Does δ¹⁸O of O₂ record meridional shifts in tropical rainfall?

Abstract. Marine sediments, speleothems, paleo-lake elevations, and ice core methane and δ¹⁸O of O₂ (δ¹⁸O_atm) records provide ample evidence for repeated abrupt meridional shifts in tropical rainfall belts throughout the last glacial cycle. To improve understanding of the impact of abrupt events on the global terrestrial biosphere, we present composite records of δ¹⁸O_atm and inferred changes in fractionation by the global terrestrial biosphere (Δε_LAND) from discrete gas measurements in the WAIS Divide (WD) and Siple Dome (SD) Antarctic ice cores. On the common WD timescale, it is evident that maxima in Δε_LAND are synchronous with or shortly follow small-amplitude WD CH₄ peaks that occur within Heinrich stadials 1, 2, 4, and 5 – periods of low atmospheric CH₄ concentrations. These local CH₄ maxima have been suggested as markers of abrupt climate responses to Heinrich events. Based on our analysis of the modern seasonal cycle of gross primary productivity (GPP)-weighted δ¹⁸O of terrestrial precipitation (the source water for atmospheric O₂ production), we propose a simple mechanism by which Δε_LAND tracks the centroid latitude of terrestrial oxygen production. As intense rainfall and oxygen production migrate northward, Δε_LAND should decrease due to the underlying meridional gradient in rainfall δ¹⁸O. A southward shift should increase Δε_LAND. Monsoon intensity also influences δ¹⁸O of precipitation, and although we cannot determine the relative contributions of the two mechanisms, both act in the same direction. Therefore, we suggest that abrupt increases in Δε_LAND unambiguously imply a southward shift of tropical rainfall. The exact magnitude of this shift, however, remains under-constrained by Δε_LAND.