In formal nonfiction genres like encyclopedias and academic writing, parenthetical citations and footnote references are common. Some examples:

• It has a long history in scholarship (Baron & Beresford 2014), but many key aspects of its production and significance have not yet been fully understood.
• Indeed, some commentators have called replication the “gold standard” of research evidence (Jasny, Chin, Chong, & Vignieri, 2011, p. 1225) and a “linchpin of the scientific process” (Let’s replicate, 2006, p. 330).
• Climate change is one of the priorities of the Sixth Community Environment Action Programme, laid down by Decision No 1600/2002/EC of the European Parliament and of the Council [3].
• As an example, research shows that greater diversity within a community or group can lead to improved critical thinking [1] and more creative solutions to complex tasks [2, 3] by...

UniversalDependencies/UD_English-ParTUT/issues/39 and other issues have asked for a standard.

• External relation: At the Core Group it was discussed and determined that parataxis seems like the best fit for the universal relation for attaching the citation/reference to the rest of the sentence, as these are a kind of parenthetical. (A few treebanks subtype the parataxis relation: https://universaldependencies.org/survey-deprel.html#parataxis)

• Internal structure: For references with complex structure, the English nmod:unmarked guidelines on locations and dates may be helpful: https://universaldependencies.org/en/dep/nmod-unmarked.html

• To ensure these particular kinds of intertextual entity can be found later, some English treebanks will start using ad hoc MISC features Citation=Yes and Footnote=Yes, but these are not required.