Cretaceous Oceanic Anoxic Events Research Articles

Effect of Size Fractions of Foraminifera on Reconstructing the Paleoceanography of Cretaceous Oceanic Anoxic Event 2

Abstract The globally recognized Cretaceous Oceanic Anoxic Event 2 was recorded at Integrated Ocean Drilling Program Site U1407 in the North Atlantic in sediments that were deposited under varying oxygen concentrations, conditions often associated with decreased foraminiferal test size. A >63-µm size fraction is typically used for collecting foraminifera that live in low-oxygen and deep-sea settings, particularly for paleoceanographic reconstructions where key environmental indicator species may be under-represented or absent in coarser sizes. Using 31 sediment samples from the site, we test the best size fraction to utilize when exploring biotic responses to an oceanic anoxic event. Counts of benthic and biserial (heterohelicid) planktic foraminifera from the standard >63-µm and finer 38–63-µm sieve sizes are compared to determine if critical information is lost if the smaller size fraction is not used. Results show similar trends in relative abundances for both size fractions. There are higher abundances in 38–63-µm residues, but species identification and counts are overly time-consuming for obtaining the general trends. We recommend the use of >63-µm sieves for foraminifera collected from low-oxygen sediments but encourage examination of the 38–63-µm fraction to verify intervals recognized as barren are truly absent of foraminifera.

Effect of Size Fractions of Foraminifera on Reconstructing the Paleoceanography of Cretaceous Oceanic Anoxic Event 2

Effect of Size Fractions of Foraminifera on Reconstructing the Paleoceanography of Cretaceous Oceanic Anoxic Event 2

Paleoceanographic importance of tri- and di-unsaturated alkenones through the early phase of Cretaceous Oceanic Anoxic Event 2 from southern high latitudes of the proto-Indian Ocean

Alkenones are biomarkers derived exclusively from species of haptophyte algae. The relative abundance of di- to tri-unsaturated C37 alkenones expressed as UK'37 is widely applied as a sea surface paleotemperature proxy for Cenozoic marine sediments. However, the absence of alkatrienones prior to the Eocene has precluded application of the UK'37 proxy for assessment of Cretaceous paleoclimates. Herein, we report a C40 alkatrienone (tetraconta-9E, 16E, 23E-trien-3-one; C40:3 Et) in deep-sea sediments from southern high latitudes (International Ocean Discovery Program: IODP site U1516). This discovery extends the geologic record of alkatrienones to the late Cenomanian, ∼70 million years earlier than previous reports. The parallel occurrence of higher abundances of a C40 alkadienone (tetraconta-16E, 23E-dien-3-one; C40:2 Et) allowed calculation of the UK’40 unsaturation index, comparable to UK’37. Stratigraphic variations in the δ13C of C40:2 Et revealed an elevated (∼1.5 ‰) positive carbon isotope excursion (CIE) relative to those observed in carbonate from other OAE2 sequences likely reflecting a decrease in global pCO2. The UK’40 profile suggests a concurrent drop in sea surface temperature associated with the decline in pCO2 during the early phase of OAE2. The timing of these environmental perturbations in the southern high latitude of the proto-Indian Ocean suggests they were triggered by volcanism associated with large igneous province (LIP) formation.

An overview of the Cretaceous oceanic anoxic events in Egypt, southern Tethys

Abstract The Cretaceous Period ( c. 143–66 Ma) attested to successive phases of regional tectonic activities and large-scale submarine volcanism resulting in increased atmospheric CO 2 concentrations, enhanced greenhouse climate and global warming, surface water bioproductivity and well-developed pore and bottom water anoxia, especially during the mid-Cretaceous (Aptian to Turonian). These conditions triggered widespread deposition of organic carbon (OC)-rich black shales during episodes of severe oxygen exhaustion, commonly known as oceanic anoxic events (OAEs). Oceanic anoxic events had many important consequences, including the widespread mass extinction of marine and terrestrial organisms. Several intervals of enhanced oxic ventilation and deposition of OC-poor pelagic–hemipelagic red carbonates and claystone interrupted Cretaceous OAEs, which are commonly known as Cretaceous oceanic red beds (CORBs). The mid-Cretaceous black shales from many parts of the world and the corresponding global carbon cycle changes indicated that OAE1 (Selli Level) and OAE2 (Bonarelli Event) are the most widespread and well-defined OAEs compared with the more regional OAE3 and Valanginian–Hauterivian black shales (Weissert Event). These appear to have been more restricted to the Atlantic and adjacent areas, a result of more regional conditions rather than being forced by global environmental perturbations. In the course of this review, the Egyptian territory in the southern Tethys shelf is taken as a case study area for the development, evolution and regional expression of mid-Cretaceous OAEs. The region is characterized by a thick Cretaceous sedimentary succession of OC-rich black shale and OC-poor carbonates and mudstone interbeds. The late Albian OAE1d was reported in the North Western Desert (upper Kharita Formation) based on a positive carbon isotope excursion and moderate total sulfur, total organic carbon and redox-sensitive elements. The end-Cenomanian OAE2 was reported in several regions of Egypt based on positive δ 13 C excursions. However, differential deposition between OC-rich and OC-poor facies took place, especially in marginal to shallow marine settings where black shales are absent, such as in the Eastern Desert and Sinai. During the Coniacian–Santonian, OC-poor limestone, calcareous shale and red claystone were deposited under enhanced water column respiration in Egypt. This indicates that enhanced oxygenated ocean circulation controlled organic matter decomposition during weak continental weathering and enhanced carbonate production, all of which led to the deposition of OC-poor CORBs in this region.

