Late Paleocene–early Eocene Tethyan carbonate platform evolution — A response to long- and short-term paleoclimatic change

Abstract

The early Paleogene experienced the most pronounced long-term warming trend of the Cenozoic, superimposed by transient warming events such as the Paleocene-Eocene Thermal Maximum (PETM). The consequences of climatic perturbations and associated changes on the evolution of carbonate platforms are relatively unexplored. Today, modern carbonate platforms, especially coral reefs are highly sensitive to environmental and climatic change, which raises the question how (sub)tropical reef systems of the early Paleogene reacted to gradual and sudden global warming, eutrophication of shelf areas, enhanced CO 2 levels in an ocean with low Mg/Ca ratios. The answer to this question may help to investigate the fate of modern coral reef systems in times of global warming and rising CO 2 levels. Here we present a synthesis of Tethyan carbonate platform evolution in the early Paleogene (~59-55 Ma) concentrating on coral reefs and larger foraminifera, two important organism groups during this time interval. We discuss and evaluate the importance of the intrinsic and extrinsic factors leading to the dissimilar evolution of both groups during the early Paleogene. Detailed analyses of two carbonate platform areas at low (Egypt) and middle (Spain) paleolatitudes and comparison with faunal patterns of coeval platforms retrieved from the literature led to the distinction of three evolutionary stages in the late Paleocene to early Eocene Tethys: Stage I, late Paleocene coralgal-dominated platforms at low to middle paleolatitudes; stage II, a transitional latest Paleocene platform stage with coralgal reefs dominating at middle paleolatitudes and larger foraminifera-dominated (Miscellanea, Ranikothalia, Assilina) platforms at low paleolatitudes; and stage III, early Eocene larger foraminifera-dominated (Alveolina, Orbitolites, Nummulites) platforms at low to middle paleolatitudes. The onset of the latter prominent larger foraminifera-dominated platform correlates with the Paleocene/Eocene Thermal Maximum. The causes for the change from coral-dominated platforms to larger foraminifera-dominated platforms are multilayered. The decline of coralgal reefs in low latitudes during platform stage II is related to overall warming, leading to sea-surface temperatures in the tropics beyond the maximum temperature range of corals. The overall low occurrence of coral reefs in the Paleogene might be related to the presence of a calcite sea. At the same time larger foraminifera started to flourish after their near extinction at the Cretaceous/ Paleogene boundary. The demise of coralgal reefs at all studied paleolatitudes in platform stage III can be founded on the effects of the PETM, resulting in short-term warming, eutrophic conditions on the shelves and acidification of the oceans, hampering the growth of aragonitic corals, while calcitic larger foraminifera flourished. In the absence of other successful carbonate-producing organisms, larger foraminifera were able to take over the role as the dominant carbonate platform inhabitant, leading to a stepw...

Extinctions in ancient and modern seas

“…The most recent reef crisis (approximately 56 Ma) was characterized by rapid sea surface temperature rise and a similar order of magnitude of CO 2 increase as at present [54]. However, although a marked faunal shift occurred on continental carbonate platforms [85] at this time, reef assemblages in some other settings were relatively unaffected [86].…”

Long-term legacy of massive carbon input to the Earth system: Anthropocene versus Eocene

“…It is not clear, however, whether the benthic extinction was caused by changes in oxygenation, bottom water temperatures, carbonate undersaturation as a result of the carbon input, and/or other factors [25,65]. Finally, a growing body of evidence suggests that coastal coral reef and ostracode communities experienced a significant reduction in diversity at the end of the Palaeocene [27,73], though the exact role of acidification has yet to be firmly established. In sum, it appears that the direct effects of ocean acidification on marine planktonic calcifiers during the PETM may have been limited because of a relatively 'slow' carbon input rate (slow on human time scales, rapid on geological time scales).…”

“…While the PEB marks a major extinction event of benthic foraminifera, affecting 30-50% of species globally, and the decline of coralgal reefs [25,27,73], most species of calcareous nannoplankton and zooplankton taxa appear to have survived the PEB (see discussion above). Also, terrestrial species experienced only minor extinction [87].…”

“…ocean, atmosphere and biosphere [17,21]-phenomena which could have been generated only by a rapid and massive release of carbon, causing ocean acidification. Although the surface ocean appears to have remained oversaturated, communities of marine calcifiers, primarily coralgals, phyto-and zooplankton and benthic foraminifera, experienced changes in both diversity and abundances [22][23][24][25][26][27][28]. While many species ultimately survived, the community perturbations persisted for tens of thousands of years, recovering only as carbon levels abated and the planet cooled.…”

Paleocene echinoid faunas of the eastern United States

“…One aspect of the diversity indicative of recovery is that the majority of the species are epibenthic (or semi-infaunal) forms, either regular urchins, or irregular urchins, lacking fascioles, whose modern representatives are epibenthic. Paleocene carbonate platforms were dominated by bryozoan and coralgal communities (Scheibner and Speijer, 2008), and Kroh ( , 2003 noted a preference of epibenthic forms from the Danian of Austria for coralline-algae dominated carbonate environments and a similar preference could be at play in the faunas studied here.…”