Curie temperatures and hysteresis properties of ODP Leg 183 sites, Kerguelen Plateau and Broken Ridge

In this manuscript, we present rock magnetic results of samples recovered during Leg 183. The Leg 183 cores were recovered from six drill sites and display variable rock magnetic properties. The differences in the rock magnetic properties are a function of mineralogy and alteration. Cretaceous subaerial basalt samples with titanomagnetite exhibit a strong Verwey transition in the vicinity of 110 K and have frequency-dependent susceptibility curves that resemble those of synthetic (titano) magnetites. These results are in good agreement with the thermomagnetic characteristics where titanomagnetites with Curie temperatures of ~580°C were identified. The hysteresis ratios suggest that the bulk magnetic grain size is in the psuedo-single-domain boundary. These subaerial basalts experienced high-temperature oxidation and maintained reliable paleomagnetic records. In contrast, the 34-Ma submarine pillow basalts do not show the Verwey transition during the low-temperature experiments. Thermomagnetic analysis shows that the remanent magnetization in this group is mainly carried by a thermally unstable mineral titanomaghemite. The frequency-dependent relationships are opposite of those from the first group and show little sign of titanomagnetite characteristics. Rocks from the third group are oxidized titanomagnetites and have multiple magnetic phases. They have irreversible thermaomagnetic curves and hysteresis ratios clustering toward the multidomain region (with higher Hcr/Hc ratios). The combined investigation suggests that variations in magnetic properties correlate with changes in lithology, which results in differences in the abundance and size of magnetic minerals. The rock magnetic data on Leg 183 samples clearly indicate that titanomagnetite is the dominant mineral and the primary remanence carrier in subaerial basalt. The generally good magnetic stability and other properties exhibited by titanomagnetite-bearing rocks support the inference that the ChRM isolated from the Cretaceous sites were acquired during the Cretaceous Normal Superchron. The stable inclinations identified from these samples are therefore useful for future tectonic studies.

Organic geochemistry of late Cenozoic sediments from the eastern subtropical Atlantic

Detailed organic geochemical investigations have been performed on sediment samples from upwelling Site 658 and nonupwelling Sites 657 and 659. The major objective of this study has been the relationship between organic carbon accumulation and paleoclimatic and paleoceanographic conditions in the upwelling area off northwest Africa during late Cenozoic times. The study is based on results from organic carbon, nitrogen, and hydrogen analyses, Rock-Eval pyrolysis, kerogen microscopy, gas chromatography, and gas chromatography/mass spectrometry. In general, nonupwelling Sites 657 and 659 are characterized by low organic carbon values of less than 0.5%. At Site 657, four events of high organic carbon deposition (total organic carbon of 1%-3%) occur and represent turbidites and a slump interval. The upper Pliocene to Pleistocene sediments of upwelling Site 658 display high organic carbon contents of 0.5%-4%, with higher contents concentrated in the upper Pliocene. Accumulation rates of organic carbon vary between 0.1 and 0.5 gC/cm-**2/1000 yr, with maximum values between 3.5 and 3.1 Ma. Short-term cyclic ("Milankovitch-type") variations in organic carbon accumulation suggest climate-controlled mechanisms causing these fluctuations. The quality of organic matter at Site 658 is a mixture of kerogen type II and HI, with a dominance of the marine type. This is indicated by high hydrogen-index values of 200-400 mgHC/gC, low C/N ratios of 5-15, atomic H/C ratios of 1.0-1.5, and high amounts of marine macerals (alginite and liptodetrinite). We have estimated paleoproductivity for Sites 658 and 659 based on the amount of marine organic carbon. At open-marine Site 659, mean paleoproductivity varies between 20 and 50 gC/m**2/yr. At Site 658, mean paleoproductivity reaches high values of 160 to 320 gC/m**2/yr, very similar to those recorded in modern upwelling areas. The changes in productivity off northwest Africa are linked to changes in nutrient supply caused by both upwelling and fluvial input. The change from a dominantly humid climate to one characterized by fluctuations between humid and fully arid climates in northwest Africa occurs between 3.1 and 2.45 Ma.

