1000 resultados para Baikal Drilling Project


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We have studied the sedimentary and basaltic inputs of lithium to subduction zones. Various sediments from DSDP and ODP drill cores in front of the Mariana, South Sandwich, Banda, East Sunda and Lesser Antilles island arcs have been analysed and show highly variable Li contents and d7Li values. The sediment piles in front of the Mariana and South Sandwich arcs largely consist of pelagic sediments (clays and oozes). The pelagic clays have high Li contents (up to 57.3 ppm) and Li isotope compositions ranging from +1.3? to +4.1?. The oozes have lower Li contents (7.3-16 ppm) with d7Li values of the diatom oozes from the South Sandwich lower (+2.8? to +3.2?) than those of the radiolarian oozes from the Mariana arc (+8.1? to +14.5?). Mariana sediment also contains a significant portion of volcanogenic material, which is characterised by a moderate Li content (14 ppm) and a relatively heavy isotope composition (+6.4?). Sediments from the Banda and Lesser Antilles contain considerable amounts of continental detritus, and have high Li contents (up to 74.3 ppm) and low d7Li values (around 0?), caused by weathering of continental bedrock. East Sunda sediments largely consist of calcareous oozes. These carbonate sediments display intermediate to high Li contents (2.4-41.9 ppm) and highly variable d7Li values (-1.6? to +12.8?). Basaltic oceanic crust samples from worldwide DSDP and ODP drill cores are characterised by enrichment of Li compared to fresh MORB (6.6-33.1 vs. 3.6-7.5 ppm, respectively), and show a large range in Li isotope compositions (+1.7? to +11.8?). The elemental and isotopic enrichment of Li in altered basalts is due to the uptake of isotopically heavy seawater Li during weathering. However, old oceanic crust samples from Sites 417/418 exhibit lighter Li isotope compositions compared to young basaltic crust samples from Sites 332B and 504B. This lighter Li isotope signature in old crust is unexpected and further research is needed to explore this issue.

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Leg 27 sediments were analyzed for total carbon and acid-insoluble (organic) carbon using a LECO acid-base Analyzer. The 3-cc sediment samples were first dried at 105°-110°C and then ground to a homogeneous powder. The ground sediment was redried and two samples, a 0.1-g and a 0.5-g sample, were then weighed into LECO clay crucibles. The 0.5-g sample was acidified with diluted hydrochloric acid and washed with distilled water. The sample was then dried and analyzed for acid-insoluble carbon, listed in the table as "organic" carbon. The 0.1-g sample was analyzed for total carbon without further treatment. If the result showed less than 10% CaCO3, an additional 0.5-g sample was analyzed for greater accuracy. The calcium carbon percentages were calculated as follows: (% total C-% organic C) * 8.33 = % CaCO3. Although other carbonates may be present, all acid-soluble carbon was calculated as calcium carbonate. All results are given in weight percent.

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In the largest global cooling event of the Cenozoic Era, between 33.8 and 33.5 Myr ago, warm, high-CO2 conditions gave way to the variable 'icehouse' climates that prevail today. Despite intense study, the history of cooling versus ice-sheet growth and sea-level fall reconstructed from oxygen isotope values in marine sediments at the transition has not been resolved. Here, we analyse oxygen isotopes and Mg/Ca ratios of benthic foraminifera, and integrate the results with the stratigraphic record of sea-level change across the Eocene-Oligocene transition from a continental-shelf site at Saint Stephens Quarry, Alabama. Comparisons with deep-sea (Sites 522 (South Atlantic) and 1218 (Pacific)) d18O and Mg/Ca records enable us to reconstruct temperature, ice-volume and sea-level changes across the climate transition. Our records show that the transition occurred in at least three distinct steps, with an increasing influence of ice volume on the oxygen isotope record as the transition progressed. By the early Oligocene, ice sheets were ~25% larger than present. This growth was associated with a relative sea-level decrease of approximately 105 m, which equates to a 67 m eustatic fall.

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During DSDP Leg 70, a 1.60 m thick manganese oxide layer was sampled in hole 509B. This deposit is formed of alternating layers of hard plates of pure todorokite, about 2 mm thick, and of a more powdery material deeply impregnated with manganese oxide, about 3 mm thick. A SEM study of the plates and the associated powder shows that the powdery material is a transformation of a pre-existing sediment, while the plates are a direct precipitation from a hydrothermal solution. The uranium series disequilibrium method was used to determine the ages of the plates. They are found to be in good chronological sequence and in accordance with the sedimentation rate of the area (4.9 cm/10^3 years) which implies that they have been formed at the sediment-seawater interface during a pulsed injection of hydrothermal solution. The powder presents systematically an "older age" which is explained by a slowing down of the injection while the normal sediment settles; the older age is due to the 230Th excess of the sediment.

