Ocean 10Be/9Be as denudation rate proxy. Does 9Be deliver?

The ratio of meteoric cosmogenic ¹⁰Be to that of stable ⁹Be in seawater has been suggested to serve as a proxy for terrestrial weathering and denudation rates (D), in the modern ocean [1], and in the past when measured in chemical sediment of known age [2, 3]. The principle is remarkably simple. The only input of ¹⁰Be is atmospheric deposition into seawater. This flux is well-known at the scale of ocean basins. The trace metal⁹Be enters the oceans after continental weathering via two potential pathways: a) direct riverine input into the coastal ocean (both dissolved and mobilised from particles); b)  the release of “reactive” terrigenous Be from particles into seawater during early marine diagenesis, called “boundary exchange”. When the dissolved ⁹Be is mixed with seawater, the unknown weathering and denudation input flux of ⁹Be can be calculated from the ¹⁰Be/⁹Be ratio.

However, because Be is an element that readily attaches to reactive particles, not all riverine ⁹Be escapes the coastal zone. We have estimated this delivery fraction (f_del) to be about 6% of the dissolved and adsorbed riverine Be [1]. For pathway a) we already suggested the possibility that with changing sediment delivery to the coastal ocean, f_del might potentially be a function of D itself [1]. However, global river data show that river particle concentration and D are not correlated. Yet, this erroneous assumption was made to suggest that ¹⁰Be/⁹Be fails to serve as a denudation rate proxy [4].

In any case such dependence does not affect pathway b) “boundary exchange” [5]. This pore water input may even dominate the marine ⁹Be budget [6].

Research is thus required to evaluate all of these potential input pathways of ⁹Be, how strongly sediment delivery onto the seafloor, also being a function of particulate riverine input flux, controls the release flux of ⁹Be, and whether its release is buffered in any way. Given the simple pathway of the known ¹⁰Be input, the ¹⁰Be(meteoric)/⁹Be ratio offers much potential to explore these fluxes, both in the terrestrial and the marine domain, and to evaluate their dependence on denudation and delivery – even three decades  after the first introduction of this system [7].

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