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Alkanone-based SST reconstructions off NW Africa and the Iberian margin

There is coastal upwelling along the NW coast of Africa, and the Iberian Margin. Satellite SST data show a strip of cold coastal waters, 3-5°C colder than open ocean conditions, in the Cape Ghir upwelling centre, with an upwelling filament extending offshore. The Holocene record of SSTs in this region was reconstructed at high-resolution from three cores using alkenone paleothermometry

Core GeoB6008-1 and short core GeoB6008-2 were recovered from 355m depth off the Moroccan coast (30°50.7 N, 10°05.9 W) and were used to estimate SST variability for the last 2000 years. Age control for GeoB6008-2 was established by 210Pb dating, and for GeoB6008-1 by both 210Pb and accelerator mass spectrometer (AMS) 14C dating.

The upper part of SST reconstructions from GeoB6008-1 and GeoB6008-2 overlap with the instrumental period for the last 90 years of the 20th century, and, taken together, they show a steady cooling trend of approximately 1.2°C over this time. In addition, the pattern of variability for the 50 years where the two cores overlap (1912-1971AD) are remarkably similar, attesting to the consistency of the age model in this part of the core, and of the SST signal in each record. When viewed in the context of the gravity core record for the last 2.5 millennia, the last 100 years of the GeoB6008 alkenone records shows that the strong decrease in SST observed for the last century is larger and more rapid than any other change seen in the record and is 0.3°C colder than the 19th century and 0.6°C colder than the next coolest period from 1050-1150AD. The 1050-1150AD period corresponds to the middle of the so called Medieval Warm Period (MWP; 850-1300AD). Relative to the MWP the alkenone SST records show a warm Little Ice Age (LIA; 1400-1850). Prior to the 20th century, the GeoB6008 record shows pronounced millennial variability.

Core GeoB5901-2 was recovered from 574m depth on the Iberian margin (36.380°N 7.071°W) and was used for high-resolution SST reconstructions during the 6000-8000 year period. Age control is based on AMS 14C dates. The alkenone SST reconstruction from shows a cooling trend of 1.3°C from 10000 to 1000 cal yrs BP. The warmest SSTs of 20.3°C occur in the early Holocene. Superimposed on the long-term trend is a multi-millennial scale oscillation, with relative peaks at 9000-9500, 6000 and 2500 cal yr BP.

The GeoB6008 cores come from the heart of the Cape Ghir upwelling system, where the upwelling is driven by alongshore winds forcing offshore Ekman transport, compensated by the upwelling of cooler, nutrient-rich water. The alkenone SST proxy record from the GeoB6008 cores are interpreted as upwelling histories for Cape Ghir, and the decrease in SSTs for the 20th Century suggests an intensification of upwelling.

The Cape Ghir SST record, including the rapid temperature changes of the last 100 years and the millennial scale variability, shows similar characteristics, though with opposite sign, to the Northern Hemisphere temperature Anomaly reconstructions. For example, from 1450-1850AD, the LIA, relative warmth is observed in the GeoB6008 SST record compared with relative cooling in the NHTA records.

Both the high decadal-scale resolution and the overlap with the 20th century instrumental/anthropogenic period make the SST reconstructions for the GeoB6008 cores a unique dataset. Through this core we are able to learn about the global warming response of ecologically and socially important marine upwelling systems and further our understanding of recent climate variability.

A high-resolution multi-proxy study reconstructed SST, river discharges and biological productivity in the Tagus deposition centre off Lisbon (Portugal) for the last 2,000 years. Century-scale SST variability of 2ºC allowed the identification of MWP and the LIA. High Fe and fine-sediment deposition with high n-alkane concentrations and freshwater diatoms during the LIA suggested increases of river discharge, whereas higher alkenone concentrations indicate increased river-induced productivity. During the MWP larger mean-grain size and low magnetic susceptibility, Fe, n-alkanes, and n-alcohols reflect decreased runoff. Increased benthic and planktonic foraminifera and upwelling related diatoms pointed to increased oceanic productivity. Based on the correlation between negative NAO phases and intensified Tagus River discharge over the last century, it was hypothesized that the increased influx of terrigenous material during the LIA reflected a negative NAO-like state or the occurrence of frequent extreme NAO minima. During the MWP, stronger upwelling was attributed to a persistent positive NAO-like state or the frequent occurrence of extreme NAO maxima. A peak in magnetic susceptibility was interpreted the 1755 AD Lisbon earthquake. This and the accompanying tsunami were estimated to have caused the loss of 39 cm of sediment and the instantaneous deposition of a 19 cm.

Reported by

Research Center Ocean Margins, University of Bremen
Leobener Strasse
28359 Bremen
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