Planktonic foraminiferal preservation in Plio/Pleistocene sediments from the western equatorial Atlantic and Caribbean

Records of mean sortable silt and planktonic foraminiferal preservation from the Ceará Rise (western equatorial Atlantic) and from the Caribbean are presented to analyze the Pliocene (3.5-2.2 Ma) to Pleistocene (1.6-0.3 Ma) evolution of near-bottom current strength and the carbonate corrosiveness of deep water. During the mid-Pleistocene climate transition (~1 Ma) a drastic decrease in glacial bottom current strength and an increase in carbonate corrosiveness is registered, demonstrating a substantial decrease in the glacial contribution of the Lower North Atlantic Deep Water (LNADW) to the Atlantic Ocean. Also, an increased sensitivity to eccentricity orbital forcing is registered after the MPT. By contrast, carbonate preservation increases considerably in the deep Caribbean in response to a strong and persistent stable contribution of Upper North Atlantic Deep Water (UNADW). We found evidence for the strongest and most stable circulation within the LNADW cell during the Northern Hemisphere cooling period between ~3.2 and 2.75 Ma. This is in agreement with the 'superconveyor model' which postulates that the highest NADW production took place prior to ~2.7 Ma. A considerable decrease in bottom current strength and planktonic foraminiferal preservation is observed synchronous with the first occurrence of large-scale continental ice sheets in the Northern Hemisphere. This documents the final termination of the 'superconveyor' at ca. 2.75 Ma. However, our data do not support a 'superconveyor' in the interval between 3.5 and 3.2 Ma when high-amplitude fluctuations in bottom current flow and preservation in planktonic foraminifera are observed. Because of the great sensitivity of NADW production to changes in surface water salinity, we assume that the high-amplitude fluctuations of LNADW circulation prior to ~3.2 Ma are linked to changes in the Atlantic salinity budget. After 2.75 Ma they are primarily controlled by ice-sheet forcing. In contrast to the stepwise deterioration of planktonic foraminiferal preservation in the western deep Atlantic, a trend toward better preservation from the Pliocene to Pleistocene is observed in the deep Caribbean. This indicates a long-term increase in the contribution of UNADW to the Atlantic Ocean.

Sea-surface temperature reconstruction in the Southwest Pacific

Uniquely in the Southern Hemisphere the New Zealand micro-continent spans the interface between a subtropical gyre and the Subantarctic Circumpolar Current. Its 20° latitudinal extent includes a complex of submerged plateaux, ridges, saddles and basins which, in the present interglacial, are partial barriers to circulation and steer the Subtropical (STF) and Subantarctic (SAF) fronts. This configuration offers a singular opportunity to assess the influence of bottom topography on oceanic circulation through Pleistocene glacial - interglacial (G/I) cycles, its effect on the location and strength of the fronts, and its ability to generate significant differences in mixed layer thermal history over short distances. For this study we use new planktic foraminiferal based sea-surface temperature (SST) estimates spanning the past 1 million years from a latitudinal transect of four deep ocean drilling sites. We conclude that: 1. the effect of the New Zealand landmass was to deflect the water masses south around the bathymetric impediments; 2. the effect of a shallow submerged ridge on the down-current side (Chatham Rise), was to dynamically trap the STF along its crest, in stark contrast to the usual glacial-interglacial (G-I) meridional migration that occurs in the open ocean; 3. the effect of more deeply submerged, downstream plateaux (Campbell, Bounty) was to dynamically trap the SAF along its steep southeastern margin; 4. the effects of saddles across the submarine plateaux was to facilitate the development of jets of subtropical and subantarctic surface water through the fronts, forming localized downstream gyres or eddies during different phases in the G-I climate cycles; 5. the deep Pukaki Saddle across the Campbell-Bounty Plateaux guided a branch of the SAF to flow northwards during each glacial, to form a strong gyre of circumpolar surface water in the Bounty Trough, especially during the mid-Pleistocene Climate Transition (MIS 22-16) when exceptionally high SST gradients existed across the STF; 6. the shallower Mernoo Saddle, at the western end of the Chatham Rise, provided a conduit for subtropical water to jet southwards across the STF in the warmest interglacial peaks (MIS 11, 5.5) and for subantarctic water to flow northwards during glacials; 7. although subtropical or subantarctic drivers can prevail at a particular phase of a G-I cycles, it appears that the Antarctic Circumpolar Current is the main influence on the regional hydrography. Thus complex submarine topography can affect distinct differences in the climate records over short distances with implications for using such records in interpreting global or regional trends. Conversely, the local topography can amplify the paleoclimate record in different ways in different places, thus enhancing its value for the study of more minor paleoceanographic influences that elsewhere are more difficult to detect. Such sites include DSDP 594, which like some other Southern Ocean sites, has the typical late Pleistocene asymmetrical saw-tooth G-I climate pattern transformed to a gap-tooth pattern of quasi-symmetrical interglacial spikes that interrupt extended periods of minimum glacial temperatures.

