Calcareous nannofossil abundance and datums of sediments from ODP Leg 170 sites

Three Pleistocene, five Pliocene, and thirteen late and middle Miocene calcareous nannofossil datums have been identified in the Leg 170 cored sequences collected from a transect across the Middle America Trench off the Nicoya Peninsula. Although some nannofossil zones could not be delineated, particularly in the Pliocene and upper Miocene, there appears to be a complete or very nearly complete Pleistocene through lower Miocene section at Sites 1039 and 1040. The oldest assemblages, observed at Site 1039 and 1040, are latest early Miocene in age (nannofossil Zone NN4). These assemblages are associated with gabbro intrusions into the basal sediments (one contact metamorphic hornfels sample contains relict nannofossils), indicating an age for the intrusion event of between 15.6 and 18.2 Ma at both Sites 1039 and 1040. Reference Site 1039, located on the Cocos plate, provides the best-preserved sequence of sediments of late Pleistocene to latest early Miocene age. The sediments cored in the prism sections at Sites 1040, 1041, 1042, and 1043 all indicate that the age of nannofossil assemblages in the prism sediments, including the toe, wedge, and apron, are all Pleistocene with a considerable amount of upper Miocene reworking. A period of low sediment accumulation rates (~5.3 m/m.y.) is recorded for Pliocene and upper Miocene sediments at Sites 1039, 1040, and 1043. Pliocene calcareous nannofossil assemblages characteristic of the ~2.5- to 3.75-m.y. time interval (nannofossil Zones NN16 and equivalent nannofossil Subzones CN12b and CN12a) were not resolved at any site. Nannofossil Zones NN15, NN14, NN13, and NN12 (early late Pliocene to early Pliocene) could not be resolved at any site either because of the absence of marker species. Within the Miocene at Sites 1039 and 1040, nannofossil Zones NN10-NN6 were difficult to differentiate because of the absence of several species that define the zonal boundaries. These intervals, where the nannofossil zones have not been resolved or are partially resolved, are primarily composed of carbonate ooze deposited during an ~8.5-m.y. (2.5-11 Ma) low sediment accumulation rate time interval. The absence of many of the marker species is attributed to warmer water conditions during those periods. Many of the same marker species are absent in the sediments recovered from nearby Deep Sea Drilling Project Site 155 in the Panama Basin.

Miocene calcareous nannofossil abundance and events in sediments of ODP Leg 189 sites

During Ocean Drilling Program (ODP) Leg 189, five sites were drilled in the Tasmanian Seaway with the objective to constrain the paleoceanographic implications of the separation of Australia from Antarctica and to elucidate the paleoceanographic developments throughout the Neogene (Shipboard Scientific Party, 2001a, doi:10.2973/odp.proc.ir.189.101.2001). Sediments ranged from Cretaceous to Quaternary in age and provided the opportunity to describe the paleoenvironments in the Tasman Seaway prior to, during, and after the separation of Australia and Antarctica. This study will focus on postseparation distribution of calcareous nannofossils through the Miocene. Miocene sediments were recovered at all five Leg 189 sites, and four of these sites were studied in detail to determine the calcareous nannofossil biostratigraphy. Hole 1168A, located on the western Tasmanian margin, contains a fairly continuous Miocene record and could be easily zoned using the Okada and Bukry (1980, doi:10.1016/0377-8398(80)90016-X) zonation. Analysis of sediments from Hole 1169A, located on the western South Tasman Rise, was not included in this study, as the recovered sediments were highly disturbed and unsuitable for further analysis (Shipboard Scientific Party, 2001c, doi:10.2973/odp.proc.ir.189.104.2001). Holes 1170A, 1171A, and 1171C are located on the South Tasman Rise south of the modern Subtropical Front (STF). They revealed incomplete Miocene sequences intersected by an early Miocene and late Miocene hiatus and could only be roughly zoned using the Okada and Bukry zonation. Similarly, Hole 1172A, located on the East Tasman Plateau, contains a Miocene sequence with a hiatus in the early Miocene and in the late Miocene and could only be roughly zoned using the Okada and Bukry (1980, doi:10.1016/0377-8398(80)90016-X) zonation. This study aims to improve calcareous nannofossil biostratigraphic resolution in this sector of the mid to high southern latitudes. This paper will present abundance, preservation, and stratigraphic distribution of calcareous nannofossils through the Miocene and focus mainly on biozonal assignment.

