HR: 1330h
AN: PP12A-0233    [PDF]
TI: Sea-level and the `Stage 11 Problem`
AU: * Bowen, D Q
EM: Bowendq@cardiff.ac.uk
AF: School of Earth, Ocean and Planetary Sciences, Museum Avenue, Cardiff, CF103YE United Kingdom
AB: Estimating an approximate relative sea level for oxygen isotope stage 11 may have a critical bearing on a solution to the `stage 11 problem` that identifies the mismatch between low eccentricity forcing and the disproportionate ice volume response - that also includes a relative sea level response. The perennial problem of separating ice volume from temperature effects has hampered attempts to estimate sea level from delta 18O data sets, even for younger odd numbered stages when comparisons with U-series ages on corals are available. Stage 11 sea levels on `stable` and uplifting coasts are recognised from geomorphic features such as terraces and shoreline angles, sediments and corals, and yield a range of estimates from over 20 m to just below present sea level. Given that the 413 ka Milankovitch pacing provides similar orbital configurations for stage 11 and the Holocene some interest attaches to the potential sea-level similarity between them, especially for the future Holocene. Attempts to derive a stage 11 sea level from coasts uplifting at different rates have used `uplift correction graphs` or uplift correction equations, but a major handicap is the dearth of appropriate geochronologic ages both for stage 11 and substage 5e (5.5) - the base line for estimating average uplift rates. Different estimates for the age of stage 11 and 5e (5.5), and the duration of 5e, have yielded a range of estimates. Earlier estimates relied on single locations or regional evidence, but it is probably misleading to rely on these. To combat this several world-wide locations are assembled and, using locality-specific data, provide a mean estimate for the stage 11 sea level of 11 m, plus-minus 10 m. But by applying a set of standardised parameters (including the peak sea level at 402 ka - event 11.3 of the Bassinott time scale) the mean sea level for stage 11 emerges as 2 m plus-minus 7 m. This closes the gap between inferences from delta 18O variability, the latest of which point to an 8 m sea level, and estimates from the actualities of coastal geology, geomorphology and sedimentary data. Higher sea level indicators from apparently `stable` locations are readily explained by consideration of contemporary coastal, especially storm wave, processes. Where did the excess ice come from for a relatively high stage 11 sea level? And what provided the heat source for ice melting? Absence of IRD in North Atlantic cores suggests that some excess ice came from Greenland; and there is evidence for repeated collapse of the isotopically heavier West Antarctic ice sheet that would have masked any inferred sea level signal from delta 18O variability. West Pacific warm pool Mg-Ca ratios have shown that stage 11 was significantly warmer than any interglacial of the last 5 Ma years. This was probably because low eccentricity and dampened tropical precession would have ensured tropical warming, from where atmospheric heat transport provided the means for melting excess ice. This mechanism also provides a possible explanation for the `stage 11 problem`.
DE: 4267 Paleoceanography
DE: 4556 Sea level variations
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting