Annotated record of the detailed examination of Mn deposits from DSDP Site 159, Leg 16 (Cores 16-159-6 and 16-159-12)

DSDP 159 is one of a series of sites in the eastern equatorial Pacific on the west flank of the East Pacific Rise. It was selected by the Pacific Site Selection Panel on the premise that if hydrothermal processes on the crest of the East Pacific Rise supply the transition metals, a broad zone of such deposits should be present immediately above basement over the entire flank of the Rise.

Annotated record of the detailed examination of Mn deposits from DSDP Site 20, Leg 3 (Holes 3-20, 3-20A and 3-20C)

Prior to arrival on this site, the only survey data available was from the Vema-20 crossing of the area. The recommended site location was over a relatively smooth valley in the bottom topography at about 4750 meters (15,580 feet) depth (uncorrected), about 10 kilometers wide E-W between peaks (or ridges) on either side. Sediment thickness was unknown. The center of the valley is near the peak of a wide (40 to 50 kilometers) positive magnetic anomaly, identified as Magnetic Anomaly 30 in the hypothesized geomagnetic time scale with an age of 72 million years.

Annotated record of the detailed examination of Mn deposits from DSDP Site 26, Leg 4 (Core 4-26-1)

Site 26 was selected on the crest of the Mid-Atlantic Ridge between 15°N and 5°S is offset to the east nearly 4000 kilometers through a series of fracture zones. One of the most prominent of these is the Vema Fracture Zone, a narrow east-west trending trough which cuts through the Mid-Atlantic Ridge at latitude 11°N.

Petrology and geochemistry at DSDP Site 89-462

The 16 samples of Deep Sea Drilling Project (DSDP) Leg 89 basalts that we analyzed for whole rock major and trace elements and for mineralogic compositions are identical to some of the basalts recovered during Leg 61. Leg 89 samples are mostly olivine-plagioclase-clinopyroxene sparsely phyric basalts and exhibit a wide variety of textures. These basalts have lower TiO2 at a given Mg/(Mg+Fe2+)*100 than MORB (midocean ridge basalt). We recognize three major chemical types of basalts in the Nauru Basin. We believe that different degrees of partial melting, modified by fractional crystallization and possibly by magma mixing at shallow depths, can explain the chemical differences among the three groups. This petrogenetic model is consistent with the observed downhole chemical-chronostratigraphic relations of the samples. New 87Sr/86Sr and U3Nd/144Nd analyses of basalt samples from DSDP Site 462 indicate that the Nauru Basin igneous complex is within the Sr-Nd isotopic range of ocean island basalt. Thus the Nauru Basin igneous complex resembles MORB in many aspects of its chemistry, morphology, and secondary alteration patterns (Larson, Schlanger, et al., 1981), but not in its isotopic characteristics. If it were not for the unambiguous evidence that the Nauru Basin complex was erupted off-ridge, the complex could easily be interpreted as normal oceanic layer 2. For this reason, we speculate that the Nauru Basin igneous complex was produced in an oceanic riftlike environment when multiple, fast-propagating rifts were formed during the fast seafloor spreading episode in the Cretaceous.

Benthic foraminiferal stable isotope record from ODP Site 138-849 on the East Pacific Rise

