Oligocene-Miocene Boundary Research Articles

Extended orbitally forced palaeoclimatic records from the equatorial Atlantic Ceara Rise

We extend existing high-resolution Oligocene–Miocene proxy records from Ocean Drilling Program (ODP) Leg 154. The extended record spans the time interval from ∼ 17.86 to 26.5 Ma. The data are age calibrated against a new astronomical solution that affords a re-evaluation of the intricate interaction between orbital (“Milankovitch”) forcing of the climate and ocean system, and the fidelity with which this forcing is recorded in oxygen and carbon stable isotope measurements from benthic foraminifera, and associated lithological proxy records of magnetic susceptibility, colour reflectance, and the measured sand fraction. Our records show a very strong continual imprint of the Earth's obliquity cycle, modulate in amplitude every ∼ 41 ka , a very strong eccentricity signal in the carbon isotope records, and a strong, but probably local, imprint of climatic precession on the coarse fraction and magnetic susceptibility records. Our data allowed us to evaluate how the interaction of long, multi-million year beats in the Earth's eccentricity and obliquity are implicated in the waxing and waning of ice-sheets, presumably on Antarctica. Our refined age model confirms the revised age of the Oligocene–Miocene boundary, previously established by analysis of the lithological data, and allows a strong correlation with the geomagnetic time scale by comparison with data from ODP Site 1090, Southern Ocean.

To the problem of the Oligocene-Miocene boundary position in the northern Caucasus (in support of the Caucasian regional stage)

Stratigraphic position of the Oligocene-Miocene (Paleogene-Neogene) boundary in the northern Caucasus and, in this connection, of the Alkun Formation and the Caucasian regional stage in geological sections of Maikop deposits, primarily in those of the Belaya and Kuban rivers, is considered. Arguments in support of the Caucasian regional stage are presented and necessity of distinguishing the Karadzhalganian regional stage is called in question. Stratigraphic implications of the Alkun Formation, the regional reference horizon traceable in many sections of the northern Caucasus, are discussed. It is substantiated based on nannoplankton, dinocysts, foraminifers, and distinctive lithological features of deposits that the Oligocene-Miocene boundary is confined to the Alkun Formation at the Belaya River.

Lower miocene sediments of the Maikop group in the Central Eastern Paratethys

The composite section of upper Maikop sediments compiled for the central part of the Eastern Paratethys is presented. The section (more than 1000 m) comprises the Karadzhalgan, Sakaraulian, and Kotsakhurian regional stages. The lower boundary of the Miocene drawn at the base of the Karadzhalgan regional stage is unambiguous only in the southern part of the central Ciscaucasia. In most areas of the Ciscaucasia, this boundary is drawn arbitrarily because of uniform lithology in the Oligocene-Miocene boundary interval and poor paleontological substantiation. Generally, the Maikop sequence is insufficiently studied and incomplete in many areas because of a discordant upper boundary of the Maikop Group. Nevertheless, materials presented in the paper characterize for the first time the composition and structure of the Lower Miocene sequence over a vast area of the Eastern Paratethys. The horizonwise reconstruction of Early Miocene basins has made it possible to reveal the major features of final stages in the formation of the Maikop clayey sequence.

Foraminiferal biostratigraphy and palaeoecology in Upper Oligocene–Lower Miocene glacial marine sequences 9, 10, and 11, CRP-2/2A drill hole, Victoria Land Basin, Antarctica

