Oceanic Anoxic Event 1b Research Articles

Abstract of “Climate-Ocean coupling off North-West Africa during the Lower Albian: The Oceanic Anoxic Event 1b”

Diagenetic minerals may provide information about the burial history of geological units and can have practical applications, for instance, for reconstructing the geochemical and thermal histories of sedimentary basins. Clumped isotope, or Δ47, thermometry on carbonates opens a new avenue for interpreting carbonate formation temperature and thermal history of rocks. Yet, most of current knowledge on Δ47 systematics has been acquired via theoretical or experimental studies with only limited validation by the rock-record at geological conditions/timescales. Here, we investigate calcitic and dolomitic cements representative of three genetically different cementation phases from a well-documented mineral paragenesis of a carbonate unit (Middle Jurassic, Paris basin, France). We compare Δ47 with fluid inclusions microthermometry (FIM) data that were independently obtained from the same calcite and dolomite crystal specimens. The range of homogenization temperatures (Th) found for Cal1, Cal2 and Dol1 fluid inclusions fit remarkably well (i.e. within < 5 °C) with the temperatures determined from the Δ47 measurements (TΔ47), for a temperature range between 60 and 100 °C and salinities between 0 and 15 wt% NaCl eq. This provides a consistent rock-based validation of the experimentally determined Δ47 calibration with formation temperature for both calcite and dolomite mineralogy. Such findings also confirm the applicability of Δ47 thermometry in low temperature diagenetic environments (i.e., below circa. 100 °C), which provides higher precision than FIM measurements (i.e., typical uncertainties of ± 6 °C with three Δ47 measurements) though significantly less time-consuming. Importantly, this study underlines how the coupling of both techniques can help to evaluate the degree of preservation of the original temperature information captured by either fluid inclusions or Δ47 compositions, and interpret each proxy as confidently/accurately as possible. Moreover, because both FIM and Δ47 measurements can provide independent constraints on the geochemistry of diagenetic paleofluids (via their salinity and δ18O composition), this study also highlights the benefits of coupling both techniques to further unravel the nature of paleofuids. Finally, we propose a practical guideline as a basis for future applications of combined FIM and Δ47 thermometry.

Elevated primary productivity of calcareous nannoplankton associated with ocean anoxic event 1b during the Aptian/Albian transition (Early Cretaceous)

Previous isotopic investigations of Aptian/Albian oceanic anoxic event (OAE) 1b from the western North Atlantic (Blake Nose) posited that increased sea surface temperatures and decreased salinity led to stratification of the upper water column, resulting in lowered dissolved oxygen and enhanced organic matter preservation. We examined calcareous nannofossils from the same site in the western North Atlantic (Blake Nose) to evaluate changes in surface water conditions prior to, during, and after the Aptian/Albian OAE1b. The results of our analysis conflict somewhat with the previous interpretation that OAE1b at Blake Nose was linked to water column stratification. The signal from calcareous nannofossils indicates that in the late Aptian, prior to OAE1b, oligotrophic conditions in the surface waters were replaced by mesotrophic conditions that persisted throughout the OAE event and into the early Albian. We speculate that increased surface water productivity facilitated to some degree the development of OAE1b at Blake Nose.

Further examples of archaeal-derived hydrocarbons in mid-Cretaceous oceanic anoxic event (OAE) 1b sediments

Biomarker and total organic carbon (TOC) analyses were carried out on laminated black shales and massive marls from the Goguel level (OAE1a) and the Paquier and Kilian levels (OAE1b) of the Vocontian Basin of southeast France. TOC content was high (ca. 3–8%) throughout the Goguel, the Paquier and Kilian levels. Archaeal-derived hydrocarbons such as 2,6,15,19-tetramethylicosane (TMI) and 2,6,10,15,19-pentamethylicosane (PMI) were present in the OAE1b sediments, as reported previously [Kuypers, M.M.M., Blokker, P., Erbacher, J., Kinkel, H., Pancost, R.D., Schouten, S., Sinninghe Damsté, J.S., 2001. Massive expansion of marine archaea during a mid-Cretaceous oceanic anoxic event. Science 293, 92–94]. However, we did not detect these archaeal biomarkers in the Goguel level, confirming that the prevalence of archaea producing TMI and PMI was restricted to OAE1b.

