Speakers
John Lowe presenting the developments in the past 50 years of geochronology

John Lowe presenting the developments in geochronology over the past 50 years

John Lowe (Royal Holloway University of London, Surrey)

Mike Walker (Aberystwyth University, Aberystwyth)

Chris Bronk-Ramsey (University of Oxford, Oxford)

Siwan Davies (Swansea University, Swansea)

 

Why are dating methods important?

In order to successfully reconstruct past climate and environmental change it is vital to understand when these changes occurred, and construct a ‘chronology’. Our ability to date environmental change is reliant on the quality of the dating methods we use. Developing accurate and precise dating methods allow for the best control on the timing of changes, and the best opportunity to understand leads, lags, and correlations within climatic and environmental systems. Advances in our dating methods have also allowed us to date different materials (e.g. organics, sand, bone, rocks), which are important if we want to study a range of environments. To use geochronology in environmental science successfully, there needs to be synergy between the chronological tool and the environmental tool in which it is being used. For example, we need to ensure that the dating method that we use is appropriate for the environment and the material that we are trying to date.

What have been the major developments in geochronology?

Over the past 50 years there have been major developments in dating techniques, and a number of important paradigm shifts. These improvements have been methodological – through developments in our understanding of how techniques should be used.

Radiocarbon dating is one of the most important geochronological tools, used for a wide range of different organic materials within many environmental settings. Since the discovery of radiocarbon dating by Willard Libby in 1949, the method has been revolutionised.

The main revolutions have been:

  • Calibration of radiocarbon dates: using tree rings and detailed records from Lake Suigetsu (http://www.suigetsu.org/), the Bahamas, and Hulu cave, radiocarbon dates can be securely calibrated. Marine records have also allowed calibration back to 50 ka BP.
  • The development of accelerated mass spectrometry (AMS) has allowed small samples to be dated, as well as different types of sample material.
  • The use of Bayesian statistics in radiocarbon dating has allowed precise age-depth models to be generated for sequences, allowing more robust comparison of records.
  • The development of chemical methods for radiocarbon dating has helped to push the upper limits of achievable ages to 50 ka BP.

The long marine oxygen isotope stratigraphy (developed by Imbrie and Shackleton) has been one of the most influential developments in geochronology. Its use has actually led to major developments in how we both interact with, and interpret environmental data. Ice core records from Antarctica and Greenland have been very influential in understanding past climatic changes. Ice cores have annual resolution (each layer represents a year of snow deposition and compaction into ice), allowing us to look at changes over decadal and sometimes annual timescales. Other records and dating methods are now attempting to emulate this high precision. These developments have given records the highest possible precision, led to the development of integrated age models, and allow for inter-calibration.

Tephrachronology is a recently developed method, and has the potential to link multiple records across a wide geographical area. The use of tephra emerged in the 1970s and 1980s through work recognising Icelandic tephra in UK peat sequences. The method became more useful with the development of electron microprobe analysis, and the use of laser ablation ICP-MS to geochemically fingerprint different trace elements from tephra shards. Tephrachronology not only works to provide a chronology for sequences, but the presence of tephra in multiple sequences can allow climatic leads and lags to be investigated in high precision.

What are the major challenges?

  • Methodological progress will continue to be made in the future, improving the utility of dating methods.
  • For radiocarbon dating the use of atmospheric, marine, and reservoir radiocarbon data together will allow us to better understand the global cycle of carbon in the past. These developments will reduce the errors margins on the chronology (which, at the moment, are frequently centennial or millennial), to improve precision.
  • In the past, aligning records to one another – or wiggle-matching – has been used. This is problematic as it can remove variability between records.
  • Future research will use other methods – possibly tephrachronology – to correctly correlate records based on precise, independent evidence.