a b s t r a c tDrill cores obtained from Lake Peté n Itzá , Peté n, Guatemala, contain a $85-kyr record of terrestrial climate from lowland Central America that was used to reconstruct hydrologic changes in the northern Neotropics during the last glaciation. Sediments are composed of alternating clay and gypsum reflecting relatively wet and dry climate conditions, respectively. From $85 to 48 ka, sediments were dominated by carbonate clay indicating moist conditions during Marine Isotope Stages (MIS) 5a, 4, and early 3. The first gypsum layer was deposited at $48 ka, signifying a shift toward drier hydrologic conditions and the onset of wet-dry oscillations. During the latter part of MIS 3, Peté n climate varied between wetter conditions during interstadials and drier states during stadials. The pattern of clay-gypsum (wet-dry) oscillations during the latter part of MIS 3 ($48-23 ka) closely resembles the temperature records from Greenland ice cores and North Atlantic marine sediment cores and precipitation proxies from the Cariaco Basin. The most arid periods coincided with Heinrich Events when cold sea surface temperatures prevailed in the North Atlantic, meridional overturning circulation was reduced, and the Intertropical Convergence Zone (ITCZ) was displaced southward. A thick clay unit was deposited from 23 to 18 ka suggesting deposition in a deep lake, and pollen accumulated during the same period indicates vegetation consisted of a temperate pine-oak forest. This finding contradicts previous inferences that climate was arid during the Last Glacial Maximum (LGM) chronozone (2172 ka). At $18 ka, Peté n climate switched from moist to arid conditions and remained dry from 18 to 14.7 ka during the early deglaciation. Moister conditions prevailed during the warmer Bolling-Allerod (14.7-12.8 ka) with the exception of a brief return to dry conditions at $13.8 ka that coincides with the Older Dryas and meltwater pulse 1A. The onset of the Younger Dryas at 12.8 ka marked the return of gypsum and hence dry conditions. The lake continued to precipitate gypsum until $10.3 ka when rainfall increased markedly in the early Holocene.

Figure 1. Earthquake catalog of Switzerland (Swiss Seismological Survey, 2002). Catalog is based on macroseismic and instrumental data from period A.D. 250-2001. Gray dots represent epicenters of earthquakes with M w Ͼ 5.5. Black box indicates study area, which includes Lake Lucerne and city of Lucerne situated on outflow River Reuss. Northern alpine front is marked with dashed line.

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Carbonate sediments are prone to rapid and pervasive diagenetic alterations that change the mineralogy and pore structure within carbonate rocks. In particular, cementation and dissolution processes continuously modify the pore structure to create or destroy porosity. In extreme cases these modifications can completely change the mineralogy from aragonite/calcite to dolomite, or reverse the pore distribution whereby original grains are dissolved to produce pores as the original pore space is filled with cement to form the rock (Figure 1). All these modifications alter the elastic properties of the rock and, therefore, the sonic velocity. The result is a dynamic relationship among diagenesis, porosity, poretype, and sonic velocity. The result is a wide range of sonic velocity in carbonates, in which compressional-wave velocity (V P ) ranges from 1700 to 6600 m/s and shear-wave velocity (V S ) from 600 to 3500 m/s.Porosity is the main controlling factor in determining the sonic velocity in rocks but in carbonates the pore type is nearly equally important in the elastic behavior and resultant sonic velocity (Anselmetti and Eberli, 1993, 1997). Most of the current theoretical equations do not, or insufficiently, account for this modification of the elastic behavior by the pore type. Consequently, seismic inversion, AVO analysis, and calculations of pore volumes that are based on these equations are prone to large uncertainties in carbonates.We measured acoustic velocities on modern carbonate sediments and rocks in various stages of diagenesis to reveal the relationships between original composition, porosity, pore type, and velocity. The apparatus for these laboratory experiments, constructed by VerdeGeoScience, consists of an oil-filled pressure vessel that contains the ultrasonic transmitter-receiver pair with piezoelectric transducers and the sample. Miniplug samples of one inch (2.5 cm) diameter and 1-2 inches in length are positioned between two piezoelectric transducers and sealed from the confining oil in the pressure vessel. Confining and pore-fluid pressures are chosen independently to simulate most accurately insitu stress conditions of buried rocks. The pore-fluid pressure is kept stable at 2 MPa and the confining pressure is varied between 3 and 100 MPa, resulting in an effective pressure of up to 98 MPa. The pair of transducers generates one compressional wave signal (V P ) and two perpendicularly polarized shear wave signals (V S 1, V S 2) at central frequencies close to 1 MHz.Sonic velocity of carbonate sediment. Grain size and shape, sorting, and the ratio between grain and matrix influence acoustic velocity in unconsolidated carbonate sediment. Pure carbonate mud has an average porosity of 60% and V P of ~1700 m/s. At a compression of 170 MPa, the porosity is reduced to 29% and V P increases to 2250 m/s, while V S is between 900-1200 m/s. These mud samples have a low shear modulus and, thus, a behavior similar to materials that have no rigidity (liquids). Carbonate sand (ooids and skeletal grains) s...

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