Late Pliocene stable oxygen isotope record of the Mediterranean Sea

Detailed pollen analyses and oxygen isotope records of three foraminiferal species, Globigerina bulloides, Uvigerina peregrina and Cibicides pachyderma, from the Semaforo and Vrica composite sections (Crotone, southern Italy) have been compared to the global climatic changes depicted by late Pliocene-early Pleistocene foraminiferal d18O records of Site 607 in the North Atlantic, and Hole 653A in the Tyrrhenian basin, West Mediterranean. Major overturns in the mid-altitude vegetation are shown near isotopic stages 82, 60, 58 and 50, at about 2.03 Ma, 1.6 Ma and 1.37 Ma according to the Raymo et al. (1989, doi:10.1029/PA004i004p00413) and Ruddiman et al. (1989, doi:10.1029/PA004i004p00353) timescales. At the same dates, glacial 18O maxima either became higher or display step increases in the western Mediterranean or in the open ocean as well. This suggests that size increases of Northern Hemisphere ice sheets were the driving factor for regional or local marine and continental environmental changes within the Mediterranean basin. Near isotopic stages 62-60, close to the conventional Plio-Pleistocene boundary, the climatic conditions severed enough within the Mediterranean basin to modify the continental environment, as depicted by a sudden increase of Artemisia percentages, while the first significant southward migration of the North Polar Front may have been recorded by an influx of left coiling Neogloboquadrina pachyderma in the central Mediterranean. It also appears that 'Boreal Guests' entered the Mediterranean during phases of 18O enrichment of foraminiferal calcite. There does not seem to be any discrepancy between the climatic concept of the Pliocene-Pleistocene boundary and its chronostratigraphic definition.

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.

A multiproxy analysis of late MIS 6 to early MIS 5d OSP Site 161-975 sediments

A multiproxy analysis based on planktic foraminiferal abundances, derived SSTs, and stable planktic isotopes measurements together with alkenone abundances and Uk'37 SSTs was performed on late MIS 6 to early MIS 5d sediment recovered from Site 975 (ODP Leg 161) in the South Balearic Islands basin (Western Mediterranean) with emphasis on reconstructing the climate progression of the last interglacial period. A number of abrupt climate changes related to alternative influence of nutrient rich northern and oligotrophic southern water masses were revealed. Heinrich event 11 and cooling events C27, C26, C25, C24, C23, which have been previously described in the North Atlantic, were recognized. However, in comparison to the eastern North Atlantic mid-latitude region, events C27 and C26 at Site 975 seem to be significantly more pronounced. Together with evidence of a two-phase climate optimum with maximum SSTs reached during its later phase, this implies a close similarity in climate dynamics between the Western Mediterranean and the Nordic seas. We propose that postglacial effects in the Nordic seas had an influence on the western Mediterranean climate via atmospheric circulation and that these effects competed with the insolation force.

Distribution of late Quaternary calcareous nannofossils in DSDP Leg 96 sites

Sediments from holes drilled at 11 sites in the northern Gulf of Mexico during Deep Sea Drilling Project Leg 96 were analyzed for calcareous nannofossil content. All sediments recovered are Holocene and late Pleistocene in age and are within the Emiliania huxleyi Zone. The datum level represented by the lowest stratigraphic occurrence of dominant E. huxleyi occurs at two sites (Sites 615 and 619) and can be dated at approximately 84,000 yr. ago at Site 619. Reworked Cretaceous nannofossils are generally common or abundant and dominate the floral assemblages of the late Wisconsin glacial sediments. When present, indigenous late Quaternary species are rare or few in abundance. Slight increases in the contemporaneous Quaternary component of the floral assemblages can be documented by the use of a calculated in s/fu/reworked ratio. This ratio, based on the relative abundances of the indigenous Quaternary taxa and reworked taxa, shows potential both for local correlations between drill sites and for correlation with glacio-eustatic fluctuations during the late Pleistocene.

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.

