Southern Caribbean climate variability during the mid to late Holocene from monthly resolved coral records

In the western tropical Atlantic, paleoceanographic reconstructions that robustly resolve seasonality and interannual to interdecadal variability beyond the short period of instrumental observations are extremely rare. Ocean-atmosphere interactions on these timescales play a critical role for society-relevant climate extremes such as droughts, floods and hurricanes. Consequently, a better understanding of the natural range of ocean-atmosphere variability on seasonal to interdecadal timescales is a prerequisite for more reliable projections of future climate change in this region. We present monthly resolved reconstructions of sea surface temperature (SST) and salinity variations in the southern Caribbean Sea for snapshots throughout the mid to late Holocene. Our reconstructions are derived from fossil shallow-water reef corals recovered from coastal deposits on the island of Bonaire. Well-preserved colonies of the annually-banded coral Diploria strigosa provide a unique opportunity to study past ocean-atmosphere dynamics in close proximity to the Caribbean Current and to the present-day northernmost position of the Intertropical Convergence Zone (ITCZ) in this region. Our monthly resolved records of Sr/Ca and oxygen isotopes derived from the dense theca wall of the aragonitic Diploria strigosa skeleton allows for reconstructing both temperature and delta18O seawater variations at the sea surface, robustly resolving seasonality and interannual to interdecadal variability. The Bonaire corals were accurately dated by U-series and provide a total of nearly 300 years of monthly resolved record distributed over a number of snapshots throughout the last 6200 years, with individual time windows reaching up to 68 years in length. The results indicate that the amplitude of the SST annual cycle in the southern Caribbean Sea was similar to today during most of the coral-based snapshots throughout the last 6200 years. A slightly higher SST seasonality relative to today observed at 6.2 ka BP is consistent with a climate model simulation (COSMOS) and can be explained by insolation changes on orbital timescales. However, a significantly increased SST seasonality relative to today is observed at 2.35 ka BP, which most likely results from internal dynamics of the climate system. Interestingly, the corresponding 68-year time series reveals significant variability at typical ENSO periods, suggesting a prominent ENSO signal in southern Caribbean SST at 2.35 ka BP. In contrast, a prominent ENSO signal is not observed at 6.2 ka BP. However, the 6.2 ka BP SST reconstruction indicates pronounced variability at inter- to multidecadal timescales, relative to today and 2.35 ka BP, which is also observed in the corresponding delta18O seawater reconstruction. Apparently, higher SST in the southern Caribbean Sea on these timescales was accompanied by more saline conditions and vice versa. Potential forcing mechanisms of the reconstructed mid- to late Holocene southern Caribbean SST and salinity variations with respect to both tropical Pacific and Atlantic origin are discussed, as well as potential relationships to variations in the tropical rainbelt and the Atlantic Meridional Overturning Circulation.