TEMPERATURE VARIATIONS THROUGH THE K-T BOUNDARY INFERRED FROM PEDOGENIC CARBONATE NODULES, SOUTHERN ALBERTA, CANADA

The analysis of oxygen isotope ratios of pedogenic carbonate nodules from paleosols in southern Alberta, Canada with ages ranging between 77 and 63 Ma document a cooling trend of about 3º(C). In addition, the presence of a second carbonate phase with very light oxygen isotopic composition may record temperature drops of 15°(C) that occurred some time later in the history of nodule formation. Pedogenic carbonate nodules collected from 6 stratigraphic horizons spanning the K-T boundary in southern Alberta, Canada are composed primarily of equant anhedral microspar and to a lesser extent of blocky calcite spar. The microspar portions of the nodules formed first, then desiccated and cracked, and were subsequently infilled by blocky spar. The d¹⁸O of microspar varies between –9 and –13 ‰ (PDB) and the mean oxygen isotopic composition decreases by over a per mil in paleosols that are at the K-T boundary and younger. The oxygen isotopic composition of the blocky spar consistently averages 8.5 ‰ lighter than microspar and also becomes lighter in the younger paleosols. Calculating paleotemperature is accomplished by relying on the correlation between modern surface temperatures and the oxygen isotopic composition of precipitation. Using this relationship, we calculate that average winter temperatures in southern Alberta were 6º(C) in the latest Cretaceous and dropped to about 3º(C) in the earliest Tertiary. In many of the paleosols, after periods of microspar formation, blocky spar was precipitated in fractures in the nodules. The timing of blocky spar precipitation is difficult to constrain but appears to be early in the burial history of the paleosols. If the blocky spar did form during paleosol development, then decreases in temperature of 15º(C)are recorded.