Planktonic foraminiferal oxygen isotopes of ODP Site 143-865 samples

Cool tropical sea surface temperatures (SSTs) are reported for warm Paleogene greenhouse climates based on the d18O of planktonic foraminiferal tests. These results are difficult to reconcile with models of greenhouse gas-forced climate. It has been suggested that this "cool tropics paradox" arises from postdepositional alteration of foraminiferal calcite, yielding erroneously high d18O values. Recrystallization of foraminiferal tests is cryptic and difficult to quantify, and the compilation of robust d18O records from moderately altered material remains challenging. Scanning electron microscopy of planktonic foraminiferal chamber-wall cross sections reveals that the basal area of muricae, pustular outgrowths on the chamber walls of species belonging to the genus Morozovella, contain no mural pores and may be less susceptible to postdepositional alteration. We analyzed the d18O in muricae bases of morozovellids from the central Pacific (Ocean Drilling Program Site 865) by ion microprobe using 10 ?m pits with an analytical reproducibility of ±0.34 per mil (2 standard deviations). In situ measurements of d18O in these domains yield consistently lower values than those published for conventional multispecimen analyses. Assuming that the original d18O is largely preserved in the basal areas of muricae, this new d18O record indicates Early Paleogene (~49-56 Ma) tropical SSTs in the central Pacific were 4°-8°C higher than inferred from the previously published d18O record and that SSTs reached at least ~33°C during the Paleocene-Eocene thermal maximum. This study demonstrates the utility of ion microprobe analysis for generating more reliable paleoclimate records from moderately altered foraminiferal tests preserved in deep-sea sediments.

Planktic and benthic foraminifreral isotope data for ODP Site 207-1258

The mid-Cretaceous is widely considered the archetypal ice-free greenhouse interval in Earth history, with a thermal maximum around Cenomanian-Turonian boundary time (ca. 90 Ma). However, contemporaneous glaciations have been hypothesized based on sequence stratigraphic evidence for rapid sea-level oscillation and oxygen isotope excursions in records generated from carbonates of questionable preservation and/or of low resolution. We present new oxygen isotope records for the mid-Cenomanian Demerara Rise that are of much higher resolution than previously available, taken from both planktic and benthic foraminifers, and utilizing only extremely well preserved glassy foraminifers. Our records show no evidence of glaciation, calling into question the hypothesized ice sheets and rendering the origin of inferred rapid sea-level oscillations enigmatic. Simple mass-balance calculations demonstrate that this Cretaceous sea-level paradox is unlikely to be explained by hidden ice sheets existing below the limit of d18O detection.

Stable isotope record of Cretaceous samples of DSDP Hole 71-511

Oxygen isotope data for upper Turonian planktonic foraminifera at Deep Sea Drilling Project Site 511 (Falkland Plateau, 60°S paleolatitude) exhibit an ~2 per mil excursion to values as low as -4.66 per mil (Vienna Peedee belemnite standard; PDB) coincident with the warmest tropical temperature estimates yet obtained for the open ocean. The lowest planktonic foraminifer d18O values suggest that the upper ocean was as warm as 30-32°C. This is an extraordinary temperature for 60°S latitude but is consistent with temperatures estimated from apparently coeval mollusc d18O from nearby James Ross Island (65°S paleolatitude). Glassy textural preservation, a well-defined depth distribution in Site 511 planktonics, low sediment burial temperature (~32°C), and lack of evidence of highly depleted pore waters argue against diagenesis (even solid state diffusion) as the cause of the very depleted planktonic values. The lack of change in benthic foraminifer d18O suggests brackish water capping as the mechanism for the low planktonic d18O values. However, mixing ratio calculations show that the amount of freshwater required to produce a 2 per mil shift in ambient water would drive a 7 psu decrease in salinity. The abundance and diversity of planktonic foraminifera and nannofossils, high planktonic:benthic ratios, and the appearance of keeled foraminifera argue against lower-than-normal marine salinities. Isotope calculations and climate models indicate that we cannot call upon more depleted freshwater d18O to explain this record. Without more late Turonian data, especially from outside the South Atlantic basin, we can currently only speculate on possible causes of this paradoxical record from the core of the Cretaceous greenhouse.

