Friday, 12 March 2010

Sea Levels 1meter Higher 81,000 Years Ago, While CO2 Lower

source: hidethedecline

Fig. 1 Encrusted speleothems at various levels in caves from Mallorca. (A) Geologic map of Mallorca (10) and location of sampled caves (red dots). (B) Schematic cross-section through a coastal cave in Mallorca showing multiple carbonate encrustation levels. (C and D) Present-day and paleo levels of encrusted speleothems related to higher (E) and lower (F) sea-level stands. (G) Typical morphology for tidal range–related carbonate encrustation (size of speleothem, 20 cm). (H) Bathymetric map of the western Mediterranean region and the predicted present-day rate of sea-level change due to GIA [adapted from (15)

Global sea level and Earth’s climate are closely linked. Using speleothem encrustations from coastal caves on the island of Mallorca, we determined that western Mediterranean relative sea level was ~1 meter above modern sea level ~81,000 years ago during marine isotope stage (MIS) 5a. Although our findings seemingly conflict with the eustatic sea-level curve of far-field sites, they corroborate an alternative view that MIS 5a was at least as ice-free as the present, and they challenge the prevailing view of MIS 5 sea-level history and certain facets of ice-age theory.

Sea-level rises and falls as Earth’s giant ice sheets shrink and grow. It has been thought that sea level around 81,000 years ago—well into the last glacial period—was 15 to 20 meters below that of today and, thus, that the ice sheets were more extensive. Dorale et al. (p. 860; see the Perspective by Edwards) now challenge this view. A speleothem that has been intermittently submerged in a cave on the island of Mallorca was dated to show that, historically, sea level was more than a meter above its present height. This data implies that temperatures were as high as or higher than now, even though the concentration of CO2 in the atmosphere was much lower

We therefore consider the simple interpretation of our data that eustatic sea level during MIS 5a stood around +1 m relative to present sea level, implying less ice on Earth 81,000 years ago than today. Although this interpretation conflicts with the generally accepted eustatic sea-level curve based on the far-field sites of Barbados and New Guinea, it is consistent with a number of other estimates from around the world, including those from the Bahamas, the U.S. Atlantic Coastal Plain, Bermuda, Cayman Islands, and California (4, 6, 22–26) (Fig. 2B).

We considered the simple fact that this geographically diverse suite of sites spans a wide range of presumed isostatic states, yet the suite consistently indicates a late MIS 5a highstand of ~ +0 to 3 m (Fig. 2B). Bermuda and Mallorca, for example, are both tectonically stable, and both have MIS 5e/5a estimates of 2 to 3 and 1 to 2 m above modern sea level, respectively; whereas MIS 5e/5a estimates from Barbados are ~ +5 m and ~ –18 m (2). Any appeal to GIA to account for these discrepancies must somehow take into account the unlikely outcome that different ice centers on different continents (Laurentide versus Fennoscandian) would generate the virtually identical MIS 5e/5a relative sea-level histories of tectonically stable Bermuda and Mallorca.

The very rapid onset and relatively brief nature of the MIS 5a highstand may have plausibly generated lags between the timing of sea-level changes and the timing of coral reef growth, and may provide a partial explanation as to why reefs on Barbados and New Guinea do not record a comparable eustatic height for this event. This and other factors that could be part of the apparent discrepancy are discussed in (9).

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