Planktonic Foraminiferal Data Research Articles

Assessment of Chalk as an Archive for the Lithium Isotope Composition of Seawater

AbstractThe understanding of silicate weathering and its role as a sink for atmospheric CO2 is important to get a better insight into how the Earth shifts from warm to cool climates. The lithium isotope composition (δ7Li) of marine carbonates can be used as a proxy to track the past chemical weathering of silicates. A high‐resolution δ7Li record would be helpful to evaluate the role of silicate weathering during the late Cretaceous climate cooling. Here, we assess chalk as a potential archive for reconstructing Late Cretaceous seawater Li isotope composition by comparing Maastrichtian chalk from Northern Germany (Hemmoor, Kronsmoor) to a Quaternary coccolith ooze from the Manihiki Plateau (Pacific Ocean) as a lithological analog to modern conditions. We observe a negative offset of 3.9 ± 0.6‰ for the coccolith ooze relative to the modern seawater Li isotope composition (+31.1 ± 0.3‰; 2SE; n = 54), a value that falls in the range of published offsets for modern core‐top samples and for brachiopod calcite. Further, the negative offset between the Li isotope compositions of Manihiki coccolith ooze and modern planktonic foraminifera is 2.3 ± 0.6‰. Although chalk represents a diagenetically altered modification of pelagic nannofossil ooze, manifested by changes in the composition of trace elements, we observe a consistent offset of Li isotope data between Maastrichtian chalk and Maastrichtian planktonic foraminiferal data (−1.4 ± 0. 5‰) that lies within the uncertainty of modern values. We therefore suggest that chalk can be used as a reliable archive for δ7Li reconstructions.

The response of calcareous plankton to the Sapropel S1 interval in North Ionian Sea

A high resolution study of calcareous nannofossils has been performed on samples from the Sapropel S1interval deposited in the North Ionian Sea, with the aim to assess the paleoenvironmental changes in the photic zone during this crucial interval in Mediterranean circulation. Calcareous nannofossil data have been integrated with planktonic foraminiferal data recently published from which the paleoclimatic curve has been constructed. Placoliths (namely Emiliania huxleyi) and Florisphaera profunda distributions, along with that of planktonic foraminifer Globigerinoides ruber white, evidence that, after a progressive weakening of surface water mixing, a deep chlorophyll maximum developed just prior to the sapropel deposition. We suggest that these changes took place as a response to enhanced precipitation conditions and riverine discharge as testified by increasing trend of reworked coccoliths and the occurrence of lower salinity taxon Braarudosphaera bigelowii. Calcareous nannofossils also point out that the oceanographic (water column stratification, reduced bottom water ventilation) and biogeochemical (increased primary production) processes that occurred during the S1 formation were particularly dominant during the earliest part of the older S1 warm phase (S1a). Our results support than some re-ventilation events of the shallow depth of studied site (665 m) occurred to some extent, particularly during the final phases of S1a. The distribution of holococcoliths, more abundant during the cold interruption phase S1i, seems confirm that the preservation of these tiny and delicate coccoliths, highly susceptible to dissolution, is enhanced under seafloor re-ventilation conditions. Finally, we tentatively suggest that preservation also plays a significant role in the distribution of the warm upper photic zone taxa, particularly during the warm S1b interval.

Paleocurrents in the Charlie-Gibbs Fracture Zone during the Late Quaternary

Planktonic foraminiferal and ice-rafted debris count data, as well as the mean size of mineral particles of the 10–63 μm fraction (sortable silt, $$\overline {SS} $$ ) were used as a proxy for surface and near-bottom paleocurrent intensity variations. The data obtained support our hypothesis about turbiditic origin of the lower (80–370 cm) section of the studied AMK-4515 core. Stratigraphic subdivision of the upper section (0–80 cm) makes it possible to allocate two marine isotope stages (MIS) covering the last 27 ka. The main intervals of the North Atlantic Polar Front (PF) migrations were recorded: south of the modern PF position during early MIS 2 (24–27 ka) with PF presence in the study area during MIS 2 (20–24 ka); south of the study area during the last glacial maximum (18–20 ka). Influence of the near-bottom currents within the investigated interval led to beginning of the channel-related drift formation on the northern slope of the southern channel of the Charlie-Gibbs Fracture Zone. There is a weak relationship between intensity of near-bottom contour currents and long-term climatic cyclicity. However, intervals corresponding to Heinrich events coincide with decrease in bottom currents activity.

