A new paper in Earth and Planetary Science Letters demonstrates that the water around the Antarctic recorded the isotopic signature of a warm period peaking around 750 years ago, and ending about 500, commonly called the Medieval Warm Period (MWP).
Calcium carbonate can crystallize in a hydrated form as ikaite at low temperatures. The hydration water in ikaite grown in laboratory experiments records the δ18O of ambient water, a feature potentially useful for reconstructing δ18O of local seawater. We report the first downcore δ18O record of natural ikaite hydration waters and crystals collected from the Antarctic Peninsula (AP), a region sensitive to climate fluctuations. We are able to establish the zone of ikaite formation within shallow sediments, based on porewater chemical and isotopic data. Having constrained the depth of ikaite formation and δ18O of ikaite crystals and hydration waters, we are able to infer local changes in fjord δ18O versus time during the late Holocene. This ikaite record qualitatively supports that both the Medieval Warm Period and Little Ice Age extended to the Antarctic Peninsula.The paper has been put out as evidence of a global warm period during the middle ages (MWP). There has been a great debate about the existence and scope of the MWP. It is quite clear from historical records that it was a good time in Europe. Wine grapes grew in England, crops were grown in Iceland, many contemporary writers commented on the good conditions.
|Fig. 6 from Liu et al, 2012, showing δ180 from ikaite in green, and other paleoclimate data.|
This method works because the unique rock formed in cold waters (ikaite) traps and records the 18O concentrations of seawater it forms in. Seawater becomes higher in the heavy isotope of oxygen in cold times lower during warm times because ice receives higher concentrations of the light isotope (because the heavy isotope is stripped out preferentially by precipitation before it reaches the poles). So, it essentially records the ice volume.
Because the surface ocean is fairly well mixed on a few year time scale, the Antarctic records almost essentially the same world wide decrease as water elsewhere in the world. This record demonstrates that the world ocean recorded a substantial period when the amount of ice in the world was at a substantially lower amounts than common, and perhaps less than exists now. It doesn't necessarily demonstrate that every region of the world was warmer than usual, but it strongly suggests that the world was warmer than average overall.