Late Pleistocene glaciation of the Mt Giluwe volcano, Papua New Guinea
Highlights
► A record of glaciation in Papua New Guinea extending back over 300,000 years. ► 39 new Cl-36 exposure ages identify three phases of glaciation on Mt Giluwe. ► New glacial periods are recognised at ∼65,000 and ∼150,000 years ago. ► Glaciers were present in Papua New Guinea until the start of the Holocene. ► The LGM was at least 5° colder than present.
Introduction
The tropics receive the greatest energy per unit area from the sun anywhere on Earth. Heat received in the tropics is distributed by wind, surface ocean currents and by latent heat to the rest of the planet, controlling the distribution of climate. The mountains of New Guinea lie in the western Pacific Ocean tropics near the centre of the Indo-Pacific warm pool. This region is the largest body of warm ocean on the planet and spans almost half way around the globe. Changes in the temperature of this water body not only strongly influence the climate of New Guinea but have hemispheric climatic consequences.
Knowledge of the evolution of the warm pool through the Pleistocene has grown rapidly over the last 30 years. The highly influential CLIMAP project (Climap, 1976, CLIMAP Project Members, 1981, Thompson, 1981) documented remarkably little change in tropical sea surface temperature (SST) during the last glacial maximum (LGM), an event when the lowest temperatures in the late Pleistocene would be expected. Since this time, several new techniques have been developed to estimate SST in deep sea sediments, and a large number of new cores with good chronologies covering the LGM have become available. Despite this, recent revisions of SST during the LGM indicate that the CLIMAP conclusion of little change remains robust (MARGO Project Members, 2009). Estimates of cooling in SST from three independent methods around the perimeter of New Guinea in the Western Pacific Ocean range from 0 to 3 °C (Rosell-Melé et al., 2004, Barrows and Juggins, 2005, Barker et al., 2005). This minimal cooling strongly contrasts with the observation that the snowline high in the mountains of New Guinea was as much as 1100 m lower at the same time, requiring a temperature difference of 5–6 °C (Löffler, 1972). This difference in surface cooling has been perceived as an apparent ‘paradox’ and has fuelled a long standing debate attempting to reconcile the two sets of estimates (Webster and Streten, 1978, Rind and Peteet, 1985, Peterson et al., 2002). No consensus has emerged as to an explanation that satisfactorily explains this phenomenon. Some steepening of tropical lapse rates probably occurred but this does not fully resolve the issue (e.g., Blard et al., 2007).
Despite significant progress in refining SST estimates, less progress has been made in quantifying temperature changes in tropical mountains. Glaciers that existed in the mountains of New Guinea are ideally placed to record temperature changes in the western Pacific Ocean through time. However, the maximum lowering of the snowline is not directly dated, and hence it is possible that cooling estimates may have been incorrectly attributed to the LGM. In recent reviews, the lack of chronological control was highlighted (Mark et al., 2005). The island of New Guinea represents an ideal area to reassess the tropical ‘paradox’, both because it records extensive glaciation and because the surrounding Indo-Pacific warm pool is central to the debate on the magnitude of maximum cooling in the tropics. Most moraines on New Guinea are assigned to the LGM on the basis of limiting radiocarbon dates in only a few locations (Prentice et al., 2005), and because of a lack of evidence for more than one glaciation (Löffler, 1972). Therefore the outermost are frequently assigned an LGM age. Clearly, the correct moraine sequences need to be identified before temperature estimates from snowline changes can be reliably attributed to the LGM.
In this paper we present a new study of the moraine systems on the southwest side of Mt Giluwe on the island of New Guinea. We present maps of the glacial geology and review previous glacial stratigraphies. To constrain the age of glaciation we present exposure ages on boulders from representative moraine sequences. Together with radiocarbon dates from basal sediments in associated basins, we construct the first glacial chronostratigraphy for a mountain in New Guinea. These ages are used as the basis for a regional review of the timing of glaciation in Southern Hemisphere sector of the western Pacific Ocean. Lastly, based on the heights of dated moraines, we review previous estimates of temperature change attributed to changes in the equilibrium line-altitude (ELA).
Section snippets
Background
The mountains on the island of New Guinea were the most extensively glaciated area in the Asian tropics during the late Pleistocene (Fig. 1). On the western side of the island over 3400 km2 was glaciated, mostly in the form of an almost continuous ice field from Mt Jaya and Mt Idenburg to the Star Mountains Prentice et al., (2011). On the eastern side of New Guinea, there is evidence for glacial activity on at least 20 mountains. The ice cap on Mt Giluwe was the most extensive area of
Exposure dating
The exceptionally preserved moraines of Mt Giluwe provide good targets for exposure dating. We chose sampling sites to cover the full range of glacial features from the outermost icecap moraines to the retreat sequences. MP and GH collected the GIL samples in 2001 and TTB, MP and GH collected the GLW samples in 2003. In addition, GH and MP collected peat cores in 2001 and 2003 for radiocarbon dating. Because of poor regional elevation control, we used the Shuttle Radar Topography Mission (SRTM)
Results
Exposure ages calculated using the 36Cl data are presented in Table 4. The exposure ages are discussed below in order of decreasing age.
Discussion
Exposure dating of moraines has identified at least four distinct stages of glaciation on Mt Giluwe within the last 150,000 years. These moraines are difficult to subdivide on the basis of weathering and preservation alone and this has led to them being previously classified as one (Blake and Löffler, 1971, Löffler, 1972) or two glaciations (Bik, 1972).
To interpret the glacial history of Mt Giluwe in a regional context, we have assembled two key regional SST records from the equatorial western
Acknowledgements
This work was conducted as part of ARC Discovery grants awarded to TTB (DP0557143) and GH and an NSF grant to MP (0234546). We thank Ernst Löffler for assisting GH with earlier coring.
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