Ages and inferred causes of Late Pleistocene glaciations on Mauna Kea, Hawai'i

Abstract

Glacial landforms on Mauna Kea, Hawai'i, show that the summit area of the volcano was covered intermittently by ice caps during the Late Pleistocene. Cosmogenic 36 Cl dating of terminal moraines and other glacial landforms indicates that the last two ice caps, called Older Makanaka and Younger Makanaka, retreated from their maximum positions approximately 23 ka and 13 ka, respectively. The margins and equilibrium line altitudes of these ice caps on the remote, tropical Pacific island were nearly identical, which would seem to imply the same mechanism for ice growth. But modelling of glacier mass balance, combined with palaeotemperature proxy data from the subtropical North Pacific, suggests that the causes of the two glacial expansions may have been different. Older Makanaka air atop Mauna Kea was likely wetter than today and cold, whereas Younger Makanaka times were slightly warmer but significantly wetter than the previous glaciation. The modelled increase in precipitation rates atop Mauna Kea during the Late Pleistocene is consistent with that near sea level inferred from pollen data, which suggests that the additional precipitation was due to more frequent and/or intense tropical storms associated with eastward-moving cold fronts. These conditions were similar to modern La Niñ a (weak ENSO) conditions, but persisted for millennia rather than years. Increased precipitation rates and the resulting steeper temperature lapse rates created glacial conditions atop Mauna Kea in the absence of sufficient cooling at sea level, suggesting that if similar correlations existed elsewhere in the tropics, the precipitation-dependent lapse rates could reconcile the apparent difference between glacial-time cooling of the tropics at low and high altitudes.

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“…The boundaries and interpretation of the nature of the flood deposit were based primarily on the location of poorly-sorted finer-grained material, and the presence of large cobbles and boulders ponded behind kipukas, suggesting a flow velocity greater than would normally be expected for a common glacial outwash channel carrying only sand and gravel. This hypothesis is also consistent with the deposit age estimates of both Anslow et al [22] and Pigati et al [21]. The Science Backroom also noted that the western deposit might be younger based on superposition relationships, although this relationship was not unambiguous based on rover imaging data alone.…”

“…They interpreted the valley deposit as having formed over 3-4000 years as glacial meltwater cut through and washed out portions of the moraines, redepositing clastic material downstream in channels and fans. However, Anslow et al [22] calculated a bimodal distribution of ages for boulders in the deposit, and explained this discordance with the dates of Pigati et al [21] by observing that the fan is composed of unsorted sediment with well-defined edges lying in a V-shaped gully that eroded through the distal moraine to the east of Pu'u Keonehinoe. They interpreted the deposit as having formed by catastrophic drainage of a moraine-dammed lake, a onetime event occurring around 12,000 years ago.…”

“…Attempts to date the advance and retreat of the Pleistocene glaciers [20][21][22] have led to various interpretations of the glacially-derived valley deposit. Pigati et al [21] interpreted the valley deposit as a "boulder fan" and suggested, based on boulder composition, that boulders were excavated from the Younger Makanaka moraine currently plugging the northern valley entrance, and transported a few 100 m downslope.…”

“…Pigati et al [21] interpreted the valley deposit as a "boulder fan" and suggested, based on boulder composition, that boulders were excavated from the Younger Makanaka moraine currently plugging the northern valley entrance, and transported a few 100 m downslope. They interpreted the valley deposit as having formed over 3-4000 years as glacial meltwater cut through and washed out portions of the moraines, redepositing clastic material downstream in channels and fans.…”

Timing and nature of late Quaternary mountain glaciation

“…It is essentially a palaeoclimatic lightning rod, sampling climate in the equatorial Pacific ocean. New 36 Cl chronologies from Mauna Kea, Hawaii, are assessed by Pigati et al (2008). They document the development of two summit ice caps, of similar extent, during MIS 2, with glaciers retreating from terminal positions ca.…”