270 research outputs found
North Atlantic Late Miocene Stable-Isotope Stratigraphy, Biostratigraphy, and Magnetostratigraphy
Upper Miocene foraminiferal nannofossil ooze and chalk from DSDP Hole 552A in the northeast Atlantic Ocean have been closely sampled for biostratigraphic, paleomagnetic, and stable-isotopic studies. Sampling at 10-cm intervals resulted in an uppermost Miocene isotope stratigraphy with a 1000- to 3000-yr. resolution. Covariance in benthic (Planulina wuellerstorfi) and planktonic (Globigerina bulloides) foraminiferal δ^O¸ records is taken as evidence for variability in continental ice volume. Our best estimate is that glacial maxima occurred at ~ 5.0 and ~ 5.5 Ma and lasted no more than 20,000 yrs. These events probably lowered sea level by 60 m below the latest Miocene average. There is little oxygen-isotope evidence, however, for a prolonged glaciation during the last 2 m.y. of the late Miocene. High- and low-frequency variability in the δ^13C record of foraminifers is useful for correlation among North Atlantic DSDP Sites 408, 410, 522, 610, and 611, and for correlation with sites in other oceans. Similar δ^13C changes are seen in P. wuellerstorfi and G. bulloides, but the amplitude of the signal is always greater in G. bulloides. Variability in δ^13C common to both species probably reflects variability in the δ^13C of total CO2 in seawater. Major long-term features in the δ^13C record include a latest Miocene maximum (P. wuellerstorfi = 1.5‰) in paleomagnetic Chron 7, an abrupt decrease in δ^13C at -6.2 Ma, and a slight increase at -5.5 Ma. The decrease in δ^13C at -6.2 Ma, which has been paleomagnetically dated only twice before, occurs in the upper reversed part of Chronozone 6 at Holes 552A and 611C, in excellent agreement with earlier studies. Cycles in δ^13C with a period of ~ 104 yrs. are interpreted as changes in seawater chemistry, which may have resulted from orbitally induced variability in continental biomass. Samples of P. wuellerstorfi younger than 6 Ma from throughout the North Atlantic have δ^13C near 1‰, on average ~ 1‰ greater than samples of the same age in the Pacific Ocean. Thus, there is no evidence for cessation of North Atlantic Deep Water production resulting from the Messinian "salinity crisis." Biostratigraphic results indicate continuous sedimentation during the late Miocene after about -6.5 Ma at Hole 552A. Nannofossil biostratigraphy is complicated by the scarcity of low-latitude marker species, but middle and late Miocene Zones NN7 through NN11 are recognized. A hiatus is present at -6.5 Ma, on the basis of simultaneous first occurrences of Amaurolithusprimus, Amaurolithus delicatus, Amaurolithus amplificus, and Scyphosphaera globulata. The frequency and duration of older hiatuses increase downsection in Hole 552A, as suggested by calcareous nannofossil biostratigraphy and magnetostratigraphy. Paleomagnetic results at Hole 552A indicate a systematic pattern of inclination changes. Chronozone 6 was readily identified because of its characteristic nannoflora (sequential occurrences of species assigned to the genus Amaurolithus) and the δ^13C decrease in foraminifers, but its lower reversed interval is condensed. Only the lower normal interval of Chronozone 5 was recognized at Hole 552A; the upper normal interval and the lowest Gilbert sediment are not recognized, owing to low intensity of magnetization and to coring disturbance. Interpreting magnetic reversals below Chronozone 6 was difficult because of hiatuses, but a lower normally magnetized interval is probably Chronozone 7. Correlation between DSDP Hole 552A and other North Atlantic sites is demonstrated using coiling direction changes in the planktonic foraminifer Neogloboquadrina. At most sites this genus changed its coiling preference from dominantly right to dominantly left during the late Miocene. At Hole 552A this event probably occurred about 7 m.y. ago. At the same time, P. wuellerstorfi had maximum δ^13C values. A similar δ^13C maximum and coiling change occurred together in Chron 7 at Hole 611C, and at Hole 610E. In sediment younger than -5.5 Ma, the coiling of small Neogloboquadrina species is random, but the larger species N. atlantica retains preferential left coiling
