Alkenon concentrations of Creatcesous sediments of DSDP Hole 76-534A (Table 1)

We here report the discovery of unusual distributions of long-chain alkenones (C37-C42) in two Cretaceous black shales from the Blake-Bahama Basin, western North Atlantic. These sediments are Cenomanian (c. 95 Ma) and mid-Albian (c. 105 Ma) in age, thus significantly extending the geological range of these compounds. The precise source of these lipids is, as yet, unknown, although they may derive from an ancient ancestor of Emiliania huxleyi.

Macerals in sediments

The study of particulate organic matter (OM) in Arctic Ocean sediments from the Late Cretaceous to the Eocene (IODP Expedition 302) has revealed detailed information about the aquatic/marine OM fluxes, biological sources, preservation and export of terrestrial material. Here, we present detailed data from maceral analysis, vitrinite reflectance measurements and organic geochemistry. During the Campanian/Paleocene, fluxes of land-derived OM are indicated by reworked and oxidized macerals (vitrinite, inertinite) and terrigenous liptinite (cutinite, sporinite). In the Early Eocene, drastic environmental changes are indicated by peaks in aquatic OM (up to 40-45%, lamalginite, telalginite, liptodetrinite, dinoflagellate cysts) and amorphous OM (up to 50% bituminite). These events of increased aquatic OM flux, similar to conditions favoring black shale deposition, correlate with the global d13C events "Paleocene/Eocene Thermal Maximum" (PETM) and "Elmo-event". Freshwater discharge and proximity of the source area are documented by freshwater algae material (Pediastrum, Botryococcus) and immature land-plant material (corphuminite, textinite). We consider that erosion of coal-bearing sediments during transgression time lead to humic acids release as a source for bituminite deposited in the Early Eocene black shales.

Rock eval analysis of organic matter in sediment core 159-959D

The Cretaceous Equatorial Atlantic Gateway between the Central and South Atlantic basins is of interest not only for paleoceanographic and paleoclimatic studies, but also because it provided particularly favourable conditions for the accumulation and preservation of organic-rich sediments. Deposition of carbonaceous sediments along the Côte d'Ivoire-Ghana Transform Margin (Ocean Drilling Program Leg 159) was intimately linked to the plate tectonic and paleoceanographic evolution of this gateway. Notably, the formation of a marginal basement ridge on the southeastern border of the transform margin provided an efficient shelter of the landward Deep Ivorian Basin against erosive and potentially oxidizing currents. Different subsidence histories across the transform margin were responsible for the development of distinct depositional settings on the crest and on both sides of the basement ridge. Whereas the southern, oceanward flank of the basement ridge was characterized by rapid, continuous deepening since last Albian-early Cenomanian, marine sedimentation on the northern, landward flank was interrupted by a period of uplift and erosion in the late Albian, and rapid subsidence started after the early Coniacian. Organic-rich sediments occur throughout almost the entire Cretaceous section, but hydrogen-rich marine black shales were exclusively recovered from core sections above an uplift-related unconformity. These black shales formed when separation of Africa and South America was sufficient to allow permanent oceanic midwater exchange after the late Albian. Four periods of black shale accumulation are recovered, some of them are correlated with the global oceanic anoxic events: in the last Albian-earliest Cenomanian, at the Cenomanian-Turronian boundary, during the middle Coniacian-early Campanian, and in the mid-Maastrichtian. These periods were characterized by increasing carbon flux to the seafloor, induced by enhanced palaeoproductivity and intensified supply of terrestrial organic matter. Black shale depostion appears to be intimately linked to periods of rising or maximum eustatic sea level and to the expansion of the oxygen minimum zone, as indicated by foraminiferal biofacies. Intervals between black shales units, in contrast, indicate a shrinking oxygen minimum zone and enhanced detrital flux rates, probably related to lowering sea level. Upper Cretaceous detritral limestones with high porosities may provide excellent hydrocarbon reservoirs, alsthough their areal extent appears to be limited. Palaeogene porcellanites, capped by Neogene pelagic marls and clays, extend over a wider area and max provide another target for hydrocarbon exploration.

