Geochemistry of dikes and lavas from the north wall of the Hess Deep Rift

An investigation of ~1-m.y.-old dikes and lavas from the north wall of the Hess Deep Rift (2°15'N, 101°30'W) collected during Alvin expeditions provides a detailed view of the evolution of fast spreading oceanic crust. The study area encompasses 25 km of an east-west flow line, representing ~370,000 years of crustal accretion at the East Pacific Rise. Samples analyzed exhibit depleted incompatible trace element abundances and ratios [(La/Sm)N < 1]. Indices of fractionation (MgO), and incompatible element ratios (La/Sm, Nb/Ti) show no systematic trends along flow line. Rather, over short (<4 m) and long (~25 km) distances, significant variations are observed in major and trace element concentrations and ratios. Modeling of these variations attests to the juxtaposition of dikes of distinct parental magma compositions. These findings, combined with studies of segmentation of the subaxial magma chamber and lateral magma transport in dikes along rift-dominated systems, suggest a more realistic model of the magmatic system underlying the East Pacific Rise relative to the commonly assumed twodimensional model. In this model, melts from a heterogeneous mantle feed distinct portions of a segmented axial magma reservoir. Dikes emanating from these distinct reservoirs transport magma along axis, resulting in interleaved dikes and host lavas with different evolutionary histories. This model suggests the use of axial or flow line lava compositions to infer the evolution of axial magma chambers should be approached with caution because dikes may never erupt lava or may transport magma significant distances along axis and erupt lavas far from their axial magma chamber of origin.

Magnetic properties of basalts from DSDP Hole 69-504B

The NRM intensity, AF demagnetization characteristics, hysteresis parameters, initial susceptibility, and thermomagnetic characteristics of 18 basalt specimens from Deep Sea Drilling Project Hole 504B were determined. In six samples, the grain size was large enough to allow microprobe analysis. We conclude (1) that the dominant magnetic mineral is titanomagnetite/titanomaghemite; (2) that, except for the upper few meters of the core where the grains are in the stable monodomain state, the grain size of the magnetic mineral lies in the pseudo-single-domain range (< 10 µm); (3) that maghemitization (i.e., low-temperature (< 350°C) oxidation) has taken place. We discuss possible geological histories.

Deep drilling of glaciers: Russian projects in the Arctic (1975-1995)

The data collection "Deep Drilling of Glaciers: Soviet-Russian projects in Arctic, 1975-1995" was collected by the following basic considerations: - compilation of deep (>100 m) drilling projects on Arctic glaciers, using data of (a) publications; (b) archives of IGRAN; (c) personal communication of project participants; - documentation of parameters, references. Accuracy of data and techniques applied to determine different parameters are not evaluated. The accuracy of some geochemical parameters (up to 1984 and heavy metalls) is uncertain. Most reconstructions of ice core age and of annual layer thickness are discussed; - digitizing of published diagrams (in case, when original numerical data were lost) and subsequent data conversion to equal range series and adjustment to the common units. Therefore, the equal-range series were calculated from original data or converted from digitized chart values as indicated in the metadata. For the methodological purpose, the equal-range series obtained from original and reconstructed data were compared repeatedly; the systematic difference was less then 5-7%. Special attention should be given to the fact, that the data for individual ice core parameters varies, because some parameters were originally measured or registered. Parameters were converted in equal-range series using 2 m steps; - two or more parameter values were determined, then the mean-weighted (i.e. accounting the sample length) value is assigned to the entire interval; - one parameter value was determined, measured or registered independently from the parameter values in depth intervals which over- and underlie it, then the value is assigned to the entire interval; - one parameter value was determined, measured or registered for two adjoining depth intervals, then the specific value is assigned to the depth interval, which represents >75% of sample length ; if each of adjoining depth intervals represents <75% of sample length, then the correspondent parameter value is assigned to both intervals of depth. This collection of ice core data (version 2000) was made available through the EU funded QUEEN project by S.M. Arkhipov, Moscow.

