The Little Ice Age CO2 drop: Natural, Anthropogenic or Artefact?

We present new measurements of methane (CH4) and carbon dioxide (CO2) in the Skytrain ice core, with gas ages dated around 1610AD. The aim of these measurements is to improve our understanding of why there is a significant difference between measured CO2 at that time in current ice core records.

A pronounced feature of the Law Dome record (accumulation 60 cm ice eq. yr; gas age distribution 8 years,) is a rapid decrease in CO2 of ~10 ppm over 50 years with a distinct minimum at 1610. The cause of this decrease is much debated, with complex carbon cycle feedbacks required in explanation. However, other ice cores do not show the same event. The West Antarctic Ice Sheet (WAIS) divide record (accumulation 22 cm ice eq. yr; gas age distribution 19 years) shows a steadier decline in CO2 of approximately 6 ppm over the same period, with the record also ~2-3 ppm higher than Law Dome throughout 900-1800 CE. A follow-up study using the Dronning Maud Land (DML) ice core (accumulation 7 cm ice eq. yr; gas age distribution 65 years) attempted to prove which core showed the real atmospheric signal, but results were inconclusive due to the wide gas-age distribution of the record. While Skytrain (accumulation 14 cm ice eq. yr) does not match the accumulation rate of Law Dome, we present these new, high-resolution gas measurements over the period to work towards answering the following questions: (1) if the Law Dome record is correct, what caused this amplitude of CO2 change over a short timescale? (2) Does one of the records suffer from contamination? (3) Is our understanding of gas smoothing processes in these ice cores inaccurate? We will then use these measurements, from a well-validated ‘needle-crusher’ CO2 device at the ice core labs at Oregon State University, USA, to validate a new semi-continuous ice-grating device (for which we present a preliminary outline) at the new ice core gas analysis lab at the British Antarctic Survey, UK.