SR-ISOTOPE AND C-ISOTOPE IN LOWER CAMBRIAN CARBONATES FROM THE SIBERIAN CRATON - A PALEOENVIRONMENTAL RECORD DURING THE CAMBRIAN EXPLOSION

We report Sr-87/Sr-86 measurements on a suite of well preserved sedime ntary carbonates from Lower Cambrian strata of the Lena River region o f Siberia. Stable isotopes and major and trace element chemistry have been used to identify potentially unaltered samples for Sr isotopic me asurements. The Sr data define a smooth curve of paleoseawater Sr-87/ Sr-86. values from the Tommotian through to the early Middle Cambrian. During the Tommotian-Atdabanian interval, Sr-87/Sr-86 rose rapidly fr om 0.7081 to 0.7085. The rate of change in Sr ratios decreased during the Botomian but rose to 0.7088 in the late Toyonian to early Middle C ambrian. The rate of Sr-87/Sr-86 increase during the Tommotian-Atdaban ian was ca. 0.0001/m.y., comparable to the late Miocene change in seaw ater Sr. We infer that an interval of enhanced erosion during the 'Cam brian explosion' was responsible far this increase. An important sourc e for radiogenic Sr to the oceans may have been erosion of the Pan-Afr ican orogenic belt of southern Africa. The rapid change in paleoseawat er Sr corresponds with an interval of highly variable marine delta(13) C values. Model results for the Sr and C isotopic records suggest that the quasi-periodicity in the delta(13)C record is not a consequence o f direct erosional forcing. However, our inference of high erosion rat es during the Tommotian-Atdabanian implies enhanced fluxes of nutrient elements such as P to the oceans. Phosphorite deposits and black shal e deposition in coeval strata suggest that periods of high marine prod uctivity and anoxia may be in part related to enhanced river dissolved fluxes. Our results thus provide some insight into environmental cond itions during the 'Cambrian explosion.