Recommended from our members
Orbital climate forcing of δ^13 excursions in the late Paleocene-early Eocene (chrons C24n-C25n)
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‰. 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 10^4-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
Recommended from our members
Paleogene time scale miscalibration: Evidence from the dating of the North Atlantic igneous province
Jolley et al. (2002) have proposed that the date of the Paleocene - Eocene thermal maximum is ca. 60 Ma, at least 5 m.y. older than currently estimated and, as a result, argue that the Paleogene time scale of Berggren et al. (1995) is grossly miscalibrated. The implications of this proposal are implausible, and we attribute the discrepancy in age noted by Jolley et al. (2002) to miscorrelation of the Staffa-type palynofloras and ambiguous isotopic dates from the North Atlantic igneous province
Subseries/Subepochs approved as a formal rank in the international stratigraphic guide
© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Aubry, M., Head, M. J., Piller, W. E., & Berggren, W. A. Subseries/Subepochs approved as a formal rank in the international stratigraphic guide. Episodes, 43(4), (2020): 1041-1044, doi:10.18814/epiiugs/2020/020066.The International Subcommission on Stratigraphic Classification, as the constituent body of the International Commission on Stratigraphy (ICS) responsible for the International Stratigraphic Guide, has voted to include the subseries/ subepoch as a formal rank in the next edition of the Guide. This acknowledges the recent ratification of formal subseries and their corresponding stages for the Holocene Series/Epoch but allows individual subcommissions within ICS the freedom to decide whether or not to adopt this rank for their particular stratigraphic/time interval.We are grateful to the ISSC membership for discussions and to Secretary Jochen Erbacher for organizing the vote; to Mike Walker for sharing an unpublished manuscript with us; to the many colleagues who have expressed their support for formalization of subseries; and to Dennis Kent and an anonymous reviewer for their reviews of the manuscript
Recommended from our members
Mid-Neogene Mediterranean marine-continental correlations: an alternative interpretation
Recent revised magnetostratigraphic and astrochronologic calibrations of several calcareous planktonic microfossil datum events (in particular Coccolithus miopelagicus and Neogloboquadrina acostaensis) combined with our magnetobiostratigraphic investigation of a 10 m-thick section at Ecotet, near Lyon (France), in which terrestrial micromammals (Vallesian Zone MN9) are intercalated with calcareous nannoplankton and planktonic foraminifera belonging to Zones NN6-NN8 and N14-15 (=M11 and M12), respectively, lead us to question the recent recalibration by Krijgsman et al. [(1996) Eart Planet. Sci. Lett. 142, 367-380) of the Aragonian/Vallesian (MN7-8/MN9) boundary with Chron C5r.1r (~11.1 Ma) and the lower/upper Vallesian (MN9/10) boundary with Chron C4Ar.2n (~9.6-9.7 Ma). Ecotet, with a uniform reverse polarity, is shown to be assignable uniquely to Chron C5r. Interregional correlation of Ecotet with Mediterranean terrestrial stratigraphies indicates that Progonomys (whose First Occurrence (FO) has been traditionally placed in Mediterranean Mammal Zone MN10) was already present in Mediterranean Zone MN9 at ~11.4 Ma and did not experience a 2 million-year delay in its putative prochoresis from Asia
The Neogene and Quaternary : chronostratigraphic compromise or non-overlapping magisteria?
