Cadmium isotopic composition in the ocean

Abstract

The oceanic cycle of cadmium is still poorly understood, despite its importance for phytoplankton growth and paleoceanographic applications. As for other elements that are biologically recycled, variations in isotopic composition may bring unique insights. This article presents i) a protocol for the measurement of cadmium isotopic composition (Cd IC) in seawater and in phytoplankton cells; ii) the first Cd IC data in seawater, from two full depth stations, in the northwest Pacific and the northwest Mediterranean Sea; iii) the first Cd IC data in phytoplankton cells, cultured in vitro. The Cd IC variation range in seawater found at these stations is not greater than 1.5 eCd/amu units, only slightly larger than the mean uncertainty of measurement (0.8 eCd/amu). Nevertheless, systematic variations of the Cd IC and concentration in the upper 300m of the northwest Pacific suggest the occurrence of Cd isotopic fractionation by phytoplankton uptake, with a fractionation factor of 1.6±1.4 eCd/amu units. This result is supported by the culture experiment data suggesting that freshwater phytoplankton (Chlamydomonas reinhardtii and Chlorella sp.) preferentially take up light Cd isotopes, with a fractionation factor of 3.4±1.4 eCd/amu units. Systematic variations of the Cd IC and hydrographic data between 300 and 700m in the northwest Pacific have been tentatively attributed to the mixing of the mesothermal (temperature maximum) water (eCd/amu=-0.9±0.8) with the North Pacific Intermediate Water (eCd/amu=0.5±0.8). In contrast, no significant Cd IC variation is found in the northwest Mediterranean Sea. This observation was attributed to the small surface Cd depletion by phytoplankton uptake and the similar Cd IC of the different water masses found at this site. Overall, these data suggest that i) phytoplankton uptake fractionates Cd isotopic composition to a measurable degree (fractionation factors of 1.6 and 3.4 eCd/amu units, for the in situ and culture experiment data, respectively), ii) an open ocean profile of Cd IC shows upper water column variations consistent with preferential uptake and regeneration of light Cd isotopes, and iii) different water masses may have different Cd IC. This isotopic system could therefore provide information on phytoplankton Cd uptake and on water mass trajectories and mixing in some areas of the ocean. However, the very small Cd IC variations found in this study indicate that applications of Cd isotopic composition to reveal aspects of the present or past Cd oceanic cycle will be very challenging and may require further analytical improvements. Better precision could possibly be obtained with larger seawater samples, a better chemical separation of tin and a more accurate mass bias correction through the use of the double spiking technique.