Petrology of basalts from DSDP Hole 83-504B

Basalts from Hole 504B, Leg 83, exhibit remarkable uniformity in major and trace element composition throughout the 1075.5 m of basement drilled. The majority of the basalts, Group D', have unusual compositions relative to normal (Type I) mid-ocean ridge basalts (MORB). These basalts have relatively high mg values (0.60-0.70) and CaO abundances (11.7-13.7%; Ca/Al = 0.78-0.89), but exhibit a marked depletion in compatible trace elements (Cr and Ni); moderately incompatible trace elements (Zr, Y, Ti, etc.); and highly incompatible trace elements (Nb, LREE, etc.). Petrographic and compositional data indicate that most of these basalts are evolved, having fractionated significant amounts of plagioclase, olivine, and clinopyroxene. Melting experiments on similar basalt compositions from the upper portion of Hole 504B (Leg 70; Autio and Rhodes, 1983) indicate that the basalts are co-saturated with olivine and plagioclase and often clinopyroxene on the 1-atm. liquidus. Two rarely occurring groups, M' and T, are compositionally distinct from Group D' basalts. Group T is strongly depleted in all magmaphile elements except the highly incompatible ones (Nb, La, etc.), while Group M' has moderate concentrations of both moderately and highly incompatible trace elements and is similar to Type I MORB. Groups M' and T cannot be related to Group D' nor to each other by crystal fractionation, crystal accumulation, or magma mixing. The large differences in magmaphile element ratios (Zr/Nb, La/Yb) among these three chemical groups may be accounted for by complex melting models and/or local heterogeneity of the mantle beneath the Costa Rica Ridge. Xenocrysts and xenoliths of plagioclase and clinopyroxene similar in texture and mineral composition to crystals in coarse-grained basalts from the lower portion of the hole are common in Hole 504B basalts. These suggest that addition of solid components either from conduit or magma chamber walls has occurred and may be a common source of disequilibrium crystals in these basalts. However, mixing of plagioclase-laden depleted melts (similar to the Costa Rica Ridge Zone basalts) with normal MORB magmas could provide an alternate source for some refractory plagioclase crystals found out of equilibrium in many phyric MORB. The uniformity of major element compositions in Hole 504B basalts affords an ideal situation for investigating the effects of alteration on some major and trace elements in oceanic basalts. Alteration observed in whole-rock samples records primarily two events - a high-temperature and a low-temperature phase. High-temperature phases include: chlorite, talc, albite, actinolite, sphene, quartz, and pyrite. The low-temperature phases include smectite (saponite), epistilbite or laumontite, and minor calcite. Laumontite may actually straddle the gap between the low- and high-temperature mineral assemblages. Alteration is restricted primarily to partial replacement of primary phases. Metamorphic grade, in general, increases from the top to the bottom of Hole 504B (Legs 69, 70, and 83) as seen in the change from a smectiteto- chlorite-dominated secondary mineral assemblage. However, a systematic progression for the interval recovered during Leg 83 is not apparent. Rather, the extent of alteration appears to be a function of the initial texture and fracture density. Variations in whole-rock major and trace element concentrations cannot be attributed convincingly to any differences in alteration observed. Compositional characteristics of the secondary minerals indicated that extensive remobilization of elements has not occurred; local redistribution is suggested in most cases. Thus, the major and trace element signature of these basalts remains effectively the same as the original composition prior to alteration.

Ether-derived alkanes from sedimentary organic matter

Glycerol ether lipids have been detected in the bitumen of DSDP sediments from Sites 467, 440B and 380 and from the Green River Shale. The alkyl side groups of these ethers were determined by conversion into deuteroalkanes. The presence of glycerol ethers produced by methanogenic bacteria was indicated in the DSDP bitumens by the formation of monodeuterated phytane and dideuterated biphytane. Other ethers were found with novel non-isoprenoidal side groups which may belong to sulfate-reducing or other, probably anaerobic, bacteria. Kerogen-bound alkoxy groups were determined using hydrogen iodide cleavage of the ether link followed by conversion of the iodoalkanes into corresponding deuteroalkanes. For this reaction, the kerogen was not isolated from the rock matrix. The structures so produced were found to include alkyl groups which have known bacterial precursors as well as others that are presently unknown in organisms. The Green River ether biomarker profile is interpreted as possibly indicative of bacterial diagenesis exclusive of biomethanogenesis.

