Southern Ocean Pleistocene calcareous nannofossil events: calibration with isotope and geomagnetic stratigraphies

Several cores recovered from the northern belt of the Southern Ocean were analysed to study the Pleistocene calcareous nannofossil records. Calcareous nannofossil events previously described in medium and low latitudes were identified and calibrated with the oxygen isotope and geomagnetic time scales. Although sedimentation rates, hiatuses and degree of calcareous nannofossil preservation sometimes prevent the identification and/or accurate calibration of some of these events, a useful stratigraphic framework was obtained. The possibility of using these calibrated events from high to low latitudes facilitates correlations and should facilitate isotope event identification in a region with low temperature, where calcareous plankton stratigraphies are in general restricted. In general, Pleistocene southern high latitude calcareous nannofossil events show synchronism with those observed in warm and temperate surficial waters. Small discrepancies in the assigned ages are sometimes related to low sampling resolution due to low sedimentation rates. The first occurrence (FO) of Emiliania huxleyi and the last occurrence (LO) of Pseudoemiliania lacunosa are observed in Marine Isotope Stages (MIS) 8 and 12, respectively. A reversal in abundance between Gephyrocapsa muellerae and E. huxleyi is observed close to the MIS 4/5 boundary. MIS 6 is characterised by an increase in G. muellerae and MIS 7 features a dramatic decrease in the proportion of Gephyrocapsa caribbeanica. This latter species began to increase its proportions from the MIS 13/14 boundary to MIS13, showing diachronism between the different sites. The LO of Reticulofenestra asanoi is observed at MIS 22, confirming this event as a global synchronous reference datum. By contrast, the FO of R. asanoi occurs at MIS 35 and is diachronous with the existing data from other oceanic regions. A re-entry of medium sized Gephyrocapsa (3–5 μm maximum diameters) can be identified in some cores close to MIS 25; although the low abundance of this taxon prevents an accurate calibration, it may be concluded that this event is diachronous as compared with the existing low-latitude data. The LO of large morphotypes of Gephyrocapsa is well correlated with MIS 37, showing synchronism with other oceanic regions, whereas the FO of this species is not well calibrated due to the absence of age-control points.     

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.     

Subtropical Front fluctuations south of Australia (45°09′S, 146°17′E) for the last 130 ka years based on calcareous nannoplankton

Calcareous nannoplankton assemblages from a Late Quaternary deep-sea core (GC07; 46°09′S, 146°17′E) south of Australia provide information on regional palaeoceanography and palaeoclimate changes in the Southern Ocean, in particular the movement of the Subtropical Front for the past 130 ka years. Marine Isotope Stages 1–5 are identified through changes in calcareous nannoplankton assemblages, supported by ¹⁴C dates, and oxygen isotope and %CaCO data.Two distinct assemblages are recognised: a warm water assemblage with higher abundances of Calcidiscus leptoporus, Emiliania huxleyi, Helicosphaera.carteri, Syracosphaera pulchra, Gephyrocapsa caribbeanica and Gephyrocapsa oceanica; and, a cold water assemblage with higher abundances of Gephyrocapsa muellerae and Coccolithus pelagicus. Alternation between these two assemblages downcore in GC07 reflect movement of the Subtropical Front across the location and can be correlated to Marine Isotope Stages (MIS) 1–5. Sediments with a cold water assemblage indicate the position of the Subtropical Front equatorward of the site when transitional to sub-antarctic waters were overlying the site. Conversely sediments with a warm water assemblage indicate the Subtropical Front was poleward of GC07 when warmer, subtropical waters were over the site. MIS 1 and 5 are interpreted as warmer than MIS 3 (based on species composition) with the Subtropical Front more poleward than for MIS 3. During MIS 3 the Subtropical Front is interpreted as adjacent to or immediately poleward of GC07. Some species including C. leptoporus and C. pelagicus show negative covariance and are considered to be reliable species in identifying glacial and interglacial intervals in this region.Comparison with established biostratigraphy based on calcareous nannoplankton showed the datum event for the reversal between E. huxleyi and G. muellerae of 73 ka in transitional waters is not applicable in this region. The reversal between these two species occurs between 48 and 30 cm downcore in GC07 with a ¹⁴C date of 11 020 year BP at 49–48 cm, i.e. the reversal event is younger than this date.     

