Dennis V Kent

ABSTRACT Two of the three largest impact craters found on Earth since 200 Ma (Popigai and the Chesapeake Bay Impact Structure or CBIS) are late Eocene (~35.4-35.5 Ma based on radiometric ages) and are well preserved, yet the environmental response to these near synchronous and large impacts is poorly understood. No extinction events are recorded in coccolithophorids, planktonic or benthic foraminifera at this time, and terrestrial biota appear unaffected. The late Eocene global temperature history and carbon budget are poorly constrained because of sparsely sampled delta18O and delta13C records. We present new microfauna and nannoplankton evidence and stable isotopic data that show: 1) minimal biotic and temperature response associated with the impacts; and 2) a large and transient carbon isotope excursion associated with the impacts that reflect a major change in the global carbon budget. Southern Ocean ODP Site 1090 provides an exceptional record to test if a carbon isotopic anomaly is associated with the late Eocene impacts because benthic foraminifera are well preserved, the identified ejecta horizon is marked by an Ir anomaly (~950pg/g; Kyte and Liu, 2002), and the magnetostratigraphic age control is excellent (Channell et al., 2003). A first-order correlation to the geomagnetic polarity time scale at Site 1090 places the impact ejecta horizon in Chron C16n.1n (279.01 mbsf) with a corresponding age of ~35.4 Ma, consistent with published radiometric ages. We generated a high-resolution carbon and oxygen stable isotope record of benthic foraminifera across the impact ejecta layer from 34.6-35.8 Ma (8 kyr sampling) and 33.7-36 Ma (16 kyr sampling). Our results show that a transient carbon isotope decrease (277-278 mbsf) of 0.4-0.5% is associated with the impact horizon. The delta13C anomaly persists for ~250 kyr; then the signal returns to `pre-impact' values. Following recovery from the transient excursion there were no long-term changes in global carbon isotopic values associated with the impacts. Our results show no significant changes in benthic foraminifera oxygen isotope values across the impact ejecta layers, implying that no major changes in deep-water temperatures occurred. Coeval benthic foraminiferal records also show the carbon isotopic excursion: 1) new benthic data from New Jersey slope Site 612 show a 0.5% change, though this record is partially truncated due to a hiatus; 2) Southern Ocean Site 689 shows a larger excursion (1.0%) (Vonhof et al., 2000); and 3) Pacific Ocean Site 1218 shows a smaller anomaly (0.4%) (Lear et al., 2004). We suggest this delta13C perturbation was global and directly related to the late Eocene impactor(s).

New magnetostratigraphic results from DSDP Site 608 in the North Atlantic reveal the presence of nine short-term polarity fluctuations that do not correspond to subchrons in the most recent GPTS. Comparison with published results from ODP Sites 845 and 1092, and ...

We present a summary of previously published Olenekian–Anisian boundary magnetostratigraphic and biostratigraphic results from the Kçira area of northern Albania. We focus on the stratigraphically complete Kçira-A section that represents a potential candidate Global Boundary Stratotype Section and Point (GSSP) for the base of the Anisian Stage of the Triassic System. The previously published conodont biostratigraphy from Kçira-A and ancillary sections located nearby has been updated using modern taxonomic criteria and correlated to the available ammonoid and benthic foraminifera biostratigraphy. Previously published magnetobiostratigraphic data reveal the occurrence at Kçira-A, and ancillary sections, of a well-defined magnetic polarity reversal pattern of primary origin that allows global correlations ensuring the exportability of biostratigraphic datums (e.g., the first occurrence of conodont Chiosella timorensis) falling close to the Kclr/Kc2n polarity transition. A suite of pilo...

Jurassic paleomagnetic data from North America have long been contentious, generating ambiguities in the shape of the global‐composite apparent polar wander path. Here we show from a restudy of two subdivisions of the Late Jurassic Morrison Formation at the classic locality at Norwood on the Colorado Plateau that the derived paleopoles reflect variable overprinting probably in the Cretaceous and are of limited value for apparent polar wander determination. We instead assembled an updated set of Jurassic paleopoles from parauthocthonous Adria, the African promontory, using primary paleomagnetic component directions derived from stratigraphically superposed intervals and corrected for sedimentary inclination error. These paleopoles are found to be in superb agreement with independent igneous paleopoles from the literature across the so‐called Jurassic monster polar shift, which in North American coordinates is a jump of ~30° arc distance from the 190‐ to 160‐Ma stillstand pole at 79.5...

