Carbon Isotope Ratio Variations in Bering Sea Biota: The Role of Anthropogenic Carbon Dioxide

• Published in: Limnology and oceanography 2001-06, Vol.46 (4), p.999-1000
• Main Author: Schell, Donald M.
• Format: Article
• Language: English
• Subjects: Algae; Baleen; Carbon dioxide; Chlorofluorocarbons; Comment; Isotope ratios; Nitrogen; Phytoplankton; Radiocarbon; Seas; Whales
• ISSN: 0024-3590; 1939-5590
• DOI: 10.4319/lo.2001.46.4.0999

Cullen et al. contend that isotopically "light" anthropogenic carbon dioxide (fossil fuel burning and deforestation effect) is making the major contribution to the observed decline in phytoplankton delta super(13)C measured by proxy in whale baleen. The progressive decline occurs via direct depression of delta super(13)C and enhanced fractionation from the increase of [CO sub(2)] sub(aq). This position hinges on the assumption that the increase in mixed-layer [CO sub(2)] sub(aq), which several investigators have noted in lower latitudes, is of similar magnitude in the Bering Sea. They project that the accompanying depression in dissolved inorganic carbon (DIC) delta super(13)C and increased fractionation by phytoplankton over the 30-yr period in question may be the source of at least half of the observed decline in biotic isotope ratios. Their calculations all depend on these assumptions, and they are basically straightforward. Cullen et al.'s omits the isotope data prior to the mid-1960s shown in Schell (2000). If the invasion effect is a major contributor to the decline in biotic isotope ratios since 1967, then the same effect should have been evident in the prior 20 yr, during which the delta super(13)C of atmospheric CO sub(2) declined by similar to 0.3ppt. The whale baleen delta super(13)C remained relatively constant, however, and then increased to a maximum in 1966, similar to 1.3ppt higher than the 1947 value. If the invasion of carbon dioxide was depressing the isotope ratios in [CO sub(2)] sub(aq), then the implication is that phytoplankton demand was increasing toward a maximum in 1966 that was dwarfing the invasion signal. Actually, the baleen delta super(13)C data between 1948 and 1976 show no significant decline until the North Pacific "regime shift," a major climatic change that occurred in about 1976 (Francis and Hare 1993). The effects of increasing oceanic [CO sub(2)] are unquestionably affecting all latitudes in the world oceans and depressing delta super(13)C values, including those in the Bering Sea. The devil, as usual, is in the details.