Sedimentary molybdenum and uranium cycling under seasonally contrasting redox conditions on the Namibian Shelf

Sedimentary molybdenum (Mo) and uranium (U) enrichments have been widely used as a proxy for redox conditions in oxygen-depleted marine paleo-environments. However, in a dynamic upwelling system the seasonal fluctuations from oxic to completely anoxic-sulfidic bottom waters and lateral sediment transport can modify the primary Mo and U signal of the sediment, which in turn may impact paleo-redox interpretations. In this study we present pore water and solid phase data collected at two cross shelf transects during the ‘more oxygenated’ austral winter and ‘anoxic’ austral summer to study the influence of spatially and seasonally contrasting redox conditions on the formation of authigenic Mo and U enrichments in organic carbon (TOC) rich mud belt sediments on the Namibian shelf. A mass balance was established for each element based on diffusive fluxes and element mass accumulation rates to evaluate the respective mechanisms of trace metal delivery, accumulation and recycling.Mo is delivered to the sediment in its dissolved form via diffusion across the sediment–water interface, especially during austral summer when bottom waters are anoxic and surface sediments are highly sulfidic. In the center of the inner shelf mud belt, the benthic Mo fluxes of up to 37 nmol cm−2 yr−1 into sulfidic surface sediments are the highest ever reported for reducing sulfidic systems and agree with the rate of Mo accumulation in the solid phase. Concurrently, high sedimentation rates and low terrigenous input limit solid phase Mo accumulation on the Namibian shelf. In ancient marine sediments, this mode of Mo cycling can be identified by low Mo/TOC ratios of ∼2 similar to those found in sediments deposited below the perennial oxygen minimum zone on the Peruvian shelf and to those found in deposits of the Cretaceous Oceanic Anoxic Event 2. Diffusive U fluxes into the sediment are generally too low to account for the sedimentary enrichment leading to the conclusion that U is delivered mainly in particulate form. In areas with anoxic bottom water, shallow dissolved U maxima directly below the sediment water interface and rather low sedimentary U content indicate that particulate U is recycled and largely released back into the bottom water. At sites where bottom water oxygen concentrations vary from anoxic to completely oxic on seasonal timescales, the depth at which Mo and U are removed from pore waters moves vertically within the sediment column thus defining a layer between the sediment surface and ∼20 cm depth, in which Mo and U accumulate in the solid phase. Our results emphasize the importance of short-term redox fluctuations in the bottom waters and underlying sediments, as well as lateral sediment transport for the authigenic enrichment of redox-sensitive trace metals in reducing shelf sediments. The relative enrichment patterns identified might be useful for the reconstruction of open marine anoxia in the geological past.

Carbonate uranium isotopes across Cretaceous OAE 2 in southern Mexico: New constraints on the global spread of marine anoxia and organic carbon burial

