ALBERT

Description: The oxygen isotopic composition of bulk chalk samples, the planktonic foraminifers Globotruncana and Rugoglobigerina, and the benthic bivalve Inoceramus are reported from the standard section of the Upper Cretaceous white chalk of Lägerdorf-Kronsmoor, NW Germany (Middle Coniacian to Lower Maastrichtian). The section shows the increasing impact of burial diagenesis with depth expressed by a significant negative trend in the oxygen isotopic values and increasing amounts of secondary, precipitated microspar. However, the biogenic components studied show different diagenetic characteristics. The planktonic foraminifers are completely recrystallized, whereas the prisms of Inoceramus are well preserved and only slightly overgrown by negligible amounts of secondary calcite. The original oxygen isotopic composition of the chalk is estimated on the basis of numerical elimination of the diagenetic trend and by use of a normalized carbonate content. It appears that the corrected isotopic signal of the bulk sediment, which primarily consisted of calcareous nannoplankton, significantly correlates with the isotopic composition of the Inoceramus prisms. Thus, the benthic bivalve Inoceramus and the nannoplankton probably lived in a water mass of the same isotopic composition. The Inoceramus oxygen isotope values suggest a mean temperature of approximately 16°C for the NW European Basin during the late Campanian and early Maastrichtian.

Description: During cruise ARK IV/3 with RV Polarstern (1987) volcanic rocks were recovered from the Nansen-Gakkel Ridge (NGR), a slow spreading (half rate approximately 0.5 cm) ridge with an axial depth of more than 5000 m. The NGR is one of the slowest and deepest mid-ocean ridges so far known and calculations based on the distance of sampling location from the axial valley yielded ages of approximately 600 ka for the rocks investigated here. According to petrographic and geochemical results i.e. spinifex textures, mg 〉 70 and MgO 〉 9 wt.%, the volcanics are termed komatiitic basalts. Dark spherical droplets of basanitic composition within the komatiitic basalts are believed to be relicts of an incomplete magma-mixing whose basanitic end-member could well account for the enriched character of the NGR basalts in terms of rare earth elements, Ti and incompatible trace elements. Based on Nd-isotope as well as high Sm/Nd ratios, mantle metasomatism (i.e. veined-mantle model) could be responsible for the enrichment of incompatible trace elements in the source region of komatiitic basalts of the NGR.

Description: The flux of reactive organic carbon (C(org)) into sediments of the southern and eastern Weddell Sea was estimated by modelling measured oxygen and nitrate pore-water profiles. Highest flux of reactive organic carbon into the sediment was calculated for the shelf region (500 and 600 mmol C m-2 year-1), whereas for pelagic and continental slope sediments C(org) fluxes of less than 60 mmol C m-2 year-1 and 100-200 mmol C m-2 year-1 respectively were calculated. The oxygen penetration depth (OPD) ranged from less than 2 cm in shelf sediments to much greater than 40 cm in pelagic sediments. For the first time, sediments covered by the Filchner Ice Shelf (probably cut off from a source of primary production for a few decades) were sampled. In this area a restricted vertical flux of reactive organic carbon was expected. However, the C(org), content of these sediments was as high as that of Antarctic shelf sediments, which suggests lateral transport of organic matter. In contrast, pore-water profiles and calculated reactive organic carbon fluxes off Filchner Ice Shelf are similar to those of much deeper depositional environments (3000-4000 m water depth).

