Global climate, sea level cycles, and biotic events in the Cambrian Period
Authors:
Babcock et al
Abstract:
The developing high-resolution chronostratigraphy of the Cambrian provides an updated age model for various geologic and biotic events that occurred during this critical period of Earth history. Broad, time-specific patterns of lithofacies, such as organic-rich deposits, and biofacies appear to be consistent across all Cambrian paleocontinents. Records of important evolutionary events including first appearances of certain metazoan taxa, migrations, and extinctions, tend to coincide with changes in eustatic sea level, as do the positions of many Konservat-Lagerstätten, concretion horizons, agnostoid-rich beds, and other sedimentary features. Most of these events or horizons also show a relationship to perturbations in the global carbon cycle. The positions of organic-rich deposits bear strong relationship to both paleogeographic position and sea level history. Cambrian strata show evidence of cyclicity at multiple scales. Synchronous or near-synchronous global cyclicity is inferred to be associated with oceanographic and climatic cycles characteristic of glacial expansion and deglaciation.
Benthic foraminiferal response to the Middle Eocene Climatic Optimum (MECO) in the South-Eastern Atlantic (ODP Site 1263)
Authors:
Galazzo et al
Abstract:
The response of marine biota to the Middle Eocene Climatic Optimum (MECO) is still poorly constrained. Specifically, changes in deep-sea benthic foraminiferal faunas have been documented at few locations only. We carried out a quantitative study of benthic foraminiferal assemblages at lower-bathyal ODP Site 1263 (Walvis Ridge, SE Atlantic), providing documentation of the response of benthic foraminiferal assemblage to the MECO in an open ocean setting. There was no major temporary or persistent assemblage turnover in benthic foraminiferal faunas in the SE Atlantic Ocean. The benthic foraminiferal accumulation rates (BFAR) and assemblage composition indicate that the delivery of food to the seafloor increased during the early stages of surface and deep-sea warming. During this period, the absolute and relative abundance of phytodetritus exploiters increased pronouncedly, indicating that the increase in the flux of organic matter to the sea floor was largely seasonal or otherwise pulsed. During later stages and peak warming of MECO, in contrast, the flux of organic matter to the seafloor declined markedly, as shown by a decrease in both infaunal and epifaunal benthic foraminiferal accumulation rates. The low flux of organic matter during the more extreme parts of the MECO thus did not favor one of these broad groups, but affected all common taxa. Paleoceanographic reconstructions combined with data on benthic foraminifera suggest that the MECO warming was the main cause of the reduction in the flux of organic matter to the sea floor, by increasing the metabolic rates of pelagic consumers, which thus used more food, preventing it from reaching the sea floor, as well as increasing remineralization of organic matter in the water column.
Terrestrial evidence for a two-stage mid-Paleocene biotic event
Authors:
Hyland et al
Abstract:
Marine records of the Paleocene indicate a series of hyperthermal events characterized by significant climatic and carbon cycle variability, but there are few comparable continental records. The mid-Paleocene biotic event (MPBE) is a recently described interval defined by a rapid negative carbon isotope excursion and major short-term changes in marine ecosystems, but it is as yet unclear whether the event was globally important. Here we present the first terrestrial paleoenvironmental record of the MPBE based on paleosols that document rapid and short-lived increases in temperature and precipitation and resultant shifts in plant assemblages concomitant with substantial carbon isotope excursions. The new record indicates that carbon cycle changes during the early late Paleocene may have resulted in a two-stage transient hyperthermal event that caused a significant perturbation to both the regional climate and terrestrial ecology of South America in addition to the major biotic event (MPBE) previously recognized in marine records. Overall, this suggests that the MPBE may have been a global climate event with far-reaching environmental impacts in both the marine and terrestrial realms.
Functional diversity of marine ecosystems after the Late Permian mass extinction event
Authors:
Foster et al
Abstract:
The Late Permian mass extinction event about 252 million years ago was the most severe biotic crisis of the past 500 million years and occurred during an episode of global warming. The loss of around two-thirds of marine genera is thought to have had substantial ecological effects, but the overall impacts on the functioning of marine ecosystems and the pattern of marine recovery are uncertain. Here we analyse the fossil occurrences of all known benthic marine invertebrate genera from the Permian and Triassic periods, and assign each to a functional group based on their inferred lifestyle. We show that despite the selective extinction of 62–74% of these genera, all but one functional group persisted through the crisis, indicating that there was no significant loss of functional diversity at the global scale. In addition, only one new mode of life originated in the extinction aftermath. We suggest that Early Triassic marine ecosystems were not as ecologically depauperate as widely assumed. Functional diversity was, however, reduced in particular regions and habitats, such as tropical reefs; at these smaller scales, recovery varied spatially and temporally, probably driven by migration of surviving groups. We find that marine ecosystems did not return to their pre-extinction state, and by the Middle Triassic greater functional evenness is recorded, resulting from the radiation of previously subordinate groups such as motile, epifaunal grazers.
Cause of Upper Triassic climate crisis revealed by Re-Os geochemistry of Boreal black shales
Authors:
Xu et al
Abstract:
The Triassic Period is bracketed by two of the ‘big five’ Phanerozoic mass extinctions. Though long viewed as a period of climatic stability, emerging data suggest multiple climatic swings and at least one severe ecological crisis. Linking these climatic instabilities with probable causes is hampered by poor age control within the Triassic time scale. Here we present new Re-Os ages for shale sections straddling Middle-Upper Triassic stage boundaries. Nominal Re-Os isochron ages of 236.6 and 239.3 Ma for the top and base of the Ladinian (upper Middle Triassic) bring absolute time into the contentious Triassic time scale, and place the beginning of the Late Triassic about 12 m.y. earlier than previously assigned. A marked decrease in initial 187Os/188Os in Upper Ladinian shale records input from Wrangellian flood basalts – an instigator in the Carnian (Late Triassic) Pluvial Event and accompanying radiation of key fossil groups (e.g., dinosaurs and calcareous nanoplankton). An absolute time scale is proposed for the Anisian – Ladinian – Carnian boundaries based on Re-Os geochronology.
