LATE QUATERNARY CHANGES IN CARBON CYCLING ON THE SW MARGIN OF AMAZONIA: CARBON-ISOTOPE EVIDENCE

During the late Quaternary, climatic or physiological drought conducive to the spread of dry forests and savannnas may have affected southern Amazonia under two quite distinct palaeoenvironmental scenarios: 1) lower atmospheric CO2 and sea-surface temperatures during the last glacial maximum (LGM), when the Earth’s orbital configuration was similar to today; and 2) reduced seasonality and a weaker austral summer monsoon during the early to mid-Holocene, resulting from orbital forcing. Both scenarios 1) and 2) would have favoured the spread of C4 graminoids, which exhibit significantly higher stable carbon-isotope values, at the expense of C3 trees, shrubs and herbs. This hypothesis is examined in the light of 13C data on total organic carbon (TOC) from the sediments of two lakes in eastern Bolivia: Laguna Chaplin (14 28’S, 61 04’W), which is surrounded today by humid evergreen rainforest; and Laguna La Gaiba (17 45’S, 57 35’W), which is located in closed-canopy seasonal dry forest. Sediment cores from these lakes have basal ages older than 43,400 ± 1900 and 30,670 ± 390 14C yr BP, respectively, and are being analysed for mineral magnetics, pollen, TOC, C/N ratios, 13C, and in the case of Laguna Chaplin, fossil grass cuticles. Both cores exhibit a range in 13C values of 8 – 10 per mille, with higher isotope values prior to and during the LGM, lower values centred on the late-glacial, higher values during the mid-Holocene, and a strong decrease over the last few millennia, suggesting that the relative abundance of C4 plants has varied in line with the expected occurrence of drought conditions. A comparison with a limited 13C dataset from Laguna Bella Vista (13 36’S, 61 33’W), which is also surrounded by humid evergreen rainforest, implies that the spread of C3 plants during the later Holocene progressed from north to south, consistent with a southwards shift in the position of the ITCZ during austral summer.

Mayle, F.E. et al., 2000. Millennial-scale dynamics of southern Amazonian rain forests. Science 290, 2291-2294.