Middle Miocene stable carbon and oxygen isotope record of ODP Holes 130-805B and 130-806B

Bulk carbon isotope records are an effective chemostratigraphic tool for the middle Miocene because of the large and systematic variation in first-order d13C signals. Bulk d13C measurements support the presence of a hiatus at 305 mbsf in Hole 805B (latest middle Miocene), provisionally located while on board ship using biostratigraphic and magnetostratigraphic events. Records at Holes 805B and 806B show the middle Miocene Monterey carbon isotope excursion although the record at Hole 806B is apparently more stratigraphically continuous. Detailed analysis of multispecies foraminiferal carbon isotope records during the middle Miocene ("Monterey excursion") segment at Hole 806B support the assertion that this carbon isotope excursion comprises mainly between-reservoir effects. The benthic d18O data increase after 15.3 Ma, which we suggest corresponds to the mid-Miocene cooling step/ice volume increase of other authors. Planktonic foraminiferal d18O evidence exists for steepening of the thermocline at 17.4 Ma. A second-order d13C excursion superimposed at 13.8 Ma on the first-order Monterey excursion is associated with a second-order negative d18O excursion.

Stable isotope record of foraminifera from sediment core EW9209-1JPC

Stable isotopic measurements of G. sacculifer and C. wuellerstorfi in a core from the western equatorial Atlantic imply that there are parallel, suborbital oscillations in surface water hydrography and deep water circulation occurring during oxygen isotope stages 2 and 3. Low values of G. sacculifer delta18O accompany high values of C. wuellerstorfi delta13C, linking warmer sea surface temperatures (SSTs) in the tropics with increased production of lower North Atlantic Deep Water (NADW). The amplitude of the delta18O oscillations is 0.6 per mil (or 2°-3°C), which is superimposed on a glacial/interglacial amplitude of about 2.1per mil. Using the G. sacculifer delta18O data, we calculate that surface waters were colder during stage 2 than calculated by CLIMAP [1976, 1981]. The longer-period (>2 kyr) oscillations in air temperature recorded in the Greenland and Antarctic ice cores appear to correlate with oscillations in sea surface temperature in the equatorial Atlantic. The magnitude of these oscillations in tropical SST is too large to have resulted from changes in meridional heat transport caused by the global conveyor alone. The apparent synchroneity of equatorial SST and polar air temperature changes, as well as the amplitude of the SST changes at the equator, are consistent with the climate effects expected from changes in the atmosphere's greenhouse gas content (H2Ovapor, CO2, and CH4).

Strontium and oxygen isotope data for DSDP Hole 44-390A and ODP Leg 171 holes

Differences in regional responses to climate fluctuations are well documented on short time scales (e.g., El Niño-Southern Oscillation), but with the exception of latitudinal temperature gradients, regional patterns are seldom considered in discussions of ancient greenhouse climates. Contrary to the expectation of global warming or global cooling implicit in most treatments of climate evolution over millions of years, this paper shows that the North Atlantic warmed by as much as 6°C (1.5% decrease in d18O values of planktic foraminifera) during the Maastrichtian global cooling interval. We suggest that warming was the result of the importation of heat from the South Atlantic. Decreasing North Atlantic d18O values are also associated with increasing gradients in planktic d13C values, suggesting increasing surface-water stratification and a correlated strengthening of the North Atlantic Polar Front. If correct, this conclusion predicts arctic cooling during the late Maastrichtian. Beyond implications for the Maastrichtian, these data demonstrate that climate does not behave as if there is a simple global thermostat, even on geologic time scales.

Oxygen isotope composition of benthic foraminifera ofthe Florida Street

We present a new method for the quantitative reconstruction of upper ocean flows for during times in the past. For the warm (T>5°C) surface ocean, density can be accurately reconstructed from calcite precipitated in equilibrium with seawater, as both of these properties increase with decreasing temperature and increasing salinity. Vertical density profiles can be reconstructed from the oxygen isotopic composition of benthic foraminifera. The net volume transport between two vertical density profiles can be calculated using the geostrophic method. Using benthic foraminifera from surface sediment samples from either side of the Florida Straits (Florida Keys and Little Bahama Bank), we reconstruct two vertical density profiles and calculate a volume transport of 32 Sv using this method. This agrees well with estimates from physical oceanographic methods of 30-32 Sv for the mean annual volume transport. We explore the sensitivity of this technique to various changes in the relationship between temperature and salinity as well as salinity and the oxygen isotopic composition of seawater.

Density and stable oxygen isotope profiles of four snow pits from Berkner Island, Filchner-Ronne Ice Shelf, Antarctica

The ice cap on Berkner Island is grounded on bedrock within the Filchner-Ronne Ice Shelf and is, therefore, expected to be a well-suited place to retrieve long-term ice-core records reflecting the environmental situation of the Weddell Sea region. Shallow firn cores were drilled to 11 m at the two main summits of Berkner Island and analysed in high depth resolution for electrical d.c. conductivity (ECM), stable isotopes, chloride, sulphate, nitrate and methane-sulphonate (MSA). From the annual layering of dD and non-sea-salt (nss) sulphate, a mean annual snow accumulation of 26.6 cm water at the north dome and 17.4 cm water at the south dome are obtained. As a result of ineffective wind scouring indicated by a relatively low near-surface snow density, regular annual cycles are found for all species at least in the upper 4-5 m. Post depositional changes are responsible for a substantial decrease of the seasonal dD and nitrate amplitude as well as for considerable migration of the MSA signal operating below a depth of 3-4 m. The mean chemical and isotopic firn properties at the south dome correspond to the situation on the Filchner-Ronne Ice shelf at a comparable distance to the coast, whereas the north dome is found to be more influenced by maritime air masses. Persistent high sea-salt levels in winter snow at Berkner Island heavily obscure the determination of nss sulphate probably due to sulphate fractionation in the Antartic sea-salt aerosols. Estimated time-scales predict ages at 400 m depth to be ca. 2000 years for the north and ca. 3000 years for the south dome. Pleistocene ice is expected in the bottom 200 and 300 m, respectively.