Magnetic properties, grain sizes and minerals at DSDP Site 83-504B

Twenty-seven samples from the Leg 83 section of Hole 504B have been investigated using magnetic, optical, and electron optical methods. The primary magnetic mineral to crystallize was titanomagnetite of approximate composition Fe2.4Ti0.6O4 (TM60), but none survives, nor is there evidence of titanomaghemite produced by oxidation of TM60. The average measured magnetic properties can be interpreted in terms of magnetite, Fe3O4, having average grain size of <1 µm and present in average volume concentration of - 0.5%. The intensity of the natural remanent magnetization (NRM) of the rocks could also be accounted for as being a thermoremanence carried by this mineral. Although the heterogeneity of the titanomagnetite grains could be detected optically, the texture of the intergrown phases is poorly developed. In some samples from the massive units of the lower part of the section, trellis patterns were visible. The Fe3O4 present in the intergrowths is too intimately mixed with the other intergrown phases to be revealed by electron microprobe analysis that simply returns the bulk composition of the intergrowth (oxidized TM60). The path by which the mineral assemblage evolved from TM60 to an Fe304-containing intergrowth, under the temperature and pressure conditions obtaining in the Leg 83 section, makes interesting speculation. Deuteric oxidation, maghemitization/inversion, or some hypothetical low-temperature/high-pressure oxidation by a leaching-of-iron process may all play roles.

Germanium contents, Ge/Si ratios, and Sr isotopic composition in deep-sea cherts

Nineteen chert samples from a continuous core of the DSDP (Leg 17, Hole 167) were analysed for Ge; in addition we analysed five samples from other cores. The ages range between Late Jurassic, and Late Eocene. The concentration of Ge changes with age from 0.87 ppm in the oldest samples to 0.23 ppm in the youngest (equivalent to a Ge/Si decrease from 0.00000072 to 0.00000019). The decrease in Ge/Si is well correlated with the 87Sr/86Sr ratio in sea water of the relevant age. The interpretation of this trend may reflect: (a) different levels of Ge/Si in sea water as a result of a different ratio between hydrothermal and riverine input, (b) a diagenetic trend in siliceous sediments, (c) recording (by radiolaria) a transition between a radiolaria dominated ocean (with relatively high Ge/Si ratios in sea water) and diatom domination or (d) a combination of the above.

Chemical and Sr isotopic compositions of 3 step leachates and residues of bulk samples, zeolites, and smectites from Lower Miocene sediments at ODP Site 166-1007

Massive clinoptilolite authigenesis was observed at about 1105 meters below sea floor (mbsf) in lower Miocene wellcompacted carbonate periplatform sediments from the Great Bahama Bank [Ocean Drilling Program, ODP Leg 166, Site 1007]. The diagenetic assemblage comprises abundant zeolite crystallized within foraminifer tests and sedimentary matrix, as well as Mg smectites. In carbonate-rich deposits, the formation of the zeolite requires a supply of silica. Thus, the objective of the study is to determine the origin of the silica supply, its diagenetic evolution, and consequently the related implications on interpretation of the sedimentary record, in terms of local or global paleoceanographic change. For lack of evidence for any volcaniclastic input or traces of Si-enriched deep fluids circulation, an in situ biogenic source of silica is validated by isotopic data and chemical modeling for the formation of such secondary minerals in shallow-water carbonate sequences. Geochemical and strontium isotopic data clearly establish the marine signature of the diagenetic zeolite, as well as its contemporaneous formation with the carbonate deposition (Sr model ages of 19.6-23.2 Ma). The test of saturation for the pore fluids specifies the equilibrium state of the present mineralogical assemblage. Seawater-rock modeling specifies that clinoptilolite precipitates from the dissolution of biogenic silica, which reacts with clay minerals. The amount of silica (opal-A) involved in the reaction has to be significant enough, at least 10 wt.%, to account for the observed content of clinoptilolite occurring at the most zeolite-rich level. Modeling also shows that the observed amount of clinoptilolite (~19%) reflects an in situ and short-term reaction due to the high reactivity of primary biogenic silica (opal-A) until its complete depletion. The episodic occurrence of these well-lithified zeolite-rich levels is consistent with the occurrence of seismic reflectors, particularly the P2 seismic sequence boundary located at 1115 mbsf depth and dated as 23.2 Ma. The age range of most zeolitic sedimentary levels (biostratigraphic ages of 21.5-22 Ma) correlates well with that of the early Miocene glaciation Mi-1 and Mi-1a global events. Thus, the clinoptilolite occurrence in the shallow carbonate platform environment far from volcanogenic supply, or in other sensitive marine areas, is potentially a significant new proxy for paleoproductivity and oceanic global events, such as the Miocene events, which are usually recognized in deep-sea pelagic sediments and high latitude deposits.

