Magnetic Epoch Research Articles

The Sub‐Ice Structure of Mt. Melbourne Volcanic Field (Northern Victoria Land, Antarctica) Uncovered by High‐Resolution Aeromagnetic Data

AbstractThe Mt. Melbourne Volcanic Field is a quiescent volcanic complex located in Northern Victoria Land, Antarctica, mostly covered by ice. Its inner structure has remained largely unknown, due to the paucity of outcrops and the lack of detailed multi‐disciplinary investigations. Here we present a novel high‐resolution aeromagnetic dataset, revealing strong long‐wavelength negative anomalies superimposed by short‐wavelength positive ones forming characteristic radial patterns. Automatic lineament detection, through the Hough transform technique applied to the tilt derivative of our magnetic dataset, shows prevailing NW‐SE‐to NNE‐SSW‐trending structural features, which combined with the few structural field observations contribute to define the deformation pattern. Pre‐existing and novel magnetic property measurements, coupled with available geochronological data, are used to constrain a two‐step 3D magnetic inversion. A layer‐structured Oldenburg‐Parker's inversion was utilized to model the deep and long‐wavelength components of the magnetic field, whereas a linear inversion based on a set of shallower prisms was used to model the short‐wavelength components. The final 3D model shows widespread reversely‐polarized volcanics, which are locally intruded and superimposed, respectively by swarms of normally‐polarized dikes and radial lava flows along paleo‐valleys. These results support the onset of volcanic activity in the entire field at least in the Matuyama magnetic epoch, that is, between 2.58 and 0.78 Ma.

MAGNETOSTRATIGRAPHIC INVESTIGATION OF THE UPPER SIVALIKS NEAR PINJOR, INDIA

A magnetostratigraphic study on four sections near Pinjor showed that the Tatrot stage falls in the Gauss magnetic epoch and the Pinjor corresponds to most of the Matuyama epoch. A significant faunal change, already observed by other authors in Pakistan, marks the Tatrot-Pinjor boundary and approximately coincides with the Gauss-Matuyama transition. Sedimentation becomes entirely conglomeratic in a positive episode tentatively interpreted as Jaramillo. The time interval represented by this event witnessed a deep climatic change in Europa and an extensive faunal turnover in all Eurasia, and seems also to correspond to the onset of a strong tectonic activity in the Himalayan belt.

Influence of solar wind structure on cosmic ray modulation during epochs of alternate solar magnetic polarity

The cosmic ray flux is anti-correlated with the sunspot number such that the flux maximum occurs approximately at solar minimum. Different time profiles of the cosmic ray maxima are probably related to the solar magnetic polarity influence on the balance between drift and diffusion. This study examines the differences in the flux variation of cosmic ray particles at epochs of alternate solar magnetic polarity by applying period analysis, specifically focusing on the period of solar rotation. We examine cosmic ray data from 43 neutron monitors at solar minimum years since 1954. We find that the periodicity at 27 days is stronger in positive than in negative magnetic epochs except for 2008 when it is stronger in northern neutron monitors (but not southern monitors). The yearly variation of power at 27 days shows peaks at the solar minimum years in the positive polarity, except for 2008 in the northern neutron monitors. It also shows an increase near solar maximum. Power at 22 years of the yearly variation of power at 27 days shows an increasing trend at middle-latitude northern and southern neutron monitors. Different effects of drift and diffusion could explain the different periodicity at 27 days in solar minimum years of both solar magnetic polarities and the shapes of cosmic ray flux maximum. The influence of details of the solar wind structure on cosmic ray fluxes can explain the periodicity of cosmic ray particles at the solar rotation period. The degree of this influence is clearly different during epochs of differing magnetic polarity. The most probable explanation for this is the solar polarity dependence of the cosmic ray latitude gradient.