Late Coniacian- Early Campanian planktonic foraminiferal bioevents and biostratigraphy of the northern Tunisia

Planktonic foraminifera were quantitatively analyzed across the Upper Coniacian - Lower Campanian succession at Jebel Ennahli and Ettout sections, northern Tunisia. This succession is characterized by hemipelagic facies domainated by a continuous sedimentary succession, allowing a good biostratigraphic correlation for this time interval. 55 planktonic foraminiferal species have been identified belonging to 13 genera. The distribution pattern of 17 heterohelicids and 38 trochospiral forms reveals to the identification of12 major bio-events and establishing three zones based on heterohelicids and five zones based on trochospiral forms. The trochospiral based zones are from base to top: Dicarinella primitiva Interval Zone, Dicarinella concavata Interval Zone, Dicarinella asymetrica Total Range Zone, and Globotruncanita elevata/Globotruncana arca Concurrent Range Zone, Globotruncana ventricosa Interval Zone; while the three heterohelicid based planktonic foraminiferal biozones are: Pseudotextularia nuttalli Interval Zone, Sigalia carpatica Interval Zone, Ventilabrella eggeri/Planoglobulina manuelensisConcurrent Range Zone. The bioevents and planktonic foraminiferal zones were correlated with the previously published works especially in Tunisia and other palaeolatitude provinces. There are no significant planktonic foraminiferal datums recorded across the Coniacian/Santonian boundary (CSB) except the lowest occurrence (LO) of Dicarinella asymetrica, which was recorded slightly below the LO of inoceramid Platyceramus cycloides cycloides representing a good proxy for the CSB. The Santonian/Campanian boundary of the studied sections are characterized by a major turnover represented by the highest occurrence of Sigalia, Dicarinella and Whiteinella genera; in addition to the lowest occurrence of Ventilabrella and Planoglobulina genera and above slightly the LO Globotruncana and Globotruncanita.

Planktonic foraminiferal biostratigraphy and paleoecology of Upper Cretaceous deposits from the Palmyride Region, Syria

Planktonic foraminifera were quantitatively analyzed across the Upper Coniacian - Lower Campanian succession at Jebel Ennahli and Ettout sections, northern Tunisia. This succession is characterized by hemipelagic facies domainated by a continuous sedimentary succession, allowing a good biostratigraphic correlation for this time interval. 55 planktonic foraminiferal species have been identified belonging to 13 genera. The distribution pattern of 17 heterohelicids and 38 trochospiral forms reveals to the identification of12 major bio-events and establishing three zones based on heterohelicids and five zones based on trochospiral forms. The trochospiral based zones are from base to top: Dicarinella primitiva Interval Zone, Dicarinella concavata Interval Zone, Dicarinella asymetrica Total Range Zone, and Globotruncanita elevata/Globotruncana arca Concurrent Range Zone, Globotruncana ventricosa Interval Zone; while the three heterohelicid based planktonic foraminiferal biozones are: Pseudotextularia nuttalli Interval Zone, Sigalia carpatica Interval Zone, Ventilabrella eggeri/Planoglobulina manuelensis Concurrent Range Zone. The bioevents and planktonic foraminiferal zones were correlated with the previously published works especially in Tunisia and other palaeolatitude provinces. There are no significant planktonic foraminiferal datums recorded across the Coniacian/Santonian boundary (CSB) except the lowest occurrence (LO) of Dicarinella asymetrica , which was recorded slightly below the LO of inoceramid Platyceramus cycloides cycloides representing a good proxy for the CSB. The Santonian/Campanian boundary of the studied sections are characterized by a major turnover represented by the highest occurrence of Sigalia , Dicarinella and Whiteinella genera; in addition to the lowest occurrence of Ventilabrella and Planoglobulina genera and above slightly the LO Globotruncana and Globotruncanita .

Eocene biogeochronology and magnetostratigraphic revision of ODP Hole 762C, Exmouth Plateau (northwest Australian Shelf)

We present a chronostratigraphic model for the Eocene from ODP Leg 122 Hole 762C (northwest Australian shelf), integrating calcareous nannofossil data with previously published planktonic foraminiferal data, magnetostratigraphy and stable isotopic data. This ~240m thick interval extends from magnetic polarity Chron C25r to Chron C15r, and nannofossil zones CP6/7 to CP16a (NP7/8-NP21). Examination of the calcareous nannofossil biostratigraphy showed several potential hiatuses within this stratigraphic section at Hole 762C. The presence of these hiatuses was supported by cross-correlation of planktonic foraminiferal P-zones, magnetostratigraphic reversals and δ13C and δ18O isotopic excursions. These biostratigraphic and isotopic data were used to re-evaluate the original magnetostratigraphy, and this revised framework was integrated into a new age model. The original depositional model from Leg 122 proposed a continuous section with relatively low sedimentation rates in the Lutetian and basal Ypresian (< 1.0cm/ky); however, our results suggest that sedimentation rates were relatively high but that the section is interrupted by four hiatuses, each ~1-2 myr in duration. This new age model allowed revision of sedimentation rates at Hole 762C. These revised rates are used to estimate the ages of calcareous nannofossil bioevents, which are compared to several additional, globally distributed localities.

