Geochemistry and physics on sediment cores from the Peru Basin

We studied preservation/dissolution cycles and paleoproductivity in eight sediment cores from the Peru Basin south of the highly productive surface waters of the eastern equatorial Pacific. Stratigraphy is based on stable oxygen isotopes and on combined magnetostratigraphy and biostratigraphy. Sediment cores which span the last 8 m.y., were retrieved during cruise 79 with RV SONNE close to the carbonate compensation depth (CCD). In general, sediments show Pacific-type carbonate cycles. We interpret a pronounced carbonate peak between 6 and 7 Ma as the result of a western and northern extension of the highly productive Peru Current. Decreased carbonate contents from the late Miocene to the late Pliocene might be associated with a slow contraction of the latitudinal extent of the high-productivity belt north of the study areas. During the Pliocene, carbonate variations showed 400 kyr cycles indicating the growth and decay of ice sheets, which should have been associated with pulsations of the Antarctic ice cap. An abrupt collapse of the carbonate system occurred at 2.4 Ma. Higher frequency variations of the carbonate record indicate the major increase of the northern hemisphere glaciation. During the Quaternary, carbonate fluxes are high during glacials and low during interglacials. Large amplitude variations with long broad minima and maxima, associated with small migrations of the lysocline and the CCD (< 200 m), are indicative of the preservation/dissolution history in the Peru Basin. During the early Pleistocene, climatic forcing by the 41 kyr obliquity cycle is not observed in the carbonate record. During the last 800 kyr, variability in the carbonate record was dominated by the 100 kyr eccentricity cycle. Fluxes of biogenic material (calcium carbonate, organic carbon, opal, and barium) were greatest during glacials, which imply higher productivity and export production of the Peru Current during cold climatic periods. Dissolution was greatest during interglacials as inferred from the relatively poor preservation of planktonic foraminifera and from the low accumulation rate of carbonate. After the Mid-Brunhes Event (400 ka), we observe a plateaulike shift to enhanced dissolution and to intensified productivity.

Palynology and radiocarbon dating on sediment profile PG1756 from Lake Billyakh, NE Siberia, Russia

In this study a radiocarbon-dated pollen record from Lake Billyakh (65°17'N, 126°47'E; 340 m a.s.l.) in the Verkhoyansk Mountains was used to reconstruct vegetation and climate change since about 15 kyr BP (1 kyr=1000 cal. yr). The pollen record and pollen-based biome reconstruction suggest that open cool steppe and grass and sedge tundra communities with Poaceae, Cyperaceae, Artemisia, Chenopodiaceae, Caryophyllaceae and Selaginella rupestris dominated the area from 15 to 13.5 kyr BP. On the other hand, the constant presence of Larix pollen in quantities comparable to today's values points to the constant presence of boreal deciduous conifer trees in the regional vegetation during the last glaciation. A major spread of shrub tundra communities, including birch (Betula sect. Nanae), alder (Duschekia fruticosa) and willow (Salix) species, is dated to 13.5-12.7 kyr BP, indicating a noticeable increase in precipitation toward the end of the last glaciation, particularly during the Allerød Interstadial. Between 12.7 and 11.4 kyr BP pollen percentages of herbaceous taxa rapidly increased, whereas shrub taxa percentages decreased, suggesting strengthening of the steppe communities associated with the relatively cold and dry Younger Dryas Stadial. However, the pollen data in hand indicate that Younger Dryas climate was less severe than the climate during the earlier interval from 15 to 13.5 kyr BP. The onset of the Holocene is marked in the pollen record by the highest values of shrub and lowest values of herbaceous taxa, suggesting a return of warmer and wetter conditions after 11.4 kyr BP. Percentages of tree taxa increase gradually and reach maximum values after 7 kyr BP, reflecting the spread of boreal cold deciduous and taiga forests in the region. An interval between 7 and 2 kyr BP is noticeable for the highest percentages of Scots spine (Pinus subgen. Diploxylon), spruce (Picea) and fir (Abies) pollen, indicating mid-Holocene spread of boreal forest communities in response to climate amelioration and degradation of the permafrost layer.

