Evidence for the Recovery of Terrestrial Ecosystems Ahead of Marine Primary Production Following a Biotic Crisis at the Cretaceous-Tertiary Boundary
Beerling, D. J., Lomax, B. H., Upchurch, G. R., Jr., Nichols, D. J., et al., Journal of the Geological Society
The fossil record demonstrates that mass extinction across the Cretaceous-Tertiary (K-T) boundary is more severe in the marine than the terrestrial realm. We hypothesize that terrestrial ecosystems were able
to recover faster than their marine counterparts. To test this hypothesis, we measured sedimentary delta^sup 13^C as a tracer for global carbon cycle changes and compared it with palaeovegetational changes reconstructed from palynomorphs and cuticles across the K-T boundary at Sugarite, New Mexico, USA. Different patterns of perturbation and timescales of recovery of isotopic and palaeobotanical records indicate that the delta^sup 13^C excursion reflects the longer recovery time of marine versus terrestrial ecosystems.
Keywords: Cretaceous, Tertiary, biodiversity, carbon isotopes, cuticles, palynology, productivity.
The end of the Cretaceous 65 Ma ago was followed by one of the five largest mass extinction events to occur in Earth history and was associated with major changes in the global environment, possibly resulting from an extraterrestrial impact event and the eruption of the Deccan Traps in India (reviewed in Ryder et al 1996). Plant extinctions at the K-T boundary were not on a global scale, but were locally severe in southern North America (Wolfe & Upchurch 1986; Johnson 1992). In contrast, mass extinction in the oceans was worldwide and more severe (e.g. c. 90% for planktonic foraminiferal species) (D'Hondt et al 1996), with a final recovery of marine ecosystems requiring the evolution of new species at multiple trophic levels over a period of perhaps 3 million years (D'Hondt et al 1998). Consequently because of the differences in extinction severity, terrestrial ecosystems close to the impact site were better placed to affect a more rapid recovery following environmental changes across the K T boundary than their marine counterparts - a situation aided by the potential supply of a ready influx of propagules from refugia to the ecologically disrupted area (Tschudy et al. 1984).
Intimately linked with this extinction event was a major change in the global carbon cycle. Stable carbon isotope (delta^sup 13^C) analyses of planktonic foraminifera show a negative excursion following the K-T boundary event, and a collapse in the delta^sup 13^C depth gradient (Arthur et aL 1979; Boersma & Shackleton 1981; Shackleton & Hall 1984; Hsu et al 1982; Zachos & Arthur 1986; Stott & Kennett 1989; Zachos et aL 1989; D'Hondt et aL 1998). Collectively, these observations indicate severe disruption of the global carbon cycle, probably involving a reduction in marine primary productivity following mass extinction. A shutdown or reduction in the uptake of ^sup 12^C by photosynthetic phytoplankton, and increased biomass burning (Wolbach et aL 1988; Ivany & Salawitch 1993), would together lead to an accumulation of ^sup 12^CO^sub 2^ in the atmospheric reservoir. Evidence for the transfer of this ^sup 12^CO^sub 2^ from the atmospheric to the terrestrial biosphere reservoir, via photosynthesis, is seen as a negative excursion in the delta^sup 13^C records from terrestrial biomarkers in Europe (Arinobu et al 1999), and bulk terrestrial organic matter from sites in North America (Schimmelmann & DeNiro 1984; Arens & Jahren 2000).
To test the hypothesis that terrestrial ecosystems were able to recover faster than their marine counterparts we have reconstructed changes in palaeovegetation, from fossil palynomorphs and dispersed plant cuticles, and measured delta^sup 13^C shifts in bulk organic matter from fluvial floodplain and backswamp deposits of the Raton Formation in the Raton Basin, New Mexico, USA. The Sugarite locality in New Mexico contains a K-T boundary claystone layer, characterized by an iridium anomaly and shock-metamorphosed minerals (Pillmore et al 1984, 1999; Nichols et al. 1985; Izett 1990), all of which have been considered representative of the signature of a terminal Cretaceous impact event. …