Biotic effects of impacts and volcanism
Introduction
Major mass extinctions in Earth’s history are generally attributed to impacts or major flood basalt volcanism and associated environmental extremes. A general consensus of impact-induced mass extinctions currently exists only for the Cretaceous–Tertiary (KT) boundary, whereas quantification of the environmental effects of major flood volcanism (e.g. Deccan) has remained elusive. Other mass extinctions are also closely associated with flood volcanism (e.g. Permian–Triassic boundary, late Devonian [1], [2], [3]). Although the biotic effects of flood volcanism are still poorly understood, volcanogenic sulfate aerosols (acid rain), CO2 emissions (greenhouse warming) and the dynamic effects of mantle plume activity likely contributed to the environmental extremes that led to mass extinctions [4].
Flood basalts are generally linked to hot spots, or superheated mantle plumes, which are believed to erupt over periods of 1–3 million years. Biotic effects directly attributable to flood basalts in terrestrial settings are difficult to evaluate, though they can easily be quantified in a marine environment due to the abundance of microfossils. One example is Deep Sea Drilling Project (DSDP) Site 216 on Ninetyeast Ridge in the Indian Ocean, which tracks the passage of the oceanic plate over a superheated mantle plume during the late Maastrichtian, resulting in lithospheric uplift, the formation of islands built to sea level [5], [6], and volcanic activity lasting more than 1 million years.
DSDP Site 216 thus provides an ideal locality to evaluate the biotic and environmental effects of mantle plume activity from inception to cessation and the restoration of normal marine environments. The fact that this mantle plume activity began about 2 Myr before and ended about 500 kyr prior to the KT boundary, and involved the same biotic assemblages as at KT time, permits comparison with the KT mass extinction. Recent discoveries of high stress planktonic foraminiferal assemblages in upper Maastrichtian sediments of Madagascar, Israel and central Egypt [7], [8], [9] revealed pre-KT biotic catastrophes of yet unknown source and paleogeographic extent that could be linked to volcanism.
This study evaluates the biotic effects of late Maastrichtian mantle plume activity at DSDP Site 216 based on disaster/opportunist and ecologic generalist planktonic foraminifera (more detailed analysis in preparation [10]) and compares these with the biotic effects observed in previous studies of Madagascar, Israel, Egypt and Tunisia (data published in [7], [8], [9], [11], [39] (Fig. 1). Planktonic foraminifera are unicellular species that lived in the upper water column and were highly sensitive to environmental changes. They suffered the most severe losses during the KT boundary mass extinction and are therefore ideal markers for evaluating biotic effects of catastrophic events, whether volcanism or impacts.
Section snippets
Methods
Sediment samples from DSDP Site 216 were soaked in water, washed through 38 μm and 63 μm screens and oven dried at 50°C based on standard methods [9]. Statistical counts of planktonic foraminiferal species are based on random sample splits (using an Otto microsplitter) of about 150–250 specimens (average of 5–10 species). All specimens of this aliquot were counted, picked and mounted as permanent record on microslides and archived. Because planktonic foraminifera are relatively rare in these
DSDP Site 216
DSDP Site 216 was cored on the crest of Ninetyeast Ridge, which is currently located just north of the equator (lat. 1°27.73′N, long. 90°12.48′E) and at a water depth of 2237 m. During the late Maastrichtian Site 216 was located at about 40°S [12], [13] as it passed over a mantle plume resulting in lithospheric uplift and the formation of islands, extruding amygdalar and vesicular basalt that suggest aerial or near surface lava extrusion. With the northward passage away from the mantle plume,
Amboanio, Madagascar
The Amboanio section is located in a cement quarry near the village of Amboanio about 28 km south of the city of Mahajanga. During the late Maastrichtian, Amboanio was located at about 28°S latitude (Fig. 1). The base of the section may be as much as 1 Myr older (zone CF4) than at DSDP Site 216 (zone CF3), though dating is uncertain [7]. In the lower part of the section (CF4) sediment deposition occurred in an outer shelf environment (200–300 m), followed by middle shelf (150–200 m) deposition
Qreiya, central Egypt
During the late Maastrichtian, Qreiya was located in the Asyut Basin of central Egypt at a paleodepth of 150–300 m. Similar to Amboanio, Madagascar, erosional surfaces bound most biozones as a result of sea level lowstands, though all biozones, except CF2, are present and the KT boundary event is preserved in a thin clay layer and Ir anomaly [8]. The early Danian is present albeit fragmented due to erosion as a result of sea level fluctuations (Fig. 4). Limited outcrop exposure prevents
Mishor Rotem, Israel
The Mishor Rotem section is located on the Rotem syncline of the Syrian Arc structural province near Dimona in the Negev of Israel [22]. During the late Maastrichtian the Rotem syncline was submerged at outer neritic to upper bathyal depths (300–500 m [9]). Sediments are characterized by three distinct red clay layers with Pd, Pt and minor Ir anomalies, which are interbedded in chalk and marl of the upper Maastrichtian zone CF1. The KT boundary is marked by a 3 mm thin red clay layer, altered
El Kef and Elles, Tunisia
The best-known and most expanded KT boundary records are known from the El Kef global stratotype and point (GSSP) and Elles located 75 km east of El Kef, Tunisia [11], [18], [39]. Sediment deposition occurred in a middle to outer neritic environment (150–250 m) at Elles and outer neritic–upper bathyal environment (250–500 m) at El Kef. Fig. 6 shows the more complete Elles record for the late Maastrichtian, and the early Danian of El Kef. Late Maastrichtian species richness varies between 35 and
High stress biotic environments
Most previous studies have associated planktonic foraminiferal blooms of the disaster/opportunist Guembelitria exclusively with the KT mass extinction. This study shows that high stress environments are always characterized by Guembelitria blooms, which may arise at any time (late Maastrichtian to early Tertiary) or place (nearshore to open ocean) and last from thousands to several hundred thousand years (Fig. 7). Planktonic foraminifera show the same biotic response to high stress environments
Conclusions
Although both impacts and volcanism can cause high biotic stress conditions characterized by blooms of the disaster/opportunist Guembelitria species, not every impact does. In contrast, every period of intense volcanism is associated with a high stress environment. The geographic distribution of high stress environments depends on the cause and nature of the catastrophes, and may be global or regional and associated with impacts or volcanism. Based on the current data, the biotic effects of
Acknowledgements
I gratefully acknowledge comments and suggestions by Vincent Courtillot, Joseph Kirshchvink and one anonymous reviewer, as well as Jason Morgan, Norman MacLeod and Sigal Abramovich. This study was supported by NSF Grant EAR-0207407. DSDP/ODP samples were provided by the Ocean Drilling Program.[VC]
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