Understanding the 4.2ka climate event using new proxy records and model experiments

The Holocene epoch has been shown to contain many abrupt climate change events. These are of great interest as the continental configuration of the planet has not changed in this time, allowing the Holocene to provide a window into how our current climate system responds to different perturbations. This study aims to try and understand the causes and extent of one instance of abrupt climate change, the 4.2kyr event. This has particular relevance to the modern day as it occurred subsequent to the melting of the Laurentide ice sheet, and therefore cannot be attributed to a melt water pulse entering the North Atlantic (as other climate fluctuations during the Holocene have been). This event, which is thought to have caused severe enough droughts in central Asia to result in the collapse of the Akkadian empire, was therefore forced under conditions very similar to the modern climate day. As such, it is crucial to understand the causes of this event as a similar mechanism could trigger another severe climate perturbation in the future.
The short duration of the 4.2kyr event, (<300 years), makes it difficult to observe in many climate records. The resolution of the record would need to be of decadal or higher resolution to accurately capture it. These records also need to be globally distributed and located within the Earth's major climate systems in order to explore the global impact and extent of this event. This study will focus on stalagmite records which, due to their potential for sub-decadal resolution, ability to be absolutely dated and global distribution, are ideally suited to studying this event.
This study will locate relevant samples from the world's major climate systems in order to create a global picture of the extent and effect of the 4.2kyr event. These will be used to reconstruct site rainfall at decadal-scale resolution across a 3000-year period centred at 4.2 kyr. This 3000-year period will allow for the quantification of any perturbation relative to the "background level" of the climate prior to and post this event.
Several proxies will be used to investigate this climate event. Firstly, oxygen isotopes (18O) will be measured. These are commonly used in speleothem paleoclimate reconstruction; however, their interpretation is complex. 18O values are affected by many variables, such as changes in regional moisture source, percentage rainout and temperature. In order to understand the oxygen isotope signal, isotope sensitive climate models will be used to assess the pattern of rainfall and climate variations recorded in the stalagmites. In addition, trace-metal concentrations and Ca isotopes will also be measured from each record. These both serve as proxies for changes in rainfall, with Ca isotopes able to quantify a change in rainfall amount. As such, all three of these proxies will be combined to create a reliable record of past changes in rainfall and moisture source above the cave sites.
The consideration of a global suite of records will allow this study to observe whether the 4.2kyr was an anomalous event and, if so, what the global response of the climate system was to this event. By using the distribution of any change observed, and the help of climate and isotopic modelling, this study will assess possible mechanisms that could have resulted in this pattern of change. This will hopefully aid the full understanding of how quickly and by what mechanism the current climate state can change