If there is a pile (say of leaves), and somebody drops an object into it on Shabbat, are they allowed to throw it into the wind so they can retrieve the object they just dropped? Does the prohibition of winnowing apply if what someone wants separated is something that immediately fell on Shabbat? Source(s) for this question would be appreciated.
Winnowing (zoreh) is similar to selecting (borer) in that it involves separating waste from food but uses the wind as its mechanism.
There are long discussions in halachic works regarding spitting in the wind, the use of sprays, scattering breadcrumbs off a table with some leniencies for these. However winnowing definitely applies to items that grow from the ground such as leaves and definitely applies when the person intends to scatter the items through the wind.
As such your scenario would be forbidden. The fact the object fell immediately in the pile is not relevant to these laws.
For more see R Yosef Tzi Rimon's book Shabbat, vol. 2 pp. 941-954.