Early Oligocene Larger Foraminiferal Biostratigraphy of the Qom Formation, South of Uromieh (NW Iran)

The marine strata of the Qom Formation south of Uromieh (Baranduz) yielded a moderately diverse larger benthic foraminiferal fauna resembling the fauna already described from European marine sequences. The fauna allowed the author to correlate the larger foraminiferal assemblage of northwestern Iran with the standard shallow benthic zonation. The assemblage is dominated by hyaline and porcellaneous forms including Operculina, Nummulites, Neorotalia, Penarchaias, Victoriella, Peneroplis, Praerhapydionina, Spirolina, Halkyardia, Planorbulina, Dendritina, Borelis and Bullaveolina. This faunal assemblage is also accompanied by coralline alga Subterraniphyllum thomasii. The presence of Nummulites cf. fichteli and N. cf. vascus in the absence of Lepidocyclina allows correlation of the foraminiferal fauna of the studied section with the SBZ 21 shallow benthic zone, indicating early-middle Rupelian age. Planktonic foraminiferal data from the overlying marly succession support the assigned age using larger benthic foraminifera.

Early and middle Holocene in the Aegean Sea: interplay between high and low latitude climate variability

Changes in the orbital parameters, solar output, and ocean circulation are widely considered as main drivers of the Holocene climate. Yet, the interaction between these forcings and the role that they play to produce the pattern of changes observed in different domains of the climate system remain debated. Here, we present new early to middle Holocene season-specific sea surface temperature (SST) and δ 18O seawater results, based on organic-walled dinoflagellate cyst and planktonic foraminiferal data from two sediment cores located in the central (SL21) and south-eastern (LC21) Aegean Sea (eastern Mediterranean). Today, this region is affected by high to mid latitude climate in winter and tropical/subtropical climate in summer. The reconstructed δ 18O seawater from LC21 displays a marked (∼1.3%) negative shift between 10.7 and 9.7 ka BP, which represents the regional expression of the orbitally driven African monsoon intensification and attendant freshwater flooding into the eastern Mediterranean. A virtually contemporaneous shift, of the same sign and magnitude, is apparent in the δ 18O speleothem record from Soreq Cave (Northern Israel), an important part of which may therefore reflect a change in the isotopic composition of the moisture source region (Aegean and Levantine Seas). Our SST reconstructions show that Aegean winter SSTs decreased in concert with intensifications of the Siberian High, as reflected in the GISP2 nss [K +] record. Specifically, three distinct sea surface cooling events at 10.5, 9.5–9.03 and 8.8–7.8 ka BP in the central Aegean Sea match increases in GISP2 nss [K +]. These events also coincide with dry interludes in Indian monsoon, hinting at large (hemispheric) scale teleconnections during the early Holocene on centennial timescales. A prominent short-lived (∼150 years) cooling event in core SL21 – centred on 8.2 ka BP – is coeval to the ‘8.2 ka BP event’ in the Greenland δ 18O ice, which is commonly linked to a melt-water related perturbation of the Atlantic Meridional Overturning Circulation and associated ocean heat transport. By deciphering the phasing between a recently published record of reduced overflow from the Nordic Seas into the northern North Atlantic, the Greenland δ 18O ice ‘8.2 ka BP event’ anomaly, and the short-lived cooling in SL21, we demonstrate severe far-field impacts of this North Atlantic event in the Aegean Sea. The Aegean is isolated from the North Atlantic oceanic circulation, so that signal transmission must have been of an atmospheric nature.

