| مستخلص: |
The marine biological calcification and photosynthesis, which can produce particulate inorganic and organic carbon (PIC and POC), have the opposite effects on seawater pCO2. Coccolithophores are a kind of marine unicellular algae with both of the two biological processes, and PIC and POC productions of them can shape the water column rain ratio as a dominant driver for Earth's carbon cycle. Thus, the changes in ancient coccolithophore PIC:POC can be important for the paleoceanographic and paleoclimatic studies of carbon cycle modeling. However, ancient coccolithophore PIC:POC is poorly constrained because of the occasional occurrences of intact coccospheres in deep ocean sediments, as detached coccoliths are commonly the remnants of fossilized coccolithophores. Here, we carry out the biometric analysis of coccosphere and coccolith from the living cells of Emiliania huxleyi and Gephyrocapsa oceanica in the South China Sea, and confirm a significant correlationship between their PIC:POC and lateral coccolith aspect ratio (ARL). ARL here is defined by the ratio of mean coccolith thickness with respect to coccolith length. A linear regression is given, PIC:POC=15.023∗ARL−0.083 $\mathrm{P}\mathrm{I}\mathrm{C}:\mathrm{P}\mathrm{O}\mathrm{C}=15.023\ast A{R}_{L}-0.083$ (R2 = 0.59, n = 121), for the reconstruction of ancient Noelaerhabdaceae coccolithophore PIC:POC based on individual coccoliths in marine sediments. Based on this equation, we reconstruct ancient Noelaerhabdaceae coccolithophore PIC:POC since 14 million years ago (Ma) using published coccolith data, which reveal a long‐term decrease in PIC:POC from 7 to 4 Ma. We suggest that such a change in coccolithophore physiology may be induced by a simultaneous long‐term decline in seawater calcium concentration. Plain Language Summary: Coccolithophores are a kind of marine algae which produce calcium carbonate through calcification. Photosynthesis of coccolithophores uses seawater CO2, on the contrary, calcification releases CO2. Therefore, the relative strength of calcification and photosynthesis, that is termed PIC:POC, can determine whether coccolithophore growth is a net CO2 sink or source. The knowledge of ancient coccolithophore PIC:POC is important to understand the mechanisms of Earth carbon cycle and climate changes in the geological history, as coccolithophores are one of the major components of marine carbon cycle. However, it remains unclear the characters of ancient coccolithophore PIC:POC, because detached or loosened coccoliths, which are the exoskeleton of a coccolithophore cell, are more commonly found in marine sediments, rather than intact coccospheres. Here, based on the biometric analysis of coccosphere and coccolith morphology, we refine a method to estimate ancient coccolithophore PIC:POC based on detached coccolith geometry. Key Points: Coccosphere PIC content is estimated based on coccosphere geometryPIC:POC of Emiliania huxleyi and Gephyrocapsa oceanica is estimated in the modern South China SeaAncient coccolithophore PIC:POC can be estimated by coccolith geometry in sediments [ABSTRACT FROM AUTHOR] |
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