Reel-to-Reel Re-Os Records: Earth System Transactions Preserved in Sediments

<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.

The role of LIPs in Phanerozoic mass extinctions: An Hg perspective

The Chicxulub impact in Mexico and Deccan volcanism in India are both linked to the end-Cretaceous mass extinction but the relative timing of the impact, volcanic eruptions, and environmental changes remain controversial, precluding a full assessment of their respective roles. The bulk (80%) of Deccan Trap eruptions occurred over a relatively short time interval in magnetic polarity C29r. U-Pb zircon geochronology reveals the onset of this main eruption phase 350 ky before the Cretaceous-Tertiary (KT) mass extinction. U-Pb zircon geochronology from Malwa Plateau basalts on the northern margin of the Deccan LIP are temporally correlative with the first pulse of Deccan volcanism, which is coeval with a &amp;#8764;200 kyr Late Maastrichtian warming event preserved globally in contemporaneous stratigraphic sections. This 2.5&amp;#8211;8&amp;#176;C warming has been inferred by several studies on the basis of &amp;#948;18O in benthic foraminifera, pedogenic carbonate and bivalve shells, as well as changes in leaf morphology. The onset of this excursion is temporally correlative to the initial decline in oceanic 187Os/188Os toward more radiogenic values and increasing Hg contents. This first pulse of Deccan magmatism erupted through organic-rich sedimentary Permian rocks of the Narmada-Tapi rift basin. Direct CO2 emissions from basalt are unlikely to cause this magnitude of warming, except at extreme eruption rates, which is difficult to reconcile with the likely longer duration and lower eruption rates inferred from this first eruptive pulse. Thermal contact metamorphism of these sediments could have been a source of sufficient CO2 to drive the Late Maastrichtian warming event. The aim of this study is to understand the fate of C, Hg and S during the contact metamorphism associated with the first Deccan pulse and to evaluate the importance of this process in the global C, Hg and S cycles. Our data are based on measurements of contact aureoles around several dikes and sills intruding in Permian organic-rich coal located in the Narmada-Tapi rift basin. We focused on TOC, Hg and S contents. While the sediments further away from the intrusions show high levels of TOC (&gt;20%) and significant contents in Hg and S, the samples located in the aureoles (around 5 m thick) show a nearly total loss of the same elements. Our initial results demonstrate that the global C, S and Hg cycles can be substantially perturbed after LIP-scale sill and dyke emplacement in organic-rich sedimentary rocks. Deccan volcanism likely contributed to climate instability in the late Cretaceous and may have exacerbated the environmental effects of the Chicxulub impact.

Mercury contents and isotope ratios from diverse depositional environments across the Triassic–Jurassic Boundary: Towards a more robust mercury proxy for large igneous province magmatism

Phosphorus is a biolimiting nutrient that has an important role in regulating the burial of organic matter and the redox state of the ocean-atmosphere system. The ratio of phosphorus to iron in iron-oxide-rich sedimentary rocks can be used to track dissolved phosphate concentrations if the dissolved silica concentration of sea water is estimated. Here we present iron and phosphorus concentration ratios from distal hydrothermal sediments and iron formations through time to study the evolution of the marine phosphate reservoir. The data suggest that phosphate concentrations have been relatively constant over the Phanerozoic eon, the past 542 million years (Myr) of Earth's history. In contrast, phosphate concentrations seem to have been elevated in Precambrian oceans. Specifically, there is a peak in phosphorus-to-iron ratios in Neoproterozoic iron formations dating from ∼750 to ∼635 Myr ago, indicating unusually high dissolved phosphate concentrations in the aftermath of widespread, low-latitude 'snowball Earth' glaciations. An enhanced postglacial phosphate flux would have caused high rates of primary productivity and organic carbon burial and a transition to more oxidizing conditions in the ocean and atmosphere. The snowball Earth glaciations and Neoproterozoic oxidation are both suggested as triggers for the evolution and radiation of metazoans. We propose that these two factors are intimately linked; a glacially induced nutrient surplus could have led to an increase in atmospheric oxygen, paving the way for the rise of metazoan life.

Mass extinction events are short-lived and characterized by catastrophic biosphere collapse and subsequent reorganization. Their abrupt nature necessitates a similarly short-lived trigger, and large igneous province magmatism is often implicated. However, large igneous provinces are long-lived compared to mass extinctions. Therefore, if large igneous provinces are an effective trigger, a subinterval of magmatism must be responsible for driving deleterious environmental effects. The onset of Earth’s most severe extinction, the end-Permian, coincided with an abrupt change in the emplacement style of the contemporaneous Siberian Traps large igneous province, from dominantly flood lavas to sill intrusions. Here we identify the initial emplacement pulse of laterally extensive sills as the critical deadly interval. Heat from these sills exposed untapped volatile-fertile sediments to contact metamorphism, likely liberating the massive greenhouse gas volumes needed to drive extinction. These observations suggest that large igneous provinces characterized by sill complexes are more likely to trigger catastrophic global environmental change than their flood basalt- and/or dike-dominated counterparts.