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.     

The last 100,000 years in the western Mediterranean: sea surface water and frontal dynamics as revealed by coccolithophores

A quantitative analysis was carried out on coccolith assemblages from two Pleistocene cores (K1 and K10) from the western Mediterranean. The distribution of selected coccolithophore species provides new paleoclimatic and paleoceano-graphic data. A continuous sequence from the top of Isotope Stage 5 to the Holocene was recorded. The reversal in dominance between Gephyrocapsa muellerae and Emiliania huxleyi was dated in both cores at ca. 73 ka. At about 47 ka, E. huxleyi shows a regular increase, whereas G. muellerae progressively decreases in abundance. During interglacial periods, high concentrations of coccoliths are observed, whereas in glacial times, coccoliths are more diluted and the percentage of reworked forms increases as a consequence of the higher terrigenous input. After taking careful account of the dilution factor, we conclude that the production of coccolithophores was higher during warm periods. Maxima in coccolith concentrations coincide with highstand episodes, probably as a result of the intensification of the Atlantic flux into the Mediterranean across the Gibraltar Strait. This intensification could have produced an increase in nutrient content in the surface Mediterranean waters. During cold periods, the western Mediterranean front underwent a reduction in activity, probably due to an increase in the saline and/or thermal gradients between the superficial waters, and intermediate waters in the Liguro Provençal basin.     

Revision of the early-middle pleistocene calcareous nannofossil biochronology (1.75–0.85 Ma)

The extant nannofossil biostratigraphic and biochronologic framework for the early-middle Pleistocene time interval has been tested through the micropaleontological analysis of globally distributed high-quality low- to mid-latitude deep-sea successions. The quantitative temporal distribution patterns of relative abundances of selected taxa were reconstructed in critical intervals, and the following biohorizons were defined: first occurrence of medium-sized Gephyrocapsa spp. ($\text{bmG}$); last occurrence of Calcidiscus macintyrei ($\text{tCm}$); first occurrence of large Gephyrocapsa spp. ($\text{blG}$); last occurrence of large Gephyrocapsa spp. ($\text{tlG}$); first occurrence of Reticulofenestra asanoi ($\text{bRa}$); re-entrance of medium-sized Gephyrocapsa spp. ($\text{reemG}$) and last occurrence of Reticulofenestra asanoi ($\text{tRa}$). The detailed patterns of abundance change at these biohorizons were used to generate a detailed biostratigraphy, and the biostratigraphic data were transformed into a precise biochronology by means of correlation to isotope stratigraphies and astronomical timescales. The degree of isochrony or diachrony of the biohorizons was evaluated. Biohorizons $\text{tlG}$ and $\text{tRa}$ are isochronous occurring close to marine isotope stages (MIS) 55 and MIS 22, respectively, and $\text{bmG}$ and $\text{blG}$ are slightly diachronous on the order of 30–40 kyr, whereas biohorizons $\text{tCm}$, $\text{reemG}$ and $\text{bRa}$ are confirmed as diachronous on the order of 100, 80 and 60 kyr, respectively. Some of the events are clearly controlled by environmental conditions, e.g. the last occurrence of R. asanoi, related to significant environmental changes associated with the first large-amplitude glaciation of the late Quaternary, MIS 22.     