Age determination of sediment cores of the California Current System

Alkenone sea surface temperature (SST) records were generated from the Ocean Drilling Program's (ODP) Sites 1014 and 1016 to examine the response of the California Current System to global climate change during the last 136 ka. The temperature differences between these sites (Delta SST(NEP)=SST(ODP1014)-SST(ODP1016)) reflected the intensity of the California Current and varied between 0.4 and 6.1 °C. A high Delta SST(NEP) (weaker California Current) was found for late marine isotope stage (MIS) 2 and early MIS 5e, while a low Delta SST(NEP) (stronger California Current) was detected for mid-MIS 5e and MIS 1. Spectral analysis indicated that this variation pattern dominated 23- (precession) and 30-ka periods. Comparison of the Delta SST(NEP) and SST based on data from core MD01-2421 at the Japan margin revealed anti-phase variation; the high Delta SST(NEP) (weakening of the California Current) corresponded to the low SST at the Japan margin (the southward displacement of the NW Pacific subarctic boundary), and vice versa. This variation was synchronous with a model prediction of the tropical El Niño-Southern Oscillation behavior. These findings suggest that the intensity of the North Pacific High varied in response to precessional forcing, and also that the response has been linked with the changes of tropical ocean-atmosphere interactions.

Age profiles and isotopes d18O and d13C in Globigerina angiporoides and Oridorsalis umbonatus at DSDP Site 90-593 and Holes 12-111, 29-277 and 90-592

The late Eocene through earliest Miocene stable-isotope composition of southwest Pacific microfossils has been examined in a traverse of high-quality sedimentary sequences ranging from subantarctic (DSDP Site 277) through temperate regions (DSDP Sites 592 and 593). Changes in oxygen-isotope values, measured in benthic and planktonic foraminifers, document the Oligocene development and strengthening of latitudinal thermal zonation from water masses with broad temperature gradients during the Eocene to the steeper gradients and more distinct latitudinally distributed surface water-mass belts of the Neogene. The oxygen-isotope records can be divided into three intervals: late Eocene, early Oligocene, and middle to late Oligocene. Each interval represents a successive stage in the evolution of latitudinal thermal gradients between subantarctic and temperate regions in the Southern Hemisphere. During the late Eocene, oxygen-isotope values at subantarctic Site 277 were similar to those at temperate Sites 592 and 593. The isotope values suggest that, although the inferred paleotemperatures at Site 277 are slightly cooler on average than those at the temperate sites, there is no evidence for a major thermal boundary between the regions at this time. All three sites record the well-known oxygen-isotope enrichment of about 1 per mil in both planktonic and benthic foraminifers in close association with the Eocene/Oligocene boundary. In contrast to the earliest Oligocene enrichments in the planktonic and benthic oxygen-isotope composition at Site 277, more northern Sites 592 and 593 exhibit a depletion through the early-middle Oligocene. This documents the beginning of thermal segregation as subantarctic waters cooled relative to those at temperate latitudes. During the Oligocene, this surface-water differentiation continued, as measured by planktonic d18O values. The oxygen-isotope records of the benthic foraminifers also began to diverge in the earliest Oligocene. The most enriched oxygen-isotope values in all records cluster in the middle Oligocene, marked by oscillating episodes of enrichments >0.5 per mil occurring most prominently in the subantarctic record of Site 277. These values can be interpreted as recording either the coldest oceanic temperatures of the Paleogene and/or accumulations of Antarctic ice. After this interval, latitudinal thermal differentiation developed rapidly during the middle Oligocene, especially in the surface waters which actually warmed in temperate areas. If the enriched Oligocene oxygen-isotope values indicate that ice had accumulated, this ice must have disappeared by the early Miocene, when depleted oxygen-isotope values suggest very warm conditions. The data presented in this chapter document the progressive increase of latitudinal temperature gradients from the late Eocene through the late Oligocene. This pattern of increasing isotopic offset between latitudinally distributed southwest Pacific sites is linked to the establishment and strengthening of the Circum-Antarctic Current, previously considered to have developed during the middle to late Oligocene. The intensification of this current system progressively decoupled the warm subtropical gyres from cool polar circulation, in turn leading to increased Antarctic glaciation.

