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An astronomically calibrated timescale has recently been established (Hilgen, 1991 a, b) for the Pliocene and earliest Pleistocene based on the correlation of dominantly pre- cession controlled sedimentary cycles (sapropels and carbonate cycles) in Mediterranean ma- rine sequences to the precession time series of the astronomical solution of Berger and Loutre ( 1991 ) (hereinafter referred to as Ber90). Here we evaluate the accuracy of this timescale by (1) comparing the sedimentary cycle patterns with 65 oN summer insolation time series of dif- ferent astronomical solutions and (2) a cross-spectral comparison between the obliquity-related components in the 65 oN summer insolation curves and high-resolution paleoclimatic records derived from the same sections used to construct the timescale. Our results show that the car- bonate cycles older than 3.5 m.y. should be calibrated to one precession cycle older than previ- ously proposed. Application of the astronomical solution of Laskar ( 1990) (hereinafter refer- red to as La90) with present-day values for the dynamical ellipticity of the Earth and tidal dis- sipation by the Sun and Moon results in the best fit with the geological record, indicating that this solution is the most accurate from a geological point of view. Application of Ber90, or La90 solutions with dynamical ellipticity values smaller or larger than the present-day value, results in a less obvious fit with the geological record. This implies that the change in the plane- tary shape of the Earth associated with ice loadine$ and unloading near the poles during the last 5.3 million years was too small to drive the precession into resonance with the perturbation term, s-gt+g 5, of Jupiter and Saturn. Our new timescale results in a slight but significant modi- fication of all ages of the sedimentary cycles, bioevents, reversal boundaries, chronostrati- graphic boundaries, and glacial cycles. Moreover, a comparison of this timescale with the as- tronomical timescales of ODP site 846 (Shackleton et al., 1995a, b) and ODP site 659 (Tiede- mann et al., 1994) indicates that all obliquity-related

Temporal changes in biological and isotopic tracers have successfully been used to monitor monsoon variability in the Arabian Sea. Convincing evidence that fertility indicators, such as the relative abundance of the planktic foraminifer Globigerina bulloides , and SiO 2 and Ba fluxes, can monitor monsoon changes, has also been published. Time series of the 13 C content of the thermocline foraminifer Neogloboquadrina dutertrei , and of an Upwelling Radiolarian Index (URI), have been used to reconstruct upwelling changes in the Somali Basin at 5 °N for the last 160 kyr. In order to establish a reference site for comparison with other upwelling and/or fertility records from the same area, a time series of 8 climatic and fertility proxies ( 13 C content of N. dutertrei and Globorotalia menardii , percentages of G. bulloides and of a thermocline foraminiferal group, minimal δ 18 values of Globigerinoides sacculifer, N. dutertrei and G. menardii ) are investigated in the spectral domain for the last 360 kyr, at a site located at the equator and outside the Somali upwelling centers (Core MD 85668). Chronostratigraphies for these records are developed by correlation of oxygen isotope record of G. sacculifer to the Martinson standard stacked benthic oxygen isotope record. The temporal resolution of the data, is between 2 and 5 kyr. Cross spectral comparisons with ETP (Eccentricity + Tilt + Precession-composite signal) were used to estimate coherences in conjunction with phase relationships and to quantify relationships between fertility and climatic indicators. The timing of fertility indicator cycles at the equator shows little (obliquity band) or no (other orbital bands) coincidence with monsoon proxies of the Arabian Sea. At the equator, fertility responses are dissociated, and present important leads or lags with ETP signal at the three orbital bands (with the exception of the thermocline foraminifera, in phase with maximum eccentricity). Due to significant leads with ETP in the timing of 13 C minima and URI cycles, no relationships with the Northern Hemisphere radiation can be deduced. At the obliquity and precessional frequency bands, thermocline foraminifera cycles display significant lag with ETP and ice volume minima, which suggests that radiation is not the sole forcing mechanism. Global climatic and ice volume changes may be an important forcing mechanism for these fertility proxies. The G. bulloides record is more complicated with cycles in phase with Arabian Sea monsoon proxies at the obliquity band, and 18 O minima at the precession frequency. It appears that, at the equator, the southwest monsoon winds are not the main force driving radiolarian and foraminiferal productivity.

