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Introduction - Eustatic sea-level changes refer to the world-wide changes in the height of the sea surface with respect to land areas. They also represent fluctuating water volumes within ocean basins, and over Quaternary timescales are predominantly due to changes in ice volumes (i.e., waxing and waning of continental ice sheets). The period from the peak of the Last Glacial Maximum (LGM; ca. 22–20 ka ago) to the present, the Holocene Interglacial represents the extreme end points of eustatic sea level in glacial cycles. During this time interval, sea level rose world-wide from approximately 120–125 m below present and almost attained present levels during the past 7,000 years (sidereal years). The rate and general pattern of relative sea-level change during this period, particularly during the Holocene, was spatially variable and differed according to geographic regions in response to glacio-hydro-isostatic adjustment processes, tectonism, and localized climatic changes (i.e., steric changes accompanying changes in localized sea-surface temperatures and salinity).

Barbados - The island of Barbados is located approximately 150 km east of the Lesser Antilles and is part of a fore-arc ridge. A series of 16 cores were drilled offshore through a post-glacial reef complex of Acropora palmata in an attempt to obtain a continuous time series to delineate post-glacial sea-level rise (Fairbanks, 1989). The Barbados record reveals that during deglaciation following the LGM, sea level rose rapidly and on occasions, occurred at rates approximately 20 times faster than the rate of sea-level lowering during ice sheet development in the last glacial cycle. Based on the radiocarbon dating of A. palmata at Barbados, maximum sea-level lowering attained a value of 121 5 m at 20,000 cal years BP (Fairbanks, 1989: Fig. 1). Sea level slowly rose with the onset of deglaciation, increasing by 20 m in 5,000 years (average rate of 4 mm per year).

Conclusions - Quantifying the amount of eustatic (ice-equivalent) sea-level rise from the end of the LGM to the early Holocene is complicated by the variable glaciohydro-isostatic responses of the crust-mantle system around the planet. Estimates from continental shelves in the far field of Pleistocene ice sheets suggest that sea level rose from ca. 125 5 m at 19 ka at the end of the LGM and attained present levels by 7 ka. Far field locations such as Australia record a stand of relative sea level higher than present as early as 7 ka ago, with a progressively falling sea surface since that time due to hydro-isostasy. The elevation and timing of the early Holocene highstand varies spatially and temporally. In near field areas in closer proximity to former ice sheets, an early Holocene relative sea level higher than present is not registered, apart from areas where the glacio-isostatic adjustment (uplift) following the withdrawal of ice sheets has far exceeded the rate of sea-level rise, resulting in an apparent relative fall in sea level.

 

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  • Posted on : June 03rd, 2019

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