HYDROCARBON SOURCE ROCK ASSESSMENT AND REVISED BIOSTRATIGRAPHY OF DSDP SITE 369A, OFFSHORE NORTHWEST AFRICAN MARGIN

Deep Sea Drilling Project (DSDP) Site 369A, drilled off Cape Bajador, NW Africa, provided an opportunity to examine the source rock potential and biostratigraphy of the offshore Aaiun‐Tarfaya Basin. Within the Albian‐Campanian interval, total organic carbon (TOC) contents were found to be highly variable and cyclic, ranging from less than 0.5% to nearly 12%. Variations in hydrocarbon generation potential (S1 + S2) versus depth in general paralleled variations in TOC. Organic matter was generally marine in origin and oil‐prone.The newly generated data reported in this paper expand the range of TOC values which have been reported from the study region. However, they also illustrate the kinds of problems which can be introduced as a result of sampling bias. Sampling for this study tended to focus on darker‐coloured, more organic‐rich lithologic core intervals, contrasting with previous shipboard analyses which appeared to have focused on lighter‐coloured intervals. Based on the abundance of calcareous nannofossils, the elevated TOC contents appear to have been, at least partly, driven by elevated surface productivity which in turn increased biological oxygen demand within the sediment.New biostratigraphic data was used to identify an unconformity between the late Albianl early Cenomanian and the early Campanian successions, consistent with previous interpretations. Integration of the geochemical and biostratigraphic datasets indicates that these organic‐rich sediments are in general equivalent to Cretaceous Oceanic Anoxic Events 1B, 1D and 3. Considering the apparent net thickness of these oil‐prone sediments and the possibility that they may be widely distributed throughout the basin, they may have the potential to generate commercially significant volumes of oil where buried sufficiently deeply.

Climate–ocean coupling off North-West Africa during the Lower Albian: The Oceanic Anoxic Event 1b

High-resolution records of organic and inorganic geochemical proxies document environmental changes during Oceanic Anoxic Event 1b for the Albian upwelling system at DSDP Site 545 Mazagan Plateau off NW Africa. Sea surface temperature estimates based on TEX 86 analyses show an abrupt rise by ~ 3 °C concurrent with a more than 2-fold increase in accumulation rates for organic carbon and siliciclastic sediment components. Geochemical grain-size indicators (Si/Al and Zr/Al) indicate the influx of overall finer grained sediment during the event. Our data suggest that the warming phase associated with OAE 1b resulted in an attenuation of the NE trade wind system and a weakening of local upwelling conditions. Higher amounts of precipitation over NW Africa and an associated higher continental run off may have supplied nutrients to the eastern North Atlantic and fostered the production of organic matter and black shale deposition.

Rapid warming and salinity changes of Cretaceous surface waters in the subtropical North Atlantic

Paleomarine carbon isotopic records reveal occasional and rapid release of greenhouse gases into the atmosphere. The regional consequences on surface ocean properties and time relationships between the atmosphere and the ocean, however, are still far from understood. We observe a sudden -1.5‰ shift in leaf wax n -alkane 13C values at the onset of oceanic anoxic event (OAE) 1b from early Albian sediments off northwest Africa (Deep Sea Drilling Project Site 545) that is best explained by the rapid release of isotopically light carbon into the Cretaceous atmosphere. This -1.5‰ shift in the n -alkanes precedes a negative isotope excursion of similar magnitude observed in marine carbonate, organic matter, and algal steranes. A TEX86-based record of sea surface temperature (SST) confirms almost instantaneous warming by ∼3.5 °C along with the marine isotope shifts at Site 545, paralleled by an ∼2 °C TEX86-SST increase and a freshening of surface water salinity from 43 to 41 at Blake Nose, Ocean Drilling Program Site 1049. Multiproxy evidence indicates that, once established, these warm SSTs prevailed and were stable during OAE 1b, suggesting that the emission of 13C-depleted carbon started abruptly but then continued over tens of thousands of years. The SST cooled by ∼1–2 °C at the end of the event, not reaching pre-excursion levels. The new records provide evidence for a time lag (best estimate ∼1–3 k.y.) between atmospheric and oceanic processes that we interpret as a direct response to changes in the atmospheric greenhouse gas concentrations followed by propagation into the ocean and subsequent heating of surface waters.

