How often is PhD work linked to life after the PhD? So I am currently a Master's Student in an earth sciences field, looking to get into the PhD program at my university, provided that there is funding. My current adviser likely won't have funding for my PhD. Recently, my adviser told me about an opportunity with a different adviser, working on a topic in life sciences with an influence of earth science, but one which I may be qualified for. If I do this, is it likely that I can find jobs/postdocs in earth science, where my true interest lies, or would I be irreversibly altering my career path for life science?
It depends on the exact topic and what guidance the new research group will offer you. There's plenty of opportunities to develop expertise in life sciences during PhD. Your options after may not be as limited as you perceive. There is a lot of demand for collaborative and interdisciplinary researchers both in academia and industry.

I'm doing an interdisciplinary PhD between stats and genetics. While on paper you do have to enrol in one course and will be assessed on it. In practice, you can lean on the other field. For instance, with the methods I use. My research questions are biological but my methods differ from lab trained biologists.

You ought to still have room to develop your earth science skills in this way, particularly if your new research group collaborates with people in your field that can advise you on those aspects. You should have a wider range of an advisory committee and peers to assist you with work in both disciplines. A PhD is also an independent project, especially near the end, you should be able to build up confidence ants negotiate using some of your current skills if they tie in the research topic cohesively.

In short. Consider it, it may not be a one way track to biology. It's also not too late to apply to other labs.