Glacial Timescales Research Articles

Amazonian and neotropical plant communities on glacial time-scales: The failure of the aridity and refuge hypotheses

Plants respond to Pleistocene climatic change as species, not as associations or biomes. This has been demonstrated unequivocally by paleobotanical data for temperate latitudes. In the far richer vegetations of the tropics species populations also fluctuated independently in response to climatic forcing, from their longlasting glacial states to the patterns of brief interglacials like the present and back again. We use pollen data to reconstruct the vegetation of the Amazon basin in oxygen isotope stages 3 and 2 of the last glaciation in order to measure how the plant populations of the Amazon responded to the global warming at the onset of the Holocene. We find that plant communities of the neotropics vent copious pollen to lake sediments and that this pollen yields powerful signals for community composition. Three continuous sedimentary records reaching through oxygen isotope stage 2 are available from the Amazon lowlands, those from Carajas, Lake Pata and marine deposits off the mouth of the Amazon River. All three records yield pollen histories of remarkable constancy and stability. By comparing them with deposits of equal antiquity from the cerrado (savanna) of central Brazil, we show that most of the Amazon lowlands remained under forest throughout a glacial cycle. This forest was never fragmented by open vegetation as postulated by the refugia hypothesis. Instead the intact forest of glacial times included significant populations of plants that are now montane, suggesting that the global warming of the early Holocene resulted in the expulsion of heat intolerant plants from the lowland forest. Pollen data from the Amazonian flank of the Andes and from Pacific Panama provide evidence that populations of these heat intolerant plants survive the heat of interglacials in part by maintaining large populations at cooler montane altitudes. Our conclusion that the Amazon lowlands were forested in glacial times specifically refutes the hypothesis of Amazonian glacial aridity. Accordingly we examine the geomorphological evidence for glacial aridity and find it wanting. Of the three paleodune systems reported for tropical South America, that of NE Brazil was active in the Holocene as well as the Pleistocene. Parts of NE Brazil were actually moister than now in late-glacial times. Paleodunes in the Pantanal have never been seen on the ground, and those in the Orinoco Llanos are undated and may be of any age since the Tertiary. Arkosic sands in the Amazon fan deposits came from the Andean foothills or from down cutting by rivers and cannot be evidence of a former arid land surface. White sands of Amazonia formed as podzols, not by aeolian activity. Such Amazonian stone lines as have received critical scrutiny are concretionary pisolites in stratigraphic formations that are more than ten million years old. Although the Amazon was never arid, modeling cooler glacial tropics gives plausibility to a somewhat drier Amazon in glacial times, a concept given substance by pollen data for the movement of ecotones in Rondonia, by stream histories in the Bolivian Andes, and by evidence for lowered lake levels at Carajas and Lake Pata. But this reduced precipitation was never enough to fragment the forest in the Amazon lowlands themselves. Pleistocene mammals of the Napo river valley in Ecuador were able to live along the river system in a forested landscape. Our data suggest that the Amazon forests have been stable since the start of the Pleistocene, a fact that has contributed to the storage of vast diversity. The coming anthropogenic global warming and CO 2 enrichment will add to the global warming already endured by Amazon biota in the Holocene. We think it possible that the expulsion from the lowland forests of heat intolerant species is already complete and that the forest property of maintaining its own microhabitat will allow the high species richness to survive more global warming, provided large enough tracts of forest are preserved.

Effects of climate and landscape development on the terrestrial phosphorus cycle

Past variations in the terrestrial cycle of phosphorus (P), a biolimiting nutrient, are poorly understood. Detailed geochemical analysis of P in lake-sediment cores spanning the late Pleistocene to the present from the western Appalachian Plateau, the Coast Mountains of British Columbia, and the San Bernardino Mountains of southern California reveal large changes in terrestrial P cycling. P is transformed from a mineralized form to more bioavailable forms with increased soil development and landscape stability in the catchments. This process occurs relatively rapidly (i.e., within 3,000--5,000 yr) and can be reset quickly; the degree of transformation depends on local climate and the starting point of the soil system. The release of solid-phase P from these landscapes follows modeled patterns of high rates during the initial stages of soil development and low rates upon soil and landscape stabilization. Changes in weathering style were confirmed for the southern California catchment by analysis of Ge/Si ratios of lake diatoms, which revealed substantially higher values during an interval of high P release and low values at present. These records point to significant variability in the terrestrial P mass balance on glacial time scales, a feature that needs to be incorporated into models of nutrient andmore » carbon cycling in the past.« less

Polar motion of a viscoelastic Earth due to glacial cycle mass loading

The growth and decay of ice sheets can change the symmetry axis of the global mass distribution and thus excite motion of the rotation axis of the Earth. We develop a simple, normal mode expansion of the operator which converts surface load histories into polar motion histories, and use it to characterize the polar motion response to arbitrary surface loading excitation, in terms of gain and phase, for a wide range of Earth models and excitation time scales. Uncertainties in loading history presently limit the utility of constraints on mantle rheology which can be obtained from matching the present direction and rate of motion of the pole. Because of its magnitude and distance from the pole, the Laurentide ice sheet alone can induce motion of the rotation pole by over 100 km. This will influence the pattern of incident radiation on continents and oceans, and may play a significant role in limiting ice sheet growth. Differential motion of the mantle and core, on a glacial timescale, may also influence the geomagnetic field.