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87 Sr/ 86 Sr evidence from a soil chronosequence in the Hawaiian Islands demonstrates that the atmosphere supplies >85% of putatively rock-derived Sr in older sites. Initially, bedrock is the dominant source for Sr and other lithophile elements such as Ca, but high rates of weathering and leaching of the substrate by 20 ka lead to a shift to atmospheric sources. The loss of weathering inputs coincides with other physio-chemical changes in the soil and results in a steep decline of base cations in the soil pool. While these patterns imply the potential for limitation of biological productivity by low base cation supply, the atmosphere provides a supply of base cations in excess of nutritional needs, even after nearly all rock-derived base cations have been leached from the soil. This raises the possibility that P limitation in terrestrial ecosystems may develop at least as much because of low rates of atmospheric deposition of P (relative to Ca, K, and other rock-derived elements) as because of its chemical interaction in soil.

New high-resolution δ 13 C data through the Lower Cambrian of Siberia reveal multiple, positive excursions coincident with phases in the "explosion" of invertebrate phyla (Nemakit-Daldynian to middle Botomian stages). Comparison of the δ 18 O and trace element (Mg, Fe, Mu, Sr) chemistry indicate that six new δ 13 C cycles are primary rather than diagenetic features, with potential for global correlation. Positive δ 13 C excursions up to +3‰ indicate that fractional organic carbon burial rates were high but variable through the Cambrian explosion. Values for δ 13 C dropped sharply from around +2.2‰ to -1.6‰ in Botomian time, coincident with mass extinction of the archaeocyathan reef biota. The rapid fluctuation of the δ 13 C signal and the temporal coincidence of the pronounced negative shift with the extinction event hint that the δ 13 C record may record productivity variations during the Early Cambrian radiation.

Nd and Sr isotopic measurements from a variety of marine chemical precipitates provide insight into processes controlling the chemistry of seawater during the Proterozoic. Nd isotope measurements on banded iron formations and phosphorites, and Sr isotope measurements on carbonates indicate that during both the Early and Late Proterozoic, hydrothermal input to the oceans was a significant fraction of the total input to ocean chemistry. Isotopic data from Early Proterozoic clastic sediments show systematic differences from coeval chemical sediments. These differences become less marked toward the end of the Proterozoic. This implies a higher hydrothermal water to river water flux ratio during the Early Proterozoic. The significant changes in seawater isotopic composition during the Proterozoic reflect the transition from mantle dominated Archean oceans to a typically modern system. Isotopic and mass balance constraints from banded iron formations indicate that oxidation of hydrothermal Fe2+ in seawater could have been an important control on the redox state of Proterozoic ocean-atmosphere system. The present isotopic balances of carbon and sulfur could have been decoupled as a result.

The homeostatic balance of Earth's long-term carbon cycle and the equable state of Earth's climate are maintained by negative feedbacks between the levels of atmospheric CO 2 and the chemical weathering rate of silicate rocks. Though clearly demonstrated by well-controlled laboratory dissolution experiments, the temperature dependence of silicate weathering rates, hypothesized to play a central role in these weathering feedbacks, has been difficult to quantify clearly in natural settings at landscape scale. By compiling data from basaltic catchments worldwide and considering only inactive volcanic fields (IVFs), here we show that the rate of CO 2 consumption associated with the weathering of basaltic rocks is strongly correlated with mean annual temperature (MAT) as predicted by chemical kinetics. Relations between temperature and CO 2 consumption rate for active volcanic fields (AVFs) are complicated by other factors such as eruption age, hydrothermal activity, and hydrological complexities. On the basis of this updated data compilation we are not able to distinguish whether or not there is a significant runoff control on basalt weathering rates. Nonetheless, the simple temperature control as observed in this global dataset implies that basalt weathering could be an effective mechanism for Earth to modulate long-term carbon cycle perturbations.

International audience; We combines Mg isotopic analyses with soil characterization methods to determine Mg isotopic compositions of bulk soils, basalts, and carbonate fractions at an arid (∼30 cm MAP) soil chronosequence on the Island of Hawaii. This chronosequence is developed on Pololu (350 ka) and Hawi (170 ka) lava flows. Both profiles contain pedogenic carbonates and secondary alumino- silicate and sequioxide phases. Bulk soil horizons at these sites range in δ26Mg from -0.21±0.31 ‰ to -1.75±0.22 ‰ for the Hawi and -0.01±0.31 ‰ to -0.21±0.31 ‰ for the Pololu. Basalts underlying the soil profiles have average δ26Mg values of - 0.25±0.06 ‰. Pedogenic carbonate δ26Mg values at these sites vary from -1.05±0.22‰ to -2.31±0.22‰. Integrating the soils as a whole yields bulk soil isotopic compositions of -1.35±0.16 ‰ for the Hawi and -0.12±0.12 ‰ for the Pololu. These differences in overall δ26Mg between total soils may be explained by the relative abundance of Mg in carbonate; in the Hawi soil 69±11% of Mg is hosted by carbonate phases, while in the Pololu soil 16±2% of Mg is in carbonate phases. Estimates of Mg input to the soils through time, through rainfall estimates as well as rock weathering estimates provided by the immobile index element Zr, allows the calculation of the δ26Mg of Mg exported from these systems. The isotopic composition Mg exported from the Hawi soil is not tightly constrained but must be significantly isotopically heavier than the basalt parent material. The Pololu soil is much better constrained and has exported Mg with a δ26Mg of -0.36±0.26 ‰, close to the weighted mean value of inputs to the soil from weathering and atmospheric deposition. The evolution of the soil mineralogy and morphology during progressive development of the weathering profile result in significant changes in the isotopic composition of Mg exported through time.

We report initial 87Sr/86Sr values from an Upper Proterozoic carbonate succession from Svalbard and East Greenland. This succession, now tectonically separated into three sequences, is thick, relatively continuous, and well preserved. The relative ages of the samples from within the basin are well constrained by litho-, bio-, and chemostratigraphic techniques. The data from this study and related data from the literature are used to construct a curve of 87Sr/86Sr for Upper Proterozoic seawater. The new data reported in this study substantially improve the isotopic record of Sr in seawater for the period between 650 and 800 Ma. The data indicate that delta 87Sr values of seawater were variable but low (delta 87Sr approximately -500 to -250) between 900 and 650 Ma, and rose rapidly to approximately +30 by 600 Ma. The range of variation of delta 87Sr in seawater during the Riphean-Vendian exceeds the entire range of delta 87Sr in seawater during the Phanerozoic. While variation in the average isotopic composition of Sr delivered to the oceans by rivers can account for some of the observed range, changes in the ratio of submarine hydrothermal flux to river water (continental) flux are responsible for the large variation in seawater Sr isotopic composition. Changes in the continental flux of Sr to the oceans can be related to tectonic factors. Large changes in the hydrothermal flux to river water flux ratio indicated by the data could have significant consequences for the chemistry of the ocean-atmosphere system.