Using the concept of “orbital tuning”, a continuous, high-resolution deep-sea chronostratigraphy has been developed spanning the last 300,000 yr. The chronology is developed using a stacked oxygen-isotope stratigraphy and four different orbital tuning approaches, each of which is based upon a different assumption concerning the response of the orbital signal recorded in the data. Each approach yields a separate chronology. The error measured by the standard deviation about the average of these four results (which represents the “best” chronology) has an average magnitude of only 2500 yr. This small value indicates that the chronology produced is insensitive to the specific orbital tuning technique used. Excellent convergence between chronologies developed using each of five different paleoclimatological indicators (from a single core) is also obtained. The resultant chronology is also insensitive to the specific indicator used. The error associated with each tuning approach is estimated independently and propagated through to the average result. The resulting error estimate is independent of that associated with the degree of convergence and has an average magnitude of 3500 yr, in excellent agreement with the 2500-yr estimate. Transfer of the final chronology to the stacked record leads to an estimated error of ±1500 yr. Thus the final chronology has an average error of ±5000 yr.

1) Three indices of global climate have been monitored in the record of the past 450,000 years in Southern Hemisphere ocean-floor sediments. 2) Over the frequency range 10(-4) to 10(-5) cycle per year, climatic variance of these records is concentrated in three discrete spectral peaks at periods of 23,000, 42,000, and approximately 100,000 years. These peaks correspond to the dominant periods of the earth's solar orbit, and contain respectively about 10, 25, and 50 percent of the climatic variance. 3) The 42,000-year climatic component has the same period as variations in the obliquity of the earth's axis and retains a constant phase relationship with it. 4) The 23,000-year portion of the variance displays the same periods (about 23,000 and 19,000 years) as the quasi-periodic precession index. 5) The dominant, 100,000-year climatic [See table in the PDF file] component has an average period close to, and is in phase with, orbital eccentricity. Unlike the correlations between climate and the higher-frequency orbital variations (which can be explained on the assumption that the climate system responds linearly to orbital forcing), an explanation of the correlation between climate and eccentricity probably requires an assumption of nonlinearity. 6) It is concluded that changes in the earth's orbital geometry are the fundamental cause of the succession of Quaternary ice ages. 7) A model of future climate based on the observed orbital-climate relationships, but ignoring anthropogenic effects, predicts that the long-term trend over the next sevem thousand years is toward extensive Northern Hemisphere glaciation.

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The abrupt, widespread, and simultaneous extinction of radiolarian species makes them ideal stratigraphic markers. Five species became extinct in the North Pacific during the last 3 m.y. Four of these are used to define four stratigraphic zones. The boundary between the oldest two zones correlates with the Pliocene/Pleistocene boundary, as defined in southern Italy. These .zones can be related through paleomagnetic stratigraphy to previously established radiolarian and foraminiferal zonations. One species (Eucyrtidium matuyamai) evolved and became extinct during the last 2.5 m.y. Its evolution can be related to the invasion of and adaptation to a new habitat. The extinction of E. matuyamai shows a striking correlation with the magnetic reversal at the base of the Jaramillo event. The rapid evolution of this species probably reduced the genetic variability of the population, making it more vulnerable to extinction than other less rapidly evolving species. The environmental change that caused the extinction is unclear; however, there is suggestive evidence that it is in some way related to a reversal of the earth's field. If reversals cause abrupt environmental changes, they may have played an important selective role through geologic time. 185 on July 1, 2015 memoirs.gsapubs.org Downloaded from 186 GEOLOGICAL INVESTIGATIONS OP THE NORTH PACIFIC CONTENTS

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Using relative-abundance changes of the radiolarian Cycladophora davisiana calibrated using 0 18 /0 16 values and C 14 ages, we have established a high-resolution biostratigraphy for the Antarctic and subantarctic ocean covering the past 150,000 yr. Extremes of this species' relative abundance have been estimated to have occurred approximately 18,000, 35,000, 60,000, 85,000, 105,000, and 120,000 yr ago. Crosscorrelation between 0 18 /0 16 values and the most recent C. davisiana maximum shows that this maximum is isochronous over a broad area of the Antarctic and subantarctic sea floor.Using diverse stratigraphic criteria, we selected 34 cores from the Atlantic and western Indian Ocean sectors of the Antarctic and subantarctic ocean containing a strong C. davisiana relative-abundance maximum near the core top, believing that this maximum occurred approximately 18,000 yr ago in the Antarctic and *Present address: Departamento de Geología, Universidad Nacional, Bogotá, D.E., Colombia, South America. 337 on June 29, 2015 memoirs.gsapubs.org Downloaded from 338 HAYS AND OTHERS subantarctic. Using techniques including radiolarian-based paleoecological equations, Radiolaria per gram of sediment, and percentage of calicum carbonate, we show that 18,000 yr ago the Antarctic Polar Front was displaced north of its present position by as much as 7° of latitude. The subtropical convergence was little changed from its present position ; consequently, the width of subantarctic waters was reduced.Using C. davisiana as a stratigraphie indicator, we estimate the sedimentation rate in one Antarctic core of the upper diatom-ooze layer (11.5 cm/1,000 yr) and the immediately underlying silty clay (2.7 cm/1,000 yr). The slower sedimentation rate of the silty clay strongly suggests that diatom production was inhibited during its deposition relative to production during deposition of the younger diatom ooze. We interpret this to indicate that summer ice cover extended nearly to lat 55°S 18,000 yr ago, whereas today it melts back to the Antarctic continent.