(Tables 2, 3) Element ratios of planktonic foraminifera from western Pacific surface sediments

Accurately quantifying deep-sea calcite dissolution is crucial for understanding the role of the marine carbonate system in regulating atmospheric pCO2 over millennia. We compare a foraminifer-fragmentation-based calcite dissolution proxy (Globorotalia menardii fragmentation index (MFI)) to Mg/Ca, Sr/Ca, and Mg/Sr in several species of deep dwelling planktonic foraminifers. We conducted microfossil and geochemical analyses on the same core top samples taken at different depths on the Ontong Java Plateau to maximize the dissolution signal and minimize the temperature overprint on our data. We also compare elemental ratios from planktonic foraminifer tests to modern bottom water [CO3]2- undersaturation and model-derived estimates of percent calcite dissolved in deep-sea sediments. We find clear linear decreases in Mg/Ca or Mg/Sr in G. menardii and Pulleniatina obliquiloculata with increasing (1) bottom water [CO3]2- undersaturation, (2) percent calcite dissolved in sediments calculated with biogeochemical modeling, (3) MFI, and (4) percent calcite dissolved derived from MFI. These findings lend further support to MFI as a calcite dissolution proxy for deep-sea sediments. In contrast, we find no significant correlation between Sr/Ca and independent dissolution indicators. Our results suggest that Mg/Ca and Mg/Sr from deep dwelling foraminifers could potentially be used as calcite dissolution proxies in combination with independent water temperature estimates. Likewise, establishing the relationship between MFI and dissolution-induced changes in the Mg/Ca of surface-dwelling foraminifers could provide a tool to correct Mg/Ca-derived sea surface temperature reconstructions for calcite dissolution.

Foraminifera and stable isotope record of ODP Leg 105 sites

Faunal and stable isotopic data in Sites 646 and 647 provide a ~0.9-Ma paleoclimatic and paleoceanographic record for the Labrador Sea, that is supported by a floral record for the past ~0.3 Ma. At both sites, most glacial stages generally are dominated by polar fauna and flora with low species diversity. Although minor occurrences of subpolar species also were observed in lowermost parts of several glacial stages in Site 646, the faunal classification of Ruddiman and Mclntyre (1976) suggested the presence of polar ecological water masses in the area during most of the glacial periods. In several glacial stages at Site 647, both the faunal and floral data indicate that early periods were marked by subpolar and transitional ecological water masses. The interglacials are characterized by a polar fauna at Site 646 and by polar and transitional faunas and floras at Site 647. However, several interglacial stages in Site 646 include a subpolar flora, in contrast to a planktonic foraminifer fauna similar to that found in the glacial stages. The occurrence of subpolar water masses in several glacial isotopic stages indicates significant northward advection of warmer waters into the Labrador Sea during the early glacial periods, which provided a corridor of oceanic warmth extending from mid- to high latitudes and contributed an additional source of moisture for continental ice-sheet growth. Similar conditions also were documented in the northwest Labrador Sea, Grand Banks, and the North Atlantic.

