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 (δ18Oatm) 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 δ18Oatm and inferred changes in fractionation by the global terrestrial biosphere (ΔϵLAND) 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 ΔϵLAND 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 ΔϵLAND tracks the centroid latitude of terrestrial oxygen production. As intense rainfall and oxygen production migrate northward, ΔϵLAND should decrease due to the underlying meridional gradient in rainfall δ18O. A southward shift should increase ΔϵLAND. 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 ΔϵLAND unambiguously imply a southward shift of tropical rainfall. The exact magnitude of this shift, however, remains under-constrained by ΔϵLAND.