Mineral and chemical compositions of clay sediments from the Pacific Ocean, DSDP data

Mineral and chemical compositions, as well as conditions of formation of clay sediments in major structural elements of the Pacific Ocean floor with different ages are under consideration in the monograph. Depending on evolution of the region two ways of clay sediment formation are identified: terrigenous and authigenic. It is shown that terrigenous clay sediments predominate in marginal parts of the Pacific Ocean. Authigenic mineral formation occurring in the basal part of the sedimentary cover primarily results from removal of material from underlying basalts. This material is released during secondary alteration of the basalts due to their interaction with sea water, as well as with deep solutions.

Oliogocene to Pleistocene benthic foraminifera in ODP Leg 121 sites

Oligocene to Pleistocene bathyal benthic foraminifers at Broken Ridge (Site 754) and Ninetyeast Ridge (Site 756), eastern Indian Ocean, were investigated for then- stratigraphic distribution and their response to paleoceanographic changes. Q-mode factor analysis was applied to relative abundance data of the most abundant benthic foraminifers. At Site 754, seven varimax assemblages were recognized from the upper Oligocene to the Pleistocene: the Gyroidina orbicularis-Rectuvigerina striata Assemblage in the uppermost Oligocene; the Lenticulina spp. Assemblage in the upper Oligocene to lower Miocene, and in lower Miocene to lowermost middle Miocene; the Burseolina cf. pacifica-Cibicidoides mundulus Assemblage in the lower Miocene; the Planulina wuellerstorfi Assemblage in the upper middle Miocene; the Globocassidulina spp. Assemblage in the upper Miocene; the Gavelinopsis lobatulus-Uvigerina proboscidea Assemblage in the Pliocene; and the Ehrenbergina spp. Assemblage in the Pleistocene. The major faunal changes are complex, but exist between the Lenticulina spp. Assemblage and the P. wuellerstorfi Assemblage at ~13.8 Ma, and between the Ehrenbergina spp. Assemblage and the G. lobatulus Assemblage at ~5 Ma. The development of the P. wuellerstorfi and Globocassidulina spp. Assemblages after 13.8 Ma is correlated with the decrease in temperature of the intermediate waters of the ocean, in turn related to Antarctic glacial expansion. The faunal changes at ~5 Ma are related to the development of low oxygen intermediate water, formed in the presence of a strong thermocline. At Site 756, six varimax assemblages are distributed as follows: the Cibicidoides cf. mundulus-Oridorsalis umbonatus Assemblage in the lower Oligocene; the Epistominella umbonifera-Cibicidoides mundulus Assemblage from the upper Oligocene to the lower Miocene; the Cibicidoides mundulus-Burseolinapacifica Assemblage from lower Miocene to the lower middle Miocene; the Globocassidulina spp. Assemblage from the upper lower Miocene to the Pliocene; the Uvigerina proboscidea Assemblage in the upper Miocene and the Pliocene; and the Globocassidulina sp. D Assemblage in the Pliocene. The main faunal change at this site is between the E. umbonifera Assemblage and the Globocassidulina spp. Assemblage, at ~17.1 Ma. The timing of this faunal change is coeval with faunal changes in the North Atlantic and the Pacific. The change is related to a change in bottom water characteristics caused by an increased influence of carbonate corrosive water from the Antarctic source region, and a change in surface productivity. A low oxygen event at Site 756, which started at about 7.3 Ma, occurred about 2.3 m.y. before that at Site 754. The different response to global paleoceanographic changes is not yet explained, but may be due to the difference of marine topography and the degree of upwelling