Oceanic anoxic events (OAEs) represent discrete intervals of decreased marine oxygen concentrations often associated with volcanism, enhanced organic carbon burial coupled with positive δ13C excursions, and significant biotic turnover. Cretaceous OAE 2 (ca. 94 Mya) is especially notable for globally-distributed changes in calcareous invertebrate and plankton populations. While the presence of organic-rich facies is consistent with locally anoxic environments in many cases, determining the global extent of anoxia is more problematic. To address this issue, we investigate uranium isotope (δ238U) compositions of upper Cretaceous (Cenomanian-Turonian) open marine platform carbonates from southern Mexico as a proxy for global seawater redox conditions. These data are complementary to previous δ238U studies across OAE 2 in both black shales and pelagic carbonates, which have yielded variable results that reflect both global redox and local depositional processes. In Morelos Formation carbonates, a significant and well-defined negative δ238U excursion down to a nadir of −0.6‰ is recorded over an ∼40 m interval. This is consistent with the expansion of marine anoxia, pointing to an areal extent of anoxic seawater of about 1–10% of the global seafloor (or ∼5 to 50 times the modern value). Importantly, based on biostratigraphically-controlled estimates of sediment accumulation rates, the δ238U anomaly precedes the δ13C excursion by a median of ∼45 to 51 kyr (95th percentile confidence interval, CI) or ∼105 to 120 kyr (95th percentile CI) depending on how the onset of the δ238U anomaly is estimated. These results, along with previously reported thallium isotope and trace metal data, suggest that anoxic expansion preceded carbon cycle perturbation. This observation further increases estimates of the duration of OAE 2, implying that widespread ocean anoxia may have lasted >900 kyr.

Geochemistry of the Upper Cretaceous - Miocene strata in the Tekman-Karayazı Basin (Turkey): Tectonic processes, sedimentation patterns, and the source rock quality along the northern branch of the Neo-Tethys Basins and northeastern Arabian plate

Tekman-Karayazı Basin (TKB) is one of the Eastern Anatolia basins which is located in the south of the Izmir-Ankara-Erzincan Suture Zone and is characterized by sedimentary units ranging in age from Upper Cretaceous to Pliocene. The basin began to form in Upper Cretaceous (Maastrichtian) in a tectonic depression, which over time evolved into a collisional foreland basin. While the Jurassic and Cretaceous Oceanic Anoxic Event (OAE) rocks were preserved in the NE of the Arabian Plate (SW of Iran), they are represented by an ophiolitic mélange which was formed as a result of the closure of the northern branch in the TKB and other basins on the Anatolian microplate. Four tectono-sedimentary phases are recognized over the ophiolitic mélange: Phase I (Maastrichtian-Paleocene (?), after obduction) is comprised of submarine fan deposits. Phase II (Paleocene - Lower Eocene, obduction phase) is characterized by major changes in gradually depositional environments in the TKB (clastics-nummulitic limestone), and continental to ramp carbonate to slope facies in others (e.g., Yakacık-Orhaniye, Haymana-Polatlı). Phase III (Middle – Upper Eocene, ongoing collision phase) shows the development of turbiditic and continental facies show that both TKB and/or the coeval basins have poor source rock potential. Phase IV (Oligo-Miocene, ending collision) represents from eastern to central in eastern Anatolian basins alluvial-fluvial to lacustrine and marine units during Oligocene – Miocene, respectively. The geochemical correlation between the bitumen extracted from sandstone sample and the Ağçakoca shales suggests that this formation can be considered as the corresponding source rock unit; however, according to the Tmax values (364–444 °C, mean: 428 °C), this formation is mostly immature, a scenario supported by the dry wells in the region. The regional volcanic activities triggered in Miocene by continent-continent collision seems to have provided the heat which, in turn, caused the thermal cracking of kerogen in Ağcakoca shales and locally induced petroleum expulsion. Mixing with the underground water on its way to reach a reservoir rock, the generated oil formed the oil seep. Therefore, we propose that a speculative Mescitli-Haneşdüzü (?) petroleum system may exist in this basin. We suggest that the combination of factors such as the asymmetric closure of the Tethys Ocean and the escape tectonics of the Anatolian Plate gave rise to the deposition of Paleocene to Miocene organic matter-rich shales in the NE of the Arabian plate, whereas uplift, antecedent topography, and the global paleo-climatic events disturbed the favorable conditions and caused the studied basins to lack the aforementioned generative source units.

Calcium isotope ratios of malformed foraminifera reveal biocalcification stress preceded Oceanic Anoxic Event 2

Ocean acidification causes biocalcification stress. The calcium isotope composition of carbonate producers can archive such stress because calcium isotope fractionation is sensitive to precipitation rate. Here, we synthesize morphometric observations of planktic foraminifera with multi-archive calcium isotope records from Gubbio, Italy and the Western Interior Seaway spanning Cretaceous Ocean Anoxic Event 2 (~94 million years ago). Calcium isotope ratios increase ~60 thousand years prior to the event. The increase coincides with foraminiferal abnormalities and correlates with existing proxy records for carbon dioxide release during large igneous province volcanism. The results highlight Ocean Anoxic Event 2 as a geologic ocean acidification analog. Moreover, decreasing calcium isotope ratios during the event provide evidence for ocean alkalinization, which could have shifted air-sea carbon dioxide partitioning. These data offer an explanation for the Plenus Cold Event and further have implications for refining ocean alkalinity enhancement, a leading anthropogenic carbon dioxide removal strategy.