An Empirical Modification of the Force Field Approach to Describe the Modulation of Galactic Cosmic Rays Close to Earth in a Broad Range of Rigidities

AbstractOn their way through the heliosphere, galactic cosmic rays (GCRs) are modulated by various effects before they can be detected at Earth. This process can be described by the Parker equation, which calculates the phase space distribution of GCRs depending on the main modulation processes: convection, drifts, diffusion, and adiabatic energy changes. A first‐order approximation of this equation is the force field approach, reducing it to a one‐parameter dependency, the solar modulation potential ϕ. Utilizing this approach, it is possible to reconstruct ϕ from ground‐based and spacecraft measurements. However, it has been shown previously that ϕ depends not only on the local interstellar spectrum (LIS) but also on the energy range of interest. We have investigated this energy dependence further, using published proton intensity spectra obtained by PAMELA and heavier nuclei measurements from IMP‐8 and ACE/CRIS. Our results show severe limitations at lower energies including a strong dependence on the solar magnetic epoch. Based on these findings, we will outline a new tool to describe GCR proton spectra in the energy range from a few hundred MeV to tens of GeV over the last solar cycles. In order to show the importance of our modification, we calculate the global production rates of the cosmogenic radionuclide 10Be which is a proxy for the solar activity ranging back thousands of years.

Magnetized strange quark matter in f(R, T) gravity with bilinear and special form of time varying deceleration parameter

In this paper, we have studied homogeneous and anisotropic locally rotationally symmetric (LRS) Bianchi type-I model with magnetized strange quark matter (MSQM) distribution and cosmological constant Λ in f(R, T) gravity where R is the Ricci scalar and T the trace of matter source. The exact solutions of the field equations are obtained under bilinear and special form of time varying deceleration parameter (DP). Firstly, we have considered two specific forms of bilinear DP with a single parameter of the form: q=α(1−t)1+t and q=−αt1+t, which leads to the constant or linear nature of the function based on the constant α. Second one is the special form of the DP as q=−1+β1+aβ. From the results obtained here, one can observe that in the early universe magnetic flux has more effects and it reduces gradually in the later stage. For t → ∞, we get p→−Bc and ρ → Bc. The behaviour of strange quark matter along with magnetic epoch gives an idea of accelerated expansion of the universe as per the observations of the type Ia Supernovae.

Magnetization in the South Pole-Aitken basin: Implications for the lunar dynamo and true polar wander

A number of magnetic anomalies are present along the northern edge of the lunar South Pole-Aitken (SPA) basin. A variety of hypotheses for their formation have been proposed, but an in-depth study of their properties has not been performed. Here we use two different methods to invert for their source body characteristics: one that completely searches a small parameter space of less than ten uniform-strength dipoles per anomaly, and another that uses grids of hundreds of dipoles with variable magnetization strengths. Both methods assume uniform magnetization directions at each anomaly and with one exception, produce nearly the same results. We introduce new Monte Carlo methods to quantify errors in our inversions arising from Gaussian time-dependent changes in the external field and the uncertain geometry of the source bodies. We find the errors from uncertainty in source body geometry are almost always higher. We also find a diverse set of magnetization directions around SPA, which we combine with other physical arguments to conclude that the source bodies were likely magnetized in a dynamo field. Igneous intrusions are a reasonable explanation (Purucker et al., 2012) for the directional variability, since they could be intruded over different magnetic epochs. However, the directional variability also implies either surprisingly large amounts of true polar wander or a dynamo not aligned with the lunar spin axis. We also explore the possibility that true polar wander caused by the SPA impact could allow iron-rich SPA ejecta to record a diverse set of magnetic field directions. Some of this material may have also become “sesquinary” ejecta and re-impacted across the Moon on 104–106 year timescales to capture such changes. No completely satisfactory answer emerges, except that the dipole-axis of the lunar dynamo may have been variable in direction.

