J. Thurow

Tectonic denudation of the upper mantle along passive margins: a model based on drilling results (ODP leg 103, western Galicia margin, Spain)

Abstract During ODP Leg 103, serpentinized peridotite (clinopyroxene-spinel harzburgite) was cored within the basement approximatively at the boundary between the North Atlantic oceanic curst to the west, and the thinned continental crust of the Galicia passive margin (Spain) to the east. The exposure of mantle derived peridotite on the seafloor occurred at the end of the period of rifting, roughly 110 Ma ago. Ductile shear zones observed in the cored peridotite are consistent with movements along a deep low-angle, normal fault rooted within the upper mantle and dipping eastward, beneath the Galicia margin. To explain the tectonic denudation of the mantle at the ocean-continent boundary, we use a non-uniform stretching model for the lithosphere, set up from the Wernickes model (1985).

Influence of oceanic anoxic events on the evolution of mid-Cretaceous radiolaria in the North Atlantic and western Tethys

Abstract Mid-Cretaceous (Aptian-Turonian) extinction/radiation events of radiolaria from the North Atlantic and western Tethys are summarized in relation to Oceanic Anoxic Events (OAE) from investigations of radiolarian faunas, black shales and stable carbon isotopes. Extinction and radiation events of radiolaria can be observed from the late early Aptian, the earliest Albian, the latest Albian and the Cenomanian/Turonian boundary. These events correlate well with organic carbon-rich sediments that contain type II kerogen and show positive δ13C excursions. They correspond to the Oceanic Anoxic Events (OAE) in the early Aptian (OAE 1a, Selli Level), early Albian (OAE 1b), late Albian (OAE 1d) and latest Cenomanian (OAE 2, Bonarelli Level). Detailed investigations of the two most significant events, the Selli and the Bonarelli Level in the Umbria-Marche Basin of central Italy show that extinctions coincide with the first occurrence of marine black shales (type II kerogen) and the most prominent rise of δ13Corg values. Stepwise extinctions and continuous radiations precede and follow the events. Similar patterns are evident for the OAE 1b and 1d. Black shales dominated by terrestrial organic matter (type III kerogen e.g., OAE 1c) are not characterized by isotopic excursions or by radiolarian extinction/radiation events. Contemporaneous drowning events documented from carbonate platforms around the investigated basins provide a link to a model where flooding of lowlands and leaching of nutrients during a relative rise of sea-level lead to an increased productivity and an expansion of the oxygen minimum zone (OMZ). Because of the loss of deep habitats, deeper dwelling forms become extinct. Shallower dwelling radiolarians survive. A narrowing of the OMZ during highstands and relative sea-level falls together with a coupled decrease of the nutrient supply leads to the development of new niches and therefore to radiation.

Organic-rich sedimentation at the Cenomanian-Turonian boundary in oceanic and coastal basins in the North Atlantic and Tethys

Summary One of the most striking results of the Deep Sea Drilling Project is the proof that organic-rich sediments have a widespread geographical distribution during the period from Upper Cenomanian to Middle Turonian. Such sediments were drilled at North Atlantic DSDP Sites: 105, 135, 137, 138, 367, 398, 551, 603. They are also present (from outcrops or oil wells) on the shelf of the African continental margin (Senegal, Tarfaya and Agadir Basins), in the Tethys (former Alboran Block, Southern Spain, Algeria, Tunisia and Umbrian Apennines/Italy), and in the North Sea. Although these sediments have different lithologies and depositional environments (from shelf areas to the deep sea), their study, mainly based on organic geochemistry with additional data on sedimentology, biostratigraphy and palaeo-bathymetry, suggests that a unique ‘pulse’ of organogenic accumulation characterizes the Cenomanian/Turonian Boundary Event (CTBE). The content and type of organic matter are related to the depositional environment and organic preservation. The organogenic accumulation is distributed according to various trends. Off the African continental margin the organic content increases from onshore areas to the shelf (Casamance area), and, moreover, increases also in deep sea areas, with a gradual transition from terrestrial type III to marine type II (the best preservation of the organic matter being in the deepest areas, i.e. Site 367). Off the American continental margin Site 603 shows the same TOC and type of organic matter as at Site 105. The CTBE is also well recorded in the northern part of the Atlantic (Celtic margin, North Sea) by a drastic lithological change (black shales within chalks), but the type of organic matter is mainly terrestrial. In the Tethyan area the organic matter is of marine origin and well preserved. Results are compared with those of Pratt (1984) from the Western Interior Basin of the USA. Different hypotheses to explain this synchronous widespread accumulation of organic matter are discussed.

