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.

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.

Planktonic foraminiferal stratigraphy and datum levels of ODP Leg 182 sites

Planktonic foraminifers from Ocean Drilling Program Leg 182, Holes 1126B and 1126C, 1128B and 1128C, 1130A and 1130B, 1132B, and 1134A and 1134B confirm the neritic record that during the early Miocene the Great Australian Bight region was in a cool-temperate regime with abundant Globoturborotalita woodi. Warm marine environments started to develop in the later part of the early Miocene, and the region became warm temperate to subtropical in the early middle Miocene with abundant Globigerinoides, Orbulina, and Globorotalia, corresponding to global warming at the Miocene climatic optimum. Fluctuations between cool- and warm-temperate conditions prevailed during the late Miocene, as indicated by abundant Globoconella conoidea and Menardella spp. A major change in planktonic foraminiferal assemblages close to the Miocene/Pliocene boundary not only drove many Miocene species into extinction but also brought about such new species as Globorotalia crassaformis and Globoconella puncticulata. Warm-temperate environments continued into the early and mid-Pliocene before being replaced by cooler conditions, supporting numerous Globoconella inflata and Globigerina quinqueloba. Based on data from this study and published results from the Australia-New Zealand region, we established a local planktonic foraminifer zonation scheme for separating the southern Australian Neogene (SAN) into Zones SAN1 to SAN19 characterizing the Miocene and Zones SAN20 to SAN25 characterizing the Pliocene. The Neogene sections from the Great Australian Bight are bounded by hiatuses of ~0.5 to >3 m.y. in duration, although poor core recovery in some holes obscured a proper biostratigraphic resolution. A total of 15 hiatuses, numbered 1 to 15, were identified as synchronous events from the base of the Miocene to the lower part of the Pleistocene. We believe that these are local manifestations of major third-order boundaries at about (1) 23.8, (2) 22.3, (3) 20.5, (4) 18.7, (5) 16.4, (6) 14.8, (7) 13.5, (8) 11.5, (9) 9.3, (10) 7.0, (11) 6.0, (12) 4.5, (13) 3.5, (14) 2.5, and (15) 1.5 Ma, respectively. This hiatus-bounded Neogene succession samples regional transgressions and stages of southern Australia and reveals its stepwise evolutionary history.

Planktonic foraminiferal d18O values and apparent calcification depths of core-top samples along an east-west transect across the tropical Atlantic/Caribbean

D18O values of nine tropical-subtropical planktonic foraminiferal species with different preferential habitat depths collected from 62 core-top samples along an east-west transect across the tropical Atlantic/Caribbean were used to test the applicability of interspecific d18O gradients for reconstructions of tropical upper ocean stratification. In general, the d18O difference (Delta d18O) between intermediate- and shallow-dwelling species decreases, and Delta d18O between deep and intermediate dwellers increases with increasing thermocline depth towards the west. The statistical significance of regional differences in Delta d18O highlights Delta d18O between the intermediate dwellers (in particular Globorotalia scitula and Globorotalia tumida) and the shallow dweller Globigerinoides ruber pink, as well as Delta d18O between the deep dwellers Globorotalia crassaformis or Globorotalia truncatulinoides dextral and intermediate dwellers as most sensitive to changes in tropical Atlantic thermocline depth. Based on the observed regional variations in interspecific Delta d18O, we propose a multispecies stratification index "STRAtrop" = (d18Ointermediate - d18Oshallow) / (d18Odeep - d18Oshallow) for the tropical ocean. Statistically significant differences in STRAtrop values between the E-Atlantic and the Caribbean suggest that this index may be a useful tool to monitor variations in tropical upper ocean stratification in the geological record.

