Colette Vergnaud-Grazzini

Paleoclimatic record of a long deep sea core from the eastern Mediterranean

Abstract A deep-sea core over 16 m long from the crestal area of the Mediterranean Ridge has been investigated with different techniques, including quantitative micropaleontology, stable isotopes (measured on the epipelagic species Globigerinoides ruber and on the mesopelagic species Globorotalia inflata), and clay mineralogy. The resulting record of climatic fluctuations can be cross correlated to other Mediterranean cores by means of isochronous lithologies (tephra layers and sapropels). The climatic record of the Mediterranean is similar in character, phase, and chronology to the records investigated in the equatorial Pacific and in the Caribbean. Isotope stages 1 to 17 have been recognized. Cyclically repeated stagnant cycles resulting in sapropel deposition complicate both the isotopic and the faunal signal. The isotopic investigations reveal that the temperature change in the surface layers of the eastern Mediterranean was no greater than 8°C in the late “glacial” Pleistocene. The chronostratigraphic and biostratigraphic interpretation of Core KS09 indicate that the mean sedimentation rate was 2.4 cm/1000 years, a value very close to the 2.5 cm/1000 years calculated for the entire Quaternary section at DSDP Site 125, also located in the crestal area of the Mediterranean Ridge in the Ionian Basin. The base of KS09 is likely to be very close to the Brunhes/Matuyama boundary dated at 0.7 my.

Dissolution and carbonate fluctuations in Pleistocene deep-sea cores: A review☆

Abstract Three factors govern calcite content in deep-sea sediments: dilution by terrigenous material, productivity fluctuations of calcareous organisms, and calcium carbonate dissolution. The influence of each factor is discussed. Examples from three oceans: original data from the equatorial Pacific Ocean and reexamination of previous work from the equatorial Indian and Atlantic Oceans, show that cyclic calcite content fluctuations can be correlated to glacial—interglacial phases, and that dissolution is the most important factor in these cores. Fluctuations occurring in the Atlantic Ocean are opposite to those observed in the Indo-Pacific Ocean. A weakening glacial bottom-water circulation could account for the facts as they now appear.

Crustal block movements from Holocene shorelines: Rhodes Island (Greece) *

Abstract Signs of up to eight stepped Holocene shorelines, which have been reported in a previous study on the east coast of Rhodes Island, have been reinvestigated in detail on the basis of a new geomorphological survey, identification and petrological analysis of many new samples of exposed marine organic crusts and over 30 radiocarbon datings. Confirmation was obtained of the view that the island can be charactered by considering small crustal blocks up to a dozen kilometres long with each block being affected by a specific tectonic history, and a general trend towards an uplift increasing from south to north. Two successive phases of submergence-emergence have been revealed by petrogenetic sequences in many samples, which give evidence of the occurrence of positive and negative vertical movements. This implies possible rejuvenation effects of the later submergence phase on the apparent age of pre-existing algal crusts, and those have been taken into account and estimated. The most likely sequence of vertical crustal movements to have occurred has been specified for each block and a time range has been ascribed to each former shoreline. Independent up and down movements, increasing in amplitude from south to north, appear in most blocks. There is a recurrent periodicity varying from a few hundred to one or at most two thousand years. The largest vertical displacement observed during the late Holocene—a sudden uplift movement of about 3.8 m in the northernmost part of the island—is likely to be linked with the earthquake which destroyed the Colossus in 222 B.C.

Mediterranean Late Cenozoic Stable Isotope Record: Stratigraphic and Paleoclimatic Implications

Ocean evolution during the past 100 million years (m.y.) has involved the replacement of warm Cretaceous oceans and seas by cold Neogene oceans, the closure of the Tethys, a major circum-tropical ocean, and the development of a circum-polar ocean and a north-south axis linking the Arctic with a southern ocean. The response of the Tethys (then the Mesogean and subsequently the Mediterranean) to this global climatic evolution would likely have been a specific one. Information on this climatic history is obtained by comparing the evolution of geochemical parameters, as recorded in deep-sea sediments, in the Tethys—or Mediterranean—and in the Atlantic. Among others, carbon and oxygen isotopes in marine carbonates have proved to be an indispensable tool for climatic reconstruction. Additional characteristic isotopic events have been identified that are synchronous on a world-wide scale and can serve as stratigraphie markers. These isotopic events are generally linked with paleoclimatic or hydrologie changes. The δ18O variations are controlled by temperature changes and/or ice formation on land and at the poles; among the important factors affecting the distribution of δ13C of the total dissolved inorganic carbon in the world ocean is the apparent oxygen utilization (related to the progressive oxidation of organic matter). In deep waters, longer residence time favors the addition of δ13C-depleted carbon to the dissolved bicarbonate by the oxidation of organic matter and accounts for the relation between the residence time of deep water masses and the δ13C values of the ΣCO2 (the greater the residence time, the lower the δ13C of the ΣCO2).