Gephyrocapsa occurrence during the Middle Pleistocene transition in the Northern Pacific Ocean (Shatsky Rise)

The gephyrocapsids, main component of the Pleistocene calcareous nannofossil assemblages, are here discussed as biostratigraphical and paleoclimate tools. The occurrence of the genus Gephyrocapsa is quantitatively analysed in the core ODP 198-1209B, collected in the NW Pacific Ocean. The studied stratigraphic succession covers a time interval including the Middle Pleistocene Transition (MPT), a highly investigated period characterized by important global climate changes. During the Pleistocene, Gephyrocapsa is extremely abundant and provides several bioevents used in biostratigraphy. In addition to the known standard events, we observe the occurrence of particular Gephyrocapsa morphogroups and significant changes in the relative abundance of G. caribbeanica. In the Early-Middle Pleistocene we identify four intervals based on the Gephyrocapsa content. Moreover, during the MPT, the stratigraphic distribution of Gephyrocapsa underlines a dominance of both the small morphogroup and the medium-sized G. caribbeanica that could be dependent on their paleoecology. Small Gephyrocapsa and G. caribbeanica seem to be more competitive than other coccolithophores during the global oceanographic variations and the re-organisation of the glacial-interglacial periodicity during the MPT.     

Response of nannoplankton to major changes in sea-surface temperature and movements of hydrological fronts over Site DSDP 594 (south Chatham Rise, southeastern New Zealand), during the last 130 kyr

A high-resolution history of paleoceanographic changes in the subpolar waters of the southern margin of the Subtropical Convergence Zone during the last 130 kyr, is present in foraminiferal assemblages of DSDP Site 594. The foraminifera indicate that sea-surface temperatures during the Last Interglacial Climax were warmer than today, and that between substage 5d through to the end of isotope stage 2, temperatures were mostly cooler than Holocene temperatures. The paleotemperatures suggest that (1) the Subtropical Convergence was located over the site during substage 5e, later moving further north, then moving southwards to near the site during the Holocene, and (2) the Polar Front was positioned over the Site during glacial stages 6, 4, 2 and possibly parts of stage 3. Several major events are indicated by the nannofloral assemblages during these large changes in sea-surface temperature and associated reorganization of ocean circulation. First, the time-progressive trends between E. huxleyi and medium to large Gephyrocapsa are unique to this site, with E. huxleyi dominating over medium Gephyrocapsa during stages 5c-a, middle part of stage 4 and after the middle point of stage 3. This unusual trend may (at least partly) be caused by the shift of the Polar Front across the site. Second, upwelling flora (E. huxleyi and small placoliths) increase in abundance during stages 1, 3 and 5, suggesting that upwelling or disturbance of water stratification took place during the interglacials. Thirdly, there are no significant differences between the distribution patterns of the various morphotypes of medium to large Gephyrocapsa, and the combined value of all medium Gephyrocapsa increases in abundance during glacials (stages 2 and 4 and the end of stage 6), similar to the abundance trends in benthic foraminifera. Finally, subordinate nannofossil taxa also show distinctive. climatic trends during the last glacial cycle: (1) Syracosphaera spp. are present in increased abundance during warmer extremes in climate (substages 5e, 5a, and stage 1); (2) Coccolithus pelagicus and Calcidiscus leptoporus dominate the subordinate nannofossil taxa, and their relative proportions seem to provide a useful paleoceanographic index, with C. pelagicus dominating when the Polar Front Zone is over the site (stages 6, 4 and 2), whilst C. leptoporus is relatively more abundant when the STC is positioned over the site (stages 1 and 5e). Increased abundance of C. pelagicus also can indicate intensified coastal upwelling.     

Movement of oceanic fronts south of Australia during the last 10 ka: interpretation of calcareous nannoplankton in surface sediments from the Southern Ocean