Description: Crustal xenoliths in three Cenozoic volcanic fields of West Germany, the Northern Hessian Depression (NHD), the Eifel and the Urach/Hegau, include medium to high-grade meta sedimentary and felsic to mafic meta-igneous rocks. Also present in all three suites are pyroxenites and hornblendites. For each volcanic field, a model crustal profile is proposed based on calculated or measured P-wave velocities of xenoliths and depth-Vp relationships (EGT Central Segment and Rhenish Massif traverses). The xenolith lithologies from the NHD and the Eifel show some similarities. The middle crust between the depths of about 10 and 25 km consists mainly of meta-sediments, felsic gneisses and granulites. Meta-sedimentary rock types are particularly abundant in the Eifel at depths of between about 5 and 15 km but are less common within the NHD xenolith collection. The felsic gneisses range from meta-granites to meta-tonalites (I-and S-type). Eifel meta-sediments range from meta-pelites to meta-greywackes and meta-quartzites. The NHD xenolith suite contains a few highly depleted granulite-facies meta-sedimentary fragments. At depths between 24 and 26 km, the increase in Vp from about 6.8 to 〉 8 km s−1 (28–34 km) is correlated with the presence of mafic granulites intercalated with eclogites, pyroxenites and hornblendites. Beneath North Hessia, the granulite layer problably grades into a composite eclogite-peridotite layer at the lower part of this transition zone. The crust beneath the Urach/Hegau consists largely of meta-sediments with subordinate felsic meta-igneous rocks. Most of the meta-sedimentary samples seem to be depleted in felsic components, suggesting intra-crustal differentiation by partial melting. The Urach crust contains lithologies which are similar to the outcropping Moldanubian-type para-gneisses of the Black Forest. Mafic and ultramafic xenoliths from the Urach/Hegau differ in their mineralogy and chemical composition from the Eifel and NHD mafic granulites. They represent meta-cumulates derived from alkaline magmas which intruded the base of the crust and underwent deformation and recrystallization. Petrographic and chemical differences between Urach/Hegau and Eifel/NHD are believed to reflect the contrasting styles of crustal evolution in the Rhenohercynian and the Moldanubian belts of the Variscan orogen.

Description: An improved tectonic database for the South Atlantic has been compiled by combining magnetic anomaly, Geosat altimetry and onshore geologic data. We used this database to obtain a revised plate-kinematic model. Starting with a new fit-reconstruction for the continents around the South Atlantic, we present a high-resolution isochron map from Chron M4 to present. Fit reconstructions of South America and Africa that require rigid continental plates result in substantial misfits either in the southern South Atlantic or in the equatorial Atlantic. To achieve a fit without gaps, we assume a combination of complex rift and strike-slip movements: (1) along the South American Parana-Chacos Basin deformation zone (2) within marginal basins in South America (Salado, Colorado Basin) and (3) along the Benue Trough/Niger Rift system in Africa. These faults are presumed to have been active before or during the breakup of the continents. Our model describes a successive “unzipping” of rift zones starting in the southern South Atlantic. Between 150 Ma (Tithonian) and approximately 130 Ma (Hauterivian), rifting propagated to 38 °S, causing tectonic movements within the Colorado and Salado basins. Subsequently, between 130 Ma and Chron M4 (126.5 Ma), the tip of the South Atlantic rift moved to 28 °S, resulting in intracontinental deformation along the Parana-Chacos Basin deformation zone. Between Chron M4 and Chron MO (118.7 Ma) rifting propagated into the Benue Trough and Niger Rift, inducing rift and strike-slip motion. After Chron MO, the equatorial Atlantic began to open, while rifting and strike-slip motion still occurred in the Benue Trough and Niger Rift. Since Chron 34 (84 Ma), the opening of the South Atlantic is characterized by simple divergence of two rigid continental plates.

Description: In order to characterize the sediments of the South Kolbeinsey Ridge and to determine the influence of morpho- and hydrodynamic conditions in this area, a comprehensive sedimentological investigation was combined with detailed geochemical analyses. Lithological composition and grain size are mainly controlled by the input from two different sources: the submarine, active mid-ocean ridge and Iceland. Coarse-grained volcanic material dominates in the ridge area, whereas fine-grained detritus from Iceland characterizes the adjacent basin. Further, the distribution of these sediments is largely influenced by hydrodynamic conditions and redeposition processes. Strong bottom-water currents indicated by the benthic foraminifer species Cibicides lobatulus prevent extensive deposition of fine-grained material on the top of the ridge. The ridge slope is characterized by redeposited sediments of various grain sizes. Bulk-sediment chemical analyses show element distribution patterns that are strongly correlated with the ratio of volcanic vs. detritic particle composition. This ratio reflects particle input and distribution processes. Two element associations are notable: elements coupled with (1) ridge-derived volcanic particles and (2) the detrital 〈2 μm fraction which reflects weathered material from Iceland.