Three Cape Roberts Project drillholes (CRP-1, -2/2A and -3) penetrated, in aggregate, a 1500 m succession of glacial marine Oligocene–Lower Miocene strata on the margins of Victoria Land Basin, the upper 1000+ m of which consists of a stack of more than 50 unconformity-bounded stratigraphic sequences. The benthic foraminiferal biostratigraphy, ecology, and lithofacies relationships of three contiguous sequences (Sequences 9, 10, and 11; 130.27–306.65 mbsf) from CRP-2/2A, were first determined and then tested against Motif A, a sedimentation model developed for the drillhole succession. These sequences apparently represent relatively complete glacial–deglacial cycles which span an interval of ∼450 kyr straddling the Oligocene–Miocene boundary (∼24 Ma). Sedimentological and paleontological data suggest shoreface–inner shelf to outer shelf basin margin settings with deposition under the influence of near-shore traction currents, subglacial ice contact, ice-rafting, meltwater, and turbidity associated with ice margin advance and retreat, along with sea level variations in the order of 50 m. Stratigraphic distribution data for benthic foraminifera are at an average sampling interval of ∼1 m (= ∼1000 to 3000 years), which offers stratigraphic resolution at less than orbital cycle frequencies in all major lithofacies within the three sequences. Taxa recovered represent a restricted immigrant group that repeatedly penetrated the shallow coastal waters during deglacial phases of each cycle and originated in the much more diverse assemblages of West Antarctic Rift System basins. Both eurytopic and stenotopic taxa are recognized. The former occur in all lithofacies and appear tolerant to a wide range of environmental parameters at and near the seafloor. Representative genera include, Trochammina, Cassidulinoides, Fursenkoina, Eponides, Epistominella, Cibicides, Nonionella, Melonis, Pullenia, Elphidium, Ammoelphidiella, and Cribroelphidium. Stenotopic taxa are less common, occur mainly in fine-grained lithofacies, represent maximum oceanicity, and coincide with maximum retreat of coastal glaciers and sea level highstands. Representative genera include, Quinqueloculina, Pyrgo, Triloculina, Lenticulina, Marginulina, Vaginulinopsis, Nodosaria, Pseudonodosaria, Pyrulinoides, Oolina, Fissurina, Parafissurina, Rosalina, Anomalinoides, and Hanzawaia. Planktic foraminifera are absent. Our foraminiferal data provide strong support for the Motif A model, and allow interpretation of intra-model facies and events. Motif A is an idealized glacial stratigraphic sequence construct derived from analysis of the entire Upper Oligocene–Lower Miocene sequence stack. In this model, Lowstand System Tract (LST) diamictites form the base of the sequence. Diamictites with microfaunas are considered glaciomarine, those without are more likely ice-proximal or subglacial. Fine-grained sandstone, muddy sandstone, and mudstone of mid-sequence aggradational Transgressive System Tract (TST) and Highstand System Tract (HST) strata reflect high stands of sea level, inshore penetration of open marine waters, and peak recession of ice margins, and normally contain the most diverse and abundant microfaunas (up to 17 spp/sample). Sediments of the overlying Regressive System Tract (RST) display low and upwardly-declining foraminiferal diversity in response to glacier ice advance, sediment progradation across the basin margin, and falling sea level. The time-variable Glacial Surface of Erosion (GSE) or sequence boundary terminates the Regressive System Tract (RST) and demarcates the base of the succeeding stratigraphic sequence.

Phylogeny and biogeography of Chinese sisorid catfishes re-examined using mitochondrial cytochrome b and 16S rRNA gene sequences

The family Sisoridae is one of the largest and most diverse Asiatic catfish families, most species occurring in the water systems of the Qinhai-Tibetan Plateau and East Himalayas. To date published morphological and molecular phylogenetics hypotheses of sisorid catfishes are part congruent, and there are some areas of significant disagreement with respect to intergeneric relationships. We used mitochondrial cytochrome b and 16S rRNA gene sequences to clarify existing gaps in phylogenetics and to test conflicting vicariant and dispersal biogeographical hypotheses of Chinese sisorids using dispersal–vicariance analysis and weighted ancestral area analysis in combination with palaeogeographical data as well as molecular clock calibration. Our results suggest that: (1) Chinese sisorid catfishes form a monophyletic group with two distinct clades, one represented by ( Gagata ( Bagarius, Glyptothorax)) and the other by (glyptosternoids, Pseudecheneis); (2) the glyptosternoid is a monophyletic group and Glyptosternum, Glaridoglanis, and Exostoma are three basal species having a primitive position among it; (3) a hypothesis referring to Pseudecheneis as the sister group of the glyptosternoids, based on morphological evidence, is supported; (4) the genus Pareuchiloglanis, as presently defined, is not monophyletic; (5) congruent with previous hypotheses, the uplift of Qinghai-Tibetan Plateau played a primary role in the speciation and radiation of the Chinese sisorids; and (6) an evolutionary scenario combining aspects of both vicariance and dispersal theory is necessary to explain the distribution pattern of the glyptosternoids. In addition, using a cytochrome b substitution rate of 0.91% per million years and 0.23% for 16S rRNA, we tentatively date that the glyptosternoids most possibly originated in Oligocene–Miocene boundary (19–24Myr), and radiated from Miocene to Pleistocene, along with a center of origin in the Irrawaddy–Tsangpo drainages and several rapid speciation in a relatively short time.

