Middle to late Miocene oxygen isotope stratigraphy of ODP site 1085 (SE Atlantic): new constrains on Miocene climate variability and sea-level fluctuations

The middle Miocene delta18O increase represents a fundamental change in earth's climate system due to a major expansion and permanent establishment of the East Antarctic Ice Sheet accompanied by some effect of deepwater cooling. The long-term cooling trend in the middle to late Miocene was superimposed by several punctuated periods of glaciations (Mi-Events) characterized by oxygen isotopic shifts that have been related to the waxing and waning of the Antarctic ice-sheet and bottom water cooling. Here, we present a high-resolution benthic stable oxygen isotope record from ODP Site 1085 located at the southwestern African continental margin that provides a detailed chronology for the middle to late Miocene (13.9-7.3 Ma) climate transition in the eastern South Atlantic. A composite Fe intensity record obtained by XRF core scanning ODP Sites 1085 and 1087 was used to construct an astronomically calibrated chronology based on orbital tuning. The oxygen isotope data exhibit four distinct delta18O excursions, which have astronomical ages of 13.8, 13.2, 11.7, and 10.4 Ma and correspond to the Mi3, Mi4, Mi5, and Mi6 events. A global climate record was extracted from the oxygen isotopic composition. Both long- and short-term variabilities in the climate record are discussed in terms of sea-level and deep-water temperature changes. The oxygen isotope data support a causal link between sequence boundaries traced from the shelf and glacioeustatic changes due to ice-sheet growth. Spectral analysis of the benthic delta18O record shows strong power in the 400-kyr and 100-kyr bands documenting a paleoceanographic response to eccentricity-modulated variations in precession. A spectral peak around 180-kyr might be related to the asymmetry of the obliquity cycle indicating that the response of the dominantly unipolar Antarctic ice-sheet to obliquityinduced variations probably controlled the middle to late Miocene climate system. Maxima in the delta18O record, interpreted as glacial periods, correspond to minima in 100-kyr eccentricity cycle and minima in the 174-kyr obliquity modulation. Strong middle to late Miocene glacial events are associated with 400-kyr eccentricity minima and obliquity modulation minima. Thus, fluctuations in the amplitude of obliquity and eccentricity seem to be the driving force for the middle to late Miocene climate variability.

Geological Time Scale 2012. Chapter 10. Oxygen isotope stratigraphy, Appendix 1

Variations in the 18O/16O ratios of marine fossils and microfossils record changes in seawater 18O/16O and temperature and form the basis for global correlation. Relying on previous compilations and new data, this chapter presents oxygen isotope curves for Phanerozoic foraminifera, mollusks, brachiopods, and conodonts, and for Precambrian limestones, dolostones, and cherts. Periodic oxygen-isotopic variations in deep-sea foraminifera define marine isotope stages that, when combined with biostratigraphy and astronomical tuning, provide a late Cenozoic chronostratigraphy with a resolution of several thousand years. Oxygen isotope events of early Cenozoic, Mesozoic, and Paleozoic age serve as chemostratigraphic markers for regional and global correlation. Precambrian oxygen isotope stratigraphy, however, is hampered by the lack of unaltered authigenic marine sediments.

Oxygen isotope stratigraphy Ojplus03 for the last 838,000 years

Global warming is real and has been with us for at least two decades. Questions arise regarding the response of the ocean to greenhouse forcing, including expectations for changes in ocean circulation, in uptake of excess carbon dioxide, and in upwelling activity. The large climate variations of the ice ages, within the last million years, offer the opportunity to study responses of the ocean to climate change. A histogram of sealevel positions for the last 700,000 years (based on a new d/sup 18/O stratigraphy here compiled) shows that the present is near the margin of the range of fluctuations, with only 6 percent of positions indicating a warmer climate. Thus, the future will be largely outside of experience with regard to fluctuations of the recent geologic past. The same is true for greenhouse forcing. Our inability to explain sudden climate change in the past, including the rapid rise of carbon dioxide during deglaciation, and differences in ocean productivity between glacial and interglacial conditions, demonstrates a lack of understanding that makes predictions suspect. This is the lesson from ice age studies.

Stable oxygen isotope record of Neogloboquadrina pachyderma of ODP Sites 177-1091 and 177-1094

Ocean Drilling Program (ODP) cores permit us to extend the study of millennial-scale climate variability beyond the time period that is generally accessible for piston cores (i.e., the last glacial cycle). ODP Leg 177 provided for the first time continuous high sedimentation rate cores along a north-south transect from 41°to 53°S across the main subdivisions of the Southern Ocean (Shipboard Scientific Party, 1999, doi:10.2973/odp.proc.ir.177.101.1999). The main purpose of this drilling was to investigate the Pleistocene and Holocene paleoceanographic history of this region, documented in the sedimentary records. ODP Sites 1094, 1093, 1091, and 1089 accumulated throughout the Pleistocene at rates >10 cm/k.y. and are the most detailed Pleistocene climatic records ever retrieved from the Southern Ocean. These sections provide a unique opportunity to fill an important gap in the knowledge of the paleoclimatic evolution of the high southern latitude regions. The composite sections at each site were generated shipboard using magnetic susceptibility, gamma ray attenuation (GRA) density, and reflectance data to correlate the drill holes and splice together an optimal (complete and undisturbed) record of the sedimentary sequence at each site. A preliminary magnetic polarity stratigraphy was generated on the 'archive' halves of the core sections from each hole, using the shipboard pass-through magnetometer after demagnetization at a single peak alternating field (Shipboard Scientific Party, 1999). During July 1998, we sampled core sections spanning the mid-Pleistocene interval (0.65-1.2 Ma) from Sites 1094, 1093, and 1091 at the ODP Bremen Core Repository and have since then analyzed the stable isotopic ratios of foraminifers in samples from Sites 1094 and 1091. Our goals for these studies are to establish detailed chronology for the mid-Pleistocene Southern Ocean records from Leg 177 using high-resolution stable isotope analyses, and furthermore, to trace the evolution of millennial-scale variability in proxy records from older glacial and interglacial periods characterized by higher-frequency variation. Here, we report on our stratigraphic results to date and describe the laboratory methods employed for sample preparation and stable isotope analysis. Furthermore, we provide tab-delimited text files of the age models.

Paleomagnetic, stable oxygen isotope ratios, and U/Th activities of ODP Site 172-1062

Geomagnetic excursions are recognized as intrinsic features of the Earth's magnetic field. High-resolution records of field behaviour, captured in marine sedimentary cores, present an opportunity to determine the temporal and geometric character of the field during geomagnetic excursions and provide constraints on the mechanisms producing field variability. We present here the highest resolution record yet published of the Blake geomagnetic excursion (~125 ka) measured in three cores from Ocean Drilling Program (ODP) Site 1062 on the Blake-Bahama Outer Ridge. The Blake excursion has a controversial structure and timing but these cores have a sufficiently high sedimentation rate (~10cm/ka) to allow detailed reconstruction of the field behaviour at this site during the excursion. Palaeomagnetic measurements of the cores reveal rapid transitions (<500 yr) between the contemporary stable normal polarity and a completely reversed state of long duration which spans a stratigraphic interval of 0.7 m. We determine the duration of the reversed state during the Blake excursion using oxygen isotope stratigraphy, combined with 230Th excess measurements to assess variations in the sedimentation rates through the sections of interest. This provides an age and duration for the Blake excursion with greater accuracy and with constrained uncertainty. We date the directional excursion as falling between 129 and 122 ka with a duration for the deviation of 6.5±1.3 kyr. The long duration of this interval and the fully reversed field suggest the existence of a pseudo-stable, reversed dipole field component during the excursion and challenge the idea that excursions are always of short duration.

