Memorie K Yasuda

Quaternary time scale for the Ontong Java Plateau: Milankovitch template for Ocean Drilling Program Site 806

A 2 m.y. oxygen isotope record of Globigerinoides sacculifer from the Ontong Java Plateau, based on cores from Ocean Drilling Program Leg 130, is dated by matching variations to an orbital template. The procedure allows us to present the most complete Quaternary record available for the western equatorial Pacific. The template-generating algorithm describes a balance between growth and melting of ice. Following basic Milankovitch theory, ice growth is taken as constant, while melting is taken to depend on summer insolation, current ice mass, and average past ice mass. Template settings must be changed once, between 1 and 1.2 Ma, to reflect a major shift in climate. Template fits are strikingly good over much of the record and can be used to detect and fill gaps from core breaks and other disturbances. One result of template dating is an exact age for the Brunhes-Matuyama magnetic reversal boundary, at 790 ±5 ka, as well as several other precise dates (900 ka for the middle Pleistocene climate shift; 1070, 1240, and 1450 ka for isotope stages 31, 37, and 47, respectively). Sedi- mentation rates fluctuate between 18 and 28 m/m.y., a ca. 400 ka cycle being the most prominent. Major anomalies arise within the transitional regime (1.2 to 1 Ma). The origin of the cycles is unknown; we propose productivity variations in the western equatorial Pacific.

Middle Cretaceous reef collapse linked to ocean heat transport

Biologically defined fluctuations in Cretaceous tropical reef boundaries of the Caribbean region record a dynamic rather than stable environmental history. These fluctuations may be related to major thermal changes resulting from ocean heat transport. With simultaneous poleward movement of surface and subsurface waters on sea-level highstands, the superheated middle Cretaceous tropics cooled, the reef line contracted, diversity decreased, and reef ecosystems collapsed, leading to mass extinction. Geologic data qualitatively test and support the hypothesis of enhanced Cretaceous ocean heat transport formulated from general circulation models. In these models, four times the present-day atmospheric concentration of CO 2 and twice the present-day model value of ocean heat transport cooled the superheated tropics and provided the best match to the distribution of inferred middle Cretaceous temperature data. These dynamic changes suggest an important role for large-scale disturbance in the evolution of tropical ecosystems.

Reconstruction of atmospheric CO2 from ice-core data and the deep-sea record of ontong Java plateau: the Milankovitch chron

We provide a reconstruction of atmospheric CO2 from deep-sea sediments, for the past 625000 years (Milankovitch chron). Our database consists of a Milankovitch template of sea-level variation in combination with a unique data set for the deep-sea record for Ontong Java plateau in the western equatorial Pacific. We redate the Vostok ice-core data of Barnola et al. (1987). To make the reconstructions we employ multiple regression between deep-sea data, on one hand, and ice-core CO2 data in Antarctica, on the other. The patterns of correlation suggest that the main factors controlling atmospheric CO2 can be described as a combination of sea-level state and sea-level change. For best results squared values of state and change are used. The square-of-sea-level rule agrees with the concept that shelf processes are important modulators of atmospheric CO2 (e.g., budgets of shelf organic carbon and shelf carbonate, nitrate reduction). The square-of-change rule implies that, on short timescales, any major disturbance of the system results in a temporary rise in atmospheric CO2.

Brunhes‐Matuyama Boundary: 790 k.y. date consistent with ODP Leg 130 oxygen isotope records based on fit to Milankovitch Template

The age of the Brunhes-Matuyama boundary, until recently taken as 730 k.y. in standard time scales for the Quaternary (e.g., Imbrie et al., 1984; Prell et al., 1986) and in Ocean Drilling Program results (Kroenke et al., 1991; Shackleton et al., 1992) has been revised to between 780 k.y. and 790 k.y., based on both orbital tuning of oxygen isotope data (e.g., Shackleton et al., 1990) and on new 40Ar-39Ar dates on volcanic rocks (e.g., Baksi et al., 1992). We apply the revised chronology to an oxygen isotope record for Ontong Java Plateau (ODP Leg 130, western equatorial Pacific). Using this time scale, we find excellent agreement between the record and a template based on orbital forcing. The equation for the template is where the change in ice-mass is given as the difference between constant ice growth (IGR) and variable amounts of melting. Melting depends on summer insolation at 65°N (INS), the current ice-mass present (ICE), and the average ice-mass (MEM) over a given time interval (L). The memory base L and the exponents are adjusted for best fit. The fit to the relevant portion of the record yields an age of 790±6 k.y. for the Brunhes-Matuyama boundary. The age assignment is robust relative to substantial changes in parameter settings.