Lawrence H. Tanner

Assessing the record and causes of Late Triassic extinctions

Accelerated biotic turnover during the Late Triassic has led to the perception of an end-Triassic mass extinction event, now regarded as one of the ‘‘big five’’ extinctions. Close examination of the fossil record reveals that many groups thought to be affected severely by this event, such as ammonoids, bivalves and conodonts, instead were in decline throughout the Late Triassic, and that other groups were relatively unaffected or subject to only regional effects. Explanations for the biotic turnover have included both gradualistic and catastrophic mechanisms. Regression during the Rhaetian, with consequent habitat loss, is compatible with the disappearance of some marine faunal groups, but may be regional, not global in scale, and cannot explain apparent synchronous decline in the terrestrial realm. Gradual, widespread aridification of the Pangaean supercontinent could explain a decline in terrestrial diversity during the Late Triassic. Although evidence for an impact precisely at the boundary is lacking, the presence of impact structures with Late Triassic ages suggests the possibility of bolide impact-induced environmental degradation prior to the end-Triassic. Widespread eruptions of flood basalts of the Central Atlantic Magmatic Province (CAMP) were synchronous with or slightly postdate the system boundary; emissions of CO2 and SO2 during these eruptions were substantial, but the contradictory evidence for the environmental effects of outgassing of these lavas remains to be resolved. A substantial excursion in the marine carbon-isotope record of both carbonate and organic matter suggests a significant disturbance of the global carbon cycle at the system boundary. Release of methane hydrates from seafloor sediments is a possible cause for this isotope excursion, although the triggering mechanism and climatic effects of such a release remain uncertain. D 2003 Elsevier B.V. All rights reserved.

Stability of atmospheric CO2 levels across the Triassic/Jurassic boundary

The Triassic/Jurassic boundary, 208 million years ago, is associated with widespread extinctions in both the marine and terrestrial biota. The cause of these extinctions has been widely attributed to the eruption of flood basalts of the Central Atlantic Magmatic Province. This volcanic event is thought to have released significant amounts of CO2 into the atmosphere, which could have led to catastrophic greenhouse warming, but the evidence for CO2-induced extinction remains equivocal. Here we present the carbon isotope compositions of pedogenic calcite from palaeosol formations, spanning a 20-Myr period across the Triassic/Jurassic boundary. Using a standard diffusion model, we interpret these isotopic data to represent a rise in atmospheric CO2 concentrations of about 250 p.p.m. across the boundary, as compared with previous estimates of a 2,000–4,000 p.p.m. increase. The relative stability of atmospheric CO2 across this boundary suggests that environmental degradation and extinctions during the Early Jurassic were not caused by volcanic outgassing of CO2. Other volcanic effects—such as the release of atmospheric aerosols or tectonically driven sea-level change—may have been responsible for this event.

Palustrine-Lacustrine and Alluvial Facies of the (Norian) Owl Rock Formation (Chinle Group), Four Corners Region, Southwestern U.S.A: Implications for Late Triassic Paleoclimate

ABSTRACT The Upper Triassic (Norian) Owl Rock Formation was deposited in a low-gradient floodbasin at a subtropical paleolatitude. The lower part of the formation consists predominantly of fine-grained siliciclastic lithofacies deposited by sheetflood and sinuous streams on a muddy floodplain during a period of continuous basin aggradation. Nodular calcretes are increasingly mature higher in the formation, suggesting increasingly episodic depositional conditions. The upper part of the formation consists mostly of interbedded fine-grained siliciclastic facies and laterally continuous ledges of limestone and sandstone. The predominant limestone facies has brecciated to peloidal fabrics, spar-filled circumgranular cracks, and root channeling. The subordinate limestone facies displays wavy to irregular argillaceous lamination, desiccation cracks, and oscillation ripples, and is vertically and laterally gradational with the brecciated facies. The upper Owl Rock Formation records deposition of aggrading sequences of alluvial sediments deposited during base-level rise, capped by highstand carbonates deposited in small perennial and ephemeral carbonate lakes and ponds. Base-level lowstand in an overall semiarid climate resulted in extensive pedogenesis of the limestone and laterally equivalent alluvial facies. Basin wide variations in base level are interpreted as resulting from climatic fluctuations. This depositional model is consistent with an interpreted trend towards aridification on the Colorado Plateau during the Late Triassic as Pangea drifted northward from one climate zone to another.

