Brunhes-Matuyama Research Articles

Quaternary time scale for the Ontong Java Plateau: Milankovitch template for Ocean Drilling Program Site 806

Research Article| May 01, 1994 Quaternary time scale for the Ontong Java Plateau: Milankovitch template for Ocean Drilling Program Site 806 Wolfgang H. Berger; Wolfgang H. Berger 1Scripps Institution of Oceanography, University of California, La Jolla, California 92093 Search for other works by this author on: GSW Google Scholar Memorie K. Yasuda; Memorie K. Yasuda 1Scripps Institution of Oceanography, University of California, La Jolla, California 92093 Search for other works by this author on: GSW Google Scholar Torsten Bickert; Torsten Bickert 2Fachbereich Geowissenschaften, Universität Bremen, 28334 Bremen, Germany Search for other works by this author on: GSW Google Scholar Gerold Wefer; Gerold Wefer 2Fachbereich Geowissenschaften, Universität Bremen, 28334 Bremen, Germany Search for other works by this author on: GSW Google Scholar Toshiaki Takayama Toshiaki Takayama 3Department of Geology, Kanazawa University, Kanazawa 920, Japan Search for other works by this author on: GSW Google Scholar Author and Article Information Wolfgang H. Berger 1Scripps Institution of Oceanography, University of California, La Jolla, California 92093 Memorie K. Yasuda 1Scripps Institution of Oceanography, University of California, La Jolla, California 92093 Torsten Bickert 2Fachbereich Geowissenschaften, Universität Bremen, 28334 Bremen, Germany Gerold Wefer 2Fachbereich Geowissenschaften, Universität Bremen, 28334 Bremen, Germany Toshiaki Takayama 3Department of Geology, Kanazawa University, Kanazawa 920, Japan Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1994) 22 (5): 463–467. https://doi.org/10.1130/0091-7613(1994)022<0463:QTSFTO>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Wolfgang H. Berger, Memorie K. Yasuda, Torsten Bickert, Gerold Wefer, Toshiaki Takayama; Quaternary time scale for the Ontong Java Plateau: Milankovitch template for Ocean Drilling Program Site 806. Geology 1994;; 22 (5): 463–467. doi: https://doi.org/10.1130/0091-7613(1994)022<0463:QTSFTO>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract A 2 m.y. oxygen isotope record of Globigerinoides sacculifer from the Ontong Java Plateau, based on cores from Ocean Drilling Program Leg 130, is dated by matching variations to an orbital template. The procedure allows us to present the most complete Quaternary record available for the western equatorial Pacific. The template-generating algorithm describes a balance between growth and melting of ice. Following basic Milankovitch theory, ice growth is taken as constant, while melting is taken to depend on summer insolation, current ice mass, and average past ice mass. Template settings must be changed once, between 1 and 1.2 Ma, to reflect a major shift in climate. Template fits are strikingly good over much of the record and can be used to detect and fill gaps from core breaks and other disturbances. One result of template dating is an exact age for the Brunhes-Matuyama magnetic reversal boundary, at 790 ±5 ka, as well as several other precise dates (900 ka for the middle Pleistocene climate shift; 1070, 1240, and 1450 ka for isotope stages 31, 37, and 47, respectively). Sedi- mentation rates fluctuate between 18 and 28 m/m.y., a ca. 400 ka cycle being the most prominent. Major anomalies arise within the transitional regime (1.2 to 1 Ma). The origin of the cycles is unknown; we propose productivity variations in the western equatorial Pacific. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The Matuyama-Brunhes reversal at Tecopa basin, southeastern California, revisited again

Numerous records of the Matuyama-Brunhes geomagnetic transition have been obtained from paleomagnetic studies. Because few of the reversal records are of acceptable reliability, however, the exact behavior of the field during the transition has remained enigmatic. To provide confirmation of one of the more reliable records, we have re-examined the transition at two sites, 150 m apart, in lake sediments of Tecopa basin, southeastern California. The two sites are geographically very close to that of Valet et al. [10], who previously obtained a record from that site indicating that the transitional field was non-dipolar and axisymmetric. The Matuyama-Brunhes reversal is recorded differently at each of our two sites and at that of Valet et al. [10]. Zones of mixed polarities and/or intermediate directions occur at all three sites but they differ greatly in polarity character, thickness and stratigraphic position. It appears that all three sites have provided mutually contradictory records of the transition. It is unlikely, therefore, that any of the records is acceptable for establishing the nature of the transition at this locality. Obliteration of the transition is apparently the result of acquisition of a stable, normal-polarity overprint that appears to consist of two remanence components, one acquired during post-depositional compaction and dewatering, and one during later sediment diagenesis.

