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Kapitel 3 - Hirnantian Event

Inhalte „Events in Earth History“

Introduction

Proterozoic Snowball Earth Periods

Hirnantian – Ordovician/Silurian Event

Late Devonian Events (F/F-, Kacak-, Hangenberg Event)

P/T

K/T Boundary Event

PETM

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Kapitel 4 - Late Devonian Events (F/F-, Kacak-, Hangenberg Event)

Timeliness of Devonian Mass extinction Events

Scientific interest in Devonian Extinction events as extractedfrom the ISI Science Citation data base, November 2016.

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Kapitel 4 - Late Devonian Events (F/F-, Kacak-, Hangenberg Event)

Middle Devonian

Devonian / Carboniferous Paleogeography

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Kapitel 4 - Late Devonian Events (F/F-, Kacak-, Hangenberg Event)

Mississippian

Devonian / Carboniferous Paleogeography

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Kapitel 4 - Late Devonian Events (F/F-, Kacak-, Hangenberg Event)

Causal Link between OAEs or EE and LIPs ?

Correlation between the ages of LIPs (CFBs and OPs), and those of coeval mass extinctions and oceanic anoxia events (all in Ma). Ranges are visible only when larger than the diameters of dots. The four largest recent mass extinctions and corresponding traps are in dark grey, the F/F in light grey was datedradiometrically.The F/F event is theonly Late DevonianEE/OAE linked to theemplacement of a LIP.The Hangenberg Event misses in this graph.

Courtillot & Renne

2003, C.R. Geosci.

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Kapitel 4 - Late Devonian Events (F/F-, Kacak-, Hangenberg Event)

Causal Link between OAEs or EE and LIPs ?

Gaseous emissions and input from sediments to be considered when evaluating the environment impact of LIP emplacement:

Basalt and granitoids do not release abundant volatiles. In most sandstones, the main volatile is water, whose release has little effect

on global climate. Pure limestone contains large amounts of CO2, but the thermal

decomposition of limestone into CaO and CO2 takes place at high temperatures (>850 °C) that are rarely reached in contact aureoles.

Impure limestones release large amounts of CO2 (up to 29 wt%) during the formation of calci-silicates at moderate temperatures of ~450–500 °C.

Gypsum and anhydrite in evaporites release abundant SO2 (up to 47 wt%). This reaction normally occurs at high temperatures (1400 °C) that are not observed in contact aureoles, but the reaction proceeds at temperature as low as 615 °C for impure anhydrite.

Sulfidic sediments release abundant SO2 at low temperature. Contact metamorphism of salt releases halocarbons. Organic carbon-rich shales and carbonates release methane and hydrocarbon

when heated at relatively low temperature (<300 °C). (for an opposite view, see Groecke et al., 2009)

Coal releases abundant CO2 if ignited. Ganino & Arndt

2010, Geology

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Kapitel 4 - Late Devonian Events (F/F-, Kacak-, Hangenberg Event)

Frequency of Devonian OAEs and EEs

Compilation of Devonian biotic ‘events’ with their corresponding chronostrati-graphic stage, conodont and ammonoid zonation.

Events characterised by short-term dysoxic or anoxic facies in their name areas are shown as black bars.

Stage boundaries arecommonly but not alwayspunctuated by EEs or OAEs.

The Devonian is assumed toharbour biotic events atexceptionally high frequencydue to the terrestrializationof the continents and theassociated impact on thepedo-, atmo-, hydro-, andbiosphere.

