10Be/9Be Ratios Reveal Marine Authigenic Clay Formation

A Bernhardt,H Wittmann,M Christl,M Mohtadi,F Von Blanckenburg,J Bouchez,M Oelze

doi:10.1029/2019gl086061

Abstract

AbstractAs reverse weathering has been shown to impact long‐term changes in atmospheric CO2 levels, it is crucial to develop quantitative tools to reconstruct marine authigenic clay formation. We explored the potential of the beryllium (Be) isotope ratio (10Be/9Be) recorded in marine clay‐sized sediment to track neoformation of authigenic clays. The power of such proxy relies on the orders‐of‐magnitude difference in 10Be/9Be ratios between continental Be and Be dissolved in seawater. On marine sediments collected along a Chilean margin transect we chemically extracted reactive phases and separated the clay‐sized fraction to compare the riverine and marine 10Be/9Be ratio of this fraction. 10Be/9Be ratios increase fourfold from riverine to marine sediment. We attribute this increase to the incorporation of Be high in 10Be/9Be from dissolved biogenic opal, which also serves as a Si‐source for the precipitation of marine authigenic clays. 10Be/9Be ratios thus sensitively track reverse‐weathering reactions forming marine authigenic clays.

Highlights

Reverse weathering has been recently recognized as an important factor of the ocean alkalinity balance, as HCO3− dissolved in seawater is converted into CO2 which is released back into the atmosphere with implications for global climate (Dunlea et al, 2017; Isson & Planavsky, 2018)
We attribute this increase to the incorporation of Be high in 10Be/9Be from dissolved biogenic opal, which serves as a Si‐source for the precipitation of marine authigenic clays. 10Be/9Be ratios sensitively track reverse‐weathering reactions forming marine authigenic clays
We observed a fourfold increase in 10Be/9Be isotope ratios of Be in the clay‐sized fraction of marine sediments when compared to the 10Be/9Be ratios of the corresponding terrestrial riverine source

Summary

Introduction

Reverse weathering has been recently recognized as an important factor of the ocean alkalinity balance, as HCO3− dissolved in seawater is converted into CO2 which is released back into the atmosphere with implications for global climate (Dunlea et al, 2017; Isson & Planavsky, 2018). Despite the importance of this phenomenon for the long‐term evolution of the ocean chemistry and atmospheric CO2, unambiguous proxies are still lacking to detect and quantify reverse‐weathering reactions, mostly because marine authigenic clays are difficult to distinguish from detrital material by sediment geochemistry or mineralogical analyses (Rahman et al, 2016). The stable isotope of beryllium, 9Be, is released from bedrock during weathering and introduced to the ocean in the dissolved form where it mixes with the dissolved cosmogenic and radioactive nuclide 10Be (meteoric).

Methods

Results

Discussion

Conclusion