Geochemistry of Early Toarcian carbonates

The Early Toarcian was characterized by two large perturbations to the carbon cycle: a positive trend associated with increased organic matter burial and ocean anoxia, and a pronounced negative carbon isotope excursion (CIE). We contrast the geochemical evolution in the carbonate phases of two successions: one from the Paris Basin (Sancerre core, comprising black shales), the other from the Lusitanian Basin (Peniche section with very minor lithological expression of bottom water anoxia). Our aim was to identify whether these carbon cycle perturbations were related, and differentiate between the common (global) versus regional expressions of the biogeochemical response and ocean chemistry. Our results highlight contrasts in timing of different phases of anoxia in both locations through the widely documented negative CIE. Widespread anoxic conditions were not a necessary prerequisite for generating a pronounced CIE, as required by the recycling (so-called "Küspert") model. The production of carbonate simultaneously dropped during the d13C negative shift in both locations, likely in response to lowered seawater saturation rate induced by substantial absorption of CO2 from the atmosphere. The recovery interval was accompanied by a rapid reestablishment of seawater alkalinity, and primary and carbonate productivity in epicontinental seas, as evidenced by high d13C and Sr/Ca, in contrast with the more open ocean regime in the Lusitanian Basin. Our results confirm that parallels can be draw between the ocean productivity response and feedback during the Toarcian CIE and the PETM. Both events are characterized by ocean acidification and reduced pelagic calcification followed by a peak in nearshore coccolith productivity, which could have helped the recovery from the perturbation.

A. Geology and mineral paragenesis of the Pea Ridge Iron Ore Mine, Washington County, Missouri : origin of the rare-earth-element- and goldbearing breccia pipes /

Includes bibliographical references.

Hydrothermal control on organic matter alteration and illite precipitation, Mt Isa Basin, Australia

Abundant illite precipitation, in Proterozoic rocks from Northern Lawn Hill Platform, Mt Isa Basin, Australia, occurred in organic matter-rich black shales rather than in sandstones, siltstones and organic matter-poor shales. Sandstones and siltstones acted as impermeable rocks, as early diagenetic quartz and carbonate minerals reduced the porosity-permeability. Scanning and transmission electron microscopy (SEM and TEM) studies indicate a relation between creation of microporosity-permeability and organic matter alteration, suitable for subsequent mineral precipitation. K-Ar data indicate that organic matter alteration and the subsequent illite precipitation within the organic matter occurred during the regional hydrothermal event at 1172 +/- 150 (2sigma) Ma. Hot circulating fluids are considered to be responsible for organic matter alteration, migration and removal of volatile hydrocarbon, and consequently porosity-permeability creation. Those rocks lacking sufficient porosity-permeability, such as sandstones, siltstones and organic matter poor shales, may not have been affected by fluid movement. In hydrothermal systems, shales and mudstones may not be impermeable as usually assumed because of hydrocarbons being rapidly removed by fluid, even with relatively low total organic carbon.

Paleoenvironments and geochemical signals from the late Barremian to the middle Aptian in a Tethyan marginal basin, northeast Spain: Implications for carbon sequestration in restricted basins

The hallmark of oceanic anoxic event 1a (OAE1a) (early Aptian ~125 Ma) corresponds to worldwide deposition of black shales with total organic carbon (TOC) content > 2% and a δ13C positive excursion up to ~5‰. OAE1a has been related to large igneous province volcanism and dissociation of methane hydrates during the Lower Cretaceous. However, the occurrence of atypical, coeval and diachronous organic-rich deposits associated with OAE1a, which are also characterized by positive spikes of the δ 13C in epicontinental to restricted marine environments of the Tethys Ocean, indicates localized responses decoupled from complex global forcing factors. ^ The present research is a high-resolution, multiproxy approach to assess the paleoenvironmental conditions that led to enhanced carbon sequestration from the late Barremian to the middle Aptian in a restricted, Tethyan marginal basin prior to and during OAE1a. I studied the lower 240 m of the El Pui section, Organyà Basin, Spanish Pyrenees. The basin developed as the result of extensional tectonism linked to the opening of the Atlantic Ocean. At the field scale the section consists of a sequence of alternating beds of cm – m-scale, medium-gray to grayish-black limestones and marlstones with TOC up to ~4%. ^ The results indicate that the lowest 85 m of the section, from latest Barremian -earliest Aptian, characterize a deepening phase of the basin concomitant with sustained riverine flux and intensified primary productivity. These changes induced a shift in the sedimentation pattern and decreased the oxygen levels in the water column through organic matter respiration and limited ventilation of the basin. ^ The upper 155 m comprising the earliest – late-early Aptian document the occurrence of OAE1a and its associated geochemical signatures (TOC up to 3% and a positive shift in δ13C of ~5‰). However, a low enrichment of redox-sensitive trace elements indicates that the basin did not achieve anoxic conditions. The results also suggest that a shallower-phase of the basin, coeval with platform progradation, may have increased ventilation of the basin at the same time that heightened sedimentation rates and additional input of organic matter from terrestrial sources increased the burial and preservation rate of TOC in the sediment.^