Consequences of moderate ∼ 25,000 yr lasting emission of light CO 2 into the mid-Cretaceous ocean

Future warming is predicted to shift the Earth system into a mode with progressive increase and vigour of extreme climate events possibly stimulating other mechanisms that invigorate global warming. This study provides new data and modelling investigating climatic consequences and biogeochemical feedbacks that happened in a warmer world ∼ 112 Myr ago. Our study focuses on the Cretaceous Oceanic Anoxic Event (OAE) 1b and explores how the Earth system responded to a moderate ∼ 25,000 yr lasting climate perturbation that is modelled to be less than 1 °C in global average temperature. Using a new chronological model for OAE 1b we present high-resolution elemental and bulk carbon isotope records from DSDP Site 545 from Mazagan Plateau off NW Africa and combine this information with a coupled atmosphere–land–ocean model. The simulations suggest that a perturbation at the onset of OAE 1b caused almost instantaneous warming of the atmosphere on the order of 0.3 °C followed by a longer (∼ 45,000 yr) period of ∼ 0.8 °C cooling. The marine records from DSDP Site 545 support that these moderate swings in global climate had immediate consequences for African continental supply of mineral matter and nutrients (phosphorous), subsequent oxygen availability, and organic carbon burial in the eastern subtropical Atlantic, however, without turning the ocean anoxic. The match between modelling results and stratigraphic isotopic data support previous studies [summarized in Jenkyns, H.C., 2003. Evidence for rapid climate change in the Mesozoic–Palaeogene greenhouse world. The Royal Society, 361: 1885–1916.] in that methane emission from marine hydrates, albeit moderate in dimension, may have been the trigger for OAE 1b, though we can not finally rule out alternative mechanisms. Following the hydrate mechanism a total of 1.15 × 10 18 g methane carbon ( δ 13C = − 60 ‰), equivalent to about 10% to the total modern gas hydrate inventory, generated the δ 13C carb profile recorded in the section. Modelling suggests a combination of moderate-scale methane pulses supplemented by continuous methane emission at elevated levels over ∼ 25,000 yr. The proposed mechanism, though difficult to finally confirm in the geological past, is arguably more likely to occur in a warmer world and apparently perturbs global climate and ocean chemistry almost instantaneously. This study shows that, once set-off, this mechanism can maintain Earth's climate in a perturbed mode over geological time leading to pronounced changes in regional climate.

THE USE OF CARBON AND OXYGEN STABLE ISOTOPES IN THE STUDY OF GLOBAL PALAEOCEANOGRAPHIC CHANGES: EXAMPLES FROM THE CRETACEOUS SEDIMENT ROCKS OF WESTERN GREECE

In the present paper we examine the use of carbon and oxygen stable isotopes in the study of global palaeoceanographic changes, with special reference to the oceanic anoxic events (OAEs). The analysis of stable isotopes was applied to the examination of Cretaceous sediments from the Ionian and Pindos zones of Western Greece. In the Ionian zone the carbon and oxygen stable isotopes, combined with biostratigraphic data, record the palaeoenvironmental change corresponding to the anoxic events Bonarelli (Cenomanian/Turonian, OAE2) and Paquier (Lower Albian, OAE1b). In the Pindos zone, within the Cretaceous sediments, we observed two organic-carbon-rich levels. According to the biostratigraphic and isotopie analysis, the first level corresponds to an OAE of Santonian age. This local oceanic anoxic event is described for the first time. The second level, Aptian - Albian age, possibly correlates to either the Paquier event (OAE 1b) or the Selli event (OAE 1a), which in Greece were until now known only in the Ionian zone.

