Libby A. Stern, Gary D. Johnson, C. Page Chamberlain; Carbon isotope signature of environmental change found in fossil ratite eggshells from a South Asian Neogene sequence. Geology ; 22 (5): 419–422. doi: https://doi.org/10.1130/0091-7613(1994)022<0419:CISOEC>2.3.CO;2
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A >10-m.y.-long sequence of ratite (e.g., ostrich) eggshells from Siwalik Group sedimentary deposits of northern Pakistan and India shows a dramatic (∼8‰) increase in the δ13C in biomineralogic calcite by ∼4 Ma. Values of δ13C from ratite eggshell carbonate older than 7 Ma are about the same as those from carbonate from contemporaneous fossil mammal tooth enamel and paleosols. However, after 4 Ma, the ratite eggshell carbonate δ13C values are ∼5‰ less than these coexisting materials. These findings are evidence of the development of a C3-C4 vegetative mosaic probably mirroring the sedimentologic-edaphic mosaic of the aggrading alluvial plain of the Siwalik depositional system.