Vital evidence: Change in the marine 14C reservoir around New Zealand (Aotearoa) and implications for the timing of Polynesian settlement

Fiona Petchey,Magdalena M E Schmid

doi:10.1038/s41598-020-70227-3

Fiona Petchey, Magdalena M E Schmid

Open Access

https://doi.org/10.1038/s41598-020-70227-3

Copy DOI

Abstract

Precise and accurate radiocarbon chronologies are essential to achieve tight chronological control for the ~ 750-years since Polynesian settlement of New Zealand. This goal has, however, been elusive. While radiocarbon datasets in the region are typically dominated by marine and estuarine shell dates, such chronological information has been ignored by those interpreting the timing of key events because a detailed regional calibration methodology for marine shell, comparable to the highly precise Southern Hemisphere calibration curve, is lacking. In this paper, we present the first temporal 14C marine offset (ΔR) model for New Zealand based on paired estuarine/marine and terrestrial radiocarbon dates from 52 archaeological contexts. Our dataset displays significant offsets between the measured New Zealand data and the modelled global marine radiocarbon curve. These shifts are associated with oceanographic fluctuation at the onset of the Little Ice Age ~ AD 1350–1450 (650–500 BP). The application of a regional and temporal correction to archaeological shell dates provides complimentary information to terrestrial radiocarbon production and has the potential to add structure to the blurred chronology that has plagued archaeological theories about the colonization of New Zealand, and other Pacific islands, for decades.

Highlights

The short prehistory of human occupation of New Zealand (NZ) has been vigorously debated for many decades
62% of the NZ archaeological materials dated at the Radiocarbon Dating Laboratory, University of Waikato, are marine/estuarine shell
Petchey et al.[19] identified regional variation between the different outlying islands—Norfolk, Kermadec and Chatham islands—because of the complex interplay of currents and water bodies. They settled on an average ΔR-value of − 7 ± 45 14C years for NZ as a whole, with little discernible variation around the main coastline, but identified a much higher and variable ΔR for the Chatham Islands caused by upwelling of 14C-depleted water along the Chatham Rise and Subtropical Front (Fig. 1a)

Summary

Introduction

The short prehistory of human occupation of New Zealand (NZ) has been vigorously debated for many decades. The ΔR for a specific location ‘(s)’ is calculated using the formula Rs(t) – Rg(t) = ΔR(s), where (ΔR(s)) is the difference between the global average (Rg(t)) and the actual 14C activity of the surface ocean at a particular location (Rs(t); i.e., the archaeological marine date) at that particular time (based on the terrestrial 14C date or another independent means by which to determine age, such as U-Th dates) This methodology assumes that the calibration curve accounts for temporal variation. Recent 14C values of independently U-Th dated black coral 14C ages from Tasmania (Fig. 1a)[20] indicate significant reservoir shifts These shifts are evident in archaeological ΔR from central South Pacific islands[21] and have major implications for archaeological chronologies that rely on marine and estuarine shell[22]. Contained within the archaeological literature are a host of ‘paired’ (i.e., from the same context) shell and charcoal 14C dates which can help evaluate the extent of this problem for NZ

Methods

Results

Conclusion