The regional record of Albian oceanic anoxic events at the Apulian Platform Margin (Gargano Promontory, southern Italy)

The Gargano Promontory, located on the eastern margin of the Apulia Platform, represents a distinctive Tethyan area where the transition from carbonate platform to adjacent basin is exposed on land. The Albian stratigraphic record, represented by shallow-water, slope and deep-water deposits, provides a good opportunity to investigate the regional response to oceanic anoxic events (OAEs) in different depositional settings by using an integrated, high-resolution micropalaeontological (planktonic foraminifers and calcareous nannofossils) approach. Results show that organic matter preservation is confined to the more distal areas (Marne a Fucoidi Formation), and consists of black shale intervals from the middle and late Albian ( Ticinella primula/Prediscosphaera columnata Zones; upper Ticinella praeticinensis Subzone/ R. achlyostaurion Zone, respectively). Integrated biostratigraphic data correlate the intervals of black shale deposition to the Urbino and Amadeus levels, previously identified and named in the Umbria-Marche Basin. These black shales record the effects of OAEs 1b and 1c. The biotic changes in the pelagic Albian succession of the Apulian Platform Margin provide evidence for episodes of eutrophication that correlate to the deposition of these black shales. Genetic models for the two episodes of organic matter preservation are proposed, taking into account both global and local controlling factors.

Millennial- to Centennial-Scale Interruptions of the Oceanic Anoxic Event 1b (Early Albian, mid-Cretaceous) Inferred from Benthic Foraminiferal Repopulation Events

The Early Albian Oceanic Anoxic Event 1b (OAE 1b) black shale is interrupted by one or more ventilation events that display significant changes in benthic and planktic populations. Within the OAE 1b sections studied, at ODP Site 1049, DSDP Site 545, and the Vocontian Basin, the benthic foraminiferal repopulation events last between ∼500 and ∼1,250 years and occur with a cyclicity of approximately 5.7 kyr. This period may represent an amplitude modulation of the precessional cycle. The OAE 1b sections from the marginal setting of the Vocontian Basin exhibit up to eight repopulation events. In contrast, there is only one repopulation event identified in the Atlantic OAE 1b sections from the Mazagan Plateau (DSDP 545) and Blake Nose (ODP 1049). Within the margin of dating uncertainties, this supraregional repopulation event occurred synchronously in the Vocontian Basin and the Atlantic Ocean. While the OAE 1b black shale formed under extremely warm and humid conditions, the repopulation events occurred during intervals of short-term cooling and reduced humidity at deep-water formation sites. The resulting increase in evaporation led to enhanced formation of low-latitude deep water, thus improving the ventilation of the sea floor.

Fluctuations in biosiliceous production and the generation of Early Cretaceous oceanic anoxic events in the Pacific Ocean (Shatsky Rise, Ocean Drilling Program Leg 198)

Integration of geophysical logs with sedimentological, biostratigraphic, and physical properties data from poorly recovered chert‐rich sequences at Ocean Drilling Program Leg 198 Sites 1207 and 1213 allows us to propose an Aptian–Albian (Early Cretaceous) depositional history for Shatsky Rise (North Pacific Ocean). The logs indicate that the lower Aptian and lower Albian deposits are relatively lithified and unporous compared to contiguous sediments. These characteristics are best explained by increased siliceous cementation that resulted from an elevated flux of radiolarians to the seafloor. The intervals of increased biosiliceous production may record a biotic response to enhanced nutrient delivery, coeval with (but longer in duration than) oceanic anoxic events (OAEs) 1a (early Aptian) and 1b (early Albian). On Shatsky Rise, OAE 1a is represented by organic‐carbon‐rich sediments, but OAE 1b is not. Thus “black shale” deposition in the Pacific Ocean was only possible when a “carbon‐burial threshold” was crossed. We speculate that this threshold was related to the balance between productivity and carbon oxidation and that, ultimately, the supply of biolimiting nutrients through hydrothermal alteration of basalt may have been the key to the development of “black shale” in the Pacific basin.

