Teh-Lung Ku

Milankovitch Hypothesis Supported by Precise Dating of Coral Reefs and Deep-Sea Sediments

Barbados provides a possibly unique opportunity for reconstruction of the times and elevations of late-Pleistocene high stands of the sea. The island appears to be rising from the sea at a uniform rate that is fast enough to separate in elevation coral-reef tracts formed at successive high stands of the sea. Unaltered coral found in the lower terraces enables high-precision Th230: U234 and Pa231: U235 dating. Three distinct high stands of the sea are found about 122,000, 103,000, and 82,000 years ago. New Pa231 and Th230 dates from a deep-sea core also indicate that Ericsons W-X cold-to-warm climatic change occurred close to 126,000 years ago. These data show a parallelism over the last 150,000 years between changes in Earths climate and changes in the summer insolation predicted from cycles in the tilt and precession of Earths axis.

The status of U-series methods of mollusk dating

An intensive evaluation is presented of the 400 available analyses of U-series nuclides in mollusks including 200 performed as part of the present study. It is shown that among the 60 samples of known age, Th230U234 ratios give correct ages no more than half of the time. The samples of unknown age show the following evidence of having been open systems: increase of U238 with time, increase of U234 relative to U238, and disagreement between Th230U234 ages and Pa231U235 ages. It is also shown that a recently proposed open system model (Szabo and Rosholt, 1969) is based on unlikely isotopic migration patterns. This model commonly yields ages which conflict with known ages and its apparent internal consistency in several cases is shown to be no better than that for alternative models. Finally in the vast majority of cases, resulting U234U238 evolution patterns are different from those actually observed.

Comparison of sedimentation rates measured by paleomagnetic and the ionium methods of age determination

Abstract Rates of sedimentation based on 230 Th measurements for 14 deep sea cores are compared with those derived from magnetic reversal studies on the same cores. The results are completely compatible. In some cases 231 Pa and 14 C dates are also available; again the agreement is satisfactory. The average rate of accumulation of CaCO 3 -free sediment based on our results is about 2 meters/million years. Rates below one half meter per million years have not been found. This conclusion is at variance with that of Goldberg and his coworkers who find that rates less than one half meter per million years are rather common in major portions of the deep ocean. This discrepancy is shown to be largely the result of interpretation of the 230 Th data. When ‘best’ fit lines are substituted for the ‘steepest fits’ of Goldberg and his coworkers the discrepancy between the two sets of data largely disappears.

Radiochemical studies on manganese nodules of deep-sea origin☆

Abstract Manganese nodules from different parts of the ocean floor were analyzed for their uranium, thorium, protactinium, and radium contents. (1) The uranium : thorium ratio in nodules is quite variable, and is 3 to 6 orders of magnitude lower than that in sea water. Uranium concentrations range from 4 to 13 ppm, with most values falling between 6 and 10 ppm. Thorium concentrations have a much wider spread (3 to 150 ppm) and reflect both a geographic and depth dependence. (2) The 230 Th and 231 Pa in the outermost layers of a nodule greatly exceed the amounts which may be supported by the uranium present. The decrease with depth of these two isotopes allow the growth rates of the nodules to be determined. The rates for eighteen specimens vary from 6 year; most of these fall within rather narrow limits (2–4 mm/10 6 year). (3) The 230 Th : 231 Pa ratios at the nodule surface are considerably smaller than that predicted from the production of these two nuclides by the uranium dissolved in the ocean. While an activity ratio of 11 might be anticipated, values as low as 2·4 were observed. Since 230 Th : 231 Pa ratios considerably greater than 11 are found at the tops of deep-sea sediment cores, the two species must become separated during the sedimentary process. (4) The distribution of 230 Th and 231 Pa on the surface of a given nodule is not uniform. Higher concentrations occur on the upper than on the lower sides. This suggests that these nuclides are precipitated only on those portions of the nodule in contact with sea water. (5) Measurements of 226 Ra reinforce the conclusion that this element migrates in sedimentary deposits. (6) For the oceans as a whole the amount of 230 Th and 231 Pa in manganese nodule deposits is probably no more than 5% of that generated by the uranium dissolved in the sea.