Magnetostratigraphic chronology of a late Eocene to early Miocene glacimarine succession from the Victoria Land Basin, Ross Sea, Antarctica

Drilling offshore from Cape Roberts, Antarctica, has enabled recovery of a 1472-m cumulative record of late Eocene–early Miocene history of sedimentary basin development and climate change in the Western Ross Sea. In this paper, we synthesize the results of palaeomagnetic analyses carried out on the CRP-1, CRP-2 and CRP-3 sediment cores, and present a chronology for the recovered Eocene–Miocene succession. Stepwise demagnetization data demonstrate that secondary overprints have been successfully removed and that characteristic remanent magnetizations (ChRMs) have been clearly identified in most of the samples. A close sampling interval has allowed a detailed magnetic polarity stratigraphy to be established for the composite succession. Correlation with the geomagnetic polarity time scale (GPTS) has been constrained by a number of 40Ar/ 39Ar and 87Sr/ 86Sr ages, as well as by a recently developed Antarctic siliceous microfossil zonation, and by calcareous nannoplankton biostratigraphy. The basal sediments of the Eocene–Miocene succession rest unconformably on Devonian sandstones of the Beacon Supergroup. A basal sandstone breccia, which probably represents the onset of rifting in the Victoria Land Basin (VLB), is overlain by a succession of sandstones that are interbedded with thin conglomerate beds. These sediments give way to more clearly glacially influenced mudstones and diamictite facies in the mid Oligocene, and, by the Oligocene–Miocene boundary, coincident with the Mi-1 glaciation, a permanent glacial dominance was imprinted on the sedimentary record. Average sediment accumulation rates were initially rapid in the late Eocene–early Oligocene (up to 60 cm/k.y.), but reduced to only a few cm/k.y. in the early Miocene as basin subsidence slowed.

Late Oligocene rapid transformations in the South China Sea

Lithobiostratigraphic data indicate that the double reflectors on the seismic profile through Ocean Drilling Program (ODP) Site 1148 represent two unconformities that coincide, respectively, with the lower/upper Oligocene boundary at ∼488 mcd, and Oligocene–Miocene boundary at 460 mcd. Two other unconformities, at ∼478 and 472 mcd, respectively, were also identified within the upper Oligocene section. Together they erased a sediment record of about 3 Ma from this locality in a period of very active seafloor spreading. The existence of 32.8 Ma marine sediment at the terminated depth (850 mcd) indicates that the initial breakup of the South China Sea (SCS) was probably during 34–33 Ma, close to the Eocene–Oligocene boundary. High sedimentation rates of 60–115 m/my from the much expanded, >350 m lower Oligocene section resulted from rifting and rapid subsidence between 33 and 29 Ma. The mid-Oligocene unconformity at ∼28.5 Ma, which also occurred in many parts of the Indo-West Pacific region, was probably related to a significant uplift of the Himalayan-Tibetan Plateau to the west and the initial collision between Indonesia and Australia in the south. A narrowed Indonesian seaway may have accounted for the late Oligocene warming and chalk deposition in the northern South China Sea including the Site 1148 locality. The unconformities and slumps near the Oligocene–Miocene boundary indicate a very unstable tectonic regime, probably corresponding to changes in the rotation of different land blocks and the seafloor spreading ridge from nearly E–W to NE–SW, as recognized earlier at magnetic Anomaly 7. This 25 Ma event also saw the first New Guinea terrane docking at the northern Australian craton. The low sedimentation rate of ∼15 m/my in the early to middle Miocene may correspond to another period of rapid seafloor spreading and rapid widespread subsidence that effectively caused sediment source areas to retreat with a rapidly rising sea level. The isostatic nature of these late Oligocene unconformities and slumps with several major collision-uplift events indicate that the rapid changes in the early evolutionary history of the South China Sea were mainly responding to regional tectonic reconfiguration including the uplift-driven southeast extrusion of the Indochina subcontinent.