Dome C age scale EDC1 for ice core EDC99

A tentative age scale (EDC1) for the last 45 kyr is established for the new 788 m EPICA Dome C ice core using a simple ice flow model. The age of volcanic eruptions, the end of the Younger Dryas event, and the estimated depth and age of elevated 10Be, about 41 kyr ago were used to calibrate the model parameters. The uncertainty of EDC1 is estimated to ±10 yr for 0 to 700 yr BP, up to ±200 yr back to 10 kyr BP, and up to ±2 kyr back to 41 kyr BP. The age of the air in the bubbles is calculated with a firn densification model. In the Holocene the air is about 2000 yr younger than the ice and about 5500 yr during the last glacial maximum.

Eocene-Oligocene calcareous nannofossil events of ODP Sites from the Southern Ocean

The evolution of the Southern Ocean climate during the late Eocene-late Oligocene interval is examined through highresolution, quantitative calcareous nannofossil analyses on samples from the Southern Ocean sections on Maud Rise and Kerguelen Plateau. We determined the abundance patterns of the counted species to clarify the biostratigraphy, which we correlated with high-resolution magnetostratigraphy [Roberts, A.P., Bicknell, S.J., Byatt, J., Bohaty, S.M., Florindo, F., Harwood, D.M., 2003a. Magnetostratigraphic calibration of Southern Ocean diatom datums from the Eocene-Oligocene of Kerguelen Plateau (Ocean Drilling Program Sites 744 and 748). In: Florindo, F., Cooper, A.K., O'Brien, P.A. (Eds.), Antarctic Cenozoic Palaeoenvironments: Geologic Record and Models. Palaeogeogr., Palaeoclimatol., Palaeoecol. 198 145-168; Florindo, F., Roberts, A.P., in press. Eocene-Oligocene magnetobiochronology of ODP Sites 689 and 690, Maud Rise, Weddell Sea, Antarctica. Geol. Soc. Am. Bull.], and used this data to interpret paleoceanographic changes through the late Eocene to late Oligocene. Percentage plots of the individual species, compared with R-mode principal component and cluster analysis results, allowed us to divide the assemblages into three groups: temperate-water taxa, cool-water taxa, and no temperature-affinity taxa. We attempt correlations between these paleoecological groups and the major sea-surface temperature (SST) variations with tectonic and paleoceanographic changes in the Southern Ocean. During the late Eocene, the nannofossil assemblage data reveal that there were several minor SST decreases (coolings) from 36 to 34 Ma, before the Eocene/Oligocene (E/O) boundary. A sharp cooling event, dated at 33.54 Ma (earliest Oligocene), occurred about 160 kyr after the E/O boundary, which is dated at 33.7 Ma. Relatively stable, cool conditions are interpreted to persist until the latest Oligocene, when an increase in abundance of temperate-water taxa, which corresponds to an antithetical decrease in abundance of cool-water indicators, is recorded. On the basis of our dating, the opening of the Drake Passage, allowing shallow-water circulation, began by 33.54 Ma at the latest, while the establishment of deep-water connections through the Tasmanian Gateway occurred at 33 Ma, as suggested by Exon et al. [Proc. ODP, Init. Rep. 189 (2001) 1].

Sea-surface reconstruction of the east-equatorial South Atlantic

In order to reconstruct Late Quatemary variations of surface oceanography in the eastequatorial South Atlantic, time series of sea-surface temperatures (SST) and paleoproductivity were established from cores recovered in the Guinea and Angola Basins, and at the Walvis Ridge. These records, based on sedimentary alkenone and organic carbon concentrations, reveal that during the last 350,000 years surface circulation and productivity changes in the east-equatorial South Atlantic were highiy sensitive to climate forcing at 23- and 100-kyr periodicities. Covarying SST and paleoproductivity changes at the equator and at the Walvis Ridge appear to be driven by variations in zonal trade-wind intensity, which forces intensification or reduction of coastal and equatorial upwelling, as well as enhanced Benguela cold water advection from the South. Phase relationships of precessional variations in the paleoproductivity and SST records from the distinct sites were evaluated with respect to boreal summer insolation over Africa, movements of southem ocean thermal fronts, and changes in global ice volume. The 23-kyr phasing implies a sensitivity of eastem South Atlantic surface water advection and upwelling to West African monsoon intensity and to changes in the position ofthe subtropical high pressure cell over the South Atlantic, both phenomena which modulate zonal strength of southeasterly trades. SST and productivity changes north of 20°S lack significant variance at the 41-kyr periodicity; and at the Walvis Ridge and the equator lead changes in ice volume. This may indicate that obliquity-driven clirnate change, characteristic for northem high latitudes, e.g fluctuations in continental ice masses, did not substantially influence subtropical and tropical surface circulation in the South Atlantic. At the 23-kyr cycle SST and productivity changes in the eastern Angola Basin lag those in the equatorial Atlantic and at the Walvis Ridge by about 3500 years. This lag is explained by variations in cross-equatorial surface water transport and west-east countercurrent retum flow modifying precessional variations of SST and productivity in the eastem Angola Basin relative to those in the mid South Atlantic area under the central field of zonal trade winds. Sea level-related shifts of upwelling cells in phase with global clirnate change may be also recorded in SST and productivity variability along the continental margin off Southwest Africa. They may account for the delay of the paleoceanogreaphic signal from continental margin sites with respect to that from the pelagic sites at the equator and the Walvis Ridge.

