Petrographic and isotopic evidence for Holocene long-term climate change and shorter-term environmental shifts from a stalagmite from the Serra do Courel of northwestern Spain, and implications for climatic history across Europe and the Mediterranean

Abstract

A small stalagmite from Cova da Arcoia in the Serra do Courel of northwestern Spain provides a record both of general climate change across the Holocene and of shorter-term shifts between wetter and drier conditions. Nineteen U-series dates from 9.3 ka to the present provide a chronology of the stalagmite with uncertainties of commonly only decades and at the most only 200 years. These dates constrain the age of 18 petrographic surfaces that are of two types, one (Type E) inferred to result from dissolution during enhanced flow of water and the other (Type L) inferred to result from lesser flow. Aragonite is more abundant in the earlier parts of the stalagmite and calcite is more abundant in the later part. Values of δ 13C range from − 5.6 to − 1.3‰ relative to VPDB but are greatest beneath Type L surfaces. Values of δ 18O range from − 5.4 to − 4.4‰ relative to VPDB and have a weak but statistically significant correlation with δ 13C. Greater abundance of aragonite, greater values of δ 13C, and lesser frequency of Type E surfaces early in the stalagmite suggest a drier and perhaps warmer early to middle Holocene, whereas greater abundance of calcite and frequent Type E surfaces higher in the stalagmite suggest wetter conditions later in the Holocene. δ 18O data are most compatible with this general conclusion. However, a prominent Type E surface records the 8.2 ka event as an isolated wetter episode in the otherwise-dry early Holocene. Much of the stalagmite's aragonite and calcite suggests normal stalagmite growth in moderate climatic conditions. However, the near or complete cessation of growth at Type L surfaces and greater δ 13C, greater δ 18O, and lesser fluorescence of aragonite beneath those surfaces combine to suggest at least six century-scale episodes of strikingly dry conditions. On the other hand, evidence of dissolutional erosion at Type E surfaces suggests at least ten distinct episodes of much wetter conditions. These combine to suggest highly variable climate across the Holocene at the scale of decades to a few centuries. The record from this stalagmite is similar to records from Greenland ice cores and from pollen and/or stalagmite records from France, northernmost Spain, and more broadly from northern or Atlantic-facing Europe. In contrast, it suggests trends nearly opposite those in records from eastern and southern Spain, suggesting that the Serra do Courel is north of, but perhaps not far from, the boundary between Atlantic and Mediterranean regions of Holocene climate trends in the Iberian Peninsula. That general antiphasal relationship between northwestern Spain and regions to the south and east extends to the specific observation that four of the most pronounced wetter events in the stalagmite's record coincide with periods of widespread drought and/or cultural collapse in the Mediterranean and Middle East.