Foraminiferal study of a continuous set of samples across the Upper Pliocene C2 sapropel (Singa section, southern Italy) revealed an interval containing substantial numbers of benthic foraminifera, bound by sediments in which benthic foraminifera are absent. The absence of benthic foraminifera is indicative of anoxic conditions. The interval with benthic foraminifera contains an association that is strongly dominated by the species Bulimina marginata, which is tolerant to low oxygen concentrations. The bottom water oxygenation did not improve sufficiently to allow development of a more diverse benthic foraminfferal fauna, or the return of intensively burrowing organisms. Variations in the planktic foraminiferal fauna and in the stable isotope records suggest that a surface water cooling preceded the benthic repopulation. A warming seems to have coincided with the return to bottom anoxia. The planktic productivity markers Globigerina bulloides and Neogloboquadrina show minor variations across the repopulated interval. The benthic repopulation, which reflects improved bottom water oxygenation, was probably favored by enhanced dense water formation in response to general cooling of the surface waters.

Rohling, E.J., Jorissen, F.J., Vergnaud Grazzini, C. and Zachariasse, W.J., 1993. Northern Levantine and Adriatic Quaternary planktic foraminifera; Reconstruction of paleoenvironmental gradients. Mar. Micropaleontol., 21 : 191-218. A detailed study of the planktic foraminiferal records of three cores, recovered near Crete, shows very consistent changes. Unfortunately, the record of T87/2/13G, our shallowest core, is bioturbated too much for detailed comparisons, but it can still be used to discuss the general trends. The mutual record of the other two cores, T87/2/20G and T87/2/27G, extends back to about 125,000 yr B.P. Principal Components Analyses performed on the combined records of T87/2/20G and T87/2/27G yield two axes that can be interpreted paleoecologically, namely a temperature-axis and an "annual stability" axis. The latter is a measure of stratification within the euphoric layer. In this study, we consider T87/2/20G and T87/2/27G to be reference cores for the eastern Mediterranean. Subsequently, we interprete the faunal variation in two cores from the Adriatic Sea (IN 68-5 and IN 68-9), containing records extending back to 15,000 and 18,000 yr B.P., in the context of these reference-cores. This is done by calculating scores of the Adriatic records along the two relevant PCA axes derived from the northern Levantine records. Also, cluster analyses of the Adriatic and northern Levantine records are compared. On the average, the Adriatic Sea appears to have been considerably colder than the region around Crete, during the past 18,000 years. This temperature difference was largest during the pleniglacial and only small during the Holocene. We found no distinct expression of a Younger Dryas cooling event in the Adriatic records, neither on our PCA axis, nor in the oxygen isotope record. This contradicts previous paleoclimatic reconstructions, based mainly on paleobotanical evidence, which suggests that the Younger Dryas event greatly influenced circum-Mediterranean areas. As yet, we have no ready explanation for this discrepancy. Low annual stability values dominate both the pleniglacial and the Holocene intervals of the Adriatic records. This suggests that homothermal conditions were as prevalent in the Adriatic euphoric layer at pleniglacial times as they are at present. In the present-day Adriatic, homothermal conditions are associated with deep convection and subsequent Eastern Mediterranean Deep Water (EMDW) formation, and therefore our data suggest that EMDW formation was as effective at pleniglacial times as it is at present. High annual stability values in the transitional interval, from 12,700 to 9600 yr B.P., in the Adriatic records are suggestive of low EMDW formation-rates at that time. In combination with high productivity throughout the eastern Mediterranean, which has previously been inferred from the planktic foraminiferal record, these low EMDW formation rates may have preconditioned the eastern Mediterranean for sapropel formation.

Jorissen, F.J., Asioli, A., Borsetti, A.M., Capotondi, L., De Visser, J.P., Hilgen, F.J., Rohling, E.J., Van der Borg, K., Vergnaud Grazzini, C. and Zachariasse, W.J., 1993. Late Quaternary central Mediterranean biochronology. Mar. MicropaleontoL, 21: 169-189. A high-resolution biochronology is presented for the Late Quaternary of the central Mediterranean. In the Late Pleistocene-Holocene successions three assemblage zones are distinguished on the basis of frequency patterns of planktic foraminifera. The age of these zones is determined by Accelerator Mass Spectrometry (AMS) t4C dating. The zonal boundaries are dated at 12,700 yr B.P. (the end of Termination la) and 9600 yr B.P. (the start of Termination Ib), respectively. The AMS dates show that major changes in the planktic and benthic realms occurred synchronously over wide areas, although records of individual species may show important regional differences. In the studied areas, resedimentation processes revealed by anomalous successions of 14C dates, play a far more important role than indicated by the sedimentological and micropaleontological data. Possibly these processes contribute to the very high accumulation rates in the glacial Zone IlL Although the AMS technique has increased the accuracy of laC-measurements, of older carbonate may still lead to substantial age differences between areas with different sedimentary regimes.

