(Table 1) Stable oxygen isotope ratios of Globigerinoides sacculifer of ODP Hole 130-806B

A detailed Pliocene oxygen isotope record from the Ontong Java Plateau, based on measurements of the surface-dwelling planktonic foraminifer Globigerinoides sacculifer, was produced for the period from 5 to 2 Ma. The record documents major long- and short-term climate changes. The results show periods of enhanced ice volume at 4.6 to 4.3 Ma and after 2.85 Ma, a long-term warming trend from 4.1 to 3.7 Ma, and a distinct cooling trend that was initiated at 3.5 Ma and progressed through the initiation of large-scale Northern Hemisphere glaciation after 2.85 Ma (according to the time scale of Shackleton and others proposed in 1990). Periods of high average ice volumes also show the highest d18O amplitudes. The pattern of climate cyclicity changed markedly at about 2.85 Ma. Earlier times were marked by high-frequency variability at the precessional frequencies or even higher frequencies, pointing to low-latitude processes as a main controlling factor driving planktonic d18O variability in this period. The high-frequency variability is not coherent with insolation and points to strong nonlinearity in the way the climate system responded to orbital forcing before the onset of large scale Northern Hemisphere glaciation. After 3 Ma, stronger 41-k.y. cyclicity appears in the record. The shift in pattern is clearest around 2.85 Ma (according to the time scale proposed by Shackleton and others in 1990), 100-200 k.y. before the most dramatic spread of Northern Hemisphere ice sheets. This indicates that high-latitude processes from this point on began to take over and influence most strongly the d18O record, which now reflects ice-volume fluctuations related to the climatic effects of obliquity forcing on the seasonality of high-latitude areas, most probably in the Northern Hemisphere. The general Pliocene trend is that high-latitude climate sensitivity and instability was increasing, and the causal factors producing the intensified glacial cyclicity during the Pliocene must be factors that enhance cooling and climate sensitivity in the subarctic areas.

(Table 1) Diatom abundance and preservation in ODP Hole 130-806B samples

We report on diatom abundance and preservation stratigraphy in the uppermost four cores of Ocean Drilling Program Hole 806B, which span the Quaternary period. Changes in diatom abundance and preservation show a rather complicated pattern, with much noise at high frequencies. However, in the cycles corresponding to eccentricity and obliquity variations, the picture is quite clear. Abundance and preservation follow glacial-interglacial cycles, with lowest abundances and poorest preservation observed in sediments that correspond to glacial stages. Seventy taxa compose the diatom assemblage of Hole 806B (from Samples 130-806B-1H-1, 8-9 cm, to -4H-7, 73-74 cm) with Azpeitia nodulifera as the dominant member. This species exhibits significant size variations related to glacial and interglacial stages during the Pleistocene. The distribution of power in the Fourier spectrum of the diatom signal (in the time domain) displays the expected Milankovitch frequencies (at 100,41, and 24-18 k.y.). It also shows concentration at various "odd" frequencies, especially at 62 k.y., suggesting a complicated response of productivity (and silicate chemistry) to climatic forcing.

(Table 1) Boron concentration and d11B ratios of ODP Hole 130-803D

The pH of the surface ocean is a sensitive function of its alkalinity and total inorganic carbon concentration, properties which also control the partial pressure of atmospheric carbon dioxide (Broecker and Peng, 1982). Thus, an accurate proxy for past ocean pH could yield information about variations in atmospheric CO2. Recently, it has been suggested that the boron isotopic composition of foraminiferal tests depends on the pH of sea water as well as its isotopic composition (Vengosh et al., 1991, doi:10.1016/0016-7037(91)90139-V; Hemming and Hanson, 1992, doi:10.1016/0016-7037(92)90151-8). Here we present boron isotope and elemental data for sedimentary pore fluids and isotope data for bulk foraminiferal samples from a deep-sea sediment core. The composition of the pore waters implies that sea water boron concentrations and isotopic composition have been constant during the past 21 Myr, allowing us to reconstruct past ocean pH directly from the foraminiferal isotope data. We find that 21 Myr ago, surface ocean pH was only 7.4 ±0.2, but it then increased to 8.2 ±0.2 (roughly the present value) about 7.5 Myr ago. This is consistent with suggestions (Popp et al., 1989; Cerling, 1991; Arthur et al., 1991) that atmospheric CO2 concentrations may have been much higher 21 Myr ago than today.

Mesozoic calcareous nannofossils of ODP Leg 130 holes

The distribution of Mesozoic calcareous nannofossils are tabulated for Holes 807C and 8O3D drilled on the Ontong Java Plateau in the western equatorial Pacific. Nannofossils were abundant but poorly preserved in Hole 803D and range from early Albian to Maastrichtian in age. A possibly complete and expanded K/T boundary interval yielded few diagnostic taxa because of the dissolution of Tertiary forms. The only nannofossil-bearing sample examined from Hole 803D contained the uppermost Maastrichtian zonal indicator Micula prinsii.

d11B and metal/Ca ratios versus size fractions in the Surface and deep dwelling species, ODP Holes 130-806B and 108-664C

We have assessed the reliability of several foraminifer-hosted proxies of the ocean carbonate system (d11B, B/Ca, and U/Ca) using Holocene samples from the Atlantic and Pacific oceans. We examined chemical variability over a range of test sizes for two surface-dwelling foraminifers (Globigerinoides sacculifer and Globigerinoides ruber). Measurements of d11B in G. ruber show no significant relationship with test size in either Atlantic or Pacific sites and appear to provide a robust proxy of surface seawater pH. Likewise there is no significant variability in the d11B of our Atlantic core top G. sacculifer, but we find that d11B increases with increasing test size for G. sacculifer in the Pacific. These systematic differences in d11B are inferred to be a consequence of isotopically light gametogenic calcite in G. sacculifer and its preferential preservation during postdepositional dissolution. The trace element ratio proxies of ocean carbonate equilibria, U/Ca and B/Ca, show systematic increases in both G. ruber and G. sacculifer with increasing test size, possibly as a result of changing growth rates. This behavior complicates their use in paleoceanographic reconstructions. In keeping with several previous studies we find that Mg/Ca ratios increase with increasing size fraction in our well-preserved Atlantic G. sacculifer but not in G. ruber. In contrast to previous interpretations we suggest that these observations reflect a proportionally larger influence of compositionally distinct gametogenic calcite in small individuals compared to larger ones. As with d11B this influences G. sacculifer but not G. ruber, which has negligible gametogenic calcite.

Mg/Ca ratio of Globorotalia tumida and subsurface temperature reconstruction for ODP Site 130-806

The tropical Pacific thermocline strength, depth, and tilt are critical to tropical mean state and variability. During the early Pliocene (~3.5 to 4.5 Ma), the Eastern Equatorial Pacific (EEP) thermocline was deeper and the cold tongue was warmer than today, which resulted in an mean state with a reduced zonal sea surface temperature gradient, or El Padre. However, it is unclear whether the deep thermocline was a local feature of the EEP or a basin-wide condition with global implications. Our measurements of Mg/Ca of Globorotalia tumida in a western equatorial Pacific site indicate Pliocene subsurface temperatures warmer than today; thus, El Padre included a basin-wide thermocline that was relatively warm, deep, and weakly tilted. At ~4 Ma, thermocline steepening was coupled to cooling of the cold tongue. Since ~4 Ma, the basin-wide thermocline cooled/shoaled gradually, with implications for thermocline feedbacks in tropical dynamics and the interpretation of TEX86-derived temperatures.