Sedimentology, mineral magnetism, andoxygen isotope stratigraphy of ODP Hole 133-819A

To investigate late Quaternary paleoclimatic and paleoceanographic change in the sedimentary record, preserved on the Australian Continental Margin during the late Quaternary, core material was collected from Ocean Drilling Program, Leg 133, Site 819. An expanded sequence of late Quaternary, rhythmically bedded, predominantly hemipelagic sediments were recovered from Hole 819A. The foraminiferal d18O record preserved at Hole 819A suggests that the late Quaternary section is incomplete. Both benthic and planktonic d18O stratigraphies can be traced tentatively downcore to stage 6 at about 32.5 mbsf, where a major hiatus occurs. At this level, a slump detachment surface has been identified (Shipboard Scientific Party, 1991). This slump has removed marine oxygen isotope stages 7 to 13. Below 32.5 mbsf, continuous correlation can be achieved in the planktonic d18O curve, with existing deep-sea foraminiferal oxygen isotope stratigraphies from stage 14 through stage 28. The major hiatus at 32.5 mbsf marks the position of a significant change in the character of the sedimentation at Site 819. Sediments below 32.5 mbsf, relative to those above 32.5 mbsf, are characterized by less variation in mean particle size; lower percentages of carbonate content in the coarse fraction (>63 µm); a stronger relationship between the percentage of fine fraction and magnetic mineral concentration, and lower foraminiferal abundances. Above the hiatus, large fluctuations in mean particle size occurred, which have been interpreted to be the result of high foraminiferal abundances. Early highstands show high terrigenous influx in the fine fraction above the hiatus. This is the opposite of the general idea of high terrigenous influx during lowstands of sea level on siliciclastic dominated continental margins. We are far from understanding the origin of this material and further investigation will be required (see also Glenn et al., this volume). All our records, except the planktonic foraminiferal oxygen isotope record, indicate that the major hiatus marks the position of a significant change in the environment at Site 819. The planktonic foraminiferal d18O record suggests that environmental change occurred prior to the formation of the hiatus (i.e., near the Brunhes/Matuyama [B/M] boundary). The interval between the B/M boundary and the hiatus represents a transitional period between two different patterns of ocean circulation. Throughout most of the lower part of the sequence, Site 819 was at a shallow-water depth and local oceanographic conditions were dominated by sluggish Subtropical Central Water (SCW) flow. However, near the B/M boundary, ocean circulation patterns intensified, reflecting a worldwide change in paleoenvironment. Enhanced ocean circulation patterns were possibly aided by tectonic subsidence. During this period Site 819 became progressively more under the influence of Antarctic Intermediate Water (AAIW), than SCW. In the upper part of the sequence at Hole 819 A, we see a continuation of the pattern of oceanographic reorganization suggested during stages 21 through 14. Intensification of the subsurface oceanographic circulation was also accompanied by the progressive wedging southward of surface waters associated with the East Australian Current (EAC). The change in the nature of the records in the lower and upper parts of the sequence at Site 819 are thought to reflect perturbations by the orbital eccentricity cycle.

