The Periodicity Of Change

For substantial periods western Amazonian and southern Andean sites do appear to have similar histories. Sedimentary records in the Colombian Andes, the Bolivian

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Figure 3.2. C02 concentrations from the Vostok core (Petit et al., 1999) (upper line) and the relative rate of change in C02 concentrations between samples (lower line).

Andes, the Chilean pampas, and the Cariaco Basin all reflect precessional forcing during the last 50,000 years, and so too does the paleoecological record of Lake Pata in the Hill of Six Lakes (Bush et al., 2002). Records from speleothems collected in southern and northeastern Brazil also document a precessional pattern in moisture supply (Wang et al., 2004; Cruz et al., 2005).

However, upon closer inspection, some other patterns emerge. Although preces-sional forcing is evident in all these records, the Colombian Andes are out-of-phase with the southern Andes (Bush and Silman, 2004), which is not surprising as the wet season for each occurs 6 months apart. Consequently, precipitation in Colombia is in phase with July insolation, whereas that of Titicaca correlates with December insolation. Interestingly, the highstands and lowstands of Lake Pata (0° latitude) are in phase with those of Lake Titicaca (17°S) and so a simple geographic placement of the site is not enough to predict the orbital forcing. The clue to the connection comes from the speleothem record from Botuvera Cave near Rio de Janeiro in southeastern Brazil. This record reveals an oscillation in the strength and position of the South American Summer Monsoon (SASM) resulting from changes in insolation intensity (Cruz et al., 2005) (Figure 3.3). SASM is driven by the convective activity over Amazonia and fed moisture via the South American low-level jet (SALLJ). The SALLJ is strengthened as k cal. yr BP

Tropical Rainforest Insolation

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Figure 3.3. Data for 8lsO from Brazilian speleothem records, downcore gamma radiation Salar de Uyuni and K+ concentration from Lake Pata, compared with mean insolation calculated in Analyseries 1.2 for 0°, l0oS, and 30°S (Berger, 1978). Periods selected are those used by authors in original descriptions relating data to insolation.

kcal. yr BP

Figure 3.3. Data for 8lsO from Brazilian speleothem records, downcore gamma radiation Salar de Uyuni and K+ concentration from Lake Pata, compared with mean insolation calculated in Analyseries 1.2 for 0°, l0oS, and 30°S (Berger, 1978). Periods selected are those used by authors in original descriptions relating data to insolation.

convection intensifies with the net result of drawing more moisture from the northern tropical Atlantic into Amazonia. As most of the basin heats most strongly in December, this sets the precessional rhythm for sites receiving moisture from the ALLJ (Bush, 2005). The position of SASM is influenced by convection, so that during strong convection SASM expands farther south, bringing rain to Botuvera Cave. During weaker convection Amazonia is drier and SASM is more restricted in its southerly range. SASM expands progressively southward during the austral summer and so the best correlation with orbital forcing is obtained by tracking peaks in February (late summer) insolation (Figure 3.3). SASM also transports Amazonian moisture to the Altiplano, and hence lake levels in the Titicaca and the Salar de Uyuni records, but here it is the entire wet season that is the time of critical insolation (December-February).

Another tantalizing speleothem data set from eastern Brazil shows a precessional pattern with wet peaks aligning to austral autumn (February-May) peaks in insola tion, as opposed to the December cycles of the other sites (Wang et al., 2004). As we gain more high-resolution paleoclimatic records it may be possible to test whether past dry events can truly be predicted based on precessional influences on the prevailing moisture source (Figure 3.3).

Interestingly, the long sedimentary record from the Salar de Uyuni reveals that precession is not strong enough to counteract some other drivers of precipitation change. For example, Fritz et al. (2004) suggest that global ice volume is another significant variable in Pleistocene Andean precipitation and that—until ice volume reaches a critical point—precession does not emerge as a significant factor. A similar argument was made that the climate of Panama shows poor correlation with climatic events in the North Atlantic prior to c. 45,000 years ago, but between that time and about 14,000 years ago a closer relationship is evident (Bush, 2002). Thus, in Panama at c. 7°N and in the Altiplano it appears that prevailing controls on climate were modified by global ice volume. Indeed, in Colombia the record from the High Plains of Bogota appears to reflect faithfully the three main Milankovitch cycles (Hoog-hiemstra et al., 1993), whereas in the tropical lowlands farther south, precession is by far the most important pacemaker of climate change. An observation that corresponds well with modeled data that highlights precession as a potential driver of tropical paleoclimates (Clement et al., 1999, 2001).

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