Cosmogenic dating definition


04-Aug-2019 13:34

The Mid-Holocene Transition, caused by orbital variations, brought a change in climatic mode, from solar to oceanic dominated forcing. The Blytt-Sernander sequence fell out of fashion in the 1970s when new techniques allowed a more quantitative reconstruction of past climates. The Early Holocene, up to the 8.2 Kyr event, the Middle Holocene, between the 8.2 and the 4.2 Kyr events, and the Late Holocene since the 4.2 Kyr event.

This transition displaced the climatic equator, ended the African Humid Period and increased El Niño activity. However, it captures the essence of Holocene climate as four periods of roughly 2500 years each. Although this is currently the most popular subdivision, in my opinion, it fails to properly capture the climatic trends of the Holocene.

The answer is the huge thermal inertia of the planet due mainly to its water content. But instead of an increase in temperatures, what we find is a progressive decrease from the HCO to the LIA driven by changes in insolation. Temperature and greenhouse gases changes during the Holocene.

21,000 years ago the increasing obliquity had been adding energy to the poles for 10,000 years, reducing the insolation latitudinal gradient (Raymo & Nisancioglu, 2003), and adding energy to the summers (Huybers, 2006; Tzedakis et al., 2017), and was on its way to overcome the huge cold inertia with the help of precession changes that were about to take place. Black curve, global temperature reconstruction by Marcott et al., 2013, as in figure 37. Red curve, CO levels as measured in Epica Dome C (Antarctica) ice core, reported in Monnin et al., 2004.

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This climatic optimum was when global glaciers reached their minimum extent. Finally other authors divide the Holocene in three periods.N (red) or S (blue) are the Northern or Southern Hemisphere and the three letters are the month initials. Although the annual insolation change is not too large, it accumulates over tens of thousands of years and the total change is staggering, creating a huge insolation deficit or surplus. There is only one Holocene global average temperature reconstruction available (Marcott et al., 2013; figure 37 a).