The significance of the discovery is that the water within the ikaite crystal lattice contains oxygen-18 (d18O) isotopes which are a proxy for temperature.  Therefore, analysis of the ikaite can give a record of the ambient temperature at the time the mineral formed.

According to a press release from Syracuse University:

Ikaite crystals incorporate ocean bottom water into their structure as they form. During cooling periods, when ice sheets are expanding, ocean bottom water accumulates heavy oxygen isotopes (oxygen 18). When glaciers melt, fresh water, enriched in light oxygen isotopes (oxygen 16), mixes with the bottom water. The scientists analyzed the ratio of the oxygen isotopes in the hydration water and in the calcium carbonate. They compared the results with climate conditions established in Northern Europe across a 2,000-year time frame. They found a direct correlation between the rise and fall of oxygen 18 in the crystals and the documented warming and cooling periods.

The abstract from the research paper reads:

Calcium carbonate can crystallize in a hydrated form as ikaite at low temperatures. The hydration water in ikaite grown in laboratory experiments records the d18O of ambient water, a feature potentially useful for reconstructing d18O of local seawater. We report the first downcore d18O 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 d18O of ikaite crystals and hydration waters, we are able to infer local changes in fjord d18O 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.

Note that the Medieval Warm Period and Little Ice Age disappeared in Michael Mann’s infamous hockey stick graph and from IPCC reports.  This discovery is another piece of physical evidence that the Medieval Warm Period, a time when global temperatures were as warm or warmer than today, was indeed a worldwide phenomena.  It is also another piece of evidence that recent warming is nothing unusual.

See also:

Norwegian research shows that current warming is not unusual

More evidence that current warming is not unusual