Shallow water foraminifera of ODP Hole 133-821A

Reworked shallow-water foraminifers that settled on the upper slope of the central Great Barrier Reef at Site 821 (water depth, 212.6 m) were used as indicators of the paleoclimatic and paleoenvironmental conditions that have controlled the Pleistocene evolution of the adjacent platform. Throughout the 400-m-thick sequence drilled, the nature, composition, and distribution of the shallow-water foraminiferal assemblages studied indicate that (1) all the species recorded are at present living in diverse tropical, reef-related areas of the Indo-Pacific and Atlantic provinces; (2) the composition of the microfaunal taphocoenoses is almost identical between the different stratigraphic intervals studied and the modern Great Barrier Reef environments; (3) inner-neritic, tropical environments have continued to develop since the middle Pleistocene; (4) high- to moderate-energy platform edges occurred repeatedly throughout Pleistocene time. These factors may suggest that, since the beginning of the Pleistocene, several reef-like tracts have grown successively on the central area of the northeastern Australian shelf edge. These tracts probably had a sufficiently evolved morphological zonation to act as shelters for foraminiferal biocoenoses of high species diversity.

Age model for ODP Leg 175 sites

Upwelling along the western coast of Africa south of the equator may be partitioned into three major areas, each having its own dynamics and history: (1) the eastern equatorial region, comprising the Congo Fan and the area of Mid-Angola; (2) the Namibia upwelling system, extending from the Walvis Ridge to Lüderitz; and (3) the Cape Province region, where upwelling is subdued. The highest nutrient contents in thermocline waters are in the northern region, the lowest in the southern one. Wind effects are at a maximum near the southern end of the Namibia upwelling system, and maximum productivity occurs near Walvis Bay, where the product between upwelling rate and nutrient content of upwelled waters is at a maximum. In the Congo/Angola region, opal tends to follow organic carbon quite closely in the Quaternary record. However, organic carbon has a strong precessional component, while opal does not. Despite relatively low opal content, sediments off Angola show the same patterns as those off the Congo; thus, they are part of the same regime. The spectrum shows nonlinear interference patterns between high- and low-latitude forcing, presumably tied to thermocline fertility and wind. On Walvis Ridge, as in the Congo-Angola region, the organic matter record behaves normally; that is, supply is high during glacial periods. In contrast, interglacial periods are favorable for opal deposition. The pattern suggests reduction in silicate content of the thermocline during glacial periods. The reversed phase (opal abundant during interglacials) persists during the entire Pleistocene and can be demonstrated deep into the Pliocene, not just on Walvis Ridge but all the way to the Oranje River and off the Cape Province. From comparison with other regions, it appears that silicate is diminished in the global thermocline, on average, whenever winds become strong enough to substantially shorten the residence time of silicate in upper waters (Walvis Hypothesis, solving the Walvis Paradox of reversed phase in opal deposition). The central discovery during Leg 175 was the documentation of a late Pliocene opal maximum for the entire Namibia upwelling system (early Matuyama Diatom Maximum [MDM]). The maximum is centered on the period between the end of the Gauss Chron and the beginning of the Olduvai Chron. A rather sharp increase in both organic matter deposition and opal deposition occurs near 3 Ma in the middle of the Gauss Chron, in association with a series of major cooling steps. As concerns organic matter, high production persists at least to 1 Ma, when there are large changes in variability, heralding subsequent pulsed production periods. From 3 to 2 Ma, organic matter and opal deposition run more or less parallel, but after 2 Ma opal goes out of phase with organic matter. Apparently, this is the point when silicate becomes limiting to opal production. Thus, the MDM conundrum is solved by linking planetary cooling to increased mixing and upwelling (ramping up to the MDM) and a general removal of silicate from the upper ocean through excess precipitation over global supply (ramping down from the MDM). The hypothesis concerning the origin of the Namibia opal acme or MDM is fundamentally the same as the Walvis Hypothesis, stating that glacial conditions result in removal of silicate from the thermocline (and quite likely from the ocean as a whole, given enough time). The Namibia opal acme, and other opal maxima in the latest Neogene in other regions of the ocean, marks the interval when a cooling ocean selectively removes the abundant silicate inherited from a warm ocean. When the excess silicate is removed, the process ceases. According to the data gathered during Leg 175, major upwelling started in the late part of the late Miocene. Presumably, this process contributed to the drawing down of carbon dioxide from the atmosphere, helping to prepare the way for Northern Hemisphere glaciation.

Radiogenic isotope trace composition of the Labrador Sea

Geological reconstructions and general circulation models suggest that the onset of both Northern Hemisphere glaciation, 2.7 Myr ago, and convection of Labrador Sea Water (LSW) were caused by the closure of the Panama Gateway ~4.5 Myr ago. Time series data that have been obtained from studies of ferromanganese crusts from the northwestern Atlantic suggest that radiogenic isotopes of intermediate ocean residence time (Pb and Nd) can serve as suitable tracers to reconstruct these events. However, it has been unclear until now as to whether the changes that have been observed in isotope composition at this time are the result of increased thermohaline circulation or due to the effects of increased glacial weathering. In this paper we adopt a box model approach to demonstrate that the shifts in radiogenic isotope compositions are unlikely to be due to changes in convection in LSW but can be explained in terms of increases of erosion levels due to the glaciation of Greenland and Canada. Furthermore, we provide experimental evidence for the incongruent release of a labile fraction of strongly radiogenic Pb and nonradiogenic Nd from continental detritus eroding into the Labrador Sea. This can be attributed to the glacial weathering of old continents and accounts for the paradox that one of the areas of the world most deficient in radiogenic Pb should provide such a rich supply of radiogenic Pb to the oceans. An important general conclusion is that the compositions of radiogenic isotopes in seawater are not always a reflection of their continental sources. Perhaps more importantly, the transition from chemical weathering to mechanical erosion is likely to result in significant variations in radiogenic tracers in seawater.

Benthic foraminifera and ostracoda in ODP Hole 107-650A

Twenty-three sediment intervals from top of Site 650 down to 510 m below seafloor have been studied. Their thicknesses vary between 0.25 m and about 40 m. The studied deposits are turbidites or parts of them except one which is interpreted as an ash-fall layer. The composition of the turbidites signalizes sources from shallow water/coastal areas as well as from deep water levels. Repeated mobilization and displacement seems to have been common. Volcaniclastic material is the dominant component of the whole studied part of Site 650 sedimentary sequence. Ashfall deposits as well as normal open marine sediments are rare.

Oxygen isotope composition of planktonic foraminifer Globigerinoides ruber in sediment core GeoB1413 from the eastern edge of the South Atlantic central gyre

Die Paläozeanographie versucht die Klimageschichte des Quartärs zu rekonstruieren und die Zusammenhänge zwischen Klimaänderungen und ozeanischer Zirkulation besser zu verstehen. Ein wichtiges Hilfsmittel stellen die planktischen Foraminiferen dar. Die Analyse planktischer Foraminiferengemeinschaften hat gezeigt, daß die Verbreitung dieser Protozoa durch die Umweltbedingungen in den Oberflächenwasserströmen bestimmt wird (BoLTOVSKOY, 1969; CIFELLI& BENIER, 1976; OTIENS, 1991). Durch ihre Ablagerung und Erhaltung am Meeresboden speichern sie diese Informationen und bilden einen Indikator für Wassermassen und Oberflächenwassertemperaturschichtung. Zeitliche und räumliche Veränderungen der Faunenvergesellschaftungen und der Verhältnisse stabiler Sauerstoff- und Kohlenstoffisotope einzelner Foraminiferenarten haben damit einen maßgeblichen Beitrag zur Kenntnis der spätquartären Temperatur- und Zirkulationsänderungen der Oberflächenströme geliefert (SHACKLETON & OPDYKE, 1973; BE et al., 1976; RUDDIMAN & McooYRE, 1976; VINCENT & BERGER, 1981; CLIMAP, 1981; RA VELO et al., 1990). Mit Hilfe der planktischen Foraminiferen soll diese Arbeit einen Beitrag zur Rekonstruktion der spätquartären Ozeanographie des Südatlantiks liefern. Die Oberflächenströme des Südatlantiks sind das Bindeglied im Wärmeaustausch zwischen niederen und hohen Breiten. Durch den Südäquatorialstrom (SEC) werden warme Wassermassen, die sich aufgrund der hohen Sonneneinstrahlung im tropischen Atlantik gebildet haben, in den Nordatlantik transportiert. Die Wärme wird im Nordatlantik unter Bildung des Nordatlantischen-Tiefenwassers (NADW) an die Atmosphäre abgegeben. Durch dieses Ereignis wird maßgeblich das nordeuropäische Klima beeinflußt (BROECKER & DENTON, 1989). Die Intensität des SEC wird durch den saisonal variierenden SE-, NE-Passat gesteuert, der hauptsächlich durch die Präzession der geneigten Erdachse bzw. durch die Insolation auf der Nordhalbkugel kontrolliert wird (Mc OOYRE et aI., 1989; MOLFINO & Mc INTYRE, 1990). Der SEC fließt entlang des Äquators von Ost nach West und kalte, nährstotfreiche, tiefere Wassermassen (Südatlantisches-Zentralwasser (SACW)) steigen vor allem im Osten auf und erzeugen das hochproduktive äquatoriale Auftriebsgebiet. Im Osten ist der Temperaturgradient in der Wassersäule steiler, und die Thermoklinentiefe nimmt von Ost nach West zu. Die Lage der Thermokline ist damit ein wesentlicher Faktor, der den Wärmehaushalt im Atlantik mitbestimmt. So wird z. B. im äquatorialen Auftriebsgebiet und im Auftriebsgebiet des küstennahen Benguela-Stroms, wo die Thermoklinentiefe durch aufsteigende kalte Wassermassen gering ist, eine Wärmezunahme von 100 W/qm im Wärmehaushalt erreicht (PETERSON & STRAMMA, 1991). Zur spätquartären Rekonstruktion des Wärmeflusses und der Oberflächenzirkulation im Südostatlantik ist es daher wichtig, auch die zeitlichen und räumlichen Veränderungen tieferer Wasserschichten (bis 300 m) zu erfassen.

