The late Quaternary palaeoclimate record in kauri tree rings

Abstract

Kauri (Agathis australis) contains a significant El Nino-Southern Oscillation (ENSO) signal and is a prime candidate for the creation of long tree-ring chronologies. Uniformitarianism and geographic issues central to palaeoclimate reconstruction from kauri tree rings were examined. Response function analysis supports the decision to exclude high elevation data from a region-wide chronology for palaeoENSO reconstruction. Pooling all other kauri samples helps enhance the ENSO climate signal, and opportunistic use of building timbers to supplement sample depth where needed is appropriate. Kauri <0.65m diameter at breast height had a unique climate response, which may have an impact on chronology characteristics when sample depth is low. For sub-fossil samples with pith, the age-related signal must be addressed prior to climate analysis. Regional curve standardisation (RCS) or truncation of series may be ways to deal with this issue. A swamp kauri analogue site at Waitakere Reservoir showed no prolonged negative effects associated with dam construction, suggesting transfer functions constructed from living kauri growing predominantly on north-facing hilltops and ridge sites can be applied to sub-fossil kauri sourced from lowland peat bogs. RCS was assessed for three types of kauri samples having different amounts of metadata, and shows considerable promise. Multiple error sources exist in RCS that can affect chronology characteristics, and so it is advisable to use it alongside other standardisation methods in palaeoclimate analysis. Sub-fossil kauri growth curves suggest environmental stress could represent a significant portion of the long-term growth trend for some Late Holocene swamp kauri. Climate regime classification was used to reconstruct past circulation for New Zealand, which has changed markedly during the past 4000 years. Palaeoenvironmental changes were likely caused by increased frequency of synoptic types that characterise zonal, blocking, or trough regimes that contribute to regional precipitation and temperature characteristics. Late Holocene climate regime time slices closely correspond to kauri spectral signature features, indicating a strong atmospheric circulation component is probably captured by kauri tree rings. Omaha Flats kauri chronologies dated from Oxygen Isotope Stage 3 (OIS3; 26,000-60,000 years before present) are some of the oldest annually-resolved palaeoclimate records in the World. Late Holocene and OIS3 kauri chronology similarities suggest ENSO may have been active in-part during OIS3; however, low-frequency climate drivers possibly related to polar circulation were probably more important during this time. Physical evidence of enhanced zonal flow during OIS3 is indicated by kauri tree-fall patterns at Omaha Flats that have orientations pointing to the northeast. From these findings, it is hypothesised that ENSO- driven circulation changes on sub-decadal to multi-decadal scales are potentially more important for dictating regional climate variations in New Zealand during periods of warmer global temperatures. A comparison of OIS3 and Late Holocene chronology signatures suggests the 'Little Ice Age' (LIA) in New Zealand (1650AD to the Late 19th Century AD) was anomalous in the context of the last 4000 years. When interpreted using multi-proxy information, this comparison suggests the LIA may have been most similar to climatic conditions experienced during parts of the pre-Last Glacial Maximum.