Onset Of Oceanic Anoxic Event Research Articles

Spatial heterogeneity in benthic foraminiferal assemblages tracks regional impacts of paleoenvironmental change across Cretaceous OAE2

AbstractThe impact of global climate events on local ecosystems can vary spatially. Understanding this potential heterogeneity can illuminate which environments will be most impacted and the proximal drivers of ecosystem responses. Cenomanian–Turonian marine deposits of the Western Interior Seaway (WIS) record paleoceanographic changes associated with the Greenhorn transgression and the onset of Oceanic Anoxic Event 2 (OAE2). They provide an ideal setting to study basin-wide paleoecological responses during a global perturbation. Here, we integrate benthic foraminiferal assemblages from before, during, and after OAE2 via multivariate ordination analysis to examine spatial patterns in faunal responses across the western United States on a common scale and to interrogate a previously defined faunal marker commonly used for basin-wide correlation, the Benthonic Zone (BZ). We identify oxygenation and organic matter quality as primary and secondary controls of faunal variation across the 10 stratigraphic records and use this variation to infer paleoenvironmental changes. Stratigraphic trends reveal, in contrast to previous studies, deoxygenation at the onset of OAE2. They also reveal temporal patterns in oxygenation and productivity consistent with the gradual northward migration of a southern water mass into the WIS. This spatial heterogeneity hinders the use of the BZ as a temporal marker, because assemblages change in response to diachronous environmental change, and thus timing of the BZ with respect to OAE2 is not consistent across the basin. Our study demonstrates that regional processes can overshadow ecosystem responses to global events and underscores the importance of considering how changes in the position of water masses impact the expression of global biogeochemical perturbations.

Patterns of planktonic foraminiferal extinctions and eclipses during Oceanic Anoxic Event 2 at Eastbourne (SE England) and other mid-low latitude locations

The latest Cenomanian Oceanic Anoxic Event (OAE) 2 represents one of the most extreme perturbations of the global carbon cycle. Planktonic foraminiferal events, variations in the taxonomic composition of assemblages (e.g., appearances, extinctions, temporary crisis of certain taxa) and their correlation with changes in the physico-chemical properties of surface waters are essential to reconstructing the consequences of OAE 2 on this group of calcareous microfossils.We present the results of a high-resolution biostratigraphic and taxonomic study of planktonic foraminifera performed at Eastbourne (SE England), representing the most expanded, complete and well-calibrated OAE 2 record in Europe.In this stratigraphic section, we identify a sequence of step-wise extinctions (i.e., Thalmanninella and Rotalipora species, and “Globigerinelloides” bentonensis) that are followed by an eclipse (temporary disappearance) of planispiral taxa and of hedbergellids with radially elongated chambers. These events are consistently found in approximately coeval stratigraphic intervals across low to mid-latitudes, suggesting that they were controlled by wide-scale environmental perturbations. Moreover, this study suggests that the extinction of rotaliporids might have been influenced by climate changes (i.e., warming for Thalmanninella and cooling during the Plenus Cold Event for Rotalipora) at the onset of OAE 2, whereas the eclipse of planispiral taxa and hedbergellids with radially elongated chambers during the second half of OAE 2 was likely related to enhanced productivity and mixing of surface waters potentially associated to warming after the PCE for planispirals.Finally, we identify two short-range species (Muricohedbergella kyphoma and Praeglobotruncana plenusiensis n. sp.) that co-occur with boreal macrofossils at Eastbourne and might represent the first evidence for a planktonic foraminiferal PCE fauna. The long ranging species Praeglobotrucana gungardensis n. sp. is described as new.

