Investigations on a sediment core from the Indian Ocean

The Multi-Sensor Core Logger (MSCL) enables non-destructive, quasi-continuous measuroments of physical properties, reducing the time needed for discrete sample analysis. Density, compressional wave velocity (Vp), and magnetic susceptibility are measured on water-saturated sediment cores. Rapid variations in the lithology can thus be more easily recognized. The advantages of MSCL measurements over traditional sedimentological investigation methods are illustrated using several examples. Density-Vp relationships provide detailed lithological information prior to splitting the sediment cores. In terrigenous sediments, density increases with Vp, whereas in biogenic sediments it decreases. In biogenic sediments in the South Atlantic, low densities and high Vp are associated with high opal content. In biogenic sediments in the Peru Basin, density increases with carbonate content. Carbonate, which is very important for deep-sea environmental protection and for paleoclimatic studies, can be determined quantitatively from MSCL measurements in this area. In terrigenous sediments in the Bengal Fan, the acoustic impedance (the product of density and Vp) increases with grain size. There, the grain-size distribution can be rapidly derived from the acoustic impedance. Moreover, in hemipelagic sediments in the Bengal Fan, it is possible to correlate variations in magnetic susceptibility with cyclic changes in the earth's orbital parameters - an important prerequisite for detailed stratigraphic studies.

Plio-Pleistocene age-depth points and occurrence of hiatuses in ODP sites of the Southern Ocean

Changes in Atlantic deep water circulation were reconstructed by comparing the benthic foraminiferal delta13C record at ODP Site 1090 in the South Atlantic with similar records from the North Atlantic (Sites 982, 607, 925, 929) and deep Pacific (Site 849) oceans. Important deep water circulation changes occurred in the early Pleistocene at 1.55 Myr and during the Mid-Pleistocene Transition at 0.9 Myr. At 1.55 Myr, glacial delta13C values in the Southern Ocean became significantly lower than those in the deep Pacific, establishing a pattern that persisted throughout the late Pleistocene. We propose that the lowering of delta13C values of Southern Component Water (SCW) at this time resulted from expansion of sea ice and reduced ventilation of deep water during glacial periods after marine isotope stage 52. Accompanying this change in Southern Ocean deep water circulation was enhanced interhemispheric coupling between the North and South Atlantic after 1.55 Myr. At ~0.9 Myr, the magnitude of glacial-to-interglacial variabilityin delta13C increased and shifted to a longer frequency (100 kyr) along with oceanic delta18O (ice volume). Calculation of percent Northern Component Water (NCW) using Site 1090 as the SCW end member yielded 20-30% less reduction of NCW during glacial periods of the late Pleistocene. Also, a trend toward reduced glacial suppression of NCW during the past 400 kyr is not evident. The apparent decoupling of ice volume and deep water circulation reported previously maybe an artifact of using a Pacific, rather than a Southern Ocean, carbon isotopic record to calculate past mixing ratios of NCW and SCW.

Global distributions of picophytoplankton abundance and biomass - Gridded data product (NetCDF) - Contribution to the MAREDAT World Ocean Atlas of Plankton Functional Types

The smallest marine phytoplankton, collectively termed picophytoplankton, have been routinely enumerated by flow cytometry since the late 1980s, during cruises throughout most of the world ocean. We compiled a database of 40,946 data points, with separate abundance entries for Prochlorococcus, Synechococcus and picoeukaryotes. We use average conversion factors for each of the three groups to convert the abundance data to carbon biomass. After gridding with 1° spacing, the database covers 2.4% of the ocean surface area, with the best data coverage in the North Atlantic, the South Pacific and North Indian basins. The average picophytoplankton biomass is 12 ± 22 µg C L-1 or 1.9 g C m-2. We estimate a total global picophytoplankton biomass, excluding N2-fixers, of 0.53 - 0.74 Pg C (17 - 39 % Prochlorococcus, 12 - 15 % Synechococcus and 49 - 69 % picoeukaryotes). Future efforts in this area of research should focus on reporting calibrated cell size, and collecting data in undersampled regions.

