Palynology and sea surface temperature reconstruction for the mid-Cretaceous Oceanic Anoxic Event 2 at ODP Hole 210-1276A

The mid-Cretaceous is thought to be a greenhouse world with significantly higher atmospheric pCO2 and sea-surface temperatures as well as a much flatter latitudinal thermal gradient compared to the present. This time interval was punctuated by the Cenomanian/Turonian Oceanic Anoxic Event (OAE-2, ~ 93.5 Myr ago), an episode of global, massive organic carbon burial that likely resulted in a large and abrupt pCO2 decline. However, the climatic consequences of this pCO2 drop are yet poorly constrained. We determined the first, high-resolution sea-surface temperature (SST) record across OAE-2 from a deep-marine sedimentary sequence at Ocean Drilling Program (ODP) Site 1276 in the mid-latitudinal Newfoundland Basin, NW Atlantic. By employing the organic palaeothermometer TEX86, we found that SSTs across the OAE-2 interval were extremely high, but were punctuated by a remarkably large cooling (5-11 °C), which is synchronous with the 2.5-5.5 °C cooling in SST records from equatorial Atlantic sites, and the "Plenus Cold Event". Because this global cooling event is concurrent with increased organic carbon burial, it likely acted in response to the associated pCO2 drop. Our findings imply a substantial increase in the latitudinal SST gradient in the proto-North Atlantic during this period of global cooling and reduced atmospheric pCO2, suggesting a strong coupling between pCO2 and latitudinal thermal gradients under greenhouse climate conditions.

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 < 7 µmol/kg under the Peruvian upwelling and < 5 µmol/kg in California Borderland basins and the Pakistan Margin. Seasonal to decadal changes of sediment input are necessary to create laminae of different composition. However, bottom currents may shape similar textures that are difficult to discern from primary seasonal laminae. The millimetre-sized trace fossil Chondrites was commonly found in Cretaceous strata and Recent oxygen-depleted environments where its diameter increased with oxygen levels from 5 to 45 µmol/kg. Chondrites has not been reported in Peruvian sediments but centimetre-sized crab burrows appeared around 10 µmol/kg, which may indicate a minimum oxygen value for bioturbated Cretaceous strata. Organic carbon accumulation rates ranged from 0.7 and 2.8 g C /cm2 /kyr in laminated OAE 2 sections in Tarfaya Basin, Morocco, matching late Holocene accumulation rates of laminated Peruvian sediments under Recent oxygen levels below 5 µmol/kg. Sediments deposited at > 10 µmol/kg 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.

Mitochondrial Contribution to the Anoxic Ca2+ Signal in Maize Suspension-Cultured Cells1

Anoxia induces a rapid elevation of the cytosolic Ca2+ concentration ([Ca2+]cyt) in maize (Zea mays L.) cells, which is caused by the release of the ion from intracellular stores. This anoxic Ca2+ release is important for gene activation and survival in O2-deprived maize seedlings and cells. In this study we examined the contribution of mitochondrial Ca2+ to the anoxic [Ca2+]cyt elevation in maize cells. Imaging of intramitochondrial Ca2+ levels showed that a majority of mitochondria released their Ca2+ in response to anoxia and took up Ca2+ upon reoxygenation. We also investigated whether the mitochondrial Ca2+ release contributed to the increase in [Ca2+]cyt under anoxia. Analysis of the spatial association between anoxic [Ca2+]cyt changes and the distribution of mitochondrial and other intracellular Ca2+ stores revealed that the largest [Ca2+]cyt increases occurred close to mitochondria and away from the tonoplast. In addition, carbonylcyanide p-trifluoromethoxyphenyl hydrazone treatment depolarized mitochondria and caused a mild elevation of [Ca2+]cyt under aerobic conditions but prevented a [Ca2+]cyt increase in response to a subsequent anoxic pulse. These results suggest that mitochondria play an important role in the anoxic elevation of [Ca2+]cyt and participate in the signaling of O2 deprivation.

