Records of past mid-depth ventilation: Cretaceous ocean anoxic event 2 vs. Recent oxygen minimum zones

Present day oceans are well ventilated, with the exception of mid-depth oxygen minimum zones (OMZs) under high surface water productivity, regions of sluggish circulation, and restricted marginal basins. In the Mesozoic, however, entire oceanic basins transiently became dysoxic or anoxic. The Cretaceous ocean anoxic events (OAEs) were characterised by laminated organic-carbon rich shales and low-oxygen indicating trace fossils preserved in the sedimentary record. Yet assessments of the intensity and extent of Cretaceous near-bottom water oxygenation have been hampered by deep or long-term diagenesis and the evolution of marine biota serving as oxygen indicators in today's ocean. Sedimentary features similar to those found in Cretaceous strata were observed in deposits underlying Recent OMZs, where bottom-water oxygen levels, the flux of organic matter, and benthic life have been studied thoroughly. Their implications for constraining past bottom-water oxygenation are addressed in this review. We compared OMZ sediments from the Peruvian upwelling with deposits of the late Cenomanian OAE 2 from the north-west African shelf. Holocene laminated sediments are encountered at bottom-water oxygen levels of 10 μmol kg−1 showed an inverse exponential relationship of bottom-water oxygen levels and organic carbon accumulation depicting enhanced bioirrigation and decomposition of organic matter with increased oxygen supply. In the absence of seasonal laminations and under conditions of low burial diagenesis, this relationship may facilitate quantitative estimates of palaeo-oxygenation. Similarities and differences between Cretaceous OAEs and late Quaternary OMZs have to be further explored to improve our understanding of sedimentary systems under hypoxic conditions

Late Cretaceous paleoenvironmental evolution and sea-level history of the Tarfaya Basin, SW Morocco: Evidence from XRF scanner-derived elemental records, benthic and planktonic foraminifera and bulk carbonate stable isotopes

Lithological evidence, benthic foraminiferal census counts, and high-resolution X-ray fluorescence (XRF) scanner-derived elemental data were integrated with planktonic foraminiferal biostratigraphy and bulk carbonate stable isotopes to retrace the Turonian to early Campanian paleoenvironmental evolution and sea-level history of the Tarfaya Atlantic coastal basin (SW Morocco). The lower Turonian is characterized by impoverished benthic foraminiferal assemblages, which reflect an impingement of the oxygen minimum zone on the shelf during a sea-level highstand. The appearance of low-oxygen tolerant benthic foraminiferal assemblages in the middle to upper Turonian indicates an improvement in bottom water oxygenation, probably linked to offshore retraction of the oxygen minimum zone during a regressive phase. From the late Turonian to Santonian, the presence of benthic foraminiferal with low diversity suggests relatively impoverished oxygenation in bottom water along the shelf. Three long-term oscillations in the abundance of terrigenous elements (increase of Al, Ti, K, Si and Fe normalized against Ca) are shown during the Coniacian and Santonian. This interval, which roughly corresponds to the Coniacian-Santonian Anoxic Event (OAE-3), is characterized by overall oxygen depleted to anoxic conditions at the sea-floor (indicated by the high organic carbon content, the presence of laminations and by low manganese/sulphur, high vanadium/calcium and bromine/calcium ratios in XRF scanning records). The lower Campanian transgression, only recorded in the southern part of the Tarfaya Basin, coincided with substantial deepening, enhanced accumulation of fine-grained clay-rich hemipelagic sediments and improved oxygenation at the sea-floor (highest diversity and abundance of benthic foraminiferal assemblages and low values of log(Mn/S)). The sea-level changes reconstructed in the Tarfaya Basin are correlated to the global eustatic changes. Two major unconformities (U1/U2 and U3), which punctuate the upper Turonian to lower Campanian succession in Tarfaya SN°1 and 2, are correlative to the base of the Merchantville III and Magothy III sequence boundaries of Miller et al., (2004) and Mizintseva et al. (2009), respectively. Stable isotope data of bulk carbonates (outcrop sections and composite cores) are correlated to the English Chalk, the Niobrara Formation (US Western Interior Seaway) and to the stacked carbon isotope reference curve of Wendler (2013). The Tarfaya carbon isotope curve reveals in particular the Navigation Event in the Coniacian, the Haven Brow, the Horseshoe Bay and the Buckle Events in the Santonian as well as the Santonian/Campanian Boundary Event

Unraveling the onset of Cretaceous Oceanic Anoxic Event 2 in an extended sediment archive from the Tarfaya-Laayoune Basin, Morocco

We investigated the onset and development of Cretaceous Oceanic Anoxic Event 2 (OAE2) in a newly drilled core (SN degrees 4) from the Tarfaya Basin (southern Morocco), where this interval is unusually expanded. High-resolution (centimeter-scale equivalent to centennial) analysis of bulk organic and carbonate stable isotopes and of carbonate and organic carbon content in combination with XRF scanner derived elemental distribution reveal that the ocean-climate system behaved in a highly dynamic manner prior to and during the onset of OAE2. Correlation with the latest orbital solution indicates that the main carbon isotope shift occurred during an extended minimum in orbital eccentricity (similar to 400 kyr cycle). Shorter-term fluctuations in carbonate and organic carbon accumulation and in sea level related terrigenous discharge were predominantly driven by variations in orbital obliquity. Negative excursions in organic and carbonate delta C-13 preceded the global positive delta C-13 shift marking the onset of OAE2, suggesting injection of isotopically depleted carbon into the atmosphere. The main delta C-13 increase during the early phase of OAE2 in the late Cenomanian was punctuated by a transient plateau. Maximum organic carbon accumulation occurred during the later part of the main delta C-13 increase and was associated with climate cooling events, expressed as three consecutive maxima in bulk carbonate delta O-18. The extinctions of the thermocline dwelling keeled planktonic foraminifers Rotalipora greenhornensis and Rotalipora cushmani occurred during the first and last of these cooling events and were likely associated with obliquity paced, ocean-wide expansions, and intensifications of the oxygen minimum zone, affecting their habitat space on a global scale