Carbon isotope trends are useful for stratigraphic correlation, especially for time intervals when major perturbations of the global carbon cycle occurred. Such perturbations have been documented for the Triassic–Jurassic (T–J) boundary, and several successions from this time interval are characterized by (1) an initial negative excursion, followed by (2) a pronounced positive excursion and a subsequent (3) main negative carbon isotope excursion. These features, however, are not present in all T–J boundary sections, or the stratigraphic position of the positive or the main negative excursion has variable locations. In the present study, we analysed carbon isotopes in bulk carbonate from the pelagic Csővár quarry section in Hungary and from the intra-platform basin to shallow subtidal marine Kendlbachgraben section in Austria. Both T–J boundary successions are biostratigraphically well controlled enabling – with particular focus on the bio- and chemostratigraphy of other T–J boundary sections – correlation of the carbon isotope trends. This evaluation shows that the apex of the initial negative δ13C excursion occurred slightly, but distinctly, below the mass extinction event and represents an excellent stratigraphic correlation tool.

Transition to an oxygen-rich atmosphere with an extensive overshoot triggered by the Paleoproterozoic snowball Earth

The genome sequence is an icon of early twenty-first century biology. Genomes of nearly 2000 cellular organisms, and from many thousands of organelles and viruses, are now in the public domain. For biological research in individual species, the genome sequence increasingly provides the common

Roller-coaster atmospheric-terrestrial-oceanic-climatic system during Ordovician-Silurian transition: Consequences of large igneous provinces

Evaluating Re–Os systematics in organic-rich sedimentary rocks in response to petroleum generation using hydrous pyrolysis experiments

Establishing the link between Permian volcanism and biodiversity changes: Insights from geochemical proxies

The Triassic–Jurassic transition – A review of environmental change at the dawn of modern life

ABSTRACTThe Jehol Biota represents an important phase in the history of biological evolution on Earth. Two sites with Aptian Jehol Biota, located in western Liaoning Province and in northern Hebei Province, northeastern China, have generated a major debate about the relationships between their fossil-bearing layers, resulting in confusions about their stratigraphic correlation and biogenetic sequence. We have applied carbon isotope analysis of total organic carbon to provide new data relevant to this debate and have re-examined the connection between volcanism and carbon cycle perturbations, with consideration of the paleoenvironmental conditions. The rare earth element data show that the provenances of the Jehol Group in western Liaoning and northern Hebei were similar and derived mainly from the underlying andesite. Constrained by the radiometric age, the δ13Corg curve suggests that the Dadianzi Formation in northern Hebei is comparable to the three lacustrine layers of the Yixian Formation (ranging from the Jianshangou Bed to the Jingangshan Bed) in western Liaoning. These layers are highly correlated with the early Aptian marine sequence in Oceanic Anoxic Event 1a (OAE 1a), implying that a possible teleconnection (possibly related to a large igneous province) existed between oceanic and terrestrial realms. We also propose that the Xiguayuan Formation above the Dadianzi Formation in northern Hebei is equivalent to the Huanghuashan Breccia Bed at the top of the Yixian Formation in western Liaoning. Moreover, the δ13Corg and element geochemical data suggest that the carbon cycle perturbation and the paleolimnological environments were distinctly influenced by the effects of volcanic CO2 during the early Aptian.

The Toarcian oceanic anoxic event (T-OAE, ca. 183 Ma) in the Early Jurassic was one of the most significant warming events of the Phanerozoic, associated with large-scale carbon emissions, mass extinction, and perturbations to hydrology and ocean chemistry. However, the age and duration of this hyperthermal have long been uncertain, hindering our understanding of the timing and pace of carbon release and the associated environmental and biotic changes. We present high-precision radioisotopic dates bracketing a biostratigraphically constrained record of the T-OAE in Japan. Our geochronology reveals an unexpectedly short T-OAE duration of ∼300 k.y. and a temporal coincidence with Ferrar large igneous province (LIP) magmatism. Our dates refute previous work linking the T-OAE to the earlier Karoo LIP, which was coincident with an earlier carbon cycle perturbation at the Pliensbachian–Toarcian boundary. Our results suggest both events were driven by extremely rapid (perhaps sub-millennial scale) thermogenic carbon degassing during LIP sill intrusion.