Climate over the past million years has been dominated by glaciation cycles with periods near 23,000, 41,000, and 100,000 years. In a linear version of the Milankovitch theory, the two shorter cycles can be explained as responses to insolation cycles driven by precession and obliquity. But the 100,000-year radiation cycle (arising from eccentricity variation) is much too small in amplitude and too late in phase to produce the corresponding climate cycle by direct forcing. We present phase observations showing that the geographic progression of local responses over the 100,000-year cycle is similar to the progression in the other two cycles, implying that a similar set of internal climatic mechanisms operates in all three. But the phase sequence in the 100,000-year cycle requires a source of climatic inertia having a time constant (--15,000 years) much larger than the other cycles (--5,000 years). Our conceptual model identifies massive northern hemisphere ice sheets as this larger inertial source. When these ice sheets, forced by precession and obliquity, exceed a critical size, they cease responding as linear Milankovitch slaves and drive atmospheric and oceanic responses that mimic the externally forced responses. In our model, the coupled system acts as a nonlinear amplifier that is particularly sensitive to eccentricity-driven modulations in the 23,000-year sea level cycle. During an interval when sea level is forced upward from a major low stand by a Milankovitch response acting either alone or in combination with an internally driven, higher-frequency process, ice sheets grounded on continental shelves become unstable, mass wasting accelerates, and the resulting deglaciation sets the phase of one wave in the train of 100,000-year oscillations.Whether a glacier or ice sheet influences the climate depends very much on the scale .... The interesting aspect is that an effect on the local climate can still make an ice mass grow larger and larger, thereby gradually increasing its radius of influence. Johannes Oerlemans [1991, p. 155] 1. BACKGROUND AND PURPOSE Climate over the past half-million years has been dominated by glacial cycles with periods near 23, 41, and 100 kyr [Hays 14 DSDP607 A•5•3C index of ventilation 3427

Time series of ocean properties provide a measure of global ice volume and monitor key features of the wind-driven and density-driven circulations over the past 400,000 years. Cycles with periods near 23,000, 41,000, and 100,000 years dominate this climatic narrative. When the narrative is examined in a geographic array of time series, the phase of each climatic oscillation is seen to progress through the system in essentially the same geographic sequence in all three cycles. We argue that Paper number 92PA02253 0883-8305/92/92PA-02253510.00 the 23,000-and 41,000-year cycles of glaciation are continuous, linear responses to orbitally driven changes in the Arctic radiation budget; and we use the phase progression in each climatic cycle to identify the main pathways along which the initial, local responses to radiation are propagated by the atmosphere and ocean. Early in this progression, deep waters of the Southern Ocean appear to act as a carbon trap. To stimulate new observations and modeling efforts, we offer a process model that gives a synoptic view of climate at the four end-member states needed to describe the system's evolution, and we propose a dynamic system model that explains the phase progression along causal pathways by specifying inertial constants in a chain of four subsystems. "Solutions to problems involving systems of such complexity are not born full grown like Athena from the head of Zeus. Rather they evolve slowly, in stages, each of which requires a pause to examine data at great lengths in order to guarantee a sure footing and to properly choose the next step." --Victor P. Starr Imbrie et al.' Linear Responses to Milankovitch Forcing 705 MODEL SYSTEM STATE IG G IG 1 3 PHASE © DG PG 1 4 3 2 TIME

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Climate control of nutricline depth in the equatorial Atlantic can be monitored by variations in the abundance of the phytoplankton species Florisphaera profunda. A conceptual model, based on in situ evidence, associates high abundances of F. profunda with a deep nutricline and low abundances with a shallow nutricline. A 200,000-year record of F. profunda relative abundances, obtained from a deep-sea core sited beneath the region of maximum equatorial divergence at 10 degrees W, has 52 percent of its variance centered on the 23,000-year precessional band. Cross-spectral analysis between the signals of F. profunda and sea-surface temperature, independently derived from zooplankton species, shows their 23,000-year cycles to be coherent and nearly in phase. Abundance minima of F. profunda coincide with times of December perihelion, whereas abundance maxima coincide with June perihelion. These relations indicate that nutricline dynamics in the divergence region of the equatorial Atlantic are controlled by variations in the tropical easterlies, forced by the precessional component of orbital insolation, on time scales greater than 10,000 years.