Evidence for the Critical Role of Sucrose Synthase for Anoxic Tolerance of Maize Roots using a Double Mutant

The induction of the sucrose synthase (SuSy) gene (SuSy) by low O2, low temperature, and limiting carbohydrate supply suggested a role in carbohydrate metabolism under stress conditions. The isolation of a maize (Zea mays L.) line mutant for the two known SuSy genes but functionally normal showed that SuSy activity might not be required for aerobic growth and allowed the possibility of investigating its importance during anaerobic stress. As assessed by root elongation after return to air, hypoxic pretreatment improved anoxic tolerance, in correlation with the number of SuSy genes and the level of SuSy expression. Furthermore, root death in double-mutant seedlings during anoxic incubation could be attributed to the impaired utilization of sucrose (Suc). Collectively, these data provide unequivocal evidence that Suc is the principal C source and that SuSy is the main enzyme active in Suc breakdown in roots of maize seedlings deprived of O2. In this situation, SuSy plays a critical role in anoxic tolerance.

The Effect of Nutrient-Rich Effluents from Shrimp Farming on Mangrove Soil Carbon Storage and Geochemistry Under Semi-Arid Climate Conditions in Northern Brazil

A semi-arid mangrove estuary system in the northeast Brazilian coast (Ceará state) was selected for this study to (i) evaluate the impact of shrimp farm nutrient-rich wastewater effluents on the soil geochemistry and organic carbon (OC) storage and (ii) estimate the total amount of OC stored in mangrove soils (0–40 cm). Wastewater-affected mangrove forests were referred to as WAM and undisturbed areas as Non-WAM. Redox conditions and OC content were statistically correlated (P < 0.05) with seasonality and type of land use (WAM vs. Non-WAM). Eh values were from anoxic to oxic conditions in the wet season (from − 5 to 68 mV in WAM and from < 40 to > 400 mV in Non-WAM soils) and significantly higher (from 66 to 411 mV) in the dry season (P < 0.01). OC contents (0–40 cm soil depth) were significantly higher (P < 0.01) in the wet season than the dry season, and higher in Non-WAM soils than in WAM soils (values of 8.1 and 6.7 kg m− 2 in the wet and dry seasons, respectively, for Non-WAM, and values of 3.8 and 2.9 kg m− 2 in the wet and dry seasons, respectively, for WAM soils; P < 0.01). Iron partitioning was significantly dependent (P < 0.05) on type of land use, with a smaller degree of pyritization and lower Fe-pyrite presence in WAM soils compared to Non-WAM soils. Basal respiration of soil sediments was significantly influenced (P < 0.01) by type of land use with highest CO2 flux rates measured in the WAM soils (mean values of 0.20 mg CO2 h− 1–g− 1 C vs. 0.04 mg CO2 h− 1–g− 1 C). The OC storage reduction in WAM soils was potentially caused (i) by an increase in microbial activity induced by loading of nutrient-rich effluents and (ii) by an increase of strong electron acceptors [e.g., NO3−] that promote a decrease in pyrite concentration and hence a reduction in soil OC burial. The current estimated OC stored in mangrove soils (0–40 cm) in the state of Ceará is approximately 1 million t.

Geochemistry and P and Fe fractionation in anoxic sediments

We investigated the phosphorus (P) and iron (Fe) fractionation in four cores with anoxic sediments, deposited during the mid-Cretaceous oceanic anoxic event 2 (~94 Ma) and the Paleocene-Eocene thermal maximum (?55 Ma), that were exposed to oxygen after core recovery. Surprisingly, P associated with iron oxyhydroxides (Fe-bound P) was a major P phase in these laminated sediments deposited under euxinic conditions. A significant fraction of total Fe was present as (poorly) crystalline ferric Fe. This fraction increased with increasing storage time of the investigated cores. In carbonate-poor samples, Fe-bound P accounted for up to 99% of total P and its abundance correlated with pyrite contents. In samples with higher CaCO3 contents (>5 wt% in the investigated samples), P was mostly present in authigenic Ca-P minerals, irrespective of pyrite contents. We conclude that the P fractionation in anoxic, carbonate-poor, sediments is strongly affected by pyrite oxidation that occurs when these sediments are exposed to oxygen. Pyrite oxidation produces sulfuric acid and iron oxyhydroxides. The abundance of poorly crystalline Fe oxyhydroxides provides further evidence that these were indeed formed through recent (post-recovery) oxidation rather than in situ tens of millions of years ago. The acid dissolves apatite and the released phosphate is subsequently bound in the freshly formed iron oxyhydroxides. Pyrite oxidation thus leads to a conversion of authigenic Ca-P to Fe-bound P. In more calcareous samples, CaCO3 can act as an effective buffer against acidic dissolution of Ca-P minerals. The results indicate that shielding of sediments from atmospheric oxygen is vital to preserve the in situ P fractionation and to enable a valid reconstruction of marine phosphorus cycling based on sediment records.