... Authors: Aubry, Marie-Pierre Ouda, K. Van Couvering, JA Ali, JR Berggren, WA Brinkhuis, Henk Heilmann-Clausen, Claus Hooker, Jerry Kent, Dennis Knox, RWO Laga, Piet Molina, E. Schmitz, Birger Steurbaut, Etienne Ward, DJ. Issue Date: 2002. ...

Abstract During the Late Carboniferous, large granitic bodies were emplaced on Sardinia related to a change of the tectonic setting from compressional to extensional. Simultaneously the newly formed fault-bounded basins were filled with continental clastic sediments, paleontologoically and radiometrically dated to be Late Carboniferous to Early Permian in age. These basins provide an ideal testing ground for tectonic models such as the controversial Pangea B to A transition, which should have occurred prior to the ...

Abstract One of the key elements of late Paleozoic tectonics in Europe is the continental dextral mega shear system postulated as early as 1977 by Arthaud and Matte, which is thought to control the structural evolution along the southern and eastern margins of Europe. It is rather obvious that this shear zone, which in the western Mediterranean realm is bordered by the Biscaye-North-Pyrenean Fault to the North and the Anti-Atlas-Fault to the South, is one of the most likely candidates along which the controversial Pangea B to ...

Abundant lake ice-rafted debris in Late Triassic and earliest Jurassic strata of the Junggar Basin of northwestern China (paleolatitude ~71°N) indicates that freezing winter temperatures typified the forested Arctic, despite a persistence of extremely high levels of atmospheric P co 2 (partial pressure of CO 2 ). Phylogenetic bracket analysis shows that non-avian dinosaurs were primitively insulated, enabling them to access rich deciduous and evergreen Arctic vegetation, even under freezing winter conditions. Transient but intense volcanic winters associated with massive eruptions and lowered light levels led to the end-Triassic mass extinction (201.6 Ma) on land, decimating all medium- to large-sized nondinosaurian, noninsulated continental reptiles. In contrast, insulated dinosaurs were already well adapted to cold temperatures, and not only survived but also underwent a rapid adaptive radiation and ecological expansion in the Jurassic, taking over regions formerly dominated by large noninsulated reptiles.

<p>The high frequency oscillations between wet and dry conditions plus the warmer temperatures when the Earth comes closest to the sun, might suggest weathering and hence accumulation rates should be highest during times of maximum eccentricity and maximum precessional variability in the tropics. But time series analysis of 20 Myr of continuous cores of tropical, lacustrine Late Triassic-age strata of the Newark Rift Basin (202–222 Ma) surprisingly show that that is not the case because accumulation rates are highest during the times of lowest precessional variance at the modes of the Mars–Earth (g<sub>4</sub>-g<sub>3</sub>) orbital cycle, when eccentricity is at a minimum.</p><p>            Three different methods of analysis reveal an accumulation pattern at variance with this intuitive model. 1) Tuning the depth-domain depth rank, color, and natural gamma data series to the 405 kyr, Venus–Jupiter (g<sub>2</sub>-g<sub>5</sub>) eccentricity metronome reveals oscillations in accumulation rates of ~20m to ~100m/Myr/cycle (within a total range of 70m – 250m/Myr). Spectral analysis reveals these oscillations occur with the same period (~1.8Myr) as the Mars–Earth modulation of precession for that time, with highs in accumulation rate occurring during lows in eccentricity. A weaker signal of the Mars–Earth (s<sub>4</sub>-s<sub>3</sub>) inclination cycle is also present at about 1/2 the period of the eccentricity cycle. 2) Application of the eTimeOpt method of sedimentation rate analysis reveals the same pattern and magnitudes of sedimentation rate variations in depth rank and color. 3) Spectral analyses of gamma and XRF elemental data from intervals of low- vs high-precessional variance show that significantly lower accumulation rated occurred during extended times of high- vs low-precessional variation.</p><p>            Accumulation rate oscillations in the Newark Rift Basin should be tracking weathering rates to supply the immense volumes of sediment involved in the accumulation rate variations. Such volumes could not be somehow stored in the highlands for hundreds of thousands of years, otherwise potentially shifting weathering and accumulation rates out of phase.</p><p>            The implication of these empirical data is that because pCO<sub>2</sub> should be drawn down under higher weathering rates, and the phase of eccentricity modulation of precession is global, pCO<sub>2</sub> should be oscillating in phase with the Mars–Earth eccentricity cycle. On the short-term, low-pCO<sub>2</sub> should characterize times of low-precessional variability, evidently associated with high-accumulation rates, based on these empirical data, and not vice-versa as might be intuitively modeled. In turn, the oscillations in pCO<sub>2</sub> would be expected to cause global temperature oscillations at the g<sub>4</sub>-g<sub>3</sub> frequency. These non-intuitive results, suggesting a hitherto unanticipated relationship between orbital pacing of climate and pCO<sub>2</sub>, can be tested and further explored by continuous XRF elemental scanning of these cores, currently underway, and by collection of more densely sampled soil carbonate and leaf stomatal pCO<sub>2</sub> proxy data, from proposed new cores. The mechanisms driving the relationships between these reproducible empirical data are, however, not obvious, but would seem to be related to the precession-scale variability of climate, not just the magnitude of greenhouse gas concentrations or temperatures.</p><p> </p><p> </p><p> </p><p> </p><p> </p><p> </p><p> </p>

A thick, apparently continuous section recording events of the latest Paleocene thermal maximum in a neritic setting was drilled at Bass River State Forest, New Jersey as part of ODP Leg 174AX [Miller, Sugarman, Browning et al., 1998]. Integrated nannofossil and magneto-stratigraphy provides a firm chronology supplemented by planktonic foraminiferal biostratigraphy. This chronologic study indicates that this neritic section rivals the best deep-sea sections in providing a complete record of late Paleocene climatic events. Carbon and oxygen isotopes measured on benthic foraminifera show a major (4.0% in carbon, 2.3% in oxygen) negative shift correlative with the global latest Paleocene carbon isotope excursion (CIE). A sharp increase in kaolinite content coincides with the isotope shift in the Bass River section, analogous to increases found in several other records. Carbon and oxygen isotopes remain low and kaolinite content remains high for the remainder of the depositional sequence above the CIE (32.5 ft, 9.9 m), which we estimate to represent 300-500 k.y. We interpret these data as indicative of an abrupt shift to a warmer and wetter climate along the North American mid-Atlantic coast, in concert with global events associated with the CIE.

We present a new sedimentological profile and a magnetostratigraphy of the tool-bearing Kozarnika cave sediments from Bulgaria. Modal analysis of cave infilling sedimentary texture indicates that most of the layers are produced by reworked wind-blown sediment (loess). We found evidence for a relatively thick and well defined normal magnetic polarity in the upper-middle part of the section interpreted as a record of the Brunhes Chron, followed downsection by reverse polarity directions. The Brunhes-Matuyama boundary (0.78 Ma) is placed in the upper part of Layer 13 Lower. The lowermost levels with Lower Paleolithic tools are close to - or possibly straddling the - Brunhes-Matuyama boundary. Our results are in substantial agreement with the age of onset of loess deposition in the Danube valley, which occurred shortly before the Brunhes-Matuyama boundary. Moreover, our data fit well with the hypothesis that hominins first entered Europe across a Danube-Po migration conduit during the late Early Pleistocene