Short‐term environmental changes in the Cretaceous Tethyan Ocean: micropalaeontological evidence from the Early Albian Oceanic Anoxic Event 1b

ABSTRACT To better understand the linkage between climate and ocean circulation under greenhouse conditions we have studied calcareous nannofossils, palynomorphs and benthic foraminifera from the Early Albian Oceanic Anoxic Event 1b black shale (OAE 1b) in the Vocontian Basin (SE France). We propose that monsoonal activity resulting from precessional forcing and modulated by eccentricity‐driven temperature changes represents the driving factor of OAE 1b formation in low latitudes. With the onset of OAE 1b, increasingly warm and humid conditions, stronger winds and enhanced terrestrial input led to higher surface water productivity. Therefore, increased productivity is an important factor for OAE 1b formation in the Vocontian Basin. However, monsoonally forced productivity changes are a regional climate signal only. The supraregional occurrence of the OAE 1b is probably due to a reduction of deep water formation in the low latitudes under extremely warm and humid conditions, leading to enhanced preservation of organic matter.

Forcing mechanisms for mid-Cretaceous black shale formation: evidence from the Upper Aptian and Lower Albian of the Vocontian Basin (SE France)

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.

Reconstructing nutricline dynamics of mid-Cretaceous oceans: evidence from calcareous nannofossils from the Niveau Paquier black shale (SE France)

A high-resolution calcareous nannofossil record is presented from the Lower Albian Niveau Paquier black shale from the Vocontian Basin (SE France). The Niveau Paquier black shale represents the regional equivalent of the supraregionally distributed Oceanic Anoxic Event 1b (OAE 1b). To reconstruct surface water fertility, a nutrient index based on Zeugrhabdotus erectus, Discorhabdus rotatorius (high fertility indicators), and Watznaueria barnesae (low fertility indicator) was established using principal component analysis. In addition, the distribution of Nannoconus spp. and absolute abundances of coccoliths (coccoliths per gram) were used for reconstructing nutricline dynamics of the surface waters. High surface water fertility coincides with low percentages of nannoconids and vice versa. Moreover, high percentages of nannoconids correlate with low absolute abundances of all other coccoliths. Based on the observed nannoplankton distribution pattern and a suggested similarity in ecological requirements between nannoconids and the modern taxon Florisphaera profunda, a model is proposed that couples nannoconid abundances with dynamics of the nutricline. Time series analyses of the nutrient index show fluctuations within the precessional band. The precession-controlled fluctuations of the surface water fertility may represent a monsoonal signal, with the nutrient supply in the surface waters depending on the strength of monsoonal activity. During periods of enhanced monsoonal activity, which were characterized by humid conditions and stronger winds, mixing of the upper water column was enhanced. During that time, the abundance of nannoconids decreased as a consequence of enhanced wind stress that improved vertical mixing and led to an entrainment of nutrients into the surface waters. This resulted in an increase of primary production. During periods of reduced monsoonal activity, marked by drier conditions and reduced wind stress, the surface waters were depleted in nutrients. As a result of a deep nutricline and a reduced nutrient supply to the upper photic zone in a stratified water column, percentages of nannoconids increased. According to the mechanisms outlined above, fluctuations of the nutrient index and nannoconid percentages have been used as a proxy for reconstructing the physical structure of mid-Cretaceous oceans steered by precession-forced monsoonal activity. The results of the study show that the formation of the Lower Albian OAE 1b from the Vocontian Basin occurred under strongly fluctuating surface waters fertility.

Archaeal remains dominate marine organic matter from the early Albian oceanic anoxic event 1b

The sources for both soluble and insoluble organic matter of the early Albian (∼112 Myr) oceanic anoxic event (OAE) 1b black shales of the Ocean Drilling Program (ODP) site 1049C (North Atlantic Ocean off the coast of Florida) and the Ravel section of the Southeast France Basin (SEFB) were determined using optical, chemical, stable carbon and nitrogen isotopic analyses. Archaea-derived isoprenoidal tetraether membrane lipids and free and macromolecularly bound isoprenoid alkanes are abundant in these black shales. More specifically, the presence of certain ether lipids (bi/tricyclic biphytane tetraethers) indicates an important contribution of representatives of marine planktonic archaea. The large difference (up to 12‰) in 13C/ 12C ratios between algal biomarkers and the much more abundant planktonic archaea-derived biomarkers indicates that the latter were living chemoautotrophically. This offset in 13C/ 12C ratios was used to estimate that up to ∼40% of the organic matter of the SEFB and up to ∼80% of the organic matter of ODP site 1049C preserved in the black shales is derived from archaea. Furthermore, it is shown that, even though there are apparent similarities (high organic carbon (OC) content, distinct lamination, 13C-enrichment of OC) between the black shales of OAE1b and the Cenomanian/Turonian (∼94 Myr) OAE, the origin of the organic matter (archaeal versus phytoplanktonic) and causes for 13C-enrichment of OC are completely different.

