Pliocene-Pleistocene carbon isotope record of foraminifera from DSDP Site 89-586

Oceanographic changes in the western equatorial Pacific during the past 6 m.y. are inferred from carbon isotopic analyses of planktonic and benthic foraminifers from Ontong Java Plateau (DSDP Site 586). Sample spacing is 1.5 m (ca. 35,000-75,000 yr). An overall trend of d13C toward lighter values is evident for the last 5 m.y. in all four foraminiferal taxa analyzed (G. sacculifer, Pulleniatina, P. wuellerstorfi, and O. umbonatus). This trend is interpreted as an enrichment of the global ocean with 12C, because of the addition of carbon from organic carbon reservoirs (or lack of removal of carbon to such reservoirs), as a consequence of an overall drop in sea level. Differences between shallow- and deep-water d13C decrease slightly during this time interval, suggesting a moderate drop in productivity. This drop is not sufficient to explain the drop in sedimentation rate, however, much of which apparently must be ascribed to winnowing effects. A marked convergence in the d13C values of planktonic taxa exists within the last 2 m.y. We propose that this convergence indicates nutrient depletion in thermocline waters, caused by the vigorous removal of phosphate in marginal upwelling regions, or by the stripping of intermediate waters in their source regions. No large shifts are seen in the carbon isotope record of the last 6 m.y., in contrast to the oxygen isotope record. Some indication of cyclicity is present, with a period between 0.5 and 1.0 m.y. (especially in the earlier portion of the record).

Pliocene-Pleistocene oxygen isotope record DSDP Site 586, Ontong Java Plateau, Pacific Ocean i

Oceanographic changes in the western equatorial Pacific during the past 6 Ma are inferred from oxygen isotopic analyses of planktic and benthic foraminifera from Ontong Java Plateau (DSDP Site 586). The taxa are Globigerinoides sacculifer, Pulleniatina, Cibicidoides wuellerstorfi, and Oridorsalis umbonatus. Cooling and ice buildup are indicated by an 18O enrichment of 0.3 per mil in the planktic species near 3.4 Ma. This shift apparently is compensated in the benthic data by a warming of the deep waters by between 1° and 2° C. We suggest that the dominant source of upper deep water supply to the Pacific changed from Antarctic to North Atlantic at that time, the North Atlantic-derived water being warmer. Near 2.8 Ma (approximately) the planktic foraminifera again record an enrichment in 18O (Delta delta18O=0.25 per mil). We suggest ice buildup in the northern hemisphere as the cause, because of subsequent sharp increase in fluctuations of the delta18O signal, that is, instability. The enrichment is magnified in the benthic foraminifera (Delta delta18O = 0.5 per mil) by a cooling of the deep water by 1.5° at the time, presumably signalling a glacial-type reduction of North Atlantic Deep Water (NADW) production. Episodic divergence between the signals of G. sacculifer and Pulleniatina in the Pleistocene apparently reflects periods of increased upwelling in the western equatorial Pacific. The amplitude of ice volume fluctuations cannot be reconstructed from delta18O data alone, unless there are constraints on temperature variations. The increase in amplitude of fluctuation of the benthic and planktic signals during the Pleistocene may be attributed either to an increase in maximum ice volume, or to an increase in the fractionation of continental ice, or a combination of both causes.

Late Miocene development of the western Pacific warm pool: Planktonic foraminifer and oxygen isotopic evidence

The disappearance at ~10 Ma of the deep dwelling planktonic foraminifer Globoquadrina dehiscens from the western Pacific including the South China Sea was about 3 Myr earlier than its final extinction elsewhere. Accompanying this event at ~10 Ma was a series of faunal turnover characterized by increase in mixed layer, warm-water species and decrease to a minimum in deepwater species. Paleobiological and isotopic evidence indicates sea surface warming and a deepened local thermocline that we interpret as related to the development of an early western Pacific warm pool. The stepwise decline of G. dehiscens and other deep dwelling species from the NW and SW Pacific suggests more intensive warm water pileup than equatorial localities where surface bypass flow through the narrowing Indonesia seaway appears to remain efficient during the late Miocene. Planktonic delta18O values from the South China Sea consistently lighter than the tropical western Pacific during the Miocene also suggest, similar to today, more variable hydrologic conditions along the periphery than in the core of the warm pool. Stronger hydrologic variability affected mainly by monsoons and increased thermal gradient along the western margin of the late Miocene warm pool may have contributed to the decline of deep dwelling planktonic species including the early extinction of G. dehiscens from the South China Sea region. The late Miocene warm pool became influential and paleobiologically detectable from ~10 Ma, but the modern warm pool did not appear until about 4 Ma, in the middle Pliocene.

Neogene radiolarian datum levels in the equatorial Indian and Pacific Oceans

Fifty radiolarian events of early Pleistocene and Neogene age were identified in an E-W transect of equatorial DSDP sites, extending from the Gulf of Panama to the western Pacific and eastern Indian Oceans. Our objective was to document the degree of synchroneity or time-transgressiveness of stratigraphically-useful datum levels from this geologic time interval. We restricted our study to low latitudes within which morphological variations of individual taxa are minimal, the total assemblage diversity remains high, and stratigraphic continuity is well-documented by an independent set of criteria. Each of the five sites chosen (503, 573, 289/586, 214) was calibrated to an "absolute" time scale, using a multiple of planktonic foraminiferal, nannofossil, and diatom datum levels which have been independently correlated to the paleomagnetic polarity time scale in piston core material. With these correlations we have assigned "absolute" ages to each radiolarian event, with a precision of 0.1-0.2 m.y. and an accuracy of 0.2-0.4 m.y. On this basis we have classified each of the events as either: (a) synchronous (range of ages <0.4 m.y.); (b) time-transgressive (i.e., range of ages >1.0 m.y.); and (c) not resolvable (range of ages 0.4-1.0 m.y.). Our results show that, among the synchronous datum levels, a large majority (15 out of 19) are last occurrences. Among those events which are clearly time-transgressive, most are first appearances (10 out of 13). In many instances taxa appear to evolve first in the Indian Ocean, and subsequently in the western and eastern Pacific Ocean. This pattern is particularly unexpected in view of the strong east-to-west zonal flow in equatorial latitudes. Three of the time-transgressive events have been used to define zonal boundaries: the first appearances of Spongaster pentas, Diartus hughesi, and D. petterssoni. Our results suggest that biostratigraphic non-synchroneity may be substantial (i.e., greater than 1 m.y.) within a given latitudinal zone; one would expect this effect to be even more pronounced across oceanographic and climatic gradients. We anticipate that the extent of diachroneity may be comparable for diatom, foraminiferal, and nannofossil datum levels as well. If this proves true, global "time scales" may need to be re-formulated on the basis of a smaller number of demonstrably synchronous events.