Spatial gradients of GCR protons in the inner heliosphere derived fromUlyssesCOSPIN/KET and PAMELA measurements

During the transition from solar cycle 23 to 24 from 2006 to 2009, the Sun was in an unusual solar minimum with very low activity over a long period. These exceptional conditions included a very low interplanetary magnetic field (IMF) strength and a high tilt angle, which both play an important role in the modulation of galactic cosmic rays (GCR) in the heliosphere. Thus, the radial and latitudinal gradients of GCRs are very much expected to depend not only on the solar magnetic epoch, but also on the overall modulation level. We determine the non-local radial and the latitudinal gradients of protons in the rigidity range from ~0.45 to 2 GV. This was accomplished by using data from the satellite-borne experiment Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) at Earth and the Kiel Electron Telescope (KET) onboard Ulysses on its highly inclined Keplerian orbit around the Sun with the aphelion at Jupiter's orbit. In comparison to the previous A>0 solar magnetic epoch, we find that the absolute value of the latitudinal gradient is lower at higher and higher at lower rigidities. This energy dependence is therefore a crucial test for models that describe the cosmic ray transport in the inner heliosphere.

MODELING DRIFT ALONG THE HELIOSPHERIC WAVY NEUTRAL SHEET

Drift along the wavy heliospheric neutral sheet is believed to play an important role in cosmic-ray modulation and can explain the peaked versus flat intensity profiles during consecutive solar magnetic epochs. Modulation models are becoming more and more realistic and in order to determine the role of the wavy neutral sheet more accurately, we revisit a previous calculation for drift along it. While mathematically correct, we argue that the previous expression for neutral sheet drift, which follows naturally from the standard expression for gradient and curvature drift, must be adapted in order for the drift speed to be less than particle speed. We compare the effect of both the previous and the current more accurate version of neutral sheet drift on cosmic-ray modulation with results obtained by other methods.

MODULATION OF GALACTIC COSMIC RAY PROTONS AND ELECTRONS DURING AN UNUSUAL SOLAR MINIMUM

During the latestUlyssesout-of-ecliptic orbit the solar wind density, pressure, and magneticfield strength have been the lowest ever observed in the history of space exploration. Since cosmic-ray particles respond to the heliospheric magnetic field in the expanding solar wind and its turbulence, the weak heliospheric magnetic field as well as the low plasma density and pressure are expected to cause the smallest modulation since the 1970s. In contrast to this expectation, the galactic cosmic-ray (GCR) proton flux at 2.5 GV measured by Ulysses in 2008 does not exceed the one observed in the 1990s significantly, while the 2.5 GV GCR electron intensity exceeds the one measured during the 1990s by 30%‐40%. At true solar minimum conditions, however, the intensities of both electrons and protons are expected to be the same. In contrast to the 1987 solar minimum, the tilt angle of the solar magnetic field has remained at about 30 in 2008. In order to compare the Ulysses measurements during the 2000 solar magnetic epoch with those obtained 20 years ago, the former have been corrected for the spacecraft trajectory using latitudinal gradients of 0.25% deg # 1 and 0.19% deg # 1 for protons and electrons, respectively, and a radial gradient of 3% AU # 1 . In 2008 and 1987, solar activity, as indicated by the sunspot number, was low. Thus, our observations confirm the prediction of modulation models that current sheet and gradient drifts prevent the GCR flux to rise to typical solar minimum values. In addition, measurements of electrons and protons allow us to predict that the 2.5 GV GCR proton intensity will increase by a factor of 1.3 if the tilt angle reaches values below 10 .

Improvements planned for European geomagnetic repeat stations

Direct measurements of the Earth's magnetic field are continuously made around the world at magnetic observatories; they are obtained at all kinds of locations, ranging from ocean to land and even air. All this activity has prompted the question, Why all the effort in the magnetic satellite epoch?, or, Why observe the Earth's magnetic field in such different locations? To answer these important questions, a workshop on European geomagnetic repeat station surveys was organized at the Adolf Schmidt Observatory for Geomagnetism in Niemegk, Germany

Environmental changes in pre-evaporitic Late Miocene time in the Lorca Basin (SE Spain): diatom results

