A long record of MIS 7 and MIS 5 climate and environment from a western Mediterranean speleothem (SW Sardinia, Italy)

Abstract

Here we present the first record of MArine Isotope Stage (MIS) 7 from Sardinia, based on a U-Th-dated speleothem from Crovassa Azzurra cave, which also grew during MIS 5. Carbonate precipitation was continuous during MIS 7, while two multi-millennial hiatuses interrupted growth during MIS 5. These hiatuses occurred during times of unstable climate, but local karst-specific conditions are considered to have played an important role as well. δ¹³C values mostly reflect soil bioproductivity and vegetation density. During less humid periods these values reflect bedrock δ¹³C values, because of a semi-closed system and/or sulphuric acid-driven bedrock dissolution. δ¹⁸O changes were governed by the interplay of rainfall amount and moisture source. Disentangling these two effects was possible by comparing δ¹³C and δ¹⁸O values, whereby both isotopes co-vary during periods when δ¹⁸O was dominated by the amount effect. Changes in the relative proportion of Atlantic and Mediterranean moisture sources was the main control on δ¹⁸O at times when δ¹⁸O was decoupled from δ¹³C.

δ¹⁸O variations during MIS 5 are synchronous with Greenland stadials (GS) and interstadials (GI) GS 26, GI 25, GI-GS 22, GI-GS 21. With the exception of GS 22, these climate oscillations did not affect soil bioproductivity, and δ¹⁸O mirrors the relative proportion of Atlantic and Mediterranean moisture sources. GS 22 was a prominent arid period in Sardinia, in accordance with other regional palaeoclimate archives.

High soil bioproductivity is indicated by δ¹³C values for MIS 7e and MIS 7c-a; the latter interval was characterized by a 30 kyr-long time span of warm and humid conditions in Sardinia. Soil bioproductivity decreased markedly during MIS 7d and the climate became drier. The latter period, also constrained by δ¹⁸O data, lasted significantly shorter in Sardinia than in central Europe.

As indicated by changes in δ¹³C and δ¹⁸O relationships, variations in moisture source are also recorded for MIS 7, with Mediterranean-dominated rainfall during MIS 7e and 7c followed by a shift toward Atlantic-dominated rainfall.