such as the PETM (Paleocene-Eocene thermal maximum) and the EECO (Early Eocene climatic optimum). This research also provides systematic paleontology and range data for one new genus (Hexadelus) and eight new species or variants (Calcidiscus ellipticus, Cruciplacolithus nebulosus, Cruciplacolithus opacus, Cyclicargolithus parvus, Hexadelus archus, Hayella situliformis var. ovata, Markalius latus, Pedinocyclus annulus) and addresses several taxonomic issues in other Eocene species.

Calcareous nannofossil assemblage changes from early to middle Eocene in the Levant margin of the Tethys, central Israel

Abstract. Patterns of change in calcareous nannofossil assemblages during nannozones NP11 to NP16 on the southern Levant margin of the Tethys were observed from sections of early and middle Eocene age sediments of the Avedat Plateau, central Israel. A cooling process following the Early Eocene Climate Optimum (EECO) is supported by several events of biotic change over a 4 Ma interval. The rate of pelagic sedimentation varied from 7.5 at the EECO to 23.6 m Ma–1 at the cooling transition phase. Reduced numbers of discoasters mark the end of the oligotrophic regime within the NP13 nannozone, followed by an increase in nannofossil richness especially marked by Blackites and Chiasmolithus spp. In the middle part of the cooling process a prominent peak of reworked Paleocene taxa, up to 7% of total taxa, suggests that enhanced current activity caused re-sedimentation on the Levant margin slopes. When stability resumed in the upper part of the NP15–16 interval, Coccolithus-type placoliths became rare and Reticulofenestra-type forms became dominant. Calcareous nannoplankton response to this gradual cooling became irreversible in the late Palaeogene, but the change was, however, diachronous across the Tethys.

Calcareous nannofossil changes during the Mid-Pleistocene Revolution: Paleoecologic and paleoceanographic evidence from North Atlantic Site 980/981

New quantitative data on calcareous nannofossil assemblages through 583–1138 thousand years ago (kyr) have been acquired in high sample resolution (2–3ky) at northern Atlantic ODP (Ocean Drilling Program) Site 980/981. Assemblage composition reflects a temperate paleobioprovince suggesting significant influx of warm North Atlantic Current at the site location during the mid-Pleistocene transition, between Marine Isotope Stage (MIS) 35 and 15.Three intervals have been described based on changes in nannofossil composition and abundance fluctuations of taxa. Reticulofenestrids are abundant just in the first interval (MIS 35–22), which is also characterised by the absence of Gephyrocapsa (>4μm) and C. pelagicus s.l., the latter occurring in upper MIS 24 only. The second interval corresponds to the expanded warm MIS 21 that is accompanied by high carbonate content, absence of both nannofossil reworking and ice rafted debris (IRD), higher abundance of C. leptoporus (5–8μm) and H. carteri, and absence of C. pelagicus s.l. and medium gephyrocapsids. The third interval (MIS 20–15) records the occurrence of C. pelagicus s.l. and medium Gephyrocapsa, generally higher abundance and higher amplitude fluctuations of helicoliths, and absence of reticulofenestrids. The major changes in nannofossil assemblages seem to be the result of combined paleoenvironmental factors related to sea surface water features (temperature and nutrient availability, sea-ice cover) and possibly to deep and intermediate water production of both Northern and Southern Hemisphere origin during glacial and interglacial phases. Complex inter-relationships between environmental parameters and evolutionary/ecological factors are inferred to explain the patterns of Coccolithus pelagicus subspecies, whose distribution and abundance fluctuations are compared with the available patterns of Turborotalia quinqueloba and Neogloboquadrina pachyderma (left coiled) at Site 980 and with the C. pelagicus subspecies patterns from North Atlantic DSDP (Deep Sea Drilling Project) Site 607 and South Atlantic ODP Site 1090.Data collected provide additional information on stratigraphic distribution of selected taxa; the First Occurrence (FO) of Gephyrocapsa omega is confirmed as a diachronous event, even in the Atlantic Ocean, while the First and Last Common Occurrence (FCO, LCO) of R. asanoi appear as diachronous and synchronous events, respectively, on a global scale.

