Calcium carbonate, total and organic carbon and accumulation rate characteristics at DSDP Leg 78A Holes

Bulk sediment accumulation rates and carbonate and carbonate-free accumulation rates corrected for tectonic tilting have been calculated for Leg 78A sediments. These rates are uniformly low, ranging from 0.1 to 6.8 g/(cm**2 x 10**3 yr.), reflecting the pelagic-hemipelagic nature of all the sediments drilled in the northern Lesser Antilles forearc. Rates calculated for Sites 541 and 542 [0.6-6.8 g/(cm**2 x 10**3 yr.)], located on the lower slope of the accretionary prism, are significantly greater than the Neogene rates calculated for oceanic reference Site 543 [0.1-2.4 g/(cm**2 x 10**3)]. This difference could be the result of (1) tectonic thickening of accretionary prism sediments due to folding, small-scale faulting, and layer-parallel shortening; (2) deposition in shallower water farther above the CCD (carbonate compensation depth) resulting in preservation of a greater percentage of calcareous microfossils; or (3) a greater percentage of foraminiferal sediment gravity flows. Terrigenous turbidites are not documented in the Leg 78A area because of (1) great distance from South American sources; (2) damming effects of east-west trending tectonic elements; and (3) location on the Tiburon Rise (Site 543). This lack of terrigenous material, characteristic of intraoceanic convergent margins, suggests that published sedimentation models for active continental convergent margins with abundant terrigenous influxes are not applicable to intraoceanic convergent margin settings.

Sample descriptions, d-spacing and geochemistry at DSDP Holes 68-501 and 69-504B

Mössbauer analyses were conducted on a sample of saponite selected from DSDP Leg 69 basalt core. The sample was initially placed within a nitrogen-purged container on-board Glomar Challenger approximately three hours after recovery, where it remained until analysis. The Mössbauer data revealed an original, in situ Fe2O3/FeO ratio of 0.46, with both Fe**2+ and Fe**3+ in octahedral coordination. With controlled exposure to air under ambient laboratory storage conditions, the proportion of Fe**3+ increased from an original 30% to 51% over a period of about 11.5 months. The Fe**3+ thus produced remained in octahedral coordination, and no observable changes occurred in the physical appearance of the sample.

(Table 1) Sedimentary thicknesses, lithologies, ages, and interpreted paleowater depths for ODP Sites 135-840 and 135-841

The sedimentary succession drilled at Sites 840 and 841 on the Tonga forearc allows the sedimentary evolution of the active margin to be reconstructed since shortly after the initiation of subduction during the mid Eocene. Sedimentation has been dominated by submarine fan deposits, principally volcaniclastic turbidites and mass-flows derived from the volcanic arc. Volcaniclastic sedimentation occurred against a background of pelagic nannofossil sedimentation. A number of upward-fining cycles are recognized and are correlated to regional tectonic events, such as the rifting of the Lau Basin at 5.6 Ma. Episodes of sedimentation dating from 16.0 and 10.0 Ma also correlate well with major falls in eustatic sea level and may be at least partially caused by the resulting enhanced erosion of the arc edifice. The early stages of rifting of the Lau Basin are marked by the formation of a brief hiatus at Site 840 (Horizon A), probably a result of the uplift of the Tonga Platform. Controversy exists as to the degree and timing of the uplift of Site 840 before Lau Basin rifting, with estimates ranging from 2500 to 300 m. Structural information favors a lower value. Breakup of the Tonga Arc during rifting resulted in deposition of dacite-dominated, volcaniclastic mass flows, probably reflecting a maximum in arc volcanism at this time. A pelagic interval at Site 840 suggests that no volcanic arc was present adjacent to the Tonga Platform from 5.0 to 3.0 Ma. This represents the time between separation of the Lau Ridge from the Tonga Platform and the start of activity on the Tofua Arc at 3.0 Ma. The sedimentary successions at both sites provide a record of the arc volcanism despite the reworked nature of the deposits. Probe analyses of volcanic glass grains from Site 840 indicate a consistent low-K tholeiite chemistry from 7.0 Ma to the present, possibly reflecting sediment sourcing from a single volcanic center over long periods of time. Trace and rare-earth-element (REE) analyses of basaltic glass grains indicate that thinning of the arc lithosphere had begun by 7.0 Ma and was the principle cause of a progressive depletion of the high-field-strength (HFSE), REE, and large-ion-lithophile (LILE) elements within the arc magmas before rifting. Magmatic underplating of the Tofua Arc has reversed this trend since that time. Increasing fluid flux from the subducting slab since basin rifting has caused a progressive enrichment in LILEs. Subduction erosion of the underside of the forearc lithosphere has caused continuous subsidence and tilting toward the trench since 37.0 Ma. Enhanced subsidence occurred during rifting of the South Fiji and Lau basins. Collision of the Louisville Ridge with the trench has caused no change in the nature of the sedimentation, but it may have been responsible for up to 300 m of uplift at Site 840.

