Oldest Dryas hydroclimate reorganization in the eastern Iberian Peninsula after the iceberg discharges of Heinrich Event 1

This study examines the first precisely dated and temporally highly resolved speleothem record from Iberia that reconstructs the Oldest Dryas (OD). The onset of cold conditions in the study area, contemporary with the beginning of Heinrich Stadial 1, is recorded at 18.13 ± 0.08 ka, with a pronounced drop of 6.1‰ in δ13C in 250 years. Henceforth, stadial conditions depict a period of instability in the Atlantic Meridional Overturning Circulation, peaking in freshwater input from iceberg melting during Heinrich Event 1. Anomalies in the δ18O of the stalagmite attributed to such a freshwater event are found from 16.17 to 15.89 ka. Such absolute dates given to the onset of the OD in Iberia and to the main iceberg discharges are reliable anchor points for non-absolute chronologies. Two periods are identified in the OD: OD-a (18.13–16.17 ka) is characterized by wet conditions and a faster growth rate, and OD-b (15.89–14.81 ka) exhibits relative dryness and a slower growth rate. The sudden release of fresh water is considered to be the reason for the disruption of rainfall patterns in eastern Iberia. The present study also highlights the existence of heterogeneous and complex hydrological conditions during the OD in Iberia when both Atlantic and Mediterranean realms are considered.

Sequence of Heinrich Event 1 to the Bølling-Allerød in transient climate model simulations

<p>During the last deglaciation, the North Atlantic was punctuated by evident millennial-scale climate variability – surface cooling during Heinrich Event 1 (H1), followed by abrupt warming during the Bølling-Allerød (BA). Given its abundance of available proxy records, the last deglaciation is thus a perfect testbed for us to assess the triggering dynamics of these abrupt events. Here, a water-isotope enabled, coupled atmosphere-ocean general circulation model COSMOS-wiso (Werner et al., 2016) is applied to test different mechanisms potentially responsible for a BA abrupt warming. First, two sets of experiments are conducted to test the sensitivity to background boundary conditions: one is based on the Last Glacial Maximum (LGM), and the other was 16ka BP background climate. We also consider the spatial distribution of freshwater flux (FWF) forcing. We find that during the LGM a weak freshwater forcing cannot trigger an Atlantic Meridional Overturning Circulation (AMOC) mode transition. However, the same freshwater forcing can rapidly weaken the AMOC at 16ka BP, including an abrupt AMOC resumption in the subsequent one thousand years. Our experiments support the idea that ice volume plays a dominant role in the stability of AMOC during the termination. Furthermore, we explore the impact of different initial fields on the timing of AMOC recovery. Based on the above 16ka hosing experiment mimicking H1, several phases before the AMOC recovery are selected as initial fields, also with different FWF forcing. Our experiments indicate that the larger the FWF forcing, the longer it would take for the AMOC to recover. In all simulations, we detect an overshoot behavior typically for the BA transition. Finally, we implement a transient experiment from H1 to BA with changing GHGs and orbital forcing to explore the mechanisms of the sequence of rapid climate changes during the last termination.</p><p>Werner, M., Haese, B., Xu, X., Zhang, X., Butzin, M., and Lohmann, G.: Glacial–interglacial changes in H218O, HDO and deuterium excess – results from the fully coupled ECHAM5/MPI-OM Earth system model, Geosci. Model Dev., 9, 647-670, doi:10.5194/gmd-9-647-2016, 2016.</p>

Climate warming and vegetation response after Heinrich event 1 (16 700–16 000 cal yr BP) in Europe south of the Alps

Chironomids preserved in a sediment core from Lago di Origlio (416 m a.s.l.), a lake in the foreland of the Southern Swiss Alps, allowed quantitative reconstruction of Late Glacial and Early Holocene summer temperatures using a combined Swiss–Norwegian temperature inference model based on chironomid assemblages from 274 lakes. We reconstruct July air temperatures of ca. 10 °C between 17 300 and 16 000 cal yr BP, a rather abrupt warming to ca. 12.0 °C at ca. 16 500–16 000 cal yr BP, and a strong temperature increase at the transition to the Bølling/Allerød interstadial with average temperatures of about 14 °C. During the Younger Dryas and earliest Holocene similar temperatures are reconstructed as for the interstadial. The rather abrupt warming at 16 500–16 000 cal yr BP is consistent with sea-surface temperature as well as speleothem records, which indicate a warming after the end of Heinrich event 1 (sensu stricto) and before the Bølling/Allerød interstadial in southern Europe and the Mediterranean Sea. Pollen records from Origlio and other sites in southern Switzerland and northern Italy indicate an early reforestation of the lowlands 2000–1500 yr prior to the large-scale afforestation of Central Europe at the onset of the Bølling/Allerød period at ca. 14 700–14 600 cal yr BP. Our results suggest that these early afforestation processes in the formerly glaciated areas of northern Italy and southern Switzerland have been promoted by increasing temperatures.