Global habitat suitability of deep-water Antipatharia

Antipatharia are a diverse group of corals with many species found in deep water. Many Antipatharia are habitat for associates, have extreme longevity and some species can occur beyond 8500 m depth. As they are major constituents of 'coral gardens', which are Vulnerable Marine Ecosystems (VMEs), knowledge of their distribution and environmental requirements is an important pre-requisite for informed conservation planning particularly where the expense and difficulty of deep-sea sampling prohibits comprehensive surveys. This study uses a global database of Antipatharia distribution data to perform habitat suitability modelling using the Maxent methodology to estimate the global extent of black coral habitat suitability. The model of habitat suitability is driven by temperature but there is notable influence from other variables of topography, surface productivity and oxygen levels. This model can be used to predict areas of suitable habitat, which can be useful for conservation planning. The global distribution of Antipatharia habitat suitability shows a marked contrast with the distribution of specimen observations, indicating that many potentially suitable areas have not been sampled, and that sampling effort has been disproportionate to shallow, accessible areas inside marine protected areas (MPAs). Although 25% of Antipatharia observations are located in MPAs, only 7-8% of predicted suitable habitat is protected, which is short of the Convention on Biological Diversity target to protect 10% of ocean habitats by 2020.

Physical properties of deep sea sediments

Erstmals quantitativ bearbeitete Rutschungen aus dem Tiefseebereich des äquatorialen Ostatlantiks liegen auf Hängen von 0,4 Grad und 0,7 Grad - das ist sehr viel flacher als die für statische Rutschungsauslösung benötigte kritische Hangneigung (14 Grad bis 16 Grad ). Im Gegensatz zu Flachwassergebieten kann bei Wassertiefen von über 4000 m natürlich der Einfluß von Wellenwirkung und Tidenhub auf die Hangstabilität vernachlässigt werden. Die Sedimentationsraten sind in diesem Bereich zur Bildung eines Porenwasserüberdruckes vielfach zu niedrig. Nach den Hangstabilitätsanalysen bilden hier Erdbeben den wirksamsten Auslösemechanismus für die Rutschungen. Dies gilt auch für Rutschungen an den Kontinentalrändern von Nordwest- und Westafrika sowie für das europäische Nordmeer und für Rutschungen im nördlichen Fidji-Becken. Das Alter der besonders gut datierten Rutschungen vom nordwestafrikanischen Kontinentalrand und der Tiefsee des äquatorialen Ostatlantiks schwankt zwischen 16,000 J.v.h. und 18,000 J.v.h. sowie etwa 130,000 J.v.h.. Es handelt sich dabei um Phasen des Beginns besonders starker Meeresspiegelschwankungen. Ein Vergleich der Meeresspiegelkurve mit dem Alter älterer Rutschungen zeigt ebenfalls eine Parallele mit Zeiten von Regressionen und Transgressionen. Durch die Meeresspiegelschwankungen werden isostatische Vertikalbewegungen des Tiefseebodens von bis zu 30 m bewirkt, die Spannungen in den Lithosphärenplatten erzeugen. Sie allein sind jedoch nicht groß genug, um Brüche in intakten Plattenbereichen zu verursachen. Entlang alter, ehemals aktiver Transform-Bruchzonen (Fracture Zones) können jedoch die aufgebauten Spannungen eher wieder abgebaut werden. Dabei entstehen kleinere Erdstösse und führen zur Auslösung von Rutschungen. Ein Vergleich der Verbreitung von Transform-Störungen und Rutschungen vor Norwegen, Nordwest- und Westafrika sowie vor dem südlichen Afrika zeigt, daß in diesen Gebieten Rutschmassen tatsächlich besonders häufig entlang und in der Verlängerung von Fracture Zones auftreten. Modellrechnungen, die mit typischen Werten für Hangwinkel (0,5 Grad bis 3 Grad) von Tiefseeböschungen und passive Kontinentalränder sowie für häufig ermittelte Scherfestigkeitsgradienten im Sediment (0,5 kPa/m bis 1,7 kPa/m) durchgeführt wurden, ergaben, daß in Gebieten mit normal konsolidierten Sedimenten (ohne Porenwasserüberdruck) nur Erdbeben Rutschungen ausgelöst haben können.

