Milankovitch ice sheet and paleo-eustatic sea level models have been 
confronting the same sedimentary problem -- & the parallels don't stop there 

Stuart R. Gaffin
Center for Climate Systems Research, Columbia University, 2880 Broadway, NY, NY 10025, 
USA; email: srg43@columbia.edu

A basic aspect of sedimentary systems that transcends Earth science disciplines is the 
existence of a nearly horizontal equilibrium surface intersecting with the free sedimentary 
surface.  The problem arises from this that observed changes in relative dimensions for 
such a system can be caused by an infinite combination of rates for the two surfaces.  In 
this talk I show that the solution that has implicitly been attempted in research models is 
for the time evolution of one of the surfaces to be reconstructed independently of the state 
of the other surface.  When such reconstructions are of the equilibrium surface I term them 
'A' type models and when of the sedimentary surface I term them 'B' type. I show that 
Milankovitch ice sheet models are A-type and that sediment core sea level models 
('backstripping' models), when expressed in terms of basement depth, are B-type and both 
are implicitly confronting the same problem identified above.  Mechanisms of eustatic sea 
level change (oceanic water or basin volume changes) provide A-type information and 
correspond to Milankovitch snowline forcing. In this framework, ice free geologic periods 
have a specific meaning: the equilibrium surface (sea level) did not change so rapidly or 
with such large amplitudes and this serves as an A-type reconstruction. The central target 
data for sedimentary cycles in each case -- oxygen isotope data for Late Pleistocene ice 
volume cycles and high-order (~0.5-2 Myr) Cretaceous sedimentary sequence cycles -- both 
present the unraveled problem of revealing only the combined interaction of the 
equilibrium surface and the evolving sedimentary surface.  At every level these two 
outstanding sedimentary cycles present systematic parallels.