Sea-surface tempertaure reconstruction of sediment cores from the Southeast Pacific

Applying the alkenone method, we estimated sea-surface temperatures (SSTs) for the past 33 kyr in two marine sediment cores recovered from the continental slope off mid-latitude Chile. The SST record shows an increase of 6.7°C from the last ice age (LIA) to the Holocene climatic optimum, while the temperature contrast between LIA and modern temperatures is only about 3.4°C. The timing and magnitude of the last deglacial warming in the ocean correspond to those observed in South American continental records. According to our SST record, the existence of a Younger Dryas equivalent cooling in the Southeast Pacific is much more uncertain than for the continental climate changes. A warming step of about 2.5°C observed between 8 and 7.5 cal kyr BP may have been linked to the early to mid-Holocene climatic transition (8.2-7.8 cal kyr BP), also described from equatorial Africa and Antarctica. In principal, variations in the latitudinal position of the Southern Pacific Westerlies are considered to be responsible for SST changes in the Peru-Chile current off mid-latitude Chile.

Age determination and Heinrich Events in sediment cores SU90-08 and KET80-03

The Mediterranean Sea hydrology at the time of the Heinrich formation in the North Atlantic Ocean was analyzed by comparing sea surface temperatures (SSTs) and oxygen isotope composition of seawater (dw) changes during the past 75 kyr in two marine cores. These were compared to the palynological variations derived in the Mediterranean Sea core. During the last glacial the two oceanic SST records show similar and synchronous patterns, with several long-term cooling periods, ending by abrupt SST increases. At the time of the Heinrich events, cold SSTs and low salinity prevailed in the Mediterranean Sea. The freshwater budget was similar to the modern one, permitting the presence of a mixed forest on the Mediterranean borderlands. The post-Heinrich periods are marked by a freshwater budget decrease, limiting oak and fir tree growth in the Mediterranean region. Increase of precipitation or reduction of evaporation is observed before the Heinrich episode, and is associated with a well-developed mixed Mediterranean forest.

Contusotruncana contusa abundances and morphotypes in the cretaceous mid-latitude South Atlantic

Spatial and temporal patterns in test size and shape (test conicity and spiral roundness) and absolute abundance (accumulation rate) of the planktonic foraminifer Contusotruncana contusa were studied in the South Atlantic Ocean (DSDP sites 356, 516, 525 and 527) during an interval corresponding to the last 800 kyr of the Cretaceous. The variation in absolute abundance of C. contusa was characterised by alternating periods of high and low abundance; some of these periods were traceable across the entire mid-latitude South Atlantic Ocean. While the mean spiral roundness did not show any interpretable patterns, a sudden increase of the mean test size and mean test conicity occurred between 65.3 and 65.2 Ma (based on linear interpolation within the Cretaceous part of Subchron C29R) at all sites studied, indicating a poleward migration followed by rapid withdrawal of the low-latitude C. contusa morphotypes from the mid-latitude South Atlantic Ocean. We suggest that this event was caused by a short period of surface-water warming in the southern mid-latitudes corresponding to the brief high-latitude warming event and associated faunal migrations in the Boreal and Austral realms.

