Fay Woodruff

Mid-Miocene isotope stratigraphy in the deep sea : high-resolution correlations, paleoclimatic cycles, and sediment preservation

Mid-Miocene pelagic sedimentary sections can be correlated using intermediate and high resolution oxygen and carbon isotopic records of benthic foraminifera. Precision of a few tens of thousands of years is readily achievable at sites with high sedimentation rates, for example, Deep Sea Drilling Project sites 289 and 574. The mid-Miocene carbon isotope records are characterized by an interval of high δ13C values between 17 and 13.5 Ma (the Monterey Excursion of Vincent and Berger 1985) upon which are superimposed a series of periodic or quasi-periodic fluctuations in δ13C values. These fluctuations have a period of approximately 440 kyr, suggestive of the 413 kyr cycle predicted by Milankovitch theory. Vincent and Berger proposed that the Monterey Excursion was the result of increased organic carbon burial in continental margins sediments. The increased δ13C values (called 13C maxima) superimposed on the generally high mid-Miocene signal coincide with increases in δ18O values suggesting that periods of cooling and/or ice buildup were associated with exceptionally rapid burial of organic carbon and lowered atmospheric CO2 levels. It is likely that during the Monterey Excursion the ocean/atmosphere system became progressively more sensitive to small changes in insolation, ultimately leading to major cooling of deep water and expansion of continental ice. We have assigned an absolute chronology, based on biostratigraphic and magneto-biostratigraphic datum levels, to the isotope stratigraphy and have used that chronology to correlate unconformities, seismic reflectors, carbonate minima, and dissolution intervals. Intervals of sediment containing 13C maxima are usually better preserved than the overlying and underlying sediments, indicating that the δ13C values of TCO2 in deep water and the corrosiveness of seawater are inversely correlated. This again suggests that the 13C maxima were associated with rapid burial of organic carbon and reduced levels of atmospheric CO2. The absolute chronology we have assigned to the isotopic record indicates that the major mid-Miocene deepwater cooling/ice volume expansion took 2 m.y. and was not abrupt as had been reported previously. The cooling appears abrupt at many sites because the interval is characterized by a number of dissolution intervals. The cooling was not monotonic, and the 2 m.y. interval included an episode of especially rapid cooling as well as a brief return to warmer conditions before the final phase of the cooling period. The increase in δ18O values of benthic foraminifera between 14.9 and 12.9 Ma was greatest at deeper water sites and at sites closest to Antarctica. The data suggest that the δ18O value of seawater increased by no more than about 1.1 per mil during this interval and that the remainder of the change in benthic δ18O values resulted from cooling in Antarctic regions of deepwater formation. Equatorial planktonic foraminifera from sites 237 and 289 exhibit a series of 0.4 per mil steplike increases in δ13C values. Only one of these increases in planktonic δ13C is correlated with any of the features in the mid-Miocene benthic carbon isotope record.

Biological fractionation of oxygen and carbon isotopes by recent benthic foraminifera

Abstract Recent deep-sea benthic foraminifera from five East Pacific Rise box core tops have been analyzed for oxygen and carbon isotopic composition. The five equatorial stations, with water depths of between 3200 and 4600 m, yielded fourteen specific and generic taxonomic groups. Of the taxa analyzed, Uvigerina spp. most closely approaches oxygen isotopic equilibrium with ambient sea water. Pyrgo spp. was next closest to isotopic equilibrium, being on the average 0.59 ‰ depleted in 18 O relative to Uvigerina spp. Oridorsalis umbonatus also has relatively high δ 18 O values. Most other taxa were depleted in 18 O by large amounts. In no taxa was the carbon in the CaCO 3 secreted in carbon isotopic equilibrium with the dissolved HCO 3 − of ambient sea water.

Variations in the strontium isotopic ratio of seawater during the Miocene: Stratigraphic and geochemical implications