A reproducible geomagnetic polarity template for the Late Triassic and earliest Jurassic continues to be that determined from ~5,000 meters of cored section in the Newark basin and ~2,500 meters of outcrop section in the Hartford basin, sampled at nominal ~20 kyr intervals according to a well-developed climate cyclicity that characterizes the lacustrine strata present in all but the fluviatile portions of the basins [Kent & Olsen, 1999, 2008 JGR]. The age model is based on the 405 kyr Milankovich climate cycle and pegging the sequence to high precision U-Pb dating of the Central Atlantic Magmatic Province (CAMP) at 201.6 to 200.9 Ma [Blackburn+2013 Science], the initiation of which is practically coincident with the end-Triassic extinction level (formerly set to 202 Ma) and within a climatic precession cycle after magnetochron E23r. The resulting astrochronostratigraphic polarity time scale (APTS) has 66 Poissondistributed polarity intervals from chrons E8r (~225 Ma) to H27n (~199 Ma) with a constant sedimentaccumulation rate extrapolation to chron E1r (~233 Ma). Magnetostratigraphic correlations from the most complete and usually the thickest Tethyan marine sections suggest that the Carnian/Norian boundary occurs within ~E7n [Channell+2003 PPP; Muttoni+2004 GSAB] at an APTS age of 227.5 Ma and for the Norian/Rhaetian boundary anywhere from E16n [Husing+2011 EPSL] at ~210.5 Ma to E20r [Maron+2014 Geology] at ~205.4 Ma depending on choice of conodont taxa, whereas the Hettangian/Sinemurian boundary can be placed at ~199.5 Ma within the marine equivalent of H25r [Husing+2014 EPSL]. These APTS ages are in substantive agreement with available high-precision dates in marine strata for the late Carnian [231 Ma: Furin+2006 Geology], latest Norian [205.5 Ma:Wotslaw+2014 Geology], and the boundaries of the Triassic/Jurassic [201.3 Ma: Guex+2012 PPP] and the Hettangian/Sinemurian [199.5 Ma: Schaltegger+2008 EPSL]. Carnian magnetostratigraphy needs to be improved but attempts to make a composite magnetostratigraphic sequence for the Late Triassic by merging disparate marine and non-marine records have not produced a clearer signal. The Newark-Hartford APTS already provides a framework for long-distance correlation and dating, for example, the timing of dinosaur dispersal across Pangea [Kent+2014 PNAS]

High-resolution stable carbon isotope records for upper Paleocene - lower Eocene sections at Ocean Drilling Program Sites 1051 and 690 and Deep Sea Drilling Project Sites 550 and 577 show numerous rapid (40 - 60 kyr duration) negative excursions of up to 1 per mill. We demonstrate that these transient decreases are the expected result of nonlinear insolation forcing of the carbon cycle in the context of a long carbon residence time. The transients occur at maxima in Earth's orbital eccentricity, which result in high-amplitude variations in insolation due to forcing by climatic precession. The construction of accurate orbital chronologies for geologic sections older than ~ 35 Ma relies on identifying a high-fidelity recorder of variations in Earth's orbital eccentricity. We use the carbon isotope records as such a recorder, establishing a robust orbitally tuned chronology for latest Paleocene-earliest Eocene events. Moreover, the transient decreases provide a means of precise correlation among the four sites that is independent of magnetostratigraphic and biostratigraphic data at the <10^5-year scale. While the eccentricity-controlled transient decreases bear some resemblance to the much larger-amplitude carbon isotope excursion (CIE) that marks the Paleocene/Eocene boundary, the latter event is found to occur near a minimum in the ~400-kyr eccentricity cycle. Thus the CIE occurred during a time of minimal variability in insolation, the dominant mechanism for forcing climate change on 104-year scales. We argue that this is inconsistent with mechanisms that rely on a threshold climate event to trigger the Paleocene/Eocene thermal maximum since any threshold would more likely be crossed during a period of high-amplitude climate variations.

