Reply to comment on "Mantle plume: the invisible serial killer — Application to the Permian–Triassic boundary mass extinction"

In a recent article, Heydari et al. (2008) suggested that the perturbation at the Permian–Triassic boundary (PTB) was initiated by processes associated with an end-Permian mantle plume including igneous intrusions and uplift. These events resulted in the massive release of CH_4 primarily from the dissociation of marine gas hydrates, and secondarily from maturation of organic-rich sediments and fracturing of petroleum reservoirs. Injection of CH_4 into the ocean changed seawater composition (the acid-bath ocean) leading to marine mass extinction. Transfer of CO_2 and CH_4 from the ocean to the atmosphere created a hot climate (the end-Permian inferno) which caused the terrestrial mass extinction. We suggested that the Siberian trap volcanism and marine anoxia played little role in this catastrophe. Wignall and Racki (2009-this issue) have raised three criticisms to our article. The first is that our interpretation has been previously advocated by others. Our re-evaluation indicates that our interpretation was in fact opposite of those considered by Wignall and Racki (2009-this issue) to have presented scenarios similar to ours. The second, Wignall and Racki (2009-this issue) also suggest that our proposed change in carbonate mineralogy across the PTB did not occur because such a change "should produce a large positive excursion rather than the observed negative excursion". Wignall and Racki (2009-this issue) have made a basic mathematical error in evaluating the effect of carbonate mineralogy on δ^(13)C values. Therefore, they have reached two wrong conclusions: one about the validity of a change in carbonate mineralogy and the other regarding its effect on the shift in δ^(13)C values at the PTB. A change in carbonate mineralogy produced a larger negative excursion rather than a positive shift. The third, Wignall and Racki (2009-this issue) indicate that the PTB ocean was anoxic to the rim. This criticismis not supported by the rock record because highly bioturbated stratawere deposited in environments ranging from shallow shelves to deep waters under oxygenated water column at the time of the PTB mass extinction. If the ocean were totally stratified for 20Ma, and if anoxia extended all the way to the shoreline, and if the ocean were anoxic to the rim and H_2S were oozing out of it, then we should see at least 100 m of organic-rich, varvedlaminated strata in areas ranging from the abyssal plain to the shoreline environments. Such strata have not yet been found

Diagenetic origin of carbon and oxygen isotope compositions of Permian-Triassic boundary strata

Bulk carbonate δ^(13)C and δ^(18)O compositions of profiles across Permian–Triassic (P–T) boundary sections in China, Italy, Austria, and Iran show wide varieties of trends. The δ^(13)C depletions occur in all sections and range from 2 to 8‰ PDB in magnitude. These excursions take place over intervals ranging from less than 0.1 to more than 40 m. The δ^(18)O values may increase or decrease toward the P–T boundary, but decrease sharply by 2–9‰ PDB at or above the boundary. Cross-plots of δ^(13)C and δ^(18)O values from all sections show positive covariance. Wide differences in magnitudes, trends, and position of the excursions relative to the boundary, as well as the covariance patterns suggest that P–T boundary δ^(18)O and δ^(13)C values are partially or entirely diagenetic in origin, formed in association with exposure surfaces. This interpretation implies that P–T boundary sections studied till date were subaerially exposed before, during, and after the mass extinction, resulting in the removal of strata containing key information about the extinction mechanism. This inference is consistent with the paleontological studies that have shown the presence of gaps at the boundary, and further supported by the sharp lithologic changes observed at virtually all P–T boundary sections. Subaerial exposures are documented by detailed sedimentologic and isotopic studies from central Tethyan sections in Abadeh and Shah Reza in Iran. Proposed P–T boundary extinction models are based on isotopic values that are diagenetic in origin and stratigraphic sections that are incomplete, leading to extinction mechanisms with little physical supporting evidence

Ocean's response to a changing climate: Clues from variations in carbonate mineralogy across the Permian–Triassic boundary of the Shareza Section, Iran

This investigation focuses on the original mineralogy of Wuchiapingian (Late Permian) to Induan (Early Triassic) strata of the Shahreza section of Iran to understand changes in marine carbonate system during this transition. Aragonitic carbonates precipitated during the early part of the Wuchiapingian, gradually changing to calcite during the middle part of the Wuchiapingian and continuing through the Changhsingian. Carbonate precipitation ceased during the latest Changhsingian at the end-Permian Event Horizon (EH). A sharp reversal back to aragonite precipitation occurred in the Early Triassic. These changes are interpreted as the ocean's response to a changing climate. This study proposes three types of seawater for the Late Permian to Early Triassic interval. Normal (Anahita-type) seawater during the Late Permian was hospitable to life and supported a highly active carbonate factory. Acidic (Jahi-type) seawater during the PTB transition was acidic and hostile to life, slowing or halting carbonate production. Alkaline (Amordad-type) seawater during the Early Triassic was life-nurturing and supported an active carbonate factory. It sustained life until a normal (Anahita-type) seawater could be re-established

