The very long preservation of seeds and other artifacts of the current world requires a very safe deep freeze. In a warming world even the mountain top glaciers in the northern hemisphere are melting which means they are raising above freezing on some days. Antarctica is two miles deep in ice and at the south pole is -48°F or C so it won’t be melting anytime soon. I like to put specific numbers on things to keep things from getting ridiculously nebulous as they always do when talking politically so I choose 10,000 years for this purpose because that is how long humans have been cultivating seeds. Probably most cold tolerant species of plants, that is most of them from temperate climates, will store well and be viable for 10,000 years. A location at high altitude in Antarctica will probably remain well below freezing temperature for that period of time no matter how badly human beings behave. EarthArks placed at well-chosen high altitude locations should easily remain viable and safe without any further human intervention for that long a time period.
To give some position to the Mt. Tyree valley (-78.414, -86.00) that can be considered for an EarthArk on the north flank of The Vinson Massif. These are among the highest points in the mountains near the south pole and for mental positioning, due some 8,000 miles south of Indianapolis, IN, USA. The location has possibilities because it is very high latitude and very high altitude, so it should be very cold the year around for thousands of years on into the future. A location in this valley with shipping containers fastened to rock should stay put for thousands of years. There are glaciers in the area and the site must be chosen so even in the distant future one will not form and sweep the containers away. Directly under the label Mt. Tyree there is the altitude number 4852 and near the numeral which 4 should satisfy all of these desirable conditions.
This is a start for a good EarthArk location. No doubt better ones can be found by people actually going to the sites and checking for temperature, ice flow and ground stability.
The sooner even the most basic EarthArk is in position the better.