Paleoenvironments and correlations of interglacial sediments in the North Sea

Benthic foraminiferal faunas suggesting interglacial climatic conditions are present in several shallow drillings in the North Sea and in northern Jutland. Denmark. The interglacial deposits in six of the cores arc correlated by means of isoleucine epimerization, paleomagnetic data and environmental inferences. Four episodes of interglacial circulation are recorded in sediments of Brunhes age and one warm episode is found just below the Brunhes Matuyama boundary. The Holocene and Eemian faunas of the North Sea are richer in species and individuals and contain more thermophilous species than the older interglacial faunas. However. a much more northern distribution of Bulimina marginata in sediments of Eemian age suggests that oceanic circulation was different during this period than during the Holocene. Sediments of the three other Brunhes interglacials (the Holsteinian, the Norwegian Trench and the Devils Hole Interglacial) and the Leerdam Interglacial record periods of relatively cooler conditions and possibly reduced inflow of Atlantic water.

Revisions to the age of the Brunhes ‐ Matuyama Boundary and the Pleistocene geomagnetic polarity timescale

The currently referenced age of 730 ka for the Brunhes‐Matuyama (B‐M) geomagnetic polarity boundary [Mankinen and Dalrymple, 1979] is based largely on K‐Ar and paleomagnetic data from postcollapse rhyolites in the Valles Caldera, New Mexico [Doell and Dalrymple, 1966; Doell et al., 1968]. Dating of sanidine from Valles rhyolites by the 40A/39Ar laser fusion technique indicates that samples critical to defining the B‐M boundary are up to 90 ka older than previously thought. The new data combined with earlier results yield an age for the B‐M boundary of 780 ± 10 ka. 40Ar/39Ar ages from other samples suggest the Jaramillo normal polarity subchron extended from 1010 to 915 ka.

40 Ar/ 39 Ar Dating of the Brunhes-Matuyama Geomagnetic Field Reversal

Magnetostratigraphic studies are widely used in conjunction with the geomagnetic polarity time scale (GPTS) to date events in the range 0 to 5 million years ago. A critical tie point on the GPTS is the potassium-argon age of the most recent (Brunhes-Matuyama) geomagnetic field reversal. Astronomical values for the forcing frequencies observed in the oxygen isotope record in Ocean Drilling Project site 677 suggest that the age of this last reversal is 780 ka (thousand years ago), whereas the potassium-argon-based estimate is 730 ka. Results from 4039; Ar incremental heating studies on a series of lavas from Maui that straddle the Brunhes-Matuyama reversal give an age of 783 + 11 ka, in agreement with the astronomically derived value. The astronomically based technique appears to be a viable tool for dating young sedimentary sequences.

Stratigraphy and paleomagnetism of the Jaw Face section, Wellsch Valley site, Saskatchewan

The basal part of Jaw Face section at the Wellsch Valley site, some 50 km north-northwest of Swift Current, Saskatchewan, contains mammalian fossils that are considered to be late Blancan to early Irvingtonian in age. It has been sampled for paleomagnetic studies through a thickness of 11 m. The section above 8.8 m has normal polarity; that below has reversed polarity. A tephra, which has yielded a minimum fission-track age of 0.69 ± 0.11 Ma, lies within the reversely magnetized part, near the top of the fossiliferous zone and just below the reversal at the 8.8 m level. Therefore, the change from normal to reversed polarity at the 8.8 m level is probably the Brunhes–Matuyama reversal at 0.73 Ma. Earlier studies have indicated that the Jaramillo (0.9–0.97 Ma) and Olduvai (1.67–1.87 Ma) Normal Polarity subzones occur within the lower predominantly reversed part of the Jaw Face section. In this more detailed study, we find no evidence of these normal subzones. We suggest that earlier results could have been artifacts of the procedures used. Several stratigraphic interpretations are possible. Our preferred interpretation is that the Jaw Face section extends from the upper part of the Matuyama Zone into the lower Brunhes Zone, that is, the mammalian faunas are entirely Irvingtonian in age. Alternatively, the section could contain a substantial hiatus, so the lower part would be early Matuyama in age and its faunas late Blancan – early Irvingtonisn, and the upper part could span the latest Matuyama and lower Brunhes zones. Other possibilities are discussed in the text.

