Resumen de High-resolution climate study of the past (570,000 to 300,000 years) and present interglacial in the iberian margin: a biomarkers view
Teresa Alexandra Ribeiro Rodrigues
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 570 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 of tetra-unsaturated alkenone (which in this site record 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 2ºC on a century scale that allows the identification of the Medieval Warm Period (MWP, 5501300 AD), the Little Ice Age (LIA, 13001900 AD) and the discontinuity associated to the well known 1755 AD Lisbon earthquake.
The biomarker data observed in this study shows dryer 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 with the negative NAO phase they indicated 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 warmest periods of the late Pleistocene (570-300 ka) interglacials lasted longer than the present-day interglacial and in all examined cases always ended abruptly with millennial-scale variability. On the other hand, the highest SST contrast between glacial and interglacial periods were observed in the youngest transitions. Furthermore, the high climate stability that characterizes the interglacials contrasts with the high variability observed during the glacials and glacial inceptions. The rapid SST identified in the terminations IV and V of the Estremadura spur have similar trends as that observed during the last glacial interglacial transition in the Tagus mud patch area.
This work provides an important contribution to further research on the mechanisms involved in rapid climate variability.
