Jahandar Ramezani

Calibrating the End-Permian Mass Extinction

High-precision geochronologic dating constrains probable causes of Earths largest mass extinction. The end-Permian mass extinction was the most severe biodiversity crisis in Earth history. To better constrain the timing, and ultimately the causes of this event, we collected a suite of geochronologic, isotopic, and biostratigraphic data on several well-preserved sedimentary sections in South China. High-precision U-Pb dating reveals that the extinction peak occurred just before 252.28 ± 0.08 million years ago, after a decline of 2 per mil (‰) in δ13C over 90,000 years, and coincided with a δ13C excursion of −5‰ that is estimated to have lasted ≤20,000 years. The extinction interval was less than 200,000 years and synchronous in marine and terrestrial realms; associated charcoal-rich and soot-bearing layers indicate widespread wildfires on land. A massive release of thermogenic carbon dioxide and/or methane may have caused the catastrophic extinction.

Extinction of Cloudina and Namacalathus at the Precambrian-Cambrian boundary in Oman

Biostratigraphic, carbon isotope, and U-Pb zircon geochronological data from the Ara Group of Oman indicate an abrupt last appearance of Cloudina and Namacalathus coincident with a large-magnitude, but short-lived negative excursion in the carbon isotope composition of seawater that is globally coincident with the Precambrian-Cambrian boundary. U-Pb zircon age data from an intercalated ash bed directly define this negative excursion to be at 542.0 ± 0.3 Ma, consistent with previous age constraints from Siberia and Namibia. Combined with the global biostratigraphic record, these new data strengthen hypotheses invoking mass extinction within terminal Proterozoic ecosystems at or near the Precambrian-Cambrian boundary.

Geochronologic constraints on the chronostratigraphic framework of the Neoproterozoic Huqf Supergroup, Sultanate of Oman

The Huqf Supergroup, Sultanate of Oman, contains an important record of Neoproterozoic history, including evidence for two glaciations, a massive reorganization of the global carbon cycle, and the Ediacaran-Cambrian transition. New U-Pb geochronologic data provide precise constraints on the age of several key stratigraphic levels in the Neoproterozoic Huqf Supergroup and its subjacent crystalline basement rocks. The basement ages constrain an interval of felsic magmatism to have occurred from at least 840 Ma to approximately 810 Ma. Detrital zircons from several stratigraphic levels within the Huqf Supergroup yield ages in excess of 2.5 Ga, suggesting proximity of Archean crust during the Neoproterozoic evolution of the eastern Arabian Peninsula. Volcanic ash beds intercalated within the Huqf Supergroup were dated in the Oman Mountains, and in several subsurface wells (South Oman Salt Basin). Glacial deposits of the Abu Mahara Group in the Oman Mountains (Ghubrah Formation) contain volcaniclastic rocks that are approximately 713 Ma; overlying syn-glacial turbiditic sandstones of the Fiq Formation yield a suite of detrital zircon dates ranging from 920 to 664 Ma so that deposition of at least the upper Fiq must have post-dated 664 Ma. In the South Oman Salt Basin, volcaniclastic deposits intercalated within glaciogenic strata of the Fiq Formation yielded zircons, the youngest of which is about 645 Ma. These data indicate two distinct episodes of glaciation at approximately 713 and <645 Ma. The uppermost Ara Group of the Huqf Supergoup contains multiple ash beds within its carbonate strata, where an age of roughly 547 Ma is reported for rocks that occur above strata marked by a pronounced negative (-12‰) to positive (+4‰) excursion in carbon isotope composition. Higher in the Ara sequence, three distinct ash beds dated at about 543 Ma, 542 Ma, and 541 Ma closely approximate the Ediacaran-Cambrian boundary in Oman. The dramatic carbon isotope excursion of ∼16 permil in the Shuram Formation (middle Nafun Group) has a firm maximum age of approximately 620 Ma as provided by detrital zircon ages from the base of the formation. Interpolation and downward extrapolation from the Ara Group ages, coupled with correlation to other global strata, suggests the base of the Shuram C-isotope excursion to be on the order of 560 Ma, with an estimated duration of approximately 5 to 11 m.y. This excursion is inferred to post-date the last well-documented Neoproterozoic glaciation (about 582 Ma) and is broadly coincident with the appearance of complex organisms in the fossil record.

Timing of recovery from the end-Permian extinction: Geochronologic and biostratigraphic constraints from south China

Four volcanic-ash beds bracket the Early-Middle Triassic boundary, as defined by conodont biostratigraphy, in a stratigraphic section in south China. High-precision U-Pb dates of single zircons allow us to place the Early to Middle Triassic (Olenekian-Anisian) boundary at 247.2 Ma. Magnetic-reversal stratigraphy allows global correlation. The new dates constrain the Early Triassic interval characterized by delayed biotic recovery and carbon-cycle instability to ∼5 m.y. This time constraint must be considered in any model for the end-Permian extinction and subsequent recovery.