Stratigraphy and geochemistry of a sequence of Quaternary palaeosols in the Lower Teleki Valley, Mount Kenya, East Africa — Implications for interregional correlations

AbstractThe relative weathering of a sequence of five palaesols formed in different parent materials, including bedrock of phonolitic composition, is described and assessed. The three lower palaeosols (units 3–5) are considered to be of interglacial origin. The saprolite (unit 5), in phonolite, formed prior to deposition of the lower Brunhes‐age till, is considered to be as old as the Brunhes Matuyama boundary (0.73 myr). The other two palaeosols in this group (units 3 and 4), derived from loess and till, respectively, formed over middle to late Brunhes time under palaeoclimates that were wetter (and presumably warmer), and of sufficient leaching power to move clay, as well as organically complexed extractable Fe and Al. The palaeosols in unit 3 are more complex than the lower ones. Their upper and lower horizons have about the same chemical composition as the lowest two palaeosols, while the middle horizons are similar to the upper two palaeosols (units 1 and 2). Radiocarbon dates for unit 3 are considered to be too young to account for the strong weathering in the upper and lower horizons. This interpretation is supported by high D/L ratios of aspartic acid. Overlying sediments, emplaced by episodic colluvial (unit 2) and slope wash (unit 1) processes, are less weathered; the lower colluvial unit in this group was emplaced during the late stade of the last glaciation. The surface sediments were emplaced more recently during the same interval and weathered mainly during the post‐glacial period. Tentative correlations with core 82PCS18 from the Canary Basin show that upper unit 3 is probably equivalent to Isotope Stage 5e, lower unit 3 to Isotope Stage 7, unit 4 to Isotope Stage 8, and unit 5 to Isotope Stage 9. Units 1 and 2 are firmly correlated with Isotope Stage 2.

Depth of post-depositional remanence acquisition in deep-sea sediments: a case study of the Brunhes-Matuyama reversal and oxygen isotopic Stage 19.1

Although post-depositional remanent magnetization (PDRM) in deep-sea sediments appears to be acquired during the earliest stages of sediment compaction, the natural variability of the PDRM lock-in depth in deep-sea sediments is poorly understood and as yet unquantified. Here we consider variations in the relative stratigraphic positions of oxygen isotopic interglacial Stage 19.1 and the Brunhes-Matuyama (B/M) Chronozone reversal for eight deep-sea sediment cores. Results from a similar study of the displacement between a widespread microtektite layer and the B/M boundary are also included [1]. The PDRM lock-in depth and the temporal relationships between the B/M and Stage 19.1 datums can be determined from the offsets between the paleomagnetic and the isotopic (and microtektite) stratigraphies. For cores with sedimentation rates greater than 1 cm kyr −1, the depth offset between the paleomagnetic and isotopic datums is a linear function of sedimentation rate. A simple model ( r 2 = 0.77) demonstrates that (1) PDRM acquisition occurs ∼ 16 cm below the sediment surface, and (2) the B/M reversal occurs 6 kyr (±2 kyr) after the Stage 19.1 datum, and the Stage 19.1 datum occurs 9 kyr (±3 kyr) after the deposition of the Australasian microtektite strewnfield. This example argues against simple geophysical models linking geomagnetic field reversals to climate change or impact events. The B/M boundary is anomalously deep (30–50 cm) in very low accumulation rate sediments ( < 1 cm kyr −1) and this may reflect the unusual physical properties of these sediments. A review of the geotechnical literature suggests that very low accumulation rate sediments have sufficient time to develop enhanced interparticle rigidity (structural strength) which inhibits early compaction and, hence, PDRM acquisition.