Petrology, geochemistry, and geodynamics at DSDP Hole 66-493

At Site 493, DSDP Leg 66, dioritic basement was reached below lower Miocene (NN1 Zone, 22-24 Ma) terrigenous sediments. Petrographical, mineralogical (including microprobe analyses), and chemical features of the dioritic rocks reveal their magmatic affinity with the calc-alkaline series. Furthermore, their radiometric age (35.3 m.y.) links the basement to the Sierra Madre Occidental in Mexico and to mid-Tertiary volcanic arcs in Central America. The presence of Oligocene diorite 50 km from the trench axis confirms the truncation of the south Mexico margin, which we explain as the result of a 650 to 800 km left-lateral displacement of Central America relative to North America. Truncation must have occurred in the late Miocene, after the diorite intrusion and prior to the present subduction.

Petrology and relict mineralogy of serpentinites at DSDP Leg 84 Holes

Eleven serpentine samples from DSDP Leg 84 and four serpentinized ultramafic samples from Costa Rica and Guatemala were described and their relict mineral compositions measured by electron microprobe to try to determine the origin of the Leg 84 serpentinites and their relationship to the ultramafic rocks of the onshore ophiolites. The Leg 84 samples comprise more than 90% secondary minerals, principally serpentine, with hematitic and opaque oxides, and minor talc and smectites. Four distinct textural types can be identified according to the distribution of opaque phases and smectite. Remnants of spinel, olivine, orthopyroxene, and clinopyroxene occur variously in the samples; spinal occurs in all the samples. Textural evidence suggests that the serpentinites were originally clinopyroxene-bearing harzburgites. Relict mineral compositions are refractory and relatively uniform: olivine, Fo90.6-90.9; orthopyroxene, En90-91; clinopyroxene, Wo47 En50 Fs3; spinels, Cr/Cr + Al = 0.4-0.6. 567A-29-2, 30-35 cm has slightly more magnesian olivines (Fo92) and orthopyroxene, and more aluminous spinels (Cr/Cr + Al = 0.3). These compositions are similar to those inferred for refractory upper-mantle materials and also fall within the range of compositions for relict minerals in abyssal peridotites. They could be of oceanic origin. The onshore samples include serpentinites, a clinopyroxene-bearing harzburgite, and a clinopyroxenite. They too have magnesium-rich silicate assemblages, but relative to the drilled samples have more iron-rich olivines (Fogo) and more aluminous and sodic pyroxenes; spinels which are clearly relicts are very aluminum-rich (Cr/Cr + Al = 0.1-0.25). These samples are most likely mantle materials, but significantly less depleted. Their relationship to the drilled samples is unclear. Serpentinites were the most common basement materials recovered during Leg 84, and there appears to be a bimodal assemblage (basalt/diabase and serpentine) of igneous rocks sampled from the trench slope. Diapirism of serpentine throughout the trench slope and forearc is suggested as an explanation for this distribution of samples.

Chemical composition of <0.001 mm grain size fraction from sedimentary rocks and bottom sediments of the Philippine Sea

Distriburtion and formation of clay minerals in different types of bottom sediments from the West Pacific are under consideration.

