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Decadal variability in twentieth-century ocean acidification in the California Current Ecosystem


Oceanic uptake of CO2 can mitigate climate change, but also results in global ocean acidification. Ocean acidification-related changes to the marine carbonate system can disturb ecosystems and hinder calcification by some organisms. Here, we use the calcification response of planktonic foraminifera as a tool to reconstruct the progression of ocean acidification in the California Current Ecosystem through the twentieth century. Measurements of nearly 2,000 fossil foraminifera shell weights and areas preserved in a marine sediment core showed a 20% reduction in calcification by a surface-dwelling foraminifera species. Using modern calibrations, this response translates to an estimated 35% reduction in carbonate ion concentration, a biologically important chemical component of the carbonate system. Assuming other aspects of the carbonate system, this represents a 0.21 decline in pH, exceeding the estimated global average decline by more than a factor of two. Our proxy record also shows considerable variability that is significantly correlated with Pacific Decadal Oscillation and decadal-scale changes in upwelling strength, a relationship that until now has been obscured by the relatively short observational record. This modulation suggests that climatic variations will play an important role in amplifying or alleviating the anthropogenic signal and progression of ocean acidification in this region.

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Fig. 1: Decadal population mean ANSW.
Fig. 2: Twentieth-century proxy [CO32−] record based on G. bulloides ANSW for the period 1895–2000.
Fig. 3: Stable isotope tracers of influential processes.
Fig. 4: Linking upwelling strength, the PDO and carbonate system state.
Fig. 5: Using individuals to trace habitat depth and upwelling strength.

Data availability

The authors declare that the data supporting the findings of this study are available within the article. New data generated as a part of this study have been made publicly available via PANGAEA (PDI-21505; Additional data related to this study are available from the corresponding author on request. Publicly available data used in this study are described below. Carbonate chemistry measurements made within the SBB were collected seasonally by the California Cooperative Oceanic Fisheries Investigations time-series (CalCOFI; and during several NOAA West Coast Ocean Acidification Cruises (WCOA; Observational data from the Hawaii Ocean Time-series program (HOT; were also used. Shore Station Data (salinity; were used for carbonate system calculations.


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We dedicate this manuscript to co-author Robert C. Thunell, a wonderful colleague, friend and mentor, who lost his battle to cancer on 30 July 2018. This research was supported in part by a National Science Foundation grant to R.C.T. (1631977) and the Johanna M. Resig Fellowship granted to E.B.O. by the Cushman Foundation. R.A.F. was supported by the NOAA Ocean Acidification Program (PMEL contribution number 4546). N.G. acknowledges the support of ETH Zürich and of the Swiss National Foundation through the XEBUS project. We thank D. Burdige from Old Dominion University for collecting the box core used in this study; E. Tappa for his assistance with IRMS analyses and sediment trap recovery and deployments in the SBB; and N. Umling, whose review and scientific discourse greatly improved this manuscript.

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E.B.O. and R.C.T. conceived the study and wrote the initial draft of the paper. E.B.O. designed and performed the analyses and has led the revision of the paper. N.G. and R.A.F. provided model and in situ data and contributed sustantially to the discussion and interpretation of the results and writing of the paper. C.R.B.-N. conducted radiochemistry analyses, produced an age model for the core and contributed to the interpretation and writing of the paper.

Corresponding author

Correspondence to Emily B. Osborne.

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Extended data

Extended Data Fig. 1 Scanning Electron Microscope images of the study species, G. bulloides.

B. G. bulloides shell with its final chamber broken showing a cross-sectional view of the shell wall, the key morphometric used in this study to estimate [CO32−]. Shell wall thickness is indirectly estimated by measuring area normalized shell weight (ANSW: shell weight (μg) / 2-D surface area (μm2).

Extended Data Fig. 2 Bathymetric map of the Santa Barbara Basin.

The location of the box-core (red circle) used for the down-core reconstruction as well as the sediment trap (yellow triangle) and hydrographic sampling location (Plumes and Blooms Project Station 4; orange square) used in the calibration portion of the study29.

Extended Data Fig. 3 Down core trends in G. bulloides weight (not size-normalized) and δ18O.

The increasing shell weight over the down core record coincides with a decline in δ18O (note inverted axis), which represents an increase in temperature. This comparison provides a visualization of the importance of using size-normalized shell weights to estimate changes in calcification and shell thickness. Results from this study show that while overall shell weight and shell size are increasing as a result of warming temperatures, shell thickness is declining as a result of reduced [CO32-].

Extended Data Fig. 4 The long-term shift in G. bulloides shell diameter increases in concert with warming sea surface temperatures.

B. We compare the relative changes that are recorded G. bulloides δ18O to a Northern Hemisphere Temperature anomaly for the 1900–2000 period and see a good agreement between temperature trends recorded in these records (Jones et al., 2013). C. We also compared G. bulloides δ18O to in situ sea surface temperature (SST) measurements made in the Santa Barbara Basin (SBB; 1955-Present; Shore Stations Program) and see an excellent agreements between sea surface temperature and the δ18O recorded in our G. bulloides shells.

Extended Data Fig. 5 Time-series correlations between Pacific Decadal Oscillation Index and the detrended proxy-[CO3=] and ANSW sample standard error.

Corresponding values were compared at 5-year time steps due to difference in time resolution across records. An independent t-test was conducted to compare both detrended proxy-[CO3=] and area-normalized shell weight standard area (a measure of sample variability) to the Pacific Decadal Oscillation Index. Highly significant correlation coefficients exist between PDO and the respective variables p<0.005) and the results of a paired t-tests for both records are highly significant (p<0.005), further confirming significance of the relationships.

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Osborne, E.B., Thunell, R.C., Gruber, N. et al. Decadal variability in twentieth-century ocean acidification in the California Current Ecosystem. Nat. Geosci. 13, 43–49 (2020).

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