Paleogene and early Neogene planktonic foraminifera of ODP sites 119-738 and 119-744

A virtually complete composite history of Cenozoic pelagic sedimentation was recovered from ODP Sites 738 (62°43' S) and 744 (61°35' S), drilled during Leg 119 on the Kerguelen Plateau. An excellent magnetobiochronologic record was obtained from upper Eocene through Holocene sediments at Site 744, and an expanded lower Paleocene through lower Oligocene sequence was cored at Hole 738. Analysis of the stratigraphic distribution of over 125 planktonic foraminifer taxa from these sites reveals changes in species composition that were strongly influenced by the climatic evolution of Antarctic water masses. Early Paleocene planktonic foraminifer assemblages are nearly identical in species composition to coeval assemblages from low and middle latitude sites, showing the same patterns of post-extinction recovery and taxonomic radiation. Biogeographic isolation, revealed by the absence of tropical keeled species, became apparent by late early Paleocene time. Diversity increased near the Paleocene/Eocene boundary when keeled morozovellids immigrated to the Kerguelen Plateau. Greatest diversity (23 species) was achieved by early Eocene time, corresponding to a Cenozoic warming maximum that has been recognized in lower Eocene deep sea and terrestrial sediments worldwide. A gradual decline in diversity from the late early through middle Eocene, primarily due to the disappearance of acarininids, parallels the record of cooling paleotemperatures in Southern Ocean surface waters. Chiloguembelina-dominated assemblages appeared in the late middle Eocene and persisted through the early Oligocene as Antarctic surface waters became thermally isolated. Late Eocene and early Oligocene assemblages exhibit considerably lower diversity than the older Eocene faunas, and were dominated by chiloguembelinids, subbotinids, and catapsydracids during a time of pronounced climatic cooling and development of continental glaciation on East Antarctica. The small foraminifer Globigerinit? juvenilis replaced chiloguembelinids as the dominant taxon during the late Oligocene. Diversity increased slightly toward the end of the late Oligocene with new appearances of several tenuitellid, globoturborotalitid, and globigerinid species. The trend toward diminishing planktonic foraminifer diversity was renewed during the late early Miocene as siliceous productivity increased in the Antarctic surface waters, culminating with the reduction to nearly monospecific assemblages of Neogloboqu?drin? p?chyderm? that occur in Pliocene-Holocene biosiliceous sediments. An Antarctic Paleogene zonal scheme previously devised for ODP Sites 689 and 690 in the Weddell Sea is used to biostratigraphically subdivide the Kerguelen Plateau sequence. The definition of one Antarctic Paleogene biozone is modified in the present study to facilitate correlation within the southern high latitudes. The ages of 13 late Eoceneearly Miocene datum events are calibrated based on a magnetobiochronologic age model developed for Site 744.

Oxygen and carbon isotopes from benthic and planktonic foraminifers at DSDP Leg 74 Holes

Oxygen and carbon isotope measurements have been made in picked planktonic and benthonic foraminifers from the five sites drilled on Leg 74, covering the whole Cenozoic. For the Neogene, the coverage gives good information on the development of the vertical temperature structure of Atlantic deep water. For the Paleogene, vertical gradients were weak and it is possible to combine data from different sites to obtain a very detailed record of both the temperature and carbon isotope history of Atlantic deep waters.

Photosynthetically active radiation and microalgae and protist occurrence at ice stations of POLARSTERN cruise ARK-XIX/1

Pack ice around Svalbard was sampled during the expedition ARK XIX/1 of RV "Polarstern" (March-April 2003) in order to determine environmental conditions, species composition and abundances of sea-ice algae and heterotrophic protists during late winter. As compared to other seasons, species diversity of algae (total 40 taxa) was not low, but abundances (5,000-448,000 cells/l) were lower by one to two orders of magnitude. Layers of high algal abundances were observed both at the bottom and in the ice interior. Inorganic nutrient concentrations (NO2, NO3, PO4, Si(OH)4) within the ice were mostly higher than during other seasons, and enriched compared to seawater by enrichment indices of 1.6-24.6 (corrected for losses through the desalination process). Thus, the survival of algae in Arctic pack ice was not limited by nutrients at the beginning of the productive season. Based on less-detailed physical data, light was considered as the most probable factor controlling the onset of the spring ice-algal bloom in the lower part of the ice, while low temperatures and salinities inhibit algal growth in the upper part of the ice at the end of the winter. Incorporation of ice algae probably took place during the entire freezing period. Possible overwintering strategies during the dark period, such as facultative heterotrophy, energy reserves, and resting spores are discussed.

