However, the authors may have made conclusions about materials or processes that continue today and thus should be characterized in the present tense. However, if a paper describes specific actions or experiments that happened in the past, the past tense is appropriate. Don't confuse real things with inferences. For example, geologists often talk about facies, which are bodies of rock with uniform characteristics. They try to make inferences about those facies, and often they infer that certain depositional or diagenetic facies were deposited in or affected by certain depositional or diagenetic environments.
Problems arise when the two are confused.
He took the box, which was on the top shelf. The first sentence implies that there was more than one box, and he took one of them. Thus the clause is necessary to identify the particular box. The second sentence implies that there was only one box to begin with, and the clause just provides extra information. In the first pair, the second sentence is geological untrue.