Investigation of sediment core PS2556 from Bellingshausen Sea, Antarctica

Die Rekonstruktion der glaziomarinen Sedimentationsprozesse am antarktischen Kontinentalrand des westlichen Bellingshausenmeeres erfolgte durch die sedimentologische Auswertung eines 962 cm langen Schwerelotkernes aus 3594 m Wassertiefe. Der Kern wurde während des Fahrtabschnittes ANT-XI/3 mit dem FS "Polarstern" vom Scheitel einer Sediment- "Drift" gezogen. An dem Sedimentkern wurde eine lithologische Beschreibung, sowie sedimentologische Untersuchungen und sedimentphysikalische Messungen durchgeführt. Anhand der Ergebnisse konnten signifikante Änderungen in der Zusammensetzung und Struktur der Sedimente erkannt, und drei Faziestypen unterschieden werden. Die Faziestypen charakterisieren jeweils glaziale oder interglaziale Zeiträume. Der größte Teil der Sedimentabfolge gehört der Laminitfazies an. Dabei handelt es sich um feinlaminierte Sedimentabschnitte, die vorwiegend aus feinkörnigen, terrigenen Komponenten zusammengesetzt sind. In die feinlaminierten Abschnitte sind vereinzelte, wenige Milimeter bis Zentimeter mächtige Siltlagen eingeschaltet. Die biogenen Anteile sind gering, Anzeichen für Bodenleben fehlen völlig. Die Manganfazies wird von authigen gebildeten Mangankonkretionen dominiert, die jeweils diskrete Lagen bilden. Dabei handelt es sich zum einen um Mikromanganknollen und -krusten und zum andern um manganhaltige Gangfüllungen. Biogene und terrigene Anteile sind in diesem Faziestyp unbedeutend. Die Biogenfazies ist von strukturlosen und stark bioturbierten Sedimenten gekennzeichnet. In diesen Sedimentabschnitten ist der hohe Anteil an Eisfracht (IRD) und die erhöhten Gehalte an Kalziumkarbonat und Opal in der Sandfraktion markant. Die stratigraphische Einordnung des Sedimentkernes erfolgte über die von Grobe & Mackensen (1992) entwickelte Lithostratigraphie, mit deren Einheiten die Faziestypen des Sedimentkernes korreliert werden konnten. Dabei ergaben sich zwei mögliche Altersmodelle und ein Basisalter von ca. 250.000 Jahren. Anhand der stratigraphischen Fixpunkte wurden Sedimentationsraten des Gesamtsedimentes und Akkumulationsraten des Kalziumkarbonates, des Biogenopals und des organisch gebundenen Kohlenstoffes berechnet. Dabei wurde gezeigt, daß lediglich das Kalziumkarbonat und der Biogenopal als Anzeiger für biologische Produktion dienen können, wobei Lösungsprozesse in der Wassersäule und im Sediment eine große Rolle spielen. Der Gehalt an organisch gebundenem Kohlenstoff ist in dem Sedimentkern nur erhaltungsbedingt zu erklären. Die Sedimentationsprozesse der einzelnen Faziestypen sind von den Eisverhältnissen, der biologischen Produktion, dem gravitativen Transport und der Umlagerung durch Meeresströmungen abhängig. Die Auswirkung der einzelnen Faktoren ist jeweils unterschiedlich ausgeprägt und wirkt sich spezifisch auf die einzelnen Parameter aus. In den Glazialen hatte ein Vorstoß des Schelfeises über die Schelfkante zur Anlieferung großer Sedimentmassen geführt, die über gravitativen Transport den Kontinentalhang hinunter transportiert wurden. Die Feinfracht wurde über parallel zum Kontinentalhang laufende Konturströme westwärts transportiert und in der Larninitfazies der Driftkörper abgelagert. Am Ende der Glaziale kam es zur Sedimentation der Manganfazies. Die geringen Sedimentationsraten am Kamm der Sedimentdrift kamen aufgrund reduzierter Intensität der Konturströme und fehlender Umlagerung von Schelfsedimenten in Folge rückschreitender Schelfeisrnassen zustande. In den Interglazialen kam es durch den aufsteigenden Meeresspiegel zum Aufschwimmen des Schelfeises. Der damit verbundene Abbau der Eisrnassen über dem Schelf, hatte eine hohe Sedimentation von IRD zur Folge. Mit fortschreitendem Interglazial kam es in Zeiten nur saisonaler Meereisbedeckung zu verstärkter biologischer Produktion und zur Sedimentation biogenen Materials.

Stable oxygen isotope record of planktonic foraminifera from late Quaternary sediments of the equatorial Pacific

Planktonic foraminiferal test fragmentation in three cores along a depth transect from the western equatorial Pacific (ERDC-93P, 1619 m; RC17-177, 2600 m; V28-238, 3120 m [Thompson, 1976]) were examined for the last 500 kyr at sample intervals from 2.5 to 5 kyr to study the fluctuations of dissolution in the western equatorial Pacific. The age models were constructed by correlating the delta18O records with the SPECMAP stack [Imbrie et al., 1984]. Results showed that intermediate and deep waters experienced the same patterns of dissolution through climatic cycles. Fragmentation varied with a greater amplitude, and the carbonate ion concentration changed less, in the deep than in the intermediate water. Dissolution has significant variance distributions and coherencies with delta18O over the 100, 41, and 23 kyr periods of orbital variations; dissolution maxima lag ice volume minima by 6 to 20 kyr. The dissolution variability was consistent with recent geochemical models which seek to explain the reduction of atmospheric CO2 concentration at the last glacial maximum [Broecker, 1982; Boyle, 1988].

