Cadmium isotope variations in the Southern Ocean

• Published in: Earth and Planetary Science Letters
• Main Authors: Xue, Zichen, Rehkämper, Mark, Horner, Tristan J., Abouchami, Wafa, Middag, Rob, van de Flierd, Tina, de Baar, Hein J.W.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2013
• Subjects: cadmium; HNLC; chemical oceanography; stable isotopes; isotope fractionation; marine geochemistry; Antarc*; Antarctic; Drake Passage; Southern Ocean; Weddell Sea
• Online Access: https://hdl.handle.net/11370/f6c5e9f9-ad6c-4a61-acfe-11580943b595; https://research.rug.nl/en/publications/f6c5e9f9-ad6c-4a61-acfe-11580943b595; https://doi.org/10.1016/j.epsl.2013.09.014; http://www.scopus.com/inward/record.url?eid=2-s2.0-84885356082&partnerID=MN8TOARS

Cadmium concentrations and isotope compositions were determined for 47 seawater samples from the high nutrient low chlorophyll (HNLC) zone of the Atlantic sector of the Southern Ocean. The samples include 13 surface waters from a transect of the Weddell Gyre and 3 depth profiles from the Weddell Sea and Drake Passage. The Southern Ocean mixed layer samples from two studies define a clear but broad ‘HNLC trend’ in a plot of ε114/110Cd versus [Cd], which is primarily a consequence of isotopic fractionation associated with biological uptake (ε114/110Cd is the deviation of the 114Cd/110Cd ratio of a sample from NIST SRM 3108 Cd in parts per 10,000). The trend is especially apparent in comparison to the large range of values shown by a global set of seawater Cd data for shallow depths. The Southern Ocean samples are also distinguished by their relatively high Cd concentrations (typically 0.2 to 0.6 nmol/kg) and moderately fractionated ε114/110Cd (generally between +4 and +8) that reflect the limited biological productivity of this region. Detailed assessment reveals fine structure within the ‘HNLC trend’, which may record differences in the biological fractionation factor, different scenarios of closed and open system isotope fractionation, and/or distinct source water compositions. Southern Ocean seawater from depths ⩾1000 m has an average ε114/110Cd of +2.5±0.2 (2se, n=16), and together with previous results this establishes a relatively constant ε114/110Cd value of +3.0±0.3 (2se, n=27) for global deep waters. Significant isotopic variability was observed at intermediate depths in the Southern Ocean. Seawater from 200 m to 400 m in Weddell Sea has high Cd concentrations and ε114/110Cd as low as +1, presumably due to remineralization of Cd from biomass that records incomplete nutrient utilization. Antarctic Intermediate Water, which was sampled at 150 to 750 m depth in the Drake Passage, features a distinct Cd isotope signature of ε114/110Cd≈+4, which reflects biological isotope fractionation at the surface and ...