Response of deep-sea benthic foraminifera to Late Quaternary climate changes, southeast Indian Ocean, offshore Western Australia

The Late Quaternary benthic foraminifera of four deep-sea cores off Western Australia (ODP 122-760A, ODP 122-762B, BMR96GC21 and RC9-150) have been examined for evidence of increased surface productivity to explain the anomalously low sea-surface paleotemperatures inferred by planktic foraminifera for the last and penultimate glaciations. The delta13C trends of Cibicidoides wuellerstorfi, and differences between the delta13C trends of planktics (Globigerinoides sacculifer) and benthics (C. wuellerstorfi) in the four cores indicate that during stage 6 bottom waters were significantly depleted in delta13C, and strong delta13C gradients were established in the water column, while during stage 2 and the Last Glacial Maximum, delta13C trends did not differ greatly from that of the Holocene. Two main assemblages of benthic foraminifera were identified by principal component analyses: one dominated by Uvigerina peregrina, another dominated by U. proboscidea. Abundance of these Uvigerinids, and of taxa preferring an infaunal microhabitat, and of Epistominella exigua and Bulimina aculeata indicate that episodes of high influx of particulate organic matter were established in most sites during glacial episodes, and particularly so during stage 6, while evidence for upwelling during the Last Glacial Maximum is less strong. The Penultimate Glaciation upwellings were established within the areas of low sea-surface paleotemperature indicated by planktic foraminifera. During the Last Interglacial Climax, upwelling appears to have been established in an isolated region offshore from a strengthened Leeuwin Current off North West Cape. Last Glacial Maximum delta13C values of C. wuellerstorfi at waterdepths of less than 2000 m show smaller than global mean glacial-interglacial changes suggesting the development of a deep hydrological front. A similar vertical stratification/bathyal front was also established during the Penultimate Glaciation.

Quaternary carbonate and stable isotope record of ODP Holes 133-817A and 133-818B