Formation of low-magnesium calcite at cold seeps in an aragonite sea

This study investigates the conditions of occurrence and petrographic characteristics of low-Mg calcite (LMC) from cold seeps of the Gulf of Mexico at a water depth of 2340 m. Such LMC mineral phases should precipitate in calcite seas rather than today\u27s aragonite sea. The C-depleted carbonates formed as a consequence of anaerobic oxidation of hydrocarbons in shallow subsurface cold seep environments. The occurrence of LMC may result from brine fluid flows. Brines are relatively Ca -enriched and Mg -depleted (Mg/Ca mole ratio \u3c0.7) relative to seawater, where the Mg/Ca mole ratio is ~5, which drives high-Mg calcite and aragonite precipitation. The dissolution of aragonitic mollusk shells, grains and cements was observed. Aerobic oxidation of hydrocarbons and H S is the most likely mechanism to explain carbonate dissolution. These findings have important implications for understanding the occurrence of LMC in deep water marine settings and consequently their counterparts in the geological record. © 2013 John Wiley & Sons Ltd. 13 2+ 2+

Lacustrine sedimentation by powerful storm waves in Gale crater and its implications for a warming episode on Mars

Abstract This investigation documents that the Rugged Terrain Unit, the Stimson formation, and the Greenheugh sandstone were deposited in a 1200 m-deep lake that formed after the emergence of Mt. Sharp in Gale crater, Mars, nearly 4 billion years ago. In fact, the Curiosity rover traversed on a surface that once was the bottom of this lake and systematically examined the strata that were deposited in its deepest waters on the crater floor to layers that formed along its shoreline on Mt. Sharp. This provided a rare opportunity to document the evolution of one aqueous episode from its inception to its desiccation and to determine the warming mechanism that caused it. Deep water lacustrine siltstones directly overlie conglomerates that were deposited by mega floods on the crater floor. This indicates that the inception phase of the lake was sudden and took place when flood waters poured into the crater. The lake expanded quickly and its shoreline moved up the slope of Mt. Sharp during the lake-level rise phase and deposited a layer of sandstone with large cross beds under the influence of powerful storm waves. The lake-level highstand phase was dominated by strong bottom currents that transported sediments downhill and deposited one of the most distinctive sedimentological features in Gale crater: a layer of sandstone with a 3 km-long field of meter-high subaqueous antidunes (the Washboard) on Mt. Sharp. Bottom current continued downhill and deposited sandstone and siltstone on the foothills of Mt. Sharp and on the crater floor, respectively. The lake-level fall phase caused major erosion of lacustrine strata that resulted in their patchy distribution on Mt. Sharp. Eroded sediments were then transported to deep waters by gravity flows and were re-deposited as conglomerate and sandstone in subaqueous channels and in debris flow fans. The desiccation phase took place in calm waters of the lake. The aqueous episode we investigated was vigorous but short-lived. Its characteristics as determined by our sedimentological study matches those predicted by an asteroid impact. This suggests that the heat generated by an impact transformed Mars into a warm, wet, and turbulent planet. It resulted in planet-wide torrential rain, giant floods on land, powerful storms in the atmosphere, and strong waves in lakes. The absence of age dates prevents the determination of how long the lake existed. Speculative rates of lake-level change suggest that the lake could have lasted for a period ranging from 16 to 240 Ky

Grain Size Variations in the Murray Formation: Stratigraphic Evidence for Changing Depositional Environments in Gale Crater, Mars

International audienceThe lowermost exposure of the Murray formation in Gale crater, Mars, was interpreted as sediment deposited in an ancient lake based on data collected by the Curiosity rover. Constraining the stratigraphic extent and duration of this environment has important implications for the paleohydrology of Gale. Insights into early Martian environments and paleofluid flow velocity can be obtained from grain size in rocks. Visual inspection of grain size is not always available for rocks investigated at field sites on Mars due to limited image coverage. But grain sizes can also be estimated from the Gini Index Mean Score, a grain‐size proxy that uses point‐to‐point chemical variations in ChemCam Laser Induced Breakdown Spectroscopy data. New Gini Index Mean Score results indicate that the Murray formation is dominated by mudstones with grains smaller than the spatial resolution of all rover cameras. Intervals of fine to coarse sandstone also are present, some of which are verified using observations of grain size and sedimentary structures in associated images. Overall, results demonstrate that most of the Murray consists of mudstone, suggesting settling of grains from suspension in low energy depositional environments such as lakes. Some of the mudstones contain desiccation cracks indicating periods of drying with a lowering of lake water level. However, beds and lenses of cross‐bedded sandstones are common at specific intervals, suggesting episodes of fluvial and possibly eolian deposition. The persistence of lacustrine deposits interspersed with fluvial deposits suggests that liquid water was sustained on the Martian surface for tens of thousands to millions of years