Bioestratigrafia (Foraminiferida) da sondagem 1-SCS-3B, plataforma de Florianópolis, bacia de Pelotas, Brasil

BIOSTRATIGRAPHY (FORAMINIFERIDA) OF THE 1-SCS-3B DRILL-HOLE, PLATAFORMA DE FLORIANOPOLIS, PELOTAS BASIN, BRAZIL. Planktonic foraminifers recovered from 1-SCS-3B drill hole, Florianopolis Platform, State of Santa Catarina, Brazil, represent 60 species and subspecies belonging to 13 genera, whose stratigraphic range indicates a Neogene age for the sediments herein studied. The analysis of this assemblage allowed the recognition of eight zones, in stratigraphic order: Catapsydrax dissimilis, C. stainforthi, Globorotalia fohsi fohsi, Globigerinoides ruber, Globorotalia mayeri, G. acostaensis/ G. menardii, G. margaritae evoluta and Globigerinoides trilobus fistulosus. Four important hiatuses were observed in the studied sedimentary sequence, whose are situated at Oligocene-Miocene boundary, Early Miocene, Early Miocene-Middle Miocene and Miocene-Pliocene boundary. Correlations with previous zonations for the area are discussed.

Evolutionary and biogeographic patterns of the Badidae (Teleostei: Perciformes) inferred from mitochondrial and nuclear DNA sequence data

We reconstructed phylogenetic relationships of the family Badidae using both mitochondrial and nuclear nucleotide sequence data to address badid systematics and to evaluate the role of vicariant speciation on their evolution and current distribution. Phylogenetic hypotheses were derived from complete cytochrome b (1140 base pairs) sequences of 33 individuals representing 13 badid species, and using three species of Nandidae as outgroups. Additionally, we sequenced the nuclear RAG1 (1473 base pairs) and Tmo-4C4 (511 base pairs) genes from each of the badid species and one representative of the outgroup. Our molecular data provide the first phylogenetic hypothesis of badid intrarelationships. Analysis of the mitochondrial and nuclear nucleotide sequence data sets resulted in well-supported trees, indicating a basal split between the genera Dario and Badis, and further supporting the division of the genus Badis into five species groups as suggested by a previous taxonomic revision of the Badidae. Within the genus Badis, mitochondrial and nuclear phylogenies differed in the relative position of B. kyar. We also used our molecular phylogeny to test a vicariant speciation hypothesis derived from geological evidence of large-scale changes in drainage patterns in the Miocene affecting the Irrawaddy– and Tsangpo–Brahmaputra drainages, in the southeastern Himalaya. Within both genera, Badis and Dario, we observed a divergence into Irrawaddy– and Tsangpo–Brahmaputra clades. Using a cytb substitution rate of 8.2×10−9 (substitutions×base pair−1×year−1), we tentatively date this vicariant event at the Oligocene–Miocene boundary (19–24Myr). It is concordant with a hypothesized paleo connection of the Tsangpo river with the Irrawaddy drainage that was most likely interrupted during Miocene orogenic events through tectonic uplifts in eastern Tibet. Our data, therefore, indicate a substantial role of vicariant-based speciation shaping the current distribution patterns of badids.