An overview of the post‐Messinian paleoenvironmental history of the western Mediterranean

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 stable isotope tracers of the water masses in the Central Brazil Basin

Stable isotope characteristics of the six water layers in the Central Brazil Basin are defined and compared to the various sources from which they originate. The ∂18 O−S‰ relationship allows us to discriminate between the more 18O-rich/more saline surface waters and South Atlantic Central Water formed in the tropical-subtropical zones, and the low-18O/fresher water layers (Antarctic Intermediate Water: AAIW, Circumpolar Water: CPW, North Atlantic Deep Water: NADW, Antarctic Bottom Water: AABW) generated at high latitudes. The ∂18O-∂13C relationship emphasizes the specificity of each water mass: the upper water layers, where evaporation and photosynthesis are active, are the more 18O−13C-rich waters (+0.3<∂18O<+1.5; +1.0<∂13C<+2.0), whereas intermediate, deep and bottom waters, where fresh-water dilution and organic matter oxidation are significant, have the lowest 18O−13C contents (−0.1<∂18O<+0.5; +0.5<∂13C<+1.1). Aging of the NADW in this area of the southwest Atlantic results in an average ∂13C decrease of about 0.35‰. However, discrimination of water masses by their oxygen and carbon isotope compositions may become more difficult when they are far from their source area and when lateral mixing has occurred; this is the case for the CPW layer in the Central Brazil Basin.

Identification of Heinrich Layers in core KS 01 North-Eastern Atlantic (46 °N, 17 °W), implications for their origin

Abstract Sediments of the North Atlantic contain between 40 ° and 55 °N a series of layers rich in ice-rafted material (Heinrich layers, HLs). In core KS01 from the northeastern Atlantic (46 °00.2′N, 17 ° 12.1′W) we identified HL1 to 5. Important parts of these layers are abnormally dense. In order to unravel the depositional history of the HLs and the environmental conditions which have allowed such induration, we investigated the 18O and 13C contents of planktonic foraminifers and studied the bulk physical and lithological properties of the layers. The detailed investigation of HL1, HL2 and HL4 shows that, at this site, drastic changes in the surface water and at the sediment-water interface occurred during the HL deposition. We distinguished two phases. The first phase, which includes two steps, is characterised by glacial conditions and high productivity (not reflected by planktonic foraminifers). During the second step the detrital carbonate supply increased and organic carbon content maxima occur in these HLs. The second phase is characterised by an important decrease of the surface water salinity and an important reduction of the productivity. The diagenetic precipitation of calcite within microburrows in relation with anoxic and microbial mineralisation of organic matter may have contributed to the induration of the layers.

Late Holocene sea levels and coral reef development in Vahitahi Atoll, eastern Tuamotu Islands, Pacific Ocean☆

Abstract Preliminary quantitative data are provided on the latest stage of development and morphological evolution of the closed atoll Vahitahi, in the eastern Tuamotus. From before 4300 to at least 2400 yrs B.P. sea level was at least 0.7 m higher than at present. During this time the development of a flat reef pavement, now slightly emerged, gradually separated the lagoon from the ocean. At about 1200 yrs B.P. the water level of the lagoon, already closed, was still slightly above the present level. The response of the reef was a rapid upward accretion until about 3000 yrs B.P., then an oceanward development at rates averaging from 0.04 to 0.13 m yr −1 .

A refined Pliocene to early Pleistocene chronostratigraphic frame at ODP hole 653A (West Mediterranean)

Abstract Two planktonic foraminiferal oxygen isotope records of ODP Hole 653A (Tyrrhenian Sea) are presented for the time period extending from approximately 0.8 to 3.0 Ma. Six, generally accepted, synchronous bioevents were used to precise the oxygen isotope chronology and to identify the oxygen isotope stages 22 down to 114. Subsequently, this oxygen isotope chronology was used to determine the synchronism or diachronism of various other biostratigraphic events with those recorded in the Singa and Ficarazzi land sections (Italy) and those in other DSDP/ODP sites. New results concern the diachronity of the FOD of the planktonic foraminiferal species N. atlantica, G. truncatulinoides truncatulinoides and G. inflata between ODP Hole 653A and the Italian landsections. Because many species entered the Mediterranean in short term fluxes, strongly related to the southward migration of cool North Atlantic surface waters, their time distribution through the Pliocene-Pleistocene generally corresponds to alternated intervals of presence and absence. This should explain most of the apparently diachronous appearances and disappearances. Alternating presence-absence patterns are of less importance for the various nannofossil events. The LOD of D. surculus occurs during the transition of stage 100 to 101 in both ODP Hole 653A and the Singa section, which is in perfect agreement with the disappearance of this species from the open ocean. The LOD of D. pentaradiatus in the Mediterranean occurs in stages 100-99, which seems to be consistent with the extinction of this species in the southern Hemisphere. G. oceanica , which corresponds to the 4 μm Gephyrocapsa spp μ m is recorded in stages 65 to 64 at ODP Hole 653A. The Gephyrocapsa spp. > 5.5 μ m first occurred in stage 51 at Hole 653A, which fits within the uncertainty interval for this event stretching from stage 51 to 47 in the open ocean and seems therefore a useful tool for conventional biostratigraphy in the Mediterranean.

Stable isotope record of Uvigerina peregrina from the Mediterranean Sea

Based on the glacial to postglacial delta13C 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 1.6per mil with respect to the measured delta13C values of bottom water SumCO2 and by about 0.9per mil with respect to specimens from other areas of the western Mediterranean or from the Gulf of Cadiz within the Mediterranean Outflow Water. The Uvigerina delta13C difference between the Alboran Sea and the Gulf of Cadiz (Delta delta13C), was close to 0per mil 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 Delta delta13C offset are recorded near 15 kyr and 11 kyr B.P. Fluctuations around 1.1 to 1.2per mil occurred during the early Holocene, and a maximum was reached near 9 kyr B.P. After 4 kyr the Delta delta13C offset decreased to its present-day average value of 0.9per mil. 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 delta13C 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.