As múltiplas causas do Dryas Recente (Younger Dryas)
O Dryas Recente foi um episódio climático frio que decorreu entre 12.800 e 11.500 anos antes do presente (BP) e interrompeu as condições amenas associadas à deglaciação. Os dados apontam para uma transição drástica que pode ter decorrido numa década (Alley et al. 1993) ou até menos tempo.
Fonte: Abrupt Climate Change: Inevitable Surprises, 2002. Figure 1.2
Há anos que a razão deste arrefecimento é debatida, alternando entre hipóteses de impactos extraterrestres (Firestone et al. 2007; argumentos contra em Paquay et al. 2009) e de alterações na circulação oceânica.
Esta semana dois trabalhos contribuem com mais duas explicações, uma para cada lado. Ou uma mega-inundação de água gélida nos oceanos Atlântico e Pacífico, resultante do derretimento da gigantesca massa gelada laurentidiana situada no norte do continente americano (Murton et al. 2010), que terá provocado alterações nas correntes oceânicas e sequentemente no clima global. Ou a queda de milhares de fragmentos de cometa num curtíssimo espaço de tempo, causando um arrefecimento global que pode ter ido até -8ºC (Napier in press).
Após ler os dois trabalhos, inclino-me para a primeira hipótese, que se assemelha aliás às propostas bem testadas para o posterior evento de 8.2 anos cal BP (c. 7.250 anos BP; Von Grafenstein et al. 1998).
Ainda sobre este assunto, refira-se um outro trabalho muito recente, de Broecker e colegas que sugere que rather than being a freak occurrence, the Younger Dryas is an integral part of the deglacial sequence of events that produced the last termination on a global scale.Referências:
Murton, J.B. et al (2010) - Identification of Younger Dryas outburst flood path from Lake Agassiz to the Arctic Ocean. Nature, 464 (7289): 740. DOI: 10.1038/nature08954
Sumário:
The melting Laurentide Ice Sheet discharged thousands of cubic kilometres of fresh water each year into surrounding oceans, at times suppressing the Atlantic meridional overturning circulation and triggering abrupt climate change. Understanding the physical mechanisms leading to events such as the Younger Dryas cold interval requires identification of the paths and timing of the freshwater discharges. Although Broecker et al. hypothesized in 1989 that an outburst from glacial Lake Agassiz triggered the Younger Dryas, specific evidence has so far proved elusive, leading Broecker to conclude in 2006 that “our inability to identify the path taken by the flood is disconcerting”. Here we identify the missing flood path—evident from gravels and a regional erosion surface—running through the Mackenzie River system in the Canadian Arctic Coastal Plain. Our modelling of the isostatically adjusted surface in the upstream Fort McMurray region, and a slight revision of the ice margin at this time, allows Lake Agassiz to spill into the Mackenzie drainage basin. From optically stimulated luminescence dating we have determined the approximate age of this Mackenzie River flood into the Arctic Ocean to be shortly after 13,000 years ago, near the start of the Younger Dryas. We attribute to this flood a boulder terrace near Fort McMurray with calibrated radiocarbon dates of over 11,500 years ago. A large flood into the Arctic Ocean at the start of the Younger Dryas leads us to reject the widespread view that Agassiz overflow at this time was solely eastward into the North Atlantic Ocean.
Napier, W. M. (in press) - Paleolithic extinctions and the Taurid Complex. Monthly Notices of the Royal Astronomical Society. arXiv:1003.0744v1
Sumário:
Intersection with the debris of a large (50-100 km) short-period comet during the Upper Palaeolithic provides a satisfactory explanation for the catastrophe of celestial origin which has been postulated to have occurred around 12900 BP, and which pre- saged a return to ice age conditions of duration ∼1300 years. The Taurid Complex appears to be the debris of this erstwhile comet; it includes at least 19 of the brightest near-Earth objects. Sub-kilometre bodies in meteor streams may present the greatest regional impact hazard on timescales of human concern.
