Biogenesis of landscapes: Insights from calcium and strontium isotopes

Abstract

Previous studies conducted at the Chillinup Nature Reserve (host to Lake Chillinup in southwestern WA) revealed the interconnected relationship between Eucalyptus lateral roots and clay pods hosted within its rooting zone. This bio-geological relationship between lateral roots and clay pods hosted within quartzitic sand dune, poses the problem of how Fe and Al rich soil pod structures could be formed from the percolation of freshwater through regolith depleted in such elements. It has been argued whether these clay pod structures have been produced by inorganic or via biological processes, where the former would involve chemical leaching of elements from top-soils and their accumulation at deeper depths (i.e., pod sites). Alternatively, the biological origin (i.e., phytotarium concept) involves hydraulic uplift of Fe and Al by eucalyptus trees and their subsequent release via roots at sites of pods formation. In this study, we applied elemental and isotope analyses on clay pods, soils, waters and eucalyptus organic tissues. Specifically, we used novel Ca and Sr isotopic proxies (87Sr/86Sr and δ44Ca) coupled with elemental mobility profiles (tau (τ) normalized elemental profiles) with the impetus to confirm the source(s) of material for the formation and origin of the clay pods at the Lake Chillinup site. Our Ca and Sr isotope constraints revealed that eucalyptus trees access their nutrients and water sources primarily from a perched freshwater reservoir within the shallower soils above the deeper saline groundwaters. In addition, the tau-normalised profiles confirmed systematic leaching of all analysed elements (i.e., alkali earth metals and transition metals) from the top acidic soil, with subsequent enrichments of Al, Fe, Cr and Ge within the pod horizon (~10 to 50 cm depths), followed by local enrichments of Ca and Sr in deeper and more alkaline soils, below the pod horizons (~100 cm) where calcareous soils and structures were also observed. Importantly, our 87Sr/86Sr and δ44Ca data confirmed that alkali earth metals, such as Ca and Sr, in the studied clay pods originate from the local inorganic mineral sources, and our results do not support the biogenic origin of Ca and Sr in pod structures. However, these conclusions might not be directly applicable to the origin of Fe and Al in soil pods, as these major elements may still be sourced from deeper soils via hydraulic uplift mediated by eucalyptus trees. Overall, this study contributes to the growing database of Ca and Sr isotope measurements from the earth’s surface environments and terrestrial ecosystems, with implications for the global Ca and Sr biogeochemical cycles.