In a Daquin pipe organ piece, "Noël «Suisse»" from his "Livre de Noëls", I see this notation in the Right Hand system:
This is notated this way in the Oxford Book of Christmas Organ Music. How should I play the chord that has a "+" sign under it?
I found the below footnote in a transcription on IMSLP:
I think that one could play the + sign, as a 'pinched' lower mordent.
Where this is first notated, it applies to the sign above a single note in the right hand, however, there are further places where the same notation is used on chords, and below the notes, without further comment from the transcriber, so I'd imagine the same still stands in these cases. (I'd also say it fits with the style of the music, and isn't impossible to play, no?)
This transcription is an extract from "Archives des Maitres de l'Orgue" (Archives of the masters of the organ) - written by Alex Guilmant (The annotator) and A. Pirro. It is dated 1903.
Check this out.
Thumbing is the act of playing with one or both thumbs on the keyboard below the keyboard on which the rest of fingers are playing. This technique was developed in the late 19th century, and fell out of use after 1930. While at first an organist not used to this technique will only be able to use it to play isolated sustained pitches, organists accustomed to this technique are capable of playing moving lines, although the speed possible is less than that of using all five fingers. A composer who wants to use this technique would be wise to spend some time at an organ console to find out what is physically possible as far as the reach of a hand. Failing that, a composer should refer to a piano keyboard to verify that the requested hand positions are comfortably possible. When notating a passage using this technique, one should place the notes to be thumbed on a separate staff between the "right hand" and "left hand" staves. One should also mark the notes to be thumbed with a "+" symbol.
This page is useful http://en.wikipedia.org/wiki/List_of_musical_symbols#Articulation_marks It says:
Left-hand pizzicato or Stopped note
A note on a stringed instrument where the string is plucked with the left hand (the hand that usually stops the strings) rather than bowed. On the horn, this accent indicates a "stopped note" (a note played with the stopping hand shoved further into the bell of the horn). In percussion notation this denotes, among many other specific uses, that the hi-hat is to be closed by pressing the pedal or that an instrument is to be "choked" (silenced by causing vibrations to cease).
I hope it makes sense in the context!
I have several recordings of the Noels and don't remember anything special here. In the second bar (the piece starts with your given bars, doesn't it?) it is not striking to be the standard symbol for an unspecified ornamentation. I observe however, that both systems being in the treble clef, on a one-manual instrument the right hand has to give way to the left, so it can use the same keys.