A 30\ub0 gap was recognized in a composite APW path when global poles from predominantly igneous rocks were assembled in North American coordinates using plate reconstructions (Kent & Irving 2010 JGR). The 'monster shift' occurred between a 160-190 Ma cluster of mean poles at 75-80\ub0N 90-110\ub0E to a 140-145 Ma grouping centered at 60-65\ub0N ~200\ub0E. There are hardly any intermediate igneous poles whereas the rather divergent directions from the Late Jurassic Morrison Formation published by Steiner & Helsley (1975 GSA Bulletin) are subject to adjustments for Colorado Plateau rotation and sedimentary inclination error, neither of which are precisely known for this redbed unit sampled in Colorado. On the other hand, similar large rapid swings have been recognized in the Late Jurassic APW path for Adria (Channell et al. 2010 Paleo3), suggesting a global phenomena. In an effort to fill the data gap between ~145 and 160 Ma, we sampled accessible outcrops/subcrops of kimberlites in the Timiskaming area of Ontario, Canada, that are associated with high precision U-Pb perovskite ages (Heamon & Kjarsgaard 2000 EPSL). We report initial results from two of the intrusions: the 153.6\ub12.4 Ma Peddie kimberlite from outcrop and the Triple B kimberlite that was accessible by trenching and is assumed to be the same age as the nearby 153.7\ub11.8 Ma Seed kimberlite as delineated by aeromagnetic surveys and borings. Systematic progressive thermal demagnetization indicated in each unit a dominant characteristic component with unblocking temperatures to 575\ub0 that presumably reflect a magnetite carrier that will be checked by further rock magnetic experiments. Samples from the Peddie kimberlite had stable downward (normal polarity) magnetizations whose mean direction gives a paleopole at 73\ub0N 184\ub0E. In contrast, samples from the Triple B kimberlite have upward (reverse polarity) magnetizations with a well-grouped direction whose (north) paleopole is 78\ub0N 197\ub0E, proximal to the Peddie pole. The normal and reverse polarities suggest that sufficient time elapsed between emplacement of the Triple B and Peddie to give an opportunity to average secular variation. The combined ~154 Ma Triple B and Peddie pole encouragingly lies about halfway between igneous poles from North America nearest in age: the 169 Ma Moat volcanics and the 142 Ma Ithaca kimberlites. However, preliminary paleomagnetic results from some of the other sampled kimberlite bodies are more problematical and require further paleomagnetic and geochronological work

Measurements of the NRM of a 26 m long deep-sea core from the southern Indian Ocean indicated the presence of three transitions of magnetic polarity which have been identified as the upper and lower Jaramillo and the upper Olduvai on the basis of micropaleontological riteria. Detailed studies of the magnetic reversals were made in view of the high deposition rates ( ~ 9 cm/10"J yr) present over sections of the core. The NRM was found stable against alternating fields. Magnetic mineralogy studies indicated the presence of titanomagnetite and magnetite which probably have not undergone any significant low-temperature oxidation. The three polarity changes bad the following features in common: ( 1) presence of intermediate directions of magnetization; (2) a pronounced rop in the intensity ol magnetization; (3) the drop in intensity of magnetization was coincident with the large directional f uctuations. Measurements of saturation isothermal nd anhysteretic remanence, and bulk susce...

Significance Sharply contrasting climate zonations under high atmospheric p CO 2 conditions can exert significant obstacles to the dispersal of land vertebrates across a supercontinent. This is argued to be the case in the Triassic for herbivorous sauropodomorph dinosaurs, which were confined to their initial venue in the Southern Hemisphere temperate belt of Pangea for about their first 15 million years. Sauropodomorphs only appear in the fossil record of the Northern Hemisphere temperate belt about 214 million years ago based on a composite magnetostratigraphy of the Fleming Fjord Group in East Greenland. The coincidence in timing within a major dip in atmospheric p CO 2 from published paleosol records suggests the dispersal was related to a concomitant attenuation of climate barriers in a greenhouse world.

At the Drmno open-pit coal mine near Kostolac in Serbia, a nearly complete skeleton of Mammuthus trogontherii (nicknamed Vika) was discovered in a fluvial deposit overlain by a loess–paleosol sequence where a second paleontological level named Nosak with remains of M. trogontherii was found. We studied the magnetostratigraphy of the Kostolac sedimentary sequence and found that the Vika layer dates to ~ 0.8 Ma, shortly before the Brunhes–Matuyama boundary. In addition, according to our age model and previously reported optically stimulated luminescence and electron spin resonance dates, the Nosak fossils have an estimated age of 0.19 Ma and lived during the earliest part of Marine Isotope Stage 6. It appears therefore that at Kostolac, M. trogontherii is preserved both at its earliest occurrence at ~ 0.8 Ma and close to its latest occurrence at 0.19 Ma, and may well have been present in between, albeit not yet found. We speculate that megaherbivores such as M. trogontherii entered Eu...