Planktonic foraminifera and stable isotope record of ODP Site 162-980

Detailed faunal, isotopic, and lithic marine records provide new insight into the stability and climate progression of the last interglacial period, Marine Isotope Stage (MIS) 5, which peaked approximately 125,000 years ago. In the eastern subpolar North Atlantic, at the latitude of Ireland, interglacial warmth of the ice volume minimum of substage 5e (MIS 5e) lasted ~10,000 years (10 ka) and its demise occurred in two cooling steps. The first cooling step marked the end of the climatic optimum, which was 2-3 ka long. Minor ice rafting accompanied each cooling step; the second, larger, step encompassing cold events C26 and C25 was previously identified in the northwestern Atlantic. Approximately 4 °C of cooling occurred between peak interglacial warmth and C25, and the region experienced an additional temporary cooling of at least 1-2 °C during C24, a cooling event associated with widespread ice rafting in the North Atlantic. Beginning with C24, MIS 5 was characterized by oscillations of at least 1-2 °C superimposed on a generally cool baseline. The results of this study imply that the marine climatic optimum of the last interglacial was shorter than previously thought. The finding that the eastern subpolar North Atlantic cooled significantly before C24 reconciles terrestrial evidence for progressive climate deterioration at similar and lower latitudes with marine conditions. Our results also demonstrate a close association between modest ice rafting, cooling, and deep ocean circulation even during the peak of MIS 5e and in the earliest stages of ice growth.

Age determination and planktonic foraminifera og ODP Site 159-958

Site 958 was drilled to monitor the late Neogene history of both continental aridity in northwestern Africa and the Canary Current distant from nearshore upwelling. Based on magnetostratigraphy, biostratigraphic datums, variations in carbonate, coarse fraction components, and the species composition of planktonic foraminifers, as well as using the d18O records of Globigerinoides ruber (white), we established a splice between Holes 958A and 958B and a stratigraphic age scale deciphering Milankovitch cycles. Over the last 630 k.y., sedimentation rates amount to 2.9 cm/k.y., and to 2.05-2.53 cm/k.y. back to the base of the Pleistocene. Extremely low rates of 0.4 cm/k.y. and a reworking of fossils mark the late Pliocene. The first continuous, long, sea-surface temperature (SST) record from the center of the Canary Current, which is based on foraminifer species census data, depicts a general temperature decrease in the late Pliocene, lower SST and high seasonalities of up to 6°C ~2.0-1.6 Ma, a warmer interval from 1.6 Ma to ~0.85 Ma, again lower SST and higher seasonalities until 0.33 or 0.26 Ma, and a final warmer interval, lasting until at least 50 ka, possibly reflecting the attenuated dynamics of the Canary Current. Especially over the last 400 k.y., since Stage 11, glacial stages are hardly reflected by cold SST cycles, except for various abrupt and extremely short cooling events amounting to D6°C, which possibly result from North Atlantic Heinrich events. Similar, but not necessarily synchronous, events of short-term, extremely high values occur in the paleoproductivity and (d13Cbased) paleonutrient records, which indicate a generally low primary production averaging to 180 g C m**-2 yr**-1 at 50-330 ka and about 300 g C m**-2 yr**-1 back to the base of the Pleistocene. Near 1.2-1.6 Ma, the grain-size and magnetic susceptibility records document a significant increase in the discharge of south Saharan/Sahelian dust, possibly linked to increasing aridity.

Mg/Ca and d18O measurements of planktonic foraminifera from Sargasso Sea

Paired Mg/Ca and d18O measurements on planktonic foraminiferal species (G. ruber white, G. ruber pink, G. sacculifer, G. conglobatus, G. aequilateralis, O. universa, N. dutertrei, P. obliquiloculata, G. inflata, G. truncatulinoides, G. hirsuta, and G. crassaformis) from a 6-year sediment trap time series in the Sargasso Sea were used to define the sensitivity of foraminiferal Mg/Ca to calcification temperature. Habitat depths and calcification temperatures were estimated from comparison of d18O of foraminifera with equilibrium calcite, based on historical temperature and salinity data. When considered together, Mg/Ca (mmol/mol) of all species, except two, show a significant (r = 0.93) relationship with temperature (T °C) of the form Mg/Ca = 0.38 (±0.02) exp 0.090 (±0.003)T, equivalent to a 9.0 ± 0.3% change in Mg/Ca for a 1°C change in temperature. Small differences exist in calibrations between species and between different size fractions of the same species. O. universa and G. aequilateralis have higher Mg/Ca than other species, and in general, data can be best described with the same temperature sensitivity for all species and pre-exponential constants in the sequence O. universa > G. aequilateralis = G. bulloides > G. ruber = G. sacculifer = other species. This approach gives an accuracy of ±1.2°C in the estimation of calcification temperature. The 9% sensitivity to temperature is similar to published studies from culture and core top calibrations, but differences exist from some literature values of pre-exponential constants. Different cleaning methodologies and artefacts of core top dissolution are probably implicated, and perhaps environmental factors yet understood. Planktonic foraminiferal Mg/Ca temperature estimates can be used for reconstructing surface temperatures and mixed and thermocline temperatures (using G. ruber pink, G. ruber white, G. sacculifer, N. dutertrei, P. obliquiloculata, etc.). The existence of a single Mg thermometry equation is valuable for extinct species, although use of species-specific equations will, where statistically significant, provide more accurate evaluation of Mg/Ca paleotemperature.

