Sea level changes and the continental freeboard concept: general principles and application to the Precambrian

Abstract

Eustatic sea level changes reflect variation in ocean water or ocean basin volume, or changes to the hypsometric curve, which is in itself geographically and chronologically variable. Freeboard, the elevation of a continent above mean sea level, is closely related to changes in both sea level and this curve. Relative sea level change occurs due to tectonism, sediment supply, compaction, and eustatic movements. Hydroisostatic compensation modifies first-order sea level variation by about one-third. Short term and localised change in sea level may be ascribed to waves and tides, storm winds, hurricanes, tsunamis and catastrophic sediment slumps. Wide, gently sloping shelves with more uniform circulation systems, inferred for the Precambrian, may have enhanced tide and wave-induced sea level changes. Salinity and water temperature, mutually dependent, affect eustatic and relative sea levels; elevated water temperatures postulated for the early Precambrian would have been offset partly by enhanced evaporation and increased salinity. Longer term changes in sea level, regional or eustatic, may result from orogenesis, volcanism, sediment supply, compaction and loading, isostasy, and thermal mechanisms. Glacioeustatic changes, compounded by related salinity variations, moderated by glacioisostatic effects and modified by hydroisostasy, reach ca 150 m. Isostatic rebound in interglacial episodes will produce rapid and marked sea level drops up to ca 250 m; this is a localised effect and contrasts with the global enhancement of sea level during interglacials. However, evidence for glaciation in the geological record is limited, also for most of the Precambrian; sea level oscillations in non-glacial episodes may reflect flexure of tectonic plates (change up to ∼100 m) or changes to Earth's geoid (∼250 m). Eustasy up to ∼350 m will result from change to ocean basin volume, due to variation in mid-ocean ridge volume and spreading rates, also related to supercontinent amalgamation and breakup. Intra-plate hot spots and oceanic plateaus will change eustatic sea level more slowly, up to ∼100 m. Higher heat flow in the Archaean and concomitant enhanced volcanism suggest that these were important influences on Precambrian sea levels. The exponential elevation-age relationship observed for Phanerozoic oceanic lithosphere may have been more significant in the Precambrian. As continental crust grew from ca 4.0 Ga, declining heat flow and concomitant deepening of the ocean basins maintained an approximately constant freeboard during the Archaean. Although still hotly debated, there is strong support for episodic growth models inferring rapid formation of continental crust near the Archaean–Proterozoic boundary. This would imply constant freeboard conditions after 2.5 Ga as well. The constant freeboard model, however, is one based on average global conditions. Freeboard was still subject to geographic and chronologic variation, and these potential changes lie within the bounds of eustatic and relative sea level change. Continental crustal growth, the freeboard concept and sea level changes are thus interdependent variables in Precambrian geology.