Composition of swamp waters from the lower Tom' River Basin

New data on chemical and trace component compositions of acidic and low acidic swamp waters and other types of low mineralized waters are reported in the paper. Special attention is paid to dissolved organic compounds: fulvic and humic acids, bitumen, and hydrocarbons. For the first time detailed data on organic trace components (alkanes, pentacyclic terpenoids, steranes, alkylbenzenes, naphthalenes, phenanthrenes, tetraarenes, etc.) in the swamp waters of the Western Siberia: are reported.

Organic geochemistry of lower Cretaceous sediments at DSDP Hole 77-535

At Site 535, the four lithologic units of Cretaceous age are controlled by two types of sedimentologic facies: (1) the massive light-colored limestones or marly limestones in which the total organic carbon (TOC) content is low and the organic matter more or less oxidized and (2) laminated dark facies in which the TOC content is higher and associated with a well-preserved organic matter of Type II origin. Very little typical Type III organic matter occurs in the whole series from late Berriasian to Aptian and Cenomanian. Fluctuations from oxidizing to reducing environments of deposition are proposed to account for the variations in properties of the Type II organic matter between the different facies. Dark laminated layers are good but immature potential source rocks: petroleum potential is often higher than 2 kg HC/t of rock.

Lithology and organic geochemistry at DSDP Hole 76-534

The organic facies of Early and middle Cretaceous sediments drilled at DSDP Site 534 is dominated by terrestrially derived plant remains and charcoal. Marine organic matter is mixed with the terrestrial components, but through much of this period was diluted by the terrestrial material. The supply of terrestrial organic matter was high here because of the nearness of the shore and high runoff promoted by a humid temperate coastal climate. Reducing conditions favored preservation of both marine and terrestrial organic matter, the terrestrial materials having reached the site mostly in turbidity currents or in the slow-moving, near-bottom nepheloid layer. An increase in the abundance of terrestrial organic matter occurred when the sea level dropped in the Valanginian and again in the Aptian-Albian, because rivers dumped more terrigenous elastics into the Basin and marine productivity was lower at these times than when sea level was high. A model is proposed to explain the predominance of reducing conditions in the Valanginian-Aptian, of oxidizing conditions in the late Aptian, and of reducing conditions in the Albian-Cenomanian. The model involves influx of oxygen-poor subsurface waters from the Pacific at times of high or rising sea level (Valanginian-Aptian, and Albian- Cenomanian) and restriction of that influx at times of low sea level (late Aptian). In the absence of a supply of oxygenpoor deep water, the bottom waters of the North Atlantic became oxidizing in the late Aptian, probably in response to development of a Mediterranean type of circulation. The influx of nutrients from the Pacific led to an increase in productivity through time, accounting for an increase in the proportion of marine organic matter from the Valanginian into the Aptian and from the Albian to the Cenomanian. Conditions were dominantly oxidizing through the Middle Jurassic into the Berriasian, with temporary exceptions when bottom waters became reducing, as in the Callovian. Mostly terrestrial and some marine organic matter accumulated during the Callovian reducing episode. When Jurassic bottom waters were oxidizing, only terrestrial organic matter was buried in the sediments, in very small amounts.

Analysis of the iron oxide assemblages in the ANDRILL AND-1B drill core, Ross Sea, Antarctica

The AND-1B drill core recovered a 13.57 million year Miocene through Pleistocene record from beneath the McMurdo Ice Shelf in Antarctica (77.9°S, 167.1°E). Varying sedimentary facies in the 1285 m core indicate glacial-interglacial cyclicity with the proximity of ice at the site ranging from grounding of ice in 917 m of water to ice free marine conditions. Broader interpretation of climatic conditions of the wider Ross Sea Embayment is deduced from provenance studies. Here we present an analysis of the iron oxide assemblages in the AND-1B core and interpret their variability with respect to wider paleoclimatic conditions. The core is naturally divided into an upper and lower succession by an expanded 170 m thick volcanic interval between 590 and 760 m. Above 590 m the Plio-Pleistocene glacial cycles are diatom rich and below 760 m late Miocene glacial cycles are terrigenous. Electron microscopy and rock magnetic parameters confirm the subdivision with biogenic silica diluting the terrigenous input (fine pseudo-single domain and stable single domain titanomagnetite from the McMurdo Volcanic Group with a variety of textures and compositions) above 590 m. Below 760 m, the Miocene section consists of coarse-grained ilmenite and multidomain magnetite derived from Transantarctic Mountain lithologies. This may reflect ice flow patterns and the absence of McMurdo Volcanic Group volcanic centers or indicate that volcanic centers had not yet grown to a significant size. The combined rock magnetic and electron microscopy signatures of magnetic minerals serve as provenance tracers in both ice proximal and distal sedimentary units, aiding in the study of ice sheet extent and dynamics, and the identification of ice rafted debris sources and dispersal patterns in the Ross Sea sector of Antarctica.

