Isotope ratios of osmiun, carbon and oxygen of Miocene sediments and benthic foraminifera

Seawater 187Os/188Os ratios for the Middle Miocene were reconstructed by measuring the 187Os/188Os ratios of metalliferous carbonates from the Pacific (DSDP 598) and Atlantic (DSDP 521) oceans. Atlantic and Pacific 187Os/188Os measurements are nearly indistinguishable and are consistent with previously published Os isotope records from Pacific cores. The Atlantic data reported here provide the first direct evidence that the long-term sedimentary 187Os/188Os record reflects whole-ocean changes in the Os isotopic composition of seawater. The Pacific and the Atlantic Os measurements confirm a long-term 0.01/Myr increase in marine 187Os/188Os ratios that began no later than 16 Ma. The beginning of the Os isotopic increase coincided with a decrease in the rate of increase of marine 87Sr/86Sr ratios at 16 Ma. A large increase of 1? in benthic foraminiferal delta18O values, interpreted to reflect global cooling and ice sheet growth, began approximately 1 million years later at 14.8 Ma, and the long-term shift toward lower bulk carbonate delta13C values began more than 2 Myr later around 13.6 Ma. The post-16 Ma increase in marine 187Os/188Os ratios was most likely forced by weathering of radiogenic materials, either old sediments or sialic crust with a sedimentary protolith. We consider two possible Miocene-specific geologic events that can account for both this increase in marine 187Os/188Os ratios and also nearly constant 87Sr/86Sr ratios: (1) the first glacial erosion of sediment-covered cratons in the Northern Hemisphere; (2) the exhumation of the Australian passive margin-New Guinea arc system. The latter event offers a mechanism, via enhanced availability of soluble Ca and Mg silicates in the arc terrane, for the maintenance of assumed low CO2 levels after 15 Ma. The temporal resolution (three samples/Myr) of the 187Os/188Os record from Site 598, for which a stable isotope stratigraphy was also constructed, is significantly higher than that of previously published records. These high resolution data suggest oscillations with amplitudes of 0.01 to 0.02 and periods of around 1 Myr. Although variations in the 187Os/188Os record of this magnitude can be easily resolved analytically, this higher frequency signal must be verified at other sites before it can be safely interpreted as global in extent. However, the short-term 187Os/188Os variations may correlate inversely with short-term benthic foraminiferal delta18O and bulk carbonate delta13C variations that reflect glacioeustatic events.

(Table 2) Stratigraphic position of midpoints of reddish and beige layers of lithologic cycles in the Loulja-A Section and their astronomical ages after tuning to the ETP and Summer Insolation Curves of the La2004 solution

An integrated high-resolution stratigraphy and orbital tuning is presented for the Loulja sections located in the Bou Regreg area on the Atlantic side of Morocco. The sections constitute the upward continuation of the upper Messinian Ain el Beida section and contain a well-exposed, continuous record of the interval straddling the Miocene-Pliocene (M-P) boundary. The older Loulja-A section, which covers the interval from ~5.59 to 5.12 Ma, reveals a dominantly precession-controlled color cyclicity that allows for a straightforward orbital tuning of the boundary interval and for detailed cyclostratigraphic correlations to the Mediterranean; the high-resolution and high-quality benthic isotope record allows us to trace the dominantly obliquity-controlled glacial history. Our results reveal that the M-P boundary coincides with a minor, partly precession-related shift to lighter "interglacial" values in d18O. This shift and hence the M-P boundary may not correlate with isotope stage TG5, as previously thought, but with an extra (weak) obliquity-controlled cycle between TG7 and TG5. Consequently, the M-P boundary and basal Pliocene flooding of the Mediterranean following the Messinian salinity crisis are not associated with a major deglaciation and glacio-eustatic sea level rise, indicating that other factors, such as tectonics, must have played a fundamental role. On the other hand, the onset of the Upper Evaporites in the Mediterranean marked by hyposaline conditions coincides with the major deglaciation step between marine isotope stage TG12 and TG11, suggesting that the associated sea level rise is at least partly responsible for the apparent onset of intermittently restricted marine conditions following the main desiccation phase. Finally, the Loulja-A section would represent an excellent auxiliary boundary stratotype for the M-P boundary as formally defined at the base of the Trubi marls in the Eraclea Minoa section on Sicily.