The onset of Oceanic Anoxic Event 2 on the Arabian Shelf – mechanisms between organic matter deposition, sea-level, and palaeoceanography (Shilaif Basin, United Arab Emirates)

The onset of Oceanic Anoxic Event 2 on the Arabian Shelf – mechanisms between organic matter deposition, sea-level, and palaeoceanography (Shilaif Basin, United Arab Emirates)

Unravelling the sources of carbon emissions at the onset of Oceanic Anoxic Event (OAE) 1a

The early Aptian Oceanic Anoxic Event (OAE) 1a represents a major perturbation of the Earth's climate system and in particular the carbon cycle, as evidenced by widespread preservation of organic matter in marine settings and a characteristic negative carbon isotopic excursion (CIE) at its onset, followed by a broad positive CIE. The contemporaneous emplacement of a large igneous province (LIP) is invoked as a trigger for OAE 1a (and OAEs in general), but this link and the ultimate source of the carbon perturbation at the onset of OAEs is still debated. In this study, we simultaneously assimilate an atmospheric pCO2 reconstruction along with a δ13C record from the Spanish Cau section in an Earth system model to obtain a novel transient reconstruction of emission rates and identify the primary carbon-emitting sources across the negative CIE interval at the onset of OAE 1a. We reconstruct carbon emissions of 4,300–29,200 Pg from a mixture of carbon sources. This estimate is a lower bound, as contemporaneous organic carbon burial is not accounted for. Carbon was first released at slow rates from a 13C-depleted reservoir (e.g. thermo- and/or biogenic methane from sill intrusions). Towards the end of the negative CIE the rate of emissions increased and they became more 13C-enriched, likely from a dominantly volcanic source (e.g. LIPs). New osmium isotope (187Os/188Os) measurements, from the same section as the pCO2 reconstruction and δ13C data, reveal a shift to less radiogenic values coinciding with the change towards mantle-derived carbon emissions as indicated by our modelling results, lending further support to our interpretation. These results highlight that geologically triggered carbon emissions were likely driving the OAE onset.

Palaeoenvironment and palaeoecology before and at the onset of Oceanic Anoxic Event (OAE)1a: Reconstructions from Central Tethyan archives

In this study, a new C-isotope stratigraphy combined with a revised litho-and biostratigraphy of Barremian-Aptian platform carbonates from the central-southern Apennines (Italy) is presented. The Santa Maria (Abruzzi Region) and the Monte Faito (Campania Region) sections were investigated in order to trace platform evolution during a time of major C-cycle perturbation before and during Oceanic Anoxic Event (OAE)1a. These successions are correlated at a regional scale with the Monte Raggeto reference section (Campania) where the Selli Level Equivalent (SLE) and the Magnetic Polarity Chron CM0r were recognised in earlier studies. Significant biotic and environmental changes are documented within the Late Barremian-Early Aptian interval; in particular, the Palorbitolina lenticularis-Lithocodium-Bacinella association followed by dark-brown microbial sediments reflect considerable ecological stress upon the oligotrophic carbonate platform community (mesotrophic or even eutrophic conditions) which precedes the onset of SLE by at least 1.2myr. Moreover, this integrated approach has resulted in an improved age control which offers the opportunity to correlate the Apenninic sections with a succession from the French Urgonian platform (Cluses section) and with pelagic records from the Italian Umbria-Marche (Gorgo a Cerbara) and Belluno (Cismon Apticore) Basins. Pelagic biozonations and magnetostratigraphy have been projected into carbonate platform sequences. A prevailing eustatic origin has been attributed to some of the lower-frequency transgressive-regressive cycles (T/RFTs) that reflect third order sea-level oscillations. The new correlations indicate that comparable biotic and environmental changes, documented around the Tethys, are not always synchronous. The type of response of biota to C-cycle perturbations before and during OAE1a depends upon palaeolatitude, palaeogeography, palaeobathymetry and regional climate patterns.

Basalt‐seawater interaction, the Plenus Cold Event, enhanced weathering and geochemical change: deconstructing Oceanic Anoxic Event 2 (Cenomanian–Turonian, Late Cretaceous)