Late Cenozoic paleomagnetic record of Duck Hawk Bluffs, Banks Island, Canadian Arctic Archipelago

Detailed paleomagnetic investigations have been completed on unconsolidated sediments from Duck Hawk Bluffs on Banks Island, in the Canadian Arctic Archipelago, that record some of the oldest late Cenozoic glacial and nonglacial events in Canada. The preglacial Worth Point Formation, the overlying Duck Hawk Bluffs Formation, including marine and glacial deposits laid down during the Banks Glaciation, and the lower part of the interglacial Morgan Bluffs Formation have magnetically reversed directions and therefore are of Matuyama age (&gt;790 ka). Upper Morgan Bluffs Formation organic beds and deposits of the younger Thomsen Glaciation, Cape Collinson Interglaciation, and Amundsen Glaciation are normally magnetized and therefore of Brunhes age (&lt;790 ka). The Brunhes–Matuyama boundary is recorded in the upper portion of the Morgan Bluffs Formation. Its precise position within the interglacial sequence can be identified, since the sediments document the gradual change from reversely inclined directions to normally inclined ones. These results confirm that the preglacial Worth Point Formation is at least Early Pleistocene in age and that the Banks Glaciation (the oldest and strongest continental glaciation recorded in the western Arctic) and a good part of the Morgan Bluffs Formation are of Early Pleistocene age. The study also documents a rare site in Canada where terrestrial sediments record the Brunhes–Matuyama transition and in doing so permits a precise correlation of part of the Banks Island stratigraphy with other key late Tertiary and Early to Middle Pleistocene arctic terrestrial and marine sequences.

An alternative astronomical calibration of the lower Pleistocene timescale based on ODP Site 677

ABSTRACTOcean Drilling Program (ODP) Site 677 provided excellent material for high resolution stable isotope analysis of both benthonic and planktonic foraminifera through the entire Pleistocene and upper Pliocene. The oxygen isotope record is readily correlated with the SPECMAP stack (Imbrie et al. 1984) and with the record from DSDP 607 (Ruddiman et al. 1986) but a significantly better match with orbital models is obtained by departing from the timescale proposed by these authors below Stage 16 (620 000 years). It is the stronger contribution from the precession signal in the record from ODP Site 677 that provides the basis for the revised timescale. Our proposed modification to the timescale would imply that the currently adopted radiometric dates for the Matuyama–Brunhes boundary, the Jaramillo and Olduvai Subchrons and the Gauss–Matuyama boundary underestimate their true astronomical ages by between 5 and 7%.

Equatorial and mid-latitude records of the last geomagnetic reversal from the Atlantic Ocean

Three records of the Matuyama-Brunhes reversal have been obtained from ODP deep-sea cores distributed along the West African margin in the Atlantic Ocean. These studies in addition to the record from DSDP Site 609B provide a latitudinal transect extending from the equator to 50°N. Simulations of natural smoothing by post-depositional processes show that characteristic features of a transitional field geometry dominated by axisymmetry should be preserved in the records, especially at the equator. The results do not seem to favour the hypothesis that axisymmetrical terms would dominate during this transition.

Detailed paleomagnetic records during the Brunhes-Matuyama geomagnetic reversal, and a direct determination of depth lag for magnetization in marine sediments

Detailed paleomagnetic records were studied from homoclinal bathyal siltstone deposited during the Brunhes-Matuyama geomagnetic reversal outcropping in the Boso Peninsula, central Japan. The records are based on three continuous sediment cores from three sections within a distance of 14 km. The stratigraphic horizons were correlated using a volcanic ash bed which occurs about 2 m above the Brunhes-Matuyama reversal. The sedimentation rate, according to the oxygen isotope stratigraphy for the three sections, decreases eastward and is 3.70, 2.20 and 1.83 m ka −1. The depth lag of magnetization in the sediments was calculated directly as 42 cm, based on the differences in the relative position of the key ash layer in the correlated virtual geomagnetic pole (VGP) paths for the three sections. VGP movement during the geomagnetic reversal is characterized by short intervals of very rapid motion between periods of relative stability. The last swing of the VGP from the southern hemisphere to the northern hemisphere in the Brunhes-Matuyama geomagnetic reversal took only 38 yr. Intensities of the remanent magnetization normalized by anhysteretic remanent magnetization indicate that the intensity of the geomagnetic field decreased to one-fifth for about 5.4 ka including the geomagnetic reversal. The movement of the VGP longitude is 0.36° yr −1 westward and this westward drift can be related to the rotation of the non-dipole field.