Sedimentology and Lead and Strontium isotope ratios of Miocene sediments from the Kerguelen Plateau

Characterization of sediment from Ocean Drilling Program Site 745, representing the East Kerguelen Ridge sediment drift, addresses important issues surrounding the timing of Miocene to present East Antarctic ice sheet stability and oceanic environmental change. Our results show three periods of greatly enhanced accumulation of Antarctic-derived sediment, at 6.4-5.9 Ma, 4.9-4.4 Ma and 1.1-0.8 Ma, potentially indicative of warmer, less stable ice sheets at these times. Conversely, the accumulation of Antarctic-derived material is comparatively less during the middle of the Pliocene warm epoch (4.8-3.2 Ma). The deep flow forming the Kerguelen drift was stronger during the latest Miocene and earliest Pliocene and has decreased in intensity continuously since then.

Oligocene-Miocene strontium isotopes of ODP Hole 120-747A

This study tests and improves on previously published early and middle Miocene 87Sr/86Sr marine correlations, presents Sr isotopic age correlations for this interval using the new timescale of Cande and Kent [1992 doi:10.1029/92JB01202], and evaluates Sr isotopic changes against an inferred glacioeustatic proxy. We generated a latest Oligocene to early late Miocene 87Sr/86Sr isotope record from Ocean Drilling Program (ODP) Hole 747A; this site provides an excellent magnetostratigraphic record during most of this interval for independent age estimates, very good foraminiferal preservation, and excellent core recovery. Comparisons of new 87Sr/86Sr data from Hole 747A with previously published data from Deep Sea Drilling Project (DSDP) Sites 608 [Miller et al., 1991 doi:10.1029/90PA01941] and 588 [Hodell et al., 1991 doi:10.1130/0091-7613(1991)019<0024:VITSIC>2.3.CO;2] yield the following results: (1) confirmation and refinement of the early Miocene Sr isotope changes, (2) improved definition of the timing of the changes in slope of 87Sr/86Sr near 15.4 Ma and 22.8 Ma, (3) improved Sr isotopic age resolution for the middle Miocene with resolution as good as +/- 0.7 m.y., and (4) identification of an inflection in the Sr isotope record at 28.0 Ma based on the combined records from DSDP Site 522 [Miller et al., 1988 doi:10.1029/PA003i002p00223] and ODP Hole 747A. We have been unable to determine the cause of middle Miocene offset between Site 588 and Hole 747A data, although we believe it may be attributed to problems in the age assignments for Hole 588A for the interval ~14-11 Ma and Site 747 for the interval 11-8 Ma. Because Hole 747A results provide a better chronology than Site 588 for most of the Miocene and a better middle Miocene Sr isotope record than Site 608, we propose that Hole 747A serves as the best reference section for Miocene 87Sr/86Sr variations from ca. 23 to 11 Ma. Using 87Sr/86Sr data from Sites 522, 608, and 747A, we relate late Eocene to early Miocene inflections in the 87Sr/86Sr isotope record to oxygen isotope increases and decreases inferred to represent glacioeustatic events. The decreases (deglaciations) observed in the ?18O record apparently lead the 87Sr/86Sr inflections by 1 to 1.5 m.y.

Cenozoic oxygen isotopic values for benthic foraminifera from DSDP and ODP low and high latitude marine sediment cores

The climate during the Cenozoic era changed in several steps from ice-free poles and warm conditions to ice-covered poles and cold conditions. Since the 1950s, a body of information on ice volume and temperature changes has been built up predominantly on the basis of measurements of the oxygen isotopic composition of shells of benthic foraminifera collected from marine sediment cores. The statistical methodology of time series analysis has also evolved, allowing more information to be extracted from these records. Here we provide a comprehensive view of Cenozoic climate evolution by means of a coherent and systematic application of time series analytical tools to each record from a compilation spanning the interval from 4 to 61 Myr ago. We quantitatively describe several prominent features of the oxygen isotope record, taking into account the various sources of uncertainty (including measurement, proxy noise, and dating errors). The estimated transition times and amplitudes allow us to assess causal climatological-tectonic influences on the following known features of the Cenozoic oxygen isotopic record: Paleocene-Eocene Thermal Maximum, Eocene-Oligocene Transition, Oligocene-Miocene Boundary, and the Middle Miocene Climate Optimum. We further describe and causally interpret the following features: Paleocene-Eocene warming trend, the two-step, long-term Eocene cooling, and the changes within the most recent interval (Miocene-Pliocene). We review the scope and methods of constructing Cenozoic stacks of benthic oxygen isotope records and present two new latitudinal stacks, which capture besides global ice volume also bottom water temperatures at low (less than 30°) and high latitudes. This review concludes with an identification of future directions for data collection, statistical method development, and climate modeling.

