Beginning with the orbital forcing variables as summarized by Berger (1988); Fischer & Bottjer (1991); De Boer & Smith (1994) and House (1995), herein it is assumed that the fundamental orbital forcing signal is the precessional signal (Anderson & Goodwin, 1990; 1992). The strength of this signal is directly related to the degree of eccentricity of the Earth’s orbit. As eccentricity increases and summers occur near perihelion (in the precessional cycle) insolation at high latitudes reaches a maximum (Fig. 1 Precessional hot summers.jpg). If those summers occur in a hemisphere with large areas of continent at high latitude then high insolation values may trigger the melting of accumulated continental glacial ice. When summers occur near aphelion with high eccentricity (Fig. 2 Precessional cold summers.jpg), the resulting long series of cool summers may trigger renewed build up of high-latitude, continental glacial ice.  Principles relating orbital forcing to glacial ice volume were first articulated in 1864 by James Croll (1821-1890) and reinforced by Milutin Milankovitch in 1941. 

    In concept this model leads to a pattern of sea-level fluctuation in response to varying insolation controlled by precessional cycles where the degree of rise and fall of sea level is proportional to the degree of eccentricity of the Earth’s orbit (Fig. 3  100 ka Eccentricity.jpg).   When eccentricity is high there would be large changes in summer insolation (and potentially large ice volume and sea-level changes) between the perihelion and aphelion summer positions and as the orbit approaches round the amount of change in insolation through a precessional cycle would approach zero leading to stable sea levels. The ellipticity of the Earth’s orbit varies cyclicly from near round to as much as 4% with a period of about 100 ka (see Berger, 1988). Secondarily, sets of 100 ka eccentricity cycles occur with enhanced peaks in eccentricity at 400 ka intervals (Fig. 4  400 ka Eccentricity.ppt), see Berger, 1988, p. 634, Fig. 9.

    In summary, in the assumed model, all orbitally-forced sea level rises and therefore all surfaces at allocycle boundaries are the product of the precessional signal (Anderson & Goodwin, 1992; Goodwin & Anderson 1997). These smallest-scale cycles (and their boundary surfaces) are produced by a stratigraphic process and are defined as 6th order. Stratigraphic processes are distinct from and independent of sedimentologic processes. Stratigraphic processes operate at much longer time scales than processes that can be observed in modern sedimentary environments and act independently of these sedimentologic processes. The term PAC (punctuated aggradational cycle) is exclusively equated to 6th order, precessionally forced, rock cycles (Goodwin & Anderson, 1985; 1997 and Fig. 2).  Variation in the degree of eccentricity bundles these precessional cycles (into 5th and 4th order sequences) by periodically varying the magnitude of the precessional signal (Fig. 1). This bundling also is a stratigraphic process.

    Precessional cycle boundaries are surfaces produced by rates of sea-level rise that exceed a critical value (Fig. 5  PACs and  Precession.ppt).  At rates above the critical value of sea-level change sedimentation effectively stops (see Tipper, 1997). Higher amplitude sea-level rises thus will produce discontinuities at cycle boundaries representing longer time intervals and displaying larger facies contrasts across the cycle boundary surface (see graph below). Notice that this concept of a mechanism for producing cycle boundaries is distinct from concepts employing lag-time or lag-depth (e.g. Read et al., 1986) and explains the genesis of cycle boundaries in totally sub-tidal deposits. The critical value for rate of sea-level change may also be exceeded during (precessional) sea-level falls producing sea-level fall surfaces within 6th order cycles (Fig.5). Sea-level rise surfaces are more marked and better preserved stratigraphically because rates of relative sea-level rise are amplified by subsidence while rates of relative sea-level fall are diminished.

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