A detailed geomagnetic record from Chinese loess

A collection of over 100 individually orientated hand-samples was obtained from a 335 m loess section near Lanzhou, Northern China. Previous studies defined the Brunhes-Matuyama (B-M) boundary and Jaramillo normal subchron within the section. The position of these magnetic ‘markers’ led to the suggestion of a basal age for the section of ∼ 1.3 Ma. This disagrees significantly with a date of 2.45 Ma for the onset of loess accumulation at Lochuan, 500 km to the east of Lanzhou. The present study considers the section in far greater detail and has derived a complex magnetostratigraphy, isolating seven normal events within the lower 130 m of the section. A simple interpretation of this magnetostratigraphy suggests a basal age for the section in the region of 2.4 Ma. However, the relative chronology of the loess/palaeosol sequence, although appearing to confirm this basal age, has revealed that this simple interpretation of the magnetostratigraphy may not be justified.

Davis Creek silt, an Early Pleistocene or Late Pliocene deposit in the Cypress Hills of Saskatchewan

Davis Creek silt is the informal name for a previously unreported loess and its reworked detritus encountered at several locations to the south of the east and centre blocks of the Cypress Hills. This unit intervenes between a pediment with an estimated age of 10 Ma and Late Wisconsinan glacial deposits. Because the unit has reversed magnetization, it is older than 788 ka, the astronomical age of the Matuyama–Brunhes magnetic polarity reversal. The unit also contains an undated volcanic ash from the Pearlette ash family that could represent the Mesa Falls (1.27 Ma) or the Huckleberry Ridge (2.02 Ma) ash bed. Davis Creek silt overlies an oxidized weathering zone and contains large secondary carbonate nodules near its truncated top that were, in places, reworked into a lag deposit or stone line before accumulation of the glacial overburden. At one location Davis Creek silt is separated from this overburden by a unit of cryoturbated gravelly loam with remnants of a reddish-yellow paleosolic B horizon.

The Matuyama-Brunhes transition recorded from Lake Tecopa sediments (California)

The paleomagnetic record of the Matuyama-Brunhes reversal in the sediment of ancient lake Tecopa provided the first evidence that the geomagnetic field during transition is not dipolar. We resampled the section to increase the time resolution of the record. Our new data show that the characteristic direction could not be completely isolated by AF demagnetization. The transitional directions obtained from thermal demagnetization are thus quite different from those of the previous study but the primary conclusion that the field was non-dipolar remains. The VGP paths lying close to the site longitude is consistent with an axisymmetric transition field.

Brunhes–Matuyama paleomagnetism in three lava sections in Iceland

This paper reviews previous field research on the paleomagnetism of volcanics in Iceland at localities whose age is presumed to span the Brunhes–Matuyama chron boundary. Additional mapping and laboratory measurements are presented from three localities of late Quaternary reverse-to-normal transitions. At two, Tjörnes in north Iceland and Eyjafjöll in south Iceland, the identity of the transition is confirmed by K–Ar dating. At Tjörnes and at Ingólfsfjall in southwest Iceland, short-period variations of the paleomagnetic field are demonstrated to be useful in stratigraphic mapping.

Palaeomagnetic stratigraphy of Lake Bungunnia: Plio-pleistocene precursor of aridity in the murray basin, Southeastern Australia

The Murray-Darling river system in southeastern Australia formerly flowed into a large lake, Lake Bungunnia, before reaching the sea. Fine-grained clastics of this system, the Blanchetown Clay Formation, extend over an area of about 68,000 km 2 underlying the Mallee region with its surface array of dunes and salt lakes. Palaeomagnetic analyses are used to date the major hydrologic transition from the expansive freshwater system to the saline deflation playas and dunes that characterize this landscape today. Two sites were selected for sampling; one a cliff section on the left bank of the Murray River at Chowilla, the other from within sediments of Lake Tyrrell, a saline playa cut into sediments of the older freshwater system. Two sequences were sampled at Tyrrell; one by coring to 30 m in the basin centre, the other a composite cliff and 5-m core on the western margin of the playa. Post-Bungunnia sediments representing modern landforms were sampled in both surficial dune-palaeosol units of the Woorinen Formation and in saline sediments, the Tyrrell Beds, within the modern saline playa. The sequence of multiple aeolian units of the Woorinen Formation and gypseous sediments of the Tyrrell Beds are normally magnetized indicating deposition within the past 0.7 million years. By contrast, the uppermost horizons of bedded Blanchetown Clay below a surface zone of pedogenic alteration are reversed. The Brunhes-Matuyama (B-M) boundary, key to the age of draining of Lake Bungunnia by deepening the overflow channel to the sea, is obscured within the zone of pedogenesis separating unaltered Blanchetown Clays from overlying aeolian deposits. The onset of aeolian and saline gypseous deposits characteristic of the arid-zone facies is placed soon after the B-M boundary, about 500,000 years ago. Sequences through the Blanchetown Clay pass down through the Matuyama reversed chron into the Gauss normal chron with the lowermost section on the Tyrrell west bank recording a basal reversed zone of possible Gilbert age. Thus the origins of Lake Bungunnia extend back beyond basal Pleistocene into the late Pliocene. The lake's duration covers more than 2 million years spanning the Plio-Pleistocene boundary in this important region of southeastern Australia.

