Foliar and soil N and δ15N as restoration metrics at Pūtaringamotu Riccarton Bush, Christchurch city

Growing a community's sense of 'heritage': a former mataī forest from SW Ōtautahi Christchurch and its nitrogen status, 860-1430 CE

As in many places, heritage narratives in Ōtautahi Christchurch remain focused on colonial times. To expand these narratives, we examined three subfossil mataī (Prumnopitys taxifolia) excavated along a 700 m arc during residential subdivision earthworks. Characterisation used δ 15N, radiocarbon, and ring width measurements. Conventional (uncalibrated) radiocarbon ages were 1222 BP (calibrated median 856 CE), 932 BP (1148 CE) and 545 BP (1425 CE). Mean δ 15N values of +1.0 ± 1.1‰ (±SD) were similar to nearby alluvial forest remnants, with no discernible change through the 550-year sampling interval. Ring widths in three trees from the 545 BP site were 0.58 ± 0.12 mm, half that of present day mataī from humid West Coast forests at a similar latitude. The 545 BP site included a burnt stump, and stumps with longhorn beetle (Cerambycidae) larval galleries that occur only in already dead trees. We conclude that the forest was slow growing and mesotrophic but dynamic due to periodic flooding, ending with fire around 1430 CE and inundation of the dead standing forest years or decades later. We are now using traditional and social media and artists to disseminate the narrative of a former mataī forest 'beneath our feet' through schools, community groups, and in public facilities.

Evidence for magmatic carbon bias in 14C dating of the Taupo and other major eruptions

Prehistoric timescales, volcanic hazard assessment, and understanding of volcanogenic climate events rely on accurate dating of prehistoric eruptions. Most late Quaternary eruptions are dated by ¹⁴C measurements on material from close to the volcano that may be contaminated by geologic-sourced infinite-age carbon. Here we show that ¹⁴C ages for the Taupo (New Zealand) First Millennium eruption are geographically arrayed, with oldest ages closer to the vent. The current eruption wiggle match date of 232 ± 5 years CE is amongst the oldest. We present evidence that the older, vent-proximal ¹⁴C ages were biased by magmatic CO₂ degassed from groundwater, and that the Taupo eruption occurred decades to two centuries after 232 CE. Our reinterpretation implies that ages for other proximally-dated, unobserved, eruptions may also be too old. Plateauing or declining tree ring cellulose δ¹³C and Δ¹⁴C values near a volcano indicate magmatic influence and may allow forecasting of super-eruptions.

Correlations between prehistoric eruptions and other phenomena depend on accurate dating of the eruption. Here the authors show that magmatic CO₂ in groundwater can bias radiocarbon ages for eruptions and that plateaux of carbon isotopic values in tree ring sequences biased by magmatic CO₂ foreshadow major eruptions.