Palaeogeography, Palaeoclimatology, Palaeoecology
A multiproxy palaeoecological record of Holocene lake sediments from the Rio Tapajós, eastern Amazonia
Introduction
A variety of potentially powerful mechanisms have been proposed that would have reshaped Amazonian floras in the Quaternary. These forces include: flooding (Campbell and Frailey, 1984, Nores, 1999), drought (Haffer, 1969, Van der Hammen and Absy, 1994) and cooling (Liu and Colinvaux, 1985, Bush et al., 1990, Colinvaux et al., 1996). Among biogeographers, contrasting views of community response to such climate changes are disparate, with proponents of savanna corridor formation (Van der Hammen and Hooghiemstra, 2000) or arcs of dry forest (Prado and Gibbs, 1993) suggesting direct connectivity of xeric habitat types across Amazonia. Other schools of thought proposed forest contraction due to drought, leaving isolated Pleistocene forest refugia within a sea of savanna (Haffer, 1969, Prance, 1982, Brown, 1987), relatively unchanging forest (Haberle and Maslin, 1999, Colinvaux et al., 2000, Colivaux et al., 2001, Bush et al., 2002) or a forest with markedly lowered productivity (Cowling et al., 2001). While the extreme stage of these changes is usually hypothesized to have occurred at the last glacial maximum (LGM), there are suggestions that substantial community changes could have affected Amazonia within the last 11,000 years (e.g. Servant et al., 1981).
The Holocene is clearly not the climatically constant period that was formerly portrayed. Millennial (Bond) cycles and even Dansgaard-Oeschger cycles are still evident (Hughen et al., 2000, Lea et al., 2000), though weaker than in the Pleistocene. In South America, the Early to Mid-Holocene was a time of rising sea level, weakened El Niño/Southern Oscillation (ENSO), a weakened South American Monsoon and a strengthened inter tropical convergence zone (Martin et al., 1993, Martin et al., 1997). Not surprisingly, many of these climatic events resonate in the paleoecological records of the Neotropics. A Mid-Holocene dry event caused by the weakened South American Summer Monsoon between ca. 9200 and 4400 cal. years BP (all ages are reported in calendar years Before Present (cal. years BP)) is well documented in the Andes (Seltzer et al., 1995, Thompson et al., 1995, Thompson et al., 2000, Baker et al., 2001, Moy et al., 2002, Paduano et al., 2003) and a coincident dry event has been suggested to have initiated major savanna expansion within Amazonia (Servant et al., 1981, Absy et al., 1991). A further potential climatic link is that this mid-Holocene period between 9000 and 5600 cal. years BP was one of a quiescent ENSO (Sandweiss et al., 1996, Marengo et al., 1998, Sandweiss et al., 1999).
Only a few complete Holocene paleoecological records exist for Amazonia (Absy et al., 1991, Behling, 1996, Colinvaux et al., 1996, Behling and Da Costa, 2000, Bush et al., 2000, Weng et al., 2002, Burbridge et al., 2004). In general, records from south of the equator (Absy et al., 1991, Mayle et al., 2000, Listopad, 2001) show a strong Mid-Holocene dry event, whereas this event is more weakly manifested (or not detectable) in records from central or northwestern Amazonia (Bush et al., 2000). We present a complete Holocene riverine paleoecological sequence from the lower basin of the Tapajós River, a drainage that spans the savanna-forest ecotone of southern Amazonia.
Section snippets
The site
The two sediment cores represented here are taken from “Lago Tapajós”, a 150-km long by 15-km wide basin that forms the main channel of the lower section of the Rio Tapajós. Repeated erosional cycles during Quaternary marine lowstands incised a deep, wide, valley, which when flooded resembles a lake more than a river (Fig. 1). The Tapajós drains a lowland area of ca. 489,000 km2 and has such a shallow gradient that, when sea level rose after the LGM, it impeded drainage and caused the formation
Methods
Sediment mapping was attempted using a 3.5-kHz profiler, but large concentrations of methane prevented penetration of the waves. Consequently, the bottom profile of the lake sediments was mapped using 50 soundings with a 2-cm diameter probe. Based on these soundings, a coring location was selected. Using a barge-mounted drill-rig, a 20-m long sediment core–5 cm in diameter–was taken (TAP99) in 1999. In 2002, using the same drill rig, a 50-m long core (TAP02) was raised (Fig. 2). The upper 42 m of
Results
Both the upper 20 m of the TAP99 and the TAP02 cores were analyzed for grain size and were found to be very similar. The rate of sedimentation between the two cores was also consistent. On this basis we have used both dates from TAP99 and TAP02 to derive our chronology for TAP02.
Discussion
The initial expectation was to recover lake sediments deposited 14,000 years ago. However, this turned out to be impossible because a huge sand body had been pushed onto the older sediments during the Late Pleistocene rise of the lake-level (caused by the rise in sea level). This process may be compared with the “bulldozing effect” in the ocean shelf areas where after the LGM huge masses of sands were pushed landwards by the rising sea level (Cowell et al., 1992). Consequently, the sands at the
Conclusions
The paleoecological record from a 42-m long core that spans the last 11,000 years indicates the continuous presence of a mesic forest landscape around Lago Tapajós. The data suggest that at least at a landscape level there were no substantial biome changes within the Holocene. As Holocene drying strong enough to dessicate shallow lake systems has been documented from southeastern and southwestern Amazonia (Absy et al., 1991, Mayle et al., 2000), those records are clearly more sensitive than the
Acknowledgements
The investigations were undertaken through cooperation between the Instituto Nacional de Pesquisas da Amazônia (INPA)/Manaus-Brazil and the Max-Planck-Intitut für Limnologie, AG Tropenökologie/Plön-Germany. Funding for pollen analyses was provided by NSF grant (DEB-9732951).
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