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In the anoxic Cariaco Basin of the southern Caribbean, the bulk titanium content of undisturbed sediment reflects variations in riverine input and the hydrological cycle over northern tropical South America. A seasonally resolved record of titanium shows that the collapse of Maya civilization in the Terminal Classic Period occurred during an extended regional dry period, punctuated by more intense multiyear droughts centered at approximately 810, 860, and 910 A.D. These new data suggest that a century-scale decline in rainfall put a general strain on resources in the region, which was then exacerbated by abrupt drought events, contributing to the social stresses that led to the Maya demise., Paleoclimatologists have developed an increasingly precise record of climate change for the past few millennia, coveting the same span of time over which literate human societies developed. Until recently, archaeologists [...]

Niobium-92 ([sup.92]Nb) decays to zirconium-92 ([sup.92]Zr) with a half-life of 36 million years and can be used to place constraints on the site of p-process nucleosynthesis and the timing of early solar system processes. Recent results have suggested that the initial [sup.92]Nb/[sup.93]Nb of the solar system was high (> [10.sup.-3]). We report Nb-Zr internal isochrons for the ordinary chondrite Estacado (H6) and a clast of the mesosiderite Vaca Muerta, both of which define an initial [sup.92]Nb/ [sup.93]Nb ratio of ~[10.sup.-5]. Therefore, the solar system appears to have started with a ratio of, Some extinct radionuclides were sufficiently abundant at the start of the solar system that they produced variations in the abundance of their daughter isotopes in early-formed objects (1). Such nuclides [...]

A Cenozoic record of hafnium isotopic compositions of central Pacific deep water has been obtained from two ferromanganese crusts. The crusts are separated by more than 3000 kilometers but display similar secular variations. Significant fluctuations in hafnium isotopic composition occurred in the Eocene and Oligocene, possibly related to direct advection from the Indian and At[antic oceans. Hafnium isotopic compositions have remained approximately uniform for the past 20 million years, probably reflecting increased isolation of the central Pacific. The mechanisms responsible for the increase in [sup.87]Sr/[sup.86]Sr in seawater through the Cenozoic apparently had no effect on central Pacific deep-water hafnium., Variations in the radiogenic isotope composition of seawater through time have the potential to provide information on changes in continental weathering, ocean floor hydrothermal activity, and ocean circulation (1-3) and [...]