The Quaternary history of metastable CaCO3 input and preservation within Antarctic Intermediate Water (AAIW) was examined by studying sediments from ODP Holes 818B (745 mbsl) and 817A (1015 mbsl) drilled in the Townsville Trough on the southern slope of the Queensland Plateau. These sites lie within the core of modern AAIW, and near the aragonite saturation depth (~1000 m). Thus, they are well positioned to monitor chemical changes that may have occurred within this watermass during the past 1.6 m.y. The percent of fine aragonite content, percent of fine magnesian calcite content, and percent of whole pteropods (>355 µm) were used to separate the fine aragonite input signal from the CaCO3 preservation signal. Stable d18O and d13C isotopic ratios were determined for the planktonic foraminifer Globigerinoides sacculifer and, in Hole 818B, for the benthic foraminifer Cibicidoides spp. to establish the oxygen isotope stratigraphy and to study the relationship between intermediate and shallow water d13C of Sum CO2 and the relationship between benthic foraminiferal d13C and CaCO3 preservation within intermediate waters of the Townsville Trough. Data were converted from depth to age using oxygen isotope stratigraphy, nannostratigraphy, and foraminiferal biostratigraphy. Several long hiatuses and the absence of magnetostratigraphy did not permit time series analysis. The principal results of the CaCO3 preservation study include the following (1) a general increase in CaCO3 preservation between 0.9 and 1.6 Ma; (2) a CaCO3 dissolution maximum near 0.9 Ma, primarily expressed in the Hole 818B fine aragonite record; (3) an abrupt and permanent increase of fine aragonite content between 0.86 and 0.875 Ma in both Holes 818B and 817A probably reflecting a dramatic increase of fine carbonate sediment production on the Queensland Plateau; (4) an improvement in CaCO3 preservation near 0.87 Ma, which accompanied the increase of sediment input, indicated by the first appearance of whole pteropods in the deeper Hole 817A and a "spike" in the percent whole pteropods in Hole 818B; (5) a period of strong CaCO3 dissolution during the mid-Brunhes Chron from 0.36 to 0.41 Ma; and (6) a complex CaCO3 preservation pattern between 0.36 Ma and the present characterized by a general increase in CaCO3 preservation through time with good preservation during interglacial stages and poor preservation during glacial stages. The long-term aragonite preservation histories for Holes 818B and 817A appear to be similar in general shape, although different in detail, to CaCO3 preservation records from the deep Indian and central equatorial Pacific oceans as well as from intermediate water sites in the Bahamas and the Maldives. All of these areas have experienced CaCO3 dissolution at about 0.9 Ma and during the mid-Brunhes Chron. However, the late Quaternary (0 to 0.36 Ma) glacial to interglacial preservation pattern in Holes 818B and 817A is out of phase with CaCO3 preservation records for sediments deposited in Pacific deep and bottom waters. The sharp increase in bank production and export from the Queensland Plateau and the coincident improvement of CaCO3 preservation between 0.86 and 0.875 Ma may have been synchronous with the initiation of the Great Barrier Reef and roughly coincides with an increase in carbonate accumulation on the Bahama banks, in the western North Atlantic Ocean, and on Mururoa atoll, in the central South Pacific Ocean. The development of these reef systems during the middle Quaternary may be related to the transition in the frequency and amplitude of global sea level change from 41 k.y. low amplitude cycles prior to 0.9 Ma to 100 k.y. high amplitude cycles after 0.73 Ma. Carbon isotopic analyses show that benthic foraminiferal d13C values (Cibicidoides spp.) have been heavier than planktonic foraminiferal d13C values (G. sacculifer) throughout most of the last 0.54 m.y., which may indicate that 13C-enriched intermediate water (AAIW) occupied the Townsville Trough during much of the late Quaternary. Furthermore, both planktonic and benthic foraminiferal d13C values are often observed to be heaviest during interglacial to glacial transitions, and lightest during glacial to interglacial transitions. We suggest that this pattern is the result of changes in the preformed d13C of Sum CO2 of AAIW and may reflect changes in nutrient utilization by primary producers in Antarctic surface waters, changes in the d13C of upwelled Circumpolar Deep Water, or changes in the extent and/or temperature of equilibration between surface water and atmospheric CO2 within the Antarctic Polar Frontal Zone (the source area for AAIW). Finally, the poor correlation between percent of whole pteropods (aragonite preservation) and d13C of Cibicidoides spp. may be the result of a decoupling of d13C from CO2 due to the numerous and complex variables that combine to produce the preformed d13C of AAIW.

Composition of sediments and cores from DSDP Legs 54 and 61-63

The monograph gives results of studies of sediments and rocks collected from D/S Glomar Challenger in the Pacific Ocean. These studies have been based on the lithological facial analysis applied for the first time for identificating genesis of ocean sediments. These results include new ideas on formation of the Earth's sedimentary cover and can be used for constructing regional and global schemes of ocean paleogeography, reconstructing some structures, correlating sedimentation on continents and in oceans, estimating perspectives of oil- and gas-bearing deposits and ore formation. The monograph also gives the first petrographic classification of organic matter in black shales.