(Table 1) Concentration and geological parameters of n-alkanes and n-alkenes in surface sediments from the Gulf of Gabes, Tunisia

Abstract Hydrocarbons in surface sediments were studied quantitatively and qualitatively in 18 stations along the coastline of Gabes Gulf in Tunisia. The results show that the total hydrocarbon levels vary along a wide range from 90 to 1,800 ppm. The GC-MS profiles of aliphatic hydrocarbons vary according to the stations and show that the hydrocarbons were derived from various sources. A special feature prevalent in several stations was identified: aliphatic hydrocarbons with distinctive chemical features. This includes a high abundance of even-numbered n-alkanes (n-C14 - n-C26, maximizing at n-C18, n-C20 and n-C22) and n-alk-1-enes (n-C14:1 - n-C24:1, maximizing at n-C16:1, n-C18:1, n-C20:1 and n-C22:1). This unusual predominance of even-numbered n-alkanes/alkenes is reported for the first time in the Gulf of Gabes and it thus contributes to the information on the rare occurrence of such distributions in the geosphere.

Mineralogical (bulk rock and clay fraction) and stable isotope composition (d13C, d18O) of Aptian deposits of the Bir Oum Ali succession, central Tunisia

Alternations between siliciclastic, carbonate and evaporitic sedimentary systems, as recorded in the Aptian mixed succession of southern Tunisia, reflect profound palaeoceanographic and palaeoclimatic changes in this area of the southern Tethyan margin. The evolution from Urgonian-type carbonates (Berrani Formation, lower Aptian) at the base of the series, to intervals dominated by gypsum or detrital deposits in the remainder of the Aptian is thought to result from the interplay between climate change and tectonic activity that affected North Africa. Based on the evolution of clay mineral assemblages, the early Aptian is interpreted as having been dominated by slightly humid conditions, since smectitic minerals are observed. Near the early to late Aptian boundary, the onset of a gypsiferous sedimentation is associated with the appearance of palygorskite and sepiolite, which supports the installation of arid conditions in this area of the southern Tethyan margin. The evaporitic sedimentation may have also been promoted by the peculiar tectonic setting of the Bir Oum Ali area during the Aptian, where local subsidence may have been tectonically enhanced linked to the opening of northern and central Atlantic. Stress associated with the west and central African rift systems may have triggered the development of NW-SE, hemi-graben structures. Uplifted areas may have constituted potential new sources for clastic material that has been subsequently deposited during the late Aptian. Chemostratigraphic (d13C) correlation of the Bir Oum Ali succession with other peri-Tethyan regions complements biostratigraphic findings, and indicates that a potential expression of the Oceanic Anoxic Event (OAE) 1a may be preserved in this area of Tunisia. Although the characteristic negative spike at the base of this event is not recognized in the present study, a subsequent, large positive excursion with d13C values is of similar amplitude and absolute values to that reported from other peri-Tethyan regions, thus supporting the identification of isotopic segments C4-C7 of the OAE1a. The absence of the negative spike may be linked to either non preservation or non deposition: the OAE1a occurred in a global transgressive context, and since the Bir Oum Ali region was located in the innermost part of the southern Tethyan margin during most of the Aptian, stratigraphic hiatuses may have been longer than in other regions of the Tethys. This emphasizes the importance of integrating several stratigraphic disciplines (bio-, chemo- and sequence stratigraphy) when performing long-distance correlation.

Organic carbon and calcium carbonate concentrations of K/T boundary samples from southern Tethyan margin location (Table 1)

Regional consequences of the biotic extinctions and of the changes in biological productivity that occurred at the time of the Cretaceous/Tertiary (K/T) boundary were investigated by comparison of organic matter in sediments from three southern Tethyan margin locations. Organic matter characterization comprised Rock-Eval pyrolysis and organic carbon measurements. Low concentrations of organic matter precluded additional detailed determinations. At all three locations, the organic matter has been microbially reworked and evidently was deposited in oxygenated marine environments.

Late Campanian and Maastrichtian benthic foraminifera of El Kef, Tunesia

Late Campanian through Maastrichtian sea-level changes are examined based on lithology, macrofossils and benthic foraminifera at the Elles and El Kef sections in Tunisia. Six major sea-level regressions are identified during the late Campanian (74.4-74.2 Ma, 74.0-72.5 Ma), the Campanian-Maastrichtian transition (72.2-70.3 Ma), early Maastrichtian (69.6-69.3 Ma, 68.9-68.3 Ma), and late Maastrichtian (~65.5 Ma). Correlation of the Maastrichtian sea-level regressions with the oxygen isotope record of DSDP Site 525 in the middle latitude South Atlantic reveals that they coincide with episodes of high latitude cooling and appear to be of eustatic origin.

Benthic foraminiferal carbon isotopic records and the development of abyssal circulation at DSDP Sites 9-77, 12-119, 30-289, 82-563 and 94-608

The North Atlantic at present is ventilated by overflow of the Denmark Strait, Iceland-Faeroe Ridge, Faeroe Bank Channel, and Wyville-Thompson Ridge. The evolution of Cenozoic abyssal circulation of this region was related to tectonic opening and subsidence of these sills. We used d13C records of the benthic foraminifer Cibicidoides to decipher the timing of tectonically controlled changes in bottom-water circulation in the eastern basins (Biscay and Iberian) of the northern North Atlantic. Records from Site 608 (Kings Trough, northeastern North Atlantic) show that from about 24 to 15 Ma (early to early middle Miocene), d13C values in the Kings Trough area were depleted relative to western North Atlantic values and were more similar to Pacific d13C values. This reflects less ventilation of the Kings Trough region as compared to the well-oxygenated western North Atlantic. Comparison of Oligocene d13C records from Site 119 (Bay of Biscay) with western North Atlantic records suggests that the eastern basin was also relatively isolated prior to 24 Ma. At about 15 Ma, d13C values at Site 608 attained values similar to the western North Atlantic, indicating increased eastern basin ventilation in the middle Miocene. This increased advection into the eastern basin predated a major d18O increase which occurred at about 14.6 Ma. Subsidence estimates of the Greenland-Scotland Ridge indicate that the deepening of the Iceland-Faeroe Ridge was coincident with the marked change in eastern basin deep-water ventilation.

Stable organic carbon isotope record and total organic carbon content from the Cenomanian-Turonian boundary

Ocean Drilling Program (ODP) Leg 207 recovered expanded sections of organic-carbon-rich laminated shales on Demerara Rise (western tropical Atlantic). High-resolution organic carbon isotope and total organic carbon (TOC) records are presented, which span the Cenomanian-Turonian boundary interval (CTBI), including the Oceanic Anoxic Event (OAE) 2, from four sites oriented along a NW striking depth transect. These records represent the first high-resolution carbon isotope records across OAE 2 from the South American margin of the tropical Atlantic. Due to the scarcity of age significant fossils, the main purpose of this study was to develop a detailed carbon isotope stratigraphy in order to correlate the CTBI across the depth transect and to tie this to biostratigraphically well-defined sections in the Western Interior Basin (Pueblo, USA), boreal shelf seas (Eastbourne, England), and western Tethys (Oued Mellegue, Tunisia). All four sections studied document a 6 per mil increase of d13Corg values at the base of the CTBI, which is followed by an interval of elevated d13Corg values and a subsequent decrease. Our results supply an important stratigraphic base for subsequent paleoceanographic studies on Late Cenomanian to Early Turonian sediments from Demerara Rise and elsewhere.