RADIOMETRIC AND ISOTOPIC EVIDENCE FOR CHANGING INTERGLACIAL CLIMATE OVER THE LAST 550,000 YEARS FROM SIX STALAGMITES FROM NORTHWESTERN SPAIN
Values of d13C of stalagmite CaCO3 have a large range, from –10.2 to –0.4 ‰ relative to VPDB, whereas values of d18O have a much smaller range, from –7.2 to –4.3 ‰ relative to VPDB. Values of d13C and d18O are correlative across all the data from all six stalagmites and within each stalagmite’s data from each substage. In Holocene intervals, values of d13C commonly decrease through time, suggesting progressively wetter conditions, and the same is generally true across MIS 5, MIS 7, and MIS 13. The Eemian (in MIS 5e and perhaps 5d) is characterized by greater values of d13C suggesting drier conditions punctuated by smaller values of d13C representing briefly wetter conditions. Still smaller values of d13C from MIS 5c suggest even wetter conditions that are consistent with the pattern of wetter conditions later in each interglacial. Isotopic and petrographic evidence combine with radiometric dates to suggest that interglacials were comparatively dry, whereas glacials were too wet, and at their extremes perhaps too cold, for growth of stalagmites. Across all four of the interglacials represented, stalagmite d13C is inversely proportional to d18O of the LR04 marine benthic oxygen isotope record, suggesting that the overall pattern of climate during interglacials fluctuated with global ice volume and more specifically with the extent of North Hemisphere glaciation.