Palynological, planktonic foraminiferal and stable isotope data from a sediment core in the northern Mollucca Sea have been used to evaluate regional glacial climatic and hydrographic conditions. The palynological record shows that in glacial time the climate was drier than today, indicating a weakened influence of the northwesterly winds at that time. The drier climatic conditions at glacial time were associated with a higher lapse rate, which, together with a lower sea level, resulted in a large expansion of the Lower Montane oak forests on the nearby island of Halmahera. Glacial Mollucca Sea surface water salinities were probably higher than today, while glacial surface water temperatures might have been close to present-day values. The Deep Chlorophyll Maximum (DCM) layer in the northern Molucca Sea at glacial time was more productive and cooler than today, probably the result of the inflow of cooler and nutrient-enriched Subtropical Lower Water from the Pacific. The less asymmetrical high-latitude ice coverage at glacial time and the particularly strong increase in the pole to equator thermal gradient on the Northern Hemisphere, provides theoretical evidence that the Intertropical Convergence Zone (ITCZ) shifted equatorwards and oscillated over a more narrow latitudinal range than today. This inferred configuration of the ITCZ at glacial time, however, fails to explain the observed increased aridity in the study area at that time. We propose, therefore, that the drier glacial climatic conditions are related to a possible disturbance in the El Nino Southern Oscillation (ENSO) system.

Based on the glacial to postglacial δ13C differences between endobenthic Uvigerina peregrina species from the Alboran basin and from other mediterranean basins, changes in the fertility of the western part of this basin during the last deglaciation are reconstructed. As a result of particulate organic carbon (POC) rain from the highly productive upwelling cell along the northwestern margin of the Alboran basin, U. peregrina is presently depleted by about 1.6‰ with respect to the measured δ13C values of bottom water ΣCO2 and by about 0.9‰ with respect to specimens from other areas of the western Mediterranean or from the Gulf of Cadiz within the Mediterranean Outflow Water. The Uvigerina δ13C difference between the Alboran Sea and the Gulf of Cadiz (Δδ13C), was close to 0‰ at the beginning of the last deglaciation and during the late glacial time. This suggests that highly fertile systems set in the Alboran Sea near 16 kyr B.P. Two rapid increases in the Δδ13C offset are recorded near 15 kyr and 11 kyr B.P. Fluctuations around 1.1 to 1.2‰ occurred during the early Holocene, and a maximum was reached near 9 kyr B.P. After 4 kyr the Δ δ13C offset decreased to its present-day average value of 0.9‰. Changes in the intensity of surficial production cannot account for all the observed fluctuations, especially in the early Holocene time. A strong decrease in the intermediate and deep water ventilation of the Alboran basin may have occurred near 8–9 kyr, in phase with the last stagnant phase in the eastern Mediterranean and the deposition of Sapropel S1. As a result, the redistribution and remineralization at depth of the produced organic matter was incomplete. The POC rain reaching the sediment was locally intensified and caused the lowering of the δ13C values of endobenthic foraminifers such as U. peregrina. The benthic 13C signal suggests that the difference between the Alboran Sea and the Gulf of Cadiz was at its maximum. At the same time, an important modification in the water masses structure may have occurred near 9–8 kyr B.P. The deepening of the permanent pycnocline probably related to a thicker Atlantic jet at a stage of high sea level stand is recorded by the replacement of the right coiling N. pachyderma dominance (coincident with a shallow pycnocline) by the G. inflata dominance (coincident with a deep pycnocline). Diatom abundances were strongly reduced indicating an important modification of the productive system. The glacial-postglacial evolution of productivity within the Alboran basin was therefore more complex than in the adjacent Atlantic Ocean and opposite to the global one which displays a general increase in productivity during glacial time. Although it is the global budget of paleoproductivity that would drive the partitioning of carbon within the ocean, local or regional discrepancies with the global glacial-interglacial model must be addressed. Local winds and regional atmospheric pressure systems, which are the forcing factors for circulation and exchange with the Atlantic, control the fertile systems of the Alboran basin.