Calcareous nannofossil and planktonic foraminiferal distributional patterns during deposition of sapropels S6, S5 and S1 in the Libyan Sea (Eastern Mediterranean)

In core ADE3-23 collected in the Libyan Sea, the nannofossil species Coccolithus pelagicus, Coronosphaera spp., Helicosphaera spp., Syracosphaera spp., Calcidiscus spp., small Gephyrocapsa spp., and the planktonic foraminifers Globigerina bulloides, Neogloboquadrina pachyderma, Globorotalia scitula, Turborotalita quinqueloba and Neogloboquadrina dutertrei prevail in sapropel S6 (midpoint at 172 ka b.p.), indicative of cold and highly productive surface conditions. Warm and highly stratified water-column conditions are recorded by the characteristic assemblage of Globigerinoides ruber, Globoturborotalita rubescens, Florisphaera profunda, Rhabdosphaera spp. during the sapropel S5 depositional interval (midpoint at 124 ka b.p.). Compared with S5, Globigerinita glutinata, Globorotalia inflata, Globigerinella siphonifera, Globorotalia truncatulinoides and the calcareous nannofossil Emiliania huxleyi characterise less stratified conditions within sapropel S1 (midpoint at 8.5 ka b.p.). Multivariate statistical analyses of calcareous nannofossil and planktonic foraminifers in core ADE3-23 identify planktonic assemblages which typify sapropels S6, S5 and S1 in the Libyan Sea. A warmer interval is recognised in the middle part of the cold S6, and can be associated with an influx of less saline waters and the occurrence of a faint, temporary deep chlorophyll maximum. Evidence for enhanced surface productivity and breakdown of stratification is observed in the middle–upper part of the warm S5, associated with climatic deterioration. Moreover, an increase in surface productivity in the upper S1 implies weak stratification. Our combined calcareous nannofossil and planktonic foraminiferal data add to the evidence that climate variability was more pronounced than commonly considered to date for all the three studied Eastern Mediterranean sapropel depositional intervals.

Aptian (Lower Cretaceous) biostratigraphy and cephalopods from north central Tunisia

Thick Aptian deposits in north central Tunisia comprise hemipelagic lower Aptian, reflecting the sea-level rise of OAE 1a, and an upper Aptian shallow marine environment characterized by the establishment of a carbonate platform facies. Carbon stable isotope data permit recognition of the OAE 1a event in the Djebel Serdj section. Cephalopods are rare throughout these successions, but occurrences are sufficient to date the facies changes and the position of the OAE1a event. Ammonite genera include lower Aptian Deshayesites, Dufrenoyia, Pseudohaploceras, Toxoceratoides and ? Ancyloceras; and upper Aptian Zuercherella, Riedelites and Parahoplites. Correlation of carbon isotope data with those of other Tethyan sections is undertaken together with the integration of planktonic foraminiferal data.

Integrated stratigraphic reconstruction for the last 80kyr in a deep sector of the Sardinia Channel (Western Mediterranean)

Abstract A quantitative analysis of planktonic foraminifera, coupled with petrophysical and paleomagnetic measurements and 14 C-AMS calibrations, was carried out on a deep core recovered in the Sardinia Channel (Western Mediterranean Sea), during the CIESM Sub2 survey, providing an integrated stratigraphic time-framework over the last 80 kyr. Significant changes in the quantitative distribution of planktonic foraminifera allowed the identification of several eco-bioevents useful to accurately mark the boundaries of the eco-biozones widely recognized in the Western Mediterranean records and used for large-scale correlations. Namely, 10 eco-biozones were identified based on the relative abundance of selected climate-sensitive planktonic foraminiferal species. Sixteen codified eco-bioevents were correlated with Alboran Sea planktonic foraminiferal data and several climatic global events (Sapropel S1, Younger Dryas, Greenland Isotope Interstadial 1, Greenland Isotope Stadial 2, Heinrich event H1–H6) were recognized. The eco-bioevents together with the 14 C-AMS calibrations allowed us to define an accurate age model, spanning between 2 and 83 kyr. The reliability of the age model was confirmed by comparing the colour reflectance (550 nm%) data of the studied record with the astronomically tuned record from the Ionian Sea (ODP-Site 964). A mean sedimentation rate of about 7 cm/kyr included three turbidite event beds that were chronologically constrained within the relative low stand and regressive sea-level phases of MIS 4 and 3. The deep-sea sedimentary record includes a distinct tephra occurring at the base of the core that dates 78 ka cal. BP. The paleomagnetic data provide a well-defined record of the characteristic remanent magnetization that may be used to reconstruct the geomagnetic paleosecular variation for the Mediterranean back to 83 kyr.

