Geochemistry and isotopes at DSDP Site 76-533

Concentrations and d34S and d13C values were determined on SO4, HCO3, CO2, and CH4 in interstitial water and gas samples from the uppermost 400 m of sediment on the Blake Outer Ridge. These measurements provide the basis for detailed interpretation of diagenetic processes associated with anaerobic respiration of electrons generated by organic- matter decomposition. The sediments are anaerobic at very shallow depths (<1 m) below the seafloor. Sulfate reduction is confined to the uppermost 15 m of sediment and results in a significant outflux of oxidized carbon from the sediments. At the base of the sulfate reduction zone, upward-diffusing CH4 is being oxidized, apparently in conjunction with SO4 reduction. CH4 generation by CO2 reduction is the most important metabolic process below the 15-m depth. CO2 removal is more rapid than CO2 input over the depth interval from 15 to 100 m, and results in a slight decrease in HCO3 concentration accompanied by a 40 per mil positive shift in d13C. The differences among coexisting CH4, CO2, and HCO3 are consistent with kinetic fractionation between CH4 and dissolved CO2, and equilibrium fractionation between CO2 and HCO3. At depths greater than 100 m, the rate of input of CO2 (d13C = -25 per mil) exceeds by 2 times the rate of removal of CO2 by conversion to CH4 (d13C of -60 to -65 per mil). This results in an increase of dissolved HCO3 concentration while maintaining d13C of HCO3 relatively constant at +10 per mil. Non-steady-state deposition has resulted in significantly higher organic carbon contents and unusually high (70 meq/l) pore-water alkalinities below 150 m. These high alkalinities are believed to be related more to spontaneous decarboxylation reactions than to biological processes. The general decrease in HCO3 concentration with constant d13C over the depth interval of 200 to 400 m probably reflects increased precipitation of authigenic carbonate. Input-output carbon isotope-mass balance calculations, and carbonate system equilibria in conjunction with observed CO2-CH4 ratios in the gas phase, independently suggest that CH4 concentrations on the order of 100 mmol/kg are present in the pore waters of Blake Outer Ridge sediments. This quantity of CH4 is believed to be insufficient to saturate pore waters and stabilize the CH4*6H2O gas hydrate. Results of these calculations are in conflict with the physical recovery of gas hydrate from 238 m, and with the indirect evidence (seismic reflectors, sediment frothing, slightly decreasing salinity and chlorinity with depth, and pressure core barrel observations) of gas-hydrate occurrence in these sediments. Resolution of this apparent conflict would be possible if CH4 generation were restricted to relatively thin (1-10 m) depth intervals, and did not occur uniformly at all depths throughout the sediment column, or if another methanogenic process (e.g., acetate fermentation) were a major contributor of gas.

Black shales from the Upper Aptian and Lower Albanian of the Volcontian Basin

Calcareous nannoplankton, palynomorph, benthic foraminifera, and oxygen isotope records from the supraregionally distributed Niveau Paquier (Early Albian age, Oceanic Anoxic Event 1b) and regionally distributed Niveau Kilian (Late Aptian age) black shales in the Vocontian Basin (SE France) exhibit variations that reflect paleoclimatic and paleoceanographic changes in the mid-Cretaceous low latitudes. To quantify surface water productivity and temperature changes, nutrient and temperature indices based on calcareous nannofossils were developed. The nutrient index strongly varies in the precessional band, whereas variations of the temperature index reflect eccentricity. Since polar ice caps were not present during the mid-Cretaceous, these variations probably result from feedback mechanisms within a monsoonal climate system of the mid-Cretaceous low latitudes involving warm/humid and cool/dry cycles. A model is proposed that explains the formation of mid-Cretaceous black shales through monsoonally driven changes in temperature and evaporation/precipitation patterns. The Lower Albian Niveau Paquier, which has a supraregional distribution, formed under extremely warm and humid conditions when monsoonal intensity was strongest. Bottom water ventilation in the Vocontian Basin was diminished, probably due to increased precipitation and reduced evaporation in regions of deep water formation at low latitudes. Surface water productivity in the Vocontian Basin was controlled by the strength of monsoonal winds. The Upper Aptian Niveau Kilian, which has a regional distribution only, formed under a less warm and humid climate than the Niveau Paquier. Low-latitude deep water formation was reduced to a lesser extent and/or on regional scale only. The threshold for the formation of a supraregional black shale was not reached. The intensity of increases in temperature and humidity controlled whether black shales developed on a regional or supraregional scale. At least in the Vocontian Basin, the increased preservation of organic matter at the sea floor was more significant in black shale formation than the role of enhanced productivity.

