https://boris.unibe.ch/62575/1/1-s2.0-S0277379114003485-main.pdf

A stratigraphic framework for abrupt climatic changes during the Last Glacial period based on three synchronized Greenland ice-core records: refining and extending the INTIMATE event stratigraphy

THE warming at the end of the last glaciation was characterized by a series of abrupt returns to glacial climate, the best-known of which is the Younger Dryas event1. Despite much study of the causes of this event and the mechanisms by which it ended, many questions remain unresolved1. Oxygen isotope data from Greenland ice cores2–4 suggest that the Younger Dryas ended abruptly, over a period of about 50 years; dust concentrations2,4 in these cores show an even more rapid transition (≲20 years). This extremely short timescale places severe constraints on the mechanisms underlying the transition. But dust concentrations can reflect subtle changes in atmospheric circulation, which need not be associated with a large change in climate. Here we present results from a new Greenland ice core (GISP2) showing that snow accumulation doubled rapidly from the Younger Dryas event to the subsequent Preboreal interval, possibly in one to three years. We also find that the accumulation-rate change from the Oldest Dryas to the Bo11ing/Allerod warm period was large and abrupt. The extreme rapidity of these changes in a variable that directly represents regional climate implies that the events at the end of the last glaciation may have been responses to some kind of threshold or trigger in the North Atlantic climate system.

Abrupt increase in Greenland snow accumulation at the end of the Younger Dryas event

THE timing of the last deglaciation is important to our understanding of the dynamics of large ice sheets1 and their effects on the Earth's surface2,3. Moreover, the disappearance of the glacial ice sheets was responsible for dramatic increases in freshwater fluxes to the oceans, which probably disturbed the ocean's thermohaline circulation and, hence, global climate4–7. Sea-level increases bear witness to the melting of continental ice sheets, but only two such records—from Barbados8,9 and New Guinea10,11 corals—have been accurately dated. But these corals overlie active subduction zones, where tectonic movements are large and often discontinuous (especially in New Guinea), so the apparent sea-level records may be contaminated by a complex tectonic component. Here we date fossil corals from Tahiti, which is far from plate boundaries (and thus is likely to be tectonically relatively stable) and remote from the locations of large former ice sheets. The resulting record indicates a large sea-level jump shortly before 13,800 calendar years BP, which corresponds to meltwater pulse 1A in the Barbados coral records8,9. The timing of this event is more accurately constrained in the Tahiti record, revealing that the meltwater pulse coincides with a short and intense climate cooling event12–15 that followed the initiation of the Bolling–Allerod warm period12–16, but preceded the Younger Dryas cold event by about 1,000 years.

/pdf/deglacial-sea-level-record-from-tahiti-corals-and-the-timing-1xj0pl3fae.pdf

Deglacial sea-level record from Tahiti corals and the timing of global meltwater discharge

The 3-km-long Greenland Ice Sheet Project 2 (GISP2) ice core presents a 100,000 +- year detailed oxygen isotope profile covering almost a full glacial-interglacial cycle. Measuranents of isotopic fluctuations in snow, frost, and atmospheric water vapor samples collected during summer field seasons (up to 20%0) are compatible with the large and abrupt 80/160 changes observed in accumulated tim. Snow pit 1580 profiles from the GISP2 summit area, however, show rapid smoothing of the 180/160 signal near the surface. Beyond about 2-m depth the smoothedi5180 signal is fairly well preserved and can be interpreted in terms of average local weather conditions and climate. The longer climate fluctuations also have regional and often global significance. In the older part of the record, corresponding to marine isotope stages (MIS) 5a to 5d, the effect of orbital climate forcing via the 19- and 23-kyr precession cycles and the 41-kyr obliquity cycle is obvious. From the end ofMIS 5a, at about 75,000 years B.P., till the end of the glacial at the Younger Dryas-Preboreal transition, at 11,650 years B.P., the O180/160 record shows frequent, rapid switches between intermediate interstadial and low stadial values. Fourier spectra of the oscillations that are superimposed on the orbitally induced changes contain a strong periodicity at 1.5 kyr, a broad peak at 4.0 kyr, and additional shorter periods. Detailed comparison of the GISP2 180/160 record with the Vostok, Antarctica, 15D record; Pacific Ocean foraminiferal 180/160; Grande Pile, France, tree pollen; and insolation indicates that a counterpart to many of the rapid 180/160 fluctuations of GISP2 can be found in the other records, and that the GISP2 isotopic changes clearly are the local expression of climate changes of worldwide extent. Correlation of events on the independent GISP2 and SPECMAP time scales for the interval 10,000-50,000 years B.P. shows excellent chronometric agreement, except possibly for the event labeled 3.1. The glacial to interglacial transition evidently started simultaneously in the Arctic and the Antarctic, but its development and its expression in Greenland isotopes was later suppressed by the influence of meltwater, especially from the Barents Sea ice sheet, on deep water formation and ocean circulation. Meltwaters from different ice sheets bordering the North Atlantic also influenced ocean circulation during the Bolling-Allerod interstadial complex and the Younger Dryas and led to a distinct development of European climate and Greenland 180/160 values. The Holocene interval with long-term stable mean isotopic values contains several fluauations with periods from years to millennia. Dominant is a 6.3-year oscillation with amplitude up to 3 to 4%0. Periodicities of 11 and 210 years, also found in the solar-modulated records of the cosmogenic isotopes 1oBe and 14C, suggest solar processes as the cause of these cycles. Depression of180/160 values (cooling) by volcanic eruptions is observed in stacked GISP21580 records, but the effect is small and not likely to trigger major climate changes.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97JC00880

Oxygen 18/16 variability in Greenland snow and ice with 10 -3- to 105-year time resolution

The role of ocean-atmosphere reorganizations in glacial cycles

Principles and terminology for classification of the Quaternary are discussed, including lithostratigraphy, biostratigraphy. morphostratigraphy, climatostratigraphy and chronostratigraphy. The main conclusion is a proposal for a common chronostratigraphical classification of the Quaternary in Norden (and partly continental NW Europe). The Quaternary is subdivided into the Pleistocene and the Holocene Series. The Pleistocene is further subdivided into several provisional stages (Weichselian, Eemian, etc.), based on the sequence of glacials/interglacials. but with the boundaries preferably defined by stratotypes. The Late Weichselian and the Flandrian (Holocene) are subdivided into chronozoncs (Bolling, Older Dryas, Allerod, Younger Dryas, Preboreal, Boreal, Atlantic, Subboreal, Subatlantic) with the boundaries dcfined in conventional radiocarbon years.

Joakim J. Donner

Papers

Quaternary stratigraphy of Norden, a proposal for terminology and classification