AbstractOceanic Anoxic Event 2 (Cenomanian–Turonian: ca 94 Ma) represents a major palaeoceanographic phenomenon that took place during an interval of extreme global warmth when large amounts of organic matter entered the marine burial record, probably triggered by increased availability of nutrients for planktonic biota. Three sections (Eastbourne, Sussex, UK; Raia del Pedale, Campania, Italy; and Tarfaya, Morocco) recording this event illustrate the influence on marine geochemistry of mafic volcanic rock‐seawater interaction, anoxia to euxinia, and re‐oxygenation and cooling during the so‐called ‘Plenus Cold Event’. The Eastbourne section represents the organic‐lean epicontinental pelagic deposits of the English Chalk; the Raia del Pedale section represents a shallow‐water platform carbonate on the Tethyan continental margin, also largely devoid of organic matter; and the Tarfaya core represents an Atlantic margin site where cyclically bedded organic‐rich sediments were well developed. Correlation between all three sections is readily achieved by biostratigraphy and carbon‐isotope stratigraphy (δ13Ccarb and δ13Corg) over the Oceanic Anoxic Event 2 interval, represented by a characteristic broad positive carbon‐isotope excursion. The stratigraphic range of the Plenus Cold Event, defined by the presence, in two discrete levels, of boreal fauna and an excursion to heavier oxygen‐isotope values in the English Chalk, can be identified in Raia del Pedale and Tarfaya by using the carbon‐isotope curve as a correlative tool. Similarly, a section in southern France allows its co‐existing osmium‐isotope excursion to relatively unradiogenic values to be placed in the context of the Oceanic Anoxic Event in all three analysed sections. A fall to lower osmium‐isotope values clearly pre‐dated the onset of Oceanic Anoxic Event 2, as defined by the initial rise in carbon‐isotope values, allowing the putative magmatic/mafic event as a trigger for the Oceanic Anoxic Event. An initial drop in sulphur‐isotope ratios (δ34SCAS) at Eastbourne correlates with the osmium‐isotope curve, suggesting that isotopically light sulphur could have been derived from a mafic igneous source. Re‐oxygenation of sediments of all three investigated sections during the Plenus Cold Event is variably illustrated by change in cerium:calcium, iodine:calcium, molybedenum:calcium and uranium:calcium ratios, according to the redox behaviour of the elements in question and whether controls on seawater chemistry were local or global in nature. Changes in molybdenum‐isotope ratios from Tarfaya and portions of the sulphur‐isotope curve from Eastbourne and Raia del Pedale also indicate the probable presence of more oxygen‐rich bottom waters during the Plenus Cold Event. Oxidation by such waters of previously deposited organic‐rich shales, as well as loss of anoxic/euxinic sinks, is credited with temporarily enriching global seawater in a range of other redox‐sensitive trace metals (for example, V, Cr, Co, Ni, Cu, Zn and Cd) during ongoing basalt‐seawater interaction indicated by persistent relatively non‐radiogenic osmium‐isotope seawater values. However, early diagenetic enrichment of manganese in the English Chalk over much of the Oceanic Anoxic Event interval is broadly correlative in time with relatively low osmium‐isotope values in sections elsewhere: a relationship that may be due to the lack of affinity of manganese with carbon‐rich shales, hence allowing relatively elevated concentrations of the element in marine waters to persist during leaching of mafic rocks, unlike other redox‐sensitive species. The calcium‐isotope and lithium‐isotope ratios from Eastbourne and Raia del Pedale indicate an increase in global weathering during the initial phase of Oceanic Anoxic Event 2, and the shift in strontium isotopes and osmium isotopes to more unradiogenic values during the event suggests that not only construction but also destruction of one or more Large Igneous Provinces was probably a proximal cause of this major palaeoceanographic phenomenon by elevating nutrient levels and planktonic productivity in large tracts of the world ocean. Globally widespread carbon burial and silicate weathering are both identified as important mechanisms for drawing down atmospheric carbon dioxide that, in the absence of overwhelming volcanogenic replenishment of this greenhouse gas during the early phase of Oceanic Anoxic Event 2, caused the Plenus Cold Event.

