Calcium isotope ratios in the world's largest rivers: A constraint on the maximum imbalance of oceanic calcium fluxes

摘要

The oceanic mass balance of calcium (Ca) is defined by a balance between the inputs (rivers and hydrothermal) and outputs (bulk carbonate) of Ca. Large rivers were analyzed for Ca isotope ratios (~(44)Ca/~(42)Ca, expressed as δ~(44/42)Ca) to investigate the source and cycling of riverine Ca, and to add an isotopic mass balance constraint to the oceanic budget of Ca. The new data account for approximately one-third of the total Ca supplied to the oceans by rivers. Inter-sample and seasonal variability was assessed by analyzing more than one sample for many rivers. The range in the δ~(44/42)Ca of large rivers at high water stand is extremely narrow at 0.27‰. Variations in δ~(44/42)Ca do not correlate with proxies for carbonate, silicate or evaporite derived Ca, and are more likely related either to inherent variability in the lithological sources of Ca or to process related fractionation. The spread in riverine δ ~(44/42)Ca overlaps with the spread in marine limestone δ~(44/42)Ca consistent with most riverine Ca coming from the recycling of limestones. The Ca isotope composition of continental runoff has an average δ~(44/42)Ca value of 0.38 0.04‰, identical to recent (5 M.yr) bulk carbonate ooze (0.33 0.13‰, 2S.D.). Isotopic mass balance constrains that the input and output fluxes of Ca to and from the oceans, are balanced to within 15% over time-scales similar to the residence time of Ca in the oceans (1 M.yr). A greater imbalance between the fluxes would result in a detectable difference between the δ~(44/42)Ca value of bulk carbonate and the riverine input at the current level of uncertainty. The input and output fluxes could be imbalanced over much shorter time-scales (such as glacial-interglacial cycles), in which case the ocean-carbonate system will not yet have responded, because of the long residence time of Ca. The maximum current flux imbalance of 15% would be sufficient to account for the total variations in Ca concentration over the Tertiary. Such an interpretation is not unique, but is the simplest interpretation given the similarity between the input and output isotopic compositions, and rules out hypotheses of extreme imbalance in the recent global biogeochemical cycle of Ca.