Environmental Drivers of the First Major Animal Extinction Across the Ediacaran White Sea-Nama Transition

The Ediacara Biota-the oldest communities of complex, macroscopic fossils-consists of three temporally distinct assemblages: the Avalon (ca. 575-560 Ma), White Sea (ca. 560-550 Ma), and Nama (ca. 550-539 Ma). Generic diversity varies among assemblages, with a notable decline at the transition from White Sea to Nama. Preservation and sampling biases, biotic replacement, and environmental perturbation have been proposed as potential mechanisms for this drop in diversity. Here, we compile a global database of the Ediacara Biota, specifically targeting taphonomic and paleoecological characters, to test these hypotheses. Major ecological shifts in feeding mode, life habit, and tiering level accompany an increase in generic richness between the Avalon and White Sea assemblages. We find that ∼80% of White Sea taxa are absent from the Nama interval, comparable to loss during Phanerozoic mass extinctions. The paleolatitudes, depositional environments, and preservational modes that characterize the White Sea assemblage are well represented in the Nama, indicating that this decline is not the result of sampling bias. Counter to expectations of the biotic replacement model, there are minimal ecological differences between these two assemblages. However, taxa that disappear exhibit a variety of morphological and behavioral characters consistent with an environmentally driven extinction event. The preferential survival of taxa with high surface area relative to volume may suggest that this was related to reduced global oceanic oxygen availability. Thus, our data support a link between Ediacaran biotic turnover and environmental change, similar to other major mass extinctions in the geologic record

The Geozoic Supereon

Geological time units are the lingua franca of earth sciences: they are a terminological convenience, a vernacular of any geological conversation, and a prerequisite of geo-scientific writing found throughout in earth science dictionaries and textbooks. Time units include terms formalized by stratigraphic committees as well as informal constructs erected ad hoc to communicate more efficiently. With these time terms we partition Earth’s history into utilitarian and intuitively understandable time segments that vary in length over seven orders of magnitude: from the 225-year-long Anthropocene (Crutzen and Stoermer, 2000) to the ,4-billion-year-long Precambrian (e.g., Hicks, 1885; Ball, 1906; formalized by De Villiers, 1969)

Correlation of Precambrian-Cambrian sedimentary successions across northern India and the utility of isotopic signatures of Himalayan lithotectonic zones

A common view in Himalayan geology is that differences in detrital zircon age distributions and whole-rock neodymium isotopic compositions (εNd) distinguish lithotectonic zones within the system. Such differences are used to map these zones and to locate their modern boundaries, as well as to infer ancient terrane boundaries. We test the utility of this approach using integrated geochemical, geochronological, and sedimentological data from the Himalayan successions of northern India and relatively undeformed, age-equivalent successions of the Indian craton. U-Pb geochronology of detrital zircons from cratonic successions of the Vindhyan, Ganga, and Marwar supergroups and the "inner" and "outer" Lesser Himalaya lithotectonic zones show that rocks of similar depositional age bear strikingly similar detrital zircon age distributions throughout the entire region. A sharp change in εNd occurs within the "inner" Lesser Himalaya and correlates with a regional unconformity recognized on the craton, here constrained to span a period of ~. 500. million years. Results demonstrate that isotopic differences among the lithotectonic zones relate primarily to differences in the depositional ages of their constituent rocks, and that all parts of the Himalaya were in sediment-source continuity with the Indian craton from the late Paleoproterozoic to the early Cambrian. Isotopic "signatures" may vary as much within individual Himalayan lithotectonic zones as between such zones and no lithotectonic zone can be characterized by such data alone. © 2011.Link_to_subscribed_fulltex

Age and implications of the phosphatic Birmania Formation, Rajasthan, India

© 2015 Elsevier B.V.The Birmania inlier in western Rajasthan, India, contains important phosphate deposits, the depositional age of which is poorly constrained. Here we provide the first direct age constraints for the phosphate-bearing Birmania Formation using a combination of paleontological and detrital zircon U-Pb data. The occurrence of the multicellular algal fossil Wengania exquisita in phosphatic chert of the Birmania Formation suggest that it was deposited during the Ediacaran. Detrital zircon age distributions contain prominent populations of 1.7-1.9 Ga grains, with subordinate younger grains that range from 650 to 980 Ma. The distribution broadly resembles those from Neoproterozoic strata from both cratonic and Himalayan India but, like zircon age distributions from the Marwar Group, lacks 1.0-1.2 Ga grains, which may suggest that both areas shared similar local sources. The lack of zircon grains younger than ~650. Ma is consistent with an Ediacaran depositional age because almost all Cambrian or younger strata from India have yielded Cambrian or latest Neoproterozoic age grains. These findings raise the possibility of a previously unrecognized late Neoproterozoic episode of phosphogenesis on the India craton. The presence of Wengania exquisita further supports strong palaeogeographic affinity between the India and South China during the Neoproterozoic and Cambrian.Link_to_subscribed_fulltex