Comparison of water column [CO_2aq] with sedimentary alkenone‐based estimates: A test of the alkenone‐CO₂proxy

Abstract

[1] The stable carbon isotopic compositions of alkenones have been used to interpret the long-term history of the partial pressure of atmospheric carbon dioxide ( pCO 2 ). Although extensive water column and culture studies document the potential utility and limitations of this approach, to date the accuracy of pCO 2 values derived from sedimentary alkenones remains untested. For this study we establish Holocene-aged, alkenonebased CO 2aq estimates ([CO 2aq ] alk ) from 20 sites along a central Pacific Ocean transect and compare them against both observed modern water column CO 2aq and estimated preindustrial concentrations at the depth of alkenone production at each site. Although the [CO 2aq ] alk track measured water column values, they are conspicuously lower than modern values across the subtropics. This offset likely reflects the contributions of anthropogenic CO 2 in modern surface waters relative to preindustrial concentrations at the time of alkenone production. When a model-based estimate of anthropogenic CO 2 is removed from the modern observed values, a majority (84%) of [CO 2aq ] alk falls within 20% of modeled preindustrial values. Consistency between the modeled and alkenone-based estimates of preindustrial CO 2 levels points to the relative accuracy of the alkenone-CO 2 method across a wide range of ocean and biogeographic regimes, provided that phosphate concentrations, at the depth of haptophyte production, are reasonably constrained. It further suggests that lightlimited growth and/or active carbon uptake, if they occur, have a negligible effect on reconstructed [CO 2aq ].

North Pacific seasonality and the glaciation of North America 2.7 million years ago

“…As phytoplankton, these organisms are concentrated in the upper euphotic zone of polar waters, whereas G. bulloides can live at a variety of depths and also forms a gametogenic crust in the subsurface. Moreover, coccolithophorids tend to bloom in the middle to late summer in the western subarctic Pacific, after the diatom bloom 9,10,11,12 , whereas foraminiferal production tends to follow the productivity of the entire phytoplankton pool and thus is at a maximum in the spring 9,10 . For these reasons, significant differences should be expected between alkenone-and foraminiferal-based temperature reconstructions.…”

“…During spring, as the euphotic zone deepens and the mixed layer shoals, a diatom-dominated bloom begins, lasting until early summer, when most of the nutrients are consumed, silicate in particular 9,10 . However, during late summer and autumn, when the water column is most stable, a secondary biogenic bloom typically occurs, this time dominated by coccolithophores 11,12 . Alkenones accumulating in the sediments below this region indicate a modern temperature of 10.1 °C, consistent with the late-summer and autumn growth of the coccolithophorids, which are among the prymnesiophytes that produce these compounds 11 .…”

High Earth-system climate sensitivity determined from Pliocene carbon dioxide concentrations

“…Given the available experimental results, the potential exists that different growth and environmental conditions trigger different carbon isotopic responses 23 . However, validation of the alkenone-CO 2 approach using sedimentary alkenones in the natural environment indicates that this technique can be used to resolve relatively small differences in water column [CO 2aq ] when SST and phosphate concentrations ([PO 4 3− ]) and are reasonably constrained 24 . We use alkenone unsaturation indices (U K 37 ) to determine SST and assume haptophyte production depths between 0 and 75 m at each site, bounding a range of [PO 4 3− ] determined from modern mean-annual-phosphate depth profiles (Supplementary Information) that is then applied to calculate a range of pCO 2 estimates (Fig.…”

Impact of Biogeochemical Cycling of Phytoplankton-Produced Lipid Compounds in Ocean Systems on Paleoceanographic Records Buried in Sediments

“…This simple linear relationship was challenged by a finding that cell growth rate also affects isotopic fractionation of phytoplankton organic matter and the (ε p ) is strongly related to the ratio of growth rate to CO 2 concentration (μ/C e ) rather than [CO 2 ] alone [28,29]. More studies [30][31][32][33] have documented that nutrientlimitation (either nitrate or phosphate) and Fe-limitation control the cell growth rate and thus affect isotopic fractionation. On the other hand, many field observations and laboratory experiments [34][35][36] have repeatedly shown strong evidence on isotopic depletion (negative shift) of alkenones during transport/recycling.…”