Surface fertilisation and organic matter delivery enhanced carbonate dissolution in the western South Atlantic

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Suarez-Ibarra, Jaime Y. ; Freire, Tiago M. ; Frozza, Cristiane F. ; Pinho, Taina M. L. ; Petro, Sandro M. ; Dias, Bruna B. ; Chalk, Thomas B. ; Chaabane, Sonia ; Srivastava, Medhavi ; Costa, Karen B. ; Toledo, Felipe A. L. ; de Garidel-Thoron, Thibault ; Coimbra, Joao C. ; Pivel, Maria A. G.

• Document type: Journal article
• Source: FRONTIERS IN ECOLOGY AND EVOLUTION; v. 11, p. 14-pg., 2023-11-03.

Abstract

The last glacial inception was characterised by rapid changes in temperature, atmospheric pCO(2), and changes in the water mass geometry of the major ocean basins. Although several climatic feedback mechanisms have been proposed to explain the glacial/interglacial cycles witnessed in the Quaternary, the exact mechanistic responses of these processes are still under constrained. In this study we use proxies including planktonic foraminifera compositional assemblages and oxygen stable isotopes to reconstruct past changes in sea surface productivity, stratification, and carbonate dissolution. We use core SIS-249 (2,091 mbsl, western South Atlantic 30 degrees S 47 degrees W), spanning 30-110 thousand years ago (ka), and currently bathed by modern Northern Component Water. We test existing hypotheses suggesting that the orbital obliquity cycle modulates the biological pump in the study area. Spectral analysis run on our synthesised productivity proxies recognises a similar to 43 kyr-cycle, related to the obliquity cycle. We propose that the enhanced productivity is produced by two mechanisms: i) the glacial upwelling of subsurface nutrient-rich waters and, ii) the continental (wind-driven dust and riverine outflows) fertilisation of the photic zone, with the latter process being obliquity-paced. We also suggest that not only the increased organic matter export but also a change in its bioavailability (from refractory to labile) led to calcium carbonate dissolution, as the degradation of the more soluble organic matter decreased the pH of the glacial bottom water, partially dissolving the calcium carbonate. Although our correlation analyses show a strong benthic-pelagic coupling through the relation between the enhanced biological pump and carbonate dissolution (rho<0.05, r=0.80), we cannot reject the potential of corrosive Southern Component Water bathing the site during the glacial. Finally, we highlight that these processes are not mutually exclusive and that both can be modulated by the obliquity cycle. (AU)

• FAPESP's process: 20/11452-3 - Carbon storage changes in the mid-depth South Atlantic during the penultimate glacial termination and the Anthropocene
• Grantee: Bruna Borba Dias
• Support Opportunities: Scholarships in Brazil - Post-Doctoral
• FAPESP's process: 22/01056-9 - The application of neodymium isotopes to reconstruct AMOC variability and its role in the global carbon cycle during the last two glacial terminations
• Grantee: Bruna Borba Dias
• Support Opportunities: Scholarships abroad - Research Internship - Post-doctor
• FAPESP's process: 18/15123-4 - Past perspectives on tipping elements of the climate system: the Amazon Rainforest and the Atlantic Meridional Overturning Circulation (PPTEAM)
• Grantee: Cristiano Mazur Chiessi
• Support Opportunities: Research Grants - Research Program on Global Climate Change - Young Investigators - Phase 2