Séminaire

Climate records in the western tropical Atlantic that robustly resolved seasonality and interannual to multidecadal variability beyond the short period of instrumental observations are extremely rare. On these timescales, ocean-atmosphere interactions originating from both Atlantic and Pacific basins play a critical role for society-relevant climate extremes such as droughts, floods and hurricanes in the Atlantic region. Therefore, better understanding the natural range of ocean-atmosphere variability on seasonal to multidecadal timescales is a prerequisite for more reliable projections of future climate change in this region.

Massive shallow water tropical corals are unique archives for studying climate variability on these timescales. Coral-based climate reconstructions have mainly focused on the genus Porites in the Indo-Pacific region. However, similar reconstructions in the tropical Atlantic remains underrepresented. Therefore, it is important to generate accurate records of past climate variability in this region by using the geochemistry of massive corals from the Atlantic and Caribbean.

Here we present monthly-resolved paired Sr/Ca (temperature proxy) and δ¹⁸O (a combined temperature and hydrology proxy) records from well-dated (U-series dating) annually-banded Holocene corals. Our reconstructions are derived from fossil Diploria strigosa corals recovered from coastal deposits on the island of Bonaire (southern Caribbean Sea). Both proxies reveal clear annual cycles. Accurate internal age control enables us to investigate changes in seasonality and interannual to interdecadal variability of both temperature and hydrology at the sea surface throughout the mid- to late Holocene. Well-preserved coral colonies provide a unique opportunity to study past climate dynamics in close proximity to the Caribbean Current and to the present-day northernmost position of the Intertropical Convergence Zone (ITCZ) in this region. 

The Bonaire corals provide time-windows of up to 68 years length, and the total number of 295 years of record allows for assessing the natural range of seasonality and interannual to multidecadal SST variability in the western tropical Atlantic during snapshots of the mid- to late Holocene. The amplitude of the SST annual cycle in the southern Caribbean Sea was slightly higher during the mid-Holocene compared to today which 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 that was not observed at 6.22 ka BP. Moreover, SST reconstruction from the 6.22 ka coral indicates pronounced variability at inter- to multidecadal timescales, relative to today and 2.35 ka BP, which is also observed in the corresponding δ¹⁸O seawater reconstruction. Coral records suggest that warmer SST were accompanied by more saline conditions and vice versa. Potential forcing mechanisms of the reconstructed SST and salinity variations are discussed with respect to tropical Pacific-Atlantic interactions, as well as potential relationships to variations in the tropical rain belt and the Atlantic Meridional Overturning Circulation.