Stable isotope record of benthic foraminifera from DSDP Site 68-502

Benthic foraminiferal delta13C data from site 502 in the Caribbean Sea (sill depth ?1800 m) indicate that throughout the past 2.6 m.y., glacial delta13C values in the middepth Atlantic were higher during glaciations than interglaciations. This is interpreted as indicating a greater proportion of Upper North Atlantic Deep Water (UNADW) relative to southern source waters during glaciations. The contribution of UNADW during interglaciations to the middepth Atlantic remained approximately constant, and the contribution during glaciations may have been as much as 10 % higher in the late Pleistocene than in the late Pliocene. This small increase is in striking contrast to the much larger decrease in glacial Lower North Atlantic Deep Water (LNADW) contribution relative to southern sources, from about 80% to about 20%, that occurred over the past 2.6 m.y. Glacial intensification over the past 2.6 m.y. was probably coupled with a decrease in northward heat transport by the upper limb of the North Atlantic circulation cell, as was previously suggested on the basis of a LNADW record alone. Late Pleistocene (1 Ma-present) delta13C values in the Caribbean Sea were approximately 0.2? higher than they were from 2.6 to 2.0 Ma. The delta13C rise is not due to an increase in the mean ocean delta13C value, nor can it be entirely attributed to an increase in the proportion of high-delta13C source waters. An increase in the delta13C value of the surface source waters must have contributed to the delta13C rise.

Stable isotope record of Cibicidoides spp. from Late Miocene sediments of the Southern Ocean

Deepwater circulation plays an important role in climate modulation through its redistribution of heat and salt and its control of atmospheric CO2. Oppo and Fairbanks (1987, doi:10.1016/0012-821X(87)90183-X) showed that the Southern Ocean is an excellent monitor of deepwater circulation changes for two reasons: (1) the Southern Ocean is a mixing reservoir for incoming North Atlantic Deep Water and recirculated water from the Pacific and Indian oceans; and (2) the nutrient/delta13C tracers of deepwater are not significantly changed by surficial processes within the Southern Ocean. We can extend these principles to the late Miocene because tectonic changes in the Oligocene and early and middle Miocene developed near-modern basinal configurations. However, on these time scales, changes in the oceanic carbon reservoir and mean ocean nutrient levels also affect the delta13C differences between ocean basins. From 9.8 to 9.3 Ma, Southern Ocean delta13C values oscillated between high North Atlantic values and low Pacific values. The Southern Ocean recorded delta13C values similar to Pacific values from 9.2 to 8.9 Ma, reflecting a low contribution of Northern Component Water (NCW). The delta13C differences between the NCW and Pacific Outflow Water (POW) end-members were low from 8.9 to 8.0 Ma, making it difficult to discern circulation patterns. NCW production may have completely shutdown at 8.6 Ma, allowing Southern Component Water (SCW) to fill the North Atlantic and causing the delta13C values in the North Atlantic, Pacific, and Southern oceans to converge. Deepwater delta13C patterns resembling the modern distributions evolved by 7.0 Ma: delta13C values were near 1.0 per mil in the North Atlantic; 0.0 per mil in the Pacific; and 0.5 per mil in the Southern Ocean. Development of near-modern delta13C distributions by 7.0 Ma resulted not only from an increase in NCW flux but also from an increase in deepwater nutrient levels. Both of these processes increased the delta13C difference between the North Atlantic and Pacific oceans. Deepwater circulation patterns similar to today's operated as early as 9.8 Ma, but were masked by the lower nutrient/delta13C differences. During the late Miocene, 'interglacial' intervals prevailed during intervals of NCW production, while 'glacial' intervals occurred during low NCW production.