Paleoenvironmental Implications of the Indidura Formation (Cenomanian/Turonian), Northeastern Mexico: a High Resolution Stratigraphic Study

High-resolution lithostratigraphic data from rock sequences known as the Indidura Formation near Parras de La Fuente, Coahuila, NE Mexico, led to achieve a significant improvement of our knowledge of that Formation. The results of this study indicate for the first time that the sequence at Parras de La Fuente developed from the deposition of calcareous cyanobacterial microspheroids that accumulated under perennial blooms during the Late Cenomanian through the Middle Turonian. Multi-proxy analyses included sedimentological, petrographical, scanning electron microscopy, stable isotope, trace element geochemistry, and paleontological data. The combined results allowed the correlation of δ13C and anomalies in Mo, V, and Cr with the abundance and predominance of calcareous cyanobacterial microspheroids, which were the main suppliers of the carbonate components and the organic matter throughout deposition of the Indidura Formation in the Parras de la Fuente area, under dysoxic/anoxic conditions. Conspicuous interbeds of dark and light-gray laminated marly calcilutites, and dark-gray marlstones that characterize the stratigraphic sequence formed in response to external forcing climatic factors of millennial-scale Milankovitch cycles (ca. 20 ka precession). At the microscopic level, the prominent dark and light-gray laminae were formed during cycles similar to the 10 to 15 years solar irradiance maximum, and represent alternating periods of high and low calcareous cyanobacterial microspheroids productivity.

Magnetic suceptibility, carbon and oxygen stable isotope ratios and reflectance from the Bottaccione Gorge and Furlo section, Italy

The oceans at the time of the Cenomanian-Turonian transition were abruptly perturbed by a period of bottom-water anoxia. This led to the brief but widespread deposition of black organic-rich shales, such as the Livello Bonarelli in the Umbria-Marche Basin (Italy). Despite intensive studies, the origin and exact timing of this event are still debated. In this study, we assess leading hypotheses about the inception of oceanic anoxia in the Late Cretaceous greenhouse world, by providing a 6-Myr-long astronomically-tuned timescale across the Cenomanian-Turonian boundary. We procure insights in the relationship between orbital forcing and the Late Cretaceous carbon cycle by deciphering the imprint of astronomical cycles on lithologic, geophysical, and stable isotope records, obtained from the Bottaccione, Contessa and Furlo sections in the Umbria-Marche Basin. The deposition of black shales and cherts, as well as the onset of oceanic anoxia, is related to maxima in the 405-kyr cycle of eccentricity-modulated precession. Correlation to radioisotopic ages from the Western Interior (USA) provides unprecedented age control for the studied Italian successions. The most likely tuned age for the Livello Bonarelli base is 94.17 ± 0.15 Ma (tuning #1); however, a 405-kyr older age cannot be excluded (tuning #2) due to uncertainties in stratigraphic correlation, radioisotopic dating, and orbital configuration. Our cyclostratigraphic framework suggests that the exact timing of major carbon cycle perturbations during the Cretaceous may be linked to increased variability in seasonality (i.e. a 405-kyr eccentricity maximum) after the prolonged avoidance of seasonal extremes (i.e. a 2.4-Myr eccentricity minimum). Volcanism is probably the ultimate driver of oceanic anoxia, but orbital periodicities determine the exact timing of carbon cycle perturbations in the Late Cretaceous. This unites two leading hypotheses about the inception of oceanic anoxia in the Late Cretaceous greenhouse world.

