A STALAGMITE RECORD OF MILLENNIAL-SCALE CONTINENTAL CLIMATE DURING THE LAST GLACIAL CYCLE FROM WESTERN IBERIA

While ocean conditions along the Iberian margin have been tied to millennial-scale climate variability during the last glacial cycle, continental climates in Iberia from this time period are much less well understood. Here we present a stalagmite record from cave Buraca Gloriosa, western Portugal, which shows a direct link between Iberian paleoceanographic changes and continental climate variability. This record reveals millennial-scale variability in both carbon and oxygen isotopic values similar in timing and structure to D/O events observed in Greenland ice cores. Stalagmite chronologies are anchored using 47 high precision ²³⁰Th dates and reveal intermittent stalagmite growth from 82.1 ± 0.4 to 1.2 ± 0.1 ka with growth hiatuses spanning most Heinrich stadials.

Oxygen isotopic values shift by 1.0-1.5‰ during MIS 3-4 with lower values defining D/O interstadials and showing the same characteristic asymmetry of these structures in the Greenland ice records. Although temperature effects on oxygen isotopic ratios of meteoric precipitation as recorded at the GNIP station of Porto, 179 km north of Buraca Gloriosa, are statistically significant, the slope is shallow; hence the stalagmite record from this region likely reflects precipitation dynamics rather than temperature changes. At this location, amount effects impart a prominent influence on modern isotopic ratios of precipitation. Thus amount effects, as well as the strong seasonality of precipitation in this Mediterranean climate, are thought to be the primary drivers of oxygen isotopic variability in Buraca Gloriosa stalagmites.

Carbon isotopic values shift by 3.5-4‰ between D/O events, with lower values defining D/O interstadials. These changes likely reflect increased vegetation density and reduced prior calcite precipitation during the warmer, wetter climates of D/O interstadials. Hiatuses in stalagmite growth during Heinrich stadials likely also reflect these cold and dry conditions. Future measurements of temperature, humidity and barometric pressure in Buraca Gloriosa will provide additional insight into seasonal changes in cave conditions that could influence these isotopic fluctuations.