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 δC 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 δC over the past 200 years reflects the changes in the additional flux of anthropogenic CO from the atmosphere into the surface oceans. Even so, the centennial-scale variations, and their significance, of coral δC before the Industrial Revolution remain unclear. Based on an annually resolved coral δC record from the northern South China Sea, the centennial-scale variations of coral δC over the past millennium were studied. The coral δC 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 δC by affecting the photosynthetic activity of the endosymbiotic zooxanthellae over centennial timescales. However, there was a decoupling of the coral skeletal δC and TSI during the Current Warm Period, the period in which the climate and environment became linked to anthropogenic factors. Instead, coral δC levels have a significant Pearson correlation with both the atmospheric CO concentration and δC levels in atmospheric CO. The correlation between coral δC and atmospheric CO suggests that the oceanic C Suess effect, caused by the addition of increasing amounts of anthropogenic CO to the surface ocean, has led to the decoupling of coral δC and TSI at the centennial scale.