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Comprehensive isotopic studies based on data from the Deep Sea Drilling Project have elucidated numerous details of the low- and high-temperature mechanisms of interaction between water and rocks of ocean crustal seismic Layers 1 and 2. These isotopic studies have also identified climatic changes during the Meso-Cenozoic history of oceans. Data on the abundance and isotopic composition of sulfur in the sedimentary layer as well as in rocks of the volcanic basement are more fragmentary than are oxygen and carbon data. In this chapter we specifically concentrate upon isotopic data related to specific features of the mechanisms of low-temperature interaction of water with sedimentary and volcanogenic rocks. The Leg 59 data provide a good opportunity for such lithologic and isotopic studies, because almost 600 meters of basalt flows and sills interbedded with tuffs and volcaniclastic breccias were cored during the drilling of Hole 448A. Moreover, rocks supposedly exposed to hydrothermal alteration play an important role at the deepest horizons of that mass. Sulfur isotopic studies of the character of possible biogenic processes of sulfate reduction in sediments are another focus, as well as the nature and origin of sulfide mineralization in Layer-2 rocks of remnant island arcs. Finally, oxygen and carbon istopic analyses of biogenic carbonates in the cores also enabled us to investigate the effects of changing climatic conditions during the Cenozoic. These results are compared with previous data from adjacent regions of the Pacific Ocean. Thus this chapter describes results of isotopic analyses of: oxygen and sulfur of interstitial water; oxygen and carbon of sedimentary carbonates and of calcite intercalations and inclusions in tuffs and volcaniclastic breccias interbedded with basalt flows; and sulfur of sulfides in these rocks.

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The Br/Cl, Li/Cl and B/Cl ratios and boron isotope compositions of hypersaline pore fluids from DSDP Sites 372 and 374 were measured in an attempt to evaluate the origin of the brines. In Site 374 the relationships between the Cl concentrations (up to 5000 mM) and Br/Cl (~0.012), Na/Cl (as low as 0.1), B/Cl (0.0025), and d11B values (43-55?) of the deep pore water between 380 and 405 mbsf, located within the Messinian sediments, reflect remnants of ~65-fold evaporated sea water. The original evaporated sea water was modified by: (1) dilution with overlying or less saline water by about 30%; and (2) slight dissolution of NaCl evaporites. The variations in d11B show a continuous increase in d11B values with depth in Site 374, up to 66.7? at a depth of 300 mbsf (Upper Pliocene marl sediments). The conspicuous 11B enrichment trend is consistent with elemental boron depletion, which was calculated from the expected boron concentrations of evaporated sea water with corresponding Br/Cl and Na/Cl ratios. Li/Cl variations also show a depletion of Li relative to evaporated sea water. The apparent depletions of B and Li, as well as the 11B enrichment, reflect uptake of these elements by clay minerals at low water/sediment ratios.

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Oceanic anoxic event 2 (OAE-2) occurring during the Cenomanian/Turonian (C/T) transition is evident from a globally recognized positive stable carbon isotopic excursion and is thought to represent one of the most extreme carbon cycle perturbations of the last 100 Myr. However, the impact of this major perturbation on and interaction with global climate remains unclear. Here we report new high-resolution records of sea surface temperature (SST) based on TEX86 and d 18O of excellently preserved planktic foraminifera and stable organic carbon isotopes across the C/T transition from black shales located offshore Suriname/French Guiana (Demerara Rise, Ocean Drilling Program Leg 207 Site 1260) and offshore Senegal (Cape Verde Basin, Deep Sea Drilling Project Leg 41 Site 367). At Site 1260, where both SST proxy records can be determined, a good match between conservative SST estimates from TEX86 and d 18O is observed. We find that late Cenomanian SSTs in the equatorial Atlantic Ocean (33°C) were substantially warmer than today (27°-29°C) and that the onset of OAE-2 coincided with a rapid shift to an even warmer (35°-36°C) regime. Within the early stages of the OAE a marked (4°C) cooling to temperatures lower than pre-OAE conditions is observed. However, well before the termination of OAE-2 the warm regime was reestablished and persisted into the Turonian. Our findings corroborate the view that the C/T transition represents the onset of the interval of peak Cretaceous warmth. More importantly, they are consistent with the hypotheses that mid-Cretaceous warmth can be attributed to high levels of atmospheric carbon dioxide (CO2) and that major OAEs were capable of triggering global cooling through the negative feedback effect of organic carbon-burial-led CO2 sequestration. Evidently, however, the factors that gave rise to the observed shift to a warmer climate regime at the onset of OAE-2 were sufficiently powerful that they were only briefly counterbalanced by the high rates of carbon burial attained during even the most extreme interval of organic carbon burial in the last 100 Myr.