Planktonic foraminiferal abundance and flux in the Porcupine Seabight, Northeast Atlantic

Modern planktonic foraminifera collected with a sediment trap and subfossil assemblages from surface sediments from Galway Mound in the Porcupine Seabight off southwestern Ireland, northeastern Atlantic, were studied to show recent assemblage variations. The sediment trap operated from April to August 2004 and covers the spring bloom and early summer conditions with sampling intervals of 8 days. Eleven different species were recorded. Glorotalia hirsuta, Turborotalita quinqueloba and Globigerinita glutinata appeared predominately in spring. Neogloboquadrina incompta, Globigerina bulloides and Globorotalia inflata were abundant in spring and summer. The highest foraminiferal tests flux occured in June. The faunal composition was similar to subfossil assemblages from surface sediments, but the species proportions were different. This was mainly affected by the subtropical G. hirsuta, which was frequent in 2004 and rare in surface sediment samples and in earlier plankton collections from the southern Porcupine Seabight that were performed during the 1990s. The weight of deposited foraminifera is mainly influenced by spring bloom as indicated by sea-surface chlorophyll-a data. The top three-ranked species, G. hirsuta, N. incompta and G. bulloides contributed 87 % to the foraminiferal carbonate flux at Galway Mound. Foraminiferal carbonate and shell flux as well as the shell size revealed variations, which are related to lunar periodicity. The data infer a lunar pacing of reproduction for the main species as well as for G. glutinata and G. inflata, which was not recorded before.

Stable isotope record of planktonic foraminifera from the western Iberian margin during marine isotope stage 2

The surface water hydrography along the western Iberian margin, as part of the North Atlantic's eastern boundary upwelling system, consists of a complex, seasonally variable system of equatorward and poleward surface and subsurface currents and seasonal upwelling. Not much information exists to ascertain if the modern current and productivity patterns subsisted under glacial climate conditions, such as during marine isotope stage (MIS) 2, and how North Atlantic meltwater events, especially Heinrich events, affected them. To help answer these questions we are combining stable isotope records of surface to subsurface dwelling planktonic foraminifer species with sea surface temperature and export productivity data for four cores distributed along the western and southwestern Iberian margin (MD95-2040, MD95-2041, MD99-2336, and MD99-2339). The records reveals that with the exception of the Heinrich events and Greenland Stadial (GS) 4 hydrographic conditions along the western Iberian margin were not much different from the present. During the Last Glacial Maximum (LGM), subtropical surface and subsurface waters penetrated poleward to at least 40.6°N (site MD95-2040). Export productivity was, in general, high on the western margin during the LGM and low in the central Gulf of Cadiz, in agreement with the modern situation. During the Heinrich events and GS 4, on the other hand, productivity was high in the Gulf of Cadiz and suppressed in the upwelling regions along the western margin where a strong halocline inhibited upwelling. Heinrich event 1 had the strongest impact on the hydrography and productivity off Iberia and was the only period when subarctic surface waters were recorded in the central Gulf of Cadiz. South of Lisbon (39°N), the impact of the other Heinrich events was diminished, and not all of them led to a significant cooling in the surface waters. Thus, climatic impacts of Heinrich events highly varied with latitude and the prevailing hydrographic conditions in this region.

Mg/Ca of planktonic foraminifera from tropical Pacific surface sediments

Mg/Ca in planktonic foraminifers carries two main signals: calcification temperature and postdepositional test dissolution. Shell dissolution thus distorts water temperature reconstructions made with Mg/Ca in foraminifers. This problem could be resolved by quantifying the impact of carbonate dissolution on Mg/Ca with an independent, temperature-insensitive deep-sea calcite dissolution proxy, such as the Globorotalia menardii fragmentation index (MFI). To test the validity of this approach, we measured Mg/Ca in the tests of several planktonic foraminifers and MFI in core tops collected over a wide geographic region of the tropical Pacific and covering a wide range of deep-sea calcite dissolution and seawater temperature. We confirm that Mg/Ca from different species have different susceptibility to temperature and dissolution. Mg/Ca in surface-dwelling Globigerina bulloides is controlled by calcification temperature and is largely unaffected by carbonate dissolution estimated from MFI. In contrast, Mg/Ca in deeper dwelling G. menardii is minimally sensitive to temperature and dominantly affected by dissolution. Mg/Ca in Neogloboquadrina dutertrei and Pulleniatina obliquiloculata are significantly affected by both temperature and dissolution, and MFI can be effectively used to correct temperature estimates from these species for calcite dissolution. Additional variables besides temperature and dissolution appear to control Mg/Ca in Globorotalia tumida, and their identification is a prerequisite for interpreting elemental shell composition in this species. Combining down-core measurements of Mg/Ca in multiple foraminifer species with MFI provides a powerful tool for reconstructing past changes in the upper water column temperature structure in the tropical Pacific.