Late Holocene hydrologic changes in northern New Zealand inferred from stable isotope values of aquatic cellulose in sediments from Lake Pupuke

Isotopic records of aquatic cellulose are becoming increasingly important for palaeohydrological reconstructions, but widespread application of this climate proxy is hampered by minerogenic contamination that affects oxygen isotope measures in cellulose. Few records of isotopes in aquatic cellulose are available from palaeoclimate archives in the Southern Hemisphere. In this study, we used a new bulk cellulose extraction method and determined the oxygen (δ¹⁸O) and carbon (δ¹³C) isotope values in cellulose from a Holocene lake sediment core segment (7.2–1.1 cal ka BP) from Lake Pupuke, Auckland, New Zealand. Isotope values from modern, potential sources of sedimentary cellulose revealed the aquatic origin of the cellulose extracted from the core, and hence enabled inference of past lake water δ¹⁸O values from the δ¹⁸O of measured cellulose in the core. A shift to a more positive water balance in the lake was identified around 2.8 cal ka BP by a decrease in inferred lake water δ¹⁸O values. At that time, greater epilimnetic primary productivity is indicated by the higher δ¹³C values of sedimentary cellulose. Greater divergence between the δ¹³C values of cellulose and bulk organic matter suggests stronger stratification of the lake, likely caused by greater freshwater input. We discuss a possible link to a solar minimum that occurred at that time.