Holger Gebhardt

Paleonutrient and productivity records from the subarctic North Pacific for Pleistocene glacial terminations I to V

Our study addresses fundamental questions of the mode and timing of orbital and millennial-scale changes in the meridional overturning circulation (MOC) of the subarctic North Pacific. Particular concerns are the vertical mixing, the present and past abundance of nutrients in surface waters despite strong stratification, and the North Pacific-North Atlantic seesaw of oscillations in sea surface temperature (SST). We do this by generating and interpreting multiple records for glacial terminations I-V down two long piston cores, one each from the western and eastern subarctic Pacific. Chlorins and biogenic opal are proxies for surface water productivity; delta(13)C of epibenthic foraminifera is a record of deepwater ventilation; and the delta(13)C of N. pachyderma sin. is a tracer of nutrients in subsurface waters that extend up to the sea surface during times of vertical mixing. The degree of mixing is traced by pairing SST and delta(18)O records of planktic surface and subsurface (pycnocline) dwellers. Tight age control is deduced from a suite of age-calibrated (14)C plateau boundaries for Termination I and benthic delta(18)O and geomagnetic events for the last 800 ka. Carbon 14 paleoreservoir ages record the ages of surface and deep waters to uncover short-term changes in MOC over Termination I. We have defined a standard sequence of short-term productivity events for Termination I, also evident during terminations II to V and subsequent interglacials over the last 450 ka. The peak glacial regime of stable stratification and low productivity terminated, together with the end of ice rafting and melting, near 17 ka, similar to 2000 years after the onset of Termination I. Pulses of vertical mixing and incursion of warm surface waters from the subtropics followed. Convected young water masses finally penetrated down to 3600-m water depth at 17.0 to less than 14.5 ka, significantly improving bottom water ventilation through the late deglacial and earliest interglacial. Mixing with upwelled nutrients from the pycnocline induced short-term maxima in algal production of chlorins and biogenic opal near 17-15 and 15-12 ka, respectively. Deglacial meltwater incursions in the Aleutian Current and silica input from North American rivers also promoted East Pacific productivity after 15.5 ka. Productivity decreased during the late deglacial and early interglacial, coeval with an exceptional peak in CaCO(3) preservation caused by both low organic flux and well-ventilated deepwater. Subsequently, low salinity and cool surface waters and in turn, stratification were gradually restored. A second, opal-dominated productivity maximum marked the ends of interglacials. The deglacial pulses of vertical mixing around 17-11 ka imply an important contribution of the North Pacific to the coeval release of oceanic CO(2) into the atmosphere and support the east-west seesaw model of climate change.

Extreme warmth and heat-stressed plankton in the tropics during the Paleocene-Eocene Thermal Maximum

During rapid global warming 56 million years ago, tropical sea surface temperatures exceeded 36°C and stressed eukaryotic plankton. Global ocean temperatures rapidly warmed by ~5°C during the Paleocene-Eocene Thermal Maximum (PETM; ~56 million years ago). Extratropical sea surface temperatures (SSTs) met or exceeded modern subtropical values. With these warm extratropical temperatures, climate models predict tropical SSTs >35°C—near upper physiological temperature limits for many organisms. However, few data are available to test these projected extreme tropical temperatures or their potential lethality. We identify the PETM in a shallow marine sedimentary section deposited in Nigeria. On the basis of planktonic foraminiferal Mg/Ca and oxygen isotope ratios and the molecular proxy TEX86H, latest Paleocene equatorial SSTs were ~33°C, and TEX86H indicates that SSTs rose to >36°C during the PETM. This confirms model predictions on the magnitude of polar amplification and refutes the tropical thermostat theory. We attribute a massive drop in dinoflagellate abundance and diversity at peak warmth to thermal stress, showing that the base of tropical food webs is vulnerable to rapid warming.

Cenomanian to Coniacian ostracodes from the Nkalagu area (SE Nigeria): biostratigraphy and palaeoecology

KurzfassungOstracodenfaunen von drei Profilen aus der Region um Nkalagu, einschließlich des Typ-Profils der Nkalagu Formation, werden qualitativ und quantitativ untersucht. Auf der Basis der Ergebnisse wird folgende Ostracodenzonierung vorgeschlagen: eineCythereis sp. 2-Intervallzone (spätes Turon bis Coniac), eineCythereis vitiliginosa reticulata-Intervallzone (Turon) und eineCytherella spp.-Häufigkeits-zone (Cenoman). Diese Zonierung ist wahrscheinlich nur lokal (beckenweit) gültig, sollte aber die mikropaläontologische Korrelation der Schichten im unteren Benué Trog erlauben. Im Vergleich mit sedimentologischen und paläontologischen Parametern kann eineCytherella dominierte Schelf/Sauerstoffmangel-Fauna von einerCythereis/Ovocytheridea dominierten oberbathyalen/suboxischen bis oxischen Fauna unterschieden werden. Die stratigraphischen und paläoökologischen Eigenschaften der Ostracodenfaunen werden spezifiziert und in diesem Beitrag dargestellt. Einige der Arten (Leguminocythereis sp.,Cytherella cf.comanchensis) wurden wahrscheinlich umgelagert und reflektieren die turbiditischen Ereignisse im Untersuchungsgebiet, die ihren Höhepunkt in der Ablagerung wirtschaftlich bedeutender Kalksteinlagerstätten hatten.AbstractOstracode faunas from three sections in the Nkalagu area, including the type section of the Nkalagu Formation, were analysed qualitatively and quantitatively. Based on the results, an ostracode zonation is proposed as follows: aCythereis sp. 2 Interval Zone (late Turanian to Coniacian), aCythereis vitiliginosa reticulata Interval Zone (Turonian) and aCytherella spp. Abundance Zone (Cenomanian). This zonation may be only of local (basin wide) character but should allow a micropalaeontological correlation of strata in the Lower Benue Trough. By comparison with sedimentological and palaeontological parameters, aCytherella dominated shelf/depleted oxygen-assemblage can be distinguished from aCythereis/Ovocytheridea dominated upper bathyal/suboxic to oxic-assemblage. The stratigraphic and palaeoecologic characteristics of the ostracode assemblages will be specified and presented in this contribution. Some of the species (Leguminocythereis sp.,Cytherella cf.comanchensis) are assumed to be replaced and to reflect the turbiditic activity in the area, resulting in carbonate deposits of economic significance.

