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

Abstract

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 captured in the lake record, due to: (i) close hillslope–lake connectivity, with little intervening storage of sediment compared with the Waipaoa sedimentary system; and, (ii) the preservation of event stratigraphy at Tutira compared to its reduced preservation in the dynamic marine environment. Only major storms such as Cyclone Bola leave an imprint traceable to the ocean, whereas identifiable sedimentary responses to individual earthquakes are localized, although through landscape preconditioning and sediment production they contribute to the overall high terrigenous input to the ocean. In contrast, low-frequency, high magnitude perturbations (volcanic eruptions, European deforestation) are preserved through the Source-to-Sink sedimentary system, consistent with earlier hypotheses.