Abstract
Abstract. Differences in the duration of interglacials have long been apparent in palaeoclimate records of the Late and Middle Pleistocene. However, a systematic evaluation of such differences has been hampered by the lack of a metric that can be applied consistently through time and by difficulties in separating the local from the global component in various proxies. This, in turn, means that a theoretical framework with predictive power for interglacial duration has remained elusive. Here we propose that the interval between the terminal oscillation of the bipolar seesaw and three thousand years (kyr) before its first major reactivation provides an estimate that approximates the length of the sea-level highstand, a measure of interglacial duration. We apply this concept to interglacials of the last 800 kyr by using a recently-constructed record of interhemispheric variability. The onset of interglacials occurs within 2 kyr of the boreal summer insolation maximum/precession minimum and is consistent with the canonical view of Milankovitch forcing pacing the broad timing of interglacials. Glacial inception always takes place when obliquity is decreasing and never after the obliquity minimum. The phasing of precession and obliquity appears to influence the persistence of interglacial conditions over one or two insolation peaks, leading to shorter (~ 13 kyr) and longer (~ 28 kyr) interglacials. Glacial inception occurs approximately 10 kyr after peak interglacial conditions in temperature and CO2, representing a characteristic timescale of interglacial decline. Second-order differences in duration may be a function of stochasticity in the climate system, or small variations in background climate state and the magnitude of feedbacks and mechanisms contributing to glacial inception, and as such, difficult to predict. On the other hand, the broad duration of an interglacial may be determined by the phasing of astronomical parameters and the history of insolation, rather than the instantaneous forcing strength at inception.
Highlights
Accounting for such differences can provide insights into climate forcings and feedbacks that are relevant to the onset and end of interglacials
With hydrographic effects considered minimal at this site, the δ18O of seawater (δ18Osw) provides an important insight into icevolume changes over this interval, but the propagated error of ±0.2 ‰ in calculated δ18Osw (±20 m sea-level-equivalent), estimated by Elderfield et al (2012), is too coarse to allow a precise demarcation of the onset and end of the sealevel highstand at the scale of individual interglacials
We propose that the interval between the “terminal” oscillation of the bipolar seesaw, preceding an interglacial, and its first major reactivation represents a period of minimum extension of ice sheets away from coastlines
Summary
Interglacials may be characterized by any property that changes significantly on glacial–interglacial timescales – for example, temperature, atmospheric CO2 concentration, faunal or floral composition. Whereas pinpointing the start and end of the highstand in convolved sea-level proxy records is not straightforward, an indication of the presence of ice sheets can be provided by the occurrence of interhemispheric millennial-scale climate variability This requires ice sheets large enough to extend to coastlines and produce iceberg discharges that disrupt the meridional overturning circulation (MOC), leading to rapid cooling of the North Atlantic and gradual warming of Antarctica. The placement of the interglacial onset after the terminal bipolar-seesaw oscillation does not represent the time when maximum sea level was attained, nor would it necessarily lead to the same value of relative sea level between interglacials, as rates of deglaciation may vary between terminations, depending on the location and size of ice sheets and insolation forcing
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