Distribution of planktonic foraminifera during the Mid-Pleistocene of ODP Site 181-1123

Planktonic foraminiferal assemblages and artificial neural network estimates of sea-surface temperature (SST) at ODP Site 1123 (41°47.2'S, 171°29.9'W; 3290 m deep), east of New Zealand, reveal a high-resolution history of glacial-interglacial (G-I) variability at the Subtropical Front (STF) for the last 1.2 million years, including the Mid-Pleistocene climate transition (MPT). Most G-I cycles of ~100 kyr duration have short periods of cold glacial and warm deglacial climate centred on glacial terminations, followed by long temperate interglacial periods. During glacial-deglacial transitions, maximum abundances of subantarctic and subtropical taxa coincide with SST minima and maxima, and lead ice volume by up to 8 kyrs. Such relationships reflect the competing influence of subantarctic and subtropical surface inflows during glacial and deglacial periods, respectively, suggesting alternate polar and tropical forcing of southern mid-latitude ocean climate. The lead of SSTs and subtropical inflow over ice volume points to tropical forcing of southern mid-latitude ocean-climate during deglacial warming. This contrasts with the established hypothesis that southern hemisphere ocean climate is driven by the influence of continental glaciations. Based on wholesale changes in subantarctic and subtropical faunas, the last 1.2 million years are subdivided into 4-distinct periods of ocean climate. 1) The pre-MPT (1185-870 ka) has high amplitude 41-kyr fluctuations in SST, superimposed on a general cooling trend and heightened productivity, reflecting long-term strengthening of subantarctic inflow under an invigorated Antarctic Circumpolar Current. 2) The early MPT (870-620 ka) is marked by abrupt warming during MIS 21, followed by a period of unstable periodicities within the 40-100 kyr orbital bands, decreasing SST amplitudes, and long intervals of temperate interglacial climate punctuated by short glacial and deglacial phases, reflecting lower meridional temperature gradients. 3) The late MPT (620-435 ka) encompasses an abrupt decrease in the subantarctic inflow during MIS 15, followed by a period of warm equable climate. Poorly defined, low amplitude G-I variations in SSTs during this interval are consistent with a relatively stable STF and evenly balanced subantarctic and subtropical inflows, possibly in response to smaller, less dynamic polar icesheets. 4) The post-MPT (435-0 ka) is marked by a major climatic deterioration during MIS 12, and a return to higher amplitude 100 kyr-frequency SST variations, superimposed on a long term trend towards cooler SSTs and increased mixed-layer productivity as the subantarctic inflow strengthened and polar icesheets expanded.

B/Ca ratios in planktonic foraminifera

Boron isotope systematics indicate that boron incorporation into foraminiferal CaCO3 is determined by the partition coefficient, KD = [B/Ca](CaCO3)/[B(OH)4**-/HCO3**-](seawater), and [B(OH)4?/HCO3?](seawater), providing, in principle, a method to estimate seawater pH and PCO2. We have measured B/Ca ratios in Globigerina bulloides and Globorotaliainflata for a series of core tops from the North Atlantic and the Southern Ocean and in Globigerinoides ruber (white) from Ocean Drilling Program (ODP) site 668B on the Sierra Leone Rise in the eastern equatorial Atlantic. B/Ca ratios in these species of planktonic foraminifera seem unaffected by dissolution on the seafloor. KD shows a strong species-specific dependence on calcification temperature, which can be corrected for using the Mg/Ca temperature proxy. A preliminary study of G. inflata from Southern Ocean sediment core CHAT 16K suggests that temperature-corrected B/Ca was ~30% higher during the last glacial. Correspondingly, pH was 0.15 units higher and aqueous PCO2 was 95 ?atm lower at this site at the Last Glacial Maximum. The covariation between reconstructed PCO2 and the atmospheric pCO2 from the Vostok ice core demonstrates the feasibility of using B/Ca in planktonic foraminifera for reconstructing past variations in pH and PCO2.

87Sr/86Sr ratios of planktonic foraminifers and interstitial waters at DSDP Hole 90-593

Detailed profiles of the Sr isotopic compositions of fossil planktonic foraminifers and interstitial waters have been measured from DSDP Site 593 to determine the Sr isotopic composition of seawater during the last 40 m.y. Foraminiferal recrystallization was assessed through scanning electron microscopy (SEM) and Sr/Ca ratios. Foraminifers were shown to be well preserved. Results document that the seawater 87Sr/86Sr has increased continuously but not uniformly, since the latest Eocene.

