Decoupling of coral skeletal δ13C and solar irradiance over the past millennium caused by the oceanic Suess effect

Many factors influence the seasonal changes in delta C-13 levels in coral skeletons; consequently, the climatic and environmental significance of such changes is complicated and controversial. However, it is widely accepted that the secular declining trend of coral delta C-13 over the past 200 years reflects the changes in the additional flux of anthropogenic CO2 from the atmosphere into the surface oceans. Even so, the centennial-scale variations, and their significance, of coral delta C-13 before the Industrial Revolution remain unclear. Based on an annually resolved coral delta C-13 record from the northern South China Sea, the centennial-scale variations of coral delta C-13 over the past millennium were studied. The coral delta C-13 and total solar irradiance (TSI) have a significant positive Pearson correlation and coupled variation during the Medieval Warm Period and Little Ice Age, when natural forcing controlled the climate and environment. This covariation suggests that TSI controls coral delta C-13 by affecting the photosynthetic activity of the endosymbiotic zooxanthellae over centennial timescales. However, there was a decoupling of the coral skeletal delta C-13 and TSI during the Current Warm Period, the period in which the climate and environment became linked to anthropogenic factors. Instead, coral delta C-13 levels have a significant Pearson correlation with both the atmospheric CO2 concentration and delta C-13 levels in atmospheric CO2. The correlation between coral delta C-13 and atmospheric CO2 suggests that the oceanic C-13 Suess effect, caused by the addition of increasing amounts of anthropogenic (CO2)-C-12 to the surface ocean, has led to the decoupling of coral delta C-13 and TSI at the centennial scale.