A review of the Pliocene-Pleistocene paleoenvironmental history of the western Mediterranean is presented based on micropaleontological and geochemical studies of Ocean Drilling Program (ODP) leg 107 material and marine outcrops in southern Italy. The calcareous plankton biochronology proposed for the Mediterranean by Rio et al. (1990 and 1991a) provides the necessary age control for determining the timing of major climatic and oceanographic changes in the region. The early Pliocene period in the Mediterranean was not a time of warm, stable climate; rather, it was marked by relatively high-amplitude climatic oscillations. In marginal areas, these climatic oscillations resulted in the rhythmic deposition of limestone-marl couplets which have an average duration of approximately 19,000 years. It is proposed that these sedimentary couplets are the result of precessionally induced changes in monsoon intensity which in turn cause variability in coastal divergence and upwelling. The δ18O and the calcareous plankton (foraminifera and coccoliths) records for Tyrrhenian Sea ODP site 653 reveal a cooling of surface waters at ∼3.1 Ma. Coincident with this cooling is an increase in the frequency of diatomite formation in southern Italy (Bianco section). The global cooling at ∼2.4 Ma and the associated expansion of northern hemisphere glaciation resulted in a shift to more arid conditions in the Mediterranean region. This climate change may have altered the Mediterranean water balance, resulting in the establishment of antiestuarine water exchange with the Atlantic at this time. The δ18O record for site 653 is also marked by a sharp cooling across the Pliocene/Pleistocene boundary. In the boundary stratotype section at Vrica, this cooling is manifested as an increase in the abundance of the planktonic foraminifer Neogloboquadrina pachyderma.

Oxygen and carbon isotope records of twelve Mediterranean cores raised from west (Alboran sea) to east (off Israel), and of four Atlantic cores raised from the western side of Gibraltar and located respectively inside, below and beyond the modern Mediterranean outflow are compared to open ocean records of core 71–139C from the North Atlantic and core V19–30 from the East Pacific. Oxygen isotope stratigraphies can be established for deep Mediterranean records and the major events of the last 150 ka are easily identifiable. For instance, the five substages of stage 5 or the 18 ka event (stage 2) are identified but the 10 ka event (= Younger Dryas) is only recorded by Globigerina bulloides and benthic foraminifera, suggesting that the cooling affected only winter SST. 14C dates for the two terminations, IA and IB, of the last deglaciation which are recorded in sediment cores from areas with high sedimentation rates (Faro Ridge, Alboran Sea, Sicilian Strait), are in good agreement with the dates published for the same termination in Atlantic cores. An intermediate step, however, has been identified in some cores at 14 ka and divides termination IA into two phases. Oxygen isotope anomalies are recorded by the shallow water species Globigerinoides ruber. They correspond to (1) a higher amplitude glacial-interglacial change in δ18O values which exceeds that of the open ocean by 1‰ in the western basin and by 1.7‰ in the eastern basin; (2) large isotopic depletions during the deposition of sapropel S1, S3 and S5. They are interpreted as the result of fresh water run-off responsible for a strong dilution in the shallower layers of the east Mediterranean waters. The isotopic effect of this dilution averages −0.75‰ for sapropel S1 and −1.6‰ for sapropel S5. During the last glacial maximum, the Mediterranean still acted as a concentration basin and a gradient of δ18O values similar to the modern one (≅ +0.75‰) existed from west to east in the deep waters. Apparently, deep water temperatures were lower during the last glacial but the decrease did not exceed 4°C in the eastern basin. Major anomalies are also contained in the 13C records of G. bulloides and benthic foraminifera. The 13C records from surface, intermediate and deep waters of the eastern basin and from intermediate and deep waters of the western basin are opposite to that of the open ocean. During glacial time δ13C values were higher in intermediate and deep waters of the whole Mediterranean, and also in surface water of the eastern basin. At the same time, the 13C gradient of surface to deep waters was low. This gradient increased during the last deglaciation while δ13C values of G. bulloides and benthic foraminifera decreased. This decrease, interrupted at 10 ka, may have been related to increased fresh water run-off from the bordering contintents due to increased precipitation, especially in the tropical belt. The 13C depletion is clearly recorded in the Levantine and deep water masses and can be traced westward in the Mediterranean outflow. Oxygen and carbon isotope anomalies recorded simultaneously in the east Mediterranean by the shallow water species Globigerinoides ruber during sapropel deposition indicate that increased precipitation on the bordering continents and strong fresh water run-off during the time of the last deglaciation, may have caused a density stratification and deep stagnation in the eastern basin. Uncertainties arise from the fact that, in the mixing model, deglacial Mediterranean waters are mixed with continental fresh waters, the isotopic characteristics of which are not entirely understood. Although increased surface water production of biogenic material can not be excluded as a possible cause of 13C depletion, it appears that the river discharge including the Nile, as well as Black Sea input, are also plausible sources for the fresh water layering.