Analysis of organic matter from DSDP Leg 81 Holes

Organic geochemical and visual kerogen analyses were carried out on approximately 50 samples from Leg 81 (Rockall Plateau, North Atlantic). The sediments are from four sites (Sites 552-555), Pleistocene to Paleocene in age, and represent significantly different depositional environments and sources of organic matter. The Pleistocene glacial-interglacial cycles show differences in sedimentary organic matter based on Rock-Eval pyrolysis, organic phosphorus, and pyrolysis/mass-spectrometry analyses. Glacial samples contain more organic carbon, with a larger proportion of reworked organic matter. This probably reflects increased erosion of continental and shelf areas as a result of low sea level stands. Inter glacial samples contain a larger proportion of marine organic matter as determined by organic phosphorus and pyrolysis analyses. This immature, highly oxidized marine organic matter may be associated with the skeletal organic matrix of calcareous organisms. In addition, Rock-Eval data indicate no significant inorganic-carbonate contribution to the S3 pyrolysis peak. The Pliocene-Miocene sediments consist of pelagic, biogenic carbonates. The organic matter is similar to that of the Pleistocene interglacial periods; a mixture of oxidized marine organic matter and reworked, terrestrial detritus. The Paleocene-Oligocene organic matter reflects variations in source and depositional factors associated with the isolation of Rockall from Greenland. Paleocene sediments contain primarily terrestrial organic matter with evidence of in situ thermal stress resulting from interbedded lava flows. Late Paleocene and early Eocene organic matter suggests a highly oxidized marine environment, with major periods of deposition of terrestrially derived organic matter. These fluctuations in organic-matter type are probably the result of episodic shallowing and deepening of Rockall Basins. The final stage of Eocene/Oligocene sedimentation records the accelerating subsidence of Rockall and its isolation from terrestrial sources (Rockall and Greenland). This is shown by the increasingly marine character of the organic matter. The petroleum potential of sediments containing more than 0.5% organic carbon is poor because of their thermal immaturity and their highly oxidized and terrestrial organic-matter composition.

Mineralogy and pyrolysis at DSDP Hole 62-463

A 15-meter sequence of early Aptian organic-matter-rich sediments, cored at Deep Sea Drilling Project Site 463 (western Mid-Pacific Mountains) has been submitted for detailed mineralogical studies (XRD, SEM) and organiccarbon characterization. Although intense diagenesis has obscured the sedimentary record of depositional conditions, the history has been tentatively reconstructed. Through sustained volcanic activity and alteration processes on the archipelago, large amounts of silica were released into the sea water, resulting in a "bloom" of radiolarians. Hard parts settled in large amounts, yielding a hypersiliceous sediment; amorphous silica was diagenetically transformed into chalcedony, opal-CT and clinoptilolite through dissolution and recrystallization. Oxidization of part of the radiolarian soft parts (1) depleted the sea water in dissolved oxygen, allowing the burial of organic matter, and (2) generated carbon dioxide which led to dissolution of most of the calcareous tests. Moderate depositional depth and a high sedimentation rate are though to have prevailed during this episode. An immature stage of evolution is assigned to the studied organic matter, which is of two origins: autochthonous marine material, and allochthonous humic compounds and plant debris. Rhythmic sedimentation characterizes the distribution of the organic matter; each sequence shows (1) an upward progressive increase in organic-carbon content, and (2) an upward enrichment in marine organic matter.

