TY - JOUR
T1 - The Zealandia Switch
T2 - Ice age climate shifts viewed from Southern Hemisphere moraines
AU - Denton, George H.
AU - Putnam, Aaron E.
AU - Russell, Joellen L.
AU - Barrell, David J.A.
AU - Schaefer, Joerg M.
AU - Kaplan, Michael R.
AU - Strand, Peter D.
N1 - Funding Information:
Funding provided by the Comer Family Foundation , the Quesada Family Foundation , the National Science Foundation , and from the New Zealand Government through the GNS Science ‘Global Change through Time’ research program (David Barrell). George Denton received support from National Science Foundation grant EAR-1102782. Aaron Putnam and Peter Strand acknowledge support from a National Science Foundation CAREER grant ( EAR-1554990 ). Michael Kaplan acknowledges support from National Science Foundation grant EAR-0745781 . Roseanne Schwartz and Jeremy Frisch assisted with laboratory work. Alice Doughty, Samuel Kelley, Kathryn Ladig, and Bess Koffman assisted in the field. We thank the New Zealand Department of Conservation and the individual landowners for permission to carry out the fieldwork. Thanks to Robert F. Anderson who suggested the term Mercer’s Paradox and kindly allowed us to make use of it. Peter Huybers kindly provided guidance on constructing the 65°S seasonal duration curve. We thank Helen Bostock for providing us with a vector-based version of the SST diagram ( Bostock et al., 2015 ) used in Fig. 6 . Finally, we thank Andrew Lorrey and an anonymous reviewer for thoughtful comments that enabled us to improve the clarity and presentation of the manuscript. This is LDEO contribution no. 8461.
Publisher Copyright:
© 2020 Elsevier Ltd
PY - 2021/4/1
Y1 - 2021/4/1
N2 - Two fundamental questions about the ice-age climate system await satisfactory resolution. First, if summer solar radiation intensity truly controls the orbital signature of the last glacial cycle, then why were major climatic shifts, including the last termination, globally synchronous? Second, what caused the millennial-scale climate oscillations superimposed on this cycle? We address these questions from a Southern Hemisphere perspective focused on mid-latitude mountain ice fields. We put particular emphasis on the last glacial termination, which involved both orbital-scale and millennial-scale climate elements and has generally well-resolved chronological control. Sustained retreat of mountain glaciers, documented by detailed mapping and chronology of glacial landforms in the Southern Alps and southern Andes, marked the termination of the last ice age, beginning ∼18 kyrs ago and involved a change from glacial to near-interglacial atmospheric temperature within a millennium or two. A rapid poleward shift of the Subtropical Front, delineating the northern margin of the Southern Ocean, ∼18 kyrs ago implies a concurrent poleward shift of the austral westerlies and leads us to hypothesize a southern origin for the dominant phase of the last glacial termination. Together with interhemispheric paleoclimate records and with results of coupled ocean-atmosphere climate modeling, these findings suggest a big, fast, and global end to the last ice age in which a southern-sourced warming episode linked the hemispheres. We posit that a shift in the Southern Ocean circulation and austral westerly wind system, tied to southern orbital forcing, caused this global warming episode by affecting the tropical heat engine and hence global climate. Central to this hypothesis, dubbed the ‘Zealandia Switch’, is the location of the Australia and Zealandia continents relative to Southern Hemisphere oceanic and atmospheric circulation. Coupled ocean-atmosphere climate modeling shows that the locus of the austral westerlies, whether in a more equatorward position representing a glacial-mode climate or in a poleward-shifted position marking interglacial-mode climate, has profound effects on oceanic and associated atmospheric linkages between the tropical Pacific and the Southern Ocean. Shifts in the austral westerlies have global climatic consequences, especially through resulting changes in the greenhouse gas content of the atmosphere and altered heat flux from the tropical Pacific into the Northern and Southern Hemispheres. We suggest that the last glacial termination was a global warming episode that led to extreme seasonality in northern latitudes by stimulating a flush of meltwater and icebergs into the North Atlantic from adjoining ice sheets. This fresh-water influx resulted in widespread North Atlantic sea ice that caused very cold boreal winters, thus amplifying the annual southward shift of the Intertropical Convergence Zone and the monsoonal rain belts. We further suggest that muted manifestations of the Zealandia Switch mechanism were responsible for smaller, recurring millennial-scale climate oscillations within the last glacial cycle.
AB - Two fundamental questions about the ice-age climate system await satisfactory resolution. First, if summer solar radiation intensity truly controls the orbital signature of the last glacial cycle, then why were major climatic shifts, including the last termination, globally synchronous? Second, what caused the millennial-scale climate oscillations superimposed on this cycle? We address these questions from a Southern Hemisphere perspective focused on mid-latitude mountain ice fields. We put particular emphasis on the last glacial termination, which involved both orbital-scale and millennial-scale climate elements and has generally well-resolved chronological control. Sustained retreat of mountain glaciers, documented by detailed mapping and chronology of glacial landforms in the Southern Alps and southern Andes, marked the termination of the last ice age, beginning ∼18 kyrs ago and involved a change from glacial to near-interglacial atmospheric temperature within a millennium or two. A rapid poleward shift of the Subtropical Front, delineating the northern margin of the Southern Ocean, ∼18 kyrs ago implies a concurrent poleward shift of the austral westerlies and leads us to hypothesize a southern origin for the dominant phase of the last glacial termination. Together with interhemispheric paleoclimate records and with results of coupled ocean-atmosphere climate modeling, these findings suggest a big, fast, and global end to the last ice age in which a southern-sourced warming episode linked the hemispheres. We posit that a shift in the Southern Ocean circulation and austral westerly wind system, tied to southern orbital forcing, caused this global warming episode by affecting the tropical heat engine and hence global climate. Central to this hypothesis, dubbed the ‘Zealandia Switch’, is the location of the Australia and Zealandia continents relative to Southern Hemisphere oceanic and atmospheric circulation. Coupled ocean-atmosphere climate modeling shows that the locus of the austral westerlies, whether in a more equatorward position representing a glacial-mode climate or in a poleward-shifted position marking interglacial-mode climate, has profound effects on oceanic and associated atmospheric linkages between the tropical Pacific and the Southern Ocean. Shifts in the austral westerlies have global climatic consequences, especially through resulting changes in the greenhouse gas content of the atmosphere and altered heat flux from the tropical Pacific into the Northern and Southern Hemispheres. We suggest that the last glacial termination was a global warming episode that led to extreme seasonality in northern latitudes by stimulating a flush of meltwater and icebergs into the North Atlantic from adjoining ice sheets. This fresh-water influx resulted in widespread North Atlantic sea ice that caused very cold boreal winters, thus amplifying the annual southward shift of the Intertropical Convergence Zone and the monsoonal rain belts. We further suggest that muted manifestations of the Zealandia Switch mechanism were responsible for smaller, recurring millennial-scale climate oscillations within the last glacial cycle.
KW - Climate dynamics
KW - Climate modeling
KW - Cosmogenic isotopes
KW - Geomorphology
KW - Glacial
KW - Glaciation
KW - Paleoclimatology
KW - Quaternary
KW - South Pacific Ocean
KW - Southern Hemisphere westerlies
KW - Southern Ocean
UR - http://www.scopus.com/inward/record.url?scp=85102261054&partnerID=8YFLogxK
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U2 - 10.1016/j.quascirev.2020.106771
DO - 10.1016/j.quascirev.2020.106771
M3 - Article
AN - SCOPUS:85102261054
VL - 257
JO - Quaternary Science Reviews
JF - Quaternary Science Reviews
SN - 0277-3791
M1 - 106771
ER -