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Kapitel 4 - Late Devonian Events (F/F-, Kacak-, Hangenberg Event)

Terrestrialization and Devonian Events

The Devonian Period was characterized by major changes in both the terrestrial biosphere, the evolution of trees and seed plants and the appearance of multi-storied forests, and in the marine biosphere, an extension of biota in tropical marine benthos, especially the stromatoporoid-tabulate coral reef community. Teleconnections between these terrestrial and marine events may arise from soils as a geochemical interface linking litho/atmo/hydrosphere, and from land plants mediating weathering processes at this interface. The effectiveness of terrestrial floras in weathering was significantly enhanced as a consequence of increases in the size and geographic extent of vascular land plants during the Devonian. In this regard, the most important palaeobotanical innovations were:

(1) arborescence (tree stature), which increased maximum depths of root penetration and rhizoturbation, and

(2) the seed habit, which freed land plants from reproductive dependence on moist lowland habitats (e.g. ferns) and allowed colonization of drier upland and primary successional areas.

These developments resulted in a transient intensification of pedogenesis and to large increases in the thickness and areal extent of soils. Enhanced chemical weathering may have led to increased riverine nutrient fluxes that promoted development of eutrophic conditions in epicontinental seaways, resulting in algal blooms, widespread bottomwater anoxia, and high sedimentary organic carbon fluxes. Long-term effects included drawdown of atmospheric pCO2 and global cooling, leading to a brief but multistage Late Devonian (Famenne) glaciation.

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Kapitel 4 - Late Devonian Events (F/F-, Kacak-, Hangenberg Event)

Terrestrialization and Devonian Events

Palaeobotanical and palaeoecologicalreconstructions of:(a) an Early Devonian (Siegenian or Pragian to Emsian) upland floodplain, (b) a Middle Devonian (Eifelian toGivetian) upland floodplain, and (c) a Late Devonian (Famennian) upland flood-plain.

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Kapitel 4 - Late Devonian Events (F/F-, Kacak-, Hangenberg Event)

Terrestrialization and Devonian Events

Relative sizes, morphologies, and penetration depths of the root systems of selected Early, Middle, and Late Devonian plants. Abbreviation: rhy. rhyniophytes, e.g. Agalophyton orHorneophyton; tri. trimerophytes such as Psilophyton; lyc-he. early herbaceous lycopods such asAsteroxylon or Drepanophycus; lyc-tr. early tree lycopods such as Lepido-sigillaria or Cyclostigma; prog-an. aneurophyte progymnosperms such asTetraxylopteris; prog-arc. Archaeopteris progymnosperms; gym. early gymnosperms, Elkinsia or Moresnetiaand zyg. zygopterid ferns such as Rhacophyton.

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Kapitel 4 - Late Devonian Events (F/F-, Kacak-, Hangenberg Event)

Terrestrialization and Devonian Events

Changes in atmospheric and soil pCO2 as a function of the spread of vascular land plants during the Devonian. (a) In the largely non-vegetated pre-

Devonian world, atmosphere and soil CO2 levels were similar;

(b) during the Devonian (accelerating in the Late Devonian), atmospheric pCO2 was drawn down through active CO2 pumping by land plants, leading to elevated soil CO2 levels;

(c) in the post-Devonian, terrestrial floras dynamically maintain low atmospheric pCO2 via the CO2 pumping mechanism; soil CO2 levels are approximately equal to those prior to the spread of land plants owing to quasi-steady-state CO2 supply from volcanic and metamorphic sources and the requirement for long-term flux balance.

As a consequence, silicate weatheringrates were comparable in the pre- andpost-Devonian, but experienced a transient maximum during the Devonian.

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Kapitel 4 - Late Devonian Events (F/F-, Kacak-, Hangenberg Event)

Terrestrialization and Devonian Events

Alluvial facies, channel and floodplain characteristics for Cambrian to Devonian strata linked to evolutionary vegetation stages. Stars indicate a first appearance. A secular change in alluvial characteristics and correlation with vegetational events suggests a first-order causal link.

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Kapitel 4 - Late Devonian Events (F/F-, Kacak-, Hangenberg Event)

Terrestrialization and Devonian Events

Model linking development of arborescence and seed habit among vascular land plants during the Devonian to coeval terrestrial and marine events. Events are arrayed by relative duration with transient effects on the left and long-term effects on the right. Solid outlines indicate documented geological records; dashed outlines indicate processes inferred to link records.