Author Posting. © Micropaleontology Press, 2009. This article is posted here by permission of Micropaleontology Press for personal use, not for redistribution. The definitive version was published in Stratigraphy 6 (2009): 1-16.The International Commission on Stratigraphy (ICS) together with its subcommissions on Neogene Stratigraphy (SNS) and Quaternary Stratigraphy (SQS) are facing a persistent conundrum regarding the status of the Quaternary, and the implications for the Neogene System/Period and the Pleistocene Series/Epoch. The SQS, in seeking a formal role for the Quaternary in the standard time scale, has put forward reasons not only to truncate and redefine the Neogene in order to accommodate this unit as a third System/Period in the Cenozoic, but furthermore to shift the base of the Pleistocene to c. 2.6 Ma to conform to a new appreciation of when “Quaternary climates” began. The present authors, as members of SNS, support the well-established concept of a Neogene extending to the Recent, as well as the integrity of the Pleistocene according to its classical meaning, and have published arguments for workable options that avoid this conflict. In this paper, we return to the basic principles involved in the conversion of the essentially marine biostratigraphic/ biochronologic units of Lyell and other 19th-century stratigraphers into the modern hierarchical arrangement of chronostratigraphic units, embodied in the Global Standard Stratotype-section and Point (GSSP) formulation for boundary definitions. Seen in this light, an immediate problem arises from the fact that the Quaternary, either in its original sense as a state of consolidation or in the more common sense as a paleoclimatic entity, is conceptually different from a Lyellian unit, and that a Neogene/Quaternary boundary may therefore be a non sequitur. Secondly, as to retaining the base of the Pleistocene at 1.8 Ma, the basic hierarchical principles dictate that changing the boundary of any non-fundamental or “higher” chronostratigraphic unit is not possible without moving the boundary of its constituent fundamental unit. Therefore, to move the base of the Pleistocene, which is presently defined by the Calabrian GSSP at 1.8 Ma, to be identified with the Gelasian GSSP at 2.6 Ma, requires action to formally redefine the Gelasian as part of the Pleistocene. Finally, it is important to keep in mind that the subject under discussion is chronostratigraphy, not biostratigraphy. Both systems are based on the fossil record, but biostratigraphic units are created to subdivide and correlate stratigraphic sequences. The higher-level units of chronostratigraphy, however, were initially selected to reflect the history of life through geological time. The persistence of a characteristic biota in the face of environmental pressures during the last 23 my argues strongly for the concept of an undivided Neogene that extends to the present. Several ways to accommodate the Quaternary in the standard time scale can be envisaged that preserve the original concepts of the Neogene and Pleistocene. The option presently recommended by SNS, and most compatible with the SQS position, is to denominate the Quaternary as a subperiod/subsystem of the Neogene, decoupled from the Pleistocene so that its base can be identified with the Gelasian GSSP at c. 2.6 Ma. A second option is to retain strict hierarchy by restricting a Quaternary subperiod to the limits of the Pleistocene at 1.8 Ma. As a third option, the Quaternary could be a subera/suberathem or a supersystem/ superperiod, decoupled from the Neogene and thus with its base free to coincide with a convenient marker such as the base of the Pleistocene at 1.8 Ma, or to the Gelasian at 2.6 Ma, as opinions about paleoclimatology dictate. If no compromise can be reached within hierarchical chronostratigraphy, however, an alternative might be to consider Quaternary and Neogene as mutually exclusive categories (climatostratigraphic vs. chronostratigraphic) in historical geology. In this case, we would recommend the application of the principle of NOMA, or Non-Overlapping Magisteria, in the sense of the elegant essay by the late Stephen J. Gould (1999) on the mutually exclusive categories of Religion and Science. In this case the Quaternary would have its own independent status as a climatostratigraphic unit with its own subdivisions based on climatic criteria
Recommended from our members
Integrated Paleocene calcareous plankton magnetobiochronology and stable isotope stratigraphy: DSDP Site 384 (NW Atlantic Ocean)
