Abstract
AbstractThe concept of Quaternary average conditions has gained popularity over the past few decades, especially with studies of long-term landscape evolution. In this paper, we critically assess this concept by analyzing the marine isotope record (LR04 δ18O stack) relative to the Quaternary. This shows that the frequency and amplitude of climate glacial-interglacial cycles are not constant throughout the Quaternary, with a clear change during the Middle Pleistocene Transition (MPT), and that many minor oscillations exist within each cycle. For this reason, the identification of pre- and post-MPT most-frequent and, cumulatively, longest-lasting (rather than average) conditions is recommended. The most-frequent pre-MPT δ18O value of 3.725 ± 0.025‰ last occurred during 11.31–11.47 ka, while the most-frequent post-MPT δ18O value of 4.475 ± 0.025‰ last occurred during 14.81–15.04 ka. However, many other δ18O values were almost as frequent throughout the Quaternary and we present geomorphological reasons as to why it is unlikely that the present-day landscape reflects Quaternary average or, indeed, most-frequent conditions. Collectively, our results indicate that extreme caution should be taken when attempting to infer long-term landscape evolution processes (including the buzzsaw hypothesis) based on average Quaternary conditions.
Highlights
The Quaternary covers the past 2580 ka
Through the use of three isotope records from marine sediment cores, it was calculated that the “average” Quaternary glacial conditions were last met ca. 12,000 yr BP, and were generally associated with an equilibrium-line altitude (ELA) depression of ∼500 m, roughly halfway between the last glacial maximum (LGM) ELA and the presentday ELA
Porter’s (1989) intuition that Quaternary climate extremes have had little influence on present day landscape is likely correct, and relevant since many landscape reconstructions tend to focus on these extremes, indirectly suggesting that what happened in-between is of secondary importance
Summary
The Quaternary covers the past 2580 ka. Human evolution largely occurred during this period, which is characterized by a dynamic climate, dominated by alternating glacial and interglacial conditions driven by orbital forcing. Most paleo-climatology, -geomorphology, and -glaciology studies have focused on the most recent portion of the Quaternary (i.e., the last glacial-interglacial cycle). Those that have ventured beyond the last cycle have been interested in the coldest and warmest “extremes” of the many glacial-interglacial cycles that characterized the Quaternary (e.g., looking for an analogue of the current interglacial). This was especially true some thirty years ago, when, in order to counter this excessive focus on Quaternary “extremes,” a popular paper (Porter, 1989, cited >200 times) advocated the need to shift attention to “average” Quaternary conditions, which were calculated using the available oxygen isotope record. The glacial cirques of the US Cascade Range (which have a Younger Dryas ELA ∼450 m above the LGM and ∼450 m below the modern), as well as the fjords of Norway, were presented amongst the most spectacular examples of the influence of “average” Quaternary glacial conditions on landscape evolution
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