Late Holocene Climate Variability in the North Atlantic based on biomarker reconstruction: The lake Azul (São Miguel, Azores archipielago) case

Abstract

The principal objective of this PhD thesis is to obtain a high resolution reconstruction of the climate (temperature and precipitation) in the north Atlantic for the late Holocene. This new reconstruction is from a sedimentary lacustrine record located in Azores (Lake Azul) that has been compared and discussing with principal other climate reconstructions published on the North Atlantic region. Climate in the North Atlantic is modulated by the conjunction of atmospheric patterns and ocean dynamics. The main atmospheric pattern is the North Atlantic Oscillation (NAO) but, other climatic modes such as the Eastern Atlantic (EA) and the Atlantic Meridional Oscillation (AMO) are also present. The use of biomarkers has appeared as a cutting edge technique to reconstruct climate and their validity has been proved by previous works. The Azul lacustrine sedimentary sequence occupied the last 660 years, and on it has been applied a multiproxy study including: facies analysis, X-Ray Diffraction (XRD), X-Ray

Fluorescense (XRF), Total Organic Carbon (TOC), Total Nitrogen (TN), bulk isotopes of Carbon (δ13C) and Nitrogen (δ15N) and biomarkers (GDGTs and δD leaf waxes). Facies were grouped in two main facies types according to the velocity of sedimentation and the organic matter origin: (1) “Continuous facies” accumulated by a more or less continuously sedimentation processes and dominated by lake organic matter, and (2) “rapid facies” deposited from flood events and transporting mainly terrestrial organic matter. An accurate age-depth model based on 14C and 210Pb dates and XRF data was built to figure out those sudden changes (the Dynamic Age Model, DAM). The DAM calculates the age of the samples redistributing the time along the profile according to the amount of crystalline terrestrial particulated material. DAM can acquire short and long-term sedimentation rate (SR) changes and previous models only acquire long- term fluctuations. The biomarkers (GDGTs and δD leaf waxes) were extracted and studied on the continuous lacustrine facies to reconstruct temperature and precipitation. The relative percentage of GDGTs was transferred into temperature by the use of Pearson et al., (2011) calibration. The climatic factor affecting δD

fractionation was assessed by the weekly collection of rain water showing a clear depletion with the amount of precipitation. Results from realized reconstruction suggested a climatic model that converge oceanic currents with the principal atmospheric patterns, defining the NAO as the main factor triggering the others. The temperature in Azores depends on the position of the Gulf Stream (GS). A positive

(negative) NAO phase enhances (reduces) the Gulf Stream (GS) velocity that increases (reduce) the amount of Eddies around Bermuda driving towards north (east) the GS and heating up high (medium) latitudes. Therefore, a negative phase of NAO favors the GS heat to reach Azores. Since the temperature in Azores is modulated by the oceanic circulation, the AMO also plays an important role. The AMO is reflected in Ponta Delgada by the amplitude between maximum and minimum mean annual temperatures, which means an increase on seasonality. The AMO positive phase is reflected as an increase on temperatures in Lake Azul reconstruction. Otherwise, the AMO negative phase is masked by other climatic patterns. The precipitation in Azores is mainly triggered by the NAO phase, increasing (decreasing) precipitation during NAO negative (positive). The comparison with other reconstructions in the North Atlantic area suggested that an increase of precipitation in São Miguel produces an increase at other Azorean islands, the Iberian Peninsula and Morocco. This pattern has been observed for the last 700 years except for the period comprised between 1680 and 1920 AD where a possible southeastern shift of the NAO south dipole seemed to be produced. We suggest that this movement is produced by the occurrence, simultaneously, of an in phase and positive NAO and EA conjunction.