Nannofossil biostratigraphy of ODP Hole 206-1256B sediments

Site 1256 of Ocean Drilling Program Leg 206 to the Guatemala Basin on the eastern flank of the East Pacific Rise yielded a near-complete, middle Miocene-Quaternary carbonate-rich section that provides an opportunity to study low-latitude biostratigraphic and paleoceanographic events. The sedimentary sequence in Hole 1256B has been zoned using calcareous nannofossils according to the biostratigraphic schemes by Martini of 1971 (modified by Martini and Müller in 1986) and Okada and Bukry of 1980. The nannofossil assemblage is characteristic of the low latitudes, with abundant Gephyrocapsa, Discoaster, and Sphenolithus, and is in general moderately to well preserved, depending on nannofossil abundance and the presence of diatoms. Age estimates for the first occurrence and last occurrence of Reticulofenestra rotaria were derived from biostratigraphy and magnetostratigraphy independently and assigned to 7.18 and 6.32 Ma, respectively. Linear sedimentation rates, calculated using 28 nannofossil datums and age estimates, are high in the middle Miocene, decrease from the late Miocene to the Pliocene, then increase upsection. The abrupt drop in carbonate mass accumulation rates during the early late Miocene is referred to as the "carbonate crash." This pattern reflects (1) the long-trend decrease of productivity as the site moves away from the upwelling system at the equatorial divergence as well as (2) fluctuation in the chemistry of the bottom waters associated with production of the North Atlantic Bottom Water and ventilation via the Panama Gateway. A basement age of 14.5 Ma was obtained by extrapolating the 39.1-m/m.y. rate in the middle Miocene to the basement at 250.7 meters below seafloor, and is consistent with the ~15-Ma age of the oceanic crust estimated from marine magnetic anomalies. Reworked nannofossils and lithologic changes were used to unravel postdepositional history, and three episodes were recognized, one of which in the latest Miocene can be widely correlated.

Late Miocene to Holocene calcareous nannofossils from ODP sites of Leg 125

During Leg 125 of the Ocean Drilling Program, nine sites were drilled in the Mariana and Izu-Bonin areas. The sediments recovered range in age from early Pliocene to late Pleistocene in the Mariana Region and from middle Eocene to late Pleistocene in the Izu-Bonin region. This contribution concerns the biostratigraphic study of the latest Miocene (CN9b Subzone) to late Pleistocene interval. Aquantitative analysis of all calcareous nannofossil associations was conducted for the interval encompassing late Miocene to the top of the early Pliocene. Moreover, the genera Discoaster, Amaurolithus, and Ceratolithus were quantitatively investigated from the late Miocene to late Pliocene interval. Some bioevents were identified, and variations in the composition of assemblages were linked to climatic changes.

Cenozoic calcareous nannofossils from the Lesser Antilles Forearc Region

During Leg 110 of the Ocean Drilling Program, sediment was recovered from six sites in the vicinity of the Lesser Antilles Forearc. Hole 671B, drilled near the toe of the Barbados deformation front, was the first-ever penetration of the decollement between the underthrusting Atlantic Plate and the off scraped Barbados accretionary prism. Stratigraphic repetitions in sequence associated with tectonic movement along the decollement zone, first observed on DSDP Leg 78A, were further documented at four ODP Leg 110 sites. A significant biostratigraphic inversion is present at Site 671 at 128 mbsf in which upper Miocene sediments rest atop lower Pleistocene strata. Smaller repetitions in sequence are recorded at Sites 671, 673, 674, and 676. Leg 110 sediments range from middle Eocene to early Pleistocene in age. Pliocene/Pleistocene assemblages are generally well preserved; however, Miocene assemblages have undergone extensive dissolution at all Leg 110 sites. Paleogene sediments are sometimes recrystallized and the nannofossils contained within exhibit a range in preservation from poor to good.

Stratigraphic first occurrence of Emiliania huxleyi in sediments of the Chatham Rise (Table 2)

