Stephen J. Burns

High-Latitude Forcing of the South American Summer Monsoon During the Last Glacial

Monsoon Forcing Dansgaard/Oeschger (D/O) events—the sudden, millennial-scale periods of warmth that punctuate the cold climate of the Last Glacial period, and Heinrich events—cold intervals characterized by tremendous discharges of icebergs into the North Atlantic, have been observed at many locations in the Northern Hemisphere but not extensively in the Southern Hemisphere. Kanner et al. (p. 570, published online 12 January; see the Perspective by Rodbell) present a stable isotope record from the central Peruvian Andes, which represents a record of the South American Summer Monsoon from 50,000 to 16,000 years ago and which contains the signals of D/O and Heinrich events. The Southern Hemisphere monsoon displayed an antiphase relationship to Northern Hemisphere monsoon intensity at the millennial scale, and Antarctic millennial-scale climate fluctuations influenced the South American Summer Monsoon. High-latitude processes in the Northern and Southern Hemispheres both influence the South American Summer Monsoon. The climate of the Last Glacial period (10,000 to 110,000 years ago) was characterized by rapid millennial-scale climate fluctuations termed Dansgaard/Oeschger (D/O) and Heinrich events. We present results from a speleothem-derived proxy of the South American summer monsoon (SASM) from 16,000 to 50,000 years ago that demonstrate the occurrence of D/O cycles and Heinrich events. This tropical Southern Hemisphere monsoon reconstruction illustrates an antiphase relationship to Northern Hemisphere monsoon intensity at the millennial scale. Our results also show an influence of Antarctic millennial-scale climate fluctuations on the SASM. This high-resolution, precisely dated, tropical precipitation record can be used to establish the timing of climate events in the high latitudes of the Northern and Southern Hemispheres.

Climatic backdrop to the terminal Pleistocene extinction of North American mammals

North American terminal Pleistocene mammal extinctions are the subject of a long-running scientifi c debate. Although the role of climate has fi gured centrally, we lack clear knowledge of the timing and nature of terminal Pleistocene climate variability. Herein we document lengthy terminal Pleistocene drought in the southwestern United States (USA) using δ 13 C and δ 234 U effective moisture proxy data in speleothem calcite (stalagmite FS2) from Fort Stanton Cave, New Mexico, supplemented with age data from pool basin shelfstone speleothems from the Big Room in Carlsbad Cavern. This terminal Pleistocene drought, defi ned by a sharp rise in both δ 13 C and δ 234 U values, began just before 14.5 k.y. ago and lasted at least until 12.9 k.y. ago, when it was briefl y and only mildly interrupted by the Younger Dryas. The timing and length of this drought (~1500 yr) match the Northern Hemisphere Bolling-Allerod oscillation preserved in Greenland ice cores and exhibited in the δ 18 O record of stalagmite FS2. Rapid transition from cool moist Late Glacial to warm dry Holocenelike climatic conditions was likely unfavorable to many species of Pleistocene mammals in the southwestern USA. A climate-induced extinction implies that this last glacial cycle and its termination were more extreme than previous glacial cycles and/or glacial terminations.

Milankovitch-paced Termination II in a Nevada speleothem?

[1]xa0The Devils Hole (DH) calcite record from the Great Basin, Nevada, provided the first radiometrically dated time series of late-Pleistocene climate fluctuations,. In doing so, DH suggested that the penultimate deglaciation, Termination II (T-II), occurred at 142 ± 3 ka, which is ∼10 kyr before the associated rise in summer insolation, helping to launch the “causality problem” for Milankovitch. The interpretation of the DH record has been subject to considerable discussion ever since. We present the first attempt to reproduce the DH record of an early T-II in the Great Basin based on a stalagmite from Lehman Caves, Nevada, which is ∼400 km northeast of Devils Hole. Our speleothem records at least the latter part of T-II over its period of growth from ∼133–129 ka, which appears to disagree with the early timing of T-II in DH. DH also exhibits important differences from many other paleoclimate records over the past two deglaciations, though these are reduced somewhat after ice-volume correcting DH δ18O. We suggest that the climatic signal in the DH record remains to be fully explained.