Stable isotope record, Mg/Ca ratios and age model of St. Stephens Quarry, Alabama

In the largest global cooling event of the Cenozoic Era, between 33.8 and 33.5 Myr ago, warm, high-CO2 conditions gave way to the variable 'icehouse' climates that prevail today. Despite intense study, the history of cooling versus ice-sheet growth and sea-level fall reconstructed from oxygen isotope values in marine sediments at the transition has not been resolved. Here, we analyse oxygen isotopes and Mg/Ca ratios of benthic foraminifera, and integrate the results with the stratigraphic record of sea-level change across the Eocene-Oligocene transition from a continental-shelf site at Saint Stephens Quarry, Alabama. Comparisons with deep-sea (Sites 522 (South Atlantic) and 1218 (Pacific)) d18O and Mg/Ca records enable us to reconstruct temperature, ice-volume and sea-level changes across the climate transition. Our records show that the transition occurred in at least three distinct steps, with an increasing influence of ice volume on the oxygen isotope record as the transition progressed. By the early Oligocene, ice sheets were ~25% larger than present. This growth was associated with a relative sea-level decrease of approximately 105 m, which equates to a 67 m eustatic fall.

Changes in calcareous nannofossil assemblages across the Paleocene/Eocene transition from the paleo-equatorial Pacific Ocean

Quantitative analyses of selected calcareous nannofossils in deep-sea sections recovered from the paleo-equatorial Pacific (ODP Leg 199) provide new information about biostratigraphy, biochronology and the evolutionary history of calcareous nannofossils across the Paleocene/Eocene transition interval. The sediment cores from ODP Leg 199 represent the first continuous Paleocene/Eocene boundary sections ever to be sampled in the central equatorial Pacific Ocean. Calcareous nannofossil assemblages are studied to document the distribution of biostratigraphically useful taxa such as Ericsonia, Discoaster, Fasciculithus, Rhomboaster and Tribrachiatus. Focus is given to the evolution of the Rhomboaster-Tribrachiatus lineage in the lower Eocene interval at Site 1215, and on the stratigraphic relationship of these taxa relative to species in the genus Fasciculithus. Critical intervals of North Atlantic DSDP Site 550 have also been re-examined. The Tribrachiatus digitalis morphotype was described at Site 550 from an interval affected by down-hole contamination, partly originating from within the Tribrachiatus orthostylus range. The T. digitalis morphotype represents an evolutionary transitional form between T. contortus and T. orthostylus, entering the stratigraphic record within the range of the former species and disappearing within the lower part of the range of the latter species. The subzonal subdivision of Zone NP10 hence collapses. Lithological and colour variability reflecting orbital cyclicity occur in the lower Eocene of Site 1215, permitting a relative astronomical age calibration of the Tribrachiatus taxa. The distinct Rhomboaster spp.-Discoaster araneus association also occurs in the paleo-equatorial Pacific Ocean, together with a marked decrease in diversity of Fasciculithus spp. Site 1220 reveals a short peak abundance of Thoracosphaera spp. just above the P/E boundary interval, which probably reflects a stressed surface water environment.

Eocene-Oligocene calcareous nannofossils from ODP sites 115-711 and 120-748 in the Indian Ocean