High-precision temporal calibration of Late Permian vertebrate biostratigraphy: U-Pb zircon constraints from the Karoo Supergroup, South Africa

Therapsid and other tetrapod fossils from the South African Karoo Supergroup provide the most detailed and best studied terrestrial vertebrate record of the Middle and Late Permian. The resulting biostratigraphic scheme has global applicability. Establishing a temporal framework for these faunas has proven difficult: magnetostratigraphy has been hampered by a Jurassic overprint, and intercorrelation with Permian marine sequences has been equivocal. Here we report U-Pb zircon isotope dilution–thermal ionization mass spectrometry (ID-TIMS) dates for five volcanic ashes interbedded with fossils from the Pristerognathus , Tropidostoma , and Cistecephalus vertebrate biozones of the Beaufort Group. This temporal framework allows correlation to marine zonations and improves understanding of rates of faunal evolution and patterns of basin evolution. Our results identify no correlative vertebrate extinctions in the Karoo Supergroup to the marine end-Guadalupian mass extinction and raise the question of whether there is any record of a terrestrial extinction related to the Emeishan large igneous province.

Paleoproterozoic intraplate magmatism and basin development on the Kaapvaal Craton: Age, paleomagnetism and geochemistry of ~1.93 to ~1.87 Ga post-Waterberg dolerites

We report U–Pb baddeleyite crystallization ages of ~1927 and ~1879 to ~1872 Ma for dolerite sills intruding the Waterberg Group in Botswana and South Africa. These data increase the known extent of ~1.9 Ga intraplate magmatism in southern Africa and place tighter age constraints on the Waterberg Group than previously available. In South Africa, ~1.88 Ga dolerite intrudes upper Waterberg strata, constraining most, if not all, of the succession to have accumulated between ~2.06 Ga (age of the underlying Bushveld Complex) and ~1.88 Ga. This is consistent with derivation of much of the group from uplifted sources in reactivated segments of the Limpopo Belt. The dolerites are typical continental tholeiites, but their trace-element contents discriminate them from dolerite sills of the 1.1 Ga Umkondo Igneous Province, which occur in the same region. Paleomagnetic samples from dolerite intrusions in the Waterberg Group in South Africa (including one sill with a U–Pb baddeleyite age of ~1872 Ma), and from dolerite sills and basalt flows in the Soutpansberg Group to the east-northeast, yield antipodal directions with a site mean pole at 15.6°north, 17.1°east, A95 = 8.9°. These data are interpreted to indicate that the ~1879 to ~1872 Ma dolerites were intruded into the Waterberg Group during voluminous magmatism associated with development of the Soutpansberg rift basin. Older, ~1927 Ma dolerite in Botswana is similar in age and geochemistry to basalts in the craton-margin Olifantshoek Supergroup, suggesting that the mafic magmatism in those two regions is genetically related.

Geology and thermochronology of Tertiary Cordilleran-style metamorphic core complexes in the Saghand region of central Iran

An ~100-km-long north-south belt of metamorphic core complexes is localized along the boundary between the Yazd and Tabas tectonic blocks of the central Iranian micro-continent, between the towns of Saghand and Posht-e-Badam. Amphibolite facies mylonitic gneisses are structurally overlain by east-tilted supracrustal rocks including thick (>1 km), steeply dipping, nonmarine siliciclastic and volcanic strata. Near the detachment (the Neybaz-Chatak fault), the gneisses are generally overprinted by chlorite brecciation. Crosscutting relationships along with U-Pb zircon and ^(40)Ar/^(39)Ar age data indicate that migmatization, mylonitic deformation, volcanism, and sedimentation all occurred in the middle Eocene, between ca. 49 and 41 Ma. The westernmost portion of the Tabas block immediately east of the complexes is an east-tilted crustal section of Neoproterozoic–Cambrian crystalline rocks and metasedi-mentary strata >10 km thick. The ^(40)Ar/^(39)Ar biotite ages of 150–160 Ma from structurally deep parts of the section contrast with ages of 218–295 Ma from shallower parts, and suggest Late Jurassic tilting of the crustal section. These results define three events: (1) a Late Jurassic period of upper crustal cooling of the western Tabas block that corresponds to regional Jurassic–Cretaceous tectonism and erosion recorded by a strong angular unconformity below mid-Cretaceous strata throughout central Iran; (2) profound, approximately east-west middle Eocene crustal extension, plutonism, and volcanism (ca. 44–40 Ma); and (3) ~2–3 km of early Miocene (ca. 20 Ma) erosional exhumation of both core complex and Tabas block assemblages at uppermost crustal levels, resulting from significant north-south shortening. The discovery of these and other complexes within the mid-Tertiary magmatic arcs of Iran demonstrates that Cordilleran-style core complexes are an important tectonic element in all major segments of the Alpine-Himalayan orogenic system.