A quantitative analysis was carried out of planktonic diatoms (biogenic opal) and calcareous nannofossils (biogenic calcite) in late Quaternary sediments (MIS 1-6) from four cores along a N-S transect east of New Zealand from 39°50'S to 50°04'S across the E-W-trending submarine ridge, the Chatham Rise. This was done to trace movements of oceanic fronts and to improve calcareous nannofossil stratigraphy for the last 130 000 yr in the SW Pacific. Sites ODP 1123 and Q 858 are below present day subtropical surface waters north of Chatham Rise. Site DSDP 594 is below present-day mixed temperate-subantarctic surface water south of the rise, and site ODP 1120 is below subantarctic surface water. The more diverse and opportunistic planktonic diatoms provided marker species for subtropical surface waters (Alveus marina, Fragilariopsis doliolus, Rhizosolenia bergonii and Azpeitia nodulifer) and others for subantarctic surface waters (Nitzschia kerguelensis, Thalassiosira lentiginosa). Application of these tracers permits the following conclusions: (1) subtropical conditions persisted north of Chatham Rise throughout the past 130 000 yr, in spite of the cooling of surface waters during colder periods; (2) during warm times (MIS 5 and MIS 3, and in MIS 1), the sporadic occurrence of subtropical species south of Chatham Rise indicates occasional admixture of subtropical surface waters that far south; (3) subantarctic waters extended to the southern slopes of the Chatham Rise during MIS 5b, late MIS 5a to early MIS 4, during the warmer time intervals in early MIS 3, and during latest MIS 3 to early MIS 2; (4) subantarctic frontal conditions existed over southern Chatham Rise during early MIS 4 and late MIS 3 to early MIS 2; and (5) it is probable that during cooler times, MIS 6, MIS 5b, and in MIS 2, intensified particle transport from the Bounty Trough to the northern flank of Chatham Rise occurred by intensified boundary currents. The larger abundance fluctuations in both microfossil groups at the sites south of Chatham Rise than north of Chatham Rise reflect northward shifts of the Circumpolar Subantarctic Water (CSW) and a contemporaneous disappearance of Australasian Subantarctic Water (ASW), implying an elevated temperature gradient between the surface water masses north and south of the Chatham Rise at the times of such northward shifts of CSW. Calcareous nannofossils are less diverse than diatoms, and are less specialised. Some calcareous nannofossil species show abundance shifts at the same time at different latitudes. Two of these abundance shifts can be used for correlation between subtropical and subantarctic sediments in the SW Pacific: (1) reversal in the relative abundance of Calcidiscus leptoporus and Coccolithus pelagicus associated with the MIS 2/1 boundary; and (2) drop in abundance of Gephyrocapsa muellerae or medium-sized Gephyrocapsa at the MIS 4/3 boundary. An additional abundance shift seems to be restricted to subtropical to mixed temperate-subtropical-subantarctic surface waters: (3) increase in abundance of G. muellerae or medium-sized Gephyrocapsa at the beginning of MIS 2 below the Okareka tephra.

Calcareous nannofossils of ODP sites 177-1088 and 177-1090

During ODP Leg 177, a Miocene to Pliocene calcareous nannofossil record was recovered at Sites 1088 and 1090. Site 1088, located at 41°8'S, shows a continuous middle-upper Miocene to Pliocene carbonate sequence that was deposited at relatively high sedimentation rates (Shipboard Scientific Party, 1999a, doi:10.2973/odp.proc.ir.177.103.1999). Moreover, Site 1088 proved suitable for calcareous nannofossil analysis as a means to improve the biostratigraphy at this southern latitude. Site 1090 was drilled at 42°54'S; a tephra layer marks a significant disconformity at the Miocene/Pliocene boundary of this sequence (Shipboard Scientific Party, 1999b, doi:10.2973/odp.proc.ir.177.105.1999). Although nannofossil assemblages are poorly preserved at this site (Shipboard Scientific Party, 1999b), they may help in determining the age of the disconformity and its paleoceanographic significance.

The Global Plankton Database: an inventory and data from the Former Soviet Union expeditions

At present time, there is a lack of knowledge on the interannual climate-related variability of zooplankton communities of the tropical Atlantic, central Mediterranean Sea, Caspian Sea, and Aral Sea, due to the absence of appropriate databases. In the mid latitudes, the North Atlantic Oscillation (NAO) is the dominant mode of atmospheric fluctuations over eastern North America, the northern Atlantic Ocean and Europe. Therefore, one of the issues that need to be addressed through data synthesis is the evaluation of interannual patterns in species abundance and species diversity over these regions in regard to the NAO. The database has been used to investigate the ecological role of the NAO in interannual variations of mesozooplankton abundance and biomass along the zonal array of the NAO influence. Basic approach to the proposed research involved: (1) development of co-operation between experts and data holders in Ukraine, Russia, Kazakhstan, Azerbaijan, UK, and USA to rescue and compile the oceanographic data sets and release them on CD-ROM, (2) organization and compilation of a database based on FSU cruises to the above regions, (3) analysis of the basin-scale interannual variability of the zooplankton species abundance, biomass, and species diversity.