Chemical composition of DSDP and ODP cherts

Rare earth element (REE), major, and trace element abundances and relative fractionations in forty nodular cherts sampled by the Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) indicate that the REE composition of chert records the interplay between terrigenous sources and scavenging from the local seawater. Major and (non-REE) trace element ratios indicate that the aluminosilicate fraction within the chert is similar to NASC (North American Shale Composite), with average Pacific chert including ~7% NASC-like particles, Indian chert ~11% NASC, Atlantic chert ~17% NASC, and southern high latitude (SHL) chert 53% NASC. Using La as a proxy for sum REE, approximations of excessive La (the amount of La in excess of that supplied by the detrital aluminosilicate fraction) indicate that Pacific chert contains the greatest excessive La (85% of total La) and SHL chert the least (38% of total La). As shown by interelement associations, this excessive La is most likely an adsorbed component onto aluminosilicate and phosphatic phases. Accordingly, chert from the large Pacific Ocean, where deposition occurs relatively removed from significant terrigenous input, records a depositional REE signal dominated by adsorption of dissolved REEs from seawater. Pacific chert Ce/Ce* <<1 and normative La/Yb ~ 0.8-1, resulting from adsorption of local Ce-depleted seawater and preferential adsorption of LREEs from seawater (e.g., normative La/Yb ~0.4), which increases the normative La/Yb ratio recorded in chert. Chert from the Atlantic basin, a moderately sized ocean basin lined by passive margins and with more terrigenous input than the Pacific, records a mix of adsorptive and terrigenous REE signals, with moderately negative Ce anomalies and normative La/Yb ratios intermediate to those of the Pacific and those of terrigenous input. Chert from the SHL region is dominated by the large terrigenous input on the Antarctic passive margin, with inherited Ce/Ce* ~1 and inherited normative La/Yb values of ~1.2-1.4. Ce/Ce* does not vary with age, either throughout the entire data base or within a particular basin. Overall, Ce/Ce* does not correlate with P2O5 concentrations, even though phosphatic phases may be an important REE carrier.

Revised chronology of Neogene DSDP holes from the World Ocean

After nearly 30 years of growth in geochronologic knowledge, the originally published age models for many older deep sea marine sections have become badly outdated. In this report we present newly revised age models for Neogene sediments from 94 DSDP holes. Biostratigraphic data for planktonic foraminifers, calcareous nannofossils, diatoms and radiolarians, paleomagnetic and other stratigraphic data were compiled from the original Initial Reports volumes of DSDP. The Berggren et al. (1985 doi:10.1130/0016-7606(1985)96<1407:CG>2.0.CO;2) scale was used for the age of magnetic reversals, and a variety of recent papers were used to establish a standard modern set of calibrations for marine microfossil events to the magnetic reversal scale. New age vs depth plots were made for each hole, and for each a new line of correlation was created. All tabulated stratigraphic data, new age models, and age depth plots are given as appendices to the report.

Geochemistry of marine sediments of the world's major subduction zones

This paper presents new major and trace-element data and Lu-Hf and Sm-Nd isotopic compositions for representative suites of marine sediment samples from 14 drill sites outboard of the world's major subduction zones. These suites and samples were chosen to represent the global range in lithology, Lu/Hf ratios, and sediment flux in subducting sediments worldwide. The data reported here represent the most comprehensive data set on subducting sediments and define the Hf-Nd isotopic variations that occur in oceanic sediments and constrain the processes that caused them. Using new marine sediment data presented here, in conjunction with published data, we derive a new Terrestrial Array given by the equation, epsilon-Hf = 1.55 * epsiolon-Nd + 1.21. This array was calculated using >3400 present-day Hf and Nd isotope values. The steeper slope and smaller y-intercept of this array, compared to the original expression (epsilon-Hf = 1.36 * epsilonNd + 2.89; Vervoort et al., 1999, doi:10.1016/S0012-821X(99)00047-3) reflects the use of present day values and the unradiogenic Hf of old continental samples included in the array. In order to examine the Hf-Nd isotopic variations in marine sediments, we have classified our samples into 5 groups based on lithology and major and trace-element geochemical compositions: turbidites, terrigenous clays, and volcaniclastic, hydrothermal and hydrogenetic sediments. Compositions along the Terrestrial Array are largely controlled by terrigenous material derived from the continents and delivered to the ocean basins via turbidites, volcaniclastic sediments, and volcanic inputs from magmatic arcs. Compositions below the Terrestrial Array derive from unradiogenic Hf in zircon-rich turbidites. The anomalous compositions above the Terrestrial Array largely reflect the decoupled behavior of Hf and Nd during continental weathering and delivery to the ocean. Both terrigenous and hydrogenetic clays possess anomalously radiogenic Hf, reflecting terrestrial sedimentary and weathering processes on the one hand and marine inheritance on the other. This probably occurs during complementary processes involving preferential retention of unradiogenic Hf on the continents in the form of zircon and release of radiogenic Hf from the breakdown of easily weathered, high Lu-Hf phases such as apatite.