A Southern Hemisphere record of global trace‐metal drawdown and orbital modulation of organic‐matter burial across the Cenomanian–Turonian boundary (Ocean Drilling Program Site 1138, Kerguelen Plateau)

AbstractDespite its assumed global nature, there are very few detailed stratigraphic records of the late Cenomanian to the early Turonian Oceanic Anoxic Event 2 from the Southern Hemisphere. A highly resolved record of environmental changes across the Cenomanian–Turonian boundary interval is presented from Ocean Drilling Program Site 1138 on the central Kerguelen Plateau (southern Indian Ocean). The new data lead to three key observations. Firstly, detailed biostratigraphy and chemostratigraphy indicate that the record of Oceanic Anoxic Event 2 is not complete, with a hiatus spanning the onset of the event. A decrease in glauconite and highly weathered clays after the onset of Oceanic Anoxic Event 2 marks the end of the hiatus interval, which can be explained by a relative sea‐level rise that increased sediment accommodation space on the Kerguelen Plateau margin. This change in depositional environment controlled the timing of the delayed peak in organic‐matter burial during Oceanic Anoxic Event 2 at Site 1138 compared with other Oceanic Anoxic Event 2 locations worldwide. A second key observation is the presence of cyclic fluctuations in the quantity and composition of organic matter being buried on the central Kerguelen Plateau throughout the latter stages of Oceanic Anoxic Event 2 and the early Turonian. A close correspondence between organic matter, sedimentary elemental compositions and sediments recording sea‐floor oxygenation suggests that the cycles were mainly productivity‐driven phenomena. Available age‐control points constrain the periodicity of the coupled changes in sedimentary parameters to ca 20 to 70 ka, suggesting a link between carbon burial and astronomically forced climatic variations (precession or obliquity) in the Southern Hemisphere mid‐latitudes both during, and after, Oceanic Anoxic Event 2: fluctuations that were superimposed on the impact of global‐scale processes. Finally, trace‐metal data from the black‐shale unit at Site 1138 provide the first evidence from outside of the proto‐North Atlantic region for a global drawdown of seawater trace‐metal (Mo) inventories during Oceanic Anoxic Event 2.

Onset and demise of Cretaceous oceanic anoxic events: The coupling of surface and bottom oceanic processes in two pelagic basins of the western Tethys

AbstractThe upper Albian–lower Turonian pelagic successions of the Tethys record processes acting during the onset, core, and recovery from perturbed conditions across oceanic anoxic event (OAE) 1d, OAE 2, and the mid‐Cenomanian event I (MCE I) relative to intervening intervals. Five sections from Umbria‐Marche and Belluno Basins (Italy) were analyzed at high resolution to assess processes in surface and deep waters. Recurrent facies stacking patterns (SP) and their associations record periods of bottom current activity coupled with surface changes in trophic level. Climate changes appear to have been influential on deep circulation dynamics. Under greenhouse conditions, vigorous bottom currents were arguably induced by warm and dense saline deep waters originated on tropical shelves in the Tethys and/or proto‐Atlantic Ocean. Tractive facies postdating intermittent anoxia during OAE 1d and in the interval bracketed by MCE I and OAE 2 are indicative of feeble bottom currents, though capable of disrupting stratification and replenish deep water with oxygen. The major warming at the onset of OAE 2 might have enhanced the formation of warm salty waters, possibly producing local hiatuses at the base of the Bonarelli Level and winnowing at the seafloor. Hiatuses detected at the top of the Bonarelli Level possibly resulted from most effective bottom currents during the early Turonian thermal maximum. Times of minimal sediment displacement correlate with cooler climatic conditions and testify a different mechanism of deep water formation, as further suggested by a color change to reddish lithologies of the post‐OAE 1d and post‐OAE 2 intervals.

Paleoenvironmental and paleoceanographic changes across the Cenomanian–Turonian Boundary Event (Oceanic Anoxic Event 2) as indicated by foraminiferal assemblages from the eastern margin of the Cretaceous Western Interior Sea