No extinctions during Oceanic Anoxic Event 1b: the Aptian-Albian benthic foraminiferal record of ODP Leg 171

Abstract Outstandingly well-preserved benthic foraminiferal successions from upper Aptian-lower Albian sediments at Site 1049 (Leg 171, Blake Nose escarpment, western North Atlantic) provide a detailed record of the faunal turnover across Oceanic Anoxic Event 1b (OAE 1b). Changes in abundance, diversity and species composition reflect strong fluctuations in carbon flux and bottom-water oxygenation. Before the onset of black shale sedimentation, the originally diverse assemblages are replaced by low-diversity associations, dominated by species inferred to be opportunistic phytodetritus feeders and thriving on an enhanced carbon flux to the sea floor. The 46 cm thick laminated black shale horizon corresponding to OAE 1b is virtually devoid of benthic formainifers or contains highly impoverished assemblages, suggesting that intense eutrophication and/or strong stratification triggered near anoxia at the sea floor during black shale deposition. Above the black shale, reoccurrence of the pre-black shale fauna points to relatively rapid bottom-water reoxygenation. The benthic formainiferal record of Leg 171 provides clear evidence that no major extinctions occurred across OAE 1b, as most of the species occurring below the black shale reappear above it. In contrast to other Cretaceous anoxic events, OAE 1b may have been more limited in duration or in geographical and water-depth extent, allowing recolonization from adjacent, more hospitable areas, once local conditions improved at the sea floor. Prolific radiation within the suborder Rotaliina and diversification of Textulariina with calcareous cement appear to have started in the Aptian time before OAE 1b, and continued into early Albian time to give rise to many of the modern lineages.

Deposition of sedimentary organic matter in black shale facies indicated by the geochemistry and petrography of high-resolution samples, Blake Nose, western North Atlantic

Abstract A transect of three holes drilled across the Blake Nose, western North Atlantic Ocean, retrieved cores of black shale facies related to the Albian Oceanic Anoxic Events (OAE) 1b and 1d. Sedimentary organic matter (SOM) recovered from Ocean Drilling Program Hole 1049A from the eastern end of the transect showed that before black shale facies deposition organic matter preservation was a Type III–IV SOM. Petrography reveals that this SOM is composed mostly of degraded algal debris, amorphous SOM and a minor component of Type III–IV terrestrial SOM, mostly detroinertinite. When black shale facies deposition commenced, the geochemical character of the SOM changed from a relatively oxygen-rich Type III–IV to relatively hydrogen-rich Type II. Petrography, biomarker and organic carbon isotopic data indicate marine and terrestrial SOM sources that do not appear to change during the transition from light-grey calcareous ooze to the black shale facies. Black shale subfacies layers alternate from laminated to homogeneous. Some of the laminated and the poorly laminated to homogeneous layers are organic carbon and hydrogen rich as well, suggesting that at least two SOM depositional processes are influencing the black shale facies. The laminated beds reflect deposition in a low sedimentation rate (6m Ma −1 ) environment with SOM derived mostly from gravity settling from the overlying water into sometimes dysoxic bottom water. The source of this high hydrogen content SOM is problematic because before black shale deposition, the marine SOM supplied to the site is geochemically a Type III–IV. A clue to the source of the H-rich SOM may be the interlayering of relatively homogeneous ooze layers that have a widely variable SOM content and quality. These relatively thick, sometimes subtly graded, sediment layers are thought to be deposited from a Type II SOM-enriched sediment suspension generated by turbidites or direct turbidite deposition.

