Anomalously cool clumped isotope temperatures in tropical lagoon carbonates

Carbonate clumped isotopes are a powerful tool for paleoclimate reconstruction due to the ability to reconstruct past changes in both temperature and precipitation-evaporation balance. Here we test the utility of this method on last millennium carbonate lagoonal sediments from Kiritimati, a coral atoll where modern climate variability is driven by interannual changes in the El Niño-Southern Oscillation. We find last millennium lagoonal temperatures from clumped isotopes are cooler than anticipated compared to modern measurements and other paleoclimate reconstructions. This discrepancy is probably due to sediments containing a mixture of high-magnesium calcite derived from primary precipitates and benthic foraminifera and aragonite derived from warm water corals. We employed an inverse mixing model to minimise the impact of vital effects related to coral growth on clumped isotope compositions and found an increasing difference between modelled and measured T(Δ₄₇) values through time. This potentially indicates that the composition of lagoon water became increasingly unique from the coral carbonate formation waters through the last millennium. This study highlights the necessity of detailed understanding of carbonate mineralogy, sedimentology and provenance in interpreting clumped isotope temperature reconstructions.