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Kapitel 4 - Late Devonian Events (F/F-, Kacak-, Hangenberg Event)

Isotopic excursions at the F/F-event

Carbon isotope excursions related to sea-level fluctuations and broad continental shelfs in tropical and subtropical regions. Anoxia in particular extinguished aerobic reefal communities widely distributed in these regions.

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Kapitel 4 - Late Devonian Events (F/F-, Kacak-, Hangenberg Event)

Isotopic excursions at the F/F-event

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Kapitel 4 - Late Devonian Events (F/F-, Kacak-, Hangenberg Event)

Isotopic excursions at the F/F-event

Regional character of F/F-event isotopic excursion in NW-Europe is evident. Similar isotopic excursions occur in North Africa and in Mid-Continent U.S.

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Kapitel 4 - Late Devonian Events (F/F-, Kacak-, Hangenberg Event)

Isotopic excursions at the F/F-event

Carbon isotope excursion is not exclusively bound to carbonate but shifts organic matter C isotopes in the same direction. OM isotopes are consistent for primary photosynthate arguing against input variation affecting d13Corg.

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Kapitel 4 - Late Devonian Events (F/F-, Kacak-, Hangenberg Event)

Regional continuity of F/F-event

Compilation of the main redox ‘events’ recognisedat the F/F-boundary.

The peak intensity and spread of anoxia coincides with deposition of the Upper Kellwasser Horizon in Germany.

In contrast the Lower Kellwasser Horizon of Germany does not mark a European-wide anoxic event (note that oxygenation improves at this level in both La Serre and Kowala).

Anoxia assessment is largely based on biotic indicators, trace element distributions, and pyrite diameter size.

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Kapitel 4 - Late Devonian Events (F/F-, Kacak-, Hangenberg Event)

Causal Link between OAEs or EE and LIPs ?

Comparison of model proposed for the development of anoxia during the F/F anoxic events: (a) restricted epicontinental basins and (b) shelf anoxia as a consequence of oceanic upwelling of deep anoxic waters.

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Kapitel 4 - Late Devonian Events (F/F-, Kacak-, Hangenberg Event)

Biomarker indicators of evolutionary crisis

Algal community changes (increase in prasinophtyes disaster species) occurred in a punctuated fashion during short term Devonian events but returned to starting values after event termination. A major evolutionary re-organization occurred at the D/C-boundary, with a continuous dominance of C28-algae.

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Kapitel 4 - Late Devonian Events (F/F-, Kacak-, Hangenberg Event)

Biomarker indicators of evolutionary crisis

Algal community changes (increase in prasinophtyes disaster species) occurred in a punctuated fashion during short term Devonian events but returned to starting values after event termination. A major evolutionaryre-organization occured at the D/C-boundary, with a continuous dominance.

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Kapitel 4 - Late Devonian Events (F/F-, Kacak-, Hangenberg Event)

Devonian biogeochemical events and mass extinctions

Emplacement of LIPS has been proposed as one cause for Devonian events but seems of less importance compared to other epochs.

Terrestrialization of the continents is a key factor in reorganization of weathering intensities, nutrient fluxes from land to ocean, ocean chemistry (anoxia), CO2 budgets and biological cycles.

Reorganization of biological cycles was associated with shifts in marine plankton composition and may have triggered important evolutionary radiations.

Short-lived „events“ in the Devonian reveal pronounced isotopic as well as biomarker excursions. These events may have been associated with anoxic conditions in epicontinental basins extinguishing in particular deeper dwelling or reefal faunas and floras. Anoxia was triggered by increased bioproductivity due to weathering intensity-linked nutrient supply. Plankton blooms then initiated black shale deposition, whereby large export of isotopically light OM caused heavy carbonate isotope signatures.

Formation of warm saline shelfal waters is assumed to have caused regional establishment of anoxic conditions, leading to isotope perturbations and mass extinctions, particular in tropical deeper-water environments. Causes for temperature variations and sea level fluctuations remain elusive but may be associated to LIPs.