Sedimentary cycles on the Exmouth Plateau

Sedimentary cycles are observed in the nearly complete Lower Cretaceous to Eocene pelagic carbonates at Site 762 on the Exmouth Plateau off northwest Australia. The high-frequency cycles of variable clay and foraminifers in nannofossil chalk appear as color cycles repeating on a scale of centimeters to meters in thickness. Measured cycle thickness indicate that the dominant cycles appear to be related to the precession and obliquity periods. To evaluate the high-frequency variance observed on the gamma-ray curve, spectral analysis of the log was performed on two intervals: 260 to 365 mbsf in the Cenozoic, and 555 to 685 mbsf in the Mesozoic. Average Cenozoic sedimentation rates of 10.5 m/m.y. are high enough to show that variance is present in the full suite of eccentricity bands (413-123-95 k.y.). Spectral analysis of the Mesozoic section failed to produce dominant peaks that could be correlated to predicted orbital periods. The bioturbation observed in the cores in this interval may be responsible for diluting the signal and producing high-frequency noise, which is manifested in the spectra as low, broad amplitude peaks. Orbital forcing may be affecting sedimentation on the Exmouth Plateau by influencing cycles of increased carbonate production or dissolution. Alternatively, clay abundance cycles may be related to eolian deposition during cycles of increased aridity in western Australia. Four low-frequency events were also identified at Site 762 from the core and log data. The duration of these events is approximately 13 m.y., and the conformable boundaries of these sedimentary cycles correlate with observed nondepositional surfaces in other wells in western Australia. The causal mechanism for the onset of these events may be eustatic, but alternatively may be regional tectonism with associated circulation pattern changes.

Petrology and geochemistry of altered ocenaic crust of ODP Hole 140-504B

Altered basalt dikes from Hole 504B were partially melted at 1150°C and 1180°C to determine the composition of the first melts as oceanic Layer 2C is assimilated by a magma chamber. The partial melts are chemically similar to actinolite, the most abundant secondary mineral, but the melts are not simply melted actinolite. High TiO2, P2O5, and K2O abundances of the melts indicate that minor secondary minerals that are enriched in these elements also contribute to the melt. The incorporation of partial melts into a ridge-crest magma chamber may explain the local variability that is sometimes found in ocean ridge basalts that are not readily explained fractional crystallization or partial melting.

Long sedimentary records of sediment cores from the Chilean margin

The Antarctic Intermediate Water (AAIW) is a key player in global-scale oceanic overturning processes and an important conduit for heat, fresh water, and carbon transport. The AAIW past variability is poorly understood mainly due to the lack of sedimentary archives at intermediate water depths. We present records of benthic stable isotopes from sediments retrieved with the seafloor drill rig MARUM-MeBo at 956 m water depth off northern Chile (GeoB15016, 27°29.48'S, 71°07.58'W) that extend back to 970 ka. The sediments at this site are presently deposited at the boundary between AAIW and Pacific Deep Water (PDW). For previous peak interglacials, our results reveal similar benthic d13C values at site GeoB15016 and of a newly generated stack of benthic d13C from various deep Pacific cores representing the "average PDW." This suggests, unlike today, the absence of AAIW at the site and the presence of nearly pure PDW. In contrast, more positive d13C values at site GeoB15016 compared to the stack imply a considerable AAIW contribution during cold phases of interglacials and especially during glacials. Besides, we used three short sediment cores to reconstruct benthic d13C values from the AAIW core during the last glacial and found a d13C signature similar to today's. Assuming that this was the case also for the past 970 kyr, we demonstrate that sea level changes and latitudinal migrations of the AAIW formation site can only account for about 50% of the full range of past d13C increases at site GeoB15016 during cold periods. Other processes that could explain the remaining of the positive d13C anomalies are increases in glacial AAIW production and/or deeper convection of the AAIW with respect to preceding interglacials.

Chemical and mineral compositions of recent and ancient sediments and sedimentary rocks from the Mariana Trench

On the bed and on the ocean slope of the southern latitudinal part of the Mariana Trench ancient sediments, as well as sedimentary and igneous rocks are exposed. In the lower part of the sampled part of the studied section Late Oligocene to Early Miocene chalk-like limestones and marls occur. Upward marly tuffites and tuffs (apparently alternating with carbonate rocks) occur. These rocks are overlain by Early Miocene tuffaceous clays and siliceous-clayey muds. In the upper part of the section there are Pleistocene pelagic clays and ethmodiscus oozes.

Geochemistry and petrology of basalts from Celebes Sea

Major-, trace-, and rare-earth element analyses are presented from a suite of basaltic rocks from the basement of the Celebes Sea. The major elements and trace-elements were determined by X-ray fluorescence techniques, and the rare-earth elements were analyzed by instrumental neutron activation analysis. Compositionally the Celebes Sea basalts are very similar to typical normal mid-ocean ridge basalts, such as those described from the Indian Ocean triple junction. Petrogenetic modeling shows that all of the basalts analyzed can be formed by 10% to 20% partial melting of a light rare-earth element-depleted spinel lherzolite followed by fractional crystallization of mixtures of olivine, Plagioclase, and iron oxide. The Celebes Sea is interpreted as a fragment of the basement of the Jurassic Argo abyssal plain trapped during the Eocene to the north of Australia.

