Paleoclimatic reconstruction of past interglacial periods and their transitions in the Iberian Peninsula and its surrounding seas

Abstract

[eng] This study combines cave speleothems and marine sediments to gain an insight into the climate variability of the Iberian Peninsula (IP) and its surrounding seas associated with previous warm interglacial periods. In particular, it centres in the interglacials comprised within the Marine Isotopic Stages (MIS) 13, 11, 9, 7, and 5, and their associated transitions. The speleothems are used as archives for changes in atmospheric moisture availability by means of stable isotopes and Mg/Ca analysis. The studied speleothems correspond mostly from the Balearic Islands (Minorca and Mallorca) but also from the Pyrenees. Marine conditions for the penultimate termination (T-II) and the MIS 5 have been reconstructed in the base to three sediment cores from three different marine locations around the IP: the Cantabrian Sea, the Alboran Sea, and the Balearic Sea. Surface conditions were explored by means of pair analyses of δ18O and Mg/Ca ratios measured in carbonate shells of the planktonic foraminifera Globigerina bulloides. These measurements let to the reconstruction of Sea Surface Temperatures (SST) and sea water δ18Osw values as a proxy of major changes in the regional precipitation/evaporation balance or as an indicator of the arrival of melting waters. In addition, changes in the intensity of western Mediterranean deep convection are explored through additional measurements of grain size distribution and XRF-geochemical ratios performed in the Balearic core (MD99- 2343). The studied speleothem collection provides a nearly continuous 450 kyr record, unique for the Mediterranean region. Interglacial periods are characterized by light δ13C and low Mg/Ca ratios pointing to enhanced precipitation and climate amelioration that allowed the expansion of the vegetation cover. On the contrary, heavier/higher δ13C and Mg/Ca ratios during glacial stages highlight their dry conditions with reduced vegetation activity. The δ18Ospeleo records reveal also the overall dominance of lighter values during interglacial periods in contrast to the glacial periods but with a strong 23-kyr (precessional) imprint in the signal. The extraordinary resemblance between the cave and marine δ18O records support a strong influence of Mediterranean source rains over the studied caves whose δ18O signal was dominated by the precession control on the evaporation/precipitation ratio on the Mediterranean basin. The MIS 11 appears in the studied speleothem as a long and stable interglacial with rather constant water availability over the Balearic Islands. The speleothem covering the MIS 9 and MIS 7 allows identifying sub-stage structures with wetter conditions associated with the warm periods. This record presents extraordinary resemblance to previously published pollen sequences from the Mediterranean region. The MIS 5 is studied in detail through the integrated study of marine and speleothem records. During the Last Interglacial (LIG), the sea surface temperature evolution was heterogeneous around the IP with gradients among the three studied seas larger than those from today. The LIG end was coincident with an accelerated aridification phase that marked the glacial inception at 116.5 kyr BP. This was the first of a series of stadials that punctuated the early glaciation and where the sea thermal gradient almost disappeared around the IP. These intense coolings during stadials led the development of drier but intense westerlies over southern European latitudes that favoured deep convection in the western Mediterranean Sea. In contrast to this regional homogeneity among the studied records during stadial periods, the interstadials periods were rather heterogeneous pointing to much complex ocean-atmosphere interconnections during these warm intervals. Glacial terminations (T-III, IV, and V) appear as periods of low rates in speleothem growth while T-II is only represented by growth interruptions. The marine δ18Osw for the T-II supports the occurrence of a major freshening event in correspondence to the Heinrich Event (HE) 11. Previous terminations are represented by an early light δ18Ospeleo anomaly, likely reflecting that this early deglacial major melting over the Atlantic Ocean was a rule for deglacial initiations.

[cat] L’estudi combinat tant de sediments marins com d’espeleotemes ha permès determinar la variabilitat climàtica durant períodes interglacials recents (MIS 13, 11, 9, 7, i 5) a la Península Ibèrica i als seus mars circumdants. S’han utilitzat sediments marins de tres localitzacions diferents, del Mar Cantàbric, el Mar d’Alboran i el Mar Balear, en els quals s’han determinat les condicions superficials del mar, a partir del càlcul de les temperatures superficials (Mg/Ca-SST) i el δ18Osw mesurats en el foraminífer planctònic Globigerina bulloides. Addicionalment, s’han caracteritzat canvis en la intensitat de la corrent profunda a partir de la distribució granulomètrica i l’anàlisi elemental per fluorescència de raig X dels sediments del testimoni del Mar Balear. Per un altre banda l’anàlisi geoquímic tant d’isòtops estables com de la relació Mg/Ca mesurats en espeleotemes de les illes Balears i dels Pirineus han sigut utilitzats com indicadors de precipitacions. La nova col·lecció d’espeleotemes obtinguda en aquesta tesis doctoral ha proporcionat un registre continental pràcticament continu de 450 kyr únic a la zona de la Mediterrània. Els resultats d’aquesta tesis mostren que els períodes interglacials, caracteritzats per valors lleugers de δ13C i baixes relacions de Mg/Ca en espeleotemes, varen tindre cobertures vegetals més extenses que durant els períodes glacials i varen ser períodes humits degut al augment de les precipitacions. A més, els resultats indiquen peculiaritats entre els diferents períodes interglacials estudiats i permeten identificar sub-estructures climàtiques interestadials/interglacials. En concret el MIS 5, període estudiat amb detall, mostra alts gradients tèrmics de les aigües superficials marines entre les tres localitzacions estudiades al voltant de la Península Ibèrica, indicant una interconnexió oceà-atmosfera complexa durant aquests períodes més càlids. Contràriament, durant períodes estadials freds, els gradients tèrmics pràcticament varen desaparèixer, i juntament amb el desenvolupament de condicions atmosfèriques àrides varen afavorir la convecció profunda del mediterrani occidental.