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%CaCO 3. 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.

Calcareous nannoplankton changes across the early Toarcian oceanic anoxic event in the western Tethys

Calcareous nannoplankton were profoundly affected by environmental perturbations coincident with the early Toarcian oceanic anoxic event (T‐OAE). We quantify the abundance of nannofossils across the T‐OAE at three locations in Western Europe, where the event is marked by a prominent negative carbon isotope excursion (CIE). Data were treated by statistical analysis, and the Shannon diversity index was applied in order to describe nannofossil assemblage changes related to paleoenvironmental evolution. In the basal Toarcian, before the T‐OAE, high proportions of taxa with an affinity for low temperature (e.g., P. liasicus, T. patulus, Bussonius) occurred. This observation is consistent with interpretations of published oxygen isotope records. During the T‐OAE, the lowest abundance of nannofossils is observed, but there is a peak of a coccolith (Calyculus) from an organism that probably thrived in low‐saline surface waters depleted of nitrate. At the end of the perturbation, the lowest diversities of nannofossils occurred, and assemblages are dominated by Crepidolithus crassus, a deep dweller. This interval corresponded to progressive reoxygenation of deep water and the reoccupation of the deep photic zone by nannoplankton. The highest abundance of nannofossils is recorded above the CIE and testifies to the recovery of the entire nannoplankton community. The T‐OAE was widespread (perhaps global) and probably linked to major changes in the atmosphere and hydrosphere. However, the record of this event varies in the basins surrounding the western Tethys, suggesting regional imprints on the global signal. The regional variability may attest to establishment of effective connections between the Arctic and Tethys oceans, which allowed cool, low‐saline water formed at high latitudes to stream toward the western Tethys.

Late Oligocene climatic changes: Evidence from calcareous nannofossils at Kerguelen Plateau Site 748 (Southern Ocean)

Palaeoecological changes in nannofossil assemblages in the Southern Ocean during Oligocene times are examined through high-resolution, quantitative analyses of samples from Ocean Drilling Program (ODP) Site 748 (Kerguelen Plateau). We quantitatively characterized the palaeoecological preference of groups of species and compared their general trends with those determined at Maud Rise (ODP Sites 690 and 689) (Persico, D., Villa, G., 2004. Eocene–Oligocene calcareous nannofossils from Maud Rise and Kerguelen Plateau (Antarctica): palaeoecological and palaeoceanographic implications. Marine Micropaleontology 52, 153–179). We then attempt a correlation between the main assemblage variation and sea-surface temperature (SST) changes in the Southern Ocean at this time. Relatively stable, cool conditions are interpreted to have persisted from earliest to late Oligocene times, when an increase in abundance of temperate-water taxa is recorded, both at Maud Rise and Kerguelen Plateau, before the Mi-1 event. This reveals a climatic event that probably involved both sites, and which is comparable to that indicated by the global oxygen isotope curve ([Miller, K.G., Wright, J.D., Fairbanks, R.G., 1991. Unlocking the Ice House: Oligocene–Miocene oxygen isotopes, eustasy, and margin erosion. Journal of Geophysical Research, 96, 6829–6848; Zachos, J., Pagani, M., Sloan, L., Thomas, E., Billups, K., 2001. Trends, rhythms, and aberrations in global climate 65 Ma to present. Science, 292, 686–693.]). In the uppermost Oligocene at Site 689, the temperate-water taxa index ([temperate / temperate + cool] * 100) increases from 25.2 Ma. Similarly, at Site 748, the temperate-water taxa index indicates an increase in SST in the late Oligocene, from about 26.5 Ma, at the base of Chron C8n.2n, which is offset by about 1 m.y. between the two areas. The reason for this time difference can probably be found in the location of Site 748 with respect to Site 689, and in the palaeoceanographic setting, taking into account that Site 748 lies north of a deep water passage that separates Kerguelen Plateau from Antarctica, and north of the present day front of the Antarctic Circumpolar Current. The presence of warm-water taxa, exclusively at Site 748, corroborates this hypothesis.

