Abstract
The anthropogenic input of fossil fuel carbon into the atmosphere results in increased carbon dioxide (CO2) into the oceans, a process that lowers seawater pH, decreases alkalinity and can inhibit the production of shell material. Corrosive water has recently been documented in the northeast Pacific, along with a rapid decline in seawater pH over the past decade. A lack of instrumentation prior to the 1990s means that we have no indication whether these carbon cycle changes have precedence or are a response to recent anthropogenic CO2 inputs. We analyzed stable carbon and oxygen isotopes (δ13C, δ18O) of decade-old California mussel shells (Mytilus californianus) in the context of an instrumental seawater record of the same length. We further compared modern shells to shells from 1000 to 1340 years BP and from the 1960s to the present and show declines in the δ13C of modern shells that have no historical precedent. Our finding of decline in another shelled mollusk (limpet) and our extensive environmental data show that these δ13C declines are unexplained by changes to the coastal food web, upwelling regime, or local circulation. Our observed decline in shell δ13C parallels other signs of rapid changes to the nearshore carbon cycle in the Pacific, including a decline in pH that is an order of magnitude greater than predicted by an equilibrium response to rising atmospheric CO2, the presence of low pH water throughout the region, and a record of a similarly steep decline in δ13C in algae in the Gulf of Alaska. These unprecedented changes and the lack of a clear causal variable underscores the need for better quantifying carbon dynamics in nearshore environments.
Highlights
Carbon dioxide-induced changes to the ocean carbon cycle have the potential to dramatically change the biology of the ocean [1]
Other calculations for seawater dissolved inorganic carbon (DIC) estimate a global ocean average decline of 20.015 % yr21 in d13CDIC [18]. Both are much less than our observed decline in d13C of 20.071 % yr21; the observed shell d13C decline is consistent in sign with the observed 10-year decline in pH at Tatoosh Island [2], the pH decrease was even greater, over 13 times stronger than can be explained by equilibrium with the rising atmospheric CO2 over this time period
We found no evidence that a single factor accounts for the rapid decline that we observed in d13C of M. californianus shells
Summary
Carbon dioxide-induced changes to the ocean carbon cycle have the potential to dramatically change the biology of the ocean [1]. The chemical composition and growth patterns of shells of marine organisms can be used to infer the physical and chemical features of the ocean on both recent and paleontological time scales [6], [7], [8], [9]. Shells can provide proxy chemical records spanning longer time intervals than the instrumental record. The relative proportions of light (12C) and heavy (13C) stable isotopes of carbon (d13C) in calcium carbonate-based shells can indicate carbon sources. Respiration of organic matter in subsurface ocean waters imprints intermediate and deep waters with isotopically light respired dissolved inorganic carbon (DIC), resulting in an inverse relationship between the intensity of upwelling and d13C in the shell of the California mussel (Mytilus californianus) in southern
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