Japanese doesn't have many long vowels in native vocabulary, but in Sino-Japanese it has many. But as with the native ones, these are almost invariably the result of contractions of bi-moraic diphthongs.
- 登 Middle Chinese toŋ > Middle Japanese toũ > tō
- 當 MC taŋ > MJ taũ > tɔ̄ > tō
- 通 MC tuwŋ > MJ tuũ > tū
- 刀 MC *taw > MJ tau > tɔ̄ > tō
- 答 MC *top > MJ topu > tofu > tou > tō
- 蹋 MC *dap > MJ dapu > dafu > dau > dɔ̄ > dō
- 丁 MC *teŋ > MJ teũ > tyō
- 鳥 MC *tew > MJ teu > tyō
- 笘 MC *tep > MJ tepu > tefu > teu > tyō
- 中 MC *ṭjuwŋ > MJ tiũ > tyū
- 丁 MC *teŋ > MJ (kan-on) teĩ > tē
For all of these, the length thus stems from an approximation of heavy syllables (not known to Japanese at the time) by making a bimoraic syllable.
But this is not the only origin for vowel length in Sino-Japanese. In a couple of cases fairly simply syllables in Middle Chinese take on a long vowel in Sino-Japanese in the kan-on layer but not in the go-on layer. Such cases fascinate me, because the difference is not easily explained Japanese-internally, and thus seems to suggest a real (and somewhat unexpected) phonetic difference between the go-on and kan-on layers.
The main differences between go-on and kan-on (most of which must be attributed to the underlying Chinese dialects) can be summarized as follows:
- Voicing distinction:
- go-on: voicing distinction is maintained MC t, d > MJ t, d;
- kan-on: MC t ,d > MJ t, t
- Initial nasals:
- go-on: Initial nasals are maintained MC m, n > MJ m, n;
- kan-on: become prenasalized stops MC m, n > MJ b, d
- Vowel breaking:
- go-on: MC e, æ and ɛ break before velar finals to ~ia MC *mek > *miak > MJ myaku; MC *bæk > MJ byaku; MC *tṣɛŋ > MJ syaũ > syō;
- kan-on: doesn't break (and reflects e (= e) differently from æ/ɛ (= a)): MC *mek > MJ beki; MC *bæk > MJ haku; MC tṣɛŋ > MJ saũ > sō
- Palatalization of Division III:
- go-on: Division I was still pharyngealized/ Division III non-pharyngealized initials, are not distinguished in this layer: MC 五 *ŋo [ŋˁə] > J go; MC 魚 *ŋjo [ŋə] > J go;
- kan-on: Division I has become unpharyngealized/Division III has become palatalized, they are distinguished in this layer: MC 五 *ŋo [ŋˁə] > J go; MC 魚 *ŋjo [ŋʲə] > J gyo
But there is a group of syllables that has short vowels in the go-on layer, and a long vowel in the kan-on layer. This happens in the Division I -uw and its Division III equivalent -juw rhymes.
Compare:
- MC 口 *kʰuw; go-on: ku; kan-on kou> kō
- MC 頭 *duw; go-on: du; kan-on tou > tō
- MC 母 *muw ; go-on mu, mo; kan-on bou > bō
- MC 留 *ljuw; go-on ru; kan-on riu > ryū
- MC 酒 *tśjuw; go-on syu; kan-on siu> syū
- MC守 *śjuw; go-on syu, su; kan-on siu > syū
- MC 九 *kjuw; go-on ku; kan-on kiu > kyū
As we have already seen, long vowels seem to stem from diphthongs. It therefore stands to reason that in the kan-on layer both of these finals must have been diphthongs, whereas in the go-on layer they were monophthongs. I am not able to reconstruct why Baxter & Sagart originally decided to represent these rhymes with a final w and if sino-japanese played a role in that. I would assume so, but I'm just reinventing the wheel here for my own satisfaction.
Knowing that Japanese *o can come from ə (transcribed in Middle Chinese as <o> typically) it seems attractive to reconstruct the kan-on reflex of the division I rhyme as *-əw which then naturally gives rhis to kan-on ou> ō. But the Middle Chinese that fed into the go-on layer, this must have been a monophthong (or at least a diphthong that did not allow approximation with <ou> = [əu]) so something like *-u.
口 MC kʰuw ; MC Go-on dialect: kʰˁu> go-on ku; MC kan-on dialect kʰəw> kan-on kou
As for the -juw rhyme, it is more difficult to reconstruct what the rhyme would have looked like in the Kan-on layer of Middle Chinese. But it can't have been *-jiw, as it behaves distinct from it (-jiw becomes -yū in the go-on layer). Here too -jəw seems like an option. A -jow rhyme does not exist so, we could slot it in there. But other rhymes that have MC o in it, show up with -yo- rhymes in kan-on, so presumably the vowel of the diphthong was higher, e.g. ɨ. Again, in the go-on dialect these must have monophthongized.
九 MC kjuw; MC go-on dialect ku> go-on ku; MC kan-on kʲɨw> kan-on kiu
It's interesting to note that this reconstruction (əw besides ɨw) perfectly mirrors the middle Chinese reconstruction of Zhengzhang Shangfang. Probably not a coincidence (did not check what that reconstruction is based on), but an independent conclusion on my end!
I say that in the MC go-on dialect, these diphthongs were monophthongized. But it is actually likely that we are rather dealing with a lack of of diphthongization. The -uw and -juw rhymes are reconstructed as coming originally Old Chinese monophthongs -u and -uw.
OC *-o> MC Div I -uw; Div III -ju
OC *-u > MC Div I -aw, -uw; Div III -juw
So one might in fact imagine that the go-on layer, which also in its lack of palatalization in the Division III appears appears to predate Middle Chinese proper, is actually continuing the Old Chinese vowel system in this regard.
Japanese o can come from both ə and o, so OC *-o being borrowed with Japanese -u is a bit surprising. But there are good reasons to think that the Old Japanese o was actually a rising diphthong wo, which possibly did not feel like a good match for approximating Old Chinese *o. Thus we might tentatively reconstruct the Chinese dialect that feeds into the go-on layer with this distinction maintained:
口 *kʰˁo> Go-on ku
九 *ku > Go-on ku
Whereas the Chinese dialect that feeds into the kan-on layer has all the typical Middle Chinese breaking of these vowels:
口 *kʰˁo > *kʰəw > Go-on kou
九 *ku > *kʲɨw > Go-on kiu