Late postglacial paleoenvironmental change in the northeastern Mediterranean region: Combined palynological and molecular biomarker evidence

Quaternary International (Impact Factor: 2.13). 05/2012; 261:128-137. DOI: 10.1016/j.quaint.2011.10.036

ABSTRACT Three gravity cores collected from the NE Mediterranean (NEMR) across a transect from the northern Aegean Sea (North Skyros basin) to the south Cretan margin (SCM), were investigated for pollen and terrestrial biomarkers derived from epicuticular waxes of vascular plants during the last w20 ky. Pollen data show diversified mixed temperate forest in the northern borderlands and enhanced Mediterranean vegetation in the southern areas, documenting an NeS climatic trend. Terrestrial plant biomarkers and their diagnostic geochemical indices exhibit latitudinal patterns which are interpreted in terms of the different delivery pathways (fluvial/runoff vs. atmospheric transport), resulting from the climate conditions during different periods. During the Late Glacial and early deglaciation periods (20e14 ka BP) relatively increased humidity (H-index) is recorded in the north Aegean Sea, while in the South drier climate was the limiting factor for vegetation development. During this interval, terrestrial n-alkanes showed increased accumulation rates, suggesting massive transport of terrestrial organic matter through runoffs and rivers, followed by weaker input after 14 ka BP. After w11 ka BP a major expansion of forest cover is evidenced in the NEMR, accompanied by a higher H-index because of the climatic amelioration. The forest vegetation exhibited regionally different characteristics, with cool temperate taxa being more abundant in the Aegean cores, while the SCM record is being featured by Mediterranean elements. At the onset of the Holocene and throughout the Holocene Climatic Optimum the delivery of terrestrial biomarkers increased and became more significant in the Aegean sites compared to the SCM site. Within the Holocene, the average chain length (ACL) of long chain n-alkanes exhibits lower values in the northern Aegean than in the southeastern Aegean and SCM, indicating the predominance of warmer species southwards. Finally, the H-index records a conspicuous humidity increase between 5.4 and 4.3 ka BP in the south Aegean that coincides with an increase in the terrestrial biomarker supply and the deposition of a distinct sapropel-like layer, SMH (Sapropel Mid Holocene). Similar trends in T (temperature) and H indices are slightly delayed and attenuated in the northern Aegean and are accompanied by an increase in the ACL index. A noticeable increase in the accumulation rates (ARs) of terrestrial biomarkers and the HPA index values during this period are clearly recorded in all three cores, indicative of enhanced terrigenous inputs of organic matter along with higher in-situ preservation.