CALIBRATION OF THE PLIENSBACHIAN-TOARCIAN CALCAREOUS NANNOFOSSIL ZONE BOUNDARIES BASED ON AMMONITES (BASQUE-CANTABRIAN AREA, SPAIN)

Based on a continuous succession of ammonite and calcareous nannofossil assemblages, the main purpose of this paper is the correlation between ammonite and calcareous nannofossil zone boundaries in two expanded sections from the Basque-Cantabrian area. The ammonite assemblages show clear euroboreal features and allow the identification of Pliensbachian-Toarcian zones and subzones established for NW Europe. Due to the presence of Mediterranean taxa documented at different levels, they are also helpful to improve the biocorrelations between Boreal and Tethyan successions. The calcareous nannofossil assemblage changes provide a useful set of easily recognizable events that facilitate the recognition of all the Pliensbachian-Toarcian zones described for NW Europe and Mediterranean Province. Based on the FO Similiscutum cruciulus (Jamesoni Zone), FO of Lotharingius hauffii (Stokesi/Margaritatus Zone boundary), FO of Carinolithus superbus (Serpentinus Zone), FO of Discorhabdus striatus (Serpentinus/Bifrons Zone boundary) and FO of Retecapsa incompta (Insigne Zone), the NJ3/NJ4, NJ4/NJ5, NJ5/NJ6, NJ6/NJ7 and NJ7/NJ8 Zone boundaries have been recognized. Besides, the NJ4, the NJ5 and the NJ6 span almost the same ammonite-defined time interval in both NW Europe and Basque-Cantabrian area. With respect to the ammonite zone, the zone boundaries NJ3/NJ4 and the NJ6/NJ7 coincides with the NJT3/NJT4 and the NJT6/NJT7 proposed for Mediterranean Province. Instead, the zone boundaries NJ4/NJ5 and the NJ5/NJ6 not coincides with the NJT4/NJT5 and the NJT5/NJT6 proposed for Mediterranean Province. Furthermore, in NW Europe and Mediterranean Province the FO of Retecapsa incompta is placed in the Levesquei Zone and Meneghinii Zone, whilst in the Basque Cantabrian area it lies in the Insigne Zone.

Stable isotope ‘vital effects’ in coccolith calcite

Uncertainties about the origin of the many disequilibrium or ‘vital effects’ in a variety of calcifying organisms, and whether these effects are constant or variable, have hampered paleoceanographic application of carbon and oxygen isotopic ratios. Unraveling the source of these effects will improve paleoceanographic applications and may provide new information on changes in cell physiology and ecology. Culture of eight species of coccolithophorids, a dominant marine phytoplankton group, reveals a 5‰ array of disequilibrium or ‘vital effects’ in both the carbon and oxygen isotopic composition of coccolith calcite. In moderate light and nutrient-replete cultures, oxygen isotopic fractionation and carbon isotopic fractionation correlates directly with cell division rates and correlates inversely with cell size across a range of species. However, when growth rates of a single species are increased or decreased by higher or lower light levels, ϵ18O is relatively invariant. Likewise, growth rate variations as a function of temperature do not influence coccolith ϵ18O; the slope of the ϵ18O vs. temperature relation in cultures of both Gephyrocapsa oceanica and Helicosphaera carteri is the same as for abiogenic carbonates. This suggests a constant, species-specific isotopic fractionation which does not vary with cell physiology. The constancy of vital effects suggests that coccolith stable isotopes will provide reliable phase for paleoceanographic reconstruction of temperature and seawater chemistry, as long as monospecific fractions are analyzed or changes in nannofossil assemblages are accounted for with species-specific correction factors. We suspect that the cell size, and its constraints on the rate of CO2 diffusion relative to C fixation, may be the first order influence on coccolith stable isotope vital effects. A quantitative model of this process may provide important constraints on mechanisms of carbon acquisition of coccolithophorids in both modern and extinct species.