Abstract During the Late Miocene the Lorca Basin, which was an integrated part of the Betic Strait, underwent desiccation. The present investigation was completed to determine the response of the diatom flora to environmental changes before this event. Near the city of Lorca (Province of Murcia, SE Spain) the outcrop area around the Serrata ridge was studied and a stratigraphic section of 125m has been measured and sampled (115 samples). The Serrata ridge is capped by a 25m thick gypsum bed (Gypsum Member of the Serrata Formation). The profile below these gypsum layers comprises marl, sandstone, gypsum and partially laminated diatomite (Varied Member of the Serrata Formation). The frequency of radiolaria, siliceous sponge spicules, silicoflagellates, ebridians, diatoms and diatom species has been determined quantitatively. For the lower 87.5m of the section, which contains sufficiently well preserved diatoms, an age of latest Miocene (6.6-5.3 Ma; Magnetic Epoch 6 and 5) can be assigned by the presence of the diatom species Asterolampra acutiloba and Nitzschia reinholdii , applying the stratigraphic ranges of the species used in the North and Equatorial Pacific and in the Atlantic. In total, 116 diatom species and species-groups have been identified. However, the number of species per sample varies only between 13 and 36 species (a minimum of 300 valves counted). This low diversity, together with a dominance of one species-group, Thalassionema nitzschioides (up to 80%), which tolerates considerable ecological changes (e.g. in salinity) indicate that environmental stress must have prevailed during the sedimentation of the Varied Member of the Serrata Formation. Abundance fluctuations of this species-group, as well as changes in the composition of the remaining diatom assemblage and decreases in abundances of the siliceous microfossil groups, reflect repeated periods of interruption of the still marine influenced deposits before the deposition of thick gypsum layers. Several repetitive changes are indicated, where each cycle is characterized by an increase in meroplanktonic species (e.g. Paralia sulcata, Actinoptychus senarius ) and by a contemporary decrease in the holoplanktonic species towards the top, suggesting progressive environmental stress and a shallowing of the basin. Five of these cycles are clearly distinguished characterizing the initial stages of the evaporitic desiccation event in the Lorca Basin. Benthic diatoms are present only in low abundances (2–4%) and show no correlation with these fluctuations.

Compilation of the Namibian airborne magnetic surveys: procedures, problems and results

Forty one airborne magnetic surveys flown between 1962 and 1992 over different areas of Namibia, with a total of about 700,000 km, have been compiled as part of a German-Namibian co-operation project to produce a refined compatible countrywide magnetic data set. It was required to compile the data as if the whole country was flown by a single survey at 100 m terrain clearance at the same magnetic epoch. Emphasis was therefore given to the selection of the magnetic reference field model. The creation of a digital terrain model (DTM) was compulsory. Levelling of the survey blocks was impeded since adjacent blocks were only rarely overlapping. The magnetic data set is available as both coloured contour maps (scales 1:250 000 and 1:1 000 000) and digital grids with varying cell sizes (2000 m, 500 m and 200 m). Shield areas, fold belts and anorogenic intrusions dominate the magnetic anomaly pattern of Namibia. The fold belts are characterized by parallel running, narrow (<100 km) magnetic and virtually non-magnetic zones. Shield areas, covered by a thick sequence of mostly non-magnetic sediments, are distinguished by broad, mid-amplitude anomalies with low gradients. The analysis of magnetic linear features reveals the existence of two directional sets of magnetic lineations running across both fold belts and shield areas.

Magnetostratigraphy of the Late Cenozoic glacial sequence, Pieman River basin, western Tasmania

The Pieman River basin, western Tasmania, has been partially occupied by ice on several occasions during the Late Cenozoic. Detrital remanent magnetization recorded in rhythmically laminated glaciolacustrine sediments associated with glacial diamictons from the Boco, Bobadil, Bulgobac and Que drift sheets, enables the identification of drift deposited by glacier advances during the Brunhes, Matuyama or earlier magnetic epochs. Mapping and discrimination of the drift sheets using morphostratigraphic and post‐depositional weathering criteria, and U/Th dating is supported by the magnetostratigraphy of the drifts. The palaeomagnetic results indicate that the Boco and Bobadil Glaciations were deposited during the Brunhes normal Chron (783 ka), and the reversed magnetization recorded in the Bulgobac Glaciation drift indicates that it was deposited prior to 783 ka. Absolute dating of the Bulgobac drifts is not yet possible, but analysis of post‐depositional weathering criteria suggests that it was probably depos...