Two sites near the eastern margin of the Cretaceous Western Interior Sea (WIS) were investigated. The section at Cuba, Kansas (CK) is ~630km east of the GSSP for the Cenomanian/Turonian boundary at Rock Canyon, Colorado (RC), and the section near Sioux City, Iowa (SCI) is ~315km northeast of the Cuba site. Surprisingly, planktic foraminifera dominate all the studied samples despite the relative proximity of the sites to the paleo-shoreline and presumed neritic water depths. Such dominance suggests inhospitable benthic environments, and benthic foraminiferal assemblages indicate that seafloor oxygen in the WIS decreased eastward. Therefore, the hypothesis of an influx of freshwater into the WIS from the western margin forming a fresh water cap that diminished eastward is not supported. Benthic foraminifera are scarce in most studied samples and species diversity is low. In the CK section, diversity and abundance increase abruptly in the uppermost Cenomanian Benthonic Zone at the initiation of the δ13Corg positive excursion marking the onset of Oceanic Anoxic Event (OAE 2). In the SCI section, however, the Benthonic Zone is recognized only by presence of Nodosaria bighornensis that has a stratigraphic range restricted to this interval; other benthic taxa are very rare or absent. The favorable conditions of the Benthonic Zone were short-lived. While relatively diverse planktic foraminiferal assemblages, including keeled species, characterize the initial δ13C excursion at the CK site, dwarfed specimens of Heterohelix and Hedbergella dominate the assemblages at the more proximal Sioux City site. Furthermore, most of the biostratigraphic events recognized in the basin center RC section can be traced into the CK section, but not into the SCI section, including the Rotalipora spp. and Globigerinelloides bentonensis extinctions, the Heterohelix shift event, and the brief benthic recovery event. Correlation between eastern WIS sites shows that the SCI section is thicker, suggesting a major sediment source to the east. This may explain the inhospitable benthic conditions and development of a ‘dead zone’ (seasonal hypoxia) analogous to the modern shelf of the northern Gulf of Mexico proximal to active discharge of the Mississippi River. Development of estuarine circulation in the WIS provided the eastern part of the basin with input of calcareous plankton from the south including planktic foraminifera. Foraminiferal assemblages, total organic carbon (TOC), and δ13C data of the studied sections suggest a two-fold history of Greenhorn transgression and OAE 2 development. The initial phase is a global signal characterized by a positive δ13C excursion, low TOC values, and high foraminiferal species diversity. The second phase is characterized by overprinting by local conditions in the WIS, including high fluvial input, relatively high TOC values, and low foraminiferal species diversity. These findings suggest that the eastern margin records unique depositional and biotic environments further revealing the dynamic and complex nature of the WIS and its sedimentary cycles.

The Cenomanian–Turonian Eagle Ford Group of South Texas: Insights on timing and paleoceanographic conditions from geochemistry and micropaleontologic analyses

The interbedded organic-rich marls and limestones of the Eagle Ford Group of South Texas were deposited on the shelf of the Central Texas Platform during the upper Cenomanian through the Turonian. Micropaleontological analyses of thin-sections and residue samples from subsurface cores taken in Atascosa and Karnes counties yielded 13 biostratigraphic datums, which were integrated with calcareous nannofossil datums, stable carbon isotopic and bulk inorganic geochemical analysis to gain an understanding of the paleoceanographic conditions of the study area and their relationship to regional and global events. The paired thin-section/residue sampling was necessitated by poor recovery of planktonic foraminifera in washed samples, which were, however, abundant in most thin-sections. Lower Eagle Ford marls were deposited under euxinic conditions produced by a stratified water column. Periodic enhanced water column mixing increased oxygen levels and surface-water primary productivity, which promoted calcite diagenesis and generation of limestones containing calcispheres and radiolaria. An oxygenated bottom water mass entered the basin in the uppermost Cenomanian prior to the onset of Oceanic Anoxic Event (OAE) 2, causing a spike in benthic foraminiferal diversity and decreasing preservation of organic matter. Although the lower portion of the upper Eagle Ford is age-equivalent to OAE2 as indicated by positive shift in δ13Corg, it contains less organic matter and evidence of more oxygen than the lower Eagle Ford, so deposition of the Eagle Ford black shales is not contemporaneous to OAE2.