Benthic foraminifera and sea level analyses from IODP Hole 310-M0005D

The course of sea-level fluctuations during Termination II (TII; the penultimate deglaciation), which is critical for understanding ice-sheet dynamics and suborbital climate variability, has yet to be established. This is partly because most shallow-water sequences encompassing TII were eroded during sea-level lowstands of the last glacial period or were deposited below the present sea level. Here we report a new sequence recording sea-level changes during TII in the Pleistocene sequence at Hole M0005D (water depth: 59.63 m below sea level [mbsl]) off Tahiti, French Polynesia, which was drilled during Integrated Ocean Drilling Program Expedition 310. Lithofacies variations and stratigraphic changes in the taxonomic composition, preservation states, and intraspecific test morphology of large benthic foraminifers indicate a deepening-upward sequence in the interval from Core 310-M0005D-26R (core depth: 134 mbsl) through -16R (core depth: 106 mbsl). Reconstruction of relative sea levels, based on paleodepth estimations using large benthic foraminifers, indicated a rise in sea level of about 90 m during this interval, suggesting its correlation with one of the terminations. Assuming that this rise in sea level corresponds to that during TII, after correcting for subsidence since the time of deposition, a highstand sea-level position would be 2 ± 15 m above present sea level (masl), which is generally consistent with highstand sea-level positions in MIS 5e (4 ± 2 masl). If this rise in sea level corresponds to that during older terminations, the subsidence-corrected highstand sea-level positions (30 ± 15 masl for Termination III and 54 ± 15 masl for Termination IV) are not consistent with reported ranges of interglacial sea-level highstands (-18 to 15 masl). Therefore, the studied interval likely records the rise in sea level and associated environmental changes during TII. In particular, the intervening cored materials between the two episodes of sea-level rise found in the studied interval might record the sea-level reversal event during TII. This conclusion is consistent with U/Th ages of around 133 ka, which were obtained from slightly diagenetically altered (i.e., < 1% calcite) in situ corals in the studied interval (Core 310-M0005D-20R [core depth: 118 mbsl]). This study also suggests that our inverse approach to correlate a stratigraphic interval with an approximate time frame could be useful as an independent check on the accuracy of uranium-series dating, which has been applied extensively to fossil corals in late Quaternary sea-level studies.

