Holocene sedimentary record from Lake Tutira: A template for upland watershed erosion proximal to the Waipaoa Sedimentary System, northeastern New Zealand

A Holocene lake record from northeastern New Zealand provides a detailed record of environmental controls on upper watershed sedimentation, and is proximal to the Waipaoa MARGINS Source-to-Sink focus site. In that context, Lake Tutira in Hawke's Bay was cored in 2003 to recover a complete sedimentary record since the lake's formation ca 7.2 ka. The 27.14 m-long core contains alternating lithotypes that are sedimentary responses to lacustrine organic accumulation, normal to severe rainfalls, earthquakes and volcanism. A diatom allochthonous ranking scheme, pollen counts, and C and N percentages were used to identify intra-lake and watershed-derived storm deposits and modes of lithotype deposition. The lithotypes and depositional modes are: tephras (volcanic airfall); organic-rich mud (algal-rich lake sedimentation); massive to weakly graded, brown silty clay beds (homogenites and redeposited lake sediments); grey, graded sandy mud beds (intense storm-delivered sediment); and, thin yellow clay layers (run-off from small storms). Using 12 tephras and 3 radiocarbon ages to provide a chronology, the long-term sedimentation rate is ca 3.3 mm/year, which increases to > 10 mm/year following European colonisation. Storm beds occur in response to rainfall events, with no obvious correlation to El Niño-Southern Oscillation polarity or strength. Moreover, no single climate index appears to correlate strongly with the historic rainfall event record. Having characterised and identified storm-beds over the lake's history, a hindcast relationship implies that around 53 pre-historic storms occurred with a magnitude similar to the severe Cyclone Bola event of 1988, plus 7 potentially larger storm events. Despite the prominence of storm beds, a summation of the total percent thickness as an indication of the relative modes of emplacement for each lithotype shows that proportionally, the balance of intra-lake versus storm sources preserved in the lake bed is 69% and 26%, respectively. As well as storms, lake sedimentation is strongly influenced by earthquakes that destabilize the terrigenous, sediment-laden lake margins to generate homogenites, represented by the brown silty clay beds. These deposits tend to be thicker after a hiatus in seismic activity and after sustained periods of lake-margin loading, as inferred from the occurrence of thick graded storm beds. Comparison with marine records on the adjacent continental margin suggests that more terrestrial events are captur