NALPS19: sub-orbital-scale climate variability recorded in northern Alpine speleothems during the last glacial period
Authors: Moseley Gina E. , Spötl Christoph , Brandstätter Susanne , Erhardt Tobias , Luetscher Marc , Edwards R. Lawrence 
Published: January 2020

Abstract. Sub-orbital-scale climate variability of the last glacial period provides
important insights into the rates at which the climate can change state, the
mechanisms that drive such changes, and the leads, lags, and synchronicity
occurring across different climate zones. Such short-term climate variability
has previously been investigated using δ18O from speleothems
(δ18Ocalc) that grew along the northern rim of the
Alps (NALPS), enabling direct chronological comparisons with
δ18O records from Greenland ice cores (δ18Oice). In this study, we present NALPS19, which includes a
revision of the last glacial NALPS δ18Ocalc
chronology over the interval 118.3 to 63.7 ka using 11, newly available,
clean, precisely dated stalagmites from five caves. Using only the most
reliable and precisely dated records, this period is now 90 % complete
and is comprised of 16 stalagmites from seven caves. Where speleothems grew
synchronously, the timing of major transitional events in
δ18Ocalc between stadials and interstadials (and
vice versa) are all in agreement on multi-decadal timescales. Ramp-fitting
analysis further reveals that, except for one abrupt change, the timing of
δ18O transitions occurred synchronously within
centennial-scale dating uncertainties between the NALPS19
δ18Ocalc record and the Asian monsoon composite
speleothem δ18Ocalc record. Due to the
millennial-scale uncertainties in the ice core chronologies, a comprehensive
comparison with the NALPS19 chronology is difficult. Generally, however, we
find that the absolute timing of transitions in the Greenland Ice Core
Chronology (GICC) 05modelext and Antarctic Ice Core Chronology
(AICC) 2012 are in agreement on centennial scales. The exception to this is
during the interval of 100 to 115 ka, where transitions in the AICC2012
chronology occurred up to 3000 years later than in NALPS19. In such
instances, the transitions in the revised AICC2012 chronology of Extier et
al. (2018) are in agreement with NALPS19 on centennial scales, supporting the
hypothesis that AICC2012 appears to be considerably too young between 100 and
115 ka. Using a ramp-fitting function to objectively identify the onset and the end of abrupt transitions, we show that δ18O shifts took place on multi-decadal to multi-centennial timescales in the North Atlantic-sourced regions
(northern Alps and Greenland) as well as the Asian monsoon. Given the near-complete record of
δ18Ocalc variability during the last glacial
period in the northern Alps, we also offer preliminary considerations
regarding the controls on mean δ18Ocalc for given
stadials and interstadials. We find that, as expected,
δ18Ocalc values became increasingly lighter with
distance from the oceanic source regions, and increasingly lighter with
increasing altitude. Exceptions were found for some high-elevation sites that
locally display δ18Ocalc values that are heavier
than expected in comparison to lower-elevation sites, possibly caused by a
summer bias in the recorded signal of the high-elevation site, or a winter
bias in the low-elevation site. Finally, we propose a new mechanism for the
centennial-scale stadial-level depletions in δ18O such as the
Greenland Stadial (GS)-16.2, GS-17.2, GS-21.2, and GS-23.2 “precursor”
events, as well as the “within-interstadial” GS-24.2 cooling event. Our new
high-precision chronology shows that each of these δ18O
depletions occurred in the decades and centuries following rapid rises in sea
level associated with increased ice-rafted debris and southward shifts of the
Intertropical Convergence Zone, suggesting that influxes of meltwater from
moderately sized ice sheets may have been responsible for the cold reversals
causing the Atlantic Meridional Overturning Circulation to slow down similar
to the Preboreal Oscillation and Older Dryas deglacial events.