(Table T2) Relative signal intensity of intact polar lipids in sediments from ODP Sites 207-1257 and 207-1258

We report results from the analysis of intact polar lipids (IPLs) in sediments from Ocean Drilling Program Sites 1257 and 1258. IPLs, constituting the cell membranes of living organisms, were detected in organic-lean sediments but not in underlying organic-rich black shales. Microbial activity in organic-lean sediments is likely due to sulfate-dependent oxidation of methane whereas difficulties detecting IPLs in black shales are interpreted to result from unfavorable signal-to-noise ratios due to low cell concentrations in combination with extremely high analytical noise created by uncharacterized organic matrix. IPLs found are consistent with a low-diversity community of archaea and bacteria. The concentrations of IPLs are more than one order of magnitude lower than those in Neogene deep subsurface sediments at the Peruvian margin, suggestive of significantly lower cell concentrations in Demerara Rise. This finding is consistent with inferred low rates of subsurface microbial activity.

Nd isotopes an geochemistry of bulk deep sea sediments of the Atlantic Ocean

Nd isotopes preserved in fossil fish teeth and ferromanganese crusts have become a common tool for tracking variations in water mass composition and circulation through time. Studies of Nd isotopes extracted from Pleistocene to Holocene bulk sediments using hydroxylamine hydrochloride (HH) solution yield high resolution records of Nd isotopes that can be interpreted in terms of deep water circulation, but concerns about diagenesis and potential contamination of the seawater signal limit application of this technique to geologically young samples. In this study we demonstrate that Nd extracted from the > 63 µm, decarbonated fraction of older Ocean Drilling Program (ODP) sediments using a 0.02 M HH solution produces Nd isotopic ratios that are within error of values from cleaned fossil fish teeth collected from the same samples, indicating that the HH-extractions are robust recorders of deep sea Nd isotopes. This excellent correlation was achieved for 94 paired fish teeth and HH-extraction samples ranging in age from the Miocene to Cretaceous, distributed throughout the north, tropical and south Atlantic, and composed of a range of lithologies including carbonate-rich oozes/chalks and black shales. The strong Nd signal recovered from Cretaceous anoxic black shale sequences is unlikely to be associated with ferromanganese oxide coatings, but may be derived from abundant phosphatic fish teeth and debris or organic matter in these samples. In contrast to the deep water Nd isotopic signal, Sr isotopes from HH-extractions are often offset from seawater values, suggesting that evaluation of Sr isotopes is a conservative test for the integrity of Nd isotopes in the HH fraction. However, rare earth elements (REE) from the HH-extractions and fish teeth produce distinctive middle REE bulge patterns that may prove useful for evaluating whether the Nd isotopic signal represents uncontaminated seawater. Alternatively, a few paired HH-extraction and cleaned fish teeth samples from each site of interest can be used to verify the seawater composition of the HH-extractions. The similarity between isotopic values for the HH-extraction and fish teeth illustrates that the extensive cleaning protocol applied to fish teeth samples is not necessary in typical, carbonate-rich, deep sea sediments.

Organic carbon and nitrogen, sediment composition, and clay mineralogy of Deep Sea Drilling Project Site 603, western Atlantic Ocean

Sediment and interstitial water samples recovered during DSDP Leg 93 at Site 603 (lower continental rise off Cape Hatteras) were analyzed for a series of geochemical facies indicators to elucidate the nature and origin of the sedimentary material. Special emphasis was given to middle Cretaceous organic-matter-rich turbidite sequences of Aptian to Turanian age. Organic carbon content ranges from nil in pelagic claystone samples to 4.2% (total rock) in middle Cretaceous carbonaceous mudstones of turbiditic origin. The organic matter is of marine algal origin with significant contributions of terrigenous matter via turbidites. Maturation indices (vitrinite reflectance) reveal that the terrestrial humic material is reworked. Maturity of autochthonous material (i.e., primary vitrinite) falls in the range of 0.3 to 0.6% Carbohydrate, hydrocarbon, and microscopic investigations reveal moderate to high microbial degradation. Unlike deep-basin black shales of the South and North Atlantic, organic-carbon-rich members of the Hatteras Formation lack trace metal enrichment. Dissolved organic carbon (DOC) in interstitial water samples ranges from 34.4 ppm in a sandstone sample to 126.2 ppm in an organic-matter-rich carbonaceous claystone sample. One to two percent of DOC is carbohydratecarbon.

