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FP5

PACLIVA Résumé de rapport

Project ID: EVK2-CT-2002-00143
Financé au titre de: FP5-EESD
Pays: Spain

Studies of SST in long cores from the north Atlantic and the western Mediterranean Sea

Alkenones concentration have been measured on several sediment cores from the north Atlantic and the western Mediterranean Sea, and used to reconstruct Holocene sea surface temperature changes. The main cores used were ENAM9606 in Feni Drift, MD932343 in the Norwegian Sea and MD952343 in the western Mediterranean Sea, close to Menorca. Their composition has been compared with that obtained in other cores by other partners.

Core ENAM9606 exhibits a long-term cooling during the last 10 kyr, ranging form 14ºC to 12ºC. This trend is consistent with insolation with maximal values between 11 kyr and 5.5 kyr BP and is consistent with those of the other cores included in Fig. 1 and with several continental records from the circum North Atlantic providing evidence of a long-term cooling during the Holocene. ENAM9606 can be divided in three intervals: early-Holocene (from 10 kyr to 7.7 kyr BP), mid-Holocene (form 7.7 kyr to 4 kyr BP) and late-Holocene (from 4 kyr BP to present days). The former exhibits the highest values from 14ºC to 12.7ºC. From 10 kyr to 9 kyr the SST amplitude changes about one degree every 150 years approximately, punctuated by some cooling events at 9.7 kyr, 9.4 kyr and 9.2 kyr BP. Then, SST stabilizes at around 13.5ºC between 9.2 kyr and 8.4 kyr BP punctuated by a cooling at 8.3 kyr BP. During the mid-Holocene the SST decreases from 13.7ºC to present-day values (12ºC). Finally the late-Holocene is the period that exhibits the major variability compared with the early-Holocene, although the amplitude of the SST changes is higher for the first part of the Holocene. From 1900 yr BP to present-days, the SST decreases until 12.2ºC exhibiting great variability. The most pronounced involves a drop of about 1ºC at 160 yr BP in the LIA. This event occurs nearly synchronously with temperature changes in Chesapeake Bay and humidity variations in Cariaco Basin.

In the Norwegian Sea SST changed between 10.6 and 12.7 ºC. The highest values were observed in the interval between 6000-8000 years and after this period they decreased slowly to 12ºC as average with some episodes of lower values 2700 years BP. The temperature differences between 600 years and 2000 years BP ranged between 10.6ºC and 11.4ºC.

In the Menorca core a significant SST drop from 17.6ºC at 10000 years BP to 13.74ºC at 62 years BP is observed. This decrease is consistent with the above mentioned SST decreases in the Holocene but this is the one involving highest difference among the cores included in the present project. The period between 6000 and 8000 years exhibit SST ranging between 15.8ºC and 17.6ºC with two marked drops at 7680 and 7205 years BP. Another marked drop occurred in 4753 years BP involving a change between 17ºC to 15.3ºC. The SST profile of this core is characterized by a strong well marked SST decrease between 1885 and 2530 years BP (13.9ºC) involving a decrease of 1.4ºC. Temperature changes in the order of 0.9-1.5ºC are observed in the last 2000 year period.

The SST estimates in all cores studied shows a decrease from the beginning of the Holocene to present days. These high resolution studies confirm previous findings and reinforce the general trend in the marine environment. This trend is also observed in the continental masses, e.g. Mann et al., Nature, 392, 779-787 (1998), and likely constitutes one of the most robust trends of the climate evolution of the Holocene.

In contrast, in the last 2000 years, the expected SST changes for the MWP and the LIA are observed in some of the cores studied but not in all. In some cases, e.g. ENAM9606 there are features that can be related to these climate episodes but they do not expand over equivalent time periods as in the continent. The marine records in which SST changes have been observed to exhibit a better defined pattern in agreement with these two episodes are located nearby the coast of Portugal (namely MWP) and nearby Iceland (core MD992270). Climate in the marine environments of these two records SST was maybe under significant influence of the nearby continental masses.

Reported by

Institute of Environmental Assessment and Water Research
Jordi Girona, 18
08034 Barcelona
Spain
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