Manganese Nodules: Their Evolution

That manganese nodules and adjacent deep-sea sediments are accumulating manganese at almost the same rate has been established by thorium-230 dating of both sediments and nodules of known manganese content. The rate of manganese deposition is nearly constant over the world oceans. A relatively simple model of nodule evolution explains the distribution of manganese nodules between sediment column and sediment-water interface; the model appears to apply to other trace elements such as copper, nickel, and cobalt.

Accumulation rates of manganese in pelagic sediments and nodules

Accumulation rates of manganese in sediments of thirty-eight deep-sea cores range from 0.1 to 3.4 mg cm−210−3yr−1. Manganese accumulation rates in five nodules range from 0.2 to 1.0 mg cm−210−3yr−1. In the North Pacific, manganese is apparently incorporated into sediments in association with clay or a related phase; in other ocean basins, no generalizations can be drawn from the data presented here. The similarity between nodule and sediment manganese accumulation rates could be coincidence or could reflect the possibility that nodules and sediments accumulate manganese by the same mechanism.

Rates of sedimentation in the Arctic ocean

Abstract The sedimentation rate in the Arctic deep-sea over the last 150,000 years has undergone no significant changes. Based on both radiocarbon and uranium series isotope analyses, the deposition rate is estimated to be about 0.2 cm/1000 years. These findings imply that: (1) the observed variations in the Foraminifera abundance in most Arctic cores reflect changes in the biological productivity; (2) the Arctic Foraminifera abundance curves cannot be simply correlated with those observed in the Atlantic deep-sea cores; and (3) the Arctic Ocean has not been open during the last 150,000 years.

Uranium, thorium, and protactinium in a manganese nodule

Abstract Isotopic compositions of uranium, thorium and protactinium have been determined in a manganese nodule dredged from the North Pacific Ocean. Both the230Th and231Pa activities are found to decay exponentially with depth in the nodule. The corresponding accretion rate of the nodule, averaged over the past several hundred thousands of years, is about 4 mm/106 y. The depth distribution of the unsupported234U activities in the nodule indicate that the234U/238U ratio of the oceanic uranium has undergone a rather minor change for the last few hundred thousand years. The fluctuation has been no more than ±2% of the present value of 1.15.

Atlantic Deep-Sea Stratigraphy: Extension of Absolute Chronology to 320,000 Years

Thorium-230 measurements on a core of globigerina ooze from the Caribbean Sea substantiate the prediction of Ericson et al. that the paleontological boundary U-V (Sangamon-Illinoian boundary in their scheme) in the Atlantic sediments has an age of close to 320,000 years. As the ages derived by Ericson et al. were based on extrapolations of mean sedimentation rates established by carbon-14 and protactinium-231 dating of the upper sections of this and other cores, this result confirms the assumption that sedimentation rates in the Caribbean Sea have not changed significantly during the past several hundred thousand years. The uranium content of the ocean as indicated by the deposition rate of thorium-230 was no more than 30 percent higher during glacial than during interglacial periods.

Radiocarbon Chronology of Red Sea Sediments

Radiocarbon dates have been obtained on seven cores taken from the central part of the Red Sea in the axial trough. The observed sedimentation rates (over the past 20,000 years) range from about 5cm/1,000 yr to more than 60cm/1,000 yr. Variations are primarily due to the episodic precipitation of minerals associated with the geothermal activity of the hot-brine deeps. The brine-derived materials precipitate at a rate of more than 40cm/1,000 yr, whereas accumulation rates of the detrital silicates and calcareous shells are on the order of 2cm/1,000 yr and 8cm/1,000 yr respectively, for the area studied.