New insights into Cenomanian paleoceanography and climate evolution from the Tarfaya Basin, southern Morocco

Highlights • Complete upper Albian to early Turonian climate archive in drilled core from Tarfaya Basin. • Eccentricity pacing of mid Cretaceous OAE isotope excursions. • MCE and OAE2 associated with climate cooling and sea level fall. Abstract A 325 m long continuous succession of uppermost Albian to lower Turonian pelagic (outer shelf) deposits was recovered from a new drill site in the central part of the Tarfaya Basin (southern Morocco). Natural gamma ray wireline logging, carbonate and organic carbon content, bulk carbonate and organic carbon stable isotopes and X-ray fluorescence (XRF)-scanner derived elemental distribution data in combination with planktonic foraminiferal biostratigraphy indicate complete recovery of the Cenomanian Stage. This exceptional sediment archive allows to identify orbitally driven cyclic sedimentation patterns and to evaluate the pacing of climatic events and regional environmental change across the Albian-Cenomanian boundary (ACB), the mid-Cenomanian Event (MCE) and Oceanic Anoxic Event 2 (OAE2) in the latest Cenomanian. The deposition of organic-rich sediments in the Tarfaya Basin, likely driven by upwelling of nutrient-rich water masses, started during the latest Albian and intensified in two major steps following the MCE and the onset of OAE2. The duration and structure of the MCE and OAE2 carbon isotope excursions exhibit striking similarities, suggesting common driving mechanisms and climate-carbon cycle feedbacks. Both events were also associated with eustatic sea level falls, expressed as prominent sequence boundaries in the Tarfaya Basin. Based on the 405 kyr signal imprinted on the Natural Gamma Ray (NGR) and XRF-scanner derived Log(Zr/Rb) records, we estimate the duration of the Cenomanian Stage to be 4.8 ± 0.2 Myr

Cretaceous oceanic anoxic events prolonged by phosphorus cycle feedbacks

Oceanic Anoxic Events (OAEs) document major perturbations of the global carbon cycle with repercussions on the Earth’s climate and ocean circulation that are relevant to understand future climate trends. Here, we compare sedimentation patterns, nutrient cycling, organic carbon accumulation and carbon isotope variability across Cretaceous Oceanic Anoxic Events OAE1a and OAE2 in two drill cores with unusually high sedimentation rates from the Vocontian Basin (southern France) and Tarfaya Basin (southern Morocco). OAE1a and OAE2 exhibit remarkable similarities in the evolution of their δ13C excursion with long-lasting negative carbon isotope excursions preceding the onset of both anoxic events, supporting the view that OAEs were triggered by massive emissions of volcanic CO2 into the atmosphere. Based on analysis of cyclic sediment variations, we estimated the duration of the individual phases within the carbon isotope excursions. For both events, we identify: (1) a precursor phase lasting ~ 430 kyr and ~ 130 kyr, (2) an onset phase of ~ 390 and ~ 70 kyr, (3) a peak phase of ~ 600 and ~ 90 kyr, (4) a plateau phase of ~ 1400 and ~ 200 kyr and (5) a recovery phase of ~ 630 and ~ 440 kyr, respectively. The total duration of the positive carbon isotope excursion is estimated as 3400 kyr and 790 kyr and that of the main carbon accumulation phase as 980 kyr and 180 kyr, for OAE1a and OAE 2 respectively. The extended duration of the peak, plateau and recovery phases requires fundamental changes in global nutrient cycles either (1) through excess nutrient inputs to the oceans by increasing continental weathering and river discharge or (2) through nutrient-recycling from the marine sediment reservoir. We investigated the role of phosphorus on the development of carbon accumulation by analysing phosphorus speciation across OAE2 and the mid-Cenomanian Event (MCE) in the Tarfaya Basin. The ratios of organic carbon and total nitrogen to reactive phosphorus (Corg/Preact and Ntotal/Preact) prior to OAE2 and the MCE hover close to or below the Redfield ratio characteristic of marine organic matter. Decreases in reactive phosphorus resulting in Corg/Preact and Ntotal/Preact above the Redfield ratio during the later phase of OAE2 and the MCE indicate leakage from the sedimentary column into the water column under the influence of intensified and expanded oxygen minimum zones. These results suggest that a positive feedback loop, rooted in the benthic phosphorus cycle, contributed to increased marine productivity and carbon burial over an extended period of time during OAEs

Stable isotope analysis (organic material) of core SN4 in Tarfaya Basin, southern Morocco

Stable isotope analysis (organic material) of core SN4 in Tarfaya Basin, southern Morocc

Natural Gamma Ray of core SN4 in Tarfaya Basin, southern Morocco

Natural Gamma Ray of core SN4 in Tarfaya Basin, southern Morocc

Elemental raw data, analysed with XRF core scanner on core SN4 in Tarfaya Basin, southern Morocco

Elemental raw data, analysed with XRF core scanner on core SN4 in Tarfaya Basin, southern Morocc

Geochemical analysis of core SN4 in Tarfaya Basin, southern Morocco

Geochemical analysis of core SN4 in Tarfaya Basin, southern Morocc

Stable isotope analysis (bulk carbonate) of core SN4 in Tarfaya Basin, southern Morocco

Stable isotope analysis (bulk carbonate) of core SN4 in Tarfaya Basin, southern Morocc