Paper No. 4
Presentation Time: 9:45 AM
HEINRICH STADIAL EVENT 4 RECORDED IN BRAZILIAN STALAGMITES
The precise timing and duration of Heinrich Stadial (HS) Events remains a topic of active debate. We aim to study, in detail, a sequence of NE Brazilian stalagmites known to grow during HS Events1. Stalagmite growth in this region is caused by increased precipitation resulting from the southerly displacement of the Intertropical Convergence Zone during HS Events. We focus on stalagmite TBV-40 collected from Toca da Boa Vista Cave (40o51’39”W 10o09’36”S). We dated TBV-40 using 230Th dating methods. The stalagmite grew rapidly over two short time intervals: first, during a portion of HS-6 from 66.4-65.4 ka B.P. and second, during HS-4 from 39.6-38.9 ka B.P. We discovered, using powder X-ray diffractometry, that the HS-6 growth phase and the oldest portion of the HS-4 growth phase were aragonitic, whereas the youngest portion of the HS-4 growth phase was calcitic. The switch from aragonite to calcite occurred at approximately 39.5 ka B.P. We infer that conditions within the cave shifted from lower to higher humidity at this time. In addition, we have discovered banding in this sample using confocal laser fluorescence microscopy. If the banding is annual, band-counts constrain the duration of the HS-4 growth phases. Bands are brightest and most distinct in the calcitic region, with band counts yielding a duration of 284 + 6 years. Banding in the aragonitic region, while less defined, yields a duration of 106 + 10 years. The total estimated duration of the HS-4 growth phases, 390 years, is consistent within the much larger errors of the 230Th dates. The aragonite and calcite growth phases may correlate in detail with the HS-4 portion of the Hulu Cave record2 from China, with the aragonite portion correlating to the weak monsoon immediately following Chinese Interstadial 9 and the calcitic portion correlating with the subsequent yet weaker monsoon. Using a combination of 230Th dating, confocal microscopy, and comparison with other paleoclimate records, we aim to provide additional constraints on the timing and duration of the Brazilian pluvial anomalies associated with HS events.
1Wang et al. (2004) “Wet periods in northeastern Brazil over the past 210 kyr linked to distant climate anomalies” Nature 432, 740-743
2Wang et al. (2001) “A High-Resolution Absolute-Dated Late Pleistocene Monsoon Record from Hulu Cave, China” Science 294, 2345