Sedimentary calcite Zn isotope composition as a possible paleo pH proxy – a case study from eutrophic Lake Greifen, Switzerland

The pH is a fundamental determinant for physico-chemical reactions in aquatic environments but notoriously difficult to reconstruct in paleo records. Experimental studies have shown that the δ⁶⁶Zn isotopic composition of calcite is strongly pH dependent in the 7.0 ≤ pH ≤ 8.0 range, with lower δ⁶⁶Zn at higher pH, making it a suitable pH proxy in settings with endogenic calcite precipitation (Mavromatis et al. 2019). Here we applied the calcite δ⁶⁶Zn pH proxy to a sediment core covering the recent anthropogenic eutrophication at Lake Greifen, Switzerland to test its applicability in a natural setting that has undergone a productivity-driven shift in surface water pH. Lake Greifen is a hard water lake with a surface water pH ranging between >8.5 during spring and summer algal blooms and ~7.5 in the remainder of the year. Sediments deposited in the center of the lake are predominantly composed of endogenic calcite (>75 %wt) with the remainder comprising varying amounts of organic matter and biogenic silica and only minor detrital matter. Pure calcite samples for this study were collected from sediments deposited during both the oligotrophic and eutrophic lake phases to capture a wide range in pH. Lake Greifen sedimentary calcite δ⁶⁶Zn values relative to the JMC3-0749L standard range between 0.71 ± 0.05 ‰ in sediments deposited under oligotrophic conditions to 0.29 ± 0.01 ‰ in sediments deposited under eutrophic conditions. Lower δ⁶⁶Zn values during the more recent eutrophic lake phase may thus be explained by a productivity-driven change in surface water pH. Interestingly, δ⁶⁶Zn values around 0.3 ‰ have been reported for calcite precipitating at pH >8 in experimental studies, which is very close to Lake Greifen’s modern surface water pH recorded during spring and summer algal blooms. δ⁶⁶Zn values around 0.7 ‰, reported in samples originating from the less productive oligotrophic phase of Lake Greifen, are indicative for a pH <7 in experimental studies. Additional data from other settings with varying δ⁶⁶Zn water isotope compositions are required to further confirm our findings but the preliminary results of this study suggest δ⁶⁶Zn_calcite to be a promising pH proxy candidate in paleo records.

 

Reference

Mavromatis, V.; Gonzalez, A.G.; Dietzel, M.; Schott, J. (2019) Zinc isotope fractionation during the inorganic precipitation of calcite – Towards a new pH proxy. Geochimica et Cosmochimica Acta 244, 99 – 112.