Sea surface temperatures during Marine Isotope Stage (MIS) 11

The Marine Isotope Stage (MIS) 11 (424-374 ka) was characterized by a protracted deglaciation and an unusually long climatic optimum. It remains unclear to what degree the climate development during this interglacial reflects the unusually weak orbital forcing or greenhouse gas trends. Previously, arguments about the duration and timing of the MIS11 climatic optimum and about the pace of the deglacial warming were based on a small number of key records, which appear to show regional differences. In order to obtain a global signal of climate evolution during MIS11, we compiled a database of 78 sea surface temperature (SST) records from 57 sites spanning MIS11, aligned these individually on the basis of benthic (N = 28) or planktonic (N = 31) stable oxygen isotope curves to a common time frame and subjected 48 of them to an empirical orthogonal function (EOF) analysis. The analysis revealed a high commonality among all records, with the principal SST trend explaining almost 49% of the variability. This trend indicates that on the global scale, the surface ocean underwent rapid deglacial warming during Termination V, in pace with carbon dioxide rise, followed by a broad SST optimum centered at ~410 kyr. The second EOF, which explained ~18% of the variability, revealed the existence of a different SST trend, characterized by a delayed onset of the temperature optimum during MIS11 at ~398 kyr, followed by a prolonged warm period lasting beyond 380 kyr. This trend is most consistently manifested in the mid-latitude North Atlantic and Mediterranean Sea and is here attributed to the strength of the Atlantic meridional overturning circulation. A sensitivity analysis indicates that these results are robust to record selection and to age-model uncertainties of up to 3-6 kyr, but more sensitive to SST seasonal attribution and SST uncertainties >1 °C. In order to validate the CCSM3 (Community Climate System Model, version 3) predictive potential, the annual and seasonal SST anomalies recorded in a total of 74 proxy records were compared with runs for three time slices representing orbital configuration extremes during the peak interglacial of MIS11. The modeled SST anomalies are characterized by a significantly lower variance compared to the reconstructions. Nevertheless, significant correlations between proxy and model data are found in comparisons on the seasonal basis, indicating that the model captures part of the long-term variability induced by astronomical forcing, which appears to have left a detectable signature in SST trends.

Stable carbon and oxygen isotope compositions of benthic foraminifera N. truempyi samples and coarse fraction analyses from ODP Site 198-1209

We have compiled the first stratigraphically continuous high-resolution benthic foraminiferal stable isotope record for the Paleocene from a single site utilizing cores recovered at Pacific ODP Site 1209. The long-term trend in the benthic isotope record suggests a close coupling of volcanic CO2 input and deep sea warming. Over the short-term the record is characterized by slow excursions with a pronounced periodic beat related to the short (100-kyr) and long (405-kyr) eccentricity cycle. The phase relationship between the benthic isotope record and eccentricity is similar to patterns documented for the Oligocene and Miocene confirming the role of orbital forcing as the pace maker for paleoclimatic variability on Milankovitch time scales. In addition, the record documents an unusual transient warming of 2°C coeval with a 0.6 per mil carbon isotope excursion and a decrease in carbonate content at 61.75 Ma. This event, which bears some resemblance to Eocene hyperthermals, marks the onset of a long-term decline in d13C. The timing indicates it might be related to the initiation of volcanism along Greenland margin.

Planktonic foraminifera in the Maastrichtian of Hor-Hahar

The lengthy warm, stable climate of the Cretaceous terminated in the Campanian with a cooling trend, interrupted in the early and latest Maastrichtian by two events of global warming, at ~70-68 Ma and at 65.78-65.57 Ma. These climatic oscillations had a profound effect on pelagic ecosystems, especially on planktic foraminiferal populations. Here we compare biotic responses in the tropical-subtropical (Tethyan) open ocean and mesotrophic (Zin Valley, Israel) and oligotrophic (Tunisia) slopes, which correlate directly with global warming and cooling. The two warming events coincide with blooms of Guembelitria, an extreme opportunist genus best known as the main survivor of the Cretaceous-Paleogene (K-Pg) catastrophe. In the Maastrichtian, Guembelitria bloomed in the uppermost surface water above shelf and slope environments but failed to reach the open ocean as it did at K-Pg. The coldest interval of the late Maastrichtian (~68-65.78 Ma) is marked by an acme of the otherwise rare species Gansserina gansseri, a deep-dwelling keeled globotruncanid. The G. gansseri acme event can be traced from the deep ocean even onto the Tethyan slope, marking copious production and circulation of cold intermediate water. This acme is abruptly terminated by extinction of the species, a dramatic reversal attributed to a short-term global warming episode. This extinction corresponds precisely with the second bloom of Guembelitria that began ~300 kyr prior to the K-Pg event. The antithetical relationship between blooming of Guembelitria and the G. gansseri acme reflects planktic foraminiferal sensitivity to warm-cool-warm-cool climatic oscillations marking the end of the Cretaceous.

