Abstract
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.
Highlights
The last glacial cycle shows two modes of abrupt climate variability, both with a center of action in the North Atlantic
The seasonal cycle is an imperfect analog for abrupt climate change during the last glacial period, the analysis presented in Sect. 3.2 highlights the importance of the meridional distribution of oxygen production for the gross primary productivity (GPP)-weighted mean δ18O of photosynthetic source water
Based on an analysis of the modern seasonal cycles of terrestrial oxygen production and production-weighted δ18Oprecip, we propose a simple relationship between spatial shifts in terrestrial oxygenesis and δ18O of atmospheric O2 (δ18Oatm)
Summary
The last glacial cycle shows two modes of abrupt climate variability, both with a center of action in the North Atlantic. The second mode of abrupt climate change is Heinrich events (HEs), periods of extreme cold in the North Atlantic associated with extensive layers of ice-rafted detritus in ocean. We quantitatively explore a simple relationship between meridional shifts in terrestrial oxygen production and δ18O of oxygen production-weighted terrestrial rainfall (δ18Oprecip) over the modern seasonal cycle using spatially gridded monthly observations. We use these observations as a guide to consider instantaneous changes in O2 fractionation during HEs and discuss the implications of these changes, and related changes in atmospheric CH4, for the meridional distribution of tropical rainfall associated with these events
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