Radiogenic ⁴⁰Ca in Seawater: Implications for Modern and Ancient Ca Cycles

Abstract

Radiogenic 40Ca is preferentially concentrated in the continental crust through the decay of radioactive 40K and may have the potential to be used as a tracer for Ca fluxes to the ocean through time. Numerous published flux estimates suggest that rivers are the dominant source of Ca to the oceans. This conflicts, however, with conclusions drawn from previous radiogenic Ca data suggesting that seawater Ca has been dominated by weathering/hydrothermal alteration of oceanic crust throughout Earth history. We attempt to address this discrepancy by carrying out a larger number of radiogenic Ca measurements on materials that represent modern seawater, marine carbonates, and ocean floor basalt. We find that the 40Ca/44Ca composition of the oceanic crust and mantle appear to be different from modern seawater and marine carbonates, such that the latter are measurably enriched in radiogenic 40Ca (εCa = +1.1 ± 0.3, 2SE) relative to basalts and deep-sea hydrothermal fluid. This observation is consistent with most other data available in the literature. The results are also consistent with Sr isotope data and confirm that continental sources of Ca (mainly from rivers and groundwater) dominate the modern seawater budget. We find that off-axis Ca fluxes from the low temperature alteration of the oceanic crust are not large enough to change this balance. The Ca isotope data measured and compiled here also suggest that bulk-silicate earth 40Ca/44Ca is 1.2 ± 0.3 ε-units lower than reference material SRM915a and that variations in seawater εCa in the geologic past are likely too small to be resolved with current analytical techniques.

High-precision analysis of calcium isotopes using a Nu Sapphire collision cell (CC)-MC-ICP-MS

“…Previous studies have demonstrated that 3 40 Ca is a robust tracer in petrogenesis, early crustmantle evolution, the oceanic Ca cycle and post-formation K depletion of thelower continental crust. 2,3,[33][34][35][36][37] A new generation dual-path MC-ICP-MS equipped with a collision cell (CC-MC-ICP-MS), known as the Sapphire, was launched by Nu Instruments in 2017. The collision cell technique has been used in conjunction with quadrupole ICP-MS for nearly 20 years.…”

“…Previous studies have demonstrated that ε 40 Ca is a robust tracer in petrogenesis, early crust-mantle evolution, the oceanic Ca cycle and post-formation K depletion of thelower continental crust. 2,3,33–37…”

Decoupling of physical and chemical erosion in the Himalayas revealed by radiogenic Ca isotopes

“…However, unlike Sr, calcium is a major element that plays a pivotal role in the carbon cycle and thus can provide more direct constraints on the relative contribution of silicate and carbonate weathering to riverine dissolved fluxes. Previous investigations of 40 K- 40 Ca systematics of crustal rocks showed that silicate lithologies of the upper crust have variably radiogenic 40 Ca excesses whose magnitude depends on their K/Ca ratio and crustal residence time (Antonelli et al, 2021(Antonelli et al, , 2019Kreissig and Elliott, 2005;DePaolo, 1989, 1982;Mills et al, 2018;Nelson and Mcculloch, 1989). Considering an average K/Ca ratio of 0.95 (Taylor and McLennan, 1985) for the upper continental crust, and a constant rate of crustal growth starting 4 Gyr ago, a total radiogenic ingrowth of 2-3 ε-units over the mantle value can be estimated for the present-day upper crust (Caro et al, 2010;Antonelli et al, 2021).…”

“…Previous investigations of 40 K- 40 Ca systematics of crustal rocks showed that silicate lithologies of the upper crust have variably radiogenic 40 Ca excesses whose magnitude depends on their K/Ca ratio and crustal residence time (Antonelli et al, 2021(Antonelli et al, , 2019Kreissig and Elliott, 2005;DePaolo, 1989, 1982;Mills et al, 2018;Nelson and Mcculloch, 1989). Considering an average K/Ca ratio of 0.95 (Taylor and McLennan, 1985) for the upper continental crust, and a constant rate of crustal growth starting 4 Gyr ago, a total radiogenic ingrowth of 2-3 ε-units over the mantle value can be estimated for the present-day upper crust (Caro et al, 2010;Antonelli et al, 2021). In contrast, mafic and ultramafic rocks have low K/Ca on the order of 0.01-0.1, resulting in negligible 40 Ca ingrowth (0.1 -1 ε-units) over the entire Earth's history.…”

“…The 40 K-40 Ca decay scheme results from the decay branch of Potassium-40, which represents 89% of total K decay (λ TOT = 5.543 x 10 -10 year -1 ; λ  = 4.962 x 10 -10 year -1 ) (Begemann et al, 2001;Nägler and Villa, 2000;Naumenko-Dèzes et al, 2018). Despite its evident potential as a source tracer in the exogenic Ca cycle, the 40 K-40 Ca system has rarely been applied as an environmental tracer (Antonelli et al, 2021;Caro et al, 2010;Davenport et al, 2014;Farkas et al, 2011;Hindshaw et al, 2014;Lehn et al, 2017;Ryu et al, 2011;Zhu and Mac Dougall, 1998). This is mainly due to analytical difficulties associated with detecting small radiogenic excesses on the large 40 Ca isotope (96.9%) using the minor 42 Ca isotope to correct for instrumental mass fractionation.…”