Last glacial maximum

Evidence for sea level rise of 17 meters in less than 350 years

During the last deglaciation, about 19,000 to 11,000 years before the present, there were several episodes of rapid sea-level rise associated with the injection of significant amounts of fresh water from melting ice sheets, from the Antarctica ice sheet (AIS) and from the Laurentian ice sheet (LIS) in the Northern Hemisphere. In one of the events, called MWP-1A, sea level rose dramatically in a very short period of time. The precise timing, duration, sea level rise and mechanism of this meltwater pulse has remained uncertain making it difficult to relate it to known warming and cooling events during the deglaciation. In a recent study multiple cores from corals drilled offshore from Tahiti were used to determine that the MWP-1A started no earlier than 14,650 years ago and ended before 14,310 years ago and that the increase in sea level at Tahiti was about 17 meters over a period that does not exceed 350 years, but could be as low as a century.

Introduction

The observed increase in surface melting in Greenland and the collapse of sea ice associated with glaciers in both Greenland and Antarctica has raised questions about the vulnerability of the Greenland and Antarctic ice sheets as the climate warms. Past research into the into the last deglaciation has provided evidence for rapid and massive discharges of fresh water from continental ice sheets. The evidence suggests that there were several brief periods of extremely rapid sea-level rise. The precise timing, duration, sea level rise and mechanism of these very short-term events, referred to as meltwater pulses (MWP), remain uncertain.

The most extreme of these events is designated MWP-1A and is associated with a sea level rise of approximately 20 meters over about 500 years. The precise timing of this event has been uncertain making it difficult to associate it with other major cooling and warming events of the last deglaciation such as the Bølling warming that occurred from 14,700 and 14,100 years ago and ended with the Older Dryas cold event. Additionally the source of the massive meltwater pulse or pulses, whether from the Antarctic or Northern Hemisphere ice sheets, is also unclear. One scenario that has been proposed is that partial melting of the Northern ice sheets caused a massive injection of fresh water into the North Atlantic weakening the major Atlantic south-north current, called the Atlantic overturning current or AMOC, causing the consequent Older Dryas cooling of the Southern Hemisphere. An alternative scenario is that the fresh water pulse originated from the West Antarctica Ice Sheet (WAIS) which would have strengthened the Atlantic south-north current to initiate the Bølling warming in the north.

Data

In a recent study uranium-thorium isotope dating (U–Th dating) has been used to determine the chronology of coral cores collected from the Tahiti reef slope during the and Integrated Ocean Drilling Program expedition. An advantage of Tahiti is that it is located far from the former major ice sheets in the north and south. The coring operations recovered post-glacial reef material, ranging from 122 to 40 meters below modern sea level (m.b.s.l.). Eighty U–Th ages were determined on coral samples recovered from twenty-three holes drilled at fourteen different sites. The Tahiti data cover the last 16,000 years before the present.

Tahiti sea level record during last deglaciation
a, Sea level reconstructed from U–Th dated corals recovered in long holes drilled onshore and offshore Tahiti island. Coral depths are expressed in meters below present sea level (m.b.s.l.). b, Magnified view of the MWP-1A time window.

Results

The MWP-1A can be seen in a major discontinuity in the data points captured from the Tahiti data. Based on this analysis it is estimated that the change in sea level associated with MWP-1A was 17 meters, with uncertainty bounds of 12 and 22 meters. The dating of the event can be narrowed down to 14,310 to 14,650 years before the present indicating that the longest possible duration of MWP-1A is about 350 years. The error bars indicate that the duration of the MWP-1A event could have been as short as a century.  This corresponds to an annual sea level rise exceeding 4 centimeters per year.

Conclusion

The precise dating makes it clear that the MWP-1A event happened at the same time as the Bølling warming. The evidence indicates that the MWP-1A event could not have been the trigger for the Older Dryas cooling event that terminated the Bølling period. Instead, MWP-1A coincided with the beginning of the Bølling warming period.

Possible mechanisms

The results together with evidence from other sources suggest that at least half of the sea level rise associated with MWP-1A resulted from of melting ice from Antarctica. It remains difficult to determine the phasing of meltwater pulses in the the Northern Hemisphere and the Antarctic and the effect on the major Atlantic south-north current during the Bølling warming. One possibility is that a rapid freshwater discharge from the Antarctic Ice Sheet could have strengthened the Atlantic current and carrying heat to the north initiating the Bølling warming in the Northern Hemisphere and a rapid melting of the Laurentian Ice Sheet.  Another possibility is an intensification of the Atlantic current caused northward ocean heat transport resulting in the Bølling warming. The subsequent rapid melting of the Laurentian Ice Sheet raised sea levels causing a a rapid collapse of the Antarctic ice sheet.

Source

Ice-sheet collapse and sea-level rise at the Bølling warming 14,600?years ago, Pierre Deschamps, Nicolas Durand, Edouard Bard, Bruno Hamelin, Gilbert Camoin, Alexander L. Thomas, Gideon M. Henderson, Jun’ichi Okuno & Yusuke Yokoyama, Nature 483, 559–564 (29 March 2012) doi:10.1038/nature10902

Artist impression of ice-age Earth: Crowley, Thomas J. (1995). “Ice age terrestrial carbon changes revisited”. Global Biogeochemical Cycles. 9 (3): 377–389. doi:10.1029/95GB01107.