Age models, organic analyses and major-element analyses of sediment cores on a transect of the tropical African rainbelt

The distribution of rainfall in tropical Africa is controlled by the African rainbelt**1, which oscillates on a seasonal basis. The rainbelt has varied on centennial to millennial timescales along with changes in Northern Hemisphere high-latitude climate**2, 3, 4, 5, the Atlantic meridional overturning circulation**6 and low-latitude insolation**7 over the past glacial-interglacial cycle. However, the overall dynamics of the African rainbelt remain poorly constrained and are not always consistent with a latitudinal migration**2, 4, 5, 6, as has been proposed for other regions**8, 9. Here we use terrestrially derived organic and sedimentary markers from marine sediment cores to reconstruct the distribution of vegetation, and hence rainfall, in tropical Africa during extreme climate states over the past 23,000 years. Our data indicate that rather than migrating latitudinally, the rainbelt contracted and expanded symmetrically in both hemispheres in response to changes in climate. During the Last Glacial Maximum and Heinrich Stadial 1, the rainbelt contracted relative to the late Holocene, which we attribute to a latitudinal compression of atmospheric circulation associated with lower global mean temperatures**10. Conversely, during the mid-Holocene climatic optimum, the rainbelt expanded across tropical Africa. In light of our findings, it is not clear whether the tropical rainbelt has migrated latitudinally on a global scale, as has been suggested**8,9.

Paleomagnetic characterization and age of several ODP Leg 198 sites, Shatsky Rise

Shatsky Rise, a medium-sized large igneous province in the west Central Pacific Ocean, has three main topographic highs that preserve a thick sedimentary record from Cretaceous through Cenozoic. During Ocean Drilling Program (ODP) Leg 198 to Shatsky Rise, a total of ~768 m of late Miocene-Holocene sediments was recovered from six sites. Sites 1207 and 1208 were drilled on the Northern and Central Highs, respectively, and yielded expanded late Miocene-Holocene sequences. Sites 1209, 1210, 1211, and 1212 were drilled on the Southern High and yielded shorter sequences of similar age. Clearly interpretable magnetic stratigraphies were obtained from all sites using the shipboard pass-through magnetometer. These results were augmented using discrete sample cubes (7 cm**3) collected shipboard and measured postcruise. Miocene age sediments are separated by a hiatus from Oligocene, Eocene, and Cretaceous age sediments beneath. An astrochronological age model was developed for the six sites based on cycles observed in reflectance data, measured shipboard. This age model is in good agreement with published astrochronological polarity chron ages in the 1 to 6 Ma interval.

Age and clay mineralogy on sediment core GeoTü SL112 from the Mediterranean Sea

The present-day clay mineral distribution in the southeastern Levantine Sea and its borderlands reveals a complex pattern of different sources and distribution paths. Smectite dominates the suspended load of the Nile River and of rivers in the Near East. Illite sources are dust-bearing winds from the Sahara and southwestern Europe. Kaolinite is prevalent in rivers of the Sinai, in Egyptian wadis, and in Saharan dust. A high-resolution sediment core from the southeastern Levantine Sea spanning the last 27 ka shows that all these sources contributed during the late Quaternary and that the Nile River played a very important role in the supply of clay. Nile influence was reduced during the glacial period but was higher during the African Humid Period. In contrast to the sharp beginning and end of the African Humid Period recorded in West African records (15 and 5.5 ka), our data show a more transitional pattern and slightly lower Nile River discharge rates not starting until 4 ka. The similarity of the smectite concentrations with fluctuations in sea-surface temperatures of the tropical western Indian Ocean indicates a close relationship between the Indian Ocean climate system and the discharge of the Nile River.

Age models of northern North Atlantic sediment cores

Benthic foraminiferal stable isotope records from four high-resolution sediment cores, forming a depth transect between 1237 m and 2303 m on the South Iceland Rise, have been used to reconstruct intermediate and deep water paleoceanographic changes in the northern North Atlantic during the last 21 ka (spanning Termination I and the Holocene). Typically, a sampling resolution of ~100 years is attained. Deglacial core chronologies are accurately tied to North Greenland Ice Core Project (NGRIP) ice core records through the correlation of tephra layers and changes in the percent abundance of Neogloboquadrina pachyderma (sinistral) with transitions in NGRIP. The evolution from the glacial mode of circulation to the present regime is punctuated by two periods with low benthic d13C and d18O values, which do not lie on glacial or Holocene water mass mixing lines. These periods correlate with the late Younger Dryas/Early Holocene (11.5-12.2 ka) and Heinrich Stadial 1 (14.7-16.8 ka) during which time freshwater input and sea-ice formation led to brine rejection both locally and as an overflow exported from the Nordic seas into the northern North Atlantic, as earlier reported by Meland et al. (2008). The export of brine with low ?13C values from the Nordic seas complicates traditional interpretations of low d13C values during the deglaciation as incursions of southern sourced water, although the spatial extent of this brine is uncertain. The records also reveal that the onset of the Younger Dryas was accompanied by an abrupt and transient (~200-300 year duration) decrease in the ventilation of the northern North Atlantic. During the Holocene, Iceland-Scotland Overflow Water only reached its modern flow strength and/or depth over the South Iceland Rise by 7-8 ka, in parallel with surface ocean reorganizations and a cessation in deglacial meltwater input to the North Atlantic.