A fast and easy method to derive highest-resolution time-series datasets from drillcores and rock samples

Abstract We present a fast method to obtain highest-resolution greyvalue datasets from digital core images. These datasets are processed using a software package developed by us. Artificially introduced trends in the datasets are corrected, disturbances (i.e. cracks, voids) are eliminated. To reduce the high-level noise in data we have developed a vector filter that effectively removes such noise. Application of the method to ODP drillcores from the Santa Barbara Basin allowed exact correlation of two drillcores from the same basin. The influence of the termination of the last glacial cold period on the flux rates in the basin are reflected in the greyvalue datasets. We found a transitional period of about 3000 yr between the end of the last glacial cold period and the onset of post-glacial climatic oscillation in the basin. Fourier analyses show that the high-resolution greyvalue time series record climatic cycles with frequencies as high as 1 cycle/yr (varves).

Assessing past temperature and soil pH estimates from bacterial tetraether membrane lipids: Evidence from the recent lake sediments of Lochnagar, Scotland

Past variation in soil pH and air temperature can potentially be reconstructed from the relative abundance in sediments of branched glycerol dialkyl glycerol tetraethers (GDGTs), synthesized by anaerobic bacteria. Specifically, the cyclization of branched tetraethers (CBT) is believed to be a function of pH, whereas temperature can be estimated from a combination of the extent of both the CBT and methylation of branched tetraethers. Here we explore this potential by comparing a recent sedimentary GDGT profile from Lochnagar, Scotland, with reconstructed air temperature (statistically extrapolated from regional instrumental data sets) and diatom-inferred lake water pH for the past similar to 200 years. Branched glycerol dialkyl glycerol tetratether and diatom-inferred pH generally agree throughout the core, supporting the use of cyclization of branched tetraethers to reconstruct pH. During the period of rapid industrial acidification (similar to 1860-1970 A. D.), changes in diatom-inferred pH lag behind those inferred using branched tetraethers by between 10 and 50 years, possibly due to differing nonlinear responses to acid deposition within soil and lake water environments. However, branched-GDGT-derived temperatures are both lower than extrapolated mean annual air temperature estimates (by similar to 5 degrees C) and exhibit at least double the rate of reconstructed warming (similar to 2.5 degrees C in 200 years). At Lochnagar, methylation and cyclization of branched tetraethers are closely correlated (r(2) = 0.96) suggesting that in this setting the underlying controls over the two indices may not significantly differ. Therefore the validity of branched-GDGT-derived temperature is uncertain and further research is required to address the environmental controls over branched glycerol dialkyl glycerol tetratether synthesis and thus their value as palaeoclimate proxies.

Mid-Cretaceous of the Gibraltar Arch Area

Summary Mid-Cretaceous depositional environments of the Gibraltar Arch and adjacent areas exhibit at the Cenomanian-Turonian boundary a special type of organic carbon-rich siliceous sedimentation which is intercalated in all the different environments—from the shelf down to the deep—of this area. This will be called the Cenomanian-Turonian Boundary Event (CTBE). Summarizing the most important features of the CTBE it can be pointed out: (1) the initiation of the CTBE is nearly coeval; (2) very high TOCs, especially at the base of Turonian; (3) kerogen is exclusively of type (I-) II in the undiluted strata; (4) independent of given palaeogeographic setting (e.g. E. and W. margin of the Atlantic ocean); (5) bituminous sedimentation is also very pronounced in environments with high detrital input and strong dilution (e.g. deep sea fans). Here too, most strata comprise kerogen of type II; (6) as far as we know, there is a striking co-occurrence of these sediments with strong enrichment in biogenic silica from diagenetically altered radiolaria, but there is no link between good preservation of radiolaria and bituminous sediments (e.g. Scaglia-facies); (7) rich radiolarian faunas are common in the carbon-rich and carbon-free sediments of shallow marine and deep sea environments; (8) the important change in the evolution of planktonic organisms (foraminifera/radiolaria) which marks the Cenomanian-Turonian boundary coincides with the peak of anoxic conditions and biogenic silica-rich sedimentation. All these features of the CTBE observed in the Gibraltar Arch area are comparable with coeval DSDP-facies in the North Atlantic and allow on-shore-off-shore tracing of the CTBE.