Preservation of calcareous nannoplankton in surface sediment samples from the Southern Ocean south of Australia and adjacent to New Zealand record a single assemblage. The dominant species are Emiliania huxleyi, Gephyrocapsa muellerae, Calcidiscus leptoporus, Helicosphaera carteri and Coccolithus pelagicus. The assemblage varies little in abundance and diversity with minor correlation to present-day overlying surface water masses and oceanic fronts. Increase in abundance of H. carteri and C. pelagicus in the region of the Subtropical Front may reflect higher nutrients associated with this front. The assemblage, although altered by dissolution, represents a warmer climatic interval than present-day with the presence of preferentially dissolved, warm-water species preserved as far south as the Polar Front. The presence of warm-water species under sub-Antarctic waters at the Polar Front is interpreted as a relic population from the Holocene climatic optimum of 10–8 ka. The absence of coccoliths in sediments poleward of the Polar Front suggests an equatorward shift of this front following the climatic optimum, resulting in increased productivity of siliceous phytoplankton associated with the colder waters and increased dissolution of coccoliths. Movement of the Subtropical Front for the same interval is not recorded in the preserved coccoliths. The more heavily calcified form of E. huxleyi which dominates the living assemblage north of the Subtropical Front is subject to dissolution in this region and is poorly preserved in the sediment assemblage.     

Long-term upwelling evolution in tropical and equatorial Pacific during the last 800 kyr as revealed by coccolithophore assemblages

The coccolithophore assemblages in two ODP Sites (1237 and 1238) are studied in order to reconstruct the paleoenvironmental conditions in the tropical and equatorial Pacific during the last 800 kyr. Both ODP Sites are located in the two most significant upwelling zones of the tropical and equatorial Pacific: Peru and Equatorial upwelling, respectively. The two sites are considered to have had similar evolutions. The coccolith relative abundance, the nannofossil accumulation rate (NAR) and the N ratio (namely, the proportion of < 3 μm placoliths in relation to Florisphaera profunda) allow us to identify three different intervals. Interval I (0.86-0.45 Ma) and interval III (0.22-0 Ma) are related to weak upwelling and weak Trade Winds, as suggested by coccolithophore assemblages with low N ratios. Interval II (0.45-0.22 Ma), characterized by dominant Gephyrocapsa caribbeanica and very abundant “small” Gephyrocapsa and Gephyrocapsa oceanica, is conversely related to intense upwelling and enhanced Trade Winds.     

Coccolith distribution patterns in surface sediments of Equatorial and Southeastern Pacific Ocean

This study aims to contribute to a more detailed knowledge of the biogeography of coccolithophores in the Equatorial and Southeastern Pacific Ocean. Census data of fossil coccoliths are presented in a suite of core-top sediment samples from 15°N to 50.6°S and from 71°W to 93°W. Following standard preparation of smear slides, a total of 19 taxa are recognized in light microscopy and their relative abundances are determined for 134 surface sediment samples. Considering the multivariate character of oceanic conditions and their effects on phytoplankton, a Factor Analysis was performed and three factors were retained. Factor 1, dominated by Florisphaera profunda and Gephyrocapsa oceanica, includes samples located under warm water masses and indicates the occurrence of calcite dissolution in the water column in the area offshore Chile. Factor 2 is related to cold, low-salinity surface-water masses from the Chilean upwelling, and is dominated by Emiliania huxleyi, Gephyrocapsa sp. < 3 μm, Coccolithus pelagicus and Gephyrocapsa muellerae. Factor 3 is linked to more saline, coastal upwelling areas where Calcidiscus leptoporus and Helicosphaera carteri are the dominant species.     

Upper Campanian–Maastrichtian chronostratigraphy of the Skælskør‐1 core,Denmark: correlation at the basinal and global scale and implications for changes in sea‐surface temperatures

