87Sr/86Sr and Element Concentration in Kauri Tree Rings to Determine Provenance of Archaeological Wood and to Detect Climate Signals

Abstract

A 4,500 year tree ring chronology (2488 BCE – 2002 AD) has been constructed for New Zealand kauri (Agathis australis). Kauri has been demonstrated to be sensitive to the southern oscillation component of the El-Niño Southern Oscillation (“ENSO”) and therefore has potential as a climate proxy. Part of the Late Holocene Kauri Chronology (“LHKC”) is comprised of archaeological material from historic buildings from the Auckland region and is of uncertain ecological provenance from within the historical kauri tree growth area. The LHKC has a westerly skew from sub-fossil swamp kauri material, and resolving the provenance of the archaeological wood will help clarify the climate signal. This thesis investigates whether ecological provenance can be determined using strontium (“Sr”) isotope ratio 87Sr/86Sr and a suite of elemental concentrations. The use of strontium ratios in trees to provenance artefact wood has precedent, but not in the New Zealand context, and has not been previously explored in kauri trees. Archaeological kauri tree wood samples from St. Paul’s Church in Kawakawa and a geographically diverse reference set were examined to establish geographic provenance. This study found that using 87Sr/86Sr ratios to establish provenance for archaeological kauri timber wood has potential, but requires a substantially larger sample population to prove its efficacy, and may be confounded by the significant influence of oceanic sea spray and rainfall that is prevalent in the island-nation of New Zealand. A central question raised during the provenance study was whether there are temporal fluctuations of 87Sr/86Sr ratios within kauri tree cores/samples, since this could confound findings and conclusions. The kauri tree cores investigated in this study revealed substantial changes in 87Sr/86Sr ratios over time, and these changes correlated with the age of the tree. The observed temporal fluctuations of 87Sr/86Sr ratios in kauri tree cores also provided an opportunity to assess if these changes reflected variability in rainfall either due to an ENSO event or due to trends in climate change. Since ecosystem 87Sr/86Sr ratios from geologic weathering and atmospheric deposition are homogenised within trees and plants, and since strontium flux is affected by the relative contribution of strontium inputs, it was hypothesised that 87Sr/86Sr ratios are impacted by changing weather, especially rainfall. The Sr ratio and elemental concentration analysis did not find strong evidence of dendrochemical imprinting of changes in rainfall. There may have been one or more dynamics that may have obscured or masked the investigated data, and the evidence suggests that biological, airborne dust deposition and hydrological forces in particular may have influenced the dendrochemical results, and these are important areas of future study.