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The postrifting subsidence of the Lomonosov Ridge is assumed to follow the simple thermal cooling pathway that is a function of the square root of its postrifting age (Sclater et al., 1971). In applying this simple model, we assume that the initial rifting process ended by 54 Ma and then thermal subsidence began. The subsidence constant used in the equation and the progressive increase in the sediment load was adjusted so that the sediment surface reached the modern water depth of the sites at “zero” age (Fig. F2). Sediment accumulation rates and simple Airy isostasy are used to estimate the amount of sediment loading on basement.

Errors in estimating paleowater depth of the recovered section using this model could derive from two main sources. First, the simple model developed by Sclater et al. (1971) is based on the depth versus age profile of oceanic crust that is believed to indicate a gradual cooling of that crust as it ages. Thus, its application to the subsidence of a rifted fragment of an outer continental margin is associated with some uncertainty—especially in the early stages of rifting, when the role of faulting, possible uplift, erosion, and early isostatic adjustment is unknown. In applying this model to the Lomonosov Ridge we assume that true thermal subsidence did not begin until 2–3 m.y. after initial rifting and after most of the immediate impact of tectonic adjustments had diminished.

The second source of error is associated with the estimates of paleowater depth and paleosea level. Paleowater depth estimates from the benthic fauna recovered from the base of the section only extend up to Core 302-M0004A-27X (estimated age = ~54 Ma) and indicate an inner neritic to neritic depth environment. This qualitative term is usually thought to indicate very shallow to shallow water and is not usually associated with large error bars (i.e., <100 m). Quantitative estimates of paleosea level, however, have substantial errors associated both with the timing and the magnitude of sea level change (Miller et al., 2005)—especially in the older part of the Cenozoic and in preCenozoic times. Errors in estimated age of the sea level changes in the early Cenozoic could range from 0.5 to 1 m.y., whereas errors in the magnitude of sea level changes could be 20–30 m or more.