Calcareous nannofossil biostratigraphy of ODP Leg 108 sediments

Calcareous nannofossils were examined from the 400 cores recovered at 12 sites during Ocean Drilling Program Leg 108 in the eastern equatorial Atlantic Ocean and along the northwest African margin, representing a transect spanning 24° of latitude. Thirty calcareous nannofossil biohorizons were recognized in the Neogene and Quaternary sequences; only Site 661, located in water depths of 3500 m, contains a fossiliferous record older than the Oligocene. At Site 661, a 200-m-thick sequence of Upper Cretaceous sediments yielded Maestrichtian and uppermost Campanian nannofossils, yet a continuous Cretaceous/Tertiary boundary was not recovered. Widespread sediment slumps and turbidites deposited at many sites interrupted the pelagic sedimentation. A careful study of calcareous nannofossil and foraminifer assemblages correlated to paleomagnetic records suggests that "slumped" units at most sites were added as extra sediments to rapidly deposited pelagic sediments, with minor disturbance of the surrounding layers. Nannofossils are generally common to abundant and moderately preserved at all sites except for those located in two upwelling areas, where placoliths are etched and discoasters overgrown. Typical low-latitudinal zonal markers were used during this study, yet some of them were considered to be of little biostratigraphic value because of their inconsistent stratigraphic ranges and low abundances. This is especially apparent for the intervals representing the Miocene/Pliocene and Oligocene/Miocene boundaries. Characteristic nannofossils of cool-water conditions and low discoaster abundances occur at the coastal African upwelling and along the south equatorial divergence sites, signifying a stronger advection of cold waters toward the equator within the Canary and Benguela eastern boundary currents.

Microfossil occurrence in Pliocene-Quaternary sediments of the North Atlantic

Planktonic foraminiferal and nannoplankton stratigraphy of the Pliocene-Quatemary Sediments of the northern half of the Atlantic Ocean from the equator up to the Rockall Plateau and the Norwegian Sea, is considered. Lowlatitude zonations were used for the subdivision of the Pliocene and Quaternary Sediments of different climatic belts, and certain subglobal zonal units were recognized. Variations in the degree of resolution of the zonation in different latitudes were revealed; the resolution of zonal scales based on calcareous plankton diminishes northwards. Changes of taxonomic composition of the zonal foraminifer and nannoplankton assemblages within various latitudinal belts of the Atlantic were analyzed taking into consideration the influence of climatic factors and of local bionomic conditions. Correlation with the magnetostratigraphic time-scale permitted the establishment of the most reliable appearance and disappearance datums (datum planes) of planktonic foraminifer and nannoplankton species. Paleontologic plates demonstrate some guide forms of two groups of calcareous plankton, and a short description of the taxa is given in the text. Major stratigraphic problems of Pliocene and Quaternary marine deposits are discussed. The monograph can be used in different geological investigations by specialists in geology, paleontology, and oceanology.

Distribution of late Quaternary calcareous nannofossils in DSDP Leg 96 sites

Sediments from holes drilled at 11 sites in the northern Gulf of Mexico during Deep Sea Drilling Project Leg 96 were analyzed for calcareous nannofossil content. All sediments recovered are Holocene and late Pleistocene in age and are within the Emiliania huxleyi Zone. The datum level represented by the lowest stratigraphic occurrence of dominant E. huxleyi occurs at two sites (Sites 615 and 619) and can be dated at approximately 84,000 yr. ago at Site 619. Reworked Cretaceous nannofossils are generally common or abundant and dominate the floral assemblages of the late Wisconsin glacial sediments. When present, indigenous late Quaternary species are rare or few in abundance. Slight increases in the contemporaneous Quaternary component of the floral assemblages can be documented by the use of a calculated in s/fu/reworked ratio. This ratio, based on the relative abundances of the indigenous Quaternary taxa and reworked taxa, shows potential both for local correlations between drill sites and for correlation with glacio-eustatic fluctuations during the late Pleistocene.

Calcareous nannofossil stratigraphy and datum levels of ODP Site 182-1127, Great Australian Bight

Ocean Drilling Program (ODP) Leg 182 drilled at nine sites on the Great Australian Bight, which is located directly south of the Australian continent. Leg 182 proposed to examine the paleoceanographic evolution of a midlatitude, cool-water carbonate platform. During drilling on the Great Australian Bight, three sites (1127, 1129, and 1131) recovered highly expanded Pleistocene sections. This paper presents the detailed calcareous nannofossil biostratigraphy of the most distal site. This report should provide a useful Pleistocene biostratigraphic reference for this previously unknown area.

