Quantitative assessment of volcanogenic aluminosilicate weathering in the northern Hikurangi margin sediments and its global implication

Abstract:

Volcanogenic aluminosilicates (VAs), present either as discrete tephra layers or dispersed fine grain materials within the sediment matrix, have been recognized as important components of marine sediments. Due to their abundance and high reactivity, VAs play a crucial role in local and probably global element cycles. In particular, VA weathering can influence the long-term atmospheric CO₂ concentration and global carbon cycle, thereby affecting global climate change. Moreover, microbial activity both enhances the alteration of volcanic glasses and benefits from the cations released by VA alteration, thus sustaining the subsurface biosphere. However, since the alteration of VAs represents a set of coupled reactions with unconstrained kinetics, it is difficult and challenging to quantify its in-situ rate.

Here we present an example from the Hikurangi margin offshore New Zealand, where ongoing VA alteration is documented and quantified by the concurrent downcore increase in pore-water Sr concentration and decrease in ⁸⁷Sr/⁸⁶Sr in the upper 150 mbsf of ash-bearing sediments of four sites drilled during IODP Expeditions 372/375, and one site previously sampled by the MeBo seafloor drill. Higher model-derived ⁸⁷Sr/⁸⁶Sr ratios of VAs (0.7082-0.7084) than that measured in volcanic glass imply a concomitant alteration of other volcanogenic phases and/or clay mineral formation. The non-steady-state modeling exercise shows a significant increase in the seafloor flux of VAs at sites that experienced slide emplacement, which leads to enhanced VA alteration rates compared to sites not impacted by mass transport deposition.

All the Ca released from VA alteration was fixed as authigenic carbonate at all four sites, accounting for the majority of Ca consumption. Si released by VA alteration was predominantly precipitated as authigenic clay minerals. For site GeoB20802, we also show that the iron released from VA alteration serves as an electron acceptor for anaerobic methane oxidation. This study illustrates that sediment deposition patterns can impact the VA alteration rates, that cations released by VA alteration are mostly recycled as the formation of authigenic minerals, and that these cations can influence biotic and abiotic carbon turnover with implications for global carbon and silicon cycles.