Late Pleistocene and Holocene sea-level change in the Australian region and mantle rheology

Abstract

Spatial and temporal variations in sea-level are produced by the melting of the Late Pleistocene ice and by the Earth's response to the redistribution in surface loads. By examining different parts of the sea-level curves of the past 20 000 yr from geographically widely distributed regions it becomes possible to constrain models of the melting history of the ice sheets and of the Earth's rheology. Observations from sites away from the former Arctic ice sheets, such as the Australian and South Pacific region, are particularly important for constraining the total meltwater volumes added into the oceans in the past 20 O00 yr and the rates at which this occurred. These observations indicate that the Antarctic ice sheets provided a significant contribution to the sea-level rise at a rate that was approximately synchronous with the melting of the Laurentide ice sheet, except for the interval 9OOO-6ooo yr ago when it may have lagged behind. Minor melting of the Antarctic ice sheet appears to have continued throughout the Late Holocene. Differential observations of the Late Holocene sea-level change recorded at sites in the same region are particularly useful for estimating parameters describing the Earth's non-elastic response to surface loading. The effective parameters used here are a lithospheric thickness, an upper mantle viscosity, and a lower mantle viscosity describing the response below 670 km depth. With the observations used here, it is not possible to separate the lithospheric thickness H from the upper mantle viscosity and the viscosity results are based on the assumption that 50 5 H s 100 km. Neither do these observations provide a resolution of the depth dependence of viscosity in the upper mantle and the resulting estimates are effective parameters only. Differential sea-levels along continental margins and along the shores of large gulfs and bays constrain the effective upper mantle viscosity to be about (1-2) x Ido Pa s-' while differential values from islands of different sizes are suggestive of a somewhat lower value. The lower mantle (taken to be below 670 km depth) viscosity is about two orders of magnitude greater than this. The estimated ice and rheological models explain many of the Holocene sea-level observations throughout the Australian and southern Pacific region. The tilting of continental margins, as exemplified by observations of variable amplitudes of the Holocene high-stands and the variable times at which sea-levels first reached their present level along the north Queensland coast and Great Bamer Reef, is well represented by these models. Differential Holocene sea-levels observed along the narrow Spencers Gulf of South Australia are also well explained by the models and in neither case is it necessary to invoke tectonic motions. Predicted Late Holocene sea-levels at small-to medium-sized islands are characterized by small amplitude high-stands that reached their maximum values about 4000-2000 yr ago, consistent with observations from the Society, Cook and Tuamotu Islands.

Improved Constraints on Models of Glacial Isostatic Adjustment: A Review of the Contribution of Ground-Based Geodetic Observations

“…In this way, since elastic properties are independently obtained from seismologic studies (e.g., Dziewonski and Anderson 1981), the inversion process can be optimized to constrain the ice history and the vertical viscosity structure of the Earth (e.g., Peltier 1976;Nakada and Lambeck 1989;Tushingham and Peltier 1991). The resulting ice history and viscosity structure are coupled, meaning that unrealistic GIA predictions might result from using those deglaciation models in combination with arbitrary viscosity profiles.…”

Holocene Relative Sea-Level Changes from Near-, Intermediate-, and Far-Field Locations

“…The RSL signal of many far-field locations is characterized by a mid-Holocene sea-level maximum, or highstand, at the time meltwater production decreased [37]. The fall in RSL to present is due to hydro-isostatic loading (continental levering) [18,30] and a global fall in the ocean surface due to both hydro-and glacio-isostatic loading of the Earth's surface (equatorial ocean siphoning) [38]. Perturbations to Earth's rotation driven by mass redistribution also cause RSL changes in far-field regions to depart from the eustatic value [39].…”

“…The color scheme denoting rates of RSL change was chosen to remain consistent with previous GIA modeling studies from the University of Toronto range of intermediate-field sites (Fig 1). In far-field regions at increasing distances from major ice centers, eustatic contributions to RSL change exceeded glacio-isostatic contributions [4,5,15,16,[30][31][32][33][34][35][36]. The RSL signal of many far-field locations is characterized by a mid-Holocene sea-level maximum, or highstand, at the time meltwater production decreased [37].…”

IODP Expedition 310 Reconstructs Sea Level, Climatic and Environmental chagnes in the South Pacific during the Last Deglaciation

“…The timing and course of the last deglaciation (19,000-6,000 years BP) are essential components for understanding the dynamics of large ice sheets (Lindstrom and MacAyeal, 1993) and their effects on Earth's isostasy (Nakada and Lambeck, 1989;Lambeck, 1993;Peltier, 1994), as well as the complex relationship between freshwater fluxes to the ocean, thermohaline circulation, and, hence, global climate during the Late Pleistocene and the Holocene. Moreover, the last deglaciation is generally seen as a possible analogue for the environmental changes and increased sea level that Earth may experience because of the greenhouse effect, related thermal expansion of oceans, and the melting of polar ice sheets.…”