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Lower and Upper Cretaceous sediments of the Maurice Ewing Bank, Site 511 (black shales, mudstones, zeolitic clays, and nannofossil chalk and ooze, 361 m thick) are characterized by an assemblage of planktonic foraminifers of low systematic diversity, including over 50 species. Representatives of Hedbergella, Globigerinelloides, Archaeoglobigerina, Whiteinella, Rugoglobigerina, and Heterohelix are predominant; species of Ticinella, Praeglobotruncana, Globotruncana, Schackoina, and Planoglobulina associated with some interbeds occur in smaller numbers. Planktonic foraminifers enable us to subdivide the Cretaceous sediments into Barremian-Aptian, Albian, upper Cenomanian, Turonian, Coniacian-Santonian, Santonian, Campanian, and upper Campanian-Maestrichtian intervals. The Lower Cretaceous (Albian) and Upper Cretaceous (upper Cenomanian-Turonian) are separated by a distinct hiatus and unconformity. In the Upper Cretaceous section, a hiatus may be present at the top of the Campanian. The upper Cenomanian-Santonian sediments are reduced in thickness, whereas the Campanian-Maestrichtian interval is expanded. In the Barremian-Aptian black shales, planktonic foraminifers are very rare: they were deposited in shallow water under anoxic conditions. In the Albian, when sedimentation conditions became oxidizing and the depth increased to 200-400 meters, they became more common. By the end of the Upper Cretaceous, depths appear to increase to 2000 meters. In the interbeds of calcareous sediments, planktonic foraminifers are common; in interbeds of zeolitic clays they are rare or absent (dissolution facies). Alternation of these types of sediments is especially characteristic of the Coniacian-lower Campanian, testifying to abrupt CCD fluctuations. The planktonic foraminifers of the Falkland Plateau belong to the Austral Province of the Southern Hemisphere. In their systematic composition they are extremely similar to microfauna of the Boreal Province of the Northern Hemisphere.

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Use of the hydraulic piston corer during DSDP Leg 70 in the Galapagos mounds area allowed recovery of an undisturbed sedimentary sequence down to the basement. It thus became possible to establish the chronology of different events. Several holes on and off the mounds were studied, using uranium series disequilibrium methods of age determination and oxygen isotope stratigraphy. The following sequence was thereby established: 1) From 600,000 to 300,000 years ago there was normal pelagic sedimentation, with an injection of uranium-rich solution, probably of hydrothermal origin, between 400,000 and 300,000 years ago. 2) From 300,000 to 90,000 years ago, nontronitic clay formed, replacing a pre-existing sediment. 3) From 60,000 to 20,000 years ago, manganese oxide deposits formed, probably also replacing pre-existing sediments. 4) About 19,000 years ago there occurred a uranium injection from seawater, attributed to the end of the hydrothermal circulation. In some holes, especially Hole 424, Leg 54, younger manganese oxides have been found, indicating that some mounds may be presently active.

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Quantitative analyses of selected calcareous nannofossils in deep-sea sections recovered from the paleo-equatorial Pacific (ODP Leg 199) provide new information about biostratigraphy, biochronology and the evolutionary history of calcareous nannofossils across the Paleocene/Eocene transition interval. The sediment cores from ODP Leg 199 represent the first continuous Paleocene/Eocene boundary sections ever to be sampled in the central equatorial Pacific Ocean. Calcareous nannofossil assemblages are studied to document the distribution of biostratigraphically useful taxa such as Ericsonia, Discoaster, Fasciculithus, Rhomboaster and Tribrachiatus. Focus is given to the evolution of the Rhomboaster-Tribrachiatus lineage in the lower Eocene interval at Site 1215, and on the stratigraphic relationship of these taxa relative to species in the genus Fasciculithus. Critical intervals of North Atlantic DSDP Site 550 have also been re-examined. The Tribrachiatus digitalis morphotype was described at Site 550 from an interval affected by down-hole contamination, partly originating from within the Tribrachiatus orthostylus range. The T. digitalis morphotype represents an evolutionary transitional form between T. contortus and T. orthostylus, entering the stratigraphic record within the range of the former species and disappearing within the lower part of the range of the latter species. The subzonal subdivision of Zone NP10 hence collapses. Lithological and colour variability reflecting orbital cyclicity occur in the lower Eocene of Site 1215, permitting a relative astronomical age calibration of the Tribrachiatus taxa. The distinct Rhomboaster spp.-Discoaster araneus association also occurs in the paleo-equatorial Pacific Ocean, together with a marked decrease in diversity of Fasciculithus spp. Site 1220 reveals a short peak abundance of Thoracosphaera spp. just above the P/E boundary interval, which probably reflects a stressed surface water environment.

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The western warm pools of the Atlantic and Pacific oceans are a critical source of heat and moisture for the tropical climate system. Over the past five million years, global mean temperatures have cooled by 3-4 °C. Yet, current reconstructions of sea surface temperatures indicate that temperature in the warm pools has remained stable during this time. This stability has been used to suggest that tropical sea-surface temperatures are controlled by some sort of thermostat-like regulation. Here we reconstruct sea surface temperatures in the South China Sea, Caribbean Sea and western equatorial Pacific Ocean for the past five million years, using a combination of the Mg/Ca, TEXH86-and Uk'37 surface temperature proxies. Our data indicate that during the period of Pliocene warmth from about 5 to 2.6 million years ago, the western Pacific and western Atlantic warm pools were about 2 °C warmer than today. We suggest that the apparent lack of warming seen in the previous reconstructions was an artefact of low seawater Mg/Ca ratios in the Pliocene oceans. Taking this bias into account, our data indicate that tropical sea surface temperatures did change in conjunction with global mean temperatures. We therefore conclude that the temperature of the warm pools of the equatorial oceans during the Pliocene was not limited by a thermostat-like mechanism.