Late-Glacial Cooling in Amazonia Inferred from Pollen at Lagoa do Caçó, Northern Brazil

Abstract

New pollen data from a core at Lagoa do Caçó, Maranhão state, Brazil (2°58′S 43°25′W; 120 m elevation), show higher frequencies of Podocarpus at the end of the Pleistocene than today. The increase in Podocarpus, which follows the successive increase of various pioneer species such as Didymopanax, Melastomataceae/Combretaceae, and Cecropia, implies a progressive late-glacial increase of moist and cool climatic conditions. A comparable increase in Podocarpus is found in other lowland records in Amazonia. A review of published pollen data from Amazonia suggests that the moisture source was from the southeast. By contrast, present-day moisture comes from the tropical Atlantic and from the Amazon basin, with its convective precipitation. The likely cause for the southeastern moisture source between ca. 15,000 and 14,500 cal yr B.P. was enhanced polar (Antarctic) advection that reached low latitudes and maintained year-round the meteorological equator in its austral-winter position at northern latitudes or reduced drastically its southward summer displacement. This hypothesis is consistent with marine and ice core records.

Paleohydrological changes during the last deglaciation in Northern Brazil

“…Our ability to track more sensible hydrological changes will depend on future work on calibration that are now required in the tropics. (Ledru et al, 2001;Sifeddine et al, 2003). Low AP values reflect the development of a savana whereas high values attest to the presence of a humid forest.…”

“…The very low AP/NAP ratios (arboreal against herbaceous pollens) during the considered time span indicate sparse vegetation dominated by grasses, typical of semi-arid climate (Ledru et al, 2001;. This is also depicted by low δ 13 C values for triacontanic acid that attest to a strong contribution of C4 plants and/or lower stomatal conductance of C3 plants (Figure 3).…”

“…0.65 year. Modern vegetation that is governed by dune dynamics and topography ranges from littoral steppe vegetation "restinga" to sandy savanna "cerrado" with "restinga" species admixed (Ledru et al, 2001 …”

Contrasted distributions of triterpene derivatives in the sediments of Lake Caçó reflect paleoenvironmental changes during the last 20,000 yrs in NE Brazil

“…These rather humid conditions allowed forest development around the lake (Ledru et al, 2001) that became perennial with a notable water level. Nevertheless, the lake waters were well oxygenated as a result of a strong mixing by the westerlies.…”

Pentacyclic triterpene methyl ethers in recent lacustrine sediments (Lagoa do Caçó, Brazil)

“…1). The sedimentary filling has been studied using various approaches to better document palaeoclimatic development since the Last Glacial Maximum (LGM) in the Tropics (Ledru et al, 2001;2002;Sifeddine et al, 2003;Jacob, 2003;Jacob et al, 2004a and b). Here we report on the GC-MS characteristics of several series of pentacyclic triterpene methyl ethers (PTMEs) that are present in the lipid extracts of Quaternary sediments from this tropical setting and their significance as plant source indicators or diagenetic indicators.…”