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The fightback cometh … all of a quiet

7 March 2012

Not CAGW magic tricks this time but the YD boundary impact hypothesis has suddenly made a comeback – in PNAS (see www.pnas.org/cgi/doi/10.1073/pnas.1110614109 or http://cosmictusk.com/pnas-evidence-from-central-mexico-supporting-the-Y…). Further articles from new authors on the subject, including scientists from a variety of disciplines, are due to publish shortly, a surprising and unexpected bounce back after all the angst of last year.

The first fusilade comes from a paper by Isobel Israde-Alcontara and Gabriela Dominguez-Vasquez (Mexico) and Ho-Chun Li (Taiwan) et al (a whole list of collaboratores). Basically, at Lake Cuitzeo in central Mexico a black carbon rich lacustrine layer has been found – containing among other goodies, nano-diamonds and micro-spherules etc. The were found in a 27m long sediment core as part of an attempt to extract a palaeo-climate record back into the Ice Age. They discovered a 10cm thick layer at a depth of 2.8m that they say dates back to 12,900 years ago, the Younger Dryas boundary. Further, it coincides with a suite of accretions coeval with environmental and biotic changes independently recognised in other Mexican lake sequences. Collectively, these changes have produced the most distinctive boundary layer in the Late Quaternaryrecord, they add, and it seems to be a developing meme to look at the Younger Dryas event as something rare when in fact it can be compared quite amicably with the Heinrich events. I can see why they want to do this as they want to make the YD a special event – otherwise the criticism would revolve around the possibility of seven extraterrestrial impacts in the last 100,000 years, there being seven Heinrich events. This means an impact event would be harder to sell – but what if it was reality?

The layer contains an assemblage of impact related markers immediately beneath a layer containing the highest peak of charcoal in the sediment core, and the discovery is consistent with the YD boundary at other sites in North America, Greenland, and Europe, it is argued – which is probably the result of input from the et al component (see also www.sciencedaily.com/releases/2012/03/120305160814.htm).

However, reading the actual article itself something very different may have emerged from the lake sediment core – something quite exciting but as yet unexplained. The YD boundary event has been set at 2.8m via the use of C14 methodology. However, when refining the model, from earlier cores as well as the one under study, a 20cm tephra layer was found at 4.7 to 4.5m that originated from a nearby volcano, Los Axufres, which by various means is dated at 31,000 years ago. This tephra was used as an anchor, a point of synchronism with data other than lake cores. The author then adds, the data between 9m to 3.35m and from 2 to 0m shows a consistent linear increase with depth ranging in age from 46,000 years ago to the present. The sample at 3.35m and 1.95m have C14 calibrated ages of 18.8 thousand and 9.9 thousand years ago consistent with the linear interpolation of the rest of the core. However, and here is the 'something yet to be explained', six samples between these two levels provides radiocarbon ages older than the interpolation predicts. They represent a major radiocarbon reversal of thousands of years, with older sediment apparently overlying younger, a situation indicating a reworking of older organic material. The reversal begins with a date of 18.8 thousand years ago and drifts to an anomalously older date – normalising at 9.9 thousand years ago, somewhat higher in the core sequence. Within this anomalous part of the core, total organic carbon content is very high. Although it yields a date of 32 thousand years ago liner interpolation indicates it should date 13,000 years ago. Rather, that is the lowest estimate. The source of the old carbon has yet to be identified but we may note this anomaly could perhaps have been caused by the hypothetical impact event at 12,900 years ago, and this is the line taken by the authors of the article. The idea of course is to emphasize the anomaly at the YD boundary but the period also includes whatever it was that brought the Late Glacial Maximum to an end, somewhere between 18 thousand and 15 thousand years ago. We may also wonder how reliable are all those C14 dates at this time elsewhere in the world – were they affected by the same old carbon mixture?

The YD boundary event is not solely fixed by radiocarbon, it should be added, as the authors mention biostratigraphy was used to determine its placement – including pollen sequences from the lake and comparing them with other pollen sequences elsewhere. They found that climate change associated with the Younger Dryas could then be identified in various lakes in Mexico, Guatemala, Costa Rica, Columbia, Ecuador, and Peru.

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