An Eocene-Oligocene calcareous nannofossil biostratigraphic framework for Ocean Drilling Program (ODP) Site 748 in the southern Indian Ocean is established, which provides a foundation for this and future quantitative biogeographic studies. This biostratigraphic analysis, together with quantitative nannofossil data, enables a reinterpretation of the preliminary magnetostratigraphy and a new placement for magnetic Subchron CBN in the lowermost Oligocene. Calcareous nannofossil species diversity is low at Site 748 relative to lower latitude sites, with about 13 taxa in the middle Eocene, gradually decreasing to about 6 in the late Oligocene. There is, however, no apparent mass extinction at any stratigraphic level. Similarly, no mass extinctions were recorded at or near the Eocene/Oligocene boundary at Site 711 in the equatorial Indian Ocean. Species diversity at the equatorial site is significantly higher than at Site 748, with a maximum of 39 species in the middle Eocene and a minimum of 14 species in the late Oligocene. The abundance patterns of nannofossil taxa are also quite different at the two sites, with chiasmoliths, Isthmolithus recurvus, and Reticulofenestra daviesii abundant and restricted to the high-latitude site and Coccolithus formosus, discoasters, and sphenoliths abundant at the equatorial site but impoverished at the high-latitude site. This indicates a significant latitudinal biogeographic gradient between the equatorial site and the high-latitude site in the Indian Ocean for the middle Eocene-Oligocene interval. The abundance change of warm-water taxa is similar to that of species diversity at Site 711. There is a general trend of decreasing abundance of warm-water taxa from the middle Eocene through the early Oligocene at Site 711, suggesting a gradual cooling of the surface waters in the equatorial Indian Ocean. The abundance of warm-water taxa increased in the late Oligocene, in association with an increase in species diversity, and this may reflect a warming of the surface waters in the late Oligocene. An abrupt increase in the abundance of cool-water taxa (from ~20% to over 90%) occurred from 36.3 to 35.9 Ma at high-latitude Site 748. Coincident with this event was a ~1.0 per mil positive shift in the delta18O value of planktonic foraminifers and the occurrence of ice-rafted debris. This abrupt change in the nannofossil population is a useful biostratigraphic event for locating the bottom of magnetic Subchron C13N in the Southern Ocean. The sharp increase in cool-water taxa coeval with a large positive shift in delta18O values suggests that the high-latitude surface waters drastically cooled around 36.3-35.9 Ma. The temperature drop is estimated to be 4°C or more at Site 748 based on the nannofossil population change relative to the latitudinal biogeographic gradient established in the South Atlantic Ocean during previous studies. Consequently, much of the delta18O increase at Site 748 appears to be due to a temperature drop in the high latitudes rather than an ice-volume signal. The ~0.1 per mil delta18O increase not accounted for by the temperature drop is attributed to an ice-volume increase of 4.6 * 10**3 km**3, or 20% the size of the present Antarctic ice sheet.

Radioisotope stratigraphy, sedimentology and geochemistry of Late Quaternary sediments from the Eastern Arctic Ocean

To reconstruct Recent and past sedimentary environments, marine sediments of Upper Pleistocene and Holocene ages from the eastern Arctic Ocean and especially from the Nansen-Gakkel Ridge (NGR) were investigated by means of radioisotopic, geochemical and sedimentological methods. In combination with mass physical property data and lithological analysis these investigations allow clearly to characterize the depositional environments. Age dating by using the radioisotope 230Th gives evidence that the investigated sediments from the NGR are younger than 250,000 years. Identical lithological sediment sequences within and between sediment cores from the NGR can be related to sedimentary processes which are clearly controlled by palaeoclimate. The sediments consist predominantly of siliciclastic, terrigenous ice-rafted detritus (IRD) deriving from assorted and redeposited sediments from the Siberian shelfs. By their geochemical composition the sediments are similar to mudstone, graywacke and arcose. Sea-ice as well as icebergs play a major roll in marine arctic sedimentation. In the NGR area rapid change in sedimentary conditions can be detected 128,000 years ago. This was due to drastic change in the kind of ice cover, resulting from rapid climatic change within only hundreds of years. So icebergs, deriving mostly from Siberian shelfs, vanished and sea-ice became dominant in the eastern Arctic Ocean. At least three short-period retreats of the shelf ice between 186,000 and 128,000 years are responsible for the change of coarse to fine-grained sediments in the NGR area. These warmer stages lasted between 1,000 and 3,000 years. By monitoring and comparing the distribution patterns of sedimentologic, mass physical and geochemical properties with 230Th ex activity distribution patterns in the sediment cores from the NGR, there is clear evidence that sediment dilution is responsible for high 230Th ex activity variations. Thus sedimentation rate is the controlling factor of 230Th ex activity variations. The 230Th flux density in sediments from the NGR seems to be highly dependent On topographic Position. The distribution patterns of chemical elements in sediment cores are in general governed by lithology. The derivation of a method for dry bulk density determination gave the opportunity to establish a high resolution stratigraphy on sediment cores from the eastern Arctic Ocean, based on 230Thex activity analyses. For the first time sedimentation and accumulation rates were determined for recent sediments in the eastern Arctic Ocean by 230Th ex analyses. Bulk accumulation rates are highly variable in space and time, ranging between 0.2 and 30 g/cm**2/ka. In the sediments from the NGR highly variable accumulation rates are related to the kind of ice cover. There is evidence for hydrothermal input into the sediments of the NGR. Hydrothermal activity probably also influences surficial sediments in the Sofia Basin. High contents of As are typical for surficial sediments from the NGR. In particular SL 370-20 from the bottom of the rift valley has As contents exceeding in parts 300 ppm. Hydrothermal activity can be traced back to at least 130,000 years. Recent to subrecent tectonic activity is documented by the rock debris in KAL 370 from the NGR. In four other sediment cores from the NGR rift valley area tectonically induced movements can be dated to about 130,000 years ago, related most probably to the rapid climate change. Processes of early diagenesis in sediments from the NGR caused the aobilization and redeposition of Fe, Mn and Mo. These diagenetic processes probably took place during the last 130,000 years. In sediment cores from the NGR high amounts of kaolinite are related to coarse grained siliciclastic material, probably indicating reworking and redeposition of siberian sandstones with kaolinitic binding material. In contrast to kaolinite, illite is correlated to total clay and 232Th contents. Aragonite, associated with serpentinites in the rift valley area of the NGR, was precipitated under cold bottom-water conditions. Preliminary data result in a time of formation about 60 - 80 ka ago. Manganese precipitates with high Ni contents, which can be related to the ultrabasic rocks, are of similar age.

