The very strong caveat that I don't know much about PIE linguistics is in effect, I'm mostly just throwing out ideas to see if they help those who actually know what they're talking about.
kanejam wrote:As Tropylium said, Kortlandt points out that roots with mobile stress tend to have voiced aspirates, whereas static stress tend to have voiceless. Unfortunately they give no examples or figures and I'm sure there are plenty of exceptions, but it is tantalising to connect the stop system with the accent system. This is possibly backed up by a handful of correspondences such as *-bhi (a case/adverbial suffix) with *pi (a preposition? this may not actually exist, I'm drawing from memory) but has a bit of trouble explaining the large number of voiceless consonants in unstressed suffixes.
Original /t 'd/ system, older layer of words with mobile stress and productive voicing of t>d (traditional t>dh), newer layer of words with fixed stress and no voicing rules? The voiceless consonants in unstressed syllables would then be a younger generation of grammaticalization. Don't know if that can be matched up with other theories about the relative chronology of things, though.
Along with connecting the IE stops with the single and geminate stops in Uralic, I have also seen an attempt to connect them with the Uralic nasals, of which there are more than in IE and could conceivably come from voiced stops.
Which, again, is compatible with the theory that the original PIE system was /T 'D D/ rather than /T D Dh/, with the tradiational voiced/possible glottalized set > nasal in Uralic, maintained in PIE as some kind of glottalized sound:
Late PIE: T 'D D
Germanic: D series fricitivizes, 'D series devoices to (')T. I don't see a lot of precedence for this, but from the other direction Korean and Javanese both have a voiceless, unaspirated series that gained elements of glottalization. Obviously not an exact match, but it seems especially compatible with hwhatting's chronology, adjusted to T 'D D > Tʰ 'D D > Þ (')D Ð > Þ (')T Ð.
Italic: D series fricitivizes, leaving room for the 'D series to lose its (pre)glottalization
Balto-Slavic: 'D denatures into ' + D, with ' becoming part of the vowel nucleus as a (glottalizing) rising tone (compare Chinese, Northern Athabascan where -ʔ > high tone, Tzeltal ɓ>ʔb,ʔC > creaky voice on vowel)
In the other branches, either the glottalization becomes pure phonation (creaky voice) and, in Greco-Armenian-Aryan, then pushes the voiced towards breathy, or preglottalization is lost directly (Khmer, some Mayan ɓ>ʔb>b), after the voiced series becomes breathy in Greco-Aryan. I wouldn't say one is more likely that the other; despite Sindhi having full-blown implosives, it seems just as likely that it was creaky voice that Sindhi reinforced back to full implosion, given that implosives aren't preserved anywhere else (and, with a lot of stretching, might be able to claim areal influence from pre-Indo-European languages, if the Harrapan language was indeed related to Austroasiatic). However, I at least think there's decent reason to believe it was still preglottalized/implosive at least when Germanic and Balto-Slavic split off, as glottalization precedes the consonant rather than following it as is common with stiff/creaky voice (where glottalization is usually realized on the following vowel).
The problem with this is I'm not sure this matches up well with Tocharian and Anatolian, like, at all.
Sumelic wrote:Aside from this, though, are there any attested examples of breathy-voiced stops being more prone to fricatization than simple-voiced stops?
I'm not sure. Gujarati has fricative allophones of breathy stops intervocally, and less widespread of aspirated stops as well. It's only one language but then, not many languages have breathy stops in the first place so one language that fricativizes them is a lot more than one language that fricativizes voiced stops. The only other changes to breathy stops I know of involve vowels, either with tone (Chinese, Punjabi, Zulu) or vowel quality (Khmer).