Foraminiferal area density as a proxy for ocean acidification over the last 200 years in the California Current System
Abstract
Anthropogenic activities have resulted in an increase in atmospheric CO2 from 280 ppm to 400 ppm over the last 250 years. It is estimated that approximately one-third of this anthropogenically produced CO2 is sequestered in the global ocean, increasing the inventory of bicarbonate (HCO3-) and hydrogen ions (H+) and consuming carbonate (CO32-) as a result of carbonate buffering reactions. This increase in [H+] lowers seawater pH, the phenomenon known as ocean acidification (OA). Estimates indicate that mean seawater pH has already decreased by 0.1 pH units since 1750 and IPCC reports indicate it is likely that CO2 concentrations will reach 790 ppm by 2100 further reducing pH by 0.3 units. Marine calcifiers, such as foraminifera, utilize CO32- dissolved in seawater during calcification, a process that is highly sensitive to changes in pH due to the chemical reactions described above. The reduction in surface ocean carbonate ion concentration ([CO32-]) caused by OA has impaired calcification of planktonic foraminifera and other marine calcifiers. It has been proposed that planktonic foraminiferal shell weight or shell thickness is positively correlated with ambient [CO32-] and has been used as proxy to reconstruct past changes in the surface ocean carbonate system. An ideal location for the application of this proxy is the California Current System (CSS), an Eastern Boundary Upwelling System (EBUS), which is characterized as having naturally lower pH. Upwelling introduces CO2-enriched bottom waters to the surface ocean, intensifying the effects of increasing dissolved CO2 as a result of anthropogenic activities. Upwelling produces a wide range of surface water [CO32-] making the Santa Barbara Basin (SBB) an ideal site to carry out a foraminiferal shell weight calibration study. Area density (ρA) is a new method for collecting size-normalized shell weights that will be used in this study. Species-specific calibrations have been derived for two symbiont-barren planktonic foraminifera, N. pachyderma (dextral) and G. bulloides. ρA measurements of these species collected from 35 months of sediment trap material are regressed with corresponding calcification depth-specific [CO32-]. Preliminary results show that ρA for both species demonstrate a positive linear relationship with ambient [CO32-]. Seasonal upwelling patterns are clearly recorded in ρA measurements with lower values occurring during periods of peak upwelling, which typically initiates in early spring. Preliminary observations suggest that distinguishing different morphotypes and ontogenic stages of the planktonic foraminifera used in this study could optimize the calibration equations. Final calibration equations will be applied to ρA measurements for a 200-year core record collected near the sediment trap mooring in the SBB. This reconstruction will quantify changes in [CO32-] as a result of OA since the onset of the industrial revolution, providing insights for future reduction in calcification efficiency of foraminifera as a result of OA and increasing carbon emissions.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMPP11C1848O
- Keywords:
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- 4902 PALEOCEANOGRAPHY Anthropogenic effects;
- 0428 BIOGEOSCIENCES Carbon cycling;
- 0473 BIOGEOSCIENCES Paleoclimatology and paleoceanography;
- 1635 GLOBAL CHANGE Oceans