Description: Helium (RA = 3.0 to 5.6) and carbon (δ13C from −7.2 to −3.4‰) isotopic compositions, and relative CO2, CH4, N2, He and Ar contents of CO2-rich gases from Lakes Nyos and Monoun, Cameroon; Laacher See, Germany; Dieng Volcanic Plateau, Indonesia, and a well at Mt. Gambier, Australia, point to a common, essentially magmatic origin. Absorption of the original magmatic gases into deeply circulating groundwater and equilibration of the resulting solutions with crustal rock at temperatures of about 300°C fix CO2 and CH4 contents. On further rise, the solutions start to boil separating gas-rich vapors which, on encountering an impermeable barrier, may accumulate to form gas pockets with steadily increasing pressures. In the case of sufficiently high gas contents, the pressures may exceed lithostatic pressures leading to a blow-out or a “pneumatic” eruption (Dieng). Otherwise, gas may accumulate to form a stable pocket (Mt. Gambier). Minor leakage from such pockets may lead to surface discharges of CO2-rich gases as at Laacher See, re-absorption into shallow groundwater to the formation of the low-salinity, CO2-charged waters encountered at Lakes Nyos and Monoun. The occurrence of these high-CO2, low-temperature systems is likely to be favored in tectonically active regions, allowing deep, possibly mantle gases to rise, but with sufficiently low regional heat flows to prevent the establishment of large-scale geothermal activity.

Description: The inner Scotian Shelf off the Eastern Shore of Nova Scotia forms an irregular surface that extends some 25–30 km seaward of the present coastline to water depths of about 100 m where it drops off into Emerald Basin. The distribution of Late Quaternary deposits is highly variable both along and across the shelf. These sediments preserve a record of Late Wisconsinan glaciation, ice recession, and late- and post-glacial changes of relative sea level. Glaciomarine deposits occur in a valley complex extending seaward from Halifax Harbour. East of Halifax, we observe a three-part zonation across the inner shelf. The innermost zone extends to water depths of about 50 m. It is characterized by acoustic basement (Meguma Group metasediments), either outcropping or overlain by acoustically unstratified deposits, interpreted as glacial diamict, and by a unit interpreted as stratified outwash. These units are overlain by stratified valleyfill deposits representing Holocene lacustrine and estuarine facies, which have been sampled in a number of cores. Much of the inner shelf is covered by a thin veneer of sand and gravel, generally less than 1 m thick. Further seaward, the sea floor is an erosional unconformity that truncates acoustic units interpreted as glacial diamict and stratified drift. The stratified estuarine deposits found in the inner zone appear to be absent here, but thin patches of transgressive lag deposits occur throughout the area. The outer part of the inner shelf is dominated by outcrop of acoustic basement, with very limited surficial sediment cover. This zone of rugged outcrop occupies half or more of the inner shelf width over much of the study area. The extensive outcrop is attributed to a combination of glacial and/or glaciofluvial erosion, limited recessional deposition, and reworking of any remaining sediment cover by energetic long-period surface gravity waves under lower postglacial sea levels. Seaward of the outcrop zone, there is a transitional area between the inner shelf and Emerald Basin. This zone is characterized by high relief, with exposures of acoustic basement rising up to 60 m above intervening depressions. The depressions are partially filled by stratified glaciomarine and marine deposits up to 55 m thick. Lateral transitions between stratified and unstratified facies along a morainal ridge in northern Emerald Basin suggest the presence of a partially grounded floating ice margin in this area. Late-glacial relative sea level changes remain poorly defined. Several lines of evidence suggest high relative sea level early in the process of glacial recession from the inner shelf, followed by a rapid drop resulting from glacio-isostatic rebound. Samples of estuarine and salt-marsh deposits collected in cores from the inner part of the inner shelf provide evidence of Holocene marine transgression from below −40 m at 11,000 years BP, continuing to the present.

