Organic geochemistry of sediments of the Gulf of California

The effects of intrusive thermal stress have been studied on a number of Pleistocene sediment samples obtained from Leg 64 of the DSDP-IPOD program in the Gulf of California. Samples were selected from Sites 477, 478 and 481 where the organic matter was subjected to thermal stress from sill intrusions. For comparison purposes, samples from Sites 474 and 479 were selected as representative of unaltered material. The GC and GC-MS data show that lipids of the thermally unaltered samples were derived from microbial and terrestrial higher-plant detritus. Samples from sill proximities were found to contain thermally-derived distillates and those adjacent to sills contained essentially no lipids. Curie point pyrolysis combined with GC and GC-MS was used to show that kerogens from the unaltered samples reflected their predominantly autochthonous microbial origin. Pyrograms of the altered kerogens were much less complex than the unaltered samples, reflecting the thermal effects. The kerogens adjacent to the sills produce little or no pyrolysis products since these intrusions into unconsolidated, wet sediments resulted in in situ pyrolysis of the organic matter. Examination of the kerogens by ESR showed that spin density and line width pass through a maximum during the course of alteration but ESR g-values show no correlation with maturity. Stable carbon isotope (d13C) values of kerogens decrease by 1-1.5 per mil near the sills at Sites 477 and 481 and the atomic N/C decreases slightly with proximity to a smaller sill at Site 478. Differences in maturation behavior between Site 477 and 481 and Site 478 are attributed to dissimilarities in thermal stress and to chemical and isotopic heterogeneity of Guaymas Basin protokerogen.

Apatite fission track analytical data for samples from ODP Leg 152

We test a new approach to understanding the tectonic evolution of passive margins by using fission-track analysis on detrital apatites from sediments deposited offshore East Greenland. These apatites have not undergone postdepositional track annealing and therefore reflect provenance. The apatites preserve a component of the source rocks' thermal history that otherwise may not be retained within the present-day outcrop. Fission-track derived denudational histories from samples at Ocean Drilling Program drill sites offshore East Greenland at lat 63°N are compared with data from the onshore Singertat Complex. Previous apatite fission-track studies and geomorphic mapping of the East Greenland coast have shown that locally up to 6 km of denudation may have occurred, implying significant tectonic or magmatic activity starting as much as 30 m.y. after breakup at 56 Ma. In contrast, apatite fission-track data presented here record <2 km of Cenozoic denudation in southeast Greenland, probably driven by magmatic underplating at the time of breakup. Large-magnitude, postrift denudation of East Greenland is restricted to the area around Kangerdlugssuaq (68°N). The timing (<40-50 Ma) and magnitude are in accord with revised plume track models suggesting that the Iceland plume crossed the margin here during the late Eocene.

Nd and Pb isotope data for DSDP and ODP sediments from the Bengal Fan

Correlation of new multichannel seismic profiles across the upper Indus Fan and Murray Ridge with a dated industrial well on the Pakistan shelf demonstrates that ~40% of the Indus Fan predates the middle Miocene, and ~35% predates uplift of the Murray Ridge (early Miocene, ~22 Ma). The Arabian Sea, in addition to the Makran accretionary complex, was therefore an important repository of sediment from the Indus River system during the Paleogene. Channel and levee complexes are most pronounced after the early Miocene, coincident with an increase in sedimentation rates. Middle Eocene sandstones from Deep Sea Drilling Project Site 224 on the Owen Ridge yield K-feldspars whose Pb isotopic composition, measured by in situ ion microprobe methods, indicates an origin in, or north of, the Indus suture zone. This observation requires that India-Asia collision had occurred by this time and that an Indus River system, feeding material from the suture zone into the basin, was active soon after collision. Pleistocene provenance was similar to that during the Eocene, albeit with greater contribution from the Karakoram. A mass balance of the erosional record on land with deposition in the fan and associated basins suggests that only ~40% of the Neogene sediment in the fan is derived from the Indian plate.

Chemical and boron isotopic compositions of volcanics from ODP Sites 135-834 to 135-840

The influence of fluid flux on petrogenesis in the Tonga-Kermadec Arc was investigated using ion microprobe measurements of B/Be and boron isotope ratios (11B/10B) to document the source and relative volumes of the fluids released from the subducting oceanic plate. We analyzed young lavas from eight different islands along the Tonga-Kermadec Arc, as well as glass shards in volcanic sediments from Ocean Drilling Program (ODP) Site 840, which record the variations in the chemistry of Tonga magmatism since 7 Ma. B/Be is variable (5.8-122), in young Tonga-Kermadec Arc lavas. In contrast, glass shards from around 3 to 4 Ma old volcanic sediments at Site 840 have the highest B/Be values yet reported for arc lavas (18-607). These values are too high to be related simply to a sediment influence on petrogenesis. Together with very high d11B values (-11.6 to +37.5) for the same shards and lavas these data indicate that most of the B is derived from fluid escaped from the subducting altered Pacific oceanic crust, rather than from sediment. High d11B values also reflect large degrees of isotopic fractionation in this cold fast subduction zone. Lower d11B values noted in the Kermadec Arc (17 to -4.4) are related to the influence of sediment eroded from New Zealand and slower convergence. High fluid flux (B/Be) is synchronous in Tonga and the Marianas at 3 to 4 Ma and may be related to acceleration of the Pacific Plate just prior to this time. The timing of maximum B/Be at 3 to 4 Ma correlates with maximum light rare earth (LREE) and high field strength element depletion. This suggests maximum degrees of partial melting at this time. Although thinning of the arc lithosphere during rifting to form the Lau Basin is expected to influence the arc geochemistry, variable aqueous fluid flux from the subducting plate alone appears capable of explaining boron and other trace element systematics in the Tonga-Kermadec Arc with no indication of slab melting.

Tab. 3: Concentrations of selected chemical species at various depths in Ace Lake

This article reviews the history, chemical stratification, biology and biogeochemistry of Ace Lake, which is one of the many marine-derived meromictic (permanently stratified) lakes in the Vestfold Hills, Eastern Antarctica. The lake has an area of 18 ha, a maximum depth of 25 m, and a salinity range from 7 to 43 g l**-1. The lake mixes to a depth of 7 m in late winter as a result of brine freeze out during ice formation. Deeper mixing is precluded by a sharp halocline. The water beneath 12 m is permanently anoxic, The lake was formed approximately 10,800 yr BP as the polar ice cap melted. Sea level rise 7,800 yr BP resulted in invasion of seawater into the initially freshwater lake. Subsequently, sea level dropped, and the now saline lake became isolated from the ocean. The biota of the lake was derived from species trapped when the connection between the lake and the ocean was cut off. The oxic zone above 12 m supports a relatively simple community which includes microbial mats, four major species of phytoplankton (including a picocyanobacterium), two copepod species, and a variety of heterotrophic flagellates and ciliates. The anoxic zone contains populations of photosynthetic sulfur, sulfate reducing, fermentative and methanogenic bacteria, which combine to remineralise organic carbon which sediments from the upper waters. Research on the physics, biology and chemistry of Ace Lake has contributed significantly to knowledge of Antarctic meromictic lakes.

Sedimentary and subsidence history of ODP Sites 135-840 and 135-841

Sedimentary sections recovered from the Tonga platform and forearc during Ocean Drilling Program Leg 135 provide a record of the sedimentary evolution of the active margin of the Indo-Australian Plate from late Eocene time to the Present. Facies analyses of the sediments, coupled with interpretations of downhole Formation MicroScanner logs, allow the complete sedimentary and subsidence history of each site to be reconstructed. After taking into account the water depths in which the sediments were deposited and their subsequent compaction, the forearc region of the Tofua Arc (Site 841) can be seen to have experienced an initial period of tectonic subsidence dating from 35.5 Ma. Subsidence has probably been gradual since that time, with possible phases of accelerated subsidence, starting at 16.2 and 10.0 Ma. The Tonga Platform (Site 840) records only the last 7.0 Ma of arc evolution. However, the increased accuracy of paleowater depth determinations possible with shallow-water platform sediments allows the resolution of a distinct increase in subsidence rates at 5.30 Ma. Thus, sedimentology and subsidence analyses show the existence of at least two, and possibly four, separate subsidence events in the forearc region. Subsidence dating from 35.5 Ma is linked to rifting of the South Fiji Basin. Any subsidence dating from 16.2 Ma at Site 841 does not correlate with another known tectonic event and is perhaps linked to localized extensional faulting related to slab roll back during steady-state subduction. Subsidence from 10.0 Ma coincides with the breakup of the early Tertiary Vitiaz Arc because of the subduction polarity reversal in the New Hebrides and the subsequent readjustment of the plate boundary geometry. More recently, rapid subsidence and deposition of a upward-fining cycle from 5.30 Ma to the Present at Site 840 is thought to relate to rifting of the Lau Basin. Sedimentation is principally controlled by tectonic activity, with variations in eustatic sea level playing a significant, but subordinate role. Subduction of the Louisville Seamount Chain seems to have disrupted the forearc region locally, although it had only a modest effect on the subsidence history and sedimentation of the Tonga Platform as a whole.

Lithology of sediments from the Arabian Sea

The role of hotter than ambient plume mantle in the formation of a rifted volcanic margin in the northern Arabian Sea is investigated using subsidence analysis of a drill site located on the seismically defined Somnath volcanic ridge. The ridge has experienced >4 km of subsidence since 65 Ma and lies within oceanic lithosphere. We estimate crustal thickness to be 9.5-11.5 km. Curiously <400 m of the thermal subsidence occurred prior to 37 Ma, when subsidence rates would normally be at a maximum. We reject the hypothesis that this was caused by increasing plume dynamic support after continental break-up because the size of the thermal anomalies required are unrealistic (>600°C), especially considering the rapid northward drift of India relative to the Deccan-Réunion hotspot. We suggest that this reflects very slow lithospheric growth, possibly caused by vigorous asthenospheric convection lasting >28 m.y., and induced by the steep continent-ocean boundary. Post-rift slow subsidence is also recognized on volcanic margins in the NE Atlantic and SE Newfoundland and cannot be used as a unique indicator of plume mantle involvement in continental break-up.