This is one of the satellite pieces of Many Pink Butterflies, derived from that work's Solo II. Each solo in MPB is about a minute long, and explores a single compositional technique. A satellite piece based on a solo develops that technique further. In this case, the technique was randomized pitches: a through-composed melody would be followed by its version with randomized pitches; dynamics, tempi, and pedal are then used to create a coherent sequence.
In this piece, the technique is the same, but there are more melodies. Each is shorter than the last, usually by the factor of 2, until the last bars in which just a tiny snippet of music is followed by its version with randomized pitches. The Chinese title can be translated as Once a Deep Blue Sea, Now Mulberry Fields, which is a famous idiom to talk about the vicissitudes of time, and how the fortunes of a person or a country can suddenly change. The way this piece treats its melodies seemed to be the same; it is also somewhat similar to Chinese grammar in the sentence, because the literal translation is "deep blue, sea/ocean, mulberry, field". Finally, the entire score is divided by pauses into regular strips of music – the fields of the title.
I have already composed another monophonic satellite for Many Pink Butterflies, a solo piccolo piece called Retroactive Camouflage. That piece immediately spawned a solo piano version of itself, and looking at the score of 沧海桑田, I felt that it too could have a piano counterpart. The solo piano version keeps approximately 60% of the original piece, while the randomized parts were re-randomized, and tinkered with; there were also other changes, resulting in a very different piece of music.