Apparent long-term cooling of the sea surface in the northeast Atlantic and Mediterranean during the Holocene

Abstract

Reconstructions of upper ocean temperature (T) during the Holocene (10-0 ka B.P.) were established using the alkenone method from seven, high accumulation sediment cores raised from the northeast Atlantic and the Mediterranean Sea (361N-751N). All these paleo-T records document an apparent long-term cooling during the last 10 kyr. In records with indication of a constant trend, the apparent cooling ranges from À0.27 to À0.151C kyr À1 . Records with indication of time-variable trend show peak-to-peak amplitudes in apparent temperatures of 1.2-2.91C. A principal component analysis shows that there is one factor which accounts for a very large fraction (67%) of the total variance in the biomarker paleo-T records and which dominates these records over other potential secondary influences. Two possible contributions are (1) a widespread surface cooling, which may be associated with the transition from the Hypsithermal interval (B9-5.7 ka B.P.) to the Neoglaciation (B5.7-0 ka B.P.); and (2) a change in the seasonal timing and/or duration of the growth period of alkenone producers (prymnesiophyte algae). The first contribution is consistent with many climate proxy records from the northeast Atlantic area and with climate model simulations including Milankovitch forcing. The second contribution is consistent with the divergence between biomarker and summer faunal paleo-T from early to late Holocene observed in two cores. Further work is necessary, and in particular the apparent discordance between biomarker and faunal T records for the relative stable Holocene period must be understood, to better constrain the climatic and ecological contributions to the apparent cooling observed in the former records. r

Holocene climate variability as derived from alkenone sea surface temperature and coupled ocean-atmosphere model experiments

“…4b). The records from the eastern North Atlantic and from the western Mediterranean Sea indicate a general cooling from the early to late Holocene, consistent with a previous study (Marchal et al 2002). The record 18 (GeoB5844-2) from the northern Red Sea indicates a general warming from the early to the late Holocene.…”

“…This warming period is also reflected in other proxy records from many parts of the world, although the dating of the climatic episode is variable and there are geographical differences in the timing of the thermal maximum (e.g. Marchal et al 2002). This mode captures also the relatively cool periods at the beginning and end of the Holocene.…”

“…The EOFs also served as a data-filtering procedure to eliminate the noise. A similar procedure was used to identify the dominant mode of Holocene temperature variability in the eastern North Atlantic (Marchal et al 2002) as well as to analyze the climate variability during the last deglaciation (Clark et al 2002). We used the EOF analysis also to derive the dominant modes of Northern Hemisphere atmospheric circulation variability at interannual to multi-centennial time scales as resulted from a long-term simulation of the ECHO-G coupled ocean-atmosphere general circulation model.…”

Holocene and Eemian sea surface temperature trends as revealed by alkenone and Mg/Ca paleothermometry

“…Although less spectacular than glacial climate instabilities, significant long-term Sea Surface Temperature (SST) trends arising from paleotemperature reconstructions clearly mark the Holocene epoch (e.g. Marchal et al, 2002;. A consistent regional SST evolution can be identified when comparing multiple SST records from various ocean basins .…”

Tropical climate variability from the last glacial maximum to the present

“…These issues also hamper multiproxy SST reconstructions outside of upwelling areas. For example, alkenone and foraminiferal faunal assemblage SST reconstructions from three North Atlantic cores were found to disagree systematically, with alkenones suggesting a long-term cooling from 6 ka to present that was not apparent in the faunal data (Marchal et al, 2002, and references therein).…”