Annually-resolved multi-parameter assessment

CEZA along with partners of the Johannes Gutenberg University Mainz, Geo-Forschungszentrum Potsdam and University of Hohenheim want to uncover the climate variability and solar activity at the time of the 8.2k event. Radiocarbon and stable isotope analysis will be used on wood from tree-rings for climate reconstruction. This project is funded by the DFG starting by the end of 2021.

Participants: Alexander Land, Ronny Friedrich, Gerhard Helle, Denis Scholz, Jan Esper.

Abrupt climate fluctuations occurred multiple times during the Holocene. They are of special interest to study the role of internal (e.g. North Atlantic Ocean circulation) and external (e.g. solar, volcanic) forcing. These events are studied in various archives, such as lake sediments, polar ice cores, tree rings, pollen profiles and marine sediments. To identify causes and effects accurate time scales of these archives are crucial, with radiocarbon (14C) the common backbone (except for ice cores) of chronology. 

One of such a prominent climate relapse happened ~8200 years before the year 1950 CE (henceforth 8.2 ka BP event) and vast parts of the northern hemisphere suffered from this cooling event. In central Greenland temperatures decreased by 4–8 K (Alley et al., 1997) and in the northeastern part of the North Atlantic temperatures dropped by ~2 K (Bond et al., 1997; Grafenstein et al., 1998). The sudden catastrophic drainage of the Laurentide Lakes Agassiz and Ojibway combined with Hudson Bay Ice Saddle collapse have been considered as triggers, both freshening the North Atlantic.The particular causes, as well as potential implications for terrestrial ecosystems may be more complex than previously thought. Some other triggers might have played a significant role, including decreasing solar activity (Bond et al., 2001) and internal climate system variability (Renssen et al., 2007). 

The 8.2 ka BP cold event is widespread and evident in many marine and terrestrial climate proxy records around the North Atlantic as well as in continental Europe (Fig. 1). Nonetheless, well-dated high-resolution pollen records revealed a spatial structure in northern Europe in-duced by particularly pronounced cold temperatures during winter and spring with different effects on vegetation along the gradient from the southern arctic to the temperate boreal zone (Seppä et al., 2007). The forcing and oceanic spatial extent of the 8.2 ka BP event is still deeply investigated and is being debated intensively e.g. regarding the triggers causing the rapid globally climate fluctuation, the spatio-temporal dynamics in different regions at high temporal resolution and the sun`s role prior and during the event. However, our knowledge pertains to the temporal dynamics on annual (Estrella-Martínez et al., 2019) / seasonal precision and is limited due to insufficient temporal resolution and dating uncertainty in most of the marine and terrestrial records (see also Fig. 1). This leads to the fact that very little is known about the precise onset, duration, ending and temporal persistence of the 8.2 ka BP event. In addition, hydroclimate variability has only rarely been examined for central Europe (e.g. Land et al., in review) and if so, temporal resolution is not satisfactory. Therefore no conclusions can be drawn about e.g. the year-to-year precipitation variability, short- and long-term shifts and warm-season vs. off-season dynamic prior and during the event (Waltgenbach et al., 2020). Changes in the thermohaline circulation caused by the freshwater influx from the final collapse of the Laurentide Ice Sheet and the cooling event was probably superimposed upon a longer-term cooling trend (Rohling and Pälike, 2005). Studying the 8.2 ka BP event dynamics with annual resolution and dating accuracy is crucial to understand stratosphere-troposphere inter-action, present-day freshening of the North Atlantic and thus will contribute to our understanding of future climate change.