The deviation from isotopic equilibrium has been evaluated for eight foraminiferal species from the Mediterranean Basin. A detailed study of some forms presenting an outer calcitic crust in late ontogenetic stages and in sedimented specimens allows us to propose another hypothesis for the biological fractionation: it should be a function of metabolic processes which condition the secretion of the tests.

Oxygen isotope changes of the foraminiferal species Globigerina bulloides and Uvigerina peregrina , eleven radiocarbon dates on the sediment fraction above 50 μm and 13 biostratigraphical events allow us to correlate and to date the sediment record of the last climatic cycle in four cores from the Alboran Sea. Nine ecozones are recognized which differ from those previously established for Atlantic sediments cored immediately west of Gibraltar. These ecozones can be grouped into two broad phases. From the last glacial maximum up to near 9 500 yr B.P., cold North Atlantic microfaunas are the major component of the assemblages. During this interval, dominant species replace each other according to the varying temperature and density stratification of the water masses. From 9 500 yr B.P. up to the present time, subtropical forms from the Central Atlantic dominate the Northern Atlantic forms. An abrupt increase in the percentages of G. inflata occurred near 7 000 B.P.; this event was diachronous within the Mediterranean. During the last glacial G. inflata was endemic inside the Mediterranean and its abrupt increase near the end of the deglaciation did not result from replenishment from the Atlantic. Between 4 000 and 5 000 yr B.P., percentages of G. bulloides also increased abruptly. Warming conditions associated with the onset of a seasonal thermal stratification and a contrasting winter vertical convection are strongly suggested.

An oozy core sampled on the top of an abyssal hill in the Ligurian Sea, and characterized by a very regular sedimentary process, was studied by means of three paleoclimatological analyses founded on Foraminifera, oxygen isotopes 18O/16O and clay minerals. Their results were plotted in three diagrams; their comparison leads us to point out several climatic oscillations that are interpreted as follows: Wurm IIc from 810 to 730 cm; Wurm II-Wurm III from 730 to 620 cm; Wurm III and Wurm IV from 620 to 100 cm; and the post-Glacial period from 100 cm to the surface. Consequently, the mean rate of sedimentation occurring on the hill is about 15 cm per thousand years since the end of the Wurm II. These results should be tested by absolute dating with 14C and I/Th methods.

Vergnaud Grazzini, C. and Oberhansli, H., 1985. Isotopic events at the Eocene/Oligocene transition. A review.

There is evidence that in Mesozoic and Palaeogene times both north and south subpolar areas had a mild climate1,2. Unlike the present day climatic symmetry, with warm equatorial regions and ice-covered polar regions, a strong planetary asymmetry developed in the middle Miocene, when the Antarctic ice cap was established while the northern high latitudes remained unglaciated3. The symmetry was restored when a Northern Hemisphere continental ice cover was established ∼3 Myr ago in the middle Pliocene4. These changes influenced the formation of deep water in the ocean. At present the cold, dense bottom waters originate in only two high latitude areas—the Weddell Sea producing Antarctic Bottom Water, and the Norwegian Sea, producing Norwegian Sea Overflow Water which after mixing is a major component of the North Atlantic Deep Water. The production of cold deep water in the Southern Ocean started at the Eocene–Oligocene boundary3. We present here new evidence from isotope analyses of benthic Foraminifera from DSDP site 116 indicating that the production of oxygenated deep water in the North Atlantic Ocean started in the late middle-Miocene, ∼12 Myr ago.

RECONSTRUCTION of the climatic and hydrologic history of ocean basins has been based on either oxygen isotope palaeothermometry or studies of foraminiferal assemblages preserved in deep sea sediments. We report here the application of both methods to deep sea sediments collected from the Arctic Ocean by the Lamont–Doherty Geological Observatory. Oxygen isotope determinations were made on two selected cores from the central Arctic basin to test the validity of the palaeoceanographic interpretations based on microfaunal analysis1–3. Details of the cores are given in Fig. 1 and Table 1. Analyses of the cores have already been published in detail: here we wish to summarize what we have discovered about the history of the Arctic Ocean1–4.