At Deep Sea Drilling Site 384 (J-Anomaly Ridge, Grand Banks Continental Rise, NW Atlantic Ocean) Paleocene nannofossil chalks and oozes (∼70 m thick) are unconformably/disconformably underlain (∼168 m; upper Maastrichtian) and overlain (∼98.7 m; upper lower Eocene) by sediments of comparable lithologies. The chalks are more indurated in stratigraphically higher levels of the Paleocene reflecting increasing amounts of biosiliceous (radiolarians and diatoms) components. This site serves as an excellent location for an integrated calcareous and siliceous microfossil zonal stratigraphy and stable isotope stratigraphy. We report the results of a magnetostratigraphic study which, when incorporated with published magnetostratigraphic results, reveals an essentially complete magnetostratigraphic record spanning the interval from Magnetochron C31n (late Maastrichtian) to C25n (partim) (late Paleocene, Thanetian). Integrated magnetobiochronology and stable isotope stratigraphy support the interpretation of, and constrain the estimated duration of, a short hiatus (∼0.9 my) within the younger part of Chron C29r (including the K/P boundary) and an ∼6 my hiatus separating upper Paleocene (Magnetozone C25n) and upper lower Eocene (Magnetozone C22r) sediments. Some 30 planktonic foraminiferal datum levels [including the criteria used to denote the Paleocene planktonic foraminiferal (sub)tropical zonal scheme of Berggren and Miller, Micropaleontology 34 (4) (1988) 362–380 and Berggren et al., SEPM Spec. Publ. 54 (1995) 129–212, Geol. Soc. Am. Bull. 107 (11) (1995) 1272–1287], and nearly two dozen calcareous nannoplankton datum levels have been recognized and calibrated to the magnetochronology. Planktonic foraminiferal Subzones P4a and P4b of (upper Paleocene) Zone P4 are emended/redefined based on the discovery of a longer stratigraphic extension of Acarinina subsphaerica (into at last Magnetozone C25n). Stable isotope stratigraphies from benthic foraminifera and fine fraction (<38 μm) carbonate have been calibrated to the biochronology and magnetostratigraphy. A minimum in benthic foraminifer δ13C was reached near the Danian/Selandian boundary (within Chron C26r, planktonic foraminiferal Zone P3a and calcareous nannoplankton Zone NP4) and is followed by the rise to maximum δ13C values in the late Thanetian (near the base of C25n, in Zone P4c and NP9a, respectively) that can be used for global correlation in the Paleocene
Recommended from our members
Oligocene-Miocene biostratigraphy, magnetostratigraphy, and isotopic stratigraphy of the western North Atlantic
Magnetostratigraphic records from western North Atlantic Deep Sea Drilling Project (DSDP) Sites 563 and 558 are correlated with the geomagnetic polarity time scale (GPTS; Berggren et aI., 1984a, 1984b) using marine magnetic anomalies and selected biostratigraphic datum levels. The magnetochronology established is used to make direct magnetobiostratigraphic correlations that agree with previous Oligocene-early Miocene studies. However, we show that Zones NN8 partim and NN9 and associated Epoch 11 correlate with Magnetic Anomaly 5 (= Chron C5n). This contrasts with previous indirect correlations of Epoch 11 with Anomaly 5A and requires an upward adjustment of 1.5-2.0 m.y. for middle-late Miocene calcareous nannofossil zones. We correlate the middle/late Miocene boundary with Zone NN8 and earliest Chron C5n (10.4 Ma)
Recommended from our members
Late Paleocene event chronology: unconformities, not diachrony
The chronology of the events associated with the late Paleocene thermal maximum (LPTM, Chron C24r) has been established through the construction of a composite reference section that involved chemomagnetobiostratigraphic correlations and assumed minimum diachrony of biostratigraphic events. On this basis, discrepancies between correlations in different sections were explained by inferred unconformities. However, diachrony between distant sections cannot be ruled out. We report here on two geographically close sections drilled onshore New Jersey that yield different records of chemomagnetobiostratigraphic correlations in the interval representing Chron C24r. Because of their proximity (approximately 40 km apart), diachrony of biostratigraphic events between the two sections can be ruled out. In contrast, the marked lithologic disconformities in the sections explain well the different records of events. We thus conclude that the current relative chronology for Chron C24r is firmly based and that the upper Paleocene-lower Eocene stratigraphic record yields multiple unconformities, with Subzone NP9b rarely sampled. We examine the implications that undeciphered unconformities may have on the identification of proxies for paleoceanographic reconstruction, in particular with regard to the identification of the carbon isotope excursion (CIE) that reflects a dramatic latest Paleocene disturbance of the carbon cycle. We propose biostratigraphic means (short-lived calcareous nannoplankton and planktonic foraminifera taxa) that permit the unequivocal identification of the CIE not only in the oceanic realm but also in neritic settings
- …