A Mechanism for the Formation of Shallowing-Upward Sequences in Hothouse Periods

Abstract:

The geological record hosts pervasive shallowing-upward sequences indicative of small-amplitude, cyclic sea-level changes. These sequences appear throughout Earth history, even during hothouse periods, when it was likely too warm for the growth and retreat of ice sheets to induce the requisite sea-level changes. Astronomically controlled exchanges of water mass between land and ocean reservoirs (e.g., exchanges caused by changing insolation) could be an alternative mechanism for these inferred sea-level changes, one that does not require the existence of ice cover.

We investigate this possibility by first examining the effects of orbital forcing on groundwater storage by running the Community Earth System Model (CESM 1.1.1) under present day conditions. This is done twice, both times until the shallow ocean equilibrates, with the only differences between the two runs being the adopted values for insolation. The seasonal signal is smoothed out by averaging over 20 years in order to isolate the long-term signal, and the resulting change in total water storage on land is used as input to a gravitationally self-consistent sea-level model that accounts for perturbations in the deformational, gravitational and rotational state of a viscoelastic Earth. The amplitude of the resulting sea level cycles is characterized by significant geographic variability and peaks at values consistent with meter-scale sediment deposition. This suggests that astronomically forced changes in groundwater storage provide a plausible mechanism for sea-level changes and shallowing-upward sequences formed during hothouse climate conditions.