Late Blancan (Pliocene) microtine rodents from Jackson Hole, Wyoming: biostratigraphy and biogeography

ABSTRACT Fossil voles (Pliophenacomys primaevus, Ophiomys meadensis), a muskrat (Pliopotamys near meadensis), and a lemming (Synaptomys) are described from the Shooting Iron Formation in Jackson Hole, Teton County, Wyoming. These microtines indicate that the base of the formation is of late Blancan age (=latest Gauss or early Matuyama magnetic epochs, between 2.4 and 1.8 million years old). All four taxa show affinity with species to the east, implying faunal continuity between Jackson Hole and the Great Plains in the late Blancan. After the late Blancan, barriers to microtine dispersal developed or were accentuated near Jackson Hole, so that the modern microtine fauna of the region lacks a strong eastern affinity. Probable mechanisms that helped to develop barriers and redefine microtine distributions near Jackson Hole include: (1) climate change, which altered vegetation and glacial limits; (2) competition from immigrant taxa; and (3) tectonism, all of which took place during the Pleistocene.

Diatom stratigraphy of the late Pleistocene (Brunhes) subarctic Pacific

Three piston cores in the subarctic region of the North Pacific show consistent changes in relative abundance of diatom species throughout the Brunhes magnetic epoch. These events can be used both as stratigraphic markers to subdivide the interval and as indicators of oceanographic conditions. The stratigraphic record shows an acme for A. ochotensis from about 625-350 Kyr (Isotope Stages 15-10); a marked increase in amplitude of abundance fluctuations for R. hebetata 300-0 Kyr (Stages 8-1), the extinction of R. curvirostris at 276 Kyr (Stage 8), and abundance fluctuations of D. seminae roughly in phase with the global oxygen isotope record from 450-0 Kyr (Stages 12-1). In the subarctic Pacific, an abrupt change in species abundance is not a reliable indicator of hiatus occurrence. Although details are not clear, there appears to have been a fundamental change of the subarctic gyre during the middle Brunhes (Stages 10-8 time), with more intense glacial intervals and stronger glacial—interglacial contrast occurring after that time. Coarse clastic detritus is not restricted to glacial intervals, suggesting that ice-rafting has occurred throughout the Brunhes interval; peak levels of ice-rafting may occur during ice-growth and decay, as well as during short intervals within a stage.

Correlation of Monterey Shale to Paleo-Oceanographic and Paleoclimatic Events: ABSTRACT

Three distinct oxygen isotope events, which appear to record successive stages in the growth of the east Antarctic ice sheet, have been identified and dated paleomagnetically in middle and upper Miocene sediments from the Pacific Ocean. These stages are as follows: (1) late Magnetic Epoch 15 to early Magnetic Epoch 12 (c. 15 to 12 m.y.B.P.); (2) early Magnetic Epoch 11 (c. 11 m.y.B.P.); and (3) early Magnetic Epoch 6 (c. 6.7 m.y.B.P.). An additional slight cooling is recorded in Magnetic Epoch 10 at c. 10 m.y.B.P. These events can be tied to changes in paleo-oceanography, paleobiogeography, and opal accumulation in the Pacific Ocean, and, in turn, can be identified in the Monterey. The initiation of the east Antarctic ice sheet at c. 14 to 12 m.y.B.P., and its attendant c anges in surface circulation and paleoproductivity, is correlated to the beginning of the Monterey Shale. In addition to the beginning of massive opal accumulation along the California coast, increased silica accumulation in the equatorial Pacific is noted. This is accompanied by changes in diatom communities marking the beginning of present-day circulation patterns. The role of the Southern Ocean as an arbiter in the Miocene and Pliocene silica budget is noted and discussed. End_of_Article - Last_Page 908------------