Geochemistry of Chixulub ejecta in ODP Leg 207 holes

An up to 2-cm thick Chicxulub ejecta deposit marking the Cretaceous-Paleogene (K-Pg) boundary (the "K-T" boundary) was recovered in six holes drilled during ODP Leg 207 (Demerara Rise, tropical western Atlantic). Stunning features of this deposit are its uniformity over an area of 30 km2 and the total absence of bioturbation, allowing documentation of the original sedimentary sequence. High-resolution mineralogical, petrological, elemental, isotopic (Sr-Nd), and rock magnetic data reveal a distinct microstratigraphy and a range of ejecta components. The deposit is normally graded and composed predominantly of rounded, 0.1- to max. 1-mm sized spherules. Spherules are altered to dioctahedral aluminous smectite, though occasionally relict Si-Al-rich hydrated glass is also present, suggesting acidic precursor lithologies. Spherule textures vary from hollow to vesicle-rich to massive; some show in situ collapse, others include distinct Fe-Mg-Ca-Ti-rich melt globules and lath-shaped Al-rich quench crystals. Both altered glass spherules and the clay matrix (Site 1259B) display strongly negative epsilon-Nd (T=65Ma) values (-17) indicating uptake of Nd from contemporaneous ocean water during alteration. Finally, Fe-Mg-rich spherules, shocked quartz and feldspar grains, few lithic clasts, as well as abundant accretionary and porous carbonate clasts are concentrated in the uppermost 0.5-0.7 mm of the deposit. The carbonate clasts display in part very unusual textures, which are interpreted to be of shock-metamorphic origin. The preservation of delicate spherule textures, normal grading with lack of evidence for traction transport, and sub-millimeter scale compositional trends provide evidence for this spherule deposit representing a primary air-fall deposit not affected by significant reworking. The ODP Leg 207 spherule deposit is the first known dual-layer K-Pg boundary in marine settings; it incorporates compositional and stratigraphic aspects of both proximal and distal marine sites. Its stratigraphy strongly resembles the dual-layer K-Pg boundary deposits in the terrestrial Western Interior of North America (although there carbonate phases are not preserved). The occurrence of a dual ejecta layer in these quite different sedimentary environments - separated by several thousands of kilometers - provides additional evidence for an original sedimentary sequence. Therefore, the layered nature of the deposit may document compositional differences between ballistic Chicxulub ejecta forming the majority of the spherule deposit, and material falling out from the vapor (ejecta) plume, which is concentrated in the uppermost part.

Annotated record of the detailed examination of Mn deposits from R/V Xiang Yang Hong 16 stations (1983) in the Pacific Ocean

Electron microprobe and X-ray diffraction data for north Pacific manganese nodules reveal that the transition metal distributions are controlled by the mineralogy. Microlayers rich in 10Å-manganates generally have high Mn/Fe ratios and positive correlations between Ni, Cu and Mn, and between Co and Fe. Microlayers rich in vernadite, on the other hand, show low Mn/Fe ratios, and Co, Ni and Cu all show positive correlations with Mn. The 10Å-manganates form mainly in porewaters with high Mn/Fe ratios. The Ni2+ and Cu2+ ions are post-depositionally incorporated into the interlayers of the manganates, whereas Co3+ is substituted for Fe3+ in ferric oxyhydroxides. In seawater with a low Mn/Fe ratio, on the other hand, the adsorption of positively charged ferric oxyhydroxides on negatively charged [MnO6] octahedral layers suppresses the growth of 10Å-manganates, enhancing the formation of vernadite. Positively charged hydroxides of Co3+, Ni2+ and Cu2+ are also adsorbed on the [MnO6] layers. These mechanisms of mineral formation and metal uptake are corroborated by data for other oceanic non-hydrothermal manganese nodules and crusts.