Megareefs in Middle Devonian supergreenhouse climates

Abstract

A newly refined reef database, modified to calculate reef tracts in relation to major tectonic plates, and with new paleogeographic maps, indicates that the largest known, and latitudinally most widespread Phanerozoic reefs developed during the Middle Paleozoic (Siluro-Devonian), with an acme in the Middle Devonian. Expanding during times of exceptional sea-level highstands and widespread epicontinental shallow seas, this 26 m.y. long acme of coral-sponge reef growth coincided with the warmest global temperatures known for the Phanerozoic, i.e., with a "supergreenhouse" climate mode well above Holocene interglacial norms. During the Middle Paleozoic, reefs were particularly abundant, occupying large, continental seaboard, carbonate platforms, and vast inland epicontinental seas. Examples of such "extremes" occurred mostly on passive margin settings, and extensive flooded continental interiors, e.g., the 1700-3000 km long tracts of the Western Canada Sedimentary Basin, Canadian arctic (Innuitian platform), eastern Laurentia "Old Red Continent" (United Kingdom to Poland), eastern Russian Platform (northeast Laurentia), Ural "Fold Belt" (eastern slopes of Urals), Siberia, northwest Africa, and South China. Smaller scale reef belts between 700 and 1300 km long were constructed on isolated tectonic terranes facing Gondwana on the north (Pyrenees, Afghanistan-Pakistan), Mongolia, Kolyma-Chukot, and North China. Large basins and flooded shelf areas, and the reefs featured within them, were not persistently developed throughout the Middle Paleozoic. They especially characterized the middle Emsian through Givetian (late Early Devonian-Middle Devonian). The following Frasnian (Late Devonian) showed more restricted and confined distribution of coralstromatoporoid reefs, and during the Famennian, coral-stromatoporoid reefs "crashed" and were replaced by calcimicrobial reefs and platforms. During the latter phases of the Frasnian/Famennian mass extinctions, such microbial reefs were confined to relatively small areas, and metazoan reefs were nearly entirely obliterated, being confined to rare stromatoporoid patch reefs or lithistid mounds. Coral reefs were completely absent during the 21 m.y. long Famennian interval, and no real recovery of "keystone" frame-building, colonial corals took place in reef settings. The Famennian coincided with repeated glaciations, sharp sea-surface cooling events, sea-level drawdowns, and concurrent, matching stable isotope excursions.

On the Sensitivity of the Devonian Climate to Continental Configuration, Vegetation Cover, Orbital Configuration, CO 2 Concentration, and Insolation

“…For example, Harada () reports significant changes in the biogeochemical cycling due to the anthropogenic climate change in the western Arctic Ocean at all trophic levels. As marine life in the Devonian underwent considerable changes (Copper & Scotese, ; Dahl et al, ; McGhee, ), it is important to further investigate the described mechanism, although verification from proxy data might prove difficult due to the short timescale and the regional focus of the effect. Furthermore, it is crucial for a meaningful interpretation of the data of this study to take into account these temperature variations.…”

“…By the Late Devonian, vascular plants had vastly diversified, going hand in hand with the evolution of more advanced leaves and root systems (Algeo & Scheckler, ; Algeo et al, ). In the ocean, coral stromatoporoid reefs reached their largest extent during the Phanerozoic in the Middle Devonian (Copper & Scotese, ) and fish evolved into rich diversity (Dahl et al, ). Finally, in the Late Devonian, the first tetrapods moved from ocean to land (Brezinski et al, ; Clack, ).…”

Post-tectonic limitations on Early Devonian (Emsian) reef development in the Gobi-Altai region, Mongolia

“…During the Early Devonian, shallow‐marine carbonate depositional systems across the world, including the Siberian, Timan‐Pechora, Alberta, central Europe, Morocco and South China platforms, supported reef development (James & Bourque 1992; Kiessling et al. 1999; Copper & Scotese 2003; Kiessling 2006). Generally, the reefs were constructed by a diversity of stromatoporoid sponges, tabulates and colonial rugose corals characterized by domal growth forms.…”

“…In the Lower Devonian (Emsian) Chuluun Formation, well‐preserved stromatoporoid sponges and tabulate corals secreted massive, domal skeletons but failed to produce reefs as indicated by the low abundance and low density of colonies that form isolated ‘heads’ in well‐bedded limestone. Comparative palaeocommunity composition suggests that the region was located within 35° of the equator (Copper & Scotese 2003). In addition, global sea surface temperatures are estimated to have been on average 30°C in the Early Devonian (Copper 2002), thus temperature does not explain the lack of reef development in the south Gobi.…”

On the sensitivity of the Devonian climate to continental configuration, vegetation cover and insolation

“…By the Late Devonian, vascular plants had vastly diversified, going hand in hand with the evolution of more advanced leaves and root systems (Algeo et al, 1995;Algeo and Scheckler, 1998). In the ocean, coral stromatoporoid reefs reached their largest extent during the Phanerozoic in the Middle Devonian (Copper and Scotese, 2003) and fish evolved into rich diversity (Dahl et al, 2010). Finally, in the Late Devonian, the first tetrapods moved from ocean to land (Clack, 2007;Brezinski et al, 2009).…”