Preservation Factors during Cretaceous Oceanic Anoxic Events in the Espírito Santo Basin, Southeast Brazil

The oceanic anoxic events (OAEs) are characterized by enhanced accumulation of organic matter in marine sediments. However, there is still an ongoing debate regarding the interplay between production and preservation during these events. Moreover, few studies provide quantitative estimations of primary productivity and/or the amount of carbon preserved during the OAEs. Here, we used geochemical data from multiple wells located at the Espírito Santo Basin that cover the intervals of events OAE1d and OAE2 to provide quantitative estimates of preservation factors. Our results show enhanced preservation during OAEs compared to modern conditions and a stronger preservation during OAE1d compared to OAE2 in the Espírito Santo Basin. The amount of preserved carbon could reach up to 8.6% during OAE1d, depending on the productivity of the system. In addition, we show that such improvement in preservation is linked to the bottom water with low-O2 concentrations and not due to fast burial caused by high sedimentation rates. Our findings are extremally relevant for organic carbon and source rock modelling studies since model simulations need quantitative estimations.

Orbitally paced global oceanic deoxygenation decoupled from volcanic CO2 emission during the middle Cretaceous Oceanic Anoxic Event 1b (Aptian-Albian transition)

Abstract The ongoing oceanic oxygen loss will have a profound impact on the distribution and density of life on Earth. However, drivers of the initiation and termination of global oceanic deoxygenation are poorly understood. Here we present a thallium isotope record that reveals three rapid (50 k.y.) global deoxygenation intervals during the mid-Cretaceous that postdate massive volcanism by at least 1 m.y. New strontium isotope data reveal gradually enhanced continental weathering under a warmer climate following volcanism. However, global deoxygenation occurred only under the combined influences of a long-term increase in weathering rates in a warmer climate and short-term orbital modulation that led to atmospheric-circulation reorganization. Interactions of multiple drivers are consistent with the abrupt termination of each deoxygenation interval. Dynamic oxygenation responses in the mid-Cretaceous highlight the role of these processes for understanding the consequences and potential termination of the current oceanic deoxygenation.

Biotic and Paleoceanographic Changes Across the Late Cretaceous Oceanic Anoxic Event 2 in the Southern High Latitudes (IODP Sites U1513 and U1516, SE Indian Ocean).

Oceanic Anoxic Event 2, spanning the Cenomanian/Turonian boundary (93.9 Ma), was an episode of major perturbations in the global carbon cycle. To investigate the response of biota and the paleoceanographic conditions across this event, we present data from International Ocean Discovery Program sites U1513 and U1516 in the Mentelle Basin (offshore SW Australia; paleolatitude 59°–60°S in the mid‐Cretaceous) that register the first complete records of OAE 2 at southern high latitudes. Calcareous nannofossils provide a reliable bio‐chronostratigraphic framework. The distribution and abundance patterns of planktonic and benthic foraminifera, radiolaria, and calcispheres permit interpretation of the dynamics of the water mass stratification and provide support for the paleobathymetric reconstruction of the two sites, with Site U1513 located northwest of the Mentelle Basin depocenter and at a deeper depth than Site U1516. The lower OAE 2 interval is characterized by reduced water mass stratification with alternating episodes of enhanced surface water productivity and variations of the thickness of the mixed layer as indicated by the fluctuations in abundance of the intermediate dwelling planktonic foraminifera. The middle OAE 2 interval contains lithologies composed almost entirely of radiolaria reflecting extremely high marine productivity; the low CaCO3 content is consistent with marked shoaling of the Carbonate Compensation Depth and ocean acidification because of CaCO3 undersaturation. Conditions moderated after deposition of the silica‐rich, CaCO3‐poor rocks as reflected by the microfossil changes indicating a relatively stable water column although episodes of enhanced eutrophy did continue into the lower Turonian at Site U1516.