The lithostratigraphy, calcareous nannofossil biostratigraphy, carbon‐ and oxygen‐isotope stratigraphy and gamma‐ray profile are presented for the Skælskør‐1 core, eastern Denmark. The correlation of carbon isotopes to Gubbio (Italy) and ODP Site 762C (Indian Ocean) provides the chronostratigrahical framework of the core through a tie to magnetostratigraphy. Two new carbon‐isotope excursions are defined for the uppermost Maastrichtian of the core and prove useful for long‐distance correlation. Twenty stratigraphic tie‐points are used for correlation of the upper Campanian–Maastrichtian interval by combining carbon‐isotope and gamma‐ray variations. Significant dissimilarities in the gamma‐ray profiles of the Danish Basin cores preclude the sole use of this tool for basin‐scale correlations. Bulk oxygen‐isotopes and semi‐quantitative abundance changes in the warm‐water calcareous nannofossil Watznaueria barnesiae and the cool‐water Kamptnerius magnificus highlight the following past changes in sea‐surface temperatures (SSTs): relatively warm late Campanian SSTs, cooling across the Campanian–Maastrichtian boundary and through the early Maastrichtian, warming across the early–late Maastrichtian transition, cooling in the late Maastrichtian, intense warming in the latest Maastrichtian chron C29r, followed by a very short episode of cooling immediately before the Cretaceous–Palaeogene boundary. The late Campanian–Maastrichtian evolution in sea water temperatures inferred from the Danish Basin is similar to that delineated at tropical latitude oceanic sites.     

Integrated quantitative biostratigraphy of the latest Tortonian-early Messinian Pissouri section (Cyprus): An evaluation of calcareous plankton bioevents

A quantitative study was performed on planktonic foraminifera and calcareous nannofossils of the astronomically dated Late Miocene Pissouri section (Cyprus). Our results confirm the reliability of well-known planktonic foraminiferal events as Catapsydraxparvulus LO (Last Occurrence), sinistral coiling change of Globorotaliascitula, Globorotaliamiotumida group FRO (First Regular Occurrence), Globorotalianicolae FO (First occurrence) and LO, sinistral coiling change of Neogloboquadrinaacostaensis and also of several important calcareous nannofossil events (Amaurolithusprimus FO, Amaurolithusdelicatus FO, Reticulofenestrarotaria FO and FCO). Integrated planktonic foraminifera and calcareous nannoplankton data contribute to an enhanced time resolution of the Tortonian - early Messinian interval in the Levantine basin, and contribute to detailed correlations throughout the Eastern Mediterranean. In addition, we compare methodologies commonly used in calcareous plankton biostratigraphy, and shortly outline sources of bias that can influence the results of stratigraphic studies.     

Integrated stratigraphy and astronomical calibration of the Serravallian/Tortonian boundary section at Monte Gibliscemi (Sicily, Italy)

Results are presented of an integrated stratigraphic (calcareous plankton biostratigraphy, cyclostratigraphy and magnetostratigraphy) study of the Serravallian/Tortonian (S/T) boundary section of Monte Gibliscemi (Sicily, Italy). Astronomical calibration of the sedimentary cycles provides absolute ages for calcareous plankton bio-events in the interval between 9.8 and 12.1 Ma. The first occurrence (FO) of Neogloboquadrina acostaensis, usually taken to delimit the S/T boundary, is dated astronomically at 11.781 Ma, pre-dating the migratory arrival of the species at low latitudes in the Atlantic by almost 2 million years. In contrast to delayed low-latitude arrival of N. acostaensis, Paragloborotalia mayeri shows a delayed low-latitude extinction of slightly more than 0.7 million years with respect to the Mediterranean (last occurrence (LO) at 10.49 Ma at Ceara Rise; LO at 11.205 Ma in the Mediterranean). The Discoaster hamatus FO, dated at 10.150 Ma, is clearly delayed with respect to the open ocean. The ages of D. kugleri first and last common occurrence (FCO and LCO), Catinaster coalitus FO, Coccolithus miopelagicus last (regular) occurrence (L(R)O) and the D. hamatus/neohamatus cross-over, however, are in good to excellent agreement with astronomically tuned ages for the same events at Ceara Rise (tropical Atlantic), suggesting that both independently established timescales are consistent with one another. The lack of a reliable magnetostratigraphy hampers a direct comparison with the geomagnetic polarity timescale of Cande and Kent (1995; CK95), but ages of calcareous nannofossil events suggests that CK95 is significantly younger over the studied time interval. Approximate astronomical ages for the polarity reversals were obtained by exporting astronomical ages of selected nannofossil events from Ceara Rise (and the Mediterranean) to eastern equatorial Pacific ODP Leg 138 Site 845, which has a reliable magnetostratigraphy.Our data from the Rio Mazzapiedi–Castellania section reveal that the base of the Tortonian stratotype corresponds almost exactly with the first regular occurrence (FRO) of N. acostaensis s.s. as defined in the present study, dated at 10.554 Ma. An extrapolated age of 11.8 Ma calculated for the top of the Serravallian stratotype indicates that there is a gap between the top of the Serravallian and the base of the Tortonian stratotype, potentially rendering all bio-events in the interval between 11.8 and 10.554 Ma suitable for delimiting the S/T boundary. Despite the tectonic deformation and the lack of a magnetostratigraphy, Gibliscemi remains a candidate to define the S/T boundary by means of the Tortonian global boundary stratotype section and point (GSSP).     