Late Oligocene to Early Miocene foraminifers and ostracods from Karben (Wetterau, Hesse State, Germany): stratigraphic occurrence and palaeoecological implications

Foraminiferal and ostracod assemblages from two upper Oligocene to lower Miocene sections located in the Wetterau region (NE Frankfurt a. M.) were analysed. The lithologie units investigated encompass Cyrenenmergel, Glimmersand, Vilbeler Kies, Cerithiensand, Cerithienkalk andCorbicula-Schichten. Hemicyprideis basiliensis, H. helvetica andCypridopsis? cyclocypriformae are characteristic of Cyrenenmergel deposits, representing the last marine influence around the Rupelian/Chattian boundary. The ostracod assemblages from the Cerithiensand are characterized by variousHemicyprideis species, of whichH. rhenana is the most frequent. The microfossil assemblages of the Cerithiensand indicate normal marine conditions for the basal and upper portions of the unit and slightly brackish to normal marine conditions for the middle portion. Very low salinities are indicated for some levels close to the base (around levels of emergence above sealevel) and for the clay layer around sample 30 in the lower part of the Cerithiensand. The Cerithienkalk represents a transitional facies from normal marine to brackish/freshwater conditions as indicated by successive higher contents of stress tolerant foraminifera and freshwater ostracods. TheCorbicula-Schichten are characterized by largeEucypris-moulds and various proportions ofCypridopsis spp. (freshwater indicators). When foraminifera (Bolivina spp.) are present, the fauna indicates slightly higher but still low salinities for a few levels of theCorbicula-Schichten. Oxygenation levels were generally high (presence of large ostracods and epifaunal foraminifera) and vary from suboxic to high oxic. As in the Mainz Basin, the last occurrence ofHemicyprideis rhenana marks the upper boundary of the Cerithienschichten.Protelphidium nonioninoides can be used to identify the Cyrenenmergel when it occurs in high numbers. Cerithiensand-samples show the highest diversities and contain high portions ofUvigerinella michelsi. Overall, highest diversities and normal marine salinities are reached during deposition of the basal and upper parts of the Cerithiensand, which may correspond to global sealevel rises at the end of the Chattian.KurzfassungForaminiferen- und Ostracodenfaunen zweier oberoligozäner bis untermiozäner Profile aus der Wetterau (NE Frankfurt a.M.) wurden stratigraphisch analysiert. Die untersuchten lithologischen Einheiten umfassen Cyrenenmergel, Glimmersand, Vilbeler Kies, Cerithiensand, Cerithienkalk undCorbicula-Schichten.Hemicyprideis basiliensis, H. Helvetica undCypridopsis? cyclocypriformae sind charakteristisch für die Ablagerungen des Cyrenenmergels, die die letzten marinen Einflüsse an der Rupelium/Chattium-Grenze anzeigen. Die Ostracodenvergesellschaftungen des Cerithiensandes sind charakterisiert durch verschiedeneHemicyprideis-Arten; am häufigsten istH. rhenana. Die Zusammensetzung der Mikrofossilen des Cerithiensandes zeigt normal marine Bedingungen für die basalen und oberen Anteile und leicht brackische bis normal marine Bedingungen für den mittleren Teil dieser Einheit an. Sehr geringe Salinitäten herrschten in einigen Niveaus nahe der Basis (während Auftauchphasen) und im Bereich der tonigen Lage im unteren Teil des Cerithiensandes vor. Der Cerithienkalk repräsentiert eine Übergangsfazies von normal marinen zu brackischen/Süßwasser-Bedingungen, angezeigt durch sukzessiv höhere Anteile stresstoleranter Foraminiferen und Süßwasserostracoden. GroßeEucypris-Steinkerne und unterschiedliche Anteile vonCypridopsis spp. (Süßwasserindikatoren) kennzeichnen dieCorbicula-Schichten. Wenn Foraminiferen auftreten (Bolivina spp.), zeigen die Faunen etwas erhöhte, aber immer noch geringe Salinitäten für einige wenige Lagen in denCorbicula-Schichten an. Die Sauerstoffgehalte waren im allgemeinen hoch (Anwesenheit großer Ostracoden und epifaunaler Foraminiferen) und schwankten zwischen suboxisch bis hoch oxisch. Wie im Mainzer Becken markiert das letzte Auftreten vonHemicyprideis rhenana die obere Grenze der Cerithienschichten.Protelphidium nonioninoides kann zur Identifizierung des Cyrenenmergels verwendet werden, wenn sie in großer Anzahl auftritt. Die Cerithiensandproben zeigten die höchsten Diversitäten und enthielten besonders hohe Anteile vonUvigerinella michelsi. Die insgesamt höchsten Diversitäten und normal-marine Salinitäten wurden während der Ablagerung des basalen und oberen Teils des Cerithiensandes erreicht. Diese Phasen könnten globalen Meeresspiegelanstiegen am Ende des Chattium entsprechen.