The Fohsella chronocline in middle Miocene sediments from the Ontong Java Plateau, Pacific Ocean

Micropaleontologists have traditionally recognized the mid-Miocene Fohsella lineage as a flagship for phyletic gradualism within the planktic foraminifera. However, study of a deep-sea record from the western equatorial Pacific (ODP Site 806) reveals that coiling ratios within this clade suddenly (<5 kyr) shift after a prolonged, ancestral state of near randomness (~50%) to a transient phase (13.42-13.43 Ma) of dextral dominance (~75%) immediately following the first common occurrence of keeled fohsellids. This brief period of dextral dominance was abruptly (<5 kyr) succeeded by an irreversible change to sinistral dominance (~96%). Fohsellid abundances decline markedly through the interval in which the sinistral preference is established. The shift to sinistrality (13.42 Ma) predated the deepening of fohsellid depth ecology by ~240-488 kyr, indicating that these two events were unrelated. This view is supported by a lack of delta 18O evidence for depth-habitat differences between the two chiral forms, which refutes the notion that sinistral fohsellids were "pre-adapted" for ensuing hydrographic change because they occupied a deeper depth habitat than their dextral counterparts. Planktic foraminiferal assemblages become strongly oligotrophic in character through the interval in which the fohsellid delta 18O increase is recorded, indicating that the migration to deeper depths was fostered by an expansion of the mixed layer in the western equatorial Pacific. Salient aspects of this brief, but conspicuous faunal change are a marked increase in the abundance of symbiont-bearing globigerinoidids, a concomitant collapse of local Jenkinsella mayeri/siakensis populations, and reduced fohsellid abundances. The rapid and permanent nature of the Fohsella sinistral shift provides a distinct, unequivocal datum that may prove useful for correlating mid-Miocene sections throughout the Caribbean Sea and tropical regions in the western sectors of the Pacific and Atlantic. The coiling ratio changes that occurred during the evolution of the Fohsella chronocline probably reflect changing population dynamics between cryptic genotypes with different coiling preferences.

Age model and color measurements of sediment cores from the eastern equatorial Pacific