Late Eocene-early Oligocene radiolarian events and abundances in ODP Leg 199 sediments

During Ocean Drilling Program Leg 199, eight sites (Sites 1215-1222) were cored in the Central Pacific. Late Eocene-early Oligocene thick radiolarian-rich biogenic sediments were collected from Holes 1218A, 1219A, and 1220A. This is the first attempt to calibrate the ages of Paleogene radiolarian events using magnetostratigraphy in this region. A total of 107 species and species groups, which are valuable for stratigraphic correlation, are listed with numeric data and figures. Among these three holes, a total of 77 radiolarian events were recognized and their ages were calibrated by correlation with paleomagnetic events recorded in Hole 1220A.

Alkenones and coccolithophorids in late Quaternary sediments from the Walvis Ridge

Sea surface temperatures (SSTs) derived from the alkenone UK'37) record of Quaternary sediments may be subject to bias if algae with different temperature sensitivities have contributed to the sedimentary alkenone record. The alkenone-derived SST records are usually based on a UK'37-temperature relationship which was measured in culture experiments using the coccolithophorid Emiliania huxleyi (F.G. Prahl, L.A. Muehlhausen and D.L. Zahnle, 1988. Further evaluation of long-chain alkenones as indicators of paleoceanographic conditions. Geochim. Cosmochim. Acta 52, 2303-2310). To assess possible effects of past species changes on the UK'37-temperature signal, we have analyzed long-chain alkenones and coccolithophorids in a late Quaternary sediment core from the Walvis Ridge and compared the results to SST estimates extracted from the d18O record of the planktonic foraminifer Globigerinoides ruber. Alkenones and isotopes were determined over the entire 400-kyr core record while the coccolithophorid study was confined to the last 200 kyr when the most pronounced changes in alkenone content occurred. Throughout oxygen-isotope stages 6 and 5, species of the genus Gephyrocapsa were the predominating coccolithophorids. E. huxleyi began to increase systematically in relative abundance since the stage 5/4 transition, became dominant over Gephyrocapsa spp. during stage 3 and reached the highest abundances in the Holocene. Carbon-normalized alkenone concentrations are inversely related to the relative abundances of E. huxleyi, and directly related to that of Gephyrocapsa spp., suggesting that species of this genus were the principal alkenone contributors to the sediments. Nevertheless, SST values obtained from the UK'37-temperature relationship for E. huxleyi compare favourably to the isotope-derived temperatures. The recently reported UK'37-temperature relationship for a single strain of Gephyrocapsa oceanica (J.K. Volkman. S.M. Barrett, S.I. Blackburn and E.L. Sikes, 1995. Alkenones in Gephyrocapsa oceanica: Implications for studies of paleoclimate. Geochim. Cosmochim. Acta 59, 513-520) produces unrealistically high SST values indicating that the temperature response of the examined strain is not typical for the genus Gephyrocapsa. This is supported by the C37:C38, alkenone ratios of the sediments which are comparable to average ratios reported for E. huxleyi, but significantly higher than for the G. oceanica strain. Most notably, the general accordance of the alkenone characteristics between sediments and E. huxleyi persists through stages 8 to 5 and even in times that predate the first appearance of this species (268 ka; H.R. Thierstein, K.R. Geitzenauer and B. Molfino, 1977. Global synchroneity of late Quaternary coccolith datum levels: Validation by oxygen isotopes. Geology 5, 400-404). Our results suggest that UK'37-temperature relationships based on E. huxleyi produce reasonable paleo-SST estimates even for late Quaternary periods when this species was scarce or absent because other alkenone-synthesizing algae, e.g. of the genus Gephyrocapsa.