The Early Devensian (MIS 3) Dilemma

 

Thanks to Prof Ian Shennen for drawing this to my attention.  It's a very interesting article from 2021, drawing together vast amounts of data to give us a picture of absolure sea level change (not RSL !!) over the last 80,000 years -- the approx span of the Devensian.

Evan J. Gowan, Xu Zhang, Sara Khosravi, Alessio Rovere, Paolo Stocchi, Anna L. C. Hughes, Richard Gyllencreutz, Jan Mangerud, John-Inge Svendsen & Gerrit Lohmann. 2021. A new global ice sheet reconstruction for the past 80 000 years. Nature Communications 12, 1199. https://doi.org/10.1038/s41467-021-21469-w

Abstract
The evolution of past global ice sheets is highly uncertain. One example is the missing ice
problem during the Last Glacial Maximum (LGM, 26 000-19 000 years before present) – an
apparent 8-28 m discrepancy between far-field sea level indicators and modelled sea level
from ice sheet reconstructions. In the absence of ice sheet reconstructions, researchers often
use marine δ18O proxy records to infer ice volume prior to the LGM. We present a global ice
sheet reconstruction for the past 80 000 years, called PaleoMIST 1.0, constructed independently
of far-field sea level and δ18O proxy records. Our reconstruction is compatible with
LGM far-field sea-level records without requiring extra ice volume, thus solving the missing
ice problem. However, for Marine Isotope Stage 3 (57 000-29 000 years before present) - a
pre-LGM period - our reconstruction does not match proxy-based sea level reconstructions,
indicating the relationship between marine δ18O and sea level may be more complex than
assumed.

The tail end of the Ipswichian interglacial is represented at the left edge of the diagram.  Then we can see an Early Devensian cooling with ice sheet development in MIS 3, a milder episode in MIS 4 with some ice wastage, and then the last great surge in ice sheet growth during the LGM of MIS 2.  Then follows the Holocene, with ice wastage and the return of vast quantities of meltwater into the oceans, with sea level returning to its interglacial level.  We can assume that these three graphs are pretty accurate, since they combine modelling and empirical data.  

The authors point out that the only serious mismatch occurs in MIS 3, where observed sea levels appear to have been lower than the models suggest they should have been.  This might be due to problems in extrapolating from oxygen 18 levels to assumptions about sea level positions -- but no doubt that problem will be ironed out in due course.

The nice thing about these curves is that they fit very nicely into the climate change scheme which I worked out in 1965 on the basis of the West Wales Quaternary sediment sequence.  I worked out that the Ipswichian high sea level was followed on the Pembrokeshire coast by a long period of lower relative sea-level during which there were many oscillations of periglacial climate with slope breccia accumulations.  Then came the onset of full glacial conditions in the LGM, with ice affecting all parts of the county.  The Irish Sea till horizon was the definitive stratigraphic marker.   I was quite sure that whatever was going on elsewhere in the Early and Mid Devensian, glacier ice did not affect Pembrokeshire.

The full glacial cycle at Aber-mawr, which I described in 1965.  The Ipswichian raised beach was not visible at Aber-mawr at that time.  The "main head" episode, with a number of facies of slope breccia, was thought to represent the Early and Middle Devensian (MIS 4 and MIS 3).

So far so good, until geomorphologists working in SE Ireland started to discover evidence of a substantial ice load just on the other side of St George's Channel, sufficient for a good deal of crustal depression, in MIS 4.  Of which more in due course.........