Matuyama Epoch Research Articles

Preliminary magnetostratigraphic record of Pleistocene deposits, Lake Natron Basin, Tanzania

Summary The declination of characteristic magnetization has been used to determine the polarity changes in two volcano-sedimentary sequences exposed in the Lake Natron basin. Radiometric dating and biostratigraphic data allow us to link the magnetostratigraphy to the standard geomagnetic polarity time scale. In the Lake Natron palaeomagnetic record, the Matuyama Epoch is well defined, possibly including the Olduvai Event (1.87–1.67 Ma). Other normal polarity zones have to be considered cautiously.

A geological and paleomagnetic study of the Hyblean volcanic rocks, Sicily

The geological and paleomagnetic study of the Hyblean volcanic rocks in southeastern Sicily permits the following conclusions: 1. (1) The Upper Cretaceous volcanics are considerably more extensive than earlier studies indicated, they stretch from north of Augusta to the vicinity of Siracusa. Their magnetization is identical to that of the better known outcrops around Capo Passero. Of the two new age dates reported here, one is younger than any so far reported. 2. (2) The Upper Tortonian flows are normally magnetized, suggesting a correlation of the Carlentini Formation with magnetic interval 7. The flows at the top of the Tellaro Formation are also normally magnetized and are tentatively correlated with the Carlentini Formation flows. Shorter reversed intervals within interval 7 may be represented by two less well-dated flows. 3. (3) The lowest Pliocene flows have an adequate stratigraphie control and their magnetization is consistent with the extrusion of the pillow lavas during the Mammoth event. The Militello flow sequence together with the Francofonte flows appear by indirect argument to lie within the oldest reversed interval, i.e. pre-Kaena, of the Matuyama epoch. The individual flows in which both normal and reversed directions have been found cannot be assigned positions in magneto-stratigraphie scale. 4. (4) The geomagnetic pole positions are consistent with that of the African plate during the Cretaceous, and a European plate location during both Pliocene and Miocene time. The pole positions are: Upper Cretaceous 212.3°E, 69.3°N. Upper Miocene: 200.9°E, 82.1°N. Pliocene 339.3°E, 85.9°N.

Palaeomagnetic evidence for early Pleistocene in the central and northern North Sea

The palaeomagnetic evidence presented here refutes the earlier hypothesis that most of the Quaternary succession in the central North Sea area is of Middle to Upper Weichselian age. A study of the Quaternary seismic-stratigraphy has been tied to 11 cored boreholes which are being examined for their physical properties and faunal content. Initial palaeomagnetic studies show a major reversely magnetized zone which can be correlated from borehole to borehole and with the seismic stratigraphy. This reversely magnetized zone is thought to represent the Matuyama Epoch which ended ∼790,000 yr BP; consequently this suggests the presence of early Pleistocene strata in the central and northern North Sea.

Baffin Bay: Present-day analog of the central Arctic during late Pliocene to mid-Pleistocene time

Olive-green and olive-gray “glacial marine” sandy clays with rare planktonic foraminifera, dominated by a euryhaline and eurytherm species, blanket the northern Baffin Bay floor today. These sediments are analogous to those of the central Arctic Basin deposited between ∼3 and ∼0.9 m.y. ago. While today the central Arctic is covered with perennial sea ice, during part of the Gauss and most of the Matuyama epochs (unit II), hydrological conditions were very similar to those in northern Baffin Bay today—generally ice-free year round. These data lend support to the hypothesis of an ice-free Arctic during late Pliocene to mid-Pleistocene time.

Palaeomagnetic and palaeontological dating of a section at Gioia Tauro, Italy: Identification of the Blake Event

Palaeomagnetic and palaeontological studies on samples withdrawn from a 250-m-long unorientated core from Reggio di Calabria, Italy (38°N, 16°E) are described. The lower 200 m of core penetrated off-shore clays and visual examination of the split sections of core indicated no obvious breaks in deposition. After alternating field demagnetization in 200 Oe the palaeomagnetic inclination log shows the Brunhes/Matuyama boundary at 110 m below which the Jaramillo and Olduvai Events within the Matuyama Epoch are identified. This interpretation of the magnetostratigraphy is supported by palaeontological evidence. The rate of accumulation of clay is estimated to have ranged between about 60 and 190 mm/kyr with an overall average through the Matuyama Epoch of about 90 mm/kyr. Assuming that this average rate continued through the Brunhes Epoch, the age of the top of the clay unit is estimated to be about 90,000 yr B.P. About 5 m from the unconformable top surface of the clay, a split sequence of reversed inclinations is interpreted as a record of the Blake Event, and the overall average deposition rate implies that its duration may have been as long as 50,000 yr. No other reversed event is recorded by the palaeomagnetic inclination log through the Brunhes Epoch, though there are four horizons where shallow positive inclinations are recorded.

The Cenozoic stratigraphy and structure of the western part of the Rift Valley in southern Kenya

Mapping, K–Ar dating and palaeomagnetic polarity measurements were used to study the geological history of the predominantly volcanic sequences in the western part of the Kenya rift valley S of 1º30'S. The rift was initiated as a downwarp which was later infilled by melanephelinitic agglomerates and lavas (15–12 Ma ago). Major faulting (12–7 Ma ago) downthrew these volcanics to the E and flood trachytes (7–5 Ma ago) were erupted onto the newly-formed rift floor. Minor faulting and volcanism preceded the second period of major faulting (5–3 Ma ago). Extensive flood basalts (3–2.5 Ma ago) infilled the resulting rift structure. Later subsidiary depressions in the rift floor were infiIled by two large porphyritic trachyte flows in the N (the older one 2.1 Ma ago), by flood basalts (2.1–1.7 Ma ago) in the S and by flood trachytes (1.4–0.7 Ma ago) in the E. The palaeomagnetic polarity sequence from the rift floor lavas represents a substantial part of the Matuyama epoch. A period of major faulting (most of it post-0.6 Ma ago) created much of the present rift topography and the resulting Magadi–Natron basin was later flooded by an extensive temporary lake.

Short geomagnetic episodes in the Matuyama epoch

Two short geomagnetic episodes in the middle Matuyama epoch have been identified within thin (4.3-mm) sections of two deep-sea calcareous sediment cores taken in the western equatorial Pacific. Both cores are correlated by magnetostratigraphic and micropaleontological methods. Magnetic stability and paleomagnetic reliability are tested by alternating field demagnetization of natural remanent magnetization as well as by the ratios of intensities of anhysteretic remanent magnetization to those of saturation isothermal remanent magnetization. One episode is dated to be 1.06 m.y. BP. The other is identified to be about 1.94 m.y. BP, which is presumably in agreement with the Reunion event. Both episodes seem to be accompanied by conspicuous drops in field intensity. Possible correlation of field intensity with biological productivity in the ocean is also postulated from the present paleomagnetic results.

Geomagnetic polarity measurements from pleistocene deposits in southwest France and their chronological implications

AbstractMeasurements of palaeomagnetic inclination in sediments from the French Pays Basque, representing three of the major Pleistocene biostratigraphic subdivisions previously identified, confirm normal polarity in all cases. Taken together with the palaeobotanical, stratigraphic and geomorphological evidence they suggest that the Moulignan, Senixian and Ilbarritzian stages lie within the early part of the present Brunhes normal polarity epoch and probably correlate with some unknown part of the “Cromerian Complex”. The Bidartian is thought to lie within a normal polarity event during the Matuyama epoch and to correlate with part of the Tiglian ‘A’ sub‐stage of the Netherlands.

Geomagnetic polarity measurements from Pleistocene deposits in Southwest France and their chronological implications

AbstractMeasurements of palaeomagnetic inclination in sediments from the French Pays Basque, representing three of the major Pleistocene biostratigraphic subdivisions previously identified, confirm normal polarity in all cases. Taken together with the palaeobotanical, stratigraphic and geomorphological evidence they suggest that the Moulignan, Senixian and Ilbarritzian stages lie within the early part of the present Brunhes normal polarity epoch and probably correlate with some unknown part of the “Cromerian Complex”. The Bidartian is thought to lie within a normal polarity event during the Matuyama epoch and to correlate with part of the Tiglian ‘A’ sub‐stage of the Netherlands.

近畿・東海地方の鮮新・更新統古地磁気層序

Since 1951, many studies have been published on the magnetostratigraphy of the Plio-Pleistocene strata of the Kinki district. In the 1970's, especially, various sections of the Osaka and Kobiwako Groups, including bottom sediments of Lake Biwa, as well as their correlatives in the Tokai district, have been studied repeatedly.The sediments are mostly lacustrine and fluvial, and are frequently intercalated with volcanic ash. The Osaka Group also contains interbedded marine clay layers. The sediments, therefore, are suitable for establishing a combined litho-, tephro-, magneto- and biostratigraphy and radiometric chronology. Such a chronology can be used for testing the reliability of primary remanent magnetization of the sediments by the measurements of multiple samples from various localities and correlated by means of volcanic ash intercalations. Based on the latest studies, including unpublished results of studies by the present writer, the Plio-Pleistocene magnetostratigraphy of the Kinki district has been compiled (Fig. 2). Important points include:1. The Gauss/Matuyama boundary is drawn between the Kosaji and Hazama Volcanic Ash in the Kobiwako Group, and between the T2 and Shimakumayama Volcanic Ash (2.3-2.4m.y.), in the Osaka Group.2. The Matuyama/Brunhes boundary is about 10m above the Biotite Volcanic Ash (0.7m.y.) in the Kobiwako Group, and under the Ma4 clay layer in the Osaka Group.3. Five normal polarity events have been recognized in the Matuyama Reversed Epoch. They are, in ascending order, as follows:a-. Shiba event (about 2.3m.y.)b-. Sushirodani event (about 2.2m.y.)c-. Naka-Nango event (1.7-1.8m.y.)d-. Pink-Komyoike event (1.1-1.2m.y.)e-. Kamikatsura eventSome of the five events remain uncertain as to correlation with those reported previously in the Matuyama Epoch.4. Five reversed polarity events or excursions were detected in the Brunhes Normal Epoch. The older three were named Biwa I, II and III events, and the younger two may be correlatives with the Lashamp and Blake events. The age of the Biwa III Event has been estimated to be about 0.33m.y..

Palaeomagnetic study of deep-sea sediments from the Melanesia Basin.

Detailed behavior of the palaeomagnetic field during the upper Matuyama epoch including the Jaramillo event was studied using sliced thin sections of two deep-sea sediment cores taken from the Melanesia Basin.It has been shown from the measurement of the declination and intensity of natural remanent magnetization in 785 specimens that the intensity of the geomagnetic field decreased to nearly the zero level at the beginning of the lower Jaramillo and Brunhes-Matuyama boundaries. Such a low field interval seems to have continued thereafter for an interval as long as 10, 000yrs. Detailed data for the transitional stages during the polarity reversals have further revealed that the intermediate directions hardly occurred at any time in the entire span of the time studied.

Brunhes-Matuyama polarity transition

A paleomagnetic record of the geomagnetic field during its change of polarity from the reversed Matuyama epoch to the normal Brunhes epoch has been obtained from sediments of ancient Lake Tecopa in southeastern California. The polarity switch occurs in siltstone of uniform composition, and anhysteretic magnetization experiments indicate that the magnetic mineralogy does not change markedly across the transition. Within the transition interval, intensity of the magnetization drops to a minimum of 10% of the intensity after the transition. The interval of low field intensity preceded and lasted longer than the interval during which the field direction reversed, the latter being shorter than the interval of low intensity by a factor of at least 2.5. The VGP's make a smooth transit from reversed to normal polarity, with the path lying in the sector of longitude between 30°E and 60°W. Pole paths for the Brunhes-Matuyama transition recorded in California and Japan are completely different, indicating that the dipole field decayed. The transition field appears to be nondipolar, and there is no evidence for an equatorial component. Since there is little dispersion of the VGP's about a great circle path, it is possible that large-scale drift of the nondipole field ceased during this polarity transition.

Regional deep-sea dynamic processes recorded by late Cenozoic sediments of the southeastern Indian Ocean

A large collection of USNS Eltanin deep-sea sedimentary cores and bottom photographs from the southeast Indian Ocean between long 70°E and 120°E, and between Antarctica and lat 30°S, were analyzed. Cores from the crest and flanks of the mid-ocean ridge are mostly late Quaternary in age, with only rare breaks in sedimentation. In great contrast, flanking this zone in deep basins immediately to the south of the ridge in the South Indian Basin and in a broad zone in the western sector of South Australian Basin, there are areas where bottom currents have systematically eroded or inhibited deposition of sediments ranging in age from Quaternary to Pliocene, and occasionally middle Tertiary. This regional deep-basin erosion extends northward between Broken Ridge and the Naturaliste Plateau to the Wharton Basin where sediments as old as Late Cretaceous are exposed. As indicated by disconformities, ocean-floor characteristics, and seismic-profile data, much of the shallower, north-trending Kerguelen Plateau has also undergone widespread erosion by bottom currents. The erosional disconformities in the deep basins have been created by general increase in velocities of Antarctic Bottom Water during the last 2.5 m.y., apparently with major separate pulses during the Brunhes epoch (t = 0.69 m.y. to present) and part of the Matuyama epoch (t = 2.43 to 0.69 m.y.). Extensive areas of manganese nodules have developed in conjunction with this bottom-current activity, most spectacularly as a vast pavement in the northwestern sector of the South Australian Basin. This feature, which we name the “Southeast Indian Ocean Manganese Pavement,” is approximately 106 km2 in area. The available evidence indicates long-term major erosion by the eastward-flowing Circumpolar Current across the central and southern parts of the Kerguelen Plateau. In the deep basins, high-velocity Antarctic Bottom Water flows eastward through the northern sector of the South Indian Basin, with important northward flow crossing the mid-ocean ridge at 110°E and 120°E into the South Australian Basin. This northward branch traverses the western sector of the South Australian Basin, the Southeast Indian Ocean Manganese Pavement, and then flows between Broken Ridge and Naturaliste Plateau into the Wharton Basin. Major Cenozoic to Late Cretaceous hiatuses in the Wharton Basin revealed by deep-sea drilling suggest that northward-flowing bottom water through this conduit has been a very long term feature.

A detailed paleomagnetic survey of the type location for the Gilsa geomagnetic polarity event

A total of 163 cores have been taken from a maximum of 40 separate lavas in three separate sections of the Jökuldalur, southwest of Egilsstadir, Iceland, and subjected to paleomagnetic analysis and some K-Ar dating. Previous work on the sections by McDougall and Wensink (1966) led to the establishment of the Gilságeomagnetic polarity event, with an age of about 1.60 m.y., during the reversed polarity Matuyama epoch. This earlier study described a possible reversely magnetized lava separating the Gilsáevent from a second normal polarity lava, perhaps representing the Olduvai event. Such a possibility was subsequently a source of speculation from diverse sources concerning the polarity history for the lower Matuyama. The present study indicates clearly that there is no second normal polarity event represented in the sections. Only one normal polarity event is therefore represented in the lower Matuyama of the Jökuldalur, and the age of the lavas involved is confirmed to be approximately 1.58 ± 0.08 m.y. Because of uncertainties in the interpretation of the original K-Ar results from Olduvai Gorge, it is still not possible to be certain that the Olduvai and Gilsáevents are separated in time. An incidental result of the survey is evidence to show that, contrary to recent suggestions by Einarsson (1972), there is no substantial hiatus between the major lower parts of the section and lavas believed to represent extrusions after a regional tilting and peneplanation.

Shape analysis of paleosecular variation data

Shape analysis of paleosecular variation (PSV) data, if it is properly used, can determine the statistical character of a PSV data set in many cases. Existing statistical techniques, such as the χ2 goodness of fit test, the ‘dipole moment,’ and eigenvalue methods, are reviewed with this point in mind. Two new parameters, K0 and Km, are introduced that can be used to derive a better estimate of angular standard deviation for a PSV data set, particularly if the data are most densely distributed near their mean. We have analyzed the shape of the distributions of 12 PSV data sets of the Brunhes and Matuyama epochs by means of these various statistical techniques. The results of our studies indicate that the minimum size of a PSV data set should be about 30 and that the shape analysis already mentioned should be used as one PSV data selection criterion.

Palaeomagnetism of the quaternary sediments of the locality Červený Kopec (red hill)

As the result of the palaeomagnetic research into the loesses and soils of Quaternary age from the locality of Cervený Kopec (Czechoslovakia) it was found that the viscous remanent magnetism (RM) of these rocks exceeds the stable component of the RM. Figures 1–6 show polar graphs and the demagnetization curves corresponding to cases when the rocks sedimented in a magnetic field, the direction of which corresponded to the direction of the present geomagnetic field, or when the X and Z components of the field were of the opposite sense. The positions of the magnetic N pole were computed for the interval of transition from Matuyama's epoch of negative polarity to Brunhes's epoch of positive polarity. At the time of the reversal it was found that the pole moved along an oval at time intervals of about 10 000 years and that their was an apparent west-ward drift of the pole. The reversal found in the Quaternary is compared with the reversal in the Pliocene. The agreement in the pole movements in these two cases seems to indicate constant causes of these large changes in the magnetic field and a possible relation between the pole movement and the boundaries of the lithospheric blocks.

Palaeomagnetism and Palaeosecular Variation on Lavas from Norfolk and Philip Islands, South-west Pacific Ocean

Summary Norfolk Island and the adjacent Philip Island in the South-west Pacific Ocean are built of basaltic lavas and tuffs that were erupted between 3.1 and 2-35My ago during the Gauss epoch and the early part of the Matuyama epoch. Eruptions occurred at about 3.05 My ago during the Mammoth event, and between 2.8 and 2.35 My ago with one or possibly two intervals of quiescence within this period. Palaeomagnetic studies on over 200 samples from more than 80 sites, covering the whole time range of lavas found on the islands, yields an angular standard deviation of 7.5 for the directions as well as for the virtual geomagnetic poles. This value is very low compared with that expected from current models for secular variation. The deviation of the mean virtual geomagnetic pole from the Earth's rotation axis is 14. We conclude that only a partial palaeosecular variation has been recorded in the Norfolk Island lavas because insufficient time is represented by the rocks to determine it fully, despite the fact that results on lavas of both normal and reversed polarity are included. A time span of more than 0.5 My needs to be sampled to obtain an estimate of the palaeosecular variation. The small dispersion of palaeomagnetic results from lavas in the Island of Hawaii, reported by earlier workers, may have a similar explanation to that advanced for the Norfolk Island data. If this is so then the idea that the non-dipole field under the Central Pacific has been suppressed during the Brunhes epoch needs to be re-examined. The palaeomagnetic study of a number of lava flows erupted over a period of time shows a dispersion of the measured directions of magnetization. This dispersion at least in part reflects fluctuations in the direction of the geomagnetic field, and provided that a sufficiently long period of time has been sampled an estimate of the palaeosecular variation (PSV) can be obtained from these data. Studies of the PSV for different geological times and at different localities throughout the world provides valuable information concerning the behaviour of the geomagnetic field in the past. In this paper we present the results of a palaeomagnetic study of basaltic lavas of

Pliocene-early Pleistocene paleoclimatic history recorded in Antarctic-Subantarctic deep-sea cores

Micropaleontological studies have been carried out on seven cores of middle Pliocene to early Pleistocene age (lower to upper Matuyama; t = 0.7 to 2.43 m.y. BP) from northern Antarctic and Subantarctic waters south of Australia and New Zealand. All of the cores contain abundant foraminiferal and radiolarian faunas, and have been dated by both micropaleontological and paleomagnetic methods. Fluctuations of a cold left-coiling Globigerina pachyderma fauna, with a warmer Subantarctic fauna define ten distinct intervals of warming during the Matuyama Reversed Epoch. This compares with six for the Brunhes Normal Epoch ( t = 0.0 to 0.7 m.y. BP). Peaks formed by increased frequencies of warm-water forms and right-coiling G. pachyderma are of lower amplitude than warmer-water peaks in the Brunhes. It is thus inferred that Matuyama climatic fluctuations in the Southern Ocean are of lower magnitude than those in the Brunhes. Furthermore, the foraminigferal evidence indicates that average temperatures during the Matuyama are in general cooler than during the Brunhes. This conflicts with previous paleotemperature determinations based on radiolarian assemblages which suggest average warmer conditions throughout the entire Matuyama Epoch. The disappearance of present-day subtropical radiolarians, Saturnulus planetes and Pterocanium trilobum, near the Brunhes-Matuyama boundary in the Southern Ocean has previously been regarded as strong evidence for a deterioration of climatic conditions in the Brunhes. Throughout the Matuyama, however, these forms occur in association with cold (northern Antarctic-Subantarctic) planktonic foraminiferal assemblages, and thus apparently lost their environmental tolerance for northern Antarctic-Subantarctic waters about 0.7 m.y. BP. The opposite is true for the planktonic foraminifer Globorotalia inflata which adapted to Subantarctic waters approximately 0.7 m.y. BP. Both of these apparent ecological adaptations coincide very closely with the magnetic reversal at the Brunhes-Matuyama boundary. Middle to late Pliocene (lower Matuyama) climatic oscillations within Subantarctic-northern Antarctic waters correspond rather closely with those established for the New Zealand middle to late Pliocene, but appear to be out of phase with equatorial and North Atlantic regions.

Paleomagnetic events in the brunhes and Matuyama epochs identified from magnetic profiles reduced to the pole

Eleven oceanic magnetic profiles associated with the paleomagnetic time scale younger than the beginning of the Matuyama epoch (2.43 my) have been reduced to the pole, altered to conform to a 3 cm yr-1 spreading rate, and then all halves added and averaged to obtain a representative symmetric magnetic profile. This final stacked profile emphasizes the subtle anomalies associated with minor paleomagnetic events and minimizes randomly occurring anomalies. The axial anomaly of the stacked profile shows no evidence of the Laschamp event (centered at 0.025 my); however, a minimum at 0.15 my may correspond to the Blake event. A physical model of the stacked magnetic profile consists of a thin, highly magnetized layer with a 40% magnetization decrease at 10 km from the profile center. Magnetization values were modified from Irving and Talwani and the Blake event included. The stacked profile shows two minor anomalies centered at 1.97 my and 2.17 my. The younger anomaly corresponds with the younger Olduvai event (centered at 1.965 my) on the Cox time scale and the W anomaly (centered at 1.99 my) of Emilia and Heinrichs. The older anomaly lies between the older Olduvai event (2.12 my) of Cox and the X anomaly of Heirtzler, and Emilia and Heinrichs; additional work is required to assess the significance of this older event.

South Pacific Planktonic Foraminiferal Biogeography and Late Cenozoic Paleo-Oceanography: ABSTRACT

Distinct changes in the character of South Pacific planktonic foraminiferal faunas occur at the 2 major water-mass boundaries. The southern limits of at least 10 species occur at or near the Subtropical Convergence which marks the southern limit of subtropical water. An additional 8 species have their southern limits approximately coincident with the Antarctic Convergence, leaving only 1 species characteristic of Antarctic waters. Micropaleontologic and sedimentologic studies of South Pacific deep-sea cores have enabled partial definition of the climatic and glacial history of Antarctica and the South Pacific. Pliocene and Pleistocene cores can be divided into zones on the basis of upward sequential appearance of planktonic Foraminiferida, upward sequential disappearance of Radiolaria, and stratigraphic succession of calcareous nannofossils. A chronology of South Pacific Pliocene and Pleistocene climatic oscillations has been established, based on paleomagnetic and thorium-230 dating. Alternations of cold and warm water planktonic Foraminiferida distinguish 6 intervals of climatic warming during the last 700,000 years (Brunhes epoch) and 10 between 2.4 m.y. and 700,000 years ago (Matuyama epoch). The relative m gnitudes of climatic warmings were considerably greater during the last 500,000 years than between 2.4 m.y. and 500,000 years ago. Cooling was at times as intense during the Matuyama epoch as during the Brunhes epoch. This determination conflicts with previous paleotemperature determinations based on radiolarians which suggest somewhat warmer conditions throughout the Matuyama. Several present subtropical radiolarian species, however, apparently lost their environmental tolerance for subpolar temperatures about 0.7 m.y. ago. Climatic curves for planktonic forams and radiolarians are essentially the same for the late Pleistocene, confirming the usefulness of both groups in paleoclimatic studies. Fluctuations of certain calcareous nannofossils species closely follow radiolarian and foramin feral paleotemperature oscillations for the last 400,000 years but diverge strongly in older core sections. Abundances of the silicoflagellate genera, Dictyocha and Distephanus, clearly mark warm- and cold-water intervals respectively in late Pleistocene cores. The first appearance of ice-rafted quartz in the sub-Antarctic, about 5 m.y. ago, coincides with increases in bottom-transported quartz, suggesting a relation between increased bottom-water activity and Antarctic glaciation. Distinct changes in the radiolarian assemblages also occur at this time. Studies of land-based marine sections in New Zealand and of deep-sea cores have shown that the first major late Cenozoic cooling occurred during the late Miocene and earliest Pliocene when south-central sub-Antarctic planktonic foraminiferal faunas (approximately 55°S equivalent lat.) spread over central New Zealand. A further major cooling during the middle Pliocene spans the Gauss-Matuyama boundary (t = 2.43 m.y. ago). This cooling was followed by more fluctuating climatic conditions in the late Pliocene and early Pleistocene (lower and middle Matuyama; t = 2.43 to 1.60 m.y. ago). Evidence of possibly synchronous late Miocene cooling has also been reported in California, Italy, Japan, and the equatorial Pacific. End_of_Article - Last_Page 631------------