Revised Cenozoic hiatus database

The differential effects of climate change, sea level, and water mass circulation on deposition/erosion of marine sediments can be constrained from the distribution of unconformities in the world's oceans. I identified temporal and depth patterns of hiatuses ("hiatus events") from a large and chronologically well constrained stratigraphic database of deep-sea sediments. The Paleogene is characterized by few, several million year long hiatuses. The most significant Cenozoic hiatus event spans most of the Paleocene. The Neogene is characterized by short, frequent hiatus events nearly synchronous in shallow and deep water sediments. Epoch boundaries are characterized by peaks in deep water hiatuses possibly caused by an increased circulation of corrosive bottom water and sediment dissolution. The Plio-Pleistocene is characterized by a gradual decrease in the frequency of hiatuses. Future studies will focus on the regional significance of the hiatus events and their possible causes.

Magnetic state of ocean basalts (Table 1)

The natural remanent magnetization (NRM) of ocean basalts, giving rise to the pattern of marine magnetic anomalies, is known to be of comparatively low intensity for about 20 Ma old oceanic crust. The aim of this study is to detect possible peculiarities in the rock magnetic properties of ocean basalts of this age, and to establish a link between magnetomineralogy, rock magnetic parameters, and the low NRM intensity. Ocean basalts covering ages from 0.7 to 135 Ma were selected for rock magnetic experiments and their room temperature hysteresis parameters, Curie temperature and temperature dependence of saturation magnetization MS(T) was determined and complemented by reflected light microscopy. The majority of samples is magnetically dominated by titanomagnetite and titanomaghemite with increasing oxidation state with age. For these, a strong dependence of hysteresis parameters on the age of the samples is found. The samples have a minimum in saturation magnetization and a maximum in magnetic stability in the age interval ranging from approximately 10 to 40 Ma, coinciding with the age interval of low NRM intensity. The observed change in saturation magnetization is in the same order as that for the NRM intensity. A further peculiarity of the titanomaghemites from this age interval is the shape of their MS(T) curves, which display a maximum above room temperature (Neel P-type) and, sometimes, a self-reversal of magnetization below room temperature (Neel N-type). These special rock magnetic properties can be explained by titanomagnetite low-temperature oxidation and highly oxidized titanomaghemites in the age interval 10-40 Ma. A corresponding measurement of the NRM at elevated temperature allows to identify a maximum in NRM intensity above room temperature for the samples in that age interval. This provides evidence that the NRM is equally carried by titanomaghemites and that the low NRM intensities for about 20 Ma old ocean basalts are caused consequently by the low saturation magnetization of these titanomaghemites.

Isotope record of Nd-Sr-Pb in deep sea sediments from the Pacific (Table 2)

Pelagic clay of the east-central Pacific province is shown to be a mixture of three primary detrital components, reflecting continental source areas in Asia, North America, and Central and South America. Relative contributions from each source area are a function of geography, and this distribution appears to have remained constant over the past five million years, despite changing flux rates. A Q-mode factor analysis of downcore records for Pb, Sr, and Nd isotopes identified three factors that account for 98% of the total variance. These factors represent the radiogenic isotopic signatures of 1) late Cenozoic Asian dust, which dominates in the central North Pacific; 2) North American continental hemipelagic/eolian sources, restricted mainly to the easternmost North Pacific at ~30 °N latitude; and 3) Central and South American sources, restricted to areas east of ~100 °W longitude. South of the Intertropical Convergence Zone (~6 °N), the Asian dust signature diminishes abruptly. We conclude that late Cenozoic Asian dust sources can be isotopically differentiated downcore from both North American and South and Central American sources in the eastcentral Pacific. This approach has a utility for identifying changes in long-term Cenozoic atmospheric circulation patterns.

Chemical composition of rocks and minerals from DSDP holes

The book summarizes materials obtained by Soviet scientists during participation in legs of D/S Glomar Challenger and in post-cruise studies. Results on stratigraphy and lithology of the sedimentary cover, petrography and geochemistry of magmatic rocks of the oceanic crust are discussed in the book. A modern analysis of the geophysical structure of the oceanic crust and of the tectonic structure of the ocean floor is given.