Debris-avalanche deposits of the Milo Lahar sequence and the opening of the Valle del Bove on Etna volcano (Italy)

Previously undescribed debris-avalanche deposits occur in two locations downslope from the open end of the Valle del Bove. These outcrops comprise unstratified, ungraded deposits of metre-scale lava blocks in a matrix of weathered and fractured lava clasts. The avalanche deposits are unconformably overlain by matrix- to clast-supported conglomerates, representing debris-flow and interbedded fluvial deposits, that constitute most of the Milo Lahar sequence. We present evidence that the Milo Lahar sequence, which crops out just at the exit of the Valle del Bove, formed during the opening and enlargement of this depression. The presence of the avalanche deposits at the base of the Milo Lahar sequence indicates that catastrophic landslides were involved in the formation of the Valle del Bove. The composition of lavas in the debris avalanche deposits is similar to that of most of the Ellittico volcanic sequence exposed along the northern wall of the Valle del Bove. Radiocarbon dates of 8400 and 5300 years BP from the base and top, respectively, of the debris-flow sequence indicate that the Milo Lahars are correlative with the exposed part of the Chiancone deposit. The basal lahars of the Chiancone, which contain lava blocks whose compositions partially overlap that of blocks in the avalanche deposits, may have formed by water concentration in the distal end of the avalanche causing transformation to debris, or alternatively by reworking of the avalanche deposit.

The Triassic isotope record

Abstract A variety of stable isotope measurements have been found useful in studying processes of environmental change. Measurements of δ13C, δ18O, δ34S and 87Sr/86Sr all can provide information about the conditions of the water column in which sediment deposition occurred, but the most widely applied of these is δ13C. The carbon isotope record for the Triassic System is a complex one; a pronounced negative excursion begins below the base of the Triassic System and continues into the basal Triassic. The succeeding 4 to 6 Ma Lower Triassic interval is marked by isotopic instability, with positive and negative excursions, continuing through the basal Middle Triassic. In contrast to the Lower Triassic, most of the Middle and Upper Triassic display relative isotopic stability, with rising values of δ13C likely reflecting environmental recovery and increasing storage of organic carbon in terrestrial environments. The uppermost Triassic is marked by a pronounced negative excursion near the system boundary that has been linked to significant biotic turnover. The causes of the various excursions remain under investigation, particularly those at the system boundaries, with outgassing during volcanic activity, changes in productivity, ocean anoxia, and seafloor methane releases all suggested as mechanisms both for perturbing the global carbon cycle and for forcing biotic extinction.

Chapter 4 Continental Carbonates as Indicators of Paleoclimate

Abstract The formation of carbonate sediments in continental environments is subject to numerous environmental controls, of which climate is one of the foremost. Consequently, calcareous paleosols and lacustrine, palustrine, speleothem, and tufa carbonates all have been found particularly useful as paleoclimate proxies. Calcareous paleosols, for example, are typically cited as evidence of a semiarid paleoclimate, even though modern calcrete actually forms in soils under a wide range of conditions. The maturity of calcrete is not by itself useful as a paleoclimate indicator because other variables (e.g., time, sedimentation rate) influence morphology, although micromorphology does appear to be influenced by climate. The existence of a quantifiable relationship between the depth to the carbonate horizon and mean annual precipitation has been argued, and has been demonstrated to be applicable under certain conditions. The isotopic analysis of pedogenic carbonate has become a widely applied tool for paleoenvironmental interpretation. The δ 13 C of the carbonate is used to calculate paleo- p CO 2 and ratios of C 3 /C 4 vegetation. The measurement of δ 18 O also has been used to infer changes in paleotemperature. Ancient lacustrine carbonates are often associated with deposition under semiarid climate conditions; sometimes incorrectly, because many modern carbonate lakes occur in humid, temperate, as well as semiarid climates. The δ 18 O of lacustrine carbonate from hydrologically open lakes may be used for paleotemperature calculations. In closed-basin lakes, δ 18 O and δ 13 C typically exhibit covariance due to enrichment from evaporation and atmospheric exchange. Many palustrine carbonates are lacustrine deposits that were pedogenically reworked during episodes of base-level fall, which potentially may be related to climate change. Lithologies associated with palustrine carbonates, for example, peat or evaporites, provide more detailed climatic information. Isotopic analyses of speleothem and tufa carbonates are important archives of climate data, particularly for the Quaternary. Measurements of δ 18 O in these carbonates can be related to temperature, while δ 13 C is linked to changes in vegetative cover.

Depositional facies, paleogeography and palaeoclimatology of the Lower Jurassic McCoy Brook Formation, Fundy rift basin, Nova Scotia

