Article

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

Quaternary International (Impact Factor: 2.06). 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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    • "During the so-called Holocene Climatic Optimum (HCO; approx. 10.0–6.0 ka), a distinct positive shift in the Aegean Sea's freshwater budget—possibly supplemented by precipitation and riverine contribution from the Aegean borderland and also inflow of Black Sea Water (BSW)—weakened the basin's deepwater circulation, resulting in oxygen-starved conditions at the seafloor and deposition of sapropel layer S1 (e.g., Aksu et al. 1995, 2002; Gogou et al. 2007; Kuhnt et al. 2007; Abu-Zied et al. 2008; Kotthoff et al. 2008; Geraga et al. 2010; Katsouras et al. 2010; Schmiedl et al. 2010; Kouli et al. 2012; Triantaphyllou 2014; Triantaphyllou et al. 2014). Aegean Sea sites of high sedimentation rates are associated with S1 sapropelic layers characterized by low organic carbon (OC) contents of <2% (e.g., Roussakis et al. 2004; Triantaphyllou et al. 2009a), reflecting strong dilution by lithogenic input (e.g., Mercone et al. 2000). "
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    ABSTRACT: Combined micropaleontological and geochemical analyses of the high-sedimentation gravity core M-4G provided new centennial-scale paleoceanographic data for sapropel S1 deposition in the NE Aegean Sea during the Holocene Climatic Optimum. Sapropel layer S1a (10.2–8.0 ka) was deposited in dysoxic to oxic bottom waters characterized by a high abundance of benthic foraminiferal species tolerating surface sediment and/or pore water oxygen depletion (e.g., Chilostomella mediterranensis, Globobulimina affinis), and the presence of Uvigerina mediterranea, which thrives in oxic mesotrophic-eutrophic environments. Preservation of organic matter (OM) is inferred based on high organic carbon as well as loliolide and isololiolide contents, while the biomarker record and the abundances of eutrophic planktonic foraminifera document enhanced productivity. High inputs of terrigenous OM are attributed to north Aegean borderland riverine inputs. Both alkenone-based sea surface temperatures (SSTs) and δO18G. bulloides records indicate cooling at 8.2 ka (S1a) and ~7.8 ka (S1 interruption). Sapropelic layer S1b (7.7–6.4 ka) is characterized by rather oxic conditions; abundances of foraminiferal species tolerant to oxygen depletion are very low compared with the U. mediterranea rise. Strongly fluctuating SSTs demonstrate repeated cooling and associated dense water formation, with a major event at 7.4 ka followed by cold spells at 7.0, 6.8, and 6.5 ka. The prominent rise of the carbon preference index within the S1b layer indicates the delivery of less degraded terrestrial OM. The increase of algal biomarkers, labile OM-feeding foraminifera and eutrophic planktonic species pinpoints an enhanced in situ marine productivity, promoted by more efficient vertical convection due to repeated cold events. The associated contributions of labile marine OM along with fresher terrestrial OM inputs after ~7.7 ka imply sources alternative/additional to the north Aegean riverine borderland sources for the influx of organic matter in the south Limnos Basin, plausibly related to the inflow of highly productive Marmara/Black Sea waters.
    Full-text · Article · Feb 2016 · Geo-Marine Letters
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    • "Coinciding with the interruption of the formation of sapropel S1 in the Eastern Mediterranean, the 8.2 kyr B.P. climatic event has been characterized by large-scale atmospheric circulation changes with increased influence of the Siberian High and a concurrent weakening of the impact of low-latitude monsoonal circulation (Kotthoff et al., 2008a). Various marine and terrestrial proxy records from the Aegean region show signs of temperature and/or precipitation changes during that time (Rohling et al., 2002; Dormoy et al., 2009; Marino et al., 2009; Triantaphyllou et al., 2009; Geraga et al., 2010; Kouli et al., 2012). Further, highresolution pollen records from the paleoclimate archive of Tenaghi Philippon (TP), northeastern Greece, indicate that strengthening of the Siberian High during the 8.2 kyr B.P. climatic event led to a decline in winter temperatures and precipitation seasonality, resulting in a major vegetation setback, mainly at the expense of evergreen tree taxa (Pross et al., 2009; Peyron et al., 2011). "