High-precision U-Pb zircon geochronology of the Late Triassic Chinle Formation, Petrified Forest National Park (Arizona, USA): Temporal constraints on the early evolution of dinosaurs

The Triassic successions of the Colorado Plateau preserve an important record of vertebrate evolution and climate change, but correlations to a global Triassic framework are hampered by a lack of geochronological control. Tuffaceous sandstones and siltstones were collected from the Upper Triassic Chinle Formation exposed in the Petrified Forest National Park, Arizona, USA, within a refined stratigraphic context of 31 detailed measured sections. U-Pb analyses by the isotope dilution–thermal ionization mass spectrometry (ID-TIMS) method constrain maximum depositional ages for nine tuffaceous beds and provide new insights into the depositional history of the Chinle fluvial system. The base of the Blue Mesa Member of the Chinle Formation is placed at ca. 225 Ma, and the top of the Petrified Forest Member is placed at 208 Ma or younger, bracketing an ∼280-m-thick section that spans nearly the entire Norian Stage of the Late Triassic. Estimated sediment accumulation rates throughout the section reflect extensive hiatuses and/or sediment removal by channel erosion. The new geochronology for the Chinle Formation underscores the potential pitfalls of correlation of fluvial units based solely on lithostratigraphic criteria. A mid-Norian age (ca. 219–213 Ma) for the distinctive Sonsela conglomeratic sandstone bed constrains the Adamanian-Revueltian land vertebrate faunachron boundary. Our new data permit a significant time overlap between the lower Chinle sequence and the dinosauromorph-rich Ischigualasto Formation of northwestern Argentina. Near-contemporaneity of the trans-American deposits and their faunal similarities imply that early dinosaur evolution occurred rapidly across the Americas.

U-Pb geochronology of mid-Paleozoic plutonism in western New Zealand: Implications for S-type granite generation and growth of the east Gondwana margin

New U-Pb isotope-dilution-thermal ionization mass spectrometry (ID-TIMS) ages (371–305 Ma) for 30 granitic plutons along the New Zealand sector of the East Gondwanan active margin reveal a highly episodic emplacement history and crustal growth pattern. The Late Devonian-late Carboniferous ages also establish specific links with both the mostly older, Lachlan and the mostly younger, New England fold belts of eastern Australia. Dated plutons are representative of two S- and I-type suite pairs, the volumetrically predominant Karamea-Paringa (371–360 Ma) and minor Ridge-Tobin (355–342 Ma) pulses, as well as sporadic Foulwind Suite A-type granites (350–305 Ma). Emplacement of the bulk of the dominant ∼3400 km 2 Karamea Suite S-type granite-granodiorite plutons within a 2.11 Ma interval is explained by major and intimate intrusion of mantle-derived magma into largely metasedimentary crust during intra-arc extension of previously overthickened crust. Transient emplacement rates were thus of similar magnitude as some young ignimbrite flare-ups and an order of magnitude greater than long-term averages for Mesozoic-Cenozoic cordilleran batholiths of the western Americas. Extension likely was terminated abruptly by resumption of convergence, possibly associated with amalgamation of the Buller and Takaka terranes, between 368 and 355 Ma. Significant crustal growth occurred during generation of the two S-type suites, where mantle basalt contributed mass, and heat for rapid melting, during transient intra- or backarc extensional episodes. In contrast, the I-type suites were dominated by partial melting of meta-igneous crust, and they are relatively small in volume. The Karamea S-type suite shares striking similarities in terms of age, composition, and extensional tectonic setting with S-type granites of the Melbourne terrane of the Lachlan fold belt. Both regions may have formed in a backarc position with respect to the Late Devonian-early Carboniferous subduction zone in the New England fold belt. Foulwind Suite A-type magmatism in New Zealand overlaps in age with the widespread 320–285 Ma A- and I-type magmatism in the northern New England fold belt. The likely continuation of the New England subduction system must have subsequently been removed from outboard of the New Zealand region after 320–285 Ma magmatism, and prior to Triassic accretion of a Permian oceanic arc terrane to the New Zealand margin.

Constraints on early Cambrian carbon cycling from the duration of the Nemakit-Daldynian–Tommotian boundary δ13C shift, Morocco

The Nemakit-Daldynian–Tommotian (ND-T) boundary marks the first appearance of metazoan reefs and calcite biomineralizers and is associated with the largest δ 13 C shift during the Phanerozoic Eon. Biological transitions in Earth history are often accompanied by excursions in the carbon isotopic composition (δ 13 C) of the ocean, where δ 13 C variability is interpreted to reflect changes in the global carbon cycle. The duration and thus rate of these δ 13 C anomalies are rarely known, making it difficult to constrain their possible causes and their relationship, if any, to biologic transitions. We report sedimentological and δ 13 C data from a new 2.5-km-thick section that spans the early Cambrian evolutionary “explosion” in the Moroccan Anti-Atlas Mountains. Three new zircon 206 Pb- 238 U ages from tuffs within the stratigraphy constrain the timing of the ND-T boundary to 524.84 ± 0.09 Ma. Two of the tuffs exactly bracket the ND-T transition and constrain the duration of the −8‰ δ 13 C shift to 506 ± 126 k.y. With a simple box model, we explore a range of geochemical processes that could account for such a rapid ND-T δ 13 C shift, and conclude that metamorphic and/or volcanic fluxes of carbon may have been sustained at levels 4–16 times higher than today for millions of years.