Spatial dynamics of coccolithophore assemblages in the Equatorial Western-Central Pacific Ocean

The spatial distribution of living coccolithophores was studied in the Western-Central Equatorial Pacific Ocean during November–December, 1990 and September–October, 1992. The highest local concentration of coccolithophores occurred at the thermocline in well-stratified waters, but at sea-surface level in dynamic waters. In total, 111 coccolithophore taxa were recognized, some of which exhibited hydrographically controlled variation in their absolute abundance. Gephyrocapsa oceanica and Oolithotus antillarum were abundant in the upwelling front. Most of the lower photic dwellers were abundant in the tropical to subtropical stations regardless of the water stratification. The coccolithophore flora of well-stratified waters could be distinguished from the upwelling front flora by the higher abundance of Umbellosphaera irregularis and lower abundance of G. oceanica. The temperature mixed-water flora was characterized by a high abundance of Emiliania huxleyi. The vertical distribution of all coccolithophore taxa, except three placolith-bearing species, Gephyrocapsa ericsonii, G. oceanica and E. huxleyi, was controlled by upper photic-zone temperature and water stratification. The upper or lower vertical distribution limits of many coccolithophore taxa coincided with the top of the thermocline. The most common 27 taxa were grouped into four ecological groups, Upper Photic-zone Group (UPG), Middle Photic-zone Group (MPG), Lower Photic-zone Group (LPG) and Omnipresent Group (OPG), on the basis of their vertical distribution. By analyzing the hydrographic control on the vertical distribution of these four ecological groups, four ecological assemblages were recognized: High Temperature; Warm Oligotrophic; Warm Eutrophic; and Temperate Mixed-water Assemblages. In equatorial waters, the total coccolithophore assemblage across the photic-zone was controlled by the population in the upper photic-zone. The UPG monopolized the upper photic-zone flora in the High Temperature Assemblage. In the Warn Oligotrophic-water Assemblage, common OPG accompanied abundant UPG in the upper photic-zone. The upper photic-zone of the Warm Eutrophic Assemblage consisted of UPG, MPG and OPG.Emiliania huxleyi and Gephyrocapsa oceanica, the major component of OPG, displayed intra-specific morphological variations. G. oceanica Type 1 was restricted to the upper photic-zone of well-stratified oligotrophic waters. Conversely, in these waters E. huxleyi Type C and G. oceanica Type 2 only occurred below the thermocline. These two taxa also coexist with G. oceanica Type 3 and E. huxleyi Type A in the upper photic zone of dynamic waters.     

