engleski

U-Th ages of submerged speleothems and Quaternary sea levels

U-Th ages of calcite growth phases in submerged flowstone and stalagmite sequences serve to constrain both the timing and elevation of past sea-level variation for the middle and late Pleistocene. The most robust sea-level evidence based on speleothems is the presence of continuous calcite growth phases, which provide maximum constraints on past sea-level elevation because they could only have formed when cave passages were air-filled. U-Th ages also provide minimum ages for cave passage formation, which is intimately linked to past sea level positions in carbonate island margin settings. Growth hiatuses can be caused by submergence by rising sea levels or cessation of drip during periods of aridity, lack of soil cover or fissure blockage. Ideally, then, temporal constraints on sea level change are derived from samples with unambiguous evidence of submergence (serpulid worm encrustation, phreatic overgrowths, marine borings). Elevation constraints are the most robust in regions subject to minimal tectonic movement or glacio-hydro- isostatic adjustment. We have adapted mass- spectrometric techniques (TIMS ; conventional and laser ablation MCICPMS) to achieve high precision U-Th ages for speleothem growth. To assess the accuracy of temporal constraints and potential diagenetic effects, we are focussing on increased sampling resolution at the expense of precision. We use a combination of microdrilling and laser- ablation sampling to analyse thin growth layers that might represent millennial-scale sea level fluctuations during interglacial high sea- stands. We present a range of past sea level constraints using speleothems from the Bahamas, Turks and Caicos, Croatia and Yucatan, Mexico. For the most part, material from caves in these locations is useful to constrain high sea- stands when sea levels were greater than -20 m. - Bahamas: Ages for initiation of growth after three major growth hiatuses of a flowstone sequence are ~370 ka, 302 ± 6 ka, 191 ± 4 ka, 68 ± 1.5 ka (2σ uncertainty). The hiatuses were caused by major high sea-stand events (> -18 to -10 m below present sea level) and constrain the timing of the marine isotope stage (MIS) boundaries 11/10, 9/8, 7/6 and 5/4 respectively. - Croatia: Confirmatory evidence of the timing of the double high sea-stand associated with MIS 5a also recorded in Barbados terraces - Turk and Caicos: Cave passages formed at sea level during former high sea stands (MIS 7 or older) are found at present sea level or higher and will be discussed in light of the current understanding of subsidence rates and glacio-hydro-isostatic adjustment for this region

U-Th ; submerged speleothems ; sea level ; Quaternary

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