The role of terrestrial vegetation in abrupt climate change during last Dansgaard-Oeschger cycle and Heinrich Event 1 according to palynological data

Abstract:

The role of vegetation in abrupt climate change during Dansgaard-Oeschger (D-O) cycle and Heinrich event 1 (HE1) is not well understood. There is a discordance between the rise in global CO2 since the LGM, and the response of vegetation, as depicted by pollen data. Here I examine if the vegetation signal is seen as not only responding to abrupt change but preceding it.

I analyzed pre- and post-HE1 pollen signals along a tropic to subpolar transect, using my own and published data. The general trend in biomes does not demonstrate either a strong linear spatial correlation with the CO2 records, or steady-state conditions. On the contrary, most of the pollen assemblages demonstrate a similar pattern. The most prominent feature is the appearance of saw-tooth-like (s-t) shapes in the principal variables (Trees/Herbs/Spores), independent of geographical factors. The most noticeable of these s-t shapes occur at ~18-17 ka and after HE1, at ~14 and 12 ka, which roughly corresponds to the Bølling/Allerød (Bø/Al) to Younger Dryas transition and are coherent with the temperature trend during HE1.

However, while all locations show the common chaotic features of a s-t, the trend of changes is different: at high and middle latitudes, the cold desert or steppe vegetation is replaced by forest while at low latitudes, the forest is diminishing. Further, the magnitude, timing, and response of biomes to HE1 differs according to the distance from the retreating ice sheet and the ice-rafted debris (IRD) event itself. The subsequent shutdown of the Atlantic meridional overturning circulation (AMOC) affects the response of land vegetation to the incoming climatic change. These changes are linked to the change in the ratio of C3/C4 type vegetation in response to rising atmospheric CO2. At any meridional location, during a D-O cycle, the vegetation experiences strong stress after the cold IRD event and the onset of AMOC shutdown. The universal sequence of palynological signals through HE1 is 1) stress on C3 plants; 2) short dominance of C4 plants; 3) s-t behavior during the Bø/Al; and 4) final takeover of vegetation by C3 plants in the Holocene. It is hypothesized that the role of vegetation in the regulation of CO2 in the atmosphere during periods of abrupt climate change can be a principal driving force in both the destruction and stabilization of the system.