High-resolution records of glacial-interglacial variations in biogenic carbonate, opal, and detritus (derived from non-destructive core log measurements of density, P-wave velocity and color; r >= 0.9) from 15 sediment sites in the eastern equatorial (sampling resolution is ~1 kyr) clear response to eccentricity and precession forcing. For the Peru Basin, we generate a high-resolution (21 kyr increment) orbitally-based chronology for the last 1.3 Ma. Spectral analysis indicates that the 100 kyr cycle became dominant at roughly 1.2 Ma, 200-300 kyr earlier than reported for other paleoclimatic records. The response to orbital forcing is weaker since the Mid-Brunhes Dissolution Event (at 400 ka). A west-east reconstruction of biogenic sedimentation in the Peru Basin (four cores; 91-85°W) distinguishes equatorial and coastal upwelling systems in the western and eastern sites, respectively. A north-south reconstruction perpendicular to the equatorial upwelling system (11 cores, 11°N-°3S) shows high carbonate contents (>= 50%) between 6°N and 4°S and highly variable opal contents between 2°N and 4°S. Carbonate cycles B-6, B-8, B-10, B-12, B-14, M-2, and M-6 are well developed with B-10 (430 ka) as the most prominent cycle. Carbonate highs during glacials and glacial-interglacial transitions extended up to 400 km north and south compared to interglacial or interglacial^glacial carbonate lows. Our reconstruction thus favors glacial-interglacial expansion and contraction of the equatorial upwelling system rather than shifting north or south. Elevated accumulation rates are documented near the equator from 6°N to 4°S and from 2°N to 4°S for carbonate and opal, respectively. Accumulation rates are higher during glacials and glacial-interglacial transitions in all cores, whereas increased dissolution is concentrated on Peru Basin sediments close to the carbonate compensation depth and occurred during interglacials or interglacial-glacial transitions.

Eocene-Oligocene calcareous nannofossil events of ODP Sites from the Southern Ocean

The evolution of the Southern Ocean climate during the late Eocene-late Oligocene interval is examined through highresolution, quantitative calcareous nannofossil analyses on samples from the Southern Ocean sections on Maud Rise and Kerguelen Plateau. We determined the abundance patterns of the counted species to clarify the biostratigraphy, which we correlated with high-resolution magnetostratigraphy [Roberts, A.P., Bicknell, S.J., Byatt, J., Bohaty, S.M., Florindo, F., Harwood, D.M., 2003a. Magnetostratigraphic calibration of Southern Ocean diatom datums from the Eocene-Oligocene of Kerguelen Plateau (Ocean Drilling Program Sites 744 and 748). In: Florindo, F., Cooper, A.K., O'Brien, P.A. (Eds.), Antarctic Cenozoic Palaeoenvironments: Geologic Record and Models. Palaeogeogr., Palaeoclimatol., Palaeoecol. 198 145-168; Florindo, F., Roberts, A.P., in press. Eocene-Oligocene magnetobiochronology of ODP Sites 689 and 690, Maud Rise, Weddell Sea, Antarctica. Geol. Soc. Am. Bull.], and used this data to interpret paleoceanographic changes through the late Eocene to late Oligocene. Percentage plots of the individual species, compared with R-mode principal component and cluster analysis results, allowed us to divide the assemblages into three groups: temperate-water taxa, cool-water taxa, and no temperature-affinity taxa. We attempt correlations between these paleoecological groups and the major sea-surface temperature (SST) variations with tectonic and paleoceanographic changes in the Southern Ocean. During the late Eocene, the nannofossil assemblage data reveal that there were several minor SST decreases (coolings) from 36 to 34 Ma, before the Eocene/Oligocene (E/O) boundary. A sharp cooling event, dated at 33.54 Ma (earliest Oligocene), occurred about 160 kyr after the E/O boundary, which is dated at 33.7 Ma. Relatively stable, cool conditions are interpreted to persist until the latest Oligocene, when an increase in abundance of temperate-water taxa, which corresponds to an antithetical decrease in abundance of cool-water indicators, is recorded. On the basis of our dating, the opening of the Drake Passage, allowing shallow-water circulation, began by 33.54 Ma at the latest, while the establishment of deep-water connections through the Tasmanian Gateway occurred at 33 Ma, as suggested by Exon et al. [Proc. ODP, Init. Rep. 189 (2001) 1].

Heat flow in the Central Basin of the Indian Ocean and the northern part of the Afanasy Nikitin Rise

Heat flux data obtained during Cruise 20 of R/V Akademik Mstislav Keldysh in the Central Basin of the Indian Ocean and northern part of the Afanasy Nikitin Rise are presented. Thermal conditions on the rise are not associated with an anomalous zone of the large tectonic deformation block north of it. Geothermal data indicate that the Afanasy Nikitin Rise has formed near an ancient spreading axis. Distribution of measured heat flux values indicates an additional source of heat in the Central Basin resulting from dissipative heating of the crust in the two-stage plate tectonics model.