  We investigate the dynamics of alginate gels, an important class of
biopolymer-based viscoelastic materials, by combining mechanical tests and
non-conventional, time-resolved light scattering methods. Two relaxation modes
are observed upon applying a compressive or shear stress. Dynamic light
scattering and diffusive wave spectroscopy measurements reveal that these modes
are associated with discontinuous rearrangement events that restructure the gel
network via anomalous, non-diffusive microscopic dynamics. We show that these
dynamics are due to both thermal activation and internal stress stored during
gelation and propose a scenario where a hierarchy of cross-links with different
life times is responsible for the observed complex behavior. Measurements at
various temperatures and sample ages are presented to support this scenario.
