Zarela A. Herrera

The late Ordovician carbonate sedimentation as a major triggering factor of the Hirnantian glaciation

A new approach explaining the main forcing factor of Hirnantian glaciation is proposed herein. It follows the models associating occurrences of continental glaciations with periods of low atmospheric CO2 levels. The accumulation of great volumes of carbonates during pre-Hirnantian late Ordovician, in regions where these deposits were previously absent, is suggested as a major sink of atmospheric CO2. This would have caused an important lowering of the average temperature in the early Hirnantian, after CO2 values had attained a certain threshold. This process was maintained by other positive feedbacks, such as the short-term carbonate weathering CO2 sink. An increase of the direct flux of CO2 from the atmosphere to the oceans by means of dissolution would have been driven by the enhancement of carbonate deposition. The great inundation of the low latitude Laurentia craton during Cincinnatian times and the establishment of a temperate-water carbonate sedimentation on the North Gondwana margin during pre-Hirnantian Ashgill allowed the burying of more than 840 × 1015 kg (1.9 × 1019 mol) of dissolved CO2. This mass is equivalent to nearly 350 times the present values of atmospheric CO2. This is important enough to have greatly altered the equilibrium between the CO2 dissolved in the oceans and the partial pressure of CO2 in the air, eventually causing an important reduction of the latter. The new model also offers a simple explanation for the end of the glaciation after a short time-span. Glacioeustatic lowering of the sea level, concomitant with the glaciation, would have stopped the extra-sedimentation of carbonate due to the retreat of the oceans from the platforms, closing this CO2 sink. Pre-glacial CO2 levels would then recover, due to volcanic and metamorphic CO2 outgassing. After subsequent melting of the ice cap, oceanic circulation did not recover pre-Hirnantian Ashgill strength, resulting in a strong stratification of ocean waters and precluding the recovery of an extensive carbonate deposition. The well-known positive shift in the [omega]13C at the base of the Hirnantian is assumed to have been caused by weathering and dissolution of carbonates, relatively enriched in 13C, during the glacioeustatic regression and exposure of the platforms.

Lingulate brachiopods from the Lampazar Formation (late Cambrian) of the Cordillera Oriental of northwestern Argentina

Abstract The Angosto de Lampazar, a classic locality for the study of lower Paleozoic successions in the Cordillera Oriental, NW Argentina, has yielded a late Cambrian relatively diverse, lingulate brachiopod fauna. Sandy lenses with calcareous cement from the uppermost levels of the Lampazar Formation have yielded abundant remains of articulate and phosphatic brachiopods. Among the latter, the new species Eurytreta harringtoni Mergl and Herrera, Lingulella? melonica Mergl and Herrera, Libecoviella lenticularis Mergl and Herrera, and Schizambon cardonalis Mergl and Herrera, as well as the new genus and species Saltaia lampazarensis Mergl and Herrera are formally introduced. Trilobites and conodonts from the same horizons characterize the Cordylodus proavus Zone, allowing a correlation with Stage 10 of the Furongian Series. Although the information on lingulate brachiopods from the C. proavus Zone is scarce across the world, the composition of the studied association displays a relationship with coeval and slightly younger faunas of Utah and Kazakhstan. The Argentine brachiopods, the first described from the late Cambrian Proto-Andean margin of Gondwana, are more closely related to temperate Laurentian faunas than to those from the high latitude North African margin of Gondwana.

EARLY ORTHID BRACHIOPODS FROM THE TREMADOCIAN (LOWER ORDOVICIAN) OF NORTHWESTERN ARGENTINA

Abstract Two brachiopod species of two of the earliest rhynchonelliformean families are described from one of the first studied Ordovician fossiliferous localities of northwestern Argentina. One is a new species of the family Euorthisinidae (Orthida, Plectorthoidea), Notorthisina musculosa Villas and Herrera. It displays peculiar features not observed in the family before, as well as a combination of internal and external features previously considered diagnostic of different genera, leading to emendment of the familial and generic diagnoses. Its dorsal muscle field, best impressed in large specimens, resembles that of Kvania Havlíček (Nanorthidae) in having four scars arranged radially. This feature reinforces the phylogenetic proximity of Kvania to the euorthisinids. The second studied species, Nanorthis bifurcata Harrington, has not been revised since its superficial description in 1937–1938, its type having been lost. Its description herein, based on a new collection from its type locality, allows a better knowledge of the group and helps clarify its supra-familial assignment.