Late Miocene—Pliocene (Magnetic Epoch 9—Gilbert Magnetic Epoch) calcium-carbonate stratigraphy of the equatorial Pacific Ocean: Summary

Calcium-carbonate accumulation patterns in the deep ocean are determined by the surface water, by dissolution in under-saturated deep water, and by postdepositional addition or removal by currents (Arrhenius, 1952; Berger, 1970; Broecker, 1971). In the eastern equatorial Pacific, downcore variations in Pleistoncene calcium-carbonate concentrations have been shown to be disolution cycles (Arrhenius, 1952, Hays and other, 1969; Saito and others, 1975) which have been correlated to the glacial-interglacial cycles of oxygen-isotope stratigraphy (Emiliani, 1955, Adelseck and Anderson, 1978). Extension of these dissolution cycles back to the Miocene-Pliocene bounday has been attemped (Kaneps, 1973b), but lack of data from Miocene samples has precluded further efforts of this type. SITE DESCRIPTIONS The CENOP project has undertaken the study of the late Miocene ocean, about 7 m.y. before the present, at a time when the Antarctic was glaciated. Based on faunal, floral, and oxygen-isotopic evidence, this was a time of relatively stable oceanographic conditions (Shackleton and Kennett, 1975; Douglas and Savin, 1973; Savin and others, 1975; Savin, 1977). The cores used in this study (Table 1) were chosen because they contain long, continuous sediment records that span this relatively stable late Miocene interval, with good preservation of calcareous and siliceous microfossils. During this interval, these core sites lay under the Pacific high-productivity belt associated with the equatorial Divergence (Arrhenius, 1952; Ewing and others, 1968), in which the carbonate compensation depth (CCD) is depressed due to the excess supply of calcium carbonate (Heath, 1969; van Andel and others, 1975). Because all three piston cores are under the same general surface and deep-water condition changes in the factors that control carbonate sedimentation and preservation should be reflected in a similar way in their sediments. However, the Deep Sea Drilling Project (DSDP) and piston cores used in this study cover a wide range of water depths, from a shallow drilled core (DSDP Site 158) to piston cores that currently lie below the CCD (RC12-66, RC12-65); thus, the carbonate preservation at different sites may vary due to differences in depth as well as corrosiveness of the bottom waters.

The Magnetic Epoch-6 carbon shift: A change in the ocean's 13C/ 12C ratio 6.2 million years ago

Tropical Indian Ocean planktonic and benthic foraminifera have 13C/ 12C ratios which change abruptly within Magnetic Epoch 6 about 6.2 million years ago. All species analyzed in the Late Miocene section of DSDP Site 238 show a shift towards lighter values of δ 13C by about 0.8‰. The oxygen isotope signal indicates that the pre-shift period is climatically quiet while the post-shift period has strong fluctuations. We suggest that the shift reflects a sudden increase in the rate of supply of organic carbon from coastal lowlands and from shelves exposed by regression, as well as a change in deep circulation patterns and ocean fertility. The event marks the transition of the ocean—atmosphere system from a quiet Early and Middle Neogene climatic regime toward a Late Neogene regime characterized by climatic amplifying mechanisms (albedo feedback, bottom water production) located around the northern North Atlantic. The beginning of this regime may have been strongly influenced by the isolation of the Mediterranean basin.