Evidence for an increase in cosmogenic 10Be during a geomagnetic reversal

Reversals in the geomagnetic field, which occur every few hundred thousand years, represent a dramatic change in the Earth's environment. Although there is no satisfactory theory for such reversals, it is generally accepted that the dipole field intensity decreases to <20% of its ‘normal’ value for a few thousand years during the change in direction1. Because the galactic and solar cosmic rays which impinge on the Earth's atmosphere are charged, a significant fraction (about half) of them are deflected by the geomagnetic field2. At the time of a reversal, this magnetic shielding is greatly reduced, and it has been suggested that the increased flux of high-energy particles could have effects on evolutionary3 or climatic4 processes. For example, the statistically significant coincidence in levels of some marine faunal extinctions and reversal boundaries in ocean sediments5 could be caused, directly or indirectly, by the decreased geomagnetic intensity during the reversal. We report here evidence in marine sediments for an increase in cosmogenic 10Be production in the Earth's atmosphere during the Brunhes–Matuyama reversal 730,000 yr ago. In addition to confirming an increase in cosmogenic isotope production, the results provide information on the magnitude and duration of the geomagnetic intensity decrease during such an event, and the depth at which rmanent magnetism is acquired in marine sediments.

Latitudinal dependency of geomagnetic polarity transition durations

Palaeomagnetic records of the Matuyama–Brunhes polarity transition were obtained from seven, low sedimentation rate, deep-sea cores from the Pacific Ocean. The cores were taken near the 180° meridian and provide a latitudinal transect of transition records extending from 45.3° N to 33.4° S. Examination of these records suggests that low sedimentation rate cores may not be capable of recording the fine details of transitional field behaviour, but there are indications that these cores may, in fact, provide accurate records of the more general features of the reversal. Most notable of these features is that the duration of the transition is dependent on the site latitude, with durations at mid-latitudes being more than a factor of 2 longer than at equatorial latitudes.

Brunhes-Matuyama magnetic reversal dated at 790,000 yr B.P. by marine-astronomical correlations

The presence of the Brunhes-Matuyama magnetic reversal in deep-sea core sediments makes possible an alternative to the usual K/Ar radioistope method of dating the reversal as found in rocks. The alternative method uses correlations of Northern Hemisphere summer insolation with oxygen-isotope ratios from tropical cores. The latitude-dependent insolation variations are calculated from planetary mechanics and thus provide a highly accurate astronomical time scale. The insolation variations strongly influence glacial-ice volume fluctuations that dominate the oxygenisotope ratio changes recorded in core sediments. The summer half-year insolation variations are identified with corresponding isotope-ratio changes in cores from the present through glacial Stage 20. Misleading effects of discontinuities or major nonuniformities of sediment deposition are avoided by an analysis of the uniformity of V28-238 and V28-239, the principal cores studied, and by comparisons with other cores. The top section (Stages 1 to 10) of V28-238 is uniformly deposited, and for this section an isotope-ratio time scale is chosen that agrees with the thorium-uranium date for the high sea stand of the last interglacial extreme. Over this interval, major glacial extremes (strong isotope-ratio minima) coincide consistently with major insolation minima at times of low orbital eccentricity. In addition interstadials are directly associated with precessional insolation peaks, and the envelope of isotope-ratio peaks resembles the envelope of precessionally dominated insolation peaks. The assumption that the glacial extremes depended similarly on insolation minima during Stages 10 to 20 permits minor age shifts of strong isotope-ratio minima in the two cores (relative to ages based on uniform overal deposition) to match the ages of low-eccentricity insolation minima. The age shifts reflect residual nonuniformities of deposition. The validity of this matching procedure is supported by a resulting consistent identification of principal isotope-ratio peaks with high- and low-latitude coincident insolation maxima. The Brunhes-Matuyama reversal is found intinterglacial Stage 19, and is dated on the astronomical time scale at 790,000 ± 5000 yr B.P.