Miocene stable isotope stratigraphy of ODP Hole 120-747A

We correlated Miocene d18O increases at Ocean Drilling Program Site 747 with d18O increases previously identified at North Atlantic Deep Sea Drilling Project Sites 563 and 608. The d18O increases have been directly tied to the Geomagnetic Polarity Time Scale (GPTS) at Site 563 and 608, and thus our correlations at Site 747 provide a second-order correlation to the GPTS. Comparison of the oxygen isotope record at Site 747 with records at Sites 563 and 608 indicates that three as-yet-undescribed global Miocene d18O increases may be recognized and used to define stable isotope zones. The d18O maxima associated with the bases of Zones Mila, Milb, and Mi7 have magnetochronologic age estimates of 21.8, 18.3, and 8.5 Ma, respectively. The correlation of a d18O maximum at 70 mbsf at Site 747 to the base of Miocene isotope Zone Mi3 (13.6 Ma) provides a revised interpretation of four middle Miocene normal polarity intervals observed between 77 and 63 mbsf at Hole 747A. Oxygen isotope stratigraphy indicates that the reversed polarity interval at 70 mbsf, initially interpreted as Chronozone C5AAr, should be C5ABr. Instead of a concatenated Chronozone C5AD-C5AC with distinct Chronozones C5AB, C5AA, and C5A (as in the preliminary interpretation), d18O stratigraphy suggests that these normal polarity intervals are Chronozones C5AD, C5AC, and C5AB, whereas Chronozones C5AA-C5A are concatenated. This interpretation is supported by the d13C correlations. The upper Miocene magnetostratigraphic record at Hole 747A is ambiguous. Two upper Miocene d18O events at Site 747 can be correlated to the oxygen isotope records at Site 563 and 608 using the magnetostratigraphy derived at Hole 747B. Our chronostratigraphic revisions highlight the importance of stable isotope stratigraphy in attaining an integrated stratigraphic framework for the Miocene.

Pliocene-Pleistocene stack of globally distributed benthic stable oxygen isotope records

We present a 5.3-Myr stack (the ''LR04'' stack) of benthic d18O records from 57 globally distributed sites aligned by an automated graphic correlation algorithm. This is the first benthic delta18O stack composed of more than three records to extend beyond 850 ka, and we use its improved signal quality to identify 24 new marine isotope stages in the early Pliocene. We also present a new LR04 age model for the Pliocene-Pleistocene derived from tuning the delta18O stack to a simple ice model based on 21 June insolation at 65 N. Stacked sedimentation rates provide additional age model constraints to prevent overtuning. Despite a conservative tuning strategy, the LR04 benthic stack exhibits significant coherency with insolation in the obliquity band throughout the entire 5.3 Myr and in the precession band for more than half of the record. The LR04 stack contains significantly more variance in benthic delta18O than previously published stacks of the late Pleistocene as the result of higher resolution records, a better alignment technique, and a greater percentage of records from the Atlantic. Finally, the relative phases of the stack's 41- and 23-kyr components suggest that the precession component of delta18O from 2.7-1.6 Ma is primarily a deep-water temperature signal and that the phase of d18O precession response changed suddenly at 1.6 Ma.

Stable oxygen isotope record and age determination of benthic foraminiferal faunas of the Norwegian-Greenland Sea

Fluctuations in benthic foraminiferal faunas over the last 130,000 yr in four piston cores from the Norwegian Sea are correlated with the standard worldwide oxygen-isotope stratigraphy. One species, Cibicides wuellerstorfi, dominates in the Holocene section of each core, but alternates downcore with Oridorsalis tener, a species dominant today only in the deepest part of the basin. O. tener is the most abundant species throughout the entire basin during periods of particularly cold climate when the Norwegian Sea presumably was ice covered year round and surface productivity lowered. Portions of isotope Stages 6, 3, and 2 are barren of benthic foraminifera; this is probably due to lowered benthic productivity, perhaps combined with dilution by ice-rafted sediment; there is no evidence that the Norwegian Sea became azoic. The Holocene and Substage 5e (the last interglacial) are similar faunally. This similarity, combined with other evidence, supports the presumption that the Norwegian Sea was a source of dense overflows into the North Atlantic during Substage 5e as it is today. Oxygen-isotope analyses of benthic foraminifera indicate that Norwegian Sea bottom waters warmer than they are today from Substage 5d to Stage 2, with the possible exception of Substage 5a. These data show that the glacial Norwegian Sea was not a sink for dense surface water, as it is now, and thus it was not a source of deep-water overflows. The benthic foraminiferal populations of the deep Norwegian Sea seem at least as responsive to near-surface conditions, such as sea-ice cover, as they are to fluctuations in the hydrography of the deep water. Benthic foraminiferal evidence from the Norwegian Sea is insufficient in itself to establish whether or not the basin was a source of overflows into the North Atlantic at any time between the Substage 5e/5d boundary at 115,000 yr B.P. and the Holocene.