LIP volcanism (not anoxia) tracked by Cr isotopes during Ocean Anoxic Event 2 in the proto-North Atlantic region

Chromium is a redox sensitive element that exhibits a large range of isotopic compositions in Earth’s surface environments because of Cr(VI)-Cr(III) transformations. This property of Cr has been exploited as a tracer of Earth’s oxygenation history using marine sediments. However, paleoredox applications using Cr are difficult to implement due to its complicated cycling, which creates spatial variability in seawater δ53Cr values. Applications are further hindered by the potential for variability in the major inputs of Cr, such as submarine volcanism, to mask redox processes. Two previous reports of negative excursions in sedimentary δ53Cr values during the middle Cretaceous Ocean Anoxic Event 2 (OAE 2) demonstrate these complications. Observed negative shifts in marine sediments conflict with the positive shifts expected in response to the increased drawdown of isotopically light Cr(III) prompted by the expansion of anoxic depositional sinks. In this study, a marine carbonate succession cored from the Eagle Ford Formation in Texas, USA, in the southern part of the Western Interior Seaway, depicts the negative 1.5‰ δ53Cr excursion occurring in two steps, with the second step reaching peak minimum values indistinguishable from isotopically unfractionated igneous sources. In contrast to published δ53Cr records, each step stratigraphically matches proxy evidence for increased eruption frequency and/or intensity of volcanic activity using combined 187Os/188Os, 87Sr/86Sr and Os concentration proxies previously measured from the same core, supporting higher inputs of volcanically sourced Cr to the oceans as the driver for the negative Cr isotope excursion.

Environmental change and carbon-cycle dynamics during the onset of Cretaceous oceanic anoxic event 1a from a carbonate-ramp depositional system, Abu Dhabi, U.A.E.

We report the first high-resolution sedimentological and geochemical record of the negative carbon-isotope excursion (CIE) at the onset of the early Aptian oceanic anoxic event (OAE) 1a from a carbonate-ramp depositional environment, analysed from a well core from c. 2500 m depth, 100 km offshore Abu Dhabi, United Arab Emirates. Time-series analysis of stable oxygen isotope values and concentrations of Si, Al, and Ti resulted in durations of the C3 and C4 segments of the CIE that support relative completeness of the C3 segment and high sediment preservation rates of c. 13 cm/kyr of the studied sedimentary sequence. Stable oxygen-isotope ratios of bulk carbonates are interpreted to indicate two episodes of cooling, separated by rapid warming during the peak of the negative CIE. The contributions of diagenesis and seawater pH on the bulk oxygen-isotope record will have affected the palaeoclimatic signal and are critically discussed. A major shift in oxygen isotope values at the peak of the negative CIE in the C3 segment coincides with relatively carbonate-poor, marly deposits, time-equivalent with other, global evidence for a reduction of carbonate saturation of sea-surface water. According to our chemo- and cyclostratigraphic calibration, this episode of low carbonate saturation of seawater reflects a pulse of major volcanic CO2 release from the Ontong-Java large igneous province that was sufficiently short to have escaped internal buffering by the dynamics of the ocean lysocline.

The oxic Coniacian-Santonian interval in the Kopet-Dagh Basin (NE Iran): Carbon isotope and benthic-planktic foraminiferal assemblages at the time of the last Cretaceous OAE

A middle Coniacian-middle Santonian interval has been studied in the Kopet-Dagh Basin (NE Iran) to reconstruct the paleoenvironmental conditions in the eastern part of Tethyan realm at the time of the last Cretaceous Oceanic Anoxic Event (OAE3). This succession is composed of a chalky limestone unit, sandwiched by light gray calcareous mudstone and marlstone beds. The δ 13 C curve indicates several minor positive shifts up to 1‰ in the studied section that is correlative to other carbon isotope curves from the Tethys and Boreal realms. A prominent upward increase of P/B ratio in the studied section suggests a relative sea-level rise toward the Santonian. Lithology (low carbonate contents) and faunal contents (high %benthic and higher infaunal benthic foraminifera) reveal that the lower part of the studied section was deposited in a neritic environment influenced by continental-derived nutrients and mesotrophic to eutrophic conditions. Enhanced carbonate contents, higher diversity of benthic foraminifera and dominance of epifaunal taxa suggest more oligotrophic conditions and/or higher oxygen concentrations in the interval equivalent with a δ 13 C positive shift, caused by a transgression phase at this time. The overlying deposits are characterized by the maximum flooding surface and the highstand phase. The stratification of water column and more oxygen consumption in this interval resulted in higher abundance of infaunal benthic foraminifera and decline of carbonate production in a bathyal environment. • Coniacian/Santonian boundary interval was deposited under oxic conditions in the E Tethys. • No black shales or anoxia mark this inferred OAE3 interval. • Sea-level fluctuations control foraminiferal assemblages in this study. • The δ 13 C curve reveals the diagnostic features of global Co/Sa boundary record.

