Palynological Criteria for Identification of Abrupt Climate Change During Heinrich Event 1

We consider the response of vegetation to abrupt climate change during Heinrich event 1(HE1). There is a conflict between the constant rise in global CO₂ since the LGM, and the vegetational response, as depicted by pollen data. To study this question, we analyzed pre- and post-HE1 pollen signals along a tropic to subpolar transect, using our own and published data. We find that the general trend in biomes does not demonstrate either a strong linear correlation, consistent with CO₂ records, or steady-state conditions. Despite regional differences, most of the pollen assemblages have a similar pattern. The prominent feature is the appearance of saw-tooth-like shapes in the variables of the pollen diagram, independent of site, vegetation composition and other factors. The most noticeable of these saw-tooths occurs at 18-17 Kya and after HE1, at 14 and 12 Kya, which roughly corresponds to the Bølling/Allerød to Younger Dryas transition. These oscillations are coherent with the temperature trend during the HE1. An important observation is that while all locations show the common chaotic features of a saw-tooth, the trend of changes is different: at high and middle latitudes, the cold desert or steppe vegetation is being replaced by forest while in tropical and subtropical latitudes, the forest is diminishing. We further observe that the magnitude, timing and response of biomes to HE1 differ according to the distance from the retreating ice sheet and the IRD event itself. The subsequent shutdown of the AMOC affected the response of land vegetation to the incoming climatic change. These changes can be better explained by the changing ratio of C3/C4 type vegetation in response to rising atmospheric CO₂. We speculate that at any meridional location, the vegetation experienced strong stress after the cold IRD event and the onset of AMOC shutdown. Palynological signals indicate chaotic disruption of the vegetation. The universal sequences of palynological events through HE1 is: 1) stress on C3 plants; 2) short dominance of C4 plants; 3) saw-tooth during the Bø/Al; 4) final takeover of vegetation by C3 plants in the Holocene. The role of vegetation in the regulation of CO₂ in the atmosphere is the target of next step of the investigation.