Timing of Late Pliocene to Middle Pleistocene tectonic events in Rhodes (Greece) inferred from magneto-biostratigraphy and 40Ar/ 39Ar dating of a volcaniclastic layer

We discovered a volcaniclastic layer in the Plio-Pleistocene coastal sequences on the island of Rhodes (Aegean fore-arc, Greece). Here, we present an integrated isotopic, magnetostratigraphic, and biostratigraphic (planktonic foraminifers) study for the Haraki section, where this layer is found intercalated in several meters of sedimentary rocks corresponding to the Lindos Bay clay Member of the Rhodes Formation. 40Ar/ 39Ar dating of the volcaniclastic layer provides an age of 1.89 ± 0.09 Ma, which is consistent with our planktonic foraminiferal data. Magnetostratigraphic results show that the entire Haraki section is of normal polarity and according to the isotopic results this corresponds to the Olduvai subchron (1.95–1.77 Ma). The new age determination provides severe constraints for deciphering the sedimentary and tectonic evolution of Rhodes since the Late Pliocene, which can be summarized in the following: (1) 500 to 600 m drowning during the latest Pliocene that could be related to the westward motion of the Anatolian Plate; (2) at least 520 m of uplift at around 1.4–1.3 Ma related to activity of the sinistral strike-slip of the Pliny Trench, the deep Rhodes basin being separated from the island of Rhodes; (3) a counterclockwise rotation of Rhodes, younger than 1.2–1.1 Ma, and possibly synchronous with the young clockwise rotation of the western Aegean arc.

Environmental changes off North Iceland during the deglaciation and the Holocene: foraminifera, diatoms and stable isotopes

A combined study of foraminifera, diatoms and stable isotopes in marine sediments off North Iceland records major changes in sea surface conditions since about 15 800 cal years (yr) BP. Results are presented from two gravity cores obtained at about 400 m water depth from two separate sedimentary basins on each side of the submarine Kolbeinsey Ridge. The chronology of the sedimentary record is based partly on AMS 14C dates, partly on the Vedde and the Saksunarvatn tephra markers, as well as the historical Hekla AD 1104 tephra. During the regional deglaciation, the planktonic foraminiferal assemblages are characterised by consistently high percentages of sinistrally coiled Neogloboquadrina pachyderma. However, major environmental variability is reflected by changes in stable isotope values and diatom assemblages. Low δ 18O values indicate a strong freshwater peak as well as possible brine formation by sea-ice freezing during a pre-Bølling interval (Greenland Stadial 2), corresponding to the Heinrich 1 event. The foraminifera suggest a strong concurrent influence of relatively warm and saline Atlantic water, and both the foraminifera and the diatoms suggest mixing of cold and warm water masses. Similar but weaker environmental signals are observed during the Younger Dryas (Greenland Stadial 1) around the level of the Vedde Ash. Each freshwater peak is succeeded by an interval of severe cooling both at the beginning of the Bølling–Allerød Interstadial Complex (Greenland Interstadial 1) and during the Preboreal, presumably associated with the onset of intense deep water formatiom in the Nordic Seas. The Holocene thermal optimum, between 10 200 and about 7000 cal years (yr) BP, is interrupted by a marked cooling of the surface waters around 8200 cal yr BP. This cold event is clearly expressed by a pronounced increase in the percentages of sinistrally coiled N. pachyderma, corresponding to a temperature decrease of about 3°C. A general cooling in the area is indicated after 7000–6000 cal yr BP, both by the diatom data and by the planktonic foraminiferal data. After a severe cooling around 6000 cal yr BP, the planktonic foraminiferal assemblages suggest a warmer interval between 5500 and 4500 cal yr BP. Minor temperature fluctuations are reflected both in the foraminiferal and in the diatom data in the upper part of the record, but the time resolution of the present data is not high enough to pick up details in environmental changes through the late Holocene.

The last interglacial period on the Pacific Coast of North America: Timing and paleoclimate