Evolution and biogeography of marine angelfishes (Pisces: Pomacanthidae)

Phylogenetic relationships among angelfishes (Pomacanthidae) and their putative sister taxon, the butterflyfishes (Chaetodontidae), were examined using 12S and 16S mitochondrial DNA sequences. ML and MP trees were highly congruent with good basal resolution. Monophyly of the two families was supported, although a clade comprising the Chaetodontidae and one of the outgroups, the Scatophagidae, formed the sister clade to the Pomacanthidae. All genera and subgenera within the Pomacanthidae were examined. The relationships among the 24 representative species were consistent with traditional generic boundaries, with the exception of the genus Centropyge, but differed from previous phylogenies. Estimated ages of divergence based on trans-isthmian pairs were compared with independent fossil evidence. Trans-isthmian estimates were highly conservative, while fossil-calibrated estimates were most consistent with available evidence. Fossil calibrated estimates suggest that the family has been impacted by both the Terminal Tethyan Event and the closure of the Isthmus of Panama. Within the family, ecological diversity and species-level diversification are restricted primarily to a single pygmy angelfish clade with an origin near the Oligocene–Miocene boundary.

Polynostratigraphy and palaeoenvironments across the Oligocene-Miocene boundary within the Centinela Formation, southwestern Argentina

Abstract Palynological analysis of the Centinela Formation, exposed in the foothills of the Patagonian Andes, has revealed the presence of pollen, dinoflagellate cysts, and chlorococcalean and prasinophycean algae. These groups are here reported from the Centinela Formation for the first time. Sporomorph and dinoflagellate cyst assemblages suggest a Late Oligocene and Early Miocene age. These results coincide with a 87 Sr/ 86 Sr age close to the age of the Oligocene-Miocene boundary obtained from the lower part of the section. Palynological information from the Centinela Formation permits correlation with Upper Oligocene and Lower Miocene units cropping out along the Atlantic Patagonian coast. Assemblages from the lower part of the section suggest that the beds were deposited under marine, near-shore palaeoenvironmental conditions with a strong continental influence. In the middle part of the section, high dinoflagellate cyst ratios coincide with a maximum flooding surface recorded in the Centinela Formation. Towards the top of the Centinela Formation, the sporomorph assemblages reflect the development of vegetation adapted to coastal environments, which agrees with the sparse occurrence of marine palynomorphs. A new dinoflagellate species, Hystrichostrogylon sulcatum , is proposed. This species appears to range across the Oligocene-Miocene boundary and is particularly abundant in the lowest Miocene.

Long-term subduction-erosion along the Guatemalan margin of the Middle America Trench

A new analysis of Deep Sea Drilling Project (DSDP) Leg 84 data demonstrates that the dominant process controlling the Guatemala margin tectonic evolution since ca. 25 Ma is subduction-erosion. Data from benthic foraminifera, assemblages from upper-slope DSDP Sites 568, 569, and 570 indicate long-term, progressive subsidence from upper to middle bathyal depths (600–1000 m) ca. 19 Ma to modern abyssal depths (>2000 m). Rapid subsidence migrated landward starting at the Oligocene-Miocene boundary time under the current middle slope, where it increased sharply ca. 19 Ma, reached the current upper slope by ca. 15 Ma, and arrived at the uppermost slope ca. 2 Ma. Subsidence indicates crustal thinning by basal tectonic erosion of mass from the underside of the upper plate. Under the assumption that, in the Miocene, the morphology of the forearc was similar to that of today, landward migration of the trench was at a rate of 0.8–0.9 km/m.y. This linear rate corresponds to a tectonic erosion rate of the submerged forearc of 11.3–13.1 km3·m.y.−1·km−1. The evolution of arc magmatism and superfast spreading at the East Pacific Rise since early Miocene time may have caused slab shallowing and tectonic erosion that readjusted the forearc geometry.

Glaciation across the Oligocene–Miocene boundary in southern McMurdo Sound, Antarctica: new chronology from the CIROS-1 drill hole

