Foraminiferal oxygen and carbon isotopic measurements from 5 sites of southern Atlantic and Pacific

The stratigraphy and paleoceanography of the late Miocene and early Pliocene have been examined at six sites in the South Atlantic and southwest Pacific oceans: Deep Sea Drilling Project (DSDP) sites 284, 516A, 519, 588, and 590 and two piston cores from Chain cruise 115. A consistent stratigraphy was developed among sites using graphic correlation, which resulted in age models for all sites that are tied to the revised paleomagnetic time scale of Berggren et al. (1985). Applying these chronologies, we assessed latitudinal and interocean contrasts in the stratigraphic ranges of late Miocene-early Pliocene planktonic foraminiferal and nanno - fossil datums. Salient stratigraphic results include (1) The last appearance datum (LAD) of Globoquadrina dehiscens is a late Miocene (approx. 6.4 Ma) event in the subtropics and is not useful for the placement of the Miocene/Pliocene (M/P) boundary in this biogeographic province. (2) The first appearance datum (FAD) of Globorotalia crassaformis occurred at 5.1 Ma in the South Atlantic near the M/P boundary, suggesting that Gr. crassaformis may have first evolved in the South Atlantic and later migrated to other regions. (3) In the southwest Pacific, the FADs of Gr. margaritae (5.97 Ma), Gr. puncticulata (5.09 Ma), and Gr. crassaformis (4.87 Ma) are significantly time transgressive between temperate and warm subtropical regions. Time lags of 1.0 m.y. were required for these species to adapt to physical and/or biotic conditions peripheral to their endemic biogeographic provinces. (4) Between the subtropics of the South Atlantic and southwest Pacific, many planktonic foraminiferal datums (FAD of Dentogloboquadrina altispira, Gr. cibaoensis, Gr. conomiozea, Gr. margaritae, and Gq. dehiscens and LAD of Gr. cibaoensis) markedly depart from the correlation suggested by magnetostratigraphy, indicating that these datum levels are unreliable for correlation between these ocean basins. (5) In contrast, available calcareous nannofossil datum levels fall on or near the paleomagnetic correlation line, indicating synchroneity of events within the subtropics. (6) Biostratigraphic, magnetic, and 87Sr/86Sr correlation between sites 588 and 519 and the M/P neostratotype at Capo Rossello, Sicily, suggests that the base of the Zanclean stratotype occurs at 5.1-5.0 Ma in the lower reversed subchron of the Gilbert, about 2-3 * 10**5 years above the Gilbert/Chron 5 boundary. Oxygen isotopic results from DSDP sites 284, 519, and CH115 piston cores confirm a prolonged benthic d18O increase in the latest Miocene between 5.6 and 5.0 Ma, as originally proposed by Shackleton and Kennett (1975). At DSDP site 588, the benthic d18O record in the latest Miocene is marked by high-frequency fluctuations with amplitude variations of 0.5per mill, and a long-period wavelength component of 400,000 years. Maximum d18O values, however, occurred during the late Miocene (Kapitean Stage) between 5.5 and 5.1 Ma. The late Miocene d18O changes resulted from mid- and high-latitude cooling and pulses of ice sheet expansion and contraction. Glacial events were most intense during the latest Miocene (Kapitean Stage), and occurred at 5.50-5.35 Ma and at 5.10 Ma. Glacial events are estimated to have lowered sea level by 40 to 60 m and contributed to the isolation and desiccation of the Mediterranean Basin during the late Messinian. Interglacial conditions prevailed at 5.2 Ma and between 5.0 and 4.1 Ma in the early Pliocene. The beginning of the Pliocene was marked by changes in many proxy climatic indicators at all sites, suggesting a prolonged interval of warm, interglacial conditions between 5.0 and 4.1 Ma during the earliest Pliocene.

Oxygen and carbon isotopic composition of benthic and planktonic foraminifers at DSDP Site 610

A record of carbon and oxygen isotopes in benthic and planktonic foraminifers has been obtained from the interval corresponding to the last 2.4 m.y. of Site 610, Holes 610 and 610A, with a sample resolution of about 30 kyr. The record from the late Quaternary (<0.9 Ma) shows large amplitudes and high frequencies in oxygen isotopic variation. Prior to 0.9 Ma the isotopic variability record is reduced in amplitude (but not in frequency) compared with the late Quaternary, suggesting lower ice-volume and climatic fluctuations, and higher average eustatic sea level. Left-coiling (L, polar) Neogloboquadrinapachyderma were not found in samples between 1.0 and 2.2 Ma, indicating less influence of polar front migrations in the Northeast Atlantic. Both polar planktonic faunas and larger isotope fluctuations reappear in the lowermost samples (2.3 to 2.4 Ma), pointing toward a period of larger climatic variability in the late Pliocene than in the early Quaternary. The variation in benthic d13C and hence in deep-water d13C seems to have been constant through the analyzed section, reflecting a stable variability in the production of North Atlantic Deep Water (NADW) and possibly in Norwegian-Greenland Sea Overflow. Preliminary analyses of amino-acid epimerization in N. pachyderma (L) indicate a constant rate of epimerization to approximately 0.3 Ma. Beneath this level the average epimerization rate is much reduced.

(Table 1) Stable oxygen isotopic ratios and yearly snow accumulation of firn and ice cores from Svalbard

We analyse ice cores from Vestfonna ice cap (Nordaustlandet, Svalbard). Oxygen isotopic measurements were made on three firn cores (6.0, 11.0 and 15.5 m deep) from the two highest summits of the glacier located on the SW-NE and NW-SE central ridges. Sub-annual d18O cycles were preserved and could be counted visually in the uppermost parts of the cores, but deeper layers were affected by post-depositional smoothing. A pronounced d18O minimum was found near the bottom of the three cores. We consider candidates for this d18O signal to be a valuable reference horizon since it is also seen elsewhere in Nordaustlandet. We attribute it to isotopically depleted snow precipitation, which NCEP/NCAR reanalysis shows was unusual for Vestfonna, and came from northerly air during the cold winter of 1994/95. Finding the 1994/95 time marker allows establishment of a precise depth/age scale for the three cores. The derived annual accumulation rates indirectly fill a geographical gap in mass balance measurements and thus provide information on spatial and temporal variability of precipitation over the glacier for the period spanned by the cores (1992-2009). Comparing records at the two locations also reveals that the snow net accumulation at the easternmost part of Vestfonna was only half of that in the western part over the last 17 years.

(Table T1) Stable oxygen and carbon isotopic composition of benthic foraminifers in ODP Site 199-1218 sediments

The transition from the late Oligocene warm period into the early Miocene was marked by a series of rapid and brief episodes of cryospheric expansion and global cooling. We analyzed benthic foraminifers from nannofossil oozes recovered at Ocean Drilling Program Site 1218 to construct a stable isotope stratigraphy for the deep Pacific.