Correlating environmental changes during early Albian oceanic anoxic event 1B using benthic foraminiferal paleoecology

The nature and consequences of mid-Cretaceous oceanic anoxic events (OAEs) are the subject of ongoing debate, and recent studies have shown that different scenarios are needed to explain each of these events. Nevertheless, similarities between the different OAEs can be observed. Here, we have reconstructed paleoenvironmental changes during the early Albian OAE 1b using benthic foraminiferal distributions and lithologies in three sections from different basins and paleowater depths. Eutrophic conditions, as indicated by the presence of infaunal as well as opportunistic genera such as Gyroidinoides, Pseudobolivina, Pleurostomella and bolivinitids, prevailed before and during the OAE and led to dysoxic to anoxic conditions. Dysoxia was most intense in the bathyal sections but also occurred in the outer shelf where more heterogeneous patterns of foraminiferal distributions are believed to reflect fluctuations of the upper boundary of an oxygen-minimum zone. A change from eutrophic to mesotrophic conditions caused the termination of OAE 1b and opportunistic benthic foraminifera (e.g. Pseudobolivina, Pleurostomella) were the first to subsequently repopulate the bathyal sea floor. Repopulation occurred rapidly in the shallow settings and gradually in the deeper sites, where a normal, diverse pre-event fauna was established a few tens of thousands of years after OAE 1b.

Benthic foraminiferal assemblages of late Aptian-early Albian black shale intervals in the Vocontian Basin, SE France

Benthic foraminiferal assemblages were analyzed from three black shale intervals in the upper Aptain to lower Albian of the Vocontian Basin, SE France based on Q-mode principal component analyses. Variations in the distribution patterns of benthic foraminifera around these events suggest differences in the origin of the black shales. Differences between faunas of bioturbated marly and laminated black shale facies have been observed in the Niveau Paquier, Oceanic Anoxic Event (OAE) 1b and Niveau Leenhardt. Here, the faunal composition and plankton/benthos ratios suggest eutrophic conditions during the deposition of organic-rich sediments leading to black shales. No major variations have been observed in black shales of the upper Aptain Niveau Jacob. Benthic assemblages and low plankton/benthos ratios indicate mesotrophic conditions. Third order sea-level changes are believed to control mainly the origin of the investigated black shale levels.

Influence of oceanic anoxic events on the evolution of mid-Cretaceous radiolaria in the North Atlantic and western Tethys

Mid-Cretaceous (Aptian-Turonian) extinction/radiation events of radiolaria from the North Atlantic and western Tethys are summarized in relation to Oceanic Anoxic Events (OAE) from investigations of radiolarian faunas, black shales and stable carbon isotopes. Extinction and radiation events of radiolaria can be observed from the late early Aptian, the earliest Albian, the latest Albian and the Cenomanian/Turonian boundary.These events correlate well with organic carbon-rich sediments that contain type II kerogen and show positive δ13C excursions. They correspond to the Oceanic Anoxic Events (OAE) in the early Aptian (OAE 1a, Selli Level), early Albian (OAE 1b), late Albian (OAE 1d) and latest Cenomanian (OAE 2, Bonarelli Level). Detailed investigations of the two most significant events, the Selli and the Bonarelli Level in the Umbria-Marche Basin of central Italy show that extinctions coincide with the first occurrence of marine black shales (type II kerogen) and the most prominent rise of δ13Corg values. Stepwise extinctions and continuous radiations precede and follow the events. Similar patterns are evident for the OAE 1b and 1d. Black shales dominated by terrestrial organic matter (type III kerogen e.g., OAE 1c) are not characterized by isotopic excursions or by radiolarian extinction/radiation events.Contemporaneous drowning events documented from carbonate platforms around the investigated basins provide a link to a model where flooding of lowlands and leaching of nutrients during a relative rise of sea-level lead to an increased productivity and an expansion of the oxygen minimum zone (OMZ). Because of the loss of deep habitats, deeper dwelling forms become extinct. Shallower dwelling radiolarians survive. A narrowing of the OMZ during highstands and relative sea-level falls together with a coupled decrease of the nutrient supply leads to the development of new niches and therefore to radiation.