Palaeo-CO2 reconstructions based on δ11B measured in different morphotypes of Globigerinoides ruber

Abstract:

Atmospheric CO₂ is a key component of the Earth’s radiative balance and varies by 80-100 ppm between glacial and interglacial climate states¹. Continuous ice core records of ancient atmospheric CO₂ concentrations reach as far as 800 000 years ago, limiting our understanding of the radiative forcing of climate through older intervals. The δ¹¹B-pH proxy measured in planktic foraminifera provides a powerful tool to reconstruct atmospheric CO₂ as long as: (i) the surface ocean at the selected site(s) was(were) in stable equilibrium with the atmosphere with respect to CO₂; and (ii) the targeted foraminiferal species is abundant, and (iii) the offset between ambient seawater δ¹¹B_borate and foraminiferal δ¹¹B is known. The Indian Ocean is a promising location for CO₂ reconstructions, since large areas are near air-sea CO₂ equilibrium². In addition, the surface dwelling Globigerinoides ruber occurs in a continuous fashion throughout the Pleistocene in the region and the relationship between δ¹¹B and seawater pH has been calibrated³. However, G. ruber exhibits subtle morphological variability, which potentially impacts its geochemical signature⁴ and could hamper its utility in CO₂ reconstructions.

Here we present new CO₂ reconstructions from IODP Site U1443 that are based on δ¹¹B measurements performed on different morphotypes of G. ruber (white). The data span the last 500 000 years and suggest that: (a) CO₂ derived from G. ruber sensu stricto δ¹¹B is in good agreement with atmospheric CO₂ concentrations from ice cores, which implies that the surface eastern equatorial Indian Ocean was in equilibrium with the atmosphere during the glacial/interglacial periods of last 500 000 years (b) CO₂ datasets from G. ruber sensu stricto, G. ruber sensu lato and G. ruber sensu lato extreme differ and are offset by a mean of +6.3, +45.4 and +51.9 ppm from contemporaneous ice core CO₂ data, highlighting the importance of good taxonomic control for accurate CO₂ reconstructions using the δ¹¹B proxy. We discuss the causes of these differences in the context of different habitats (water column calcification depths) reflecting different carbonate chemistry conditions.

1 Sigman & Boyle, 2000 Nature 407, 859-969 2 Takahashi, et al, 2009 Deep-Sea Res 56, 2075–2076 3 Henehan et al, 2013 Earth Pla Sci Lett 364, 111-122 4 Carter et al, 2017 Mar. Mic 131, 1-9