[1] Biogeochemical processes fractionate Ca isotopes in plants and soils along a 4 million year developmental sequence in the Hawaiian Islands. We observed that plants preferentially take up 40 Ca relative to 44 Ca, and that biological fractionation and changes in the relative contributions from volcanic and marine sources produce a significant increase in 44 Ca in soil exchangeable pools. Our results imply moderate fluxes enriched in 44 Ca from strongly nutrient-depleted old soils, in contrast with high 40 Ca fluxes in young and little weathered environments. In addition, biological fractionation controls divergent geochemical pathways of Ca and Sr in the plant-soil system. While Ca depletes progressively with increasing soil age, Sr/Ca ratios increase systematically. Sr isotope ratios provide a valuable tracer for provenance studies of alkaline earth elements in forested ecosystems, but its usefulness is limited when deciphering biogeochemical processes involved in the terrestrial Ca cycle. Ca isotopes in combination with Sr/Ca ratios reveal more complex processes involved in the biogeochemistry of Ca and Sr.

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In the past decade, we and others have compiled an extensive dataset of O, C and Sr isotope stratigraphies from sedimentary basins throughout the Paleogene North American Cordillera. In this study, we present new results from the Piceance Creek Basin of northwest Colorado, which record the evolving hydrology of the Eocene Green River Lake system. We then place the new data in the context of the broader Cordilleran dataset and summarize implications for understanding the synorogenic evolution of large-scale drainage patterns. The combined data reflect (1) a period of throughgoing foreland rivers heading in the Sevier fold-and-thrust belt and flowing east, (2) ponding of freshwater lakes in the foredeep as Laramide uplifts blocked drainage, (3) hydrologic closure that led to both intensive evaporation in the terminal sink of the Piceance Creek Basin and integration of catchments over length-scales >1000 km, (4) infilling of basin accommodation by southward migrating magmatism in distal catchments, leading to the freshening and demise of intraforeland lakes that also stepped south over time.

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