Late holocene climate variability in the western Mediterraneanan integrated organic and inorganic multiproxy approach

Resumen

This Thesis provides an integration of molecular and inorganic geochemical proxies for a detailed high resolution paleoenvironmental characterization of the Late Holocene in the westernmost Mediterranean Sea. Marine records from the western Algerian-Balearic and Alboran Sea basins have been used as archives for paleoclimatic and paleoceanographic reconstructions. Age-depth models from these records have been obtained based on 14C dating complemented with the activity depth-profiles of 210Pb. In the western Algerian-Balearic basin, a geochemical approach has been supplemented by multivariable statistical analyses for the identification of detrital input oscillations (fluvial-derived elements and eolian input fluctuations), marine productivity (total organic carbon, Br/Al and organometallic ligands), grain size distribution (sortable silt and quartz content), oxygenation conditions and post-depositional alteration (redox-sensitive elements) during the last 4000 yr. Within this time interval, the Roman Humid Period (RHP) and the Little Ice Age (LIA) are evidenced as humid periods, characterized by higher riverine influence and low Saharan eolian input while drier paleoclimatic conditions are observed during the Late Bronze Age-Iron Age (LBA-IA), the Dark Ages (DA) and the Medieval Climate Anomaly (MCA). Furthermore, more energetic paleocurrent circulation at this time is denoted by faster and well oxygenated bottom waters. The mid-late RHP appeared as the most humid during the Late Holocene, and also enhanced marine productivity is recognized. Variations in solar irradiance are claimed as the major natural forcing process contributing to climate variability in this region during the last 4000 yr, modulated by the North Atlantic Oscillation (NAO) during the MCA and the LIA. In the north-western Alboran Sea basin, two higher resolution records have enabled a comprehensive multiproxy reconstruction of climate responses during the last two millennia through the combination of a wide suite of inorganic and organic geochemical proxies. Hence, sea surface temperatures (SST) (TEXH86 and Uk37 ratios-derived SST), fluctuations in chemical elements (major and trace elements content), mineral composition of marine sediments (bulk and clay mineralogy) as well as grain size distribution have depicted paleoenvironmental (fluvial and eolian input oscillations and past SST), and paleocenographic (paleocurrent intensity and redox sensitive elements) conditions during the MCA, the LIA, the Industrial Period (IP) and the second half of the twentieth century. Additionally, further information such as paleoproductivity (related to upwelling) and paleohydrological conditions (relative contribution by land plants and provenance of the sedimentary organic matter) have been gathered from the diol index, the carbon isotopic composition of long straight chain n-alkanes and the BIT index. The organic geochemical record reveals a gradual long term decline of SST throughout the last two millennia based on TEXH86 and Uk37 ratios, with warmer temperatures during the MCA, and the LIA recorded as the coldest period. A sharp and unprecedented SST increase took place during the Industrial Era and the twentieth century. This trend is in agreement with Northern Hemisphere temperature oscillations forced by solar irradiance variations and anthropogenic greenhouse warming during the last 2000 yr. Moreover, during periods of a persistent high NAO state, lower riverine influence and an intensification of the wind-induced upwelling conditions are exposed by the BIT index and the bloom of Proboscia diatoms. Concerning hydrological conditions, the isotopic composition of n-alkanes disclosed a predominantly fluvial C3 plant contribution while the low BIT index revealed a dominant marine provenance for the organic matter. The inorganic geochemical record displayed the MCA and the second half of the twentieth century as prevalent dry periods based on decreasing trends of fluvial-derived elements ratios (Si/Zr and K/Zr) and higher Saharan eolian input (Zr/Al and Zr/Rb ratios). Wetter conditions were recognized during the LIA and the IP instead, with a concurrence of wet spells and minor Saharan eolian contribution during periods of lower sunspots activity. Two events of strengthening Atlantic waters flowing into the Mediterranean Sea were registered by a remarkable change in the paleoceanographic record (faster bottom currents and oxygenated bottom waters). An outstanding anthropic contribution during the last centuries is likewise discernable by a progressive raise in heavy metal concentrations since industrial times (Pb, Zn and Cu). This multiproxy climatic assessment, encompassing different records from the westernmost Mediterranean, has depicted a sequence of dry and humid periods with coincident signals for the overlapped periods (LIA and MCA) in the two studied basins that are also coherent with the North Atlantic climatic framework during the Late Holocene, which further support the link between this region and the North Atlantic ocean-atmosphere system.