Lower Aptian Comparative Stratigraphy of the Basco-Cantabrian Region (Spain) and Eastern Cordillera (Colombia): implications for local factors in the depositional record of Oceanic Anoxic Event 1a (OAE-1a)

An important episode of carbon sequestration, Oceanic Anoxic Event 1a (OAE-1a), characterizes the Lower Aptian worldwide, and is mostly known from deeper-water settings. The present work of two Lower Aptian deposits, Madotz (N Spain) and Curití Quarry (Colombia), is a multiproxy study that includes fossil assemblages, microfacies, X-ray diffraction bulk and clay mineralogy, elemental analyses (major, minor, trace elements), Rock-Eval pyrolysis, biomarkers, inorganic and organic carbon content, and stable carbon isotopes. The results provide baseline evidence of the local and global controlling environmental factors influencing OAE-1a in shallow-water settings. The data also improve our general understanding of the conditions under which organic-carbon-rich deposits accumulate. The sequence at Madotz includes four intervals (Unit 1; Subunits 2a, 2b and 2c) that overlap the times prior to, during and after the occurrence of OAE-1a. The Lower Unit 1(3m thick) is essentially siliciclastic, and Subunit 2a (20m) contains Urgonian carbonate facies that document abruptly changing platform conditions prior to OAE-1a. Subunit 2b (24.4 m) is a mixed carbonate-siliciclastic facies with orbitolinid-rich levels that coincides with OAE-1a δ13C stages C4-C6, and is coeval with the upper part of the Deshayesites forbesi ammonite zone. Levels with pyrite and the highest TOC values (0.4-0.97%), interpreted as accumulating under suboxic conditions, and are restricted to δ13C stages C4 and C5. The best development of the suboxic facies is at the level representing the peak of the transgression. Subunit 2c, within δ13C stage C7, shows a return of the Urgonian facies. The 23.35-m section at Curití includes a 6.3-m interval at the base of the Paja Formation dominated by organic-rich marlstones and shales lacking benthic fossils and bioturbation, with TOC values as high as 8.84%. The interval overlies a level containing reworked and phosphatized assemblages of middle Barremian to lowest Aptian ammonites. The range of values and the overall pattern of the δ13Corg (-22.05‰ to -20.47‰) in the 6.3m-interval is comparable with Lower Aptian δ13C stage C7. Thus, conditions of oxygen depletion at this site also occurred after Oceanic Anoxic Event-1a, which developed between carbon isotope stages C3 and C6. Both sites, Madotz and Curití, attest to the importance of terrigenous and nutrient fluxes in increasing OM productivity that led to episodic oxygen deficiency.

Hydrazine-hydrate intercalated halloysite under controlled-rate thermal analysis conditions

The thermal behaviour of halloysite fully expanded with hydrazine-hydrate has been investigated in nitrogen atmosphere under dynamic heating and at a constant, pre-set decomposition rate of 0.15 mg min-1. Under controlled-rate thermal analysis (CRTA) conditions it was possible to resolve the closely overlapping decomposition stages and to distinguish between adsorbed and bonded reagent. Three types of bonded reagent could be identified. The loosely bonded reagent amounting to 0.20 mol hydrazine-hydrate per mol inner surface hydroxyl is connected to the internal and external surfaces of the expanded mineral and is present as a space filler between the sheets of the delaminated mineral. The strongly bonded (intercalated) hydrazine-hydrate is connected to the kaolinite inner surface OH groups by the formation of hydrogen bonds. Based on the thermoanalytical results two different types of bonded reagent could be distinguished in the complex. Type 1 reagent (approx. 0.06 mol hydrazine-hydrate/mol inner surface OH) is liberated between 77 and 103°C. Type 2 reagent is lost between 103 and 227°C, corresponding to a quantity of 0.36 mol hydrazine/mol inner surface OH. When heating the complex to 77°C under CRTA conditions a new reflection appears in the XRD pattern with a d-value of 9.6 Å, in addition to the 10.2 Ĺ reflection. This new reflection disappears in contact with moist air and the complex re-expands to the original d-value of 10.2 Å in a few h. The appearance of the 9.6 Å reflection is interpreted as the expansion of kaolinite with hydrazine alone, while the 10.2 Å one is due to expansion with hydrazine-hydrate. FTIR (DRIFT) spectroscopic results showed that the treated mineral after intercalation/deintercalation and heat treatment to 300°C is slightly more ordered than the original (untreated) clay.