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.

Sapropel cycles in the Mediterranean Sea

Water column stratification increased at climatic transitions from cold to warm periods during the late Quaternary and led to anoxic conditions and sapropel formation in the deep eastern Mediterranean basins. High-resolution data sets on sea-surface temperatures (SST) (estimated from UK'37 indices) and d18O of planktonic foraminifer calcite (d18Ofc) across late Pleistocene sapropel intervals show that d18Ofc decreased (between 1 and 4.6 per mil) and SST increased (between 0.7° and 6.7°C). Maximal d18Oseawater depletion of eastern Mediterranean surface waters at the transition is between 0.5 and 3.0 per mil, and in all but one case exceeded the depletion seen in a western Mediterranean core. The depletion in d18Oseawater is most pronounced at sapropel bases, in agreement with an initial sudden input of monsoon-derived freshwater. Most sapropels coincide with warming trends of SST. The density decrease by initial freshwater input and continued warming of the sea surface pooled fresh water in the surface layer and prohibited deep convection down to ageing deep water emplaced during cold and arid glacial conditions. An exception to this pattern is "glacial" sapropel S6; its largest d18Oseawater depletion (3 per mil) is almost matched by the depletion in the western Mediterranean Sea, and it is accompanied by surface water cooling following an initially rapid warming phase. A second period of significant isotopic depletion is in isotope stage 6 at the 150 kyr insolation maximum. While not expressed as a sapropel due to cold SST, it is in accord with a strengthened monsoon in the southern catchment.

Stable carbon and oxygen isotope record of Cretaceous planktonic foraminifera from DSDP (Table 1)

The Cretaceous Heterohelix moremani (Cushman) was the only biserial planktonic foraminiferal species from its first appearance in the late Albian up to the Cenomanian/Turonian boundary. Within that time, it increased gradually in abundance relative to other planktonic foraminifera in five Circum-North Atlantic sections. It is generally rare in upper Albian sediments, common in most of the Cenomanian and very abundant in sediments representing the latest Cenomanian Oceanic Anoxic Event. Short-term variations on the overall abundance trend correlate with positive excursions in the bulk carbonate delta13C record. Maximum rain rates of H. moremani during OAE2 show that this species was an opportunist that did well in extreme conditions, but its overall distribution indicates that it is not necessarily a marker for very high palaeoproductivity environments. Stable oxygen and carbon isotope measurements on foraminiferal species indicate that H. moremani was a surface water dweller at least in part of its geographic range, but incorporated 13C out of equilibrium with ambient seawater. It is depleted in delta13C relative to other planktonic foraminifera, which is attributed to vital effects related to its opportunistic character.

