07 Dec 2022
07 Dec 2022

Responses of fossil coccolith morphology to preservation conditions in the deep ocean

Amanda Gerotto1,3, Hongrui Zhang2, Renata Hanae Nagai3, Heather M. Stoll2, Rubens César Lopes Figueira1, Chuanlian Liu4, and Iván Hernández-Almeida2 Amanda Gerotto et al.
  • 1Oceanographic Institute, University of São Paulo, São Paulo, Brazil
  • 2Geological Institute, ETH Zurich, Zurich, Switzerland
  • 3Center for Marine Studies, Federal University of Paraná, Pontal do Paraná, Brazil
  • 4State Key Laboratory of Marine Geology, Tongji University, Shanghai, China

Abstract. Understanding the variations in past ocean carbonate chemistry is critical in elucidating the role of the oceans in balancing the global carbon cycle. The fossil shells from marine calcifiers present in the sedimentary record are widely applied as past ocean carbon cycle proxies. However, the interpretation of these records can be challenging due to the complexity physiological and ecological response to the carbonate system during organisms' life cycle, as well as the potential for preservation at the sea-floor. Here we present a new dissolution proxy based on the morphological attributes of coccolithophores from the Noëlaerhabdaceae family (Emiliania huxleyi and Gephyrocapsa spp., > 2 µm). To evaluate the influences of coccolithophore calcification and coccolith preservation on fossil morphology, we measured morphological attributes, mass, length, thickness, and shape factor (ks), of coccoliths in a laboratory dissolution experiment and surface sediment samples in the South China Sea. The coccolith morphological data in surface sediment were also analyzed with environment settings, namely surface temperature, nutrients, pH, chlorophyll-a concentration, and carbonate saturation of bottom water by a redundancy analysis. Statistical analysis indicate that carbonate saturation of the deep ocean explains the highest proportion of variation in the morphological data instead of the environmental variables of the surface ocean. Moreover, the dissolution trajectory in the ks vs length of coccoliths is comparable between natural samples and laboratory dissolution experiments, emphasizing the importance of carbonate saturation on fossil coccolith morphology. However, the mean ks alone cannot fully explain all variations observed in our work. We propose that the mean ks and standard deviation of ks (σ) over the mean ks (σ/ks) could reflect different degrees of dissolution and size-selective dissolution, influenced by the assemblage composition. By applying together with the σ/ks ratio, the ks factor of fossil coccoliths in deep ocean sediments could be a potential proxy for a quantitative reconstruction of past carbonate dissolution dynamics.

Amanda Gerotto et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2022-1329', Anonymous Referee #1, 23 Jan 2023
  • RC2: 'Comment on egusphere-2022-1329', Anonymous Referee #2, 24 Jan 2023
  • RC3: 'Comment on egusphere-2022-1329', Anonymous Referee #3, 24 Jan 2023

Amanda Gerotto et al.

Data sets

Morphological measurements of coccoliths from surface samples of South China Sea Gerotto, A., Zhang, H., Nagai, R. H., Stoll, H. M., Figueira, R. C. L., Chuanlian, L., Hernández-Almeida, I

Amanda Gerotto et al.


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Short summary
This study demonstrates, based on morphological attributes of coccolitophores, that dissolution effects primarily affect the morphology of coccoliths preserved in the deep ocean. In the South China Sea surface sediments, bottom water calcite saturation plays a major role in the variation of the coccoliths' shape factor (ks), which has the potential, based on the current calibration, to quantitatively reconstruct past carbonate dissolution changes.