Enhanced ocean connectivity and volcanism instigated global onset of Cretaceous Oceanic Anoxic Event 2 (OAE2) ∼94.5 million years ago

Oceanic Anoxic Event 2 (OAE2) exemplifies an episode of global oceanographic and climatic changes in the mid-Cretaceous greenhouse. However, the timing and mechanism of its global initiation remain elusive. Here we report a high-resolution initial osmium isotope (187Os/188Osi, Osi) and δ13Corg record from an expanded OAE2 interval in southern Tibet, China that was deposited in the Southern Hemisphere. The Osi record documents episodic, intensifying volcanism with the highest intensity episode marked by a large Osi excursion at ∼94.5 Ma and a subsequent ∼200 kyr Osi minimum concomitant with a cooling interval, which is broadly synchronous with the Plenus Cold Event (PCE) recorded in the Northern Hemisphere. Also, the large Osi excursion lags the onset of OAE2 by ∼50 kyr and occurred during a near synchronous global transgression based on high-resolution correlation with other major OAE2 records. These results are interpreted as indicating that enhanced ocean connectivity played a critical role in helping trigger the global onset of OAE2 at ∼94.5 Ma.

Controls on sedimentary accumulation of organic matter during Cretaceous Oceanic Anoxic Event 2, IODP site U1407, Southeast Newfoundland Ridge

The pelagic sediments from Integrated Ocean Drilling Program (IODP) Site U1407 in the western North Atlantic are characterized by a sequence of organic-lean and organic-rich sediments representing a major global episode of organic matter (OM) burial during Oceanic Anoxic Event 2 (OAE2). The OM-rich deposits (up to 16% total organic carbon) consist primarily of radiolarians which are rare/absent in the organic-lean sediments. New nannofossil biostratigraphy and the correlation with the global ẟ13C curve were used to constrain the timing of paleoceanographic changes below, within, and above OAE2 sediments. Data were combined from elemental concentrations, biomarkers, and abundances of benthic foraminifera, biserial (heterohelicids) planktic foraminifera, and radiolarians to reconstruct the evolution of these changes. The ẟ13C curve records a global negative excursion within organic-lean sediments not typically recorded in other sections preceding the initial rise in isotopic values that signals OM burial worldwide. Maximum OM accumulation occurred during the second positive isotopic excursion, which is associated with higher terrigenous input (indicated by long-chain n-alkanes and terrestrially derived elements) and water column stratification (suggested by the paleoindicator gammacerane). In addition, sesquiterpene biomarkers derived from cypress and other conifers suggest flooding of gymnosperm forests from increased rainfall coupled with rising sea levels and resulting higher water tables. Within mid-OAE2 sediments, a sharp increase in radiolarians and fish teeth/debris and a decrease in benthic foraminifera implies deposition of OM from increased productivity/upwelling under low oxygen in the lower part of the water column. Upwelling and water column stratification occurring asynchronously at the surface resulted from changes in precipitation, with stratification developing during more humid/wet conditions. Greenhouse warming during OAE2 likely increased precipitation, weathering and nutrient inputs. However, prolonged anoxia caused buried phosphorus to be recycled as indicated by higher redox-sensitive trace element and biolimiting element concentrations, sustaining primary production and further enriching the sediments in OM. We suggest that increased marine paleoproductivity was the primary control on the OM-rich sediment accumulation during OAE2, while low-oxygen conditions resulting from stratification of surface waters were essential to its preservation.

Extensive jarosite deposits formed through auto-combustion and weathering of pyritiferous mudstone, Smoking Hills (Ingniryuat), Northwest Territories, Canadian Arctic – A potential Mars analogue

