Evidence of rainfall variations in Southern Brazil from trace element ratios (Mg/Ca and Sr/Ca) in a Late Pleistocene stalagmite

• DOI: 10.1016/j.gca.2007.02.005
• Publication Date: 2007-02-17T07:03:27Z
• Authors: Francisco W. Cruz; Stephen J. Burns; Michael Jercinovic; Ivo Karmann; Warren D. Sharp; Mathias Vuille
• Keywords: Stalagmite; Trace element; Speleothem; δ18O; Pluvial

Trace element ratios in the Bt2 stalagmite from Botuveracave, Southern Brazil, are explored as a proxy for changes in the local rainfall recharge during the last 116 ky. BP Mg/Ca and Sr/Ca ratios, measured with an electron microprobe, are signif- icantly positively correlated with one another throughout the entire record, and vary in a way that is very consistent with variations of d 18 O in the same speleothem during the last glacial period. We suggest that prior calcite precipitation in the vadose zone of the cave system is the main factor affecting the incorporation of Mg and Sr into calcite of the stalagmite. This interpretation is supported by trace element correlation patterns and by results from a hydrochemistry study performed in a cave located in the same region and in a similar environmental setting. Therefore, we conclude that higher (lower) Mg/Ca and Sr/Ca values are associated with lower (higher) levels of recharge into the karstic aquifer, as such conditions lead to an increase (decrease) in the volume of calcite precipitated in the unsaturated zone above the cave during dry (wet) climate periods. Trace element variations point to generally dryer (wetter) conditions during lower (high) phases of summer insolation in the southern hemisphere. These periods coincide with decreased (increased) activity of the South American summer monsoon, as revealed by d 18 O stalagmite records. In addition trace element variations show that rather wet conditions persisted through- out most of the last glacial period from approximately 70 to 17 ky BP. We suggest that during this period the glacial boundary conditions, especially ice volume buildup in the northern hemisphere, played an important role for monsoon rainfall inten- sification in the region. � 2007 Elsevier Ltd. All rights reserved.