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High-resolution planktonic foraminiferal and oxygen isotope records are presented from a Mediterranean deep marine succession, dated astronomically between 12.12 and 9.78 Ma. Planktonic and benthic oxygen isotope records are punctuated by two episodes of δ18O increase, which have astronomical ages of 11.4 and 10.4 Ma and correspond to the Mi5 and Mi6 events of Miller et al. [1991a]. These ice growth events coincide with low-amplitude variations in the 1.2 Myr obliquity cycle and are accompanied by significant faunal changes in the Mediterranean, such as the arrival of neogloboquadrinids, the increase in abundance of the G. apertura-G. obliquus group, and the areal differentiation between N. atlantica and N. acostaensis. Short-term variations in the planktonic foraminiferal and oxygen isotope records correspond to dominantly precession-controlled sedimentary cycles. Features of the sapropel/grey marl layers indicate that the short-term astronomically controlled circum-Mediterranean climate changes remained basically the same over the last 12 Myr.

For the first time, results of an integrated geochemical and micropalaeontological study of upper Miocene sapropelites in a land-based section in the Mediterranean are presented. Three sapropelites and adjacent homogeneous intervals in the Cretan Faneromeni section have been investigated in detail. The chemical and foraminiferal characteristics were found to be markedly similar to those of Plio-Pleistocene sapropelites. As the physiography and climate of the late Miocene Mediterranean were not fundamentally different from those of today, our preferred model for late Miocene sapropelite formation is similar to the one recently developed for Plio-Pleistocene sapropelites. Increased seasonality and freshwater input during northern hemisphere summer insolation maxima triggered high export production and oxygen consumption rates, which, in combination with a concomitant decrease in deep water formation rate, resulted in the observed high organic matter content of the sapropelites.

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

High-resolution micropaleontological (planktonic foraminifera and calcareous nannofossils) and geochemical (stable isotopes, organic carbon, Fe, P, S, Ca, Ba, Mn, and Al) records are presented for the first sapropel-containing carbonate cycle in the Pliocene of Sicily. The carbonate cycle is characterized by a gray to white to beige to white color layering typical of the marls of the Trubi formation. A faintly laminated sapropel is intercalated in the gray-colored bed of the carbonate cycle. CaCO3 content varies from 40% in the beige to 45-50% in the white layers. Lowest CaCO3 content of 25–30% is found in the gray layer and sapropel. Variations in carbonate and organic matter percentages can best be explained by changes in paleoproductivity rather than by variations in dilution and dissolution. Total productivity was highest during deposition of the gray layer and sapropel, as indicated by high organic carbon and Ba contents and high abundance of Globorotalia puncticulata. Carbonate production reached its highest values, however, during deposition of the white layers, as evidenced by enhanced abundances of planktonic foraminifera and nannofossils. The low carbonate content in the gray layer and sapropel is explained in terms of a collapse in carbonate production caused by extreme changes in the physical and biochemical properties of the water column, which in turn resulted in siliceous plankton and opportunistic foraminifers such as Globorotalia puncticulata outcompeting most calcareous organisms. The beige layer represents a low-productivity environment similar to the present-day eastern Mediterranean basin. Several mechanisms have previously been proposed to explain variations in productivity in the eastern Mediterranean. Both sapropels and gray layers were deposited at times when perihelion occurred in northern hemisphere summer. We envisage that the increase in seasonal contrast resulting from this orbital configuration enhanced winter mixing and stabilization of the water column during summer, both leading to favorable conditions for intensification of the spring bloom. In addition, a decrease in excess evaporation, as can be deduced from the δ18O record, led to shoaling of the pycnocline and reduced circulation, thus enhancing the availability of nutrients in the photic zone. Finally, enhanced precipitation and associated runoff should have caused an increase in river-borne nutrients.

