  Of basic interest is the quantification of the long term growth of a
language's lexicon as it develops to more completely cover both a culture's
communication requirements and knowledge space. Here, we explore the usage
dynamics of words in the English language as reflected by the Google Books 2012
English Fiction corpus. We critique an earlier method that found decreasing
birth and increasing death rates of words over the second half of the 20th
Century, showing death rates to be strongly affected by the imposed time cutoff
of the arbitrary present and not increasing dramatically. We provide a robust,
principled approach to examining lexical evolution by tracking the volume of
word flux across various relative frequency thresholds. We show that while the
overall statistical structure of the English language remains stable over time
in terms of its raw Zipf distribution, we find evidence of an enduring `lexical
turbulence': The flux of words across frequency thresholds from decade to
decade scales superlinearly with word rank and exhibits a scaling break we
connect to that of Zipf's law. To better understand the changing lexicon, we
examine the contributions to the Jensen-Shannon divergence of individual words
crossing frequency thresholds. We also find indications that scholarly works
about fiction are strongly represented in the 2012 English Fiction corpus, and
suggest that a future revision of the corpus should attempt to separate
critical works from fiction itself.
