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

Abstract

Many factors influence the seasonal changes in δ13C 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 δ13C 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 δ13C before the Industrial Revolution remain unclear. Based on an annually resolved coral δ13C record from the northern South China Sea, the centennial‐scale variations of coral δ13C over the past millennium were studied. The coral δ13C 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 δ13C by affecting the photosynthetic activity of the endosymbiotic zooxanthellae over centennial timescales. However, there was a decoupling of the coral skeletal δ13C and TSI during the Current Warm Period, the period in which the climate and environment became linked to anthropogenic factors. Instead, coral δ13C levels have a significant Pearson correlation with both the atmospheric CO2 concentration and δ13C levels in atmospheric CO2. The correlation between coral δ13C and atmospheric CO2 suggests that the oceanic 13C Suess effect, caused by the addition of increasing amounts of anthropogenic 12CO2 to the surface ocean, has led to the decoupling of coral δ13C and TSI at the centennial scale.

Links Between the Coral δ13C Record of Primary Productivity Variations in the Northern South China Sea and the East Asian Winter Monsoon

“…We suggest that the decreasing trend in the record of coral δ 13 C reflects a decrease in the δ 13 C values in the oceanic DIC due to the intrusion of 13 C-depleted anthropogenic CO 2 into the ocean, which is recognized as the oceanic 13 C Suess effect (Böhm et al, 1996). The centennial-scale decreasing trend in the coral δ 13 C values over the past 150 years has also been found in other coral records, and the mechanism has typically been attributed to the oceanic 13 C Suess effect (Dassié et al, 2013;Deng et al, 2017;Swart et al, 2010). This interpretation is further supported by the similar decreasing rate between our coral δ 13 C (−0.0096 ± 0.0038‰/year) and the δ 13 C of the atmospheric CO 2 (−0.0094 ± 0.0030‰/year) for the period 1851-2007 ( Figure 2b) (Francey et al, 1999;Keeling et al, 2001).…”

“…Previous studies have suggested that the seasonal variations in the coral δ 13 C values in the SCS were mainly controlled by solar insolation (Shimamura et al, ; Sun et al, ; Yu, ; Yu et al, ). Recently, however, several studies have shown that variations in the coral δ 13 C values in the SCS and in the solar insolation were unrelated on decadal timescales over the past 150 years (Deng, Wei, Xie, Ke, et al, ; Deng et al, ). To investigate the relationship between the solar insolation and the coral δ 13 C values in our study, we compared the decadal variability of coral ∆ δ 13 C and the local sunshine duration (duration of direct solar irradiance greater than 120 W/m 2 ) for the period 1970–2007 (Figure S11) and found a nonsignificant correlation between them ( r = −0.35, n eff = 6.77, p adj = 0.22; where p adj is the p value adjusted for the loss of degrees of freedom using the method of Trenberth ()).…”

Terrestrial Signature in Coral Ba/Ca, δ18O, and δ13C Records From a Macrotide‐Dominated Nearshore Reef Environment, Kimberley Region of Northwestern Australia

“…With respect to δ 13 C, terrestrial signal is less manifested on annual time series. The long‐term decreasing trend is often attributed to the addition of 13 C‐depleted anthropogenic CO 2 into the ocean, namely, the Suess effect (Deng et al, ; Swart et al, ; Linsley et al, ). To better understand local environmental effects on coral δ 13 C, this background trend was removed by the PAST software.…”

Carbon stable isotope record in the coral species Siderastrea stellata: A link to the Suess Effect in the tropical South Atlantic Ocean

“…However, many studies have shown a range of additional factors that may play significant roles in the seasonal variations of coral δ 13 C. Amongst these are water depth (e.g., Fairbanks and Dodge 1979;Swart et al 1996;Grottoli and Wellington 1999), heterotrophic level (Grottoli and Wellington 1999), kinetic isotope fractionation (McConnaughey 1989b), reproduction (Gagan et al 1994), bleaching (Leder et al 1991), spawning (Gagan et al 1994) and the δ 13 C of regional DIC (Swart et al 2010;Al-Rousan and Felis 2013;Dassié et al 2013;Deng et al 2017) .…”

“…Beyond the complicated short-term variations in coral δ 13 C, decadal-to centennialscale changes in coral δ 13 C are observed and related to modifications of the δ 13 C of the surface water DIC (Swart et al 1996(Swart et al , 2010Al-Rousan and Felis 2013;Dassié et al 2013;Deng et al 2017). Long-term decreases in coral δ 13 C have first been reported as early as the 1970s (Nozaki et al 1978).…”