Magnetic polarity stratigraphy and vertebrate paleontology of the upper siwalik subgroup of northern Pakistan

Two hundred and fifty three sites consisting of three samples per site were taken from eight separate stratigraphic sections from sediments of the Upper Siwalik subgroup of northern Pakistan. All samples have been partially demagnetized in alternating fields of from 150 to 300 Oe. The sediments are dipping at angles of up to 75° in a variety of directions, and the locality mean directions improve significantly after correcting for bedding attitude, providing a statistically significant fold test at the 95% level. The magnetic stratigraphy was obtained based on the statistically significant well-grouped partially demagnetized data. Two prominent bentonitized tuffs occur in several sections above and below a prominent reversal of the earth's magnetic field. Radiometric dates of 2.3 ± 0.4 m.y.B.P. and 2.5 ± 0.4 m.y.B.P. have been obtained by fission track on zircons from these tuffs. This allows the magnetic stratigraphy to be correlated to the standard reversal chronology of the Pliocene and Pleistocene. The longest stratigraphic section (1.87 km in thickness) can be shown to span the time between 0.6 m.y.B.P. and 5.5 m.y.B.P. The individual stratigraphic sections can be correlated on the basis of magnetic stratigraphy. All sections contain important vertebrate fossil localities which can be placed relative to each other in the time stratigraphic framework provided by the magnetic stratigraphy. The change from the Pinjor fauna to the Tatrot fauna occurs at about 2.47 m.y.B.P. (the Gauss/Matuyama boundary) based on the simultaneous occurrence of Equus-Elephas, Bos and cervids with antlers. Hipparion, the three-toed horse, persists to the lowermost Pleistocene just after the termination of the Olduvai event. The ranges of other important vertebrate faunas are also discussed. The folding of the eastern Salt Range has been very recent, beginning within the Brunhes normal magnetic epoch, since sediments of lower Brunhes age are folded on the flanks of the Pabbi and Rohtas anticlines and on the flanks of Chambal Ridge. The rate of uplift of these structures above the present base level is estimated to have a minimum rate of four meters per thousand years for the Rohtas anticline and three meters per thousand years for the Pabbi Hills. The rates of subsidence (i.e., rates of sedimentation) of the various sectios are estimated to range from 0.35 m/10 3 yr to 0.50 m/10 3 yr. The first occurrence of conglomerate-containing clasts derived from the Himalayan uplift occurs just prior to the Olduvai event at the abse of the Pleistocene.

Paleoclimatic trends in early and middle pliocene deep-sea sediments of the antarctic

Detailed micropaleontological investigation has established the distribution of major radiolarian assemblages in the Early to Middle Pliocene deep-sea sediments of the Antarctic region, with particular emphasis on the Gilbert Reversed Magnetic Epoch ( t = 5.18 to 3.32 m.y. B.P.). Inter-core correlations, based on paleomagnetic stratigraphy and detailed radiolarian biostratigraphy, has provided a chronological framework for a detailed paleoclimatic investigation of the Gilbert Epoch. Early Gilbert sediments ( t = 5.18 to 4.6 m.y. B.P.) contain a warm-Subantarctic assemblage marked by the presence of Anthocyrtidium ehrenbergi, “Eucyrtidium” spp., Carpocanium sp., Stylatractus universus and several collosphaerid species. Sediments of middle Gilbert age ( t = 4.6 to 3.7 m.y. B.P.) contain a cool-Subantarctic assemblage marked by the presence of Stylodicta validispina, Cenosphaera cristata, Antarctissa longa, Triceraspyris pacifica and Lychnocanium grande rugosum. The late Gilbert—early Gauss sediments ( t = 3.7 to 3.0 m.y. B.P.) contain an Antarctic assemblage marked by the presence of Antarctissa strelkovi, Antarctissa denticulata, Helotholus vema, Demospyris spongiosa and Eucyrtidium calvertense. The transition from a warm to a cold assemblage suggests a climatic deterioration occurring over a period of 1.6 m.y., significantly longer than the rapid climatic oscillations of the Pleistocene. Factor curves, produced by subjecting total faunal data to multivariate statistical analysis, have been interpreted in terms of paleoclimatic controls, resulting in a relative paleotemperature curve for the entire Gilbert Epoch. The maximum abundances of the Antarctic assemblage in the Gilbert and Gauss occurs between 3.7 and 3.2 m.y. ago coinciding closely with the 3.5 m.y. age of the first reported Patagonian glaciation in southernmost South America.