Planktic foraminiferal and sea surface temperature record during the last 1 Myr across the Subtropical Front, Southwest Pacific

Planktic foraminiferal faunas and modern analogue technique estimates of sea surface temperature (SST) for the last 1 million years (Myr) are compared between core sites to the north (ODP 1125, 178 faunas) and south (DSDP 594, 374 faunas) of the present location of the Subtropical Front (STF), east of New Zealand. Faunas beneath cool subtropical water (STW) north of the STF are dominated by dextral Neogloboquadrina pachyderma, Globorotalia inflata, and Globigerina bulloides, whereas faunas to the south are strongly dominated by sinistral N. pachyderma (80-95% in glacials), with increased G. bulloides (20-50%) and dextral N. pachyderma (15-50%) in interglacials (beneath Subantarctic Water, or SAW). Canonical correspondence analysis indicates that at both sites, SST and related factors were the most important environmental influences on faunal composition. Greater climate-related faunal fluctuations occur in the south. Significant faunal changes occur through time at both sites, particularly towards the end of the mid-Pleistocene climate transition, MIS18-15 (e.g., decline of Globorotalia crassula in STW, disappearance of Globorotalia puncticulata in SAW), and during MIS8-5. Interglacial SST estimates in the north are similar to the present day throughout the last 1 Myr. To the south, interglacial SSTs are more variable with peaks 4-7 °C cooler than present through much of the early and middle Pleistocene, but in MIS11, MIS5.5, and early MIS1, peaks are estimated to have been 2-4 °C warmer than present. These high temperatures are attributed to southward spread of the STF across the submarine Chatham Rise, along which the STF appears to have been dynamically positioned throughout most of the last 1 Myr. For much of the last 1 Myr, glacial SST estimates in the north were only 1-2 °C cooler than the present interglacial, except in MIS16, MIS8, MIS6, and MIS4-2 when estimates are 4-7 °C cooler. These cooler temperatures are attributed to jetting of SAW through the Mernoo Saddle (across the Chatham Rise) and/or waning of the STW current. To the south, glacial SST estimates were consistently 10-11 °C cooler than present, similar to temperatures and faunas currently found in the vicinity of the Polar Front. One interpretation is that these cold temperatures reflect thermocline changes and increased Circumpolar Surface Water spinning off the Subantarctic Front as an enhanced Bounty Gyre along the south side of the Chatham Rise. For most of the last 1 Myr, the temperature gradient across the STF has been considerably greater than the present 4 °C. During glacial episodes, the STF in this region did not migrate northwards, but instead there was an intensification of the temperature gradient across it (interglacials 4-11 °C; glacials 8-14 °C).

Sedimentology and faunal compositions of surface sediments from the southern Florida Strait

The water masses in the Florida Straits and Bahamas region are important sources for the Northern Atlantic surface ocean circulation. In this study, we analyse carbonate preservation in surface sediments located above the chemical lysocline in the Florida Straits and Bahamas region and discuss possible reasons for supralysoclinal dissolution. Calcite dissolution proxies such as the variation of the foraminiferal assemblage, Fragmentation Index, Benthic Foraminifera Index, and Resistance Index displayed a good preservation in both areas. The pteropod species Limacina inflata showed very good preservation in sediments of inter-platform channels from the Great Bahama Bank (Providence Channel, Exuma Sound) above the aragonite lysocline. Supralysoclinal aragonite dissolution, however, was observed at two water depth levels (800-1000 m and below 1500 m) in the Florida Straits. Our observations suggest that the supralysoclinal dissolution in the Florida Straits is due to the degradation of organic material. The presence of Antarctic Intermediate Water (AAIW) may be a contributing factor for the significant aragonite dissolution in 800-1000 m. The comparison of modern preservation patterns of the surface sediments with hydrographical measurements shows that the L. inflata Dissolution Index (LDX) might be an adequate proxy to reconstruct paleo-water mass conditions in an area which is highly saturated with respect to calcium carbonate.

Distribution of planktonic foraminifera in Quaternary sediments east of New Zealand

A series of cores from east of New Zealand have been examined to determine the paleoceanographic history of the late Quaternary in the SW Pacific using planktonic foraminiferal data. Distinct shifts of species can be seen between glacial and interglacial times especially south of Chatham Rise east of South Island. Foraminiferal fragmentation ratios and benthic/planktonic foraminiferal ratios both show increased dissolution during glacials, especially isotope stage 2 to the south of Chatham Rise. The present-day Subtropical Convergence appears to be tied to the Chatham Rise at 44°S, but during glacial times this rise separated cold water to the south from much warmer water to the north, with an associated strong thermal gradient across the rise. We estimate that this gradient could have presented as much as an 8°C temperature change across 4°C of latitude during the maximum of the last ice age. There is only weak evidence of the Younger Dryas cool event, but there is a clear climatic optimum between 8 and 6.4 ka with temperatures 1°-2°C higher than the present day. The marine changes compare well with vegetational changes on both South and North Island.