Constraint on the penultimate glacial maximum Northern Hemisphere ice topography (≈140 kyrs BP)

Differences between the last two glacial maxima and implications for ice-sheet, δ18O, and sea-level reconstructions

“…Assuming a PGM sea-level drop of 109 ± 14 m, a PGM EIS volume of 33e53 m SLE , and comparable Antarctic excess ice volume between the PGM and LGM (assuming~17 m SLE as an upper bound, based on geologically constrained glaciological modelling; Table 1), the inferred values imply a North American NAIS volume as small as 59 to 39 m SLE (±14 m SLE ), respectively. A caveat applies with respect to attribution of component contributions to the overall sea-level drop, namely that various indicators for the maximum EIS extent may represent different glacial advance phases at different locations Lambeck et al, 2006;Hughes et al, 2013;Colleoni et al, 2016) (Fig. 1).…”

“…The identified contrasts between the PGM and LGM highlight a need for future research to unravel the causes of ice-age development into either a PGM-like, or an LGM-like mode. Different PGM and LGM ice distributions between North America and Eurasia imply different moisture fluxes over the continents between these glacial cycles, partly due to interactions between NAIS size and atmospheric dynamics (e.g., Colleoni et al, 2016;Liakka et al, 2016), and partly due to complex interacting processes that drive glaciations toward large-or small-NAIS sizes (e.g., Colleoni et al, 2011). For example, not only summer insolation and GHG forcing (Table 4), but also ice-albedo feedbacks, vegetation-albedo feedbacks, dust deposition on snow/ice, sea-ice expansion, and sea surface temperature reduction need to be considered (e.g., Oglesby, 1990;Calov et al, 2009;Colleoni et al, 2011;Abe-Ouchi et al, 2013;Liakka et al, 2016).…”

“…If PGM and LGM deep-sea temperatures were similar (within about 0.5 C), then a fundamental PGMeLGM offset is implied in the relationships between mean global-ocean d sw and sea level or ice volume. This might arise from: (a) ocean circulation differences among glacial cycles, filling different volumes of the deep sea with waters of different mean d sw values, although such contrasts are thought to be averaged over multi-millennial periods (e.g., Siddall et al, 2010); (b) different moisture pathways feeding contrasting glacial configurations (e.g., Ullman et al, 2014;Colleoni et al, 2016), with impacts on atmospheric vapour isotopic fractionation and, hence, ice d 18 O; or (c) potential development of massive, largely floating Arctic ice shelves during certain glacials (e.g., PGM), and not during others (e.g., LGM) e these would cause negligible sea level change, but considerable mean ocean d 18 O change (Niessen et al, 2013;Jakobsson et al, 2016). Below, we discuss these three options in turn.…”

“…Second, temporal development of ice-age cycles provides critical information about the nature of long-term climate cooling over the past few million years, in response to CO 2 reduction and interactions among ice, land cover, and climate (e.g., Clark et al, 2006;K€ ohler and Bintanja, 2008;de Boer et al, 2010de Boer et al, , 2012Hansen et al, 2013). Third, variable amplitude of individual ice ages helps to determine the relationship between climate change, astronomical climate forcing cycles, and climate feedbacks on timescales of 10se100s of kiloyears (e.g., Oglesby, 1990;Imbrie et al, 1993;Raymo et al, 2006;Colleoni et al, 2011Colleoni et al, , 2016Ganopolski and Calov, 2011;Carlson and Winsor, 2012;Abe-Ouchi et al, 2013;Hatfield et al, 2016;Liakka et al, 2016). Fourth, the size and spatial distribution of land ice during past glacials determines crustal rebound processes when ice masses melt, which in turn affects sea-level reconstructions for subsequent interglacials.…”

“…Geological data and numerical modelling strongly suggest that the Eurasian ice sheet (EIS) covered a larger area during the PGM than during the LGM Colleoni et al, 2011Colleoni et al, , 2016 (Fig. 1), with most estimates suggesting a PGM EIS volume equivalent to a 33e53 m global sea-level fall (sea-level equivalent, SLE) (Table 1); this is approximately twice the size of the LGM EIS (14e29 m SLE ; Table 1 and Clark and Tarasov, 2014).…”