Re-OS and Pb isotope ratios of sediments from Saanich Inlet, Western Canada

Re-Os and Pb-Pb isotopic analysis of reduced varved sediments cored in the deeper basin of Saanich Inlet (B.C.) are presented. From core top to 61 cm down-core, spanning approximately the last 100 yrs of sedimentation, 187Os/188Os ratio and Os concentration respectively increase from ~0.8 to ~0.9 and from 55 to 60 ppt, whereas Re concentration decreases from 3600 to 2600 ppt. Re correlates with Corg (R2=0.6) throughout the entire section, whereas Os follows Re and Corg trends deeper down-core, suggesting a decoupling of a Re- and Os-geochemistry during burial and/or very early diagenesis. No systematic compositional differences are observed between seasonal laminae. 204Pb-normalized lead isotope ratios increase from sediment surface down to 7 cm down-core, then decrease steadily to pre-industrial levels at ~50 cm down-core. This pattern illustrates the contamination from leaded gasoline until the recent past. The measured Pb isotopic ratios point primarily toward gasoline related atmospheric lead from the USA. The osmium isotopic values measured are significantly lower than those of modern seawater-Os. In comparison with other anoxic environments, the osmium content of Saanich Inlet sediments is low, and its Os isotopic composition suggests significant inputs from unradiogenic sources (detrital and/or dissolved). Ultramafic lithologies in the watershed of the Fraser River are suspected to contribute to sedimentary inputs as well as to the input of dissolved unradiogenic osmium in the water of Saanich Inlet. The presence of some unradiogenic Os from anthropogenic contamination cannot be discounted near the core top, but since deeper, pre-anthropogenic levels also yielded unradiogenic Os results, one is led to conclude that the overall low 187Os/188Os ratios result from natural geochemical processes. Thus, the bulk sediment of Saanich Inlet does not appear to record 187Os/188Os composition of the marine end-member of the only slightly below normal salinity, fjord water. The low seawater-derived Os content of the sediment, coupled with unradiogenic Os inputs from local sources, explains the overall low isotopic values observed. As a consequence, such near-shore anoxic sediments are unlikely to record changes in the past ocean Os isotopic composition.

Cenomanian benthic foraminifera of Demerara Rise

This study is based on Cenomanian sediments of Ocean Drilling Program (ODP) Sites 1258 and 1260 from Demerara Rise (Leg 207, western tropical Atlantic, off Suriname, ~1000 and ~500 m paleo-water depth, respectively). Studied sediments consist of laminated black shales with TOC values between 3 and 18% and include the Mid Cenomanian Event (MCE), a positive carbon isotope excursion predating the well-known Oceanic Anoxic Event 2 (OAE 2). Benthic foraminiferal assemblages of the continuously eutrophic environment at Demerara Rise are characterized by low diversities (<= 9 species per sample) and large fluctuations in abundances, indicating oxygen depletion and varying organic matter fluxes. Dominant species at both sites are Bolivina anambra, Gabonita levis, Gavelinella dakotensis, Neobulimina albertensis, Praebulimina prolixa, and Tappanina cf. laciniosa. Benthic foraminiferal assemblages across the MCE show a threefold pattern: (1) stable ecological conditions below the MCE interval indicated by relatively high oxygenation and fluctuating organic matter flux, (2) decreasing oxygenation and/or higher organic matter flux during the MCE with decreasing benthic foraminiferal numbers and diversities (Site 1258) and a dominance of opportunistic species (Site 1260), and (3) anoxic to slightly dysoxic bottom-water conditions above the MCE as indicated by very low diversities and abundances or even the absence of benthic foraminifera. Slightly dysoxic conditions prevailed until OAE 2 at Demerara Rise. A comparison with other Atlantic Ocean and Tethyan sections indicates that the MCE reflects a paleoceanographic turning point towards lower bottom-water oxygenation, at least in the proto-North Atlantic Ocean and in the Tethyan and Boreal Realms. This general trend towards lower oxygenation of bottom waters across the MCE is accompanied by ongoing climate warming in combination with rising sea-level and the development of vast shallow epicontinental seas during the Middle and Late Cenomanian. These changes are proposed to have favoured the formation of warm and saline waters that may have contributed to intermediate- and deep-water masses at least in the restricted proto-North Atlantic and Tethyan Ocean basins, poor oxygenation of the Late Cenomanian sediments, and the changes in benthic foraminiferal assemblages across the MCE.