Benthic foraminifer and delta13C data from Site 849, on the west flank of the East Pacific Rise (0°11 'N, 110°31'W; 3851 m), give relatively continuous records of deep Pacific Ocean stable isotope variations between 0 and 5 Ma. The mean sample spacing is 4 k.y. Most analyses are from Cibicides wuellerstorfi, but isotopic offsets relative to Uvigerina peregrina appear roughly constant. Because of its location west of the East Pacific Rise, Site 849 yields a suitable record of mean Pacific Ocean delta13C, which approximates a global oceanic signal. The ~100-k.y.-period climate cycle, which is prevalent in delta18O does not dominate the long-term delta13C record. For delta13C, variations in the ~400- and 41-k.y. periods are more important. Phase lags of delta13C relative to ice volume in the 41- and 23-k.y. bands are consistent with delta13C as a measure of organic biomass. A model-calculated exponential response time of 1-2 k.y. is appropriate for carbon stored in soils and shallow sediments responding to glacial-interglacial climate change. Oceanic delta13C leads ice volume slightly in the 100-k.y. band, and this suggests another process such as changes in continental weathering to modulate mean river delta13C at long periods. The delta13C record from Site 849 diverges from that of Site 677 in the Panama Basin mostly because of decay of 13C-depleted organic carbon in the relatively isolated Panama Basin. North Atlantic to Pacific delta13C differences calculated using published data from Sites 607 and 849 reveal variations in Pliocene deep water within the range of those of the late Quaternary. Maximum delta13C contrast between these sites, which presumably reflects maximum influx of high-delta13C northern source water into the deep North Atlantic Ocean, occurred between 1.3 and 2.1 Ma, well after the initiation of Northern Hemisphere glaciation. Export of high-delta13C North Atlantic Deep Water from the Atlantic to the circumpolar Antarctic, as recorded by published delta13C data from Subantarctic Site 704, appears unrelated to the North Atlantic-Pacific delta13C contrast. To account for this observation, we suggest that deep-water formation in the North Atlantic reflects northern source characteristics, whereas export of this water into the circumpolar Antarctic reflects Southern Hemisphere wind forcing. Neither process appears directly linked to ice-volume variations.

Planktonic foraminiferal biostratigraphy and stable carbon isotope ratios of late middle Eocene sediments from Blake Plateau, West Atlantic Ocean

The muricate planktonic foraminiferal genera Morozovella and Acarinina were abundant and diverse during the upper Palaeocene to middle Eocene and dominated the tropical and subtropical assemblages. A significant biotic turnover in planktonic foraminifera occurred in the latest middle Eocene with a notable reduction in the acarininid lineage and the extinction of the morozovellids. These genera are extensively employed as palaeoclimatic and biostratigraphic markers and, therefore, this turnover episode is an important event in the record of the Cenozoic planktonic foraminifera. Sediments from the western North Atlantic (Ocean Drilling Program Site 1052) were examined in order to investigate these extinction events, in terms of both timing and mechanisms. Biostratigraphic events of the middle and late Eocene have been examined with a sampling resoluti on of approximately 3 kyr. These have been calibrated to the magneto- and astrochronology to accurately define the timing of key biostratigraphic events, particularly the extinction of Morozovella spinulosa which is a distinct biomarker for late middle Eocene sediments. High-resolution biostratigraphy reveals that the extinctions in the muricate group occurred in a stepwise form. The large acarininids (Acarinina praetopilensis) terminate 10 kyr prior to the extinction of M. spinulosa and small acarininids (Acarinina medizzai and Acarinina echinata) continue into the upper Eocene. High-resolution stable isotope analyses have been conducted on planktonic and benthic foraminifera from the western North Atlantic to reconstruct sea surface temperatures (SSTs) and deep water temperatures and the structure of the water column around this major biotic turnover. Whilst the extinctions of M. spinulosa and A. praetopilensis occur during a long-term cooling trend, the biotic turnover in the muricate group does not appear to be related to significant climatic change. Sea surface temperatures decrease slowly prior to the extinction events, and there is no evidence for a large-temperature shift associated with the faunal changes. The turnover event was therefore probably related to the increased surface water productivity and the deterioration of photosymbiotic partnerships with algae.

Chemistry of minerals from amphibole-chlorite veins of ODP Site 209-1270

We examined small-scale shear zones in drillcore samples of abyssal peridotites from the Mid-Atlantic Ridge. These shear zones are associated with veins consisting of chlorite + actinolite/tremolite assemblages, with accessory phases zircon and apatite, and they are interpreted as altered plagiogranite melt impregnations, which originate from hydrous partial melting of gabbroic intrusion in an oceanic detachment fault. Ti-in-zircon thermometry yields temperatures around 820°C for the crystallization of the evolved melt. Reaction path modeling indicates that the alteration assemblage includes serpentine of the adjacent altered peridotites. Based on the model results, we propose that formation of chlorite occurred at higher temperatures than serpentinization, thus leading to strain localization around former plagiogranites during alteration. The detachment fault represents a major pathway for fluids through the oceanic crust, as evidenced by extremely low d18O of altered plagiogranite veins (+3.0-4.2 per mil) and adjacent serpentinites (+ 2.6-3.7 per mil). The uniform oxygen isotope data indicate that fluid flow in the detachment fault system affected veins and adjacent host serpentinites likewise.