Strontium isotope ratios of planktonic foraminifera from late Neogene sediment samples

A curve describing the variation of the strontium isotopic composition of seawater for the late Neogene (9 to 2 Ma) was constructed from 87Sr/86Sr analyses of marine carbonate in five Deep Sea Drilling Project (DSDP) sites: 502, 519, 588, 590, and 593. The strontium isotopic composition of the oceans increased between 9 and 2 Ma with several changes in slope. From 9 to 5.5 Ma, 87Sr/86Sr values were nearly constant at ~0.708925. Between 5.5 and 4.5 Ma, 87Sr/86Sr ratios increased monotonically at a rate of not, vert, similar 1 * 10**-4 per million years. The steep slope during this interval provides the potential for high resolution strontium isotope stratigraphy across the Miocene/Pliocene boundary. The rate of change of 87Sr/86Sr decreases to near zero again during the interval 4.5-2.5 Ma, and ratios average 0.709025. The relatively rapid increase of 87Sr/86Sr between 5.5 and 4.5 Ma must be related to changes in the flux or average 87Sr/86Sr ratios of the major inputs of Sr to the oceans. Quantitative modelling of these inputs suggests that the increase was most probably caused by an increase in the dissolved riverine flux of strontium to the oceans, an increase in the average 87Sr/86Sr composition of river water, or some combination of these parameters. Modelling of this period as a transient-state requires a pulse-like increase in the input of 87Sr to the oceans between 5.5 and 4.5 Ma. Alternatively, the 5.5-4.5 Ma period can be modelled as a simple transition from one steady-state to another if the oceanic residence time of strontium was eight times less than the currently accepted value of 4 Ma. During the time interval of steep 87Sr/86Sr increase, other geochemical and sedimentologic changes also occur including an increase in sediment accumulation rates, a drop in the calcite compensation depth (CCD), and a decrease in the delta13C of dissolved bicarbonate (i.e., "carbon shift"). The simplest mechanism to explain 87Sr/86Sr variation and these related geochemical changes is to invoke an increase in the dissolved chemical fluxes carried by rivers to the oceans. This, in turn, implies increased chemical denudation rates of the continents and shelves during the late Neogene. The increase in chemical weathering rates is attributed to increased exposure of the continents by eustatic regression, intensified glacial/interglacial cycles, and accelerated rates of global tectonism beginning at 5.5 Ma during the latest Miocene.

Vertical microalgae fluxes at the mooring station LOMO-2 over the Lomonosov Ridge in the period from September 15, 1995 till August 16, 1996

The first studies of microalgae fluxes over the Lomonosov Ridge in the northern Laptev Sea were carried out with a sediment trap at the year-long mooring station LOMO-2, installed at 150 m depth from September 15, 1995 to August 16, 1996. These studies demonstrated essential seasonal variations of vertical microalgae flux. It was shown that in summer diverse flora (composed mainly of cryophylic diatoms) growed intensively beneath the permanent ice cover. Strongly pronounced seasonal variations of microalgae growth correlate closely with solar radiation. Exactly during the maximum insolation period, from the middle of July until the end of September, the microalgae flux was hundreds of times higher than that in the rest of the year. Summer values of the microalgae flux over the Lomonosov Ridge in the northern Laptev Sea were similar to those in the Weddell Sea (Antarctic) and exceeded summer flux values in the Norwegian and Greenland Seas and in the St. Anna Trough (northwestern Kara Sea).

Stable isotope record of planktonic foraminifera of the South Atlantic

The central problem of late Quaternary circulation in the South Atlantic is its role in transfer of heat to the North Atlantic, as this modifies amplitude, and perhaps phase, of glacialinterglacial fluctuations. Here we attempt to define the problem and establish ways to attack it. We identify several crucial elements in the dynamics of heat export: (1) warm-water pile-up (and lack thereof) in the Western equatorial Atlantic, (2) general spin-up (or spin-down) of central gyre, tied to SE trades, (3) opening and closing of Cape Valve (Agulhas retroflection), (4) deepwater E-W asymmetry. Means for reconstruction are biogeography, stable isotopes, and productivity proxies. Main results concern overall glacial-interglacial contrast (less pile-up, more spin-up, Cape Valve closed, less NADW during glacial time), dominance of precessional signal in tropics, phase shifts in precessional response. To generate working hypotheses about the dynamics of surface water circulation in the South Atlantic we employ Croll's paradigm that glacial - interglacial fluctuations are analogous to seasonal fluctuations. Our general picture for the last 300 kyrs is that, as concerns the South Atlantic, intensity of surface water (heat) transport depends on the strength of the SE trades. From various lines of evidence it appears that strenger SE trades appeared during glacials and cold substages during interglacials, analogous to conditions in southern winter (August).

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.