Abstract
The Greenland Stadial 1 (GS-1; ~12.9 to 11.65 kyr cal BP) was a period of North Atlantic cooling, thought to have been initiated by North America fresh water runoff that caused a sustained reduction of North Atlantic Meridional Overturning Circulation (AMOC), resulting in an antiphase temperature response between the hemispheres (the ‘bipolar seesaw’). Here we exploit sub-fossil New Zealand kauri trees to report the first securely dated, decadally-resolved atmospheric radiocarbon (14C) record spanning GS-1. By precisely aligning Southern and Northern Hemisphere tree-ring 14C records with marine 14C sequences we document two relatively short periods of AMOC collapse during the stadial, at ~12,920-12,640 cal BP and 12,050-11,900 cal BP. In addition, our data show that the interhemispheric atmospheric 14C offset was close to zero prior to GS-1, before reaching ‘near-modern’ values at ~12,660 cal BP, consistent with synchronous recovery of overturning in both hemispheres and increased Southern Ocean ventilation. Hence, sustained North Atlantic cooling across GS-1 was not driven by a prolonged AMOC reduction but probably due to an equatorward migration of the Polar Front, reducing the advection of southwesterly air masses to high latitudes. Our findings suggest opposing hemispheric temperature trends were driven by atmospheric teleconnections, rather than AMOC changes.
Highlights
The Greenland Stadial 1 (GS-1; ~12.9 to 11.65 kyr cal BP) was a period of North Atlantic cooling, thought to have been initiated by North America fresh water runoff that caused a sustained reduction of North Atlantic Meridional Overturning Circulation (AMOC), resulting in an antiphase temperature response between the hemispheres
Associated with GS-1 is a broadly synchronous change in Europe described as the Younger Dryas (YD) stadial, the pollen assemblage zone during which there was a return to near-glacial conditions and coincident with an abrupt change in radiocarbon ages (~12.70 to 11.65 kyr cal BP)[7,8,9,10,11]
We present a new decadally-resolved record of atmospheric 14C from a cohort of 40 sub-fossil kauri logs that have recently been discovered at a farm near Towai (35°30′S, 174°10′E) in Northland, producing a floating 1451-yr tree-ring chronology, supplemented by a single 533-yr tree ring record (FIN11) from Finlayson Farm, near Kai Iwi Lakes, Northland (35°50′S, 173°39′E) which together span the full GS-1 chronozone
Summary
Meridional Overturning Circulation during Greenland Stadial 1 received: 13 November 2015 accepted: 21 April 2016 Published: 19 May 2016. Regardless of the exact magnitude of the 14C offset, the observed shift to older SH ages is consistent (and synchronous within the uncertainty in the WAIS Divide chronology across this period12–see Fig. 1G) with increased ventilation of the Southern Ocean following the termination of the ACR, associated with reinvigoration of Antarctic Bottom Water formation, sea ice retreat and reduced Antarctic ice melt[15,40,41,42]. Sustained frigid conditions post 12,640 cal BP and a postulated reorganisation of atmospheric circulation[8,45] are coincident with the triplet of peaks identified in both tree ring Δ14C and 10Be in Greenland ice[46,47,48] (Fig. 2) suggesting periods of reduced solar activity may have played a similar role to those that occurred during the Little Ice Age (LIA; CE 1580–1880)[49,50,51]. Our results support a growing body of evidence that global scale changes during the GS-1 chronozone[3,16] were primarily driven by interhemispheric atmospheric teleconnections
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