Jarosite and other metal sulphates are common on Mars and are interpreted to have formed in an extreme acidic saline aqueous environment, similar to modern Earth analogues where jarosite precipitates. We examined an alternative Earth analogue, at the Smoking Hills (known as Ingniryuat by Inuvialuit), Northwest Territories, Arctic Canada. The Smoking Hills are characterised by auto-combusting pyritic mudstones of the Smoking Hills Formation in a polar desert. The Smoking Hills Formation was deposited in an outer shelf to slope marine environment, during a Late Cretaceous Ocean Anoxic Event. Oxidative weathering of this unit creates extensive jarosite-rich deposits, and banded jarosite- and phyllosilicate-rich mudstones, similar to those observed on Mars. Slumping of these mudstones exposes large masses to atmospheric oxygen leading to generation of high temperatures (sufficient to produce paralavas) through pyrite oxidation, and the subsequent formation of a diverse suite of hydrated metal-sulphate minerals. Weathered combustion sites are characterised by a simpler jarosite-rich mineralogy along with thermally-altered mudstone. Away from sites of auto-combustion (bocannes), pyrite-rich layers within the mudstone oxidise to jarosite, creating a yellow-banded appearance in outcrops. These jarosite-rich layers, similar to those observed interbedded in mudstones on Mars, reflect post depositional oxidation processes - not acidic conditions during the time of deposition. As such, this could reflect a more habitable environment than is commonly interpreted for Mars.

Processes and forcing mechanisms of the carbon cycle perturbation during Cretaceous Oceanic Anoxic Event 2

<p indent="0mm">Oceanic Anoxic Event 2 (OAE2, ~94 Ma) was one of two global oceanic anoxic events in the Cretaceous and has been one of the most studied paleoceanographic events of the Mesozoic. Owing to the hyperthermal climate state during OAE2, it serves as a key event in the study of Earth system evolution during extreme greenhouse warmth. Recently, high temporal and spatial resolution studies and the application of globally representative paleoenvironmental proxies have permitted major advances in our understanding of the triggering mechanisms and feedbacks of the Earth system during OAE2. By reviewing these findings, this paper analyzes the detailed processes involved in the OAE2 carbon cycle perturbation and associated environmental changes to elucidate the mechanisms that forced the evolution of carbon isotopes (<italic>δ</italic>¹³C) in seawater through the event. The most remarkable feature of OAE2 is a sharp positive <italic>δ</italic>¹³C excursion recorded globally, indicating a major perturbation of carbon cycle during the event. Outgassing from massive magmatism associated with large igneous province (LIP) emplacement was the major trigger of this carbon cycle perturbation. Increased primary productivity and extensive anoxia were responsible for the long-term positive <italic>δ</italic>¹³C excursion during OAE2. Interactions between paleoenvironmental parameters, such as weathering intensity, paleotemperature, CO₂ concentration, primary productivity and seawater redox condition controlled the short-term <italic>δ</italic>¹³C changes, which are superimposed on the long-term positive excursion of the entire OAE2 event. In this paper, we present a comprehensive model for the interactions between paleoenvironment changes and <italic>δ</italic>¹³C fluctuations through OAE2. The <italic>δ</italic>¹³C curve across the OAE2 has been previously divided into six segments by Li et al. (2017), labeled as C1 to C6, among which the major carbon cycle perturbation interval spans segments C2 to C5. During segment C2, a minor negative excursion in <italic>δ</italic>¹³C with a magnitude of ~1‰ is observed in open marine settings and relates to outgassing from LIP magmatic activity. However, enhanced productivity and deoxygenation of seawater during C2, especially in restricted marine settings, promoted the preservation of organic matter and this in turn led to an increase of <italic>δ</italic>¹³C. Based on simple mass balance calculation, we suggest that a 25% increase in the burial rate of organic carbon could diminish the effects of LIP outgassing on <italic>δ</italic>¹³C values in seawater and lead to relatively stable <italic>δ</italic>¹³C values during C2 in restricted seas. Segment C3 is characterized by a rapid increase in <italic>δ</italic>¹³C values that lasted for <sc>~280 ka.</sc> Significantly enhanced biological fixation of carbon is suggested to be responsible for the positive excursion in <italic>δ</italic>¹³C. A globally recorded negative excursion in <italic>δ</italic>¹³C has been found in the middle of this segment, which is consistent with an interval of maximum weathering intensity and global cooling. We suggest that reoxygenation of bottom water owing to cooling and consequent respiration of organic carbon from enhanced continental input and organic rich sediments could be the major cause of this decrease in <italic>δ</italic>¹³C in the middle of segment C3. Within the lower part of segment C4, LIP magmatism remained active and weathering intensity decreased, which led to a return to anoxic conditions and a highly fertilized state within the oceans. Consequent high burial rate of organic matter resulted in the positive trend in <italic>δ</italic>¹³C. Meanwhile, the sediment retention potential for nutrients (e.g., P) decreased under anoxic conditions, which led to sustained high primary productivity and high <italic>δ</italic>¹³C (<italic>δ</italic>¹³C plateau) that lasted for <sc>~370 ka.</sc> After this peak in <italic>δ</italic>¹³C, nutrient supply from weathering and LIP magmatism reached a minimum threshold to maintain a productivity-anoxia feedback, which in turn led to the inevitable recovery of <italic>δ</italic>¹³C (segment C5) to pre-OAE values and the termination of OAE2.