CPI, d13C, Corg and diploptene from ODP Hole 169-1034B varved sediments

Saanich Inlet has been a highly productive fjord since the last glaciation. During ODP Leg 169S, nearly 70 m of Holocene sediments were recovered from Hole 1034 at the center of the inlet. The younger sediments are laminated, anaerobic, and rich in organic material (1-2.5 wt.% Corg), whereas the older sediments below 70 mbsf are non-laminated, aerobic, with glacio-marine characteristics and have a significantly lower organic matter content. This difference is also reflected in the changes of interstitial fluids, and in biomarker compositions and their carbon isotope signals. The bacterially-derived hopanoid 17alpha(H),21beta(H)-hop-22(29)-ene (diploptene) occurs in Saanich Inlet sediments throughout the Holocene but is not present in Pleistocene glacio-marine sediments. Its concentration increases after ~6000 years BP up to present time to about 70 µg/g Corg, whereas terrigenous biomarkers such as the n-alkane C31 are low throughout the Holocene (<51 µg/g Corg) and even slightly decrease to 36 µg/g Corg at the most recent time. The increasing concentrations of diploptene in sediments younger than ~6000 years BP separate a recent period of higher primary productivity, stronger anoxic bottom waters, and higher bacterial activity from an older period with lesser activity, heretofore undifferentiated. Carbon isotopic compositions of diploptene in the Holocene are between ~31.5 and ~39.6 per mil PDB after ~6000 years BP. These differences in the carbon isotopic record of diploptene probably reflect changes in microbial community structure of bacteria living at the oxic-anoxic interface of the overlying water column. The heavier isotope values are consistent with the activity of nitrifying bacteria and the lighter isotope values with that of aerobic methanotrophic bacteria. Therefore, intermediate delta13C values probably represent mixtures between the populations. In contrast, carbon isotopic compositions of n-C31 are roughly constant at ~31.4 ± 1.1 per mil PDB throughout the Holocene, indicating a uniform input from cuticular waxes of higher plants. Prior to ~6000 years BP, diploptene enriched in 13C of up to -26.3 per mil PDB is indicative of cyanobacteria living in the photic zone and suggests a period of lower primary productivity, more oxygenated bottom waters, and hence lower bacterial activity during the earliest Holocene.

Diversity of abyssal agglutinated foraminifera from Cenomanian to Santonian sediments of the North Atlantic

Benthic foraminiferal assemblages of distinctive taxonomic composition occur at the top of benthic fossil-free black shales which correspond to the anoxic event at the Cenomanian/Turonian boundary in the North Atlantic abyssal DSDP/ODP sites 386, 398, 603 and 641. These assemblages consist of minute, thin-walled agglutinated foraminifera with low specific diversity of 2 to 4 species, variable abundance and dominance of few taxa (Haplophragmoides, Rhizammina and Glomospira). The species are inferred to be opportunistic, able to survive in low-oxygen environments and to be pioneers recolonizing the seafloor after cessation of bottom-water anoxia. Most species are characterized by test morphologies with high surface/volume ratios and single-layered wall structures, with loosely agglutinated grains, and small amounts of organic cement for agglutination. These features are best observed in material from ODP Hole 641A which has exceptional foraminiferai preservation because of its shallow burial depth. The successive appearance of benthic foraminifera after the anoxic event is probably controlled by the continuous reoccurrence of more oxygenated bottom- and interstitial-water conditions. With the final development of oxic bottom-water conditions in the Turonian, a rapid radiation of deep-water agglutinated foraminifera occurred in the North Atlantic.

Characteristics of sediments from Amvrakikos Gulf, Greece

The evolution of environmental changes during the last decades and the impact on the living biomass in the western part of Amvrakikos Gulf was investigated using abundances and species distributions of benthic foraminifera and lipid biomarker concentrations. These proxies indicated that the gulf has markedly changed due to eutrophication. Eutrophication has led to a higher productivity, a higher bacterial biomass, shifts towards opportunistic and tolerant benthic foraminifera species (e.g. Bulimina elongata, Nonionella turgida, Textularia agglutinans, Ammonia tepida) and a lower benthic species density. Close to the Preveza Strait (connection between the gulf and the Ionian Sea), the benthic assemblages were more diversified under more oxygenated conditions. Sea grass meadows largely contributed to the organic matter at this sampling site. The occurrence of isorenieratane, chlorobactane and lycopane supported by oxygen monitoring data indicated that anoxic (and partly euxinic) conditions prevailed seasonally throughout the western part of the gulf with more severe oxygen depletion towards the east. Increased surface water temperatures have led to a higher stratification, which reduced oxygen resupply to bottom waters. Altogether, these developments led to mass mortality events and ecosystem decline in Amvrakikos Gulf.