Paleoceanography of the mid-Pleistocene

The fossil record of the Pleistocene calcareous nannoplankton indicates that during the mid-Pleistocene (0.93–1.25 my) occurred an episode of overwhelming dominance of smallGephyrocapsa. During this episode normally abundant, large size specimens of this genus (mainlyGephyrocapsa oceanica) were virtually excluded from the phytoplankton of tropical and subtropical oceans. The best modern analog of this dominantly smallGephyrocapsa assemblage is the subpolarEmiliania huxleyi assemblage, which implies that nutrient content was significantly greater and water temperature was lower in the photic water column of the tropical oceans than they are today. Increased equatorial upwelling in the oceans, on a scale much greater than today, may explain the above pattern.To achieve such broad equatorial upwelling there must be a source and a drive for cold, dense water. The Arctic Ocean, which was probably seasonally free of ice during this interval of the mid-Pleistocene, is capable of providing the requisite source as well as a drive for the inferred equatorial upwelling. The energy balance of a predominantly ice-free Arctic Ocean requires an approximately three to seven fold increase of hydrospheric heat transport from the North Atlantic to the Arctic Ocean, which dictates a corresponding or even greater increase in the volume of warm water entering the Arctic Ocean at the surface and cold dense water exiting at depth to the North Atlantic. Such enhanced dense water formation in the Arctic Ocean could drive the intensified equatorial upwelling implied by the smallGephyrocapsa dominance interval.If the above scenario is correct then the climate of the earth's northern hemisphere during the mid-Pleistocene may have been very different from the younger Pleistocene climate. One manifestation of this difference may be the mid-Pleistocene shift in climatic cycle periodicity from 40 ky to 100 ky. Another important aspect is that the enhanced greenhouse effect expected during the next century because of an increase of atmospheric CO₂ is thought to lead directly to melting of the Arctic Ocean ice cover and of the Greenland ice sheet. Thus, the “greenhouse” Arctic Ocean and its attendant ocean circulation would resemble the inferred mid-Pleistocene conditions.     

长江三角洲DY03孔颗石藻及沉积环境浅析

本文研究了上海奉贤DY03孔的颗石藻化石。钻孔60.8-2m深度产颗石藻化石,涵盖上更新统的川沙组、南汇组,全新统的娄塘组、上海组和青浦组。颗石藻化石赋存于灰色泥、泥质粉砂或粉细砂中,但化石种类贫乏。群落面貌以Gephyrocapsa oceanica占绝对数量,Braarudosphaera bigelowii,Helicosphaera hyalina和Heli-cosphaera wallichii等偶现,属典型的单种类组合。研究依据颗石藻定量统计,划分出两个海相层:第一海相层,孔深61-40m,属上更新统川沙组和南汇组,颗石藻丰度低,属种单一,代表了海侵过程中的潮上带环境;第二海相层,孔深24.8-2.0m,属全新统娄塘组、上海组和青浦组,颗石藻化石丰度较高,并有数个属种共生,反映当时的滨岸潮上带-河口滨海-浅海近岸海湾的环境演化过程。综合前人的研究成果,认为长江三角洲全新世海侵之强度和范围都超过晚更新世发生的海侵。此外,本文仔细讨论了Gephyrocapsa oceanica的生态特征,显示其为一个盐度适应能力较强、分布广泛的暖水种。     

Late Quaternary high uplift rates in northeastern Sicily: evidence from calcareous nannofossils and benthic and planktonic foraminifera

The northeastern part of Sicily is characterized by intense seismic activity. Several systems of faults have been recognized in Pliocene and Pleistocene sediments in the area and, in fact, estimates of uplift rates are among the highest recorded in Sicily and south Italy. We examined calcareous nannofossil and benthic and planktonic foraminifera assemblages from pelitic sediments of the Contrada Zura section (Barcellona Pozzo di Gotto Basin, Furnari village, Messina). The occurrence of Emiliania huxleyi, a coccolithophore species which appeared in the oceanic record about 270,000 years ago, is witness to the uniqueness of this outcrop, while the planktonic/benthic foraminifera ratio indicates a deep (slope) environment, in agreement with previous observations on macrobenthic and ostracod paleo-communities. Independently from the numerical estimate of the paleo-depth, there is little doubt that the occurrence of E. huxleyi in such sediments might be explained by exceptional uplift rates. Since resulting uplift rate estimates, between 3.2 and 5.5 mm/year, exceeded by far the regional, longer-term, vertical tectonic motion, we argue that a major contribution of coseismic displacement along active faults occurred in the Furnari area.     

Calcareous nannofossil biostratigraphy of the M. del Casino section (northern Apennines,Italy) and paleoceanographic conditions at times of Late Miocene sapropel formation

A detailed quantitative calcareous nannofossil analysis has been performed on 138 samples from the astronomically dated Monte del Casino section with the aim to identify and precisely date the most important calcareous nannofossil events across the Tortonian/Messinian boundary in the Mediterranean, and to unravel paleoceanographic conditions at times of sapropel formation during the Late Miocene. From the biostratigraphic perspective, the genus Amaurolithus provides three successive first occurrences (FOs): A. primus, A. cf. amplificus and A. delicatus, dated at 7.446, 7.434 and 7.226 Ma, respectively. Other bioevents include the base and top of the `small reticulofenestrids' Acme, dated at 7.644 and 6.697 Ma, and the FO, FCO and LO of R. rotaria, dated at 7.405, 7.226 and 6.771 Ma. These events appear to be useful in improving biostratigraphic resolution in the Tortonian–Messinian boundary interval, at least for the Mediterranean. Quantitative analysis revealed changes in the calcareous nannofossil assemblage associated with the sapropels. The observed fluctuations suggest a single mechanism for sapropel formation in the Mediterranean during the late Neogene. Sapropels are characterized by a decrease in the total number of coccoliths, interpreted mainly as a reduction in calcareous nannofossil production due to increased siliceous plankton production during spring blooms; and an increase in reworked specimens, interpreted to reflect enhanced continental input via river run-off. An increase in abundance of the genus Rhabdosphaera can be explained by opportunistic behavior at the end of the spring bloom when nutrient levels start to become impoverished. As far as sea surface water temperature indicators are concerned, warm water D. pentaradiatus shows positive fluctuations in sapropels while cooler water D. intercalaris and C. pelagicus show negative fluctuations.     

Size variations of the calcareous nannofossil taxon Discoaster multiradiatus (Incertae sedis) across the Paleocene–Eocene thermal maximum in ocean drilling program holes 690B and 1209B

Size measurements of the calcareous nannofossil taxon Discoaster multiradiatus were carried out across the Paleocene–Eocene Thermal Maximum (PETM) in Ocean Drilling Program Holes 690B (Maud Rise, Weddell Sea) and 1209B (Shatsky Rise, Pacific Ocean). Morphometric investigations show that D. multiradiatus specimens are generally larger at ODP Site 1209 than at ODP Site 690. A limited increase in size of D. multiradiatus is recorded at ODP Site 1209, whereas significant enlargements characterize ODP Site 690. Preservation is comparable at both sites: nannofossils are moderately preserved with some evidence of etching/overgrowth in the PETM interval. Yet, D. multiradiatus variations do not correlate with preservation state and morphometric data most likely represent primary signals rather than diagenetic artifacts. There is a direct relationship between D. multiradiatus size and paleotemperatures: largest specimens are coeval with global warming associated with the PETM, inferred to result from excess atmospheric CO₂ due to (partial) oxidation of massive quantities of methane. Size increases and largest specimens of D. multiradiatus occur at different stratigraphic levels within PETM at ODP Sites 690 and 1209. A marked shift in diameter size was observed at the onset and peak of the Carbon Isotopic Excursion (CIE) at ODP Site 690, but only at the end of CIE and initial recovery interval at ODP Site 1209. This diachroneity is puzzling, but indeed correlates well with reconstructed changes in surface and thermocline water masses temperature and salinity in the PETM interval at low and high latitudes. The presumed high concentrations of carbon dioxide seem to have not influenced the morphometry of D. multiradiatus. The major size increase of D. multiradiatus in the CIE of ODP Site 690 could represent the migration of larger-sized allochtonus specimens that moved from peri-equatorial/subtropical areas to higher latitudes during the warmest interval of the PETM, although no direct evidence of distinct populations/subpopulations has been obtained from the frequency diagrams. As a result, we infer that D. multiradiatus is a proxy of water masses stratification and might be used for deriving temperature–salinity–nutrient conditions in the mixed layer and thermocline and their dynamics.