Memories of Pre-Jurassic Lost Oceans: How To Retrieve Them From Extant Lands

The information reflected in mid-oceanic sedimentary deposits provides critical constraints for reconstructing past global environmental changes.  Available data from extant oceans, however, are limited to the Early Jurassic and younger ages, because older oceanic plates have been subducted.  This article explains methods for obtaining information on pre-Jurassic mid-oceanic conditions by conducting fieldwork on older orogenic belts exposed on land.  The key point is the identification of ancient accretionary complexes (AC), not along currently active margins but within older orogenic belts in continental domains, particularly by recognizing ocean plate stratigraphy (OPS) that contains mid-oceanic strata, as demonstrated by studies of on-land exposed ancient AC in Japan and elsewhere.  In this paper, six examples of retrieved mid-oceanic sedimentary data are introduced, in which significant records on the following unique events in the pre-Jurassic world are archived: 1) the extinction-related Paleozoic–Mesozoic boundary superanoxia (based on data from the Jurassic AC in SW Japan); 2) the Permian Kamura cooling event in the mid-Panthalassa (ditto); 3) the Neoproterozoic snowball Earth evidence from the mid-Iapetus Ocean (based on data from the Neoproterozoic–Cambrian AC in Wales, UK); 4) the discovery of enigmatic Ediacaran (Neoproterozoic) microfossils from a mid-oceanic atoll complex (based on data from the Cambrian AC in southern Siberia, Russia); and 5) and 6) Early Archean (3.8 and 3.5 Ga) biogenic signatures in mid-oceanic deep-sea environments (based on data from the Eoarchean AC at Isua in Greenland, and the Paleoarchean one in Pilbara, Western Australia).  These results demonstrate the great utility of OPS analysis for understanding pre-Jurassic lost oceans, including the early biological and environmental evolution of the globe. SOMMAIRELes informations enregistrées dans les dépôts sédimentaires médio-océaniques constituent des contraintes logiques qui permettent de reconstituer les changements environnementaux globaux.  Cela dit, l’information sur de grands pans de fonds océaniques est limitée aux fonds océaniques Jurassiques précoces et plus jeunes, parce que les fonds océaniques plus anciens ont été subduits.  Le présent article explique des méthodes permettant d’obtenir de l’information sur les milieux médio-océaniques pré-jurassiques par des levés de terrain sur des ceintures orogéniques affleurant sur terre.  L’idée centrale consiste à circonscrire d’anciens complexes d’accrétion (AC), hors des marges actives actuelles, soit dans les ceintures orogéniques plus anciennes au sein des domaines continentaux, en y repérant des contextes stratigraphiques de plaques océaniques (OPS) qui renferment des strates médio-océaniques, comme ça a été fait lors études d’AC affleurant au Japon et ailleurs.  Le présent document décrit six exemples de contextes stratigraphiques de plaques océaniques où on trouve des indices importants des événements pré-jurassiques uniques suivants : 1) l’extinction liée à la super-anoxie de la limite Paléozoïque-Mésozoïque (à partir des données d’un AC jurassique dans le sud-ouest du Japon); 2) l’épisode de refroidissement permien de Kamura du Panthalassa moyen; 3) l’épisode néoprotérozoïque de « Terre boule de neige » conservé par l’océan mi-japétien (selon les données de l’AC néoprotérozoïque-cambrien dans les Wales au Royaume-Uni); 4) la découverte de microfossiles édiacariens (Néoprotérozoïque) d’un complexe d’atolls médio-océaniques (selon les données d’un AC cambrien du sud-est sibérien, Russie); et 5 et 6) des signatures biogéniques de milieux médio-océaniques profonds de l’Archéen précoce (3,8 et 3,5 Ga) (selon les données d’un AC éoarchéen à Isua au Groenland, et d’un AC paléoarchéen à Pilbara, Australie).  Ces résultats montrent la grande utilité de l'analyse  de la stratigraphie des plaques océaniques pour comprendre les océans pré-Jurassique, de même que l'évolution des débuts de la vie et des milieux de vie sur Terre

CO-EVOLUTION OF PROTEROZOIC CRATONIC FRAGMENTS IN WESTERN AND NORTHERN MONGOLIA

CAOB occupies a vast area that extends from the Urals to the Far East Asia and from the Siberian craton to the North China and Tarim cratons (Fig. 1, A). In order to better constrain Precambrian tectonic evolution of the CAOB, it is important to revisit Precambrian terranes of Mongolia as outlined in Badarch et al. [2002] that contain Archean to Proterozoic metamorphic basement and Neoproterozoic metasedimentary and volcanic rocks.CAOB occupies a vast area that extends from the Urals to the Far East Asia and from the Siberian craton to the North China and Tarim cratons (Fig. 1, A). In order to better constrain Precambrian tectonic evolution of the CAOB, it is important to revisit Precambrian terranes of Mongolia as outlined in Badarch et al. [2002] that contain Archean to Proterozoic metamorphic basement and Neoproterozoic metasedimentary and volcanic rocks

Coupled extinction–regression episodes revisited in mid-oceanic settings for comparative extinction study during the Palaeozoic in view of non-bolide extraterrestrial causes

Major change in the Earthâs surface temperature appears to be the most critical and universal factor for inducing coevally multiple kill mechanisms for organisms during the global environmental change episodes. Among the major extinction events during the Phanerozoic, the end-Ordovician episode shares almost the same environmental background with that of the end-Guadalupian (middle Permian). These two major extinction-relevant episodes, however, occurred respectively before and after the mid-Palaeozoic botanical revolution, i.e., the first mass development of land plants/forests. Owing to the enhanced terrestrial photosynthesis, the atmospheric CO2 content decreased irreversibly from ca. 2800â800 ppm in the Ordovician down to 400â300 ppm in the Permian. This highlights an apparent contradiction between the end-Ordovician glaciation and distinctly high atmospheric pCO2, which may suggest that one or more agents on global scale were likely responsible for the prominent global cooling besides atmospheric pCO2 with respect to the greenhouse effect. The same conundrum is much clearer in the cases of Proterozoic snowball Earth events. Ancient mid-oceanic sedimentary rocks, i.e., deep-sea cherts and atoll carbonates on top of seamounts, are valuable in recording the average regional/global changes of past oceanic domains without receiving tectonic disturbances along continental margins. The Permian mid-oceanic deep-sea cherts and paleo-atoll carbonates in South-West Japan were deposited in the mid-Panthalassa superocean that occupied nearly 70% of the Earthâs surface (the rest 30% by Pangea). The latest research results from these unique sedimentary archives in Japan are introduced with particular focus on the hiatus-bearing sea-level drop and a unique signature of extraordinarily high 3He enrichment in the extinction-relevant interval. These new lines of evidence imply non-bolide extraterrestrial agent for driving global cooling/sea-level drop. Comparative discussion with the end-Ordovician episode may open a new window for extinction study