Boron isotopes and climate change

Boron has two stable isotopes, 10B and 11B. Like all isotopes of the same element, when boron is shifted from one host to another some fractionation between its isotopes is likely. In the case of boron being taken-up by planktonic foraminifera, their shells’ 11B/10B ratios correlate with the pH of seawater. Since the pH of the oceans is dominated by the effects of dissolved CO2, itself in equilibrium with the gas’s atmospheric concentration, boron isotope ratios in foram shells are a proxy for the greenhouse effect produced by carbon dioxide. This finding dates back to 1992, but has only recently been used. It is especially revealing for the period around the Eocene-Oligocene boundary (see Lead-in to icehouse conditions in July 2009 issue of EPN) when other evidence indicates that global cooling eventually allowed glaciers to grow on Antarctica and possibly at northern high latitudes (Pearson, P.N. et al. 2009. Atmospheric carbon dioxide through the Eocene-Oligocene climate transition. Nature, v. 461, p. 1110-1113). The boron data indicate a downward shift in atmospheric CO2 from around 1100 to 750 ppm by volume from 34.2-33.5 Ma, the lower value just preceding δ18O data for a rapid increase in polar glaciers. Oddly, δ11B then rises to levels suggesting a return to CO2 levels of >1000 ppm by volume at a time of constant high δ18O that show the survival of ice caps; perhaps a result of increased albedo forcing.

Impact cause for Younger Dryas panned again

In 2007 two dozen scientists presented evidence to suggest that onset of the Younger Dryas, extinction of many North American mammal species and the sudden end of the Clovis culture at 12.9 ka followed upper atmosphere explosions of cometary material (see Whizz-bang view of Younger Dryas and Impact cause for Younger Dryas draws flak in EPN of July 2007 and May 2008). The Clovis culture of North America, signified by superbly crafted stone spear points, occupied a narrow time range between 13.3 and 12.8 ka, i.e. up to the start of the Younger Dryas interstadial. Some Clovis occupation sites are buried by organic-rich soils. Remarkably, the original proposers of a catastrophic event (Firestone, R.B. and 25 others 2007. Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling. Proceedings of the National Academy of Sciences of the United States of America, v. 104, 16016-16021) claimed that the veneers contain magnetic microspherules, magnetic grains, iridium and nickel, charcoal, soot and polycyclic hydrocarbons, carbon spherules, fullerenes that trap helium with extraterrestrial isotopic proportions, glass-like carbon, and nanodiamonds. Missing from what looks like a supportive package are shocked minerals, which are the only materials formed uniquely by impact events.

Experts on extraterrestrial influences considered the team to be ‘over-enthusiastic’. In response Firestone and co-workers made replicate samples available for independent confirmation or refutation of their claims. This offer seems not to have been followed-up, but another large team recollected the black soil veneers from two of the same sites and 5 others of similar age (Surovell, T.A. and 8 others 2009. An independent evaluation of the Younger Dryas extraterrestrial impact hypothesis. Proceedings of the National Academy of Sciences of the United States of America, v. 106, p. 18155–18158). They focussed on the claim for magnetic spherules, using the same techniques as Firestone et al. (2007), yet failed to find anomalous peaks at the time of the Clovis demise and opening of the Younger Dryas massive global cooling. Their conclusion was, ‘ In short, we find no support for the extraterrestrial impact hypothesis as proposed by Firestone et al.’. However, Surovell et al. did find magnetic spherules before, during and after the interstadial event. In fact, magnetic spherules are quite common in many sedimentary settings and have a history of controversy. In the late 1980s Robert S. Foote, an oil explorationist claimed that many oilfields were associated with geomagnetic anomalies with distinctive short wavelength ‘signatures’. He became widely regarded as a crank. But he persisted and discovered the first tangible evidence for lifeforms that thrive at high temperatures in deep oil wells – shiny, tiny magnetic spherules made of magnetite (Fe3O4). Magnetotactic bacteria living in highly reducing conditions produce them to form magnetosome chains. Magnetosomes are also present in the brains of far-migrating birds, with connections to their remarkable feats of navigation.

Just when you think it’s going to turn out alright…

The millennium of Younger Dryas global cooling from 12.8 to 11.5 ka ago caught forager-hunters on the hop as they followed herds in the wake of the general glacial retreat after 18 ka. The shut-down of the Gulf Stream when high-latitude North Atlantic surface waters freshened may have occurred in a decade or so. The end of the YD marked the start of more modern conditions in the Holocene Epoch, when northward recolonisation resumed in earnest. Climate records, such as the δ18O proxy for air temperature in the Greenland ice cores, suggest long-term but ‘noisy’ climatic constancy. That is, until one spreads out the Holocene records. At around 8200 years ago is a 200-year downward ‘blip’ in temperature to well below the Holocene average and then recovery. The perturbation also shows up in a Newfoundland mire (Daley, T.J. et al. 2009. Terrestrial climate signal of the “8200 yr B.P. cold event” in the Labrador Sea region. Geology, v. 37, p. 831-834) as a pronounced change in δ18O from moss cellulose. The event has been ascribed to slow-down in thermohaline circulation following a further freshening of North Atlantic surface water by drainage of a remaining ice-dammed lake (Lake Agassiz) on the Canadian Shield. By 8.2 Ka the northward spread of flora and fauna from refugia around the Mediterranean Sea was well underway, and included the arrival in southern Europe of Neolithic farming practices: the start of an agricultural revolution that was to reshape the entire sociocultural ethos of the ‘Old World’, from which today’s globalisation emerged. So it is interesting to learn that the ‘cold blip’ also left a signature at 41º N in northern Greece (Pross, J. et al. 2009. Massive perturbation in terrestrial ecosystems of the Eastern Mediterranean region associated with the 8.2 kyr B.P. climatic event. Geology, v. 37, p. 887-890). This study uses pollens collected from a lake-bed sediment core. The climatic event involved a rapid drop by 30 % in tree pollen abundances, matched by a 10% increase in pollen from shrubs, such as Artemisia (wormwood) normally associated with steppes further north. The end of the event involves a more sedate recolonisation by trees. From the pollen can be estimated the actual fall in winter temperature, which amounts to a devastating (for agriculture) decrease that was greater on average than 4º C. Interestingly, the German-French-Greek-Australian team ascribe some influence on the cooling to a spread of the Siberian High, a winter build-up of cold air on the steppes to the north of the Carpathians. The magnitude and extent of the Siberian High depends to a large extent on the albedo of the steppes in winter, which depends on snow cover and its persistence. This is a major influence today across much of Western Europe, as cold Siberian air spills from the continental anticyclone. At 8.2 ka it may have forced katabatic winds through Carpathian passes to cause winters that may have devastated the early farmers of northern Greece.


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