A new view on the evolution of seawater molybdenum inventories before and during the Cretaceous Oceanic Anoxic Event 2

Extreme periods of global warming in Earth's history have been strongly associated with declining oxygen concentrations in the oceans, a scenario not unlike the evolution observed in the oceans in recent times. One of the most intense Phanerozoic deoxygenation events (so called Ocean Anoxic Events (OAE)) is the late Cretaceous OAE2 approximately 94 Ma ago. Although several studies have investigated the evolution of redox sensitive geochemical proxies during the OAE2 event, geochemical records that span the time period leading up to OAE2, in particular for molybdenum (Mo) and its isotopes are rare. Here, we investigate Mo cycling in the Tarfaya upwelling system in the Cretaceous proto-North Atlantic Ocean before and during OAE 2 throughout a 5 Ma record. The observed changes in sedimentary Mo isotope compositions can be explained by a mode of Mo cycling similar to that in modern oxygen minimum zones. Based on this interpretation we estimate the pre-OAE2 Mo isotope composition of seawater to be 1.6‰ indicating that late Cretaceous pre-OAE2 seawater had a lighter Mo isotope composition than the modern ocean. We suggest that Mo burial in oxic sediments was likely diminished under conditions of globally reduced oxygen concentrations. This allows us to model the Mo isotope composition of late Cretaceous pre-OAE2 seawater without lowering of the seawater Mo inventory (e.g., −55% oxic sink, + 40% anoxic sink). Previous estimations of the Mo seawater isotope composition during OAE2 are close to the signal observed pre-OAE here. The Mo isotope variation associated with the onset of OAE2 is therefore small, which is unexpected because strong expansion of anoxic sedimentation in OAE2 is indicated by e.g. sulfur isotopes. A further decrease of Mn burial rates in the open ocean during OAE2 could “buffer” the Mo seawater isotope signal during OAE2, to account for the small offset. Our findings emphasize that changes to the relative size of not only the anoxic sink but also the oxic sink are important considerations when interpreting paleo-Mo isotope data.

Planktonic foraminiferal biostratigraphy of the Cretaceous strata, Indus Basin, Pakistan, Eastern Tethys: implications for oceanic anoxic events

Stratigraphic horizons of the Cretaceous oceanic anoxic events (OAEs) are constrained for the first time at the northwestern margin of the Indian Plate, Pakistan, using high-resolution planktonic foraminiferal biostratigraphy. In this study, 15 planktonic foraminiferal biozones have been erected, suggesting an early Aptian to early Maastrichtian age for the studied section. Based on the established biozones, the stratigraphic positions of the Cretaceous OAEs are defined. The lowermost organic-rich interval occurs in the Hedbergella delrioensis and Globigerinelloides algerianus zones of the early to late Aptian, which correlate with the Selli (OAE 1a) and Fallot events. The next interval with the abundant presence of organic matter occurs in the Ticinella bejaouaensis zone (latest Aptian) and correlates with OAE1b. The third organic-rich interval falls in the Biticinella breggiensis zone of the middle to late Albian and correlates with the OAE1c. The fourth organic-rich horizon straddles the Cenomanian − Turonian boundary and corresponds to OAE2. The fifth organic-rich interval falls in the Globotruncana ventricosa zone of middle to late Campanian age and correlates with the Campanian isotopic event. The evolution of planktonic foraminifera in this part of the Tethys Ocean, is driven by OAEs, e.g. at the base of OAE2, a mass extinction of foraminiferal species, mainly rotaliporids, occurred, and at the end of OAE2, simultaneous appearances of the whiteinellids, marginotruncanids and dicarinellids indicate the fastest rate of speciation. KEY POINTS The planktonic foraminiferal biostratigraphic investigations revealed an early Aptian to early Maastrichtian age for the Parh Formation exposed in the Mughal Kot Section, Pakistan. Based on biostratigraphic data, stratigraphic positions of the OAE1a, OAE1b, OAE1c, OAE2 and Campanian events are constrained. An evolutionary link of major planktonic foraminiferal genera with Cretaceous OAEs has been observed.