A GEOCHEMICAL ARCHIVE OF CLIMATE VARIABILITY IN KIRITIMATI ISLAND LAKE SEDIMENT
This study presents new geochemical and sedimentological data from a small brackish lake on Kiritimati Island (1.9° N, 157.4° W). The 2000-year sediment record is composed of carbonate mud, alternating with microbial mats and gypsum-rich sediment. We hypothesize the sections of carbonate mud indicate periods of high lake water balance, whereas microbial mats and gypsum-rich sediment indicate periods of reduced water balance, enhanced salinity and extended aridity. We find the presence of gypsum and microbial mats from approximately 900 to 1400 AD is coincident with the Medieval Climate Anomaly (MCA). Our results thus support other proxy evidence of a cool and dry MCA in the eastern and central tropical Pacific at this time. A transition to increased carbonate sediment, and hence higher water balance, at the advent of the Little Ice Age also supports the hypothesis of a southward shift in the ITCZ at this time, although the Little Ice Age does not appear anomalously wet in Kiritimati relative to the 19th and 20th centuries.
Investigation of higher frequency variability in the Kiritimati sediment record indicates the microbial mats grew at intervals of 20-75 years, indicating multidecadal variability in lake water balance throughout the MCA. Calcium and sulfur abundance measured by X-ray fluorescence (XRF) and loss on ignition (LOI) also supports this sedimentological interpretation of lake water balance variability. The periodicity of sub-mm scale laminations within the buried microbial mats, which has been hypothesized to reflect ENSO variability, indicates mat-carbonate laminae are too frequent to be related to seasonal or ENSO periodicity, and are more likely related to the organization of microbial communities within the microbial mats and organomineralization.