(Table 1) Phosphorus and carbon chemistry of ODP Holes 169-1033B and 169-1034B

Uncertainty currently exists about the removal of carbon (C) and phosphorus (P) from the oceanic reservoir, especially in low oxygen settings. In this paper, the cycling of C and P is examined in sediments from the anoxic Saanich Inlet, cored by Ocean Drilling Program (ODP) Leg 169S in 1996 at two sites. Although Corg/Porg ratios are high and increase with depth in the Saanich Inlet, this effect is due largely to a remobilization of P from an organic matter sink to an authigenic sink. Reducible sedimentary components act as temporary shuttles in this process even in this anoxic setting, with the ultimate burial sink for the remobilized P being carbonate fluorapatite. The effective Corg/Preactive molar ratio appears to be about 150-200, indicating some preferential loss of P compared to C during organic matter degradation, but not approaching previously reported values of over 3000 in black shales. Reactive P accumulation rates in this basin range from 10,000-60,000 µmol/cm**2/kyr, greatly exceeding the range of 500-8000 µmol/cm**2/kyr found in most continental-margin settings, including regions of modern phosphogenesis. The initiation of marine sedimentation in the Saanich Inlet occurred after deglaciation, and the high rates of P burial seen here may provide an end-member example of the effects of sea level and margin sedimentation on the distribution of P within the marine P cycle.

Stable carbon and oxygen isotope ratios of benthic and planktonic foraminifera of ODP Hole 171-1049C

Ocean anoxic events were periods of high carbon burial that led to drawdown of atmospheric carbon dioxide, lowering of bottom-water oxygen concentrations and, in many cases, significant biological extinction (Arthur et al., 1990; Erbacher et al., 1996, doi:10.1130/0091-7613(1996)024<0499:EPORAO>2.3.CO;2; Kuypers et al., 1999, doi:10.1038/20659; Jenkyns, 1997; Hochuli et al., 1999, doi:10.1130/0091-7613(1999)027<0657:EOHPAC>2.3.CO;2). Most ocean anoxic events are thought to be caused by high productivity and export of carbon from surface waters which is then preserved in organic-rich sediments, known as black shales. But the factors that triggered some of these events remain uncertain. Here we present stable isotope data from a mid-Cretaceous ocean anoxic event that occurred 112 Myr ago, and that point to increased thermohaline stratification as the probable cause. Ocean anoxic event 1b is associated with an increase in surface-water temperatures and runoff that led to decreased bottom-water formation and elevated carbon burial in the restricted basins of the western Tethys and North Atlantic. This event is in many ways similar to that which led to the more recent Plio-Pleistocene Mediterranean sapropels, but the greater geographical extent and longer duration (~46 kyr) of ocean anoxic event 1b suggest that processes leading to such ocean anoxic events in the North Atlantic and western Tethys were able to act over a much larger region, and sequester far more carbon, than any of the Quaternary sapropels.

(Table 1) Bulk Fe isotope values and pyrite framboids sizes of Cenomanian-Turonian samples

Organic carbon-rich shales from localities in England, Italy, and Morocco, which formed during the Cenomanian-Turonian oceanic anoxic event (OAE), have been examined for their total organic carbon (TOC) values together with their carbon, nitrogen, and iron isotope ratios. Carbon isotope stratigraphy (d13Corg and d13Ccarb) allows accurate recognition of the strata that record the oceanic anoxic event, in some cases allowing characterization of isotopic species before, during, and after the OAE. Within the black shales formed during the OAE, relatively heavy nitrogen isotope ratios, which correlate positively with TOC, suggest nitrate reduction (leading ultimately to denitrification and/or anaerobic ammonium oxidation). Black shales deposited before the onset of the OAE in Italy have unusually low bulk d57Fe values, unlike those found in the black shale (Livello Bonarelli) deposited during the oceanic anoxic event itself: These latter conform to the Phanerozoic norm for organic-rich sediments. Pyrite formation in the pre-OAE black shales has apparently taken place via dissimilatory iron reduction (DIR), within the sediment, a suboxic process that causes an approximately -2 per mil fractionation between a lithogenic Fe(III)oxide source and Fe(II)aq. In contrast, bacterial sulfate reduction (BSR), at least partly in the water column, characterized the OAE itself and was accompanied by only minor iron isotope fractionation. This change in the manner of pyrite formation is reflected in a decrease in the average pyrite framboid diameter from ~10 to ~7 µm. The gradual, albeit irregular increase in Fe isotope values during the OAE, as recorded in the Italian section, is taken to demonstrate limited isotopic evolution of the dissolved iron pool, consequent upon ongoing water column precipitation of pyrite under euxinic conditions. Given that evidence exists for both nitrate and sulfate reduction during the OAE, it is evident that redox conditions in the water column were highly variable, in both time and space.