Causes of /sup 87/Sr//sup 86/Sr variations in seawater of the past 750 million years

Conference · Tue Jan 01 04:00:00 EST 1985 · Geol. Soc. Am., Abstr. Programs; (United States)

OSTI ID:6317803

Keto, L S; Jacobsen, S B

The authors are presenting the results of a steady-state geochemical model for the inputs of Sr to the ocean with time. This model uses as data the /sup 87/Sr//sup 86/Sr and /sup 143/Nd//sup 144/Nd ratios of seawater through time. The authors have also used an inverse relationship between the element of/sub Sr/ and element of/sub Nd/ of river water input to the oceans. The element of/sub Nd/ of continental input is assumed to be identical to the values of element of/sub Nd/ of average ocean water. The preliminary average element of/sub Nd/ of Phanerozoic oceans varies in a range between -9 and -3. This suggests that the variation in average river water element of/sub Nd/ and element of/sub Sr has been relatively small, but not insignificant, through this time. The main component of the variation of element of/sub Sr/ of the oceans through time is shown to be caused by changes in the ratio of mass flux of Sr from continental (river water) sources relative to mantle (hydrothermal water) sources. The value of this ratio has varied by a factor of 30 in the past 750 million years, with highest values in the mass flux ratio in the Precambrian-early Cambrian, the late Ordovician-early Silurian, the Permian-early Triassic, and at the present time. The timing of these peaks corresponds to times of widespread exposure of the continents. It seems, therefore, that the dominant cause of variation of element of/sub Sr/ of seawater is the change through time in the mass flux of Sr from the continents, due to weathering and erosion of continental surfaces. This first order cause of variation is modulated by changes in the isotopic composition of continental materials eroded, including both carbonate and noncarbonate phases. Changes in hydrothermal flux of Sr during this period are likely to have been small.

• Research Organization: Harvard Univ., Cambridge, MA (USA)
• OSTI ID: 6317803
• Report Number(s): CONF-8510489-
• Conference Information: Journal Name: Geol. Soc. Am., Abstr. Programs; (United States) Journal Volume: 17
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580400* -- Geochemistry-- (-1989)
ALKALINE EARTH ISOTOPES
BETA DECAY RADIOISOTOPES
CHEMICAL COMPOSITION
CHEMISTRY
ELECTRON CAPTURE RADIOISOTOPES
EVEN-EVEN NUCLEI
EVEN-ODD NUCLEI
GEOCHEMISTRY
GEOLOGIC MODELS
HOURS LIVING RADIOISOTOPES
HYDROGEN COMPOUNDS
INTERMEDIATE MASS NUCLEI
ISOMERIC TRANSITION ISOTOPES
ISOTOPE RATIO
ISOTOPES
NEODYMIUM ISOTOPES
NUCLEI
OXYGEN COMPOUNDS
RADIOISOTOPES
SEAWATER
STABLE ISOTOPES
STRONTIUM 86
STRONTIUM 87
STRONTIUM ISOTOPES
VARIATIONS
WATER
WATER CHEMISTRY