Schaaff Valentine, Grossi V., Makou M., Garcin Yannick, Deschamps Pierre, Sebag D., Ngatcha B. N., Ménot G. (2024). Constraints on hopanes and brGDGTs as pH proxies in peat. Geochimica et Cosmochimica Acta, 373, 342-354. ISSN 0016-7037.
Titre du document
Constraints on hopanes and brGDGTs as pH proxies in peat
Schaaff Valentine, Grossi V., Makou M., Garcin Yannick, Deschamps Pierre, Sebag D., Ngatcha B. N., Ménot G.
Source
Geochimica et Cosmochimica Acta, 2024,
373, 342-354 ISSN 0016-7037
pH is one of the major parameters governing peat functioning, and pH variations in modern peatlands affect carbon and methane production and consumption. Paleo-pH reconstructions have been limited thus far, but they could help to better understand peat functioning in variety of settings and also serve as an indirect proxy for climatic and environmental variations such as precipitation. Bacterial hopanes and branched Glycerol Dialkyl Glycerol Tetraethers (brGDGTs) were investigated in a 10-ka peat record from North-East Cameroon (NGaoundaba, Western Central Africa). Recently developed pH proxies using the hopane beta beta/(alpha beta + beta beta) ratio and the brGDGT cyclization ratio (CBTpeat) were applied and compared with previously published bulk organic data from the same core. Different hypotheses are usually proposed to explain the high abundance of the "thermally mature" C31 alpha beta hopane in peats: acid-catalyzed isomerization of beta beta to alpha beta isomers with or without biological mediation or the direct input of C31 alpha beta hopanes by bacteria. In the NGaoundaba peat deposit, the opposite variation in the beta beta/(alpha beta + beta beta) ratio of C30 and C31 hopanes and the carbon isotopic composition of C31 beta beta and alpha beta hopanes suggest that an eventual transformation of C31 beta beta to alpha beta isomers is not possible without biological mediation. A linear correlation between hopane- and brGDGT-reconstructed pH is observed, suggesting a similar response to environmental changes for these two independent proxies. A large increase in reconstructed pH values coincides with changes in vegetation and precipitation at the end of the African Humid Period (AHP). Lower pH values are observed during the AHP, coinciding with a period of increased precipitation and consistent with bulk organic data and slightly higher C31 beta beta hopane delta 13C values compared to middle and late Holocene. Bulk organic data indicate an interruption of the AHP by a drier intermission that coincides with a decrease in reconstructed pH values, probably reflecting a decrease in evapotranspiration. Variations in pH values could be interpreted in terms of preservation of peat organic matter and might reflect past changes in methane cycling in the investigated peatland. The present study reinforces the idea that reconstruction of pH in peat deposits represents a promising proxy of environmental change, enabling a better understanding of changes in peat functioning over large timescales and various locations.
Plan de classement
Sciences fondamentales / Techniques d'analyse et de recherche [020]
;
Sciences du milieu [021]
;
Etudes, transformation, conservation du milieu naturel [082]