A time-slice study in Heinrich Stadial 1 and Mid Holocene sediment cores off northern Senegal

Sediment dynamics in limnic, fluvial and marine environments can be assessed by granulometric and rock-magnetic methodologies. While classical grain-size analysis by sieving or settling mainly bears information on composition and transport, the magnetic mineral assemblages reflect to a larger extent the petrology and weathering conditions in the sediment source areas. Here, we combine both methods to investigate Late Quaternary marine sediments from five cores along a transect across the continental slope off Senegal. This region near the modern summer Intertropical Convergence Zone is particularly sensitive to climate change and receives sediments from several aeolian, fluvial and marine sources. From each of the investigated five GeoB sediment cores (494-2956 m water depth) two time slices were processed which represent contrasting climatic conditions: the arid Heinrich Stadial 1 (~ 15 kyr BP) and the humid Mid Holocene (~ 6 kyr BP). Each sediment sample was split into 16 grain-size fractions ranging from 1.6 to 500 µm. Concentration and grain-size indicative magnetic parameters (susceptibility, SIRM, HIRM, ARM and ARM/IRM) were determined at room temperature for each of these fractions. The joint consideration of whole sediment and magnetic mineral grain-size distributions allows to address several important issues: (i) distinction of two aeolian sediment fractions, one carried by the north-easterly trade winds (40-63 µm) and the other by the overlying easterly Harmattan wind (10-20 µm) as well as a fluvial fraction assigned to the Senegal River (< 10 µm); (ii) identification of three terrigenous sediment source areas: southern Sahara and Sahel dust (low fine-grained magnetite amounts and a comparatively high haematite content), dust from Senegalese coastal dunes (intermediate fine-grained magnetite and haematite contents) and soils from the upper reaches of the Senegal River (high fine-grained magnetite content); (iii) detection of partial diagenetic dissolution of fine magnetite particles as a function of organic input and shore distance; (iv) analysis of magnetic properties of marine carbonates dominating the grain-size fractions 63-500 µm.

(Table 2) Actual and synthetic mass accumulation rate data and their absolute differences for ODP Site 115-707

Synthetic mass accumulation rates have been calculated for ODP Site 707 using depth-density and depth-porosity functions to estimate values for these parameters with increasing sediment thickness, at 1 Ma time intervals determined on the basis of published microfossil datums. These datums were the basis of the age model used by Peterson and Backman (1990, doi:10.2973/odp.proc.sr.115.163.1990) to calculate actual mass accumulation rate data using density and porosity measurements. A comparison is made between the synthetic and actual mass accumulation rate values for the time interval 37 Ma to the Recent for 1 Myr time intervals. There is a correlation coefficient of 0.993 between the two data sets, with an absolute difference generally less than 0.1 g/cm**2/kyr. We have used the method to extend the mass accumulation rate analysis back to the Late Paleocene (60 Ma) for Site 707. Providing age datums (e.g. fossil or magnetic anomaly data) are available the generation of synthetic mass accumulation rates can be calculated for any sediment sequence.

Mineralogy and geochemistry of Holocene sediments from the North Atlantic

Bulk mineralogy, Sm, Nd and Pb elemental and isotopic compositions of the clay-size fraction of Holocene sediments were analyzed in three deep North Atlantic cores to trace the particle provenance. The aims of the present paper are to identify the origin of the particles driven by deep currents and to reconstruct deep circulation changes over the Holocene in the North Atlantic. The three cores are retrieved in fracture zones; two of them are located in the Island Basin along the gyre of North Atlantic Deep Water, and the third core is located off the present deep circulation gyre in the Labrador Sea. Whereas sedimentary supplies in the Labrador Sea were constantly derived from proximal sources, the geochemical mixing trends in the Iceland Basin samples indicate pronounced changes in the relative contribution of continental margin inputs over the past 6 kyr. Supplies from western European margin that sharply increased at 6 kyr were progressively diluted by a larger contribution of Scandinavian margins over the last 3 kyr. Changes in composition of the particles imply significant reorganization of paleocirculation of the deep North Atlantic components in the eastern basins: mainly reorganizations for both Iceland-Scotland Overflow Water and Norwegian Sea Overflow Water. Moreover the unusual Pb isotopic composition of the oldest sediments from the southern Iceland Basin indicates that distal supplies from Greenland margin were driven into the Iceland Basin, supporting a deep connection between Labrador Sea and Iceland Basin through the Charlie Gibbs Fracture Zone prior the Holocene Transition period.