Palynology on sediment profile Tso Kar in Ladakh

Palynological investigation of a 410 cm long core section from Tso Kar (33°10'N, 78°E, 4527 m a.s.l.), an alpine lake situated in the arid Ladakh area of NW India at the limit of the present-day Indian summer monsoon, was performed in order to reconstruct post-glacial regional vegetation and climate dynamics. The area was covered with alpine desert vegetation from ca. 15.2 to 14 kyr BP (1 kyr=1000 cal. years), reflecting dry and cold conditions. High influx values of long-distance transported Pinus sylvestris type pollen suggest prevailing air flow from the west and northwest. The spread of alpine meadow communities and local aquatic vegetation is a weak sign of climate amelioration after ca. 14 kyr BP. Pollen data (e.g. influx values of Pinus roxburghii type and Quercus) suggest that this was due to a strengthening of the summer monsoon and the reduced activity of westerly winds. The further spread of Artemisia and species-rich meadows occurred in response to improved moisture conditions between ca. 12.9 and 12.5 kyr BP. The subsequent change towards drier desert-steppe vegetation likely indicates more frequent westerly disturbances and associated snowfalls, which favoured the persistence of alpine meadows on edaphically moist sites. The spread of Chenopodiaceae-dominated vegetation associated with an extremely weak monsoon occurred at ca. 12.2-11.8 kyr BP during the Younger Dryas interstadial. A major increase in humidity is inferred from the development of Artemisia-dominated steppe and wet alpine meadows with Gentianaceae after the late glacial/early Holocene transition in response to the strengthening of the summer monsoon. Monsoonal influence reached maximum activity in the Tso Kar region between ca. 10.9 and 9.2 kyr BP. The subsequent development of the alpine meadow, steppe and desert-steppe vegetation points to a moderate reduction in the moisture supply, which can be linked to the weaker summer monsoon and the accompanying enhancement of the winter westerly flow from ca. 9.2 to 4.8 kyr BP. The highest water levels of Tso Kar around 8 kyr BP probably reflect combined effect of both monsoonal and westerly influence in the region. An abrupt shift towards aridity in the Tso Kar region occurred after ca. 4.8 kyr BP, as evidenced by an expansion of Chenopodiaceae-dominated desert-steppe. Low pollen influx values registered ca. 2.8-1.3 kyr BP suggest scarce vegetation cover and unfavourable growing conditions likely associated with a further weakening of the Indian Monsoon.

Organic- and carbonate-carbon values and extract data for DSDP Holes 63-467 and 63-471

Quantity, type, and maturity of the organic matter of middle Miocene to Quaternary sediments from the eastern North Pacific (Deep Sea Drilling Project Leg 63) were determined. Hydrocarbons and fatty acids in lipid extracts were analyzed by capillary column gas chromatography and combined gas chromatography/mass spectrometry. Kerogens were investigated by Rock-Eval pyrolysis and microscopy, and vitrinite reflectance values were determined. At Site 467, in the San Miguel Gap of the outer California Continental Borderland, organic carbon contents range from 1.46% to 5.40%. Normalized to organic carbon, total extracts increase from about 10 to 36 mg/g Corg with depth. The organic matter is a mixture of both marine and terrestrial origin, with the marine organic matter representing a high proportion in some of the samples. Steroid hydrocarbons - sterenes and steradienes in the upper part of the section and steranes in the deepest sample - are the most abundant compounds in the nonaromatic hydrocarbon fractions. Perylene, alkylated thiophenes, and aromatic steroid hydrocarbons dominate in the aromatic hydrocarbon fractions of the shallower samples; increasing maturation is indicated by a more petroleumlike aromatic hydrocarbon distribution. Microscopy revealed a high amount of liptinitic organic matter and confirmed the maturation trend as observed from analysis of the extracts. The vitrinite reflectance may be extrapolated to a bottom-hole value of nearly 0.5% Ro. The liquid hydrocarbon potential of the sediments at higher maturity levels is rated to be good to excellent. At Site 471, off Baja California, organic carbon values are between 0.70% and 1.12%. Extract values increase with depth, as at Site 467. The investigation of the soluble and insoluble organic matter, despite some compositional similarities, consistently revealed a more terrigenous influx compared with Site 467. Thus the potential for liquid hydrocarbon generation is lower, the organic matter being more gas-prone. The deepest sample analyzed indicates the onset of hydrocarbon generation. At this site, frequent sand intercalations offer pathways for migration and possibly reservoir formation.

