Removal of bias in C37 alkenone-based sea surface temperature measurements by high-performance liquid chromatography fractionation.

Sea surface temperature determinations based on marine sedimentary C37 alkenone distributions have provided a wealth of data for paleoclimatic studies, including those performed at high resolution. The success of this approach results from several characteristics of alkenone compounds, e.g. their geochemical properties (such as unequivocal synthesis by certain widespread haptophyte algae, plus chemical stability/preservation of the original alkenone distributions during sedimentation), and their analytical properties (such as fast clean up procedures using alkaline hydrolysis of sediment extracts, followed by robust instrumental methods allowing large scale sample processing). Here we show that, in sediments under the influence of continental inputs, coelution of these compounds with cholest-5-enyl 3ß-undecenyl ether and 24-methylcholesta-5,22-dienyl 3ß-undecenyl ether deviate the SST measurements despite alkaline hydrolysis. Here, we report a new high performance liquid chromatrography fractionation method which eliminates these interfering compounds and gathers all the alkenones into a single fraction. These fractions can then be analysed by gas chromatography as in the initial approach, providing large amounts of data as required in high resolution studies.

Mediterranean circulation perturbations over the last five centuries: Relevance to past Eastern Mediterranean Transient-type events.

The Eastern Mediterranean Transient (EMT) occurred in the Aegean Sea from 1988 to 1995 and is the most significant intermediate-to-deep Mediterranean overturning perturbation reported by instrumental records. The EMT was likely caused by accumulation of high salinity waters in the Levantine and enhanced heat loss in the Aegean Sea, coupled with surface water freshening in the Sicily Channel. It is still unknown whether similar transients occurred in the past and, if so, what their forcing processes were. In this study, sediments from the Sicily Channel document surface water freshening (SCFR) at 1910 ± 12, 1812 ± 18, 1725 ± 25 and 1580 ± 30 CE. A regional ocean hindcast links SCFR to enhanced deep-water production and in turn to strengthened Mediterranean thermohaline circulation. Independent evidence collected in the Aegean Sea supports this reconstruction, showing that enhanced bottom water ventilation in the Eastern Mediterranean was associated with each SCFR event. Comparison between the records and multi-decadal atmospheric circulation patterns and climatic external forcings indicates that Mediterranean circulation destabilisation occurs during positive North Atlantic Oscillation (NAO) and negative Atlantic Multidecadal Oscillation (AMO) phases, reduced solar activity and strong tropical volcanic eruptions. They may have recurrently produced favourable deep-water formation conditions, both increasing salinity and reducing temperature on multi-decadal time scales.

Extraction of climatic signals from fossil organic compounds in marine sediments up to 11.7 Ma old (IODP-U1318).

This study focuses on the extraction of climate signals and processes using a combined approach which includes the analysis of a high number of lipid molecules in marine sediments, and the chemometric analysis of the acquired data. Neutral and acidic fractions of marine sediments from site IODP-U1318 (south-west of the UK, Porcupine Seabight) were quantified by GC-MS. The alkenone unsaturation index, U(k')37, was estimated from the composition of C37 alkenones and it was then used for the estimation of sea surface temperatures (SST) for reference. Principal component analysis (PCA), explained 77.45% of the total data variance, and differentiated neutral fraction GC-MS total ion current (TIC) profiles according to SST values of the different sediment sections. GC-MS TIC chromatograms were correlated to sea surface temperatures (SST) by partial least squares regression (PLSR). The compounds more robustly in line with SST values at each sediment section explained 93% of the SST variance and they were identified using the variable importance in projection (VIP) scores method. The proposed approach enables an objective identification of organic compounds sensitive to SST variability throughout complete chromatographic profiles. As a result of this multivariate unbiased approach, lipid composition of sediments was differentiated between compounds of marine (long chain n-alkanes, long chain n-alkan-1-ols) and terrestrial (short chain n-alkan1-ols, alkenols, cholesterol, squalene) origin, whose concentrations were directly and inversely correlated to SST, respectively.

Four climate cycles of recurring deep and surface water destabilizations on the Iberian margin.

Centennial climate variability over the last ice age exhibits clear bipolar behavior. High-resolution analyses of marine sediment cores from the Iberian margin trace a number of associated changes simultaneously. Proxies of sea surface temperature and water mass distribution, as well as relative biomarker content, demonstrate that this typical north-south coupling was pervasive for the cold phases of climate during the past 420,000 years. Cold episodes after relatively warm and largely ice-free periods occurred when the predominance of deep water formation changed from northern to southern sources. These results reinforce the connection between rapid climate changes at Mediterranean latitudes and century-to-millennial variability in northern and southern polar regions.

Estrogenic activity in sediments from European mountain lakes.