Coccoliths and carbonate grain-size compositions in sediment samples from the Mid-Atlantic Ridge

This study presents a differentiated carbonate budget for marine surface sediments from the Mid-Atlantic Ridge of the South Atlantic, with results based on carbonate grain-size composition. Upon separation into sand, silt, and clay sub-fractions, the silt grain-size distribution was measured using a SediGraph 5100. We found regionally characteristic grain-size distributions with an overall minimum at 8 µm equivalent spherical diameter (ESD). SEM observations reveal that the coarse particles (>8 µm ESD) are attributed to planktic foraminifers and their fragments, and the fine particles (<8 µm ESD) to coccoliths. On the basis of this division, the regional variation of the contribution of foraminifers and coccoliths to the carbonate budget of the sediments are calculated. Foraminifer carbonate dominates the sediments in mesotropic regions whereas coccoliths contribute most carbonate in oligotrophic regions. The grain size of the coccolith share is constant over water depth, indicating a lower susceptibility for carbonate dissolution compared to foraminifers. Finally, the characteristic grain-size distribution in fine silt (<8 µm ESD) is set into context with the coccolith assemblage counted and biometrically measured using a SEM. The coccoliths present in the silt fraction are predominantly large species (length > 4 µm). Smaller species (length < 4 µm) belong to the clay fraction (<2 µm ESD). The average length of most frequent coccolith species is connected to prominent peaks in grain-size distributions (ESD) with a shape factor. The area below Gaussian distributions fitted to these peaks is suggested as a way to quantitatively estimate the carbonate contribution of single coccolith species more precisely compared to conventional volume estimates. The quantitative division of carbonate into the fraction produced by coccoliths and that secreted by foraminifers enables a more precise estimate for source/sink relations of consumed and released CO2 in the carbon cycle. The allocation of coccolith length and grain size (ESD) suggests size windows for the separation or accumulation of distinct coccolith species in investigations that depend on non to slightly-mixed signals (e.g., isotopic studies).

Alkenones and coccolithophorids in late Quaternary sediments from the Walvis Ridge

Sea surface temperatures (SSTs) derived from the alkenone UK'37) record of Quaternary sediments may be subject to bias if algae with different temperature sensitivities have contributed to the sedimentary alkenone record. The alkenone-derived SST records are usually based on a UK'37-temperature relationship which was measured in culture experiments using the coccolithophorid Emiliania huxleyi (F.G. Prahl, L.A. Muehlhausen and D.L. Zahnle, 1988. Further evaluation of long-chain alkenones as indicators of paleoceanographic conditions. Geochim. Cosmochim. Acta 52, 2303-2310). To assess possible effects of past species changes on the UK'37-temperature signal, we have analyzed long-chain alkenones and coccolithophorids in a late Quaternary sediment core from the Walvis Ridge and compared the results to SST estimates extracted from the d18O record of the planktonic foraminifer Globigerinoides ruber. Alkenones and isotopes were determined over the entire 400-kyr core record while the coccolithophorid study was confined to the last 200 kyr when the most pronounced changes in alkenone content occurred. Throughout oxygen-isotope stages 6 and 5, species of the genus Gephyrocapsa were the predominating coccolithophorids. E. huxleyi began to increase systematically in relative abundance since the stage 5/4 transition, became dominant over Gephyrocapsa spp. during stage 3 and reached the highest abundances in the Holocene. Carbon-normalized alkenone concentrations are inversely related to the relative abundances of E. huxleyi, and directly related to that of Gephyrocapsa spp., suggesting that species of this genus were the principal alkenone contributors to the sediments. Nevertheless, SST values obtained from the UK'37-temperature relationship for E. huxleyi compare favourably to the isotope-derived temperatures. The recently reported UK'37-temperature relationship for a single strain of Gephyrocapsa oceanica (J.K. Volkman. S.M. Barrett, S.I. Blackburn and E.L. Sikes, 1995. Alkenones in Gephyrocapsa oceanica: Implications for studies of paleoclimate. Geochim. Cosmochim. Acta 59, 513-520) produces unrealistically high SST values indicating that the temperature response of the examined strain is not typical for the genus Gephyrocapsa. This is supported by the C37:C38, alkenone ratios of the sediments which are comparable to average ratios reported for E. huxleyi, but significantly higher than for the G. oceanica strain. Most notably, the general accordance of the alkenone characteristics between sediments and E. huxleyi persists through stages 8 to 5 and even in times that predate the first appearance of this species (268 ka; H.R. Thierstein, K.R. Geitzenauer and B. Molfino, 1977. Global synchroneity of late Quaternary coccolith datum levels: Validation by oxygen isotopes. Geology 5, 400-404). Our results suggest that UK'37-temperature relationships based on E. huxleyi produce reasonable paleo-SST estimates even for late Quaternary periods when this species was scarce or absent because other alkenone-synthesizing algae, e.g. of the genus Gephyrocapsa.