Mesozoic Tethyan strata of Thakkhola, Nepal: evidence for the drift and breakup of Gondwana

The Thakkhola region of central Nepal contains at least 1.5 km of coastal to neritic and (upper) slope deposits of Late Triassic to mid-Cretaceous (latest Albian) age. New paleomagnetic, paleobiogeographic and paleoflow data confirm that the strata were deposited on the northern Gondwana margin, bordering Tethys while Thakkhola lay at mid-latitudes (28–41° S). Late Triassic coastal deposits are overlain by Early-Mid Jurassic shelf units; thick Early Jurassic carbonates correspond with the most northerly paleolatitude recorded, probably reflecting Thakkholas rapid drift into the subtropics. The Late Jurassic to Early Cretaceous drift again in higher latitudes resulted in deposition of terrigenous clastics. A hiatus spanning late Early Callovian through Early Oxfordian time corresponds to a global transgression that may reflect accelerated sea-floor spreading in both the Atlantic and Pacific Oceans. Late Jurassic dark shales, correlative with the regionally extensive Spiti Shale, are deep shelf to slope deposits; septal strength indices for cephalopods suggest depths > 250 m. The shales contain an Indo-SW Pacific ammonite assemblage but also a diversified agglutinated foraminiferal assemblage of Boreal affinity. Such deep-water foraminiferal assemblages probably had a large cosmopolitan component. The shale is intensely deformed and its overmature condition probably reflects in part Cenozoic orogenic events. Lower Cretaceous (Berrisian through Aptian) deltaic deposits overlying the shales contain volcanoclastic material that probably was derived from coeval volcanics of the Lesser Himalayas to the south, based on compositional, paleoflow and facies analysis. Volcanism reflects the onset of sea-floor spreading between Greater India and northwestern Australia in Late Valanginian time. At the top of the Thakkhola Mesozoic sequence are hemipelagic, slope, carbonates of latest Albian age. The Thakkhola succession generally correlates well with that observed on a formerly contiguous continental margin, offshore northwestern Australia.

Morocco Basin's sedimentary record may provide correlations for Cretaceous paleoceanographic events worldwide

More than 500 m of Upper Cretaceous laminated biogenic sediments, mainly consisting of calcareous nannoplankton, dispersed biogenic silica, planktonic foraminifers, and marine organic matter, were deposited in the depocenter of the Tarfaya-Laayoune Basin, near the town of Tarfaya in southern Morocco, with sedimentation rates exceeding 10 cm/ky during transgressive phases. These sediments exhibit the worlds highest accumulation rates of marine organic matter for the Cenomanian/Turonian, and allow investigation of paleoceanographic and climate events on a centennial time scale resolution, comparable to global change studies in the Quaternary.

Mid-Cretaceous paleogeography and paleoceanography of the Betic Seaway (Betic Cordillera, Spain)

Abstract Sedimentary and micropaleontological features of Cretaceous formations in the Betic Seaway have been examined. In the External Zone of the Betic Cordillera (Southern Spain) planktic and benthic foraminiferal assemblages were studied to establish a biostratigraphic framework and to provide estimates of paleo-waterdepth. Further environmental interpretations are based on the analysis of clay minerals, sedimentary petrography and organic geochemistry. We suggest the existence of several subbasins with differing subsidence histories created by transform tectonics along the axis of the Betic Seaway. A comparison with mid-Cretaceous sequences of adjacent DSDP/ODP holes indicates that the general paleoceanographic conditions within the seaway were similar to the North Atlantic. Our results are integrated in a paleogeographic reconstruction for the Western Mediterranean during the late Albian.

Paleoproductivity, ventilation, and organic carbon burial in the Santa Barbara Basin (ODP Site 893, off California) since the last glacial

Biogenic burial rates and coccolith assemblage data were generated for the past 16 ka at ODP Site 893 in the Santa Barbara Basin, off California, to determine if dysoxic events in the basin were related to changes in marine productivity. Coccolith abundance data show that changes in surface water conditions did indeed change in concert with oxygen levels at the sea floor. However, organic carbon burial rates varied independently, indicating that oxygenation of the water column is related mainly to intermediate water ventilation. A strong correlation of organic carbon burial rates with lithogenic, and to a lesser extent with carbonate and opal accumulation rates confirms recent conclusions that the export of organic carbon from the sea surface is largely controlled by the presence of ballast minerals.