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    ABSTRACT: Marine and terrestrial biological and biogeochemical proxies in three sediment cores from North and SE Aegean and northern Levantine Seas record continuous warm and humid conditions between 5.5 and 4.0 ka BP related to the establishment of relatively stratified conditions in the upper water column. These conditions may have resulted from the concordant albeit weak Mid-Holocene South Asian monsoon forcing, combined with lighter Etesian winds. During this interval, sea surface temperatures fluctuate in the Aegean Sea, although exhibiting a strong positive shift at*4.8 ka BP. The warm and humid climatic conditions triggered upper water column stratification and enhancement of the deep chlorophyll maximum (DCM), leading to dysoxic conditions and the deposition of a sapropel- like layer, but only in the SE Aegean site. In contrast to the shallow water SE Aegean, the deeper North Aegean and the northern Levantine sites, although experiencing stratification in the upper parts of the water column, did not achieve bottom-water dysoxia. Thus, a top–bottom mechanism of stratification–DCM development accompanied by fast transport and burial of organic matter is a likely explanation for the preservation of productivity signal in the shallow sites of the SE Aegean and establishment of sapropelic conditions during the warm and humid Mid- Holocene. The termination of the Mid-Holocene warm and humid phase coincides with the ‘‘4.2 ka’’ climate event. Our data exhibit an N–S time transgressive aridification gradient around the Aegean Sea, most probably associated with the reorganization of the general atmospheric circulation during the Mid-Holocene.
    Regional Environmental Change 06/2013; DOI:10.1007/s10113-013-0495-6 · 2.26 Impact Factor
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    ABSTRACT: Lake-level records from Italy suggest that patterns of precipitation in the central Mediterranean during the Holocene were divided between the north and south, but a scarcity of reliable palaeoclimatic records in the north and central-southern Mediterranean means new evidence is needed to validate this hypothesis. We provide robust quantitative estimates of Holocene climate in the Mediterranean region using four high-resolution pollen records from northern (Lakes Ledro and Accesa) and southern (Lakes Trifoglietti and Pergusa) Italy. Multiple methods are used to provide an improved assessment of the palaeoclimatic reconstruction uncertainty. The multi-method approach uses the pollen-based weighted averaging, weighted-average partial least-squares regression, modern analogue technique, and the non-metric multidimensional scaling/generalized additive model methods. We use independent lake-level data to validate the precipitation reconstructions. Our results support a climatic partition between northern and southern Italy during the Holocene, confirming the hypothesis of opposing mid-Holocene summer precipitation regimes in the Mediterranean. The northern sites (Ledro, Accesa) are characterized by minima for summer precipitation and lake levels during the early to mid-Holocene, while the southern sites (Trifoglietti, Pergusa) are marked by maxima for precipitation and lake levels at the same time. Both pollen-inferred precipitation and lake levels indicate the opposite pattern during the late Holocene, a maximum in northern Italy and a minimum in southern Italy/Sicily. Summer temperatures show the same partitioning, with warm conditions in northern Italy and cool conditions in Sicily during the early/mid-Holocene, and a reversal during the late Holocene. Comparison with marine cores from the Aegean Sea suggests that climate trends and gradients observed in Italy show strong similarities with those recognized from the Aegean Sea, and more generally speaking in the eastern Mediterranean.
    Climate of the Past 06/2013; 9(3):1233-1252. DOI:10.5194/cp-9-1233-2013 · 3.56 Impact Factor
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    ABSTRACT: Recent paleoceanographic findings suggest that during the Holocene, NE Mediterranean climate was less stable than previously thought. Several recently generated Aegean paleoceanographic records of sapropel S1 suggest that during the Holocene short-term potentially global cooling episodes, reflecting the N. Atlantic climatic fluctuations and possibly related to solar variability, are superimposed on the underlying regional subtropical/tropical control on the hydrography and ecosystems. Detailed quantitative analyses of coccolithophores have been performed on the deep sea deposits of the NE Mediterranean Sea; four gravity core records from North Aegean Sea (SL- 152), SE Aegean (NS-14, NS-18) and South Cretan continental Margin/northern Levantine Sea (HCMR 2/22) respectively, with a focus on the regional expression of sapropel S1 deposition. In all sites, higher values of Florisphaera profunda, Helicosphaera carteri, Braarudosphaera bigelowii and the increase of stratification S-index within sapropel intervals can be interpreted as the result of elevated marine production in the deep photic zone and enhanced inflow of fresh water input during the S1 period. The stratification S-index is significantly higher in the North Aegean, in respect to SE Aegean and the South Cretan Margin (SCM), reflecting increased coccolithophore productivity at the lower photic zone, being associated with a remarkably stratified water column. The concomitant increased presence of B. bigelowii indicates the prominent riverine input in the North Aegean basins and possible influx of Black Sea waters after ∼9.0 ka BP. Consequently, the increased stratification of the water column is linked to the significant freshening of the North Aegean surface waters, particularly during the early stages of the Holocene Climatic Optimum (S1a depositional interval). A series of cooling events detected towards the end of sapropelic conditions (∼7.2 ka BP) mostly in the North Aegean site, may be linked to outbursts of cold northerly air masses and relevant pulses in the deep-intermediate water ventilation that caused the enhancement of surface marine productivity and the temporary cessation of S1 at ∼ 8.0 ka BP. In contrast, in the SE Aegean and the northern Levantine Sea, the water column remains still warm and stratified, influenced by moderate fresh water input, for at least 1 more ky.
    Quaternary International 02/2014; 345. DOI:10.1016/j.quaint.2014.01.033 · 2.13 Impact Factor

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