Stable isotope record of Cibicidoides spp. from early and middle Miocene sediments

The middle Miocene delta18O increase represents a fundamental change in the ocean-atmosphere system which, like late Pleistocene climates, may be related to deepwater circulation patterns. There has been some debate concerning the early to early middle Miocene deepwater circulation patterns. Specifically, recent discussions have focused on the relative roles of Northern Component Water (NCW) production and warm, saline deep water originating in the eastern Tethys. Our time series and time slice reconstructions indicate that NCW and Tethyan outflow water, two relatively warm deepwater masses, were produced from ~20 to 16 Ma. NCW was produced again from 12.5 to 10.5 Ma. Another feature of the early and middle Miocene oceans was the presence of a high delta13C intermediate water mass in the southern hemisphere, which apparently originated in the Southern Ocean. Miocene climates appear to be related directly to deepwater circulation changes. Deep-waters warmed in the early Miocene by ~3°C (?20 to 16 Ma) and cooled by a similar amount during the middle Miocene delta18O increase (14.8 to 12.6 Ma), corresponding to the increase (?20 Ma) and subsequent decrease (~16 Ma) in the production of NCW and Tethyan outflow water. Large (>0.6 per mil), relatively rapid (~0.5 m.y.) delta18O increases in both benthic and planktonic foraminifera (i.e., the Mi zones of Miller et al. (1991a) and Wright and Miller (1992a)) were superimposed in the long-term deepwater temperature changes; they are interpreted as reflecting continental ice growth events. Seven of these m.y. glacial/interglacial cycles have been recognized in the early to middle Miocene. Two of these glacial/interglacial cycles (Mi3 and Mi4) combined with a 2° to 3°C decrease in deepwater temperatures to produce the middle Miocene delta18O shift.

Chronostratigraphy of sediments from the Bahamas

Continuous cores drilled during the Bahamas Drilling Project (BDP) and the Ocean Drilling Program (ODP) Leg 166 along a transect from the top of Great Bahama Bank to the basin in the Straits of Florida provide a unique data set to test the assumption in seismic stratigraphy that seismic reflections are time lines and, thus, have a chronostratigraphic significance. Seismic reflections that are identified as seismic sequence boundaries (SSBs) were dated by means of biostratigraphy in the five ODP sites and by a combination of biostratigraphy, magnetostratigraphy and Sr isotope stratigraphy in the two BDP sites. The seismic reflection horizons are carried across a variety of facies belts from shallow-water carbonates over slope carbonates to drift deposits in the Straits of Florida. Within this system 17 SSBs were identified and dated. Despite the fact that the seismic reflections cross several facies belts, their ages remain remarkably constant. The average offset in all sites is 0.38 Myr. In no cases do the seismic reflections cut across time lines. The age differences are the combined result of the biostratigraphic sampling frequency, the spacing of marker species that required extrapolation of ages, and the resolution of the seismic data. In addition, uncertainties of age determination in the proximal sites where age-diagnostic fauna are rare add to the age differences between sites. Therefore, it can be concluded that the seismic reflections, which mark the SSBs along the Bahamas Transect, are time lines and can be used as stratigraphic markers. This finding implies that depositional surfaces are preferentially imaged by reflected seismic waves and that an impedance contrast exists across these surfaces. Facies successions across the sequence boundaries indicate that the sequence boundaries coincide with the change of deposition from times of high to low sea level. In the carbonate setting of Great Bahama Bank, sea-level changes produce changes in sediment composition, sedimentation rate and diagenesis from the platform top to the basin. The combination of these factors generates differences in sonic velocity and, thus, in impedance that cause the seismic reflection. The impedance contrasts decrease from the proximal to the distal sites, which is reflected in the seismic data by a decrease of the seismic amplitude in the basinal area.

Benthic foraminiferal biofacies and stable isotopic record of St. Stephens Quarry, Alabama

