(a) Annual and 10-year mean algal Ba/Ca ratios were derived from sample Kil and δ13C values from sample 2013-15–4 (plotted inversely on secondary x axis). The post-1960s departure to negative δ13C values is due to the anthropogenic introduction of isotopically light fossil fuels into the biosphere (‘Suess Effect’). A δ13C decline starting ∼1970 has commonly been observed in marine proxy archives throughout the North Atlantic1. Note that outliers defined as ±four times of interquartile range of raw δ13C values were removed from the comparison. Correlations between algal Ba/Ca and δ13C before the Suess Effect are displayed as scatter plots on (b) annual timescales (1870–1960; r=−0.53, P<0.001; Padj=0.0038; s.e. of Ba/Ca ±0.2e−5); and (c) Decadal timescales (1870–1960; r=−0.89, Padj=0.003). Significance levels were adjusted for loss in degrees of freedom using a lag-1 test that takes into account the temporal autocorrelation of the underlying time series (Padj=adjusted for loss of degrees of freedom).