Viscosity structure of Earth’s mantle is a crucial quantity in discussing mantle dynamics. One of the methods for inferring viscosity structure is to use the observed relative sea level (RSL) variations for glacial isostatic adjustment (GIA) process due to the last deglaciation during the past ~20 kyr (kilo-year). On the other hand, the rotational variations due to the GIA processes are degree-two response of the Earth, and therefore have been used to infer the lower mantle viscosity. In particular, the rate of change of degree-two harmonics of Earth’s geopotential, J2/dt (J2-dot), provides an important constraint on the lower mantle viscosity. However, the observationally derived J2-dot is affected by recent melting of glaciers and the Greenland and Antarctic ice sheets, and therefore we have to extract the recent melting component from the observation to estimate the GIA-induced J2-dot available for inferring the viscosity structure. Thus, Nakada et al. (2015) estimated the recent melting component using the data taken from the IPCC 2013 Report and obtained the GIA-induced J2-dot based on the observationally derived J2-dot.
GIA-induced J2-dot is also highly sensitive to the Late Pleistocene melting histories of the Northern and Southern hemisphere ice sheets, particularly, to the meltwater volume since the Last Glacial Maximum (~ 21 kyr BP) characterized by globally averaged (eustatic) sea level (ESL) rise. For example, Peltier’s group (Toronto University) support an ESL component of ~120 m, and Lambeck’s group (Australian National University) support the component of ~130 m.
Recently, we have examined the GIA-induce J2-dot and LGM sea level changes at Barbados and Bonaparte Gulf, Australia, to infer the viscosity structure and the ESL component. These results indicate the effective lower mantle viscosity of(5-10)x10^22 Pa s, upper mantle viscosity of (1-3)x10^20 Pa s and the preferred ESL component of ~130 m. Please see the following papers: