RADIOMETRIC AND ISOTOPIC EVIDENCE FOR CHANGING INTERGLACIAL CLIMATE OVER THE LAST 550,000 YEARS FROM SIX STALAGMITES FROM NORTHWESTERN SPAIN

Six stalagmites from northwestern Spain record deposition during the interglacials of the Holocene MIS 1 and Upper Pleistocene MIS 5, MIS 7, and MIS 13. The stalagmites come from three caves, Cova Arcoia, Cova Paleira, and Cova Tara, in the Serra do Courel in Galicia, in northwestern Spain. Culling of 83 U-Th ages leads to 70 accepted ages, 69 of which fall within interglacials as recognized in the chronology of the LR04 marine benthic oxygen isotope record, and the uncertainty of the seventieth includes part of MIS 5.

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.