  Direct detection of spontaneous spin fluctuations, or "magnetization noise",
is emerging as a powerful means of revealing and studying magnetic excitations
in both natural and artificial frustrated magnets. Depending on the lattice and
nature of the frustration, these excitations can often be described as
fractionalized quasiparticles possessing an effective magnetic charge. Here, by
combining ultrasensitive optical detection of thermodynamic magnetization noise
with Monte Carlo simulations, we reveal emergent regimes of magnetic
excitations in artificial "tetris ice". A marked increase of the intrinsic
noise at certain applied magnetic fields heralds the spontaneous proliferation
of fractionalized excitations, which can diffuse independently, without cost in
energy, along specific quasi-1D spin chains in the tetris ice lattice.