We integrate upper Eocene-lower Oligocene lithostratigraphic, magnetostratigraphic, biostratigraphic, stable isotopic, benthic foraminiferal faunal, downhole log, and sequence stratigraphic studies from the Alabama St. Stephens Quarry (SSQ) core hole, linking global ice volume, sea level, and temperature changes through the greenhouse to icehouse transition of the Cenozoic. We show that the SSQ succession is dissected by hiatuses associated with sequence boundaries. Three previously reported sequence boundaries are well dated here: North Twistwood Creek-Cocoa (35.4-35.9 Ma), Mint Spring-Red Bluff (33.0 Ma), and Bucatunna-Chickasawhay (the mid-Oligocene fall, ca. 30.2 Ma). In addition, we document three previously undetected or controversial sequences: mid-Pachuta (33.9-35.0 Ma), Shubuta-Bumpnose (lowermost Oligocene, ca. 33.6 Ma), and Byram-Glendon (30.5-31.7 Ma). An ~0.9 per mil d18O increase in the SSQ core hole is correlated to the global earliest Oligocene (Oi1) event using magnetobiostratigraphy; this increase is associated with the Shubuta-Bumpnose contact, an erosional surface, and a biofacies shift in the core hole, providing a first-order correlation between ice growth and a sequence boundary that indicates a sea-level fall. The d18O increase is associated with a eustatic fall of ~55 m, indicating that ~0.4 per mil of the increase at Oi1 time was due to temperature. Maximum d18O values of Oi1 occur above the sequence boundary, requiring that deposition resumed during the lowest eustatic lowstand. A precursor d18O increase of 0.5 per mil (33.8 Ma, midchron C13r) at SSQ correlates with a 0.5 per mil increase in the deep Pacific Ocean; the lack of evidence for a sea-level change with the precursor suggests that this was primarily a cooling event, not an ice-volume event. Eocene-Oligocene shelf water temperatures of ~17-19 °C at SSQ are similar to modern values for 100 m water depth in this region. Our study establishes the relationships among ice volume, d18O, and sequences: a latest Eocene cooling event was followed by an earliest Oligocene ice volume and cooling event that lowered sea level and formed a sequence boundary during the early stages of eustatic fall.

An integrated biostratigraphy (conodonts and foraminifers) and chronostratigraphy (paleomagnetic reversals, magnetic susceptibility, elemental chemistry, carbon isotopes and geochronology) for the Permian-Triassic strata of Guandao section

The chronostratigraphy of Guandao section has served as the foundation for numerous studies of the end-Permian extinction and biotic recovery in south China. Guandao section is continuous from the Permian-Triassic boundary to the Upper Triassic.Conodonts enable broad delineation of stage and substage boundaries and calibration of foraminifer biostratigraphy as follows. Changhsingian- Griesbachian: first Hindeodus parvus, and first appearance of foraminifers Postcladella kalhori and Earlandia sp. Griesbachian-Dienerian: first Neospathodus dieneri, and last appearance of foraminifer P. grandis. Dienerian-Smithian: first Novispathodus waageni and late Dienerian first appearance of foraminifer Hoyenella ex gr. sinensis. Smithian-Spathian: first Nv? crassatus and last appearance of foraminifers Arenovidalina n. sp. and Glomospirella cf. vulgaris. Spathian-Aegean: first Chiosella timorensis and first appearance of foraminifer Meandrospira dinarica. Aegean-Bithynian: first Nicoraella germanica and first appearance of foraminifer Pilammina densa. Bithynian-Pelsonian: after last Neogondolella regalis, prior to first Paragondolella bulgarica and first appearance of foraminifer Aulotortus eotriasicus. Pelsonian-Illyrian: first Pg. excelsa and last appearance of foraminifers Meandrospira ? deformata and Pilamminella grandis. Illyrian-Fassanian: first Budurovignathus truempyi, and first appearance of foraminifers Abriolina mediterranea and Paleolituonella meridionalis. Fassanian-Longobardian: first Bv. mungoensis and last appearance of foraminifer A. mediterranea. Longobardian-Cordevolian: first Quadralella polygnathiformis and last appearance of foraminifers Turriglomina mesotriasica and Endotriadella wirzi. The section contains primary magnetic signature with frequent reversals occurring around the Permian-Triassic, Olenekian-Anisian, and Anisian-Ladinian boundaries. Predominantly normal polarity occurs in the lower Smithian, Bithynian, and Longobardian-Cordevolian. Predominantly reversed polarity occurs in the upper Griesbachian, Induan-Olenekian, Pelsonian and lower Illyrian. Reversals match well with the GPTS. Large amplitude carbon isotope excursions, attaining values as low as -2.9 per mil d13C and high as +5.7 per mil d13C, characterize the Lower Triassic and basal Anisian. Values stabilize around +2 per mil d13C through the Anisian to Carnian. Similar signatures have been reported globally. Magnetic susceptibility and synthetic gamma ray logs show large fluctuations in the Lower Triassic and an overall decline in magnitude of fluctuation through the Middle and Upper Triassic. The largest spikes in magnetic susceptibility and gamma ray, indicating greater terrestrial lithogenic flux, correspond to positive d13C excursions. Several volcanic ash horizons occur in the Lower Triassic and Olenekian-Anisian boundary. High resolution U-Pb analysis of zircons provide a robust age of 247.2 Ma for the Olenekian-Anisian boundary.