First off, welcome to the forum.
A couple of ideas come to mind. The first is that you might want to consider controlling the power to the SD card with a free I/O line and a transistor, only powering it up when you need to write to it. The other idea is to store multiple samples in an array (which should survive going into low power mode and back) and write them to the SD card every hour or more. How many samples to store between writes to the SD card would depend on your confidence that you wouldn't lose power or have a reset condition before having a chance to move them to the card.