Sedimentary climate indicators of sediment core MD08-3167 and GeoB1711-4, southwestern Africa

The past climate evolution of southwestern Africa is poorly understood and interpretations of past hydrological changes are sometimes contradictory. Here we present a record of leaf-wax dD and View the MathML source taken from a marine sediment core at 23°S off the coast of Namibia to reconstruct the hydrology and C3 versus C4 vegetation of southwestern Africa over the last 140 000 years (140 ka). We find lower leaf-wax dD and higher View the MathML source (more C4 grasses), which we interpret to indicate wetter Southern Hemisphere (SH) summer conditions and increased seasonality, during SH insolation maxima relative to minima and during the last glacial period relative to the Holocene and the last interglacial period. Nonetheless, the dominance of C4 grasses throughout the record indicates that the wet season remained brief and that this region has remained semi-arid. Our data suggest that past precipitation increases were derived from the tropics rather than from the winter westerlies. Comparison with a record from the Congo Basin indicates that hydroclimate in southwestern Africa has evolved in antiphase with that of central Africa over the last 140 ka.

Genetic data from Staurosira (Fragilariophyceae) sedimentary DNA

Genetic investigations on eukaryotic plankton confirmed the existence of modern biogeographic patterns, but analyses of palaeoecological data exploring the temporal variability of these patterns have rarely been presented. Ancient sedimentary DNA proved suitable for investigations of past assemblage turnover in the course of environmental change, but genetic relatedness of the identified lineages has not yet been undertaken. Here, we investigate the relatedness of diatom lineages in Siberian lakes along environmental gradients (i.e. across treeline transects), over geographic distance and through time (i.e. the last 7000 years) using modern and ancient sedimentary DNA. Our results indicate that closely-related Staurosira lineages occur in similar environments and less-related lineages in dissimilar environments, in our case different vegetation and co-varying climatic and limnic variables across treeline transects. Thus our study reveals that environmental conditions rather than geographic distance is reflected by diatom-relatedness patterns in space and time. We tentatively speculate that the detected relatedness pattern in Staurosira across the treeline could be a result of adaptation to diverse environmental conditions across the arctic boreal treeline, however, a geographically-driven divergence and subsequent repopulation of ecologically different habitats might also be a potential explanation for the observed pattern.

Mineralogy and geochemistry of sedimentary deposits at DSDP Leg 55 Holes

The four holes (including a re-entry hole) drilled at Site 433 allow determination of the sedimentary sequence of Suiko Seamount in the Emperor chain. The holes are in a small graben basin situated within a lateral lagoon on the seamount. The sedimentary deposits range from the Paleocene to the upper Pliocene and are not uniform and continuous. A major hiatus exists at the top of the lower Eocene reef sediment, below the lower and upper Miocene pelagic sediments. The depositional history and succession of environments are shown by mineralogical and geochemical changes in the sediments.

Geochemistry and petrology of the volcanic rocks from the Sulu Sea

Major-, trace-, and rare-earth element analyses of the basaltic rocks recovered from the basement of the Sulu Sea and of lithic clasts from the pyroclastic unit representing the acoustic basement of the Cagayan Ridge, are presented. The major and trace elements were measured by X-ray fluorescence techniques, and rare-earth elements by instrumental neutron activation analysis. These data show that the Sulu Sea basalts are back-arc tholeiites and the lithic clasts are basalts, basaltic andesites, and andesites typical of volcanic arc suites erupted on continental crust. Petrogenetic modeling is used to show that the Sulu Sea basalts were derived from a heterogeneous mantle, probably representing subcontinental lithosphere, with contributions from a subduction component. The Sulu Sea is interpreted as a back-arc basin formed by rifting of an Oligocene to early Miocene volcanic arc leaving the Cagayan Ridge as a remnant arc. This event occurred during northward subduction of the Celebes Sea basement beneath the Oligocene to early Miocene arc.