Description: Carbonate deposition at two core sites in the subarctic Pacific (48°N, 133°W; 2.9 km and 3.7 km water depth) follows the standard Pacific carbonate cycles, with glacial values being increased over interglacial values. Benthicδ13C follows the global trend; that is, glacial values are more negative than interglacial values. Comparison with the benthicδ13C record of North Atlantic DSDP Site 552 (56°N, 23°W; 2.3 km water depth) shows the North Pacific records to be nearly in phase with and continuously more negative relative to the North Atlantic record. This suggests that concentrations of∑CO2(org) were permanently higher in the North Pacific than in the North Atlantic during the past 750,000 years conceivably supporting the hypothesis that there was no deep-water forming in the late Pleistocene North Pacific. Whereas one would expect that the North Pacific deep waters were continuously more corrosive to carbonates than deep waters in the North Atlantic, carbonate deposition at the deep North Pacific core sites is enhanced during glacial periods, and occasionally higher than at shallow North Atlantic Site 552 even though Site 552 was probably above lysocline-depth during most of the late Pleistocene. This apparent paradox can be resolved only by invoking an increase in alkalinity in the glacial North Pacific which would have increased the degree of carbonate ion saturation and thereby improved the state of carbonate preservation.

Description: We report the Sr, Nd and Pb isotopic compositions (1) of 66 lava flows and dikes spanning the circa 15 Myr subaerial volcanic history of Gran Canaria and (2) of five Miocene through Cretaceous sediment samples from DSDP site 397, located 100 km south of Gran Canaria. The isotope ratios of the Gran Canaria samples vary for 87Sr/86Sr: 0.70302–0.70346, for 143Nd/144Nd: 0.51275–0.51298, and for 206Pb/204Pb: 18.76–20.01. The Miocene and the Pliocene-Recent volcanics form distinct trends on isotope correlation diagrams. The most SiO2-undersaturated volcanics from each group have the least radiogenic Sr and most radiogenic Pb, whereas evolved volcanics from each group have the most radiogenic Sr and least radiogenic Pb. In the Pliocene-Recent group, the most undersaturated basalts also have the most radiogenic Nd, and the evolved volcanics have the least radiogenic Nd. The most SiO2-saturated basalts have intermediate compositions within each age group. Although the two age groups have overlapping Sr and Nd isotope ratios, the Pliocene-Recent volcanics have less radiogenic Pb than the Miocene volcanics. At least four components are required to explain the isotope systematics of Gran Canaria by mixing. There is no evidence for crustal contamination in any of the volcanics. The most undersaturated Miocene volcanics fall within the field for the two youngest and westernmost Canary Islands in all isotope correlation diagrams and thus appear to have the most plume-like (high 238U/204Pb) HIMU-like composition. During the Pliocene-Recent epochs, the plume was located to the west of Gran Canaria. The isotopic composition of the most undersaturated Pliocene-Recent volcanics may reflect entrainment of asthenospheric material (with a depleted mantle (DM)-like composition), as plume material was transported through the upper asthenosphere to the base of the lithosphere beneath Gran Canaria. The shift in isotopic composition with increasing SiO2-saturation in the basalts and degree of differentiation for all volcanics is interpreted to reflect assimilation of enriched mantle (EM1 and EM2) (cf. [1]) in the lithosphere beneath Gran Canaria. This enriched mantle may have been derived from the continental lithospheric mantle beneath the West African Craton by thermal erosion or delamination during rifting of Pangaea. This study suggests that the enriched mantle components (EM1 and EM2) may be stored in the shallow mantle, whereas the HIMU component may have a deeper origin.