Few Palaeogene and Neogene sediment cores from the Antarctic continental margin have been dated with sufficient precision to enable establishment of direct linkages between glacial events on the Antarctic continent and marked events in deep-sea δ 18O records. As a result, much of our knowledge of the gradual, but stepwise, shift from ‘greenhouse’ climates of the Cretaceous to the ‘ice-house’ climates of the Quaternary is inferred from well-dated and more continuously deposited deep-sea sediments. In this study, we present new magnetostratigraphic results from the CIROS-1 drill core from McMurdo Sound, Antarctica, along with a reinterpretation of a published diatom biostratigraphic zonation that is constrained by correlation to a high-precision age model from the nearby CRP-2/2A drill core. Our results suggest that most of the upper 350 m of the CIROS-1 drill core represents rapid sediment accumulation during a short time interval spanning the Oligocene–Miocene boundary. Chronostratigraphic control is precise enough to enable correlation of this interval of glacimarine sedimentation with the Mi-1 deep-sea δ 18O event, which confirms that the Mi-1 event was related to a major expansion of Antarctic ice. A major unconformity at 366 m in the CIROS-1 drill core, which is widely observed in regional seismic reflection studies, represents 9 Myr of missing time. This unconformity can be traced offshore into the Ross Sea using seismic stratigraphy and is interpreted to indicate significant East Antarctic ice sheet development during the Mi-1 glaciation. The stratigraphic expression of this ∼400-kyr glacial event is evidently multiphase and complex in the Victoria Land Basin, probably because it was punctuated by higher-frequency orbitally induced glacial oscillations. The presence of Nothofagidites pollen throughout the CIROS-1 drill core and the presence of a Nothofagus (Southern beech) leaf within the early Miocene portion of the core indicate that Antarctic mean summer temperatures did not decrease below 5°C throughout the Mi-1 glaciation. These temperatures are significantly warmer than present-day mean summer temperatures at sea level in McMurdo Sound. The persistence of a Nothofagus forest in coastal southern Victoria Land throughout this time interval suggests that the present state of deep refrigeration was not reached until some time after the Mi-1 glaciation.

Quantitative Constraints on the Origin of Stratigraphic Architecture at Passive Continental Margins: Oligocene Sedimentation in New Jersey, U.S.A.

ABSTRACT The Oligocene of the New Jersey continental margin is divisible into as many as eight sequences and 23 lithofacies associations, documented in a series of seven boreholes across the modern coastal plain. This paper summarizes the sequence architecture of these deposits, interpreted from high-resolution biostratigraphy and Sr-isotope chemostratigraphy, and evaluates the factors that governed patterns of sedimentation, making use of previously published quantitative estimates of water-depth changes and eustasy from 2-D foraminiferal paleoslope modeling and flexural backstripping. Each sequence is markedly wedge-shaped, thinning both landward of the rollover in the underlying sequence boundary (the point at which the surface steepens into a clinoform), and seaward of the rollover in the overlying boundary. Each bounding surface is associated with evidence for offlap-onlap geometry and at least locally with benthic foraminiferal evidence for abrupt upward shoaling. Most unconformities merge up dip into a single surface marking the Oligocene-Miocene boundary. Earliest Oligocene unconformities (33.5-31.6 Ma) merge downdip as a result of sediment starvation on the deep shelf. Conventional lithostratigraphic units within the New Jersey Oligocene are highly diachronous. For example, the base of Atlantic City Formation at Cape May (a downdip borehole) is at least 6.6 Myr younger than the top of the same formation at ACGS#4 (an updip borehole). Factors controlling patterns of sedimentation include: (1) a terraced physiography, with gradients ranging from 1:1,000 (0.06°) on the coastal plain and shallow shelf and 1:500 (0.11°) on the deep shelf to < 1:100 (1.0°) on an intermediate slope; (2) generally low siliciclastic sediment flux, with in situ production of authigenic glauconite, especially during times of transgression; (3) a location landward of the hinge zone of the passive margin, with slow tectonic subsidence augmented by compaction and sediment loading; (4) low to moderate amplitudes and rates of eustatic change (10-50 m over spans of 1-2 Myr); and (5) an active wave climate that permitted efficient lateral transport and complete bypass of sediment at paleodepths of at least 20 ± 10 m. Sequence architecture in the New Jersey Oligocene differs from that of the standard Exxon model. Sequences are highstand-dominated, in spite of deposition and preservation largely seaward of the rollover in each underlying sequence boundary. Transgressive systems tracts are thin. Recognizable lowstand units did not form because efficient transfer of sediment across the shallow shelf, combined with the absence of major river systems in the area of study, prevented the reorganization of sedimentation patterns commonly associated with point-source development, in spite of rates of eustatic fall considerably greater than the local rate of tectonic subsidence. Repeated eustatic rises and falls are expressed primarily by variations in paleo-water depth. Although 65-80% of the shallow shelf that had been flooded during each rise became subaerially exposed during the subsequent fall, well developed offlap at each sequence boundary is due primarily to marine bypassing and degradation rather than to forced regression. Sequence boundaries correspond in time at their correlative conformities not with the onset of falling relative sea level, but with the start of eustatic rise.

Integrated chronostratigraphic calibration of the Oligocene-Miocene boundary at 24.0 ± 0.1 Ma from the CRP-2A drill core, Ross Sea, Antarctica

[Wilson et al. (2002)][1] use 40Ar/39Ar data from the Cape Roberts Project CRP-2A core to assign absolute ages to polarity chrons in the vicinity of the Oligocene-Miocene (O-M) boundary, and estimate an age of 24.0 Ma for the O-M boundary. The identification of polarity chron 6Cn.2n, marking the O-M

Integrated chronostratigraphic calibration of the Oligocene-Miocene boundary at 24.0 ± 0.1 Ma from the CRP-2A drill core, Ross Sea, Antarctica

Integrated chronostratigraphic calibration of the Oligocene-Miocene boundary at 24.0 ± 0.1 Ma from the CRP-2A drill core, Ross Sea, Antarctica

Integrated chronostratigraphic calibration of the Oligocene-Miocene boundary at 24.0 ± 0.1 Ma from the CRP-2A drill core, Ross Sea, Antarctica

An expanded Oligocene-Miocene boundary interval recovered in the Cape Roberts Project CRP-2A core from beneath the Ross Sea, Antarctica, has yielded a high-resolution integrated chrono stratigraphy that has, in turn, enabled a new, more direct, calibra tion of magnetic polarity and biostratigraphic events. The Oligocene-Miocene boundary interval in the CRP-2A core comprises three ∼60-m-thick, rapidly deposited (>0.5 m/k.y.) sedimentary sequences (sequences 9, 10, and 11). In sequences 10 and 11, single-crystal, laser-fusion 40Ar/39Ar analyses of anorthoclase phenocrysts from two tephra horizons independently calibrate the CRP-2A magnetic-polarity stratigraphy and age model. Sequences 10 and 11 encompass subchron C6Cn.3n, which is dated as 24.3 ± 0.1 to 24.16 ± 0.1 Ma. Sequence 9 is interpreted to encompass subchron C6Cn.2n and the Oligocene-Miocene boundary, which is dated as 24.0 ± 0.1 Ma. These ages are ∼0.2 m.y. older than those of the geomagnetic polarity time scale calibrated from seafloor-spreading ridges and ∼0.9–1.3 m.y. older than the newly proposed astronomically calibrated ages. We contend that the discrepancy with the astronomically calibrated ages arises from a mismatch of three 406 k.y. eccentricity cycles or a 1.2 m.y. modulation of obliquity amplitude in the astronomical calibration of the Oligocene–Miocene time scale.

Human and ape molecular clocks and constraints on paleontological hypotheses.

Although the relationships of the living hominoid primates (humans and apes) are well known, the relationships of the fossil species, times of divergence of both living and fossil species, and the biogeographic history of hominoids are not well established. Divergence times of living species, estimated from molecular clocks, have the potential to constrain hypotheses of the relationships of fossil species. In this study, new DNA sequences from nine protein-coding nuclear genes in great apes are added to existing datasets to increase the precision of molecular time estimates bearing on the evolutionary history of apes and humans. The divergence of Old World monkeys and hominoids at the Oligocene-Miocene boundary (approximately 23 million years ago) provides the best primate calibration point and yields a time and 95% confidence interval of 5.4 +/- 1.1 million years ago (36 nuclear genes) for the human-chimpanzee divergence. Older splitting events are estimated as 6.4 +/- 1.5 million years ago (gorilla, 31 genes), 11.3 +/- 1.3 million years ago (orangutan, 33 genes), and 14.9 +/- 2.0 million years ago (gibbon, 27 genes). Based on these molecular constraints, we find that several proposed phylogenies of fossil hominoid taxa are unlikely to be correct.

Climate response to orbital forcing across the Oligocene-Miocene boundary.

Spectral analyses of an uninterrupted 5.5-million-year (My)-long chronology of late Oligocene-early Miocene climate and ocean carbon chemistry from two deep-sea cores recovered in the western equatorial Atlantic reveal variance concentrated at all Milankovitch frequencies. Exceptional spectral power in climate is recorded at the 406-thousand-year (ky) period eccentricity band over a 3.4-million-year period [20 to 23.4 My ago (Ma)] as well as in the 125- and 95-ky bands over a 1.3-million-year period (21.7 to 23.0 Ma) of suspected low greenhouse gas levels. Moreover, a major transient glaciation at the epoch boundary ( approximately 23 Ma), Mi-1, corresponds with a rare orbital congruence involving obliquity and eccentricity. The anomaly, which consists of low-amplitude variance in obliquity (a node) and a minimum in eccentricity, results in an extended period ( approximately 200 ky) of low seasonality orbits favorable to ice-sheet expansion on Antarctica.

Chronology of Exotic Mineralization at El Salvador, Chile, by 40Ar/39Ar Dating of Copper Wad and Supergene Alunite

Exotic mineralization is a complex hydrochemical process linking supergene enrichment, lateral transport, and precipitation of oxide minerals in the drainage network of a porphyry deposit. At the El Salvador porphyry deposit in northern Chile the majority of the exotic ore comprised a mixture of copper-bearing manganese oxyhydrates termed copper wad. X-ray diffraction, scanning electron microscopy, and electron probe microanalysis show that the wad is composed of copper-bearing cryptomelane [K1-2(Mn3+Mn4+)8O16 • x H2O] and birnessite [K0.33\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(Mn_{7}^{3.9+}\) \end{document}O14 • 7H2O] structures. These natural occurrences within the exotic ore provide the opportunity to directly date the formation of these deposits using recent advances in 40Ar/39Ar geochronology of supergene K-Mn oxides formed by weathering. A suite of copper-bearing cryptomelane and birnessite samples from exotic deposits within the El Salvador district were characterized and dated by 40Ar/39Ar laser step heating. Supergene alunite [KAl3(SO4)2(OH)6] found in paleospring feeder systems leading from the source zones of outward to the exotic mineralization was dated to independently constrain the age of exotic ore formation. Although the Ar retentivity of the layered birnessite structure has been questioned by others, the Ar retentivity of these samples is thought to be a function of their natural preservation and limited postcrystallization ground-water interaction in the hyperarid Atacama desert. The 40Ar/39Ar analytical results show that Ar and/or K losses after crystallization, excess 40Ar, and 39Ar recoil do not pose significant problems; therefore, in the context of exotic deposits within hyperarid environments this dating method is applicable to both cryptomelane and birnessite within wad. The 40Ar/39Ar dating of exotic mineralization at El Salvador indicates that supergene and exotic mineralization processes were active at ~35 Ma, about 5 m.y. after the emplacement of hydrothermal mineralization, and continued until the middle Miocene. The majority of exotic mineralization extends from the Oligocene-Miocene (24 Ma) boundary through the middle Miocene (11 Ma) and relates to supergene fluid emanating in multiple directions from the source of in Indio Muerto, which represents a topographic high above the exotic deposits. One reconnaissance date of exotic mineralization at the Chuquicamata porphyry deposit yielded an age of 17.03 ± 0.03 Ma coincident with the known supergene alunite dates for this deposit. Reconnaissance dating at Exotica-Mina Sur and El Abra proved problematic due to the presence of contaminant silicate minerals within the wad. Ages derived here for exotic mineralization are similar to the known supergene ages throughout northern Chile. The dates indicate that supergene exotic mineralization began at the Eocene-Oligocene boundary and continued through the Oligocene, eventually ceasing in the middle Miocene due to desiccation of the Atacama desert. A series of 10-cm-thick supergene alunite veins were microsampled across their widths and dated to address the kinetics of alunite vein growth. Apparent growth rates of the veins vary from 71 to 100 mm/m.y. in the horizontal direction to 24 mm/m.y. in the vertical, presumably caused by differences in pressure regimes due to their orientation.