Results from transient climate modeling experiments indicate an in-phase relationship between insolation forcing and Indian summer monsoonal precipitation. This is in contrast to high-resolution radioisotopically dated speleothem oxygen isotope (δ18O) records of China, which showed that East Asian Monsoon maxima lag Northern Hemisphere peak summer insolation by ∼2,700 years, while an approximately 8,000-year time lag was derived from late Pleistocene records of Arabian Sea sediments. Here, we evaluate the precession phase of the Arabian Sea signal by comparing a new high-resolution productivity and oxygen minimum zone (OMZ) intensity record from the Arabian Sea over the past 450,000 years with the results of a transient climate modeling experiment that includes glacial-bound ice volume variations. The well established tuning technique between radioisotopically dated North Atlantic cold events and the occurrence of deep-dwelling planktonic foraminifera in the Arabian Sea for the last glacial cycle was used to extend the Arabian Sea chronology, independent of orbital tuning. Cross-spectral analysis over the last 224,000 years reveals that Arabian Sea productivity maxima lag precession minima by ∼6,900 ± 200 years, i.e., in close agreement with previous reconstructions. Also our climate modeling simulations are in accord with previous studies indicating an in-phase relationship between precession minima and maximum summer monsoon intensity. We argue that the summer monsoon is most likely not the main driver of changes in Arabian Sea biological productivity and OMZ intensity at the precession frequency band, but that changes in the intensity of the Atlantic meridional overturning circulation (AMOC) have played the prominent role in controlling the nutrient delivery into the euphotic layer of the northern Indian Ocean, and hence the amount of primary productivity and intensity of the oxygen minimum zone in the Arabian Sea. Such a mechanism explains the large precession-related time lag between minimum precession and maximum productivity and OMZ conditions in the Arabian Sea, since intensified AMOC occurred during precession maxima.

[1] An integrated high-resolution stratigraphy and orbital tuning is presented for the Loulja sections located in the Bou Regreg area on the Atlantic side of Morocco. The sections constitute the upward continuation of the upper Messinian Ain el Beida section and contain a well-exposed, continuous record of the interval straddling the Miocene-Pliocene (M-P) boundary. The older Loulja-A section, which covers the interval from ~5.59 to 5.12 Ma, reveals a dominantly precession-controlled color cyclicity that allows for a straightforward orbital tuning of the boundary interval and for detailed cyclostratigraphic correlations to the Mediterranean; the high-resolution and high-quality benthic isotope record allows us to trace the dominantly obliquity-controlled glacial history. Our results reveal that the M-P boundary coincides with a minor, partly precession-related shift to lighter “interglacial” values in d18O. This shift and hence the M-P boundary may not correlate with isotope stage TG5, as previously thought, but with an extra (weak) obliquity-controlled cycle between TG7 and TG5. Consequently, the M-P boundary and basal Pliocene flooding of the Mediterranean following the Messinian salinity crisis are not associated with a major deglaciation and glacio-eustatic sea level rise, indicating that other factors, such as tectonics, must have played a fundamental role. On the other hand, the onset of the Upper Evaporites in the Mediterranean marked by hyposaline conditions coincides with the major deglaciation step between marine isotope stage TG12 and TG11, suggesting that the associated sea level rise is at least partly responsible for the apparent onset of intermittently restricted marine conditions following the main desiccation phase. Finally, the Loulja-A section would represent an excellent auxiliary boundary stratotype for the M-P boundary as formally defined at the base of the Trubi marls in the Eraclea Minoa section on Sicily.

[1] High-resolution benthic and planktonic stable oxygen isotope records are presented for the upper Miocene Ain el Beida (AEB) section in northwestern Morocco and reveal the clear imprint of the (climatic) precession, obliquity, and eccentricity cycles in the interval between 6.5 and 5.5 Ma. The δ18O depth series were transformed into time series using astronomical tuning of the sedimentary color cycles to the 65°N summer insolation curve of the La93 solution with present-day values for tidal dissipation and dynamical ellipticity. Spectral analysis and band-pass filtering show that the ratios of the astronomically related spectral peaks are not consistent in the depth and time domain. The inconsistencies are mainly due to variations in sedimentation rate that are positively correlated with precession amplitude (i.e., modulated by eccentricity). In addition, they result from a long-term trend in sedimentation rate. A simple model of nonlinear response of the sedimentation rate to the eccentricity modulation of precession was used to simulate the observed shifts in the relative position of the spectral peaks in the depth domain. Cross-spectral analysis was carried out on the δ18O time series and a combined eccentricity, tilt (obliquity), and precession (ETP) curve. The precession-controlled δ18O signal varies in-phase with ETP partly as a result of the tuning procedure, while the obliquity related signal reveals a small lag of 2–3 kyr relative to obliquity. This lag is slightly reduced at the time of the Messinian Salinity Crisis (MSC) and increases to 5–6 kyr if the tidal dissipation term in the astronomical solution is reduced to half its present-day value. The obliquity-related lag is most likely associated with the slow buildup of ice caps, whereas the precession signal is interpreted as to dominantly reflect regional climate changes comparable to those associated with Mediterranean sapropels. Finally, the benthic δ18O record is compared with open ocean records and discussed in terms of an astronomically tuned oxygen isotope stratigraphic framework.