A composite strontium isotopic seawater curve was constructed for the Miocene between 24 and 6 Ma by combining 87Sr/86Sr measurements of planktonic foraminifera from Deep Sea Drilling Project sites 289 and 588. Site 289, with its virtually continuous sedimentary record and high sedimentation rates (26 m/m.y.), was used for constructing the Oligocene to mid-Miocene part of the record, which included the calibration of 63 biostratigraphic datums to the Sr seawater curve using the timescale of Cande and Kent (1992). Across the Oligocene/Miocene boundary, a brief plateau occurred in the Sr seawater curve (87Sr/86Sr values averaged 0.70824) which is coincident with a carbon isotopic maximum (CM-O/M) from 24.3 to 22.6 Ma. During the early Miocene, the strontium isotopic curve was marked by a steep rise in 87Sr/86Sr that included a break in slope near 19 Ma. The rate of growth was about 60 ppm/m.y. between 22.5 and 19.0 Ma and increased to over 80 ppm/m.y. between 19.0 and 16 Ma. Beginning at ∼16 Ma (between carbon isotopic maxima CM3 and CM4 of Woodruff and Savin (1991)), the rate of 87Sr/86Sr growth slowed and 87Sr/86Sr values were near constant from 15 to 13 Ma. After 13 Ma, growth in 87Sr/86Sr resumed and continued until ∼9 Ma, when the rate of 87Sr/86Sr growth decreased to zero once again. The entire Miocene seawater curve can be described by a high-order function, and the first derivative (d87Sr/86Sr/dt) of this function reveals two periods of increased slope. The greatest rate of 87Sr/86Sr change occurred during the early Miocene between ∼20 and 16 Ma, and a smaller, but distinct, period of increased slope also occurred during the late Miocene between ∼12 and 9 Ma. These periods of steepened slope coincide with major phases of uplift and denudation of the Himalayan-Tibetan Plateau region, supporting previous interpretations that the primary control on seawater 87Sr/86Sr during the Miocene was related to the collision of India and Asia. The rapid increase in 87Sr/86Sr values during the early Miocene from 20 to 16 Ma imply high rates of chemical weathering and dissolved riverine fluxes to the oceans. In the absence of another source of CO2, these high rates of chemical weathering should have quickly resulted in a drawdown of atmospheric CO2 and climatic cooling through a reversed greenhouse effect. The paleoclimatic record, however, indicates a warming trend during the early Miocene, culminating in a climatic optimum between 17 and 14.5 Ma. We suggest that the high rates of chemical erosion and warm temperatures during the climatic optimum were caused by an increase in the contribution of volcanic CO2 from the eruption of the Columbia River Flood Basalts (CRFB) between 17 and 15 Ma. The decrease in the rate of CRFB eruptions at 15 Ma and the removal of atmospheric carbon dioxide by increased organic carbon burial in Monterey deposits eventually led to cooling and increased glaciation between ∼14.5 and 13 Ma. The CRFB hypothesis helps to explain the significant time lag between the onset of increased rates of organic carbon burial in the Monterey at 17.5 Ma (as marked by increased δ13C values) and the climatic cooling and glaciation during the middle Miocene (as marked by the increase in δ18O values), which did not begin until ∼14.5 Ma.

Miocene benthic foraminiferal isotope records: A synthesis

Abstract 18 O 16 O and 13 C 12 C ratios of Miocene benthic foraminifera from a number of Atlantic, Pacific and Indian Ocean DSDP sites (71, 77B, 206, 208, 238, 279, 289, 296, 329, 357 and 366A) have been compiled. These provide a rather detailed history of Miocene deep water especially in the Pacific Ocean. Bottom-water temperatures rose during the early Miocene and then declined rapidly during the middle Miocene. This decline was accompanied by an increase in Antarctic glaciation. Late Miocene bottom temperatures and Antarctic ice volumes are inferred to be similar to todays, but exhibited some fluctuation. The early Miocene ocean was less thermally stratified at intermediate and abyssal depths while the late Miocene deep ocean had a thermal structure generally similar to the modern ocean. Foraminiferal carbon isotope ratios at most of the sites varied quasi-sympathetically throughout the Miocene. These variations must reflect comparable variations in the mean 13 C 12 C of marine HCO 3 − . However, the causes of such variations are not yet clear.

Late Miocene marine carbon-isotopic shift and synchroneity of some phytoplanktonic biostratigraphic events

A search for stable-isotopic signals and biostratigraphic events in Deep Sea Drilling Project (DSDP) cores to improve chronologic resolution with an aim to reconstruct the paleoenvironment of the preglacial and postglacial Miocene oceans has led to the recognition of an apparently global decrease in the benthic foraminiferal δ 13 C in the latest Miocene. This carbon-isotopic shift is consistently bracketed by the first evolutionary appearances of several taxa of phytoplankton the ages of which have been accurately estimated from paleomagnetically dated piston cores. The first appearance of nannofossils Amaurolithus primus and A. delicatus at 6.25 m.y. B.P. and the diatoms Thalassiosira praeconvexa and Nitzschia miocenica elongata at 6.10 and 6.00 m.y. B.P., respectively, and the carbon-isotopic shift itself (dated between 6.10 and 5.90 m.y. B.P.) provide convenient synchronous events to aid in the reconstruction of the late Miocene world ocean. Magnetostratigraphically estimated ages of other useful late Miocene nannofossil events include first appearances of Discoaster quinqueramus at 8.00 m.y. B.P., D. surculus at 6.40 m.y. B.P., Amaurolithus tricorniculatus s.s. at 5.70 m.y. B.P., A. amplificus at 5.65 m.y. B.P., and Ceratolithus acutus at 5.20 m.y. B.P., and the last appearances of D. quinqueramus at 5.45 m.y. B.P. and A. amplificus at 5.30 m.y. B.P.