Age models for ODPO Leg 130 sites

The Neogene carbonate stratigraphy of five sites drilled on Ontong Java Plateau during Leg 130 reveals a number of patterns which are unexpected, and which we refer to as loss paradox, equatorial insensitivity, and climate paradox. They denote the following unresolved questions. 1 The loss of carbonate at depth (as derived from differences in accumulation rates) is much greater than suggested by the change in carbonate percentages (calculated under the assumption that carbonate dissolution is the cause of loss). This indicates an important role for redeposition processes, such as winnowing (bottom currents), sifting (resuspension and catabatic flow) and episodic sloughing or solifluction (presumably stimulated by earthquakes). 2 Accumulation rates are not markedly increased at the time a site crosses the equator. There are several possible reasons. Equatorial upwelling may be unimportant in controlling sedimentation rates this far in the western Pacific, or its output may be spread over a considerable distance from the equator. Alternatively, increased supply below the equator is compensated for by increased removal (e.g. from resuspension by bioturbation, combined with catabatic flow). It is conceivable that errors in the timescale could also produce the effect seen. 3 There is an overall tendency for agreement between the stratigraphic patterns of carbonate content and of accumulation rates, but neither pattern is readily explained by reference to changes in climate (represented by benthic delta18O) or in sea-level (as derived from sequence stratigraphy).

Cenozoic benthic foraminifers from ODP Leg 134 holes

This paper discusses the Paleobathymetric and paleoenvironmental history of the New Hebrides Island Arc and North d'Entrecasteaux Ridge during Cenozoic time based on benthic foraminiferal and sedimentological data. Oligocene and Pliocene to Pleistocene benthic foraminiferal assemblages from Sites 827, 828, 829, and 832 of Ocean Drilling Program (ODP) Leg 134 (Vanuatu) are examined by means of Q-mode factor analysis. The results of this analysis recognize the following bathymetrically significant benthic foraminiferal biofacies: (1) Globocassidulina subglobosa biofacies and Bulimina aculeata-Bolivinita quadrilatera biofacies representing the upper bathyal zone (600-1500 m); (2) Gavelinopsis praegeri-Cibicides wuellerstorfi biofacies, indicating the Pacific Intermediate Water (water depth between 1500 and 2400 m); (3) Tosaia hanzawai-Globocassidulina muloccensis biofacies, Valvulineria gunjii biofacies, and the Melonis barleeanus-Melonis sphaeroides biofacies, which characterize the lower bathyal zone; (4) the Nuttallides umbonifera biofacies, which characterizes the interval between the lysocline (approximately 3500 m) and the carbonate compensation depth (approximately 4500 m); and (5) the Rhabdammina abyssorum biofacies representing the abyssal zone below the carbonate compensation depth. Benthic foraminiferal patterns are used to construct Paleobathymetric and paleogeographic profiles of the New Hebrides Island Arc and North d'Entrecasteaux Ridge for the following age boundaries: late Miocene/Pliocene, early/late Pliocene, Pliocene/Pleistocene, and Pleistocene/Holocene.

Abundance and biomass of planktonic diatom Proboscia alata and associated species, chlorophyll a, organic carbon and nitrogen concentrations in waters over the Bering Sea middle shelf in August 2001

During most of the vegetation season from late May to early September large-sized diatom alga Proboscia alata forms local patches with high abundances and biomasses in different oceanographic domains of the eastern Bering Sea shelf. For 0-25 m layer average abundance and biomass of species in these patches are 700000 cells/l and 5 g/m**3 (wet weight), while corresponding estimates for the layer of maximal species concentrations are 40000000 cells/l and 38 g/m**3 (wet weight) or 1.6 g C/m**3. These levels of abundance and biomass are typical for the spring diatom bloom in the region. Outbursts of P. alata mass development are important for the carbon cycle in the pelagic zone of the shelf area in the summer season. The paradox of P. alata summertime blooms over the middle shelf lies in their occurrences against the background of the sharp seasonal pycnocline and deficiency in nutrients in the upper mixed layer. Duration of the outbursts in P. alata development is about two weeks and size of patches with high abundances can be as large as 200 km across. Degradation of the P. alata summertime outbursts may occur during 4-5 days. Rapid sinking of cells through the seasonal pycnocline results in intense transport of organic matter to bottom sediments. One of possible factors responsible for rapid degradation of the blooms is affect on the population by ectoparasitic flagellates. At terminal stages of the P. alata blooms percentage of infected cells can reach 70-99%.