The Cenomanian–Turonian boundary event, OAE2 and palaeoenvironmental change in epicontinental seas: New insights from the dinocyst and geochemical records

Organic-walled dinoflagellate cyst (dinocyst) and geochemical records across the Cenomanian–Turonian boundary (CTB) are presented for the NW European reference section at Eastbourne, England. Dinocyst and nannofossil fertility indexes indicate that an upwelling-driven productivity pulse accompanied a eustatic sea-level fall that preceded, by at least 40 kyr, the global rise in δ 13C values that marks the onset of Oceanic Anoxic Event 2 (OAE2) and the deposition of black shales in the deep ocean. A marine productivity collapse in the latest Cenomanian is evidenced by the falling absolute and relative abundance of peridinioid dinocysts, believed to be the product of heterotrophic dinoflagellates. This biotic change accompanied transgression and sharply rising sea-surface temperatures, following an Atlantic-wide episode of short-lived cooling. Geochemical tracers provide evidence of Caribbean–Colombian plateau volcanism. The increase in water depth caused by the latest Cenomanian transgression resulted in less eutrophic waters in epicontinental seas, where CTB biotic turnover was driven largely by water-mass changes rather than oxygen depletion. The species richness/absolute abundance ratio of dinocysts is proposed as a water-mass stability proxy.

Strontium isotope stratigraphy in the upper Cenomanian shallow-water carbonates of the southern Apennines: Short-term perturbations of marine 87Sr/ 86Sr during the oceanic anoxic event 2

Strontium isotope stratigraphy of shallow-water carbonates from the southern Apennines (Italy) indicates that the late Cenomanian−early Turonian evolution of marine 87Sr/ 86Sr is not accurately depicted by the marine reference curve. Using the low-Mg biotic calcite of well-preserved rudist shells as a study material and carbon isotope stratigraphy for correlation to the standard ammonite biozonal scheme of Northern Europe, new 87Sr/ 86Sr values are proposed for four different stratigraphic levels: the middle and uppermost part of the guerangeri zone, the lower and upper part of the geslinianum zone. The southern Apennines data suggest that a sharp positive shift at the onset of oceanic anoxic event 2 precedes the well-known Sr isotope negative shift. The positive shift is interpreted in terms of enhanced rates of chemical weathering, driven by global warming forced by volcanogenic CO 2 outgassing. A stratified ocean is invoked to reconcile the high gradient and the short lag time of the perturbation with estimates of present-day total mass and residence time of strontium in the ocean. The sharp switch to the negative shift is related to the collapse of water-column density gradient, driven by thermal instability during a late Cenomanian cool event. More active ocean circulation suddenly delivered to surface waters the signal of increased submarine volcanism that had accumulated in deep waters.

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.

Cenomanian–Turonian and δ13C, and δ18O, sea level and salinity variations at Pueblo, Colorado

Stable isotopes of the surface dwelling planktic foraminifera Hedbergella planispira, its abundance variations, and mineralogical analysis of the Cenomanian–Turonian at Pueblo, CO, reveal cyclic variations in surface salinity due to changes in precipitation, freshwater influx, marine incursions and long-term sea-level fluctuations. Hedbergella planispira is a proxy for salinity variations, as indicated by 2–4‰ more negative δ 18O values in intervals of peak abundances as compared to intervals with reduced populations. Negative δ 18O values reflect periods of brackish surface waters caused by freshwater influx during wet humid periods, accompanied by increased clastic transport. More positive δ 18O values reflect more normal marine salinities as a result of arid periods and/or marine incursions and correlate with intervals of increased biogenic carbonate deposition. The magnitude of salinity variations during the low sea-level of the Hartland Shale is twice that during the sea-level transgression of the Bridge Creek Limestone. The rapid positive δ 13C shift that marks the onset of Oceanic Anoxic Event 2 (OAE 2) at Pueblo occurred over a period of about 100 ky (93.90–94.00 Ma), and coincided with the major sea level transgression that culminated in the deposition of the basal Bridge Creek Limestone. A positive δ 13C shift also occurred in the Rotalipora cushmani zone prior to OAE 2 and coincided with a sea level rise and enhanced preservation of terrestrial organic matter. The likely cause for OAE 2 is depletion of 12C in the water column as a result of high primary productivity, whereas an earlier R. cushmani zone event was primarily caused by increased input of terrigenous organic matter. Both δ 13C events are associated with enhanced organic matter preservation and anoxic or dysoxic bottom waters.