δ13C values of Miocene Pacific benthic foraminifera: Correlations with sea level and biological productivity

13 C/ 12 C ratios of Miocene benthic foraminifera from 22 Pacific Ocean sites vary with time but are similar at almost all sites in any restricted interval. δ 13 C values are correlated with sea levels inferred from onlap/offlap curves, reflecting the deposition of greater amounts of organic matter on the continental shelves during transgressions. Differences in δ 13 C between sites are correlated with local differences in biological productivity in the overlying surface waters. 13 C/ 12 C values of benthic foraminifera show promise as indicators of marine paleoproductivity.

Response of deep-sea benthic foraminifera to Miocene paleoclimatic events, DSDP Site 289

Abstract Changes in the Miocene deep-sea benthic foraminifera at DSDP Site 289 closely correlate to the climatically induced variations in deep and bottom waters in the Pacific Ocean. In early Miocene time, oxygen and carbon isotopes indicate that bottom waters were relatively warm and poorly oxygenated. Benthic foraminiferal assemblages are characterized by various species inherited from the Oligocene. Expansion of the Antarctic icecap in the early middle Miocene, 14–16 m.y. ago, increased oxygen isotope values, produced cold, more oxygenated bottom waters and lead to a turnover in the benthic foraminifera. An Oligocene—early Miocene assemblage was replaced by a cibicidoid-dominated assemblage. Some species became extinct and benthic faunas became more bathymetrically restricted with the increased stratification of deep waters in the ocean. In mid-Miocene time, Epistominella exigua and E. umbonifera , indicative of young, oxygenated bottom waters, are relatively common at DSDP Site 289. Further glacial expansion 5–9 m.y. ago lowered sealevel, increased oceanic upwelling and associated biological productivity and intensified the oxygen minima. Abundant hispid and costate uvigerines become a dominant faunal element at shallow depths above 2500 m as E. umbonifera becomes common to abundant below 2500 m. By late Miocene time, benthic faunas similar in species composition and proportion to modern faunas on the Ontong-Java plateau, had become established.

Discrepancies in the oceanic carbon isotope record for the last fifteen million years

Abstract The S hackleton and H hall (1984) record of carbon isotope ratios for bulk CaCO3 from DSDP sites 525 to 528 in the South Atlantic has been widely used as an indicator of how the 13 C 12 C for CaCO3 leaving the ocean has changed over the last 68 million years. Of particular interest is the approximately 2%. decrease which occurred during the last 15 million years, for it suggests a change in the O2 content of the atmosphere (S hackleton , 1987). As a check on the assumption that this record serves as a global indicator, we have compiled measurements on planktonic foraminifera from a number of tropical cores. We find that this data set suggests a much smaller 13 C 12 C decrease over the same time interval. We can find no satisfactory explanation for this difference and hence can conclude only that more measurements and thought will be required before the time trend in carbon isotope composition can be established.

Miocene Deep-Sea Benthic Foraminiferal Faunal Changes in Pacific: ABSTRACT

Miocene deep-sea benthic foraminifera, analyzed from numerous Pacific DSDP sites, are found to respond to climatically induced oceanic variations by: (1) changes in depth distribution with time; (2) changes in species proportion within assemblages; and (3) becoming extinct. Because benthic species are long-ranging, many species occurring today were present in the Miocene and provide a basis for studying Miocene paleo-oceanographic changes. Analyses of ^dgrO18 and ^dgrC13 compositions of benthic foraminifera which record fluctuations in paleotemperatures and in the marine HCO3 pool reveal major changes between the early and late Miocene. Shifts in benthic foraminiferal populations and isotopic compositions during the Miocene imply the following water mass changes: (1) early Miocene deep waters appear to have been warmer with older, light ^dgrC13; and (2) a sharp middle Miocene increase in ^dgrO18 which we interpret to be a major bottom water cooling concomitant with Antarctic glacial buildup and thickening of the Antarctic bottom waters, restructuring the Miocene ocean and increasing the equatorial thermal gradient. Benthic fauna species dominance, species assemblage, and water depth indicate that by the late Miocene, both modern benthic foraminiferal assemblages and modern oceanographic conditions were approached. Intensification of the oxygen minimum zone in the late Miocene is supported by the dominance of Uvigerina fauna in the west Pacific. End_of_Article - Last_Page 1009------------