New, high-precision U-series ages of solitary corals ( Balanophyllia elegans ) coupled with molluscan faunal data from marine terraces on the Pacific Coast of North America yield information about the timing and warmth of the last interglacial sea-level highstand. Balanophyllia elegans takes up U in isotopic equilibrium with seawater during growth and shortly after death. Corals from the second terrace on San Clemente Island (offshore southern California), the third terrace on Punta Banda (on the Pacific Coast of northern Baja California), and the Discovery Point Formation on Isla de Guadalupe (in the Pacific Ocean offshore Baja California) date to the peak of the last interglacial period and have U-series ages ranging from ca. 123 to 114 ka. The first terrace on Punta Banda has corals with ages ranging from ca. 83 to 80 ka, which corresponds to a sea-level highstand formed in the late last interglacial period. U-series analyses of corals from the Cayucos terrace (central California) and the Nestor terrace at Point Loma (southern California) show that these fossils have evidence of open-system history, similar to what has been reported by other workers for the same localities. Nevertheless, a model of continuous, secondary U and Th uptake shows that two ages of corals are likely present at these localities, representing the ca. 105 and ca. 120 ka sea-level highstands reported elsewhere. U-series ages of last interglacial corals from the Pacific Coast overlap with, but are on average younger than the ages of corals from Barbados, the Bahamas, and Hawaii. This age difference is explained by the nature of the geomorphic response to sea-level change: fringing or barrier reefs on low- latitude coastlines have an accretionary growth style that keeps pace with rising sea level, whether on a tectonically rising or stable coastline. In contrast, midlatitude, high-energy coastlines are sites of platform cutting during the early part of a sea-level high stand and terrace scouring and concomitant sediment and fossil deposition as sea level starts to recede. The youngest ages of corals from the Pacific Coast suggest that sea level was still relatively high at ca. 116 ka, which is not in agreement with other estimates of relatively large global ice volume at that time. Reliably dated, ca. 120 ka marine-terrace deposits on the Pacific Coast have fossil mollusks that indicate water temperatures as warm or warmer than at present. In contrast, ca. 80 ka marine deposits reported here and elsewhere have fossil mollusks indicating cooler-than-modern water temperatures. The presence of both ca. 105 ka and ca. 120 ka corals on the Nestor and Cayucos terraces explains a previously enigmatic mixture of warm-water and cool-water mollusks. At ca. 105 ka, a relatively high sea level with cool waters may have “captured” the terrace formed during the 120 ka sea-level highstand, in areas of low uplift rate. The inference of cooler-than-modern waters off the Pacific Coast of North America at ca. 80 ka and ca. 105 ka, based on marine-terrace faunas, does not agree with estimates of sea-surface temperatures derived from alkenone studies in the Santa Barbara Basin. However, cooler water temperatures at these times are in agreement with paleotemperature estimates from planktonic foraminiferal data for the Santa Barbara Basin. All records, from central California to Baja California, whether from marine terraces or offshore cores, indicate at least seasonably warmer-than-modern waters during the peak of the last interglacial period at ca. 120 ka.

A 450-kyr record of hydrological conditions on the western Agulhas Bank Slope, south of Africa

The Agulhas Bank region, south of Africa, is an oceanographically important and complex area. The leakage of warm saline Indian Ocean water into the South Atlantic around the southern tip of Africa is a crucial factor in the global thermohaline circulation. Foraminiferal assemblage, stable isotope and sedimentological data from the top 10 m of core MD962080, recovered from the western Agulhas Bank Slope, are used to indicate changes in water mass circulation in the southeastern South Atlantic for the last 450 kyr. Sedimentological and planktonic foraminiferal data give clear signals of cold water intrusions. The benthic stable isotope record provides the stratigraphic framework and indicates that the last four climatic cycles are represented (i.e. down to marine isotope stage (MIS) 12). The planktonic foraminiferal assemblages bear a clear transitional to subantarctic character with Globorotalia inflata and Neogloboquadrina pachyderma (dextral) being the dominant taxa. Input of cold, subantarctic waters into the region by means of leakage through the Subtropical Convergence, as part of Agulhas ring shedding, and a general cooling of surface waters is suggested by increased occurrence of the subantarctic assemblage during glacial periods. Variable input of Indian Ocean waters via the Agulhas Current is indicated by the presence of tropical/subtropical planktonic foraminiferal species Globoquadrina dutertrei, Globigerinoides ruber (alba) and Globorotalia menardii with maximum leakage occurring at glacial terminations. The continuous presence of G. menardii throughout the core suggests that the exchange of water from the South Indian Ocean to the South Atlantic Ocean was never entirely obstructed in the last 450 kyr. The benthic carbon isotope record and sediment textural data reflect a change in bottom water masses over the core location from North Atlantic Deep Water to Upper Southern Component Water. Planktonic foraminiferal assemblages and sediment composition indicate a profound change in surface water conditions over the core site approximately 200–250 kyr BP, during MIS 7, from mixed subantarctic and transitional water masses to overall warmer surface water conditions.