Petrography and geochemistry of organic matter in Triassic and Cretaceous sediments from the Wombat and Exmouth Plateau

Triassic (Carnian-Rhaetian) continental margin sediments from the Wombat Plateau off northwest Australia (Sites 759, 760, 761, and 764) contain mainly detrital organic matter of terrestrial higher plant origin. Although deposited in a nearshore deltaic environment, little liptinitic material was preserved. The dominant vitrinites and inertinites are hydrogen-lean, and the small quantities of extractable bitumen contain w-alkanes and bacterial hopanoid hydrocarbons as the most dominant single gas-chromatography-amenable compounds. Lower Cretaceous sediments on the central Exmouth Plateau (Sites 762 and 763) farther south in general have an organic matter composition similar to that in the Wombat Plateau sediments with the exception of a smaller particle size of vitrinites and inertinites, indicating more distal transport and probably deposition in deeper water. Nevertheless, organic matter preservation is slightly better than in the Triassic sediments. Long-chain fatty acids, as well as aliphatic ketones and alcohols, are common constituents in the Lower Cretaceous sediments in addition to n-alkanes and hopanoid hydrocarbons. Thin, black shale layers at the Cenomanian/Turonian boundary, although present at several sites (Sites 762 and 763 on the Exmouth Plateau, Site 765 in the Argo Abyssal Plain, and Site 766 on the continental margin of the Gascoyne Abyssal Plain), are particularly enriched in organic matter only at Site 763 (up to 26%). These organic-matter-rich layers contain mainly bituminite of probable fecal-pellet origin. Considering the high organic carbon content, the moderate hydrogen indices of 350-450 milligrams of hydrocarbon-type material per gram of Corg, the maceral composition, and the low sedimentation rates in the middle Cretaceous, we suggest that these black shales were accumulated in an area of oxygen-depleted bottom-water mass (oceanwide reduced circulation?) underlying an oxygen-rich water column (in which most of the primary biomass other than fecal pellets is destroyed) and a zone of relatively high bioproductivity. Differences in organic matter accumulation at the Cenomanian/Turonian boundary at different sites off northwest Australia are ascribed to regional variations in primary bioproductivity.

Hydrous and anhydrous pyrolysis of DSDP Leg 75 kerogens

The aliphatic hydrocarbon distributions obtained from the natural bitumens of three Leg 75 sediments were compared using computerised gas chromatography-mass spectrometry (C-GC-MS). The kerogens isolated from these sediments were heated in sealed tubes at 330°C using the techniques of hydrous (i.e. heating kerogen in the presence of water) and anhydrous pyrolysis (i.e. heating dry kerogen alone). These experiments were then repeated at a lower temperature (280°C). At 330°C, under anhydrous conditions, considerable destruction of biomarkers in the ancient kerogens (i.e. pre-Tertiary) occurred, whereas with water present significant amounts of hopanes were obtained. However, with more recent kerogens (which contain larger amounts of chemically bound water), both anhydrous and hydrous pyrolysis gave a similar suite of biological markers, in which long chain acyclic isoprenoids (C40) are significant components. Lowering the temperature of pyrolysis to 280°C yielded biological markers under both hydrous and anhydrous conditions for all kerogens. n-Alkenes were not detected in any of the pyrolysates; however, a single unknown triterpene was discovered in several of the hydrous and anhydrous pyrolysates. The results tentatively indicate that the chief value to petroleum research of kerogen hydrous pyrolysis lies in its ability to increase the yield of pyrolysate. High temperature hydrous pyrolysis (280-330°C), under high pressure (2000 psi), does not appear to mimic natural conditions of oil generation. However, this study does not take into account whole rock pyrolysis.

Carbonate, organic carbon and nitrogen, bitumen and rock-eval pyrolysis at DSDP Leg 79 Holes

The lipids and kerogens of 15 sediment samples from Site 547 (ranging from Pleistocene to Early Jurassic/Triassic) and 4 from Site 545 (Cretaceous) have been analyzed. A strong terrestrial contribution of organic matter was found, and significant autochthonous inputs were also present, especially at Site 545. Both strongly reduced and highly oxidized sediments have been found in the Cenozoic and Jurassic samples of Site 547. On the contrary, all the Cretaceous sections of Sites 547 and 545 are anoxic. Sediments from anoxic paleoenvironments are immature and have a high content of sterenes, diasterenes, steradienes, hopenes, and ßß hopanes. Samples from oxic paleoenvironments are mainly mature and their content of hopenes and steriod structures is below the detection level. Nevertheless, their hopane distributions have the immature ßß homologs as the predominant molecular markers. For Site 545 the most abundant molecular markers are ring A monoaromatic steranes, and their presence is attributed to microbial and chemical transformations during early diagenesis.