The McCoy Brook Formation is a 230+ m sequence of redbeds of Early Jurassic age that accumulated in the subtropical Fundy rift basin. The basin was asymmetrical, bordered on the northern side by the Cobequid fault system and adjacent highlands. The formation is a mosaic of terrestrial facies. (a) Multi- and single-storey fluvial sandstones were deposited by rivers that collected drainage from the norhern highlands, and flowed southwest across the valley floor. (b) Graded-bed sequences of sandstone → mudstone were deposited by flood events on the fluvial floodplains and the margins of playas. (c) Interbedded sandstones and finely laminated to disrupted red mudstones represent deposits of interfingering sandflats at the toes of alluvial fans and playas, respectively. The fabrics of both lithologies are commonly disrupted by desiccation and intrastratal precipitation of calcite and minor gypsum. (d) Plane-bedded conglomerate and pebbly sandstone record deposition in shallow channels that traversed alluvial fans. (e) Non-bedded breccias of basalt boulders up to 8 m in size in a sand to mud matrix formed as talus against palaeocliffs of the Lower Jurassic North Mountain Basalt. The palaeocliffs resulted from synsedimentary faulting within the basin. (f) Basalt conglomerate beds with matrix- to clast-supported fabrics comprise sequences deposited by debris-flows, probably generated by remobilization of talus. (g) A thin sequence of limestone, siltstone, and muddy sandstone containing fish scales and bones formed in a shallow lake on a basalt flow. (h) Sandstones containing m-scale crossbeds and a three-fold hierarchy of erosional surfaces accumulated as barchans and barchanoid transverse ridges that moved southwest in the valley. All of these facies occur within an area of 18 km2 located south of the fault-bounded highlands. Synsedimentary faulting within the basin during early deposition of the McCoy Brook Formation created palaeocliffs of North Mountain Basalt. Talus and debris-flow deposits accumulated adjacent to these cliffs, interfingering with eolian, fluvial, and lacustrine sediments. During later deposition of the McCoy Brook Formation, the early fault-controlled topography was buried as alluvial fans prograded southward from the highlands. Broad sandflats extended basinward from the toes of the fans, interfingering with playa mudflats on the valley floor. The palaeoclimate was semi-arid as evidenced by gypsum-bearing playa redbeds, eolian sandstones, and caliche palaeosols.

Cyclostratigraphic record of the Triassic: a critical examination

Abstract High frequency (fourth- and fifth-order) cyclicity is a common feature of sedimentary sequences in all depositional settings. While tectonism and autocyclic processes are clearly responsible for this cyclicity in some instances, many cases are interpreted as resulting from orbitally forced variations in solar insolation at the Milankovitch frequencies, that is, the precession and short and long eccentricity cycles at scales of tens of thousands to hundreds of thousands of years. This forcing is presumed to have controlled sedimentation through periodic changes in climate or sea-level. Examples of interpreted Milankovitch-frequency cyclicity occur throughout the Triassic record, and include much of the German Triassic, the Alpine Triassic and the Newark Supergroup of North America. The cyclostratigraphy of these sections has been used as a tool for intrabasinal and interbasinal correlation, and for chronostratigraphy. These interpretations are not always without controversy, however, as conceptual arguments and radio–isotopic age data have called some of these conclusions into question.

Palaeoclimatology (Communication arising): Triassic–Jurassic atmospheric CO 2 spike

CH4 oxidizes in the atmosphere within 7–24 years to CO2, which retains a very depleted carbon-isotope composition (typically 160‰, but as low as 1110‰ dCorg) . For example, unusually depleted carbon-isotope values for Early Triassic organic matter can be taken as indications of a methane-dissociation event. An Early Triassic CO2 greenhouse effect is shown by the very low stomatal index of fossil seed ferns, but pedogenic carbonate isotopic palaeobarometers indicate low atmospheric CO2 levels in the Early Triassic . The pedogenic–isotopic palaeobarometer was not designed for methanogenic isotopic compositions, which compromise this palaeobarometer during several greenhouse transients, and perhaps also at the Triassic–Jurassic boundary. Gregory J. Retallack Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403-1272, USA e-mail: gregr@darkwing.uoregon.edu

Facies analysis and depositional mechanisms of hydroclastite breccias, Acicastello, eastern Sicily

Abstract Hydroclastite beds are interbedded with pillow lavas in an overturned section exposed near Acicastello, on the Ionian coast of Sicily. Two distinct hydroclastite facies are recognized in a continuous 15-m section in a cliff face overlying pillow lavas. The first, an hyaloclastite facies, comprises black hyaloclasts and pillow fragments, many of which retain a partial pillow shape and glassy rim, in a matrix of smaller, partially devitrified and palagonitized hyaloclasts. Beds of this facies are predominantly inversely graded, exhibit both matrix and clast support, and commonly contain outsize clasts at or near the bed top. Beds of the hyaloclastite facies beds are interpreted as the deposits of noncohesive submarine debris flows transporting fragments formed primarily by cooling-contraction granulation. The second, a pillow-fragment breccia facies, forms wedge-shaped beds that are interbedded with the hyaloclastite facies. Pillow-fragment breccias comprise non-graded breccias of basalt blocks that formed by spalling of joint blocks from pillow lavas. Beds of this facies are clast-supported and contain a matrix of marly sediment. This facies formed by accumulation of pillow fragments along the sides of pillow-lava ridges, creating wedges of talus which interfinger with the debris-flow deposits of the hyaloclastite facies.