High-frequency climate variability and forcing mechanisms in the western Mediterranean during the last 20,000 yrsreconstructing atmospheric and oceanic responses

Resumen

This Thesis provides a high-resolution paleoclimate reconstruction over the last 20,000 yrs obtained from marine records in the westernmost Mediterranean. Such reconstruction has provided new insights regarding millennial- to centennial-time scale atmospheric and oceanographic oscillations during this time interval. Thus, this multi-proxy approach allowed identifying abrupt climate oscillations and forcing mechanisms since the Last Glacial Maximum. Such oscillations have also been correlated with Greenland ice records but also with monsoonal low latitude variations, suggesting a strong connection of western Mediterranean climate with North Atlantic changes. Over the last 20,000 yrs major climate intervals studied have been: the last Heinrich event, the Bölling-Alleröd period, the Younger Dryas, and the Holocene, when significant climate oscillations were also recognized. Cold and dry conditions, supported by enhanced palygorskite and quartz contents, and significant increases in some detrital ratios (Si/Al, Zr/Al, and Ti/Al), characterize the last Heinrich event. At this time, major eolian dust contribution derived from the nowadays Senegal and Guinea regions. Such fluctuations also coincided with major oscillations in deep-water ventilation and current intensity associated with the impact of large volumes of Atlantic meltwater inflow. This has been further supported by the oxygen isotope record and lower SST (around 12ºC) derived from two organic paleothermometers (Haptophyte algae-Uk'37 and Thaumarchaeota-TEX86). Paleoproductivity proxies within this interval also indicate enhanced productivity and a subsequent organic enrichment prior to the last Organic Rich Layer described in the Alboran Sea basin. During the interstadial Bölling-Alleröd period, progressively more humid conditions are denoted by typical fluvial proxies, and short-term climate oscillations such as the Intra-Alleröd Cold Period are also recognized. Concerning the oceanographic realm, lower oxygenation conditions favoured the onset of the last Organic Rich Layer deposition, thus evidencing stable thermohaline Mediterranean circulation. This warmer period is subsequently characterized by an increase in SST (up to around 18ºC) although exhibiting rapid variations (±3.0º-4.0ºC) in the Haptophyte/alkenone paleothermometer (Uk'37) supporting the definition of the Older Dryas period. The Younger Dryas interval presents a remarkable feature; an early dry phase and more humid conditions towards the end. These distinct time-periods are also inferred by changes in paleoceanographic conditions, exhibiting high productivity levels at the onset and also an abrupt and brief cooling, reaching minimum values of 13ºC and 16ºC, using Haptophyte and Thaumarchaeota thermometers respectively. Major divergence in SST-estimation (Uk'37-18º-20ºC; TEX86-23º-26ºC) with the respective biomarkers is recorded throughout the Holocene as well as slightly warmer Haptophyte-derived temperatures (by ca. 1.5º-2ºC) in the westernmost Alboran Sea basin. These differences suggest a seasonal response in the organic proxies, as well as the influence of the hydrographic configuration of the Alboran Sea at the two locations. During the Holocene, significant detrital fluctuations are prominent at around 8.9 kyr cal. BP, correlating with forest cover declines in the Alboran borderlands, and suggest an intense eolian input from the southern African sources (Guinea and/or Senegal). At the same time, the demise of the last Organic Rich Layer deposition is recognized, which seems to be linked with major restructuration of the intermediate and deep-water circulation. A progressive decrease in values of fluvial proxies points to an early decline of the African Humid Period at 7.4 kyr cal. BP, with drier conditions prevailing up to 4.5 kyr cal. BP. Within this context, increasing detrital input during cold events, deriving from the intensification of Saharan dust export from the present Guinea and Senegal regions across the western Mediterranean, has been related to a predominant positive index of the North Atlantic Oscillations, and southward migrations of the Inter-Tropical Convergence Zone over North Africa, involving variations in the African monsoon system. Conversely, radiogenic isotopes from detrital material deposited during warmer periods support a dominant provenance from present regions of Morocco, Mali and Mauritania. Regarding climate forcing mechanisms, proxies used for reconstructing fluctuations in terrigenous input and paleoceanographic conditions, set in four paleoenvironmental groups (detrital, redox, paleoproductivity, and paleotemperature-paleosalinity) for spectral analysis, have revealed major cycles at 1,300, 1,515, 2,000, and 5,000 yrs plus secondary harmonics. Obtained periodicities at 2,000 and 5,000 yrs support a global connection with records distributed at high-, mid-, and low-latitudes linked with the solar activity and monsoonal variations, while the 1,300 and 1,515 yr cycles appear to be linked with North Atlantic climate fluctuations. Spectral periodicities reinforce the strong connection between North Atlantic climate and monsoonal variations in the western Mediterranean context, and further support the extreme sensitivity of this region to cyclic climate changes as well as the climate teleconnections triggered by the North Atlantic Oscillations, the Inter-Tropical Convergence Zone migrations and monsoon activity, probably exacerbated by solar activity variations.