Miocene radiolarians of the Sulu Sea

Radiolarians are sporadic in sediments collected in the Sulu Sea during ODP Leg 124. Due to the generally poor preservation and low abundance of radiolarians in Sulu Sea sediments, no biostratigraphic datums are well defined, although three radiolarian zones are identified. Most samples containing radiolarians are pelagic or hemipelagic clays with varying proportions of volcanic ash. Detailed analysis of Sulu Sea radiolarians was limited to Miocene successions. Pliocene and Quaternary occurrences of radiolarians were noted but have not been zoned. The late middle Miocene of Sites 769 and 771 is represented by an assemblage of radiolarians (Diartus petterssoni Zone) that is entirely replaced by massive pyrite. This type of preservation develops only under anoxic conditions. The development of widespread anoxia in Sulu Sea waters in the late middle Miocene was probably the result of hydrologic isolation of basin waters, and may be associated with eustatic sea level fall over the silled basin. Upper lower Miocene pelagic and hemipelagic sediments that overlie pyroclastics and basalt flows in the Sulu Sea sites contain moderately to very poorly preserved radiolarians of the Calocycletta costata Zone. A thin unit of marine claystone was recovered from between the thick pyroclastics and basement rocks at Site 768. Radiolarians present in these claystones are rare and very poorly preserved. This radiolarian assemblage probably represents the C. costata Zone, although very poor preservation and low abundance make this interpretation equivocal. The radiolarian zones identified constrain the age of basin formation to late early Miocene or earlier.

Late Cretaceous benthic foraminiferal assemblages from Demerara Rise, western tropical Atlantic

This paper is based on Santonian-Campanian sediments of Ocean Drilling Program Sites 1257 (2951 mbsl) and 1259 (2353 mbsl) from Demerara Rise (Leg 207, western tropical Atlantic, off Surinam). According to its position, Demerara Rise should have been influenced by the early opening of the Equatorial Atlantic Gateway and the establishment of a bottom-water connection between the central and South Atlantic Oceans during the Late Cretaceous. The investigated benthic foraminiferal faunas demonstrate strong fluctuations in bottom-water oxygenation and organic-matter flux to the sea-floor. The Santonian-earliest Campanian interval is characterised by laminated black shales without benthic foraminifera in the lowermost part, followed by an increasing number of benthic foraminifera. These are indicative of anoxic to dysoxic bottom waters, high organic-matter fluxes and a position within the oxygen minimum zone. At the shallower Site 1259, benthic foraminifera occurred earlier (Santonian) than at the deeper Site 1257 (Early Campanian). This suggests that the shallower site was characterised by fluctuations in the oxygen minimum zone and that a re-oxygenation of the sea-floor started considerably earlier at shallower water-depths. We speculate that this re-oxygenation was related to the ongoing opening of the Equatorial Atlantic Gateway. A condensed glauconitic chalk interval of Early Campanian age (Nannofossil Zone CC18 of Sissingh) overlies the laminated shales at both sites. This interval contains benthic foraminiferal faunas reflecting increasing bottom-water oxygenation and reduced organic-matter flux. This glauconitic chalk is strongly condensed and contains most of the Lower and mid-Campanian. Benthic foraminiferal species indicative of well-oxygenated and more oligotrophic environments characterise the overlying mid- to Upper Campanian nannofossil chalk. During deposition of the nannofossil chalk, a permanent deep-water connection between the central and South Atlantic Oceans is proposed, leading to ventilated and well-oxygenated bottom waters. If this speculation is true, the establishment of a permanent deep-water connection between the central and South Atlantic Oceans terminated Oceanic Anoxic Event 3 "black shale" formation in the central and South Atlantic marginal basins during the Early Campanian (Nannofossil Zone CC18) and led to well-oxygenated bottom waters in the entire Atlantic Ocean during the Late Campanian (at least from Nannofossil Zone CC22 onwards).