96/03171 - Distribution of 137Cs in water leachates of forest humus

Uncertainties about the origin of the many disequilibrium or ‘vital effects’ in a variety of calcifying organisms, and whether these effects are constant or variable, have hampered paleoceanographic application of carbon and oxygen isotopic ratios. Unraveling the source of these effects will improve paleoceanographic applications and may provide new information on changes in cell physiology and ecology. Culture of eight species of coccolithophorids, a dominant marine phytoplankton group, reveals a 5‰ array of disequilibrium or ‘vital effects’ in both the carbon and oxygen isotopic composition of coccolith calcite. In moderate light and nutrient-replete cultures, oxygen isotopic fractionation and carbon isotopic fractionation correlates directly with cell division rates and correlates inversely with cell size across a range of species. However, when growth rates of a single species are increased or decreased by higher or lower light levels, ϵ18O is relatively invariant. Likewise, growth rate variations as a function of temperature do not influence coccolith ϵ18O; the slope of the ϵ18O vs. temperature relation in cultures of both Gephyrocapsa oceanica and Helicosphaera carteri is the same as for abiogenic carbonates. This suggests a constant, species-specific isotopic fractionation which does not vary with cell physiology. The constancy of vital effects suggests that coccolith stable isotopes will provide reliable phase for paleoceanographic reconstruction of temperature and seawater chemistry, as long as monospecific fractions are analyzed or changes in nannofossil assemblages are accounted for with species-specific correction factors. We suspect that the cell size, and its constraints on the rate of CO2 diffusion relative to C fixation, may be the first order influence on coccolith stable isotope vital effects. A quantitative model of this process may provide important constraints on mechanisms of carbon acquisition of coccolithophorids in both modern and extinct species.

Interrelationships between the seasonal sea ice and biological regimes

New quantitative data on calcareous nannofossil assemblages through 583–1138 thousand years ago (kyr) have been acquired in high sample resolution (2–3 ky) at northern Atlantic ODP (Ocean Drilling Program) Site 980/981. Assemblage composition reflects a temperate paleobioprovince suggesting significant influx of warm North Atlantic Current at the site location during the mid-Pleistocene transition, between Marine Isotope Stage (MIS) 35 and 15.Three intervals have been described based on changes in nannofossil composition and abundance fluctuations of taxa. Reticulofenestrids are abundant just in the first interval (MIS 35–22), which is also characterised by the absence of Gephyrocapsa (> 4 μm) and C. pelagicus s.l., the latter occurring in upper MIS 24 only. The second interval corresponds to the expanded warm MIS 21 that is accompanied by high carbonate content, absence of both nannofossil reworking and ice rafted debris (IRD), higher abundance of C. leptoporus (5–8 μm) and H. carteri, and absence of C. pelagicus s.l. and medium gephyrocapsids. The third interval (MIS 20–15) records the occurrence of C. pelagicus s.l. and medium Gephyrocapsa, generally higher abundance and higher amplitude fluctuations of helicoliths, and absence of reticulofenestrids. The major changes in nannofossil assemblages seem to be the result of combined paleoenvironmental factors related to sea surface water features (temperature and nutrient availability, sea-ice cover) and possibly to deep and intermediate water production of both Northern and Southern Hemisphere origin during glacial and interglacial phases. Complex inter-relationships between environmental parameters and evolutionary/ecological factors are inferred to explain the patterns of Coccolithus pelagicus subspecies, whose distribution and abundance fluctuations are compared with the available patterns of Turborotalia quinqueloba and Neogloboquadrina pachyderma (left coiled) at Site 980 and with the C. pelagicus subspecies patterns from North Atlantic DSDP (Deep Sea Drilling Project) Site 607 and South Atlantic ODP Site 1090.Data collected provide additional information on stratigraphic distribution of selected taxa; the First Occurrence (FO) of Gephyrocapsa omega is confirmed as a diachronous event, even in the Atlantic Ocean, while the First and Last Common Occurrence (FCO, LCO) of R. asanoi appear as diachronous and synchronous events, respectively, on a global scale.