Contrasting Common Era climate and hydrology sensitivities from paired lake sediment dinosterol hydrogen isotope records in the South Pacific Convergence Zone

Hydroclimate on 'Uvea (Wallis et Futuna) is controlled by rainfall associated with the South Pacific Convergence Zone (SPCZ), the southern hemisphere's largest precipitation feature. To extend the short observational precipitation record, the hydrogen isotopic composition of the algal lipid biomarker dinosterol (delta H-2(dinosterol)) was measured in sediment cores from two volcanic crater lakes on 'Uvea. The modern lakes differ morphologically and chemically but both contain freshwater within the photic zone, support phytoplankton communities inclusive of dinosterol-producing dinoflagellates, and experience identical climate conditions. delta H-2(dinosterol) values track lake water isotope ratios, ultimately controlled in the tropics by precipitation amount and evaporative enrichment. However, in 88-m-deep Lac Lalolalo a steadily decreasing trend in sedimentary delta H-2(dinosterol) values from -227 parts per thousand around year 988 CE to modern values as low as -303 parts per thousand, suggests this lake's evolution from an active volcanic setting to the present system strongly influenced delta H-2(dinosterol) values. Although current hydrology and water isotope systematics may now reflect precipitation and evaporation in this lake, the interaction between these processes and large changes in basin morphology, geochemistry, and hydrology obstruct the recovery of a climate signal from Lac Lalolalo's sedimentary delta H-2(dinosterol) records. This work emphasizes the importance of site replication and the use of complementary climate reconstruction tools, especially when using molecular proxies that may be sensitive to more than one environmental parameter. Contrary to its neighbor, duplicate delta H-2(dinosterol) records from 23-m-deep Lac Lanutavake varied between -277 parts per thousand and -297 parts per thousand and indicate slightly drier conditions during the time-period known as the Medieval Climate Anomaly (MCA, 950-1250 CE). The delta H-2(dinosterol) signal in Lac Lanutavake was muted compared to published records from 'Upolu (Samoa) and Efate (Vanuatu) indicating that 'Uvea's location is not as sensitive to precipitation variability at sites farther from the SPCZ central axis. Lithogenic runoff proxies combined with delta H-2(dinosterol) support the interpretation of a relatively dry MCA on 'Uvea, 'Upolu, and Efate, potentially due to less intense precipitation, a contracted, or a more zonally oriented SPCZ