High-resolution climate study of the past, 570,000 to 300,000 years, and presen...

• Author(s): Rodrigues, Teresa
• Date: 2009
• Persistent ID: http://hdl.handle.net/10400.9/828
• Origin: Repositório do LNEG
• Subject(s): Paleoclimatologia; Mudanças climáticas; Biomarcadores; Temperatura da superfície do mar; Holocénico; Margem Continental Ibérica; Portugal
• Document Type: Doctoral Thesis
• Language: English

Description

Increasing the knowledge on the drivers of climate change during the Holocene and its consequences is fundamental to understand the processes that determine the global climate system. Accordingly, this research is focused to determine the natural evolution of the present interglacial period and to evaluate the anthropogenic contribution to it. To get insight into these objectives the present study compares the Holocene records with those of previous interglacial periods. Obviously, these warm intervals represent potential past analogues of the current interglacial not influenced by human activities. The comparison focuses in particular on the timing and duration of the interglacials and subsequent glacials, on the: i) nature of the climate response in the marine and terrestrial environments and ii) the presence of stability or instability conditions. High-resolution paleoclimate oscillations off the western Iberian margin during the Marine Isotopic Stages (MIS) 1 (covering the last 13,5 ka) and MIS 15 to 9 (between 580 and 300 ka) have been reconstructed. These are based on multiproxy analyses of several sediment archives retrieved from the Tagus mud patch and from the Estremadura spur. The multi-proxy analysis includes alkenone-based sea surface temperature reconstructions (Uk'37-SST), terrigenous biomarkers: n-alkanes and n-alcohols (which reflect the evolution of the terrestrial vascular plants), concentrations of total alkenones and percentages of total organic carbon (which provide information on the marine productivity) and percent abundance oftetra-unsaturated alkenone (which records freshwater inputs). According to the results, the last two centuries were marked by large variations of SST possibly reflecting the increasing occurrence of extreme conditions. The last 2 ka shows SST variability of 2°C on a century scale that allows the identification of the Medieval Warm Period (MWP, 550-1300 AD), the Little Ice Age (LIA, 1300-1900 AD) and the discontinuity associated to the well known 1755 AD Lisbon earthquake. The biomarker data observed in this study shows drier continental conditions and increased coastal upwelling during the MWP which was associated with the positive phase of the North Atlantic Oscillation (NAO). During the LIA that is related to the negative NAO phase, the same proxies indicate increased river influx and river induced marine productivity. The beginning of the Holocene was marked by an abrupt increase in SST until optimum climate conditions at the beginning of this period. After this episode there is a general trend of SST decrease and superimposed to it rapid cooling events have been identified. These climate patterns have also been previously recorded in other sites such as the NE Atlantic and Mediterranean areas. The glacial-interglacial transition in the Tagus area is marked by a sharp SST increase contemporaneous with a strong sea level rise synchronously with the well-known mwp-1B and consequent retreat of the coastline in the Tagus mud patch which probably precluded the input ofterrigenous material to the inner-shelf. The new results for the period from 580 to 300 ka show a general trend of warm and relative stable interglacial periods and high-frequency variability during the glacial inceptions and glacials. Periods of interglacial warmth were longer prior to MIS 9 with MIS 13 showing relative stable conditions for about 54 ka, the longest period during the last 580 ka. Cross-correlation of SST and the orbital cycles shows a change of predominant periodicity from obliquity to eccentricity around 430 ka, the time of the mid-Brunhes event. The Sub-orbital scale SST variability marks the colder periods (glacial inceptions and glacials). The most pronounced of these events are related to Heinrich-type events. In our record we have identified 8 Heinrich-type events. The general pattern recorded during the deglaciations are similar to the younger climatic cycles. The composed Iberian SST records over the last 6th climate cycles show that SST of the MIS 11.3 is analogous to the present interglacial, the Holocene. This work provides an important contribution to further research on the mechanisms involved in rapid climate variability.