Ocean environmental conditions can be inferred from the chemical composition of bamboo coral skeletons. The

high magnesium calcite internodes of these long-living octocorals may therefore represent a potential archive for

seawater properties such as salinity or temperature where instrumental time series are absent. To extend these

time series into the past using a natural archive the principles of temperature and salinity signal incorporation

into cold-water coral skeletal material need to be investigated. Since skeletal Na and S concentrations have been

proposed as environmental proxies, we mapped the spatial distribution and concentration of these elements in

two Atlantic specimens of Keratoisis grayi (family Isididae). These measurements were conducted with an electron

microprobe applying a spatial resolution of 4 μm. The mean apparent distribution coefficient of Na/Ca for the

two samples was within 2.5 and 2.8*10^{- 4}, while that of S shows a similar depletion relative to seawater with 3.8

and 3.6*10^{- 3}. The two elements show an inverse correlation in bamboo coral skeletons. The mean apparent

distribution coefficient of Na is similar to that of abiotic calcites. This similarity can be interpreted as the absence

of significant vital effects for skeletal Na/Ca. Hence it corroborates the idea that the average skeletal composition

of bamboo corals holds the potential to record past seawater conditions. In contrast, it appears unlikely that the

spatial variations of the element distribution of seemingly simultaneously precipitated material along growth

rings are exclusively controlled by environmental factors. We further exclude Rayleigh fractionation, ion-specific

pumping, and Ca/proton exchange as the driver of Na and S distribution in bamboo corals. Instead, we adapt a

calcification model originally proposed for scleractinians to bamboo corals. This model can explain the observed

distribution of Na and S in the skeleton by a combination of Ca/proton pumping, bicarbonate active transport,

and the formation of an organic skeletal matrix. The adapted model can further be used to predict the theoretical

behaviour of other elements and disentangle vital effects from external factors influencing compositional features.

It is therefore a useful tool for future studies on the potential of bamboo corals as environmental archives.