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    ABSTRACT: Throughout the Holocene, the climate of the Mediterranean region has been strongly influenced by variability in the atmospheric circulation of the high and low latitudes. A prominent example for such Holocene climate perturbations is the ‘8.2 kyr B.P. climatic event’. Reorganization of Northern Hemisphere atmospheric circulation patterns resulted in variations of temperature and precipitation distribution across the Mediterranean. The effects of changing high- and low-latitude atmospheric circulation on Mediterranean climate in relation to the 8.2 kyr B.P. climatic event are, however, not well understood. Here we present a high-resolution record of stable hydrogen isotope composition of plant-wax n-alkanes (dDwax) across the 8.2 kyr B.P. climatic event from the Tenaghi Philippon peat deposit (NE Greece) in order to characterize patterns of precipitation and changes in atmospheric circulation in the Eastern Mediterranean region. Our record reveals pronounced changes in dDwax that correlate closely with previously published palynological data. A long-term decline in dDwax values characterizes the lower part of the section. The 8.2 kyr B.P. climatic event itself is connected to two distinct positive dDwax excursions: a minor shift in dDwax around 8.2 kyr B.P. and a major shift in dDwax between ca. 8.1 and 8.0 kyr B.P.. The upper part of the section shows a progressive trend towards higher dDwax values. We link shifts in dDwax to changes in Mediterranean air mass trajectories supplying precipitation to NE Greece caused by variations in the relative contributions of northerly-derived, D-depleted moisture and southerly-derived, D-enriched moisture. Possible control mechanisms for alternating air mass trajectories include changes in the influence of the Siberian High and differences in the influence of the African and Asian monsoon circulation on anticyclonic conditions in the Mediterranean region as well as regional inflow of moist air masses from the Aegean Sea.
    Full-text · Article · Jan 2016 · Quaternary Science Reviews
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    • "In contrast to the core-SK-1, the S1 layer in NS-14 is divided into two sub-units (S1a and S1b) separated by a lightcolored interval, which corresponds to the S1 interruption (S1i) (Fig. 2A). Detailed investigations of the S1 occurrence in these cores have been reported earlier (Casford et al., 2007; Kontakiotis et al., 2009, 2013; Triantaphyllou et al., 2009a,b; Kontakiotis, 2012; Kouli et al., 2012). In NS-14, the most recent Z2 Santorini ash layer is positioned at 17 cm depth, and a sapropel-like, dark olive grey mud layer (Sapropel Mid Holocene e SMH) is deposited between 25 and 40 cm (Triantaphyllou et al., 2009a,b; 2013) (Fig. 2A). "
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    ABSTRACT: The Late Quaternary palaeoenvironmental evolution and the main palaeoceanographic changes of the Aegean Sea was reconstructed using planktonic foraminifera obtained from the sediments of a northern (SK-1) and a southern (NS-14) core site. Multivariate statistical approach, including Q-mode cluster and Principal Component (PCA) Analyses, gives an insight into the relationships between sea surface environmental changes between the north and south Aegean, and furthermore serves as a baseline data set for palaeoclimatic and palaeoceanographic reconstructions of the eastern Mediterranean Sea. Stable isotope analyses coupled with additional paleoceanographic (sea surface temperature (SST), productivity (SSP), stratification (S-index), upwelling (U-index), seasonality (Sn-index)) proxies provide insights into the sedimentological, hydrographic and trophic evolution and ecosystem dynamics of the Aegean Sea, and furthermore serve as a baseline data set for palaeoclimatic and palaeoceanographic reconstructions of the eastern Mediterranean Sea. Concentrating on the last ∼19 ka, faunal and isotopic data concur, suggesting a series of climatic zones, corresponding to well-known climatic phases of the last glacial cycle. Cold and arid conditions during the late glacial period were followed by a warmer and wetter climate trend during deglacial time. A marked seasonality, characterized by homogenous water column favored by efficient upwelling during the winter and by warm stratified water column during the summer, has also been recognized during this period (Bølling–Allerød – B–A) in both basins. An abrupt switch to cool, arid climatic event (Younger Dryas – YD) suggests a strengthening of winter convection around 12.9 ka, which seems to be more evident in the north Aegean core. However, the more pronounced environmental changes between the north and south Aegean Sea were identified during the Holocene, and they coincide with the deposition of sapropel S1 and Sapropel-Mid-Holocene (SMH). The comparison between SK-1 and NS-14 records indicated significant changes in both the productivity and stratification patterns, with the former the main factor in north Aegean and the latter in south Aegean for S1 deposition. Regarding the localized deposition of SMH in the semi-enclosed shallow south Aegean site, the fully oxygenated and less productive water column conditions prevailing during that time (5.4–4.3 ka) in the north Aegean were the limiting factor for its deposition.
    Full-text · Article · Aug 2015 · Quaternary International
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