Mismatch of glacier extent and summer insolation in Southern Hemisphere mid-latitudes

Abstract

Here we address a long-standing puzzle of ice-age climate called the "fly in the ointment of the Milankovitch theory." Using geomorphic mapping and 10 Be surface-exposure dating, we show that five moraine belts were formed during maxima of the last ice age by the Pukaki glacier in New Zealand's Southern Alps. They afford ages of 41.76 ± 1.09 ka, 35.50 ± 1.26 ka, 27.17 ± 0.68 ka, 20.27 ± 0.60 ka, and 18.29 ± 0.49 ka. These five maxima spanned an entire precessional cycle in summer insolation intensity at the latitude of the Southern Alps. A similar mismatch between summer insolation and glacier extent also characterized the Chilean Lake District in the mid-latitudes of South America. Thus, in apparent contrast to northern ice sheets linked by Milankovitch to summer insolation at 65°N latitude, the behavior of southern mid-latitude glaciers was not tied to local summer insolation intensity. Instead, glacier extent between 41.76 ka and 18.29 ka, as well as during the last termination, was aligned with Southern Ocean surface temperature and with atmospheric carbon dioxide.

The timing and cause of glacial advances in the southern mid-latitudes during the last glacial cycle based on a synthesis of exposure ages from Patagonia and New Zealand

“…The first is that many Southern Hemisphere climate proxies, including Antarctic temperature records, suggest glacial-interglacial climate change occurred broadly in-phase between the Southern and Northern Hemispheres, despite covariance of summer insolation (Mercer, 1984;Jouzel et al, 2007;Wolff et al, 2010a;Huybers & Denton, 2008). Secondly, glacial activity prior to the gLGM, as observed in our compiled chronology, is at odds with a model in which conditions suited to maximum ice growth occurred during the gLGM (Barrows et al, 2007a;Wolff et al, 2009;Doughty et al, 2015). One explanation is that, for the Southern Hemisphere, the duration of seasons may exert a greater control on climate than insolation intensity (Huybers & Denton, 2008).…”

“…This suggests that the same forcing factors may have controlled the timing of glacial activity in both regions over the last glacial cycle and strongly suggests that glaciers advanced by at least 45 ka, or mid-MIS 3, well before the gLGM. Although this study focuses on chronology, it is worth highlighting that several studies found the limits relating to these advances to be as extensive, if not significantly more extensive, than those deposited during the gLGM (Glasser et al, 2011;Putnam et al, 2013b;Kelley et al, 2014;Rother et al, 2014;Doughty et al, 2015;Darvill et al, 2015;Schaefer et al, 2015).…”

“…Recent work (Glasser et al, 2011;Putnam et al, 2013b;Kelley et al, 2014;Rother et al, 2014;Doughty et al, 2015;Schaefer et al, 2015;Darvill et al, 2015) has identified that some glaciers in Patagonia and New Zealand advanced to greater extents prior to the global Last Glacial Maximum (gLGM; ca. 26.5-19 ka; Clark et al, 2009) and Marine Isotope Stage (MIS) 2.…”

“…Secondly, the forcing factors behind southern mid-latitude glaciation during the last glacial cycle are ambiguous, as is the relationship to climatic drivers in the Northern Hemisphere. For example, insolation does not appear to directly control Southern Hemisphere climate change (Huybers & Denton, 2008;Doughty et al, 2015), whereas the movement of the southern westerly winds and oceanic frontal systems have been invoked as drivers of climate and glacial advances by controlling precipitation and sea surface temperatures (Lamy et al, 2004(Lamy et al, , 2007Barrows et al, 2007a;Denton et al, 2010). The roles of sea ice and ocean stratification, whilst likely important, also remain unclear (Allen et al, 2011;Denton et al, 2010;Putnam et al, 2013b).…”

The last deglaciation of Peru and Bolivia

“…The tropical Pacific Ocean also modulates circulation dynamics, precipitation amounts, and temperature in the Andes, and may have influenced western Andean ranges differentially over time . Other work in New Zealand and mid-latitude Chilean Lakes district (Doughty et al, 2015) also show a decoupling of glacier advances from local summer insolation that seems convincingly to pace the large Northern hemisphere ice sheets. In contrast, these southern mid-to-high-latitude sites suggest that glacier extent during the last termination was aligned with Southern Ocean surface temperature and with atmospheric carbon dioxide.…”