Maastrichtian‐danian dinoflagellate cyst biostratigraphy and biogeography from two equatorial sections in Colombia and Venezuela

Two outcrop sections from the Maastrichtian Colón and Mito Juan Formations were analyzed for dinoflagellate cyst biostratigraphy. Samples from the Río Molino section (Cesar‐Ranchería Basin, Colombia) represent a subset of samples from the 240 m‐thick hemipelagic limestones and calcareous mudstones reported in Martinez's (1989) foraminiferal study. The upper Campanian, lower Maastrichtian and uppermost Maastrichtian intervals were identified in this section based on some significant dinocyst biostratigraphic events, which are in agreement with planktonic foraminiferal data. In ascending Stratigraphic order these events include the lowest occurrence (LO) of Areoligera spp., LO of Senegalinium spp., highest occurrence (HO) of Trichodinium castanea, HO of Xenascus ceratioides, LO of Cerodinium diebelii, LO of Trithyrodinium evittii, HO of Odontochitina operculata, LO of Phelodinium tricuspe, HO of Palaeohystrichophora infusorioides, LO of Glaphyrocysta perforala, LO of Disphaerogena carposphaeropsis and LO of Manumiella seelandica. Relative higher abundance of terrestrial‐derived organic matter in the upper Colon Formation is consistent with a marine regression and consequent shallowing of depositional environments at the end of the Cretaceous in northwestern South America. The Río Loro section (Merida Andes, western Venezuela) includes 980 m of Maastrichtian, open marine, marginal marine and deltaic siliciclastic and calcareous sediments. The occurrence of Phelodinium tricuspe and Paleocystodinium australinum below the highest occurrence of Hystrichodinium sp. and Odontochitina sp. indicates that the base of the studied interval in this section is early Maastrichtian in age. The K/T boundary in the Río Loro section is placed within a 11.5 m‐unexposed interval, which is above the lowest occurrence of G. perforala and just below the lowest occurrence of the Danian marker, Damassadinium californicum. Very high sedimentation rates during the K/T transition in northern Colombia and western Venezuela may have produced one of the most expanded K/T Boundary sections reported in the literature. Spiniferites‐Achomosphaera Group and peridinioid cysts dominate most of the dinoflagellate cyst assemblage in both sections. The peridinioid assemblage reported here is assigned to the tropical to subtropical Malloy suite of Lentin and Williams (1980).

Late Quaternary dinoflagellate cysts from the Black, Marmara and Aegean seas: variations in assemblages, morphology and paleosalinity

The link between sea surface salinity (SSS) and dinoflagellate cyst morphology was studied quantitatively in cores of Late Quaternary mud from the Black, Marmara and Aegean seas, where oxygen isotopic and planktonic foraminiferal data show salinities of about 5–19, 15–22 and 36–39 ppt, respectively. In the Black Sea, late glacial muds contain low-diversity assemblages of the cruciform species Spiniferites cruciformis and Pyxidinopsis psilata, with most S. cruciformis cysts having expanded septal membranes (form 1: circular outline, form 2: irregular). The assemblage is associated with surface salinities of <7 ppt. Overlying sapropelic muds have salinity estimates of ∼14–18 ppt and contain Lingulodinium machaerophorum– Spiniferites– Cymatiosphaera assemblages, with many L. machaerophorum cysts having short processes and S. cruciformis with reduced septa (forms 3, 4 and 5). The late Holocene coccolith-rich sediments, with salinity of ∼18–20 ppt, have diverse assemblages of Brigantedinium, Peridinium ponticum and other protoperidinioids, together with normal cysts of L. machaerophorum and Operculodinium centrocarpum (sensu Wall and Dale (1966), Nature 211, 1025–1026) that bear long processes. In the northeast Aegean core, stenohaline species and morphotypes are rare, occurring only as low percentages of S. cruciformis forms 3 and 4, and O. centrocarpum var. ‘ truncatum’ in a mid-Holocene sapropel that was deposited during a period of high runoff and Black Sea water outflow. Marmara Sea cores record surface salinities of 14–18 ppt during late glacial sapropel deposition and 20–22 ppt for the overlying marine sediments. The sapropel is dominated by S. cruciformis forms 1 and 2 and by P. psilata, with common L. machaerophorum (clavate and normal forms). In surface sediments, L. machaerophorum is co-dominant with O. centrocarpum (normal and truncate forms), Brigantedinium and Spiniferites spp. Percentages of S. cruciformis (five forms) and L. machaerophorum (two forms) plotted against proxy-salinity data, however, show no clear correlation, indicating that morphological variation is not a simple function of salinity.

MAASTRICHTIAN-DANIAN DINOFLAGELLATE CYST BIOSTRATIGRAPHY AND BIOGEOGRAPHY FROM TWO EQUATORIAL SECTIONS IN COLOMBIA AND VENEZUELA

Abstract Two outcrop sections from the Maastrichtian Colon and Mito Juan Formations were analyzed for dinoflagellate cyst biostratigraphy. Samples from the Rio Molino section (Cesar‐Rancheria Basin, Colombia) represent a subset of samples from the 240 m‐thick hemipelagic limestones and calcareous mudstones reported in Martinez's (1989) foraminiferal study. The upper Campanian, lower Maastrichtian and uppermost Maastrichtian intervals were identified in this section based on some significant dinocyst biostratigraphic events, which are in agreement with planktonic foraminiferal data. In ascending Stratigraphic order these events include the lowest occurrence (LO) of Areoligera spp., LO of Senegalinium spp., highest occurrence (HO) of Trichodinium castanea, HO of Xenascus ceratioides, LO of Cerodinium diebelii, LO of Trithyrodinium evittii, HO of Odontochitina operculata, LO of Phelodinium tricuspe, HO of Palaeohystrichophora infusorioides, LO of Glaphyrocysta perforala, LO of Disphaerogena carposphaeropsis ...

Integrated Paleocene calcareous plankton magnetobiochronology and stable isotope stratigraphy: DSDP Site 384 (NW Atlantic Ocean)

At Deep Sea Drilling Site 384 ( J-Anomaly Ridge, Grand Banks Continental Rise, NW Atlantic Ocean) Paleocene nannofossil chalks and oozes (∼70 m thick) are unconformably/disconformably underlain (∼168 m; upper Maastrichtian) and overlain (∼98.7 m; upper lower Eocene) by sediments of comparable lithologies. The chalks are more indurated in stratigraphically higher levels of the Paleocene reflecting increasing amounts of biosiliceous (radiolarians and diatoms) components. This site serves as an excellent location for an integrated calcareous and siliceous microfossil zonal stratigraphy and stable isotope stratigraphy. We report the results of a magnetostratigraphic study which, when incorporated with published magnetostratigraphic results, reveals an essentially complete magnetostratigraphic record spanning the interval from Magnetochron C31n (late Maastrichtian) to C25n ( partim) (late Paleocene, Thanetian). Integrated magnetobiochronology and stable isotope stratigraphy support the interpretation of, and constrain the estimated duration of, a short hiatus (∼0.9 my) within the younger part of Chron C29r (including the K/P boundary) and an ∼6 my hiatus separating upper Paleocene (Magnetozone C25n) and upper lower Eocene (Magnetozone C22r) sediments. Some 30 planktonic foraminiferal datum levels [including the criteria used to denote the Paleocene planktonic foraminiferal (sub)tropical zonal scheme of Berggren and Miller, Micropaleontology 34 (4) (1988) 362–380 and Berggren et al., SEPM Spec. Publ. 54 (1995) 129–212, Geol. Soc. Am. Bull. 107 (11) (1995) 1272–1287], and nearly two dozen calcareous nannoplankton datum levels have been recognized and calibrated to the magnetochronology. Planktonic foraminiferal Subzones P4a and P4b of (upper Paleocene) Zone P4 are emended/redefined based on the discovery of a longer stratigraphic extension of Acarinina subsphaerica (into at last Magnetozone C25n). Stable isotope stratigraphies from benthic foraminifera and fine fraction (<38 μm) carbonate have been calibrated to the biochronology and magnetostratigraphy. A minimum in benthic foraminifer δ 13C was reached near the Danian/Selandian boundary (within Chron C26r, planktonic foraminiferal Zone P3a and calcareous nannoplankton Zone NP4) and is followed by the rise to maximum δ 13C values in the late Thanetian (near the base of C25n, in Zone P4c and NP9a, respectively) that can be used for global correlation in the Paleocene.

Response of surface water masses and circulation to Late Quaternary climate change east of New Zealand

A series of cores from east of New Zealand have been examined to determine the paleoceanographic history of the late Quaternary in the SW Pacific using planktonic foraminiferal data. Distinct shifts of species can be seen between glacial and interglacial times especially south of Chatham Rise east of South Island. Foraminiferal fragmentation ratios and benthic/planktonic foraminiferal ratios both show increased dissolution during glacials, especially isotope stage 2 to the south of Chatham Rise. The present‐day Subtropical Convergence appears to be tied to the Chatham Rise at 44°S, but during glacial times this rise separated cold water to the south from much warmer water to the north, with an associated strong thermal gradient across the rise. We estimate that this gradient could have presented as much as an 8°C temperature change across 4° of latitude during the maximum of the last ice age. There is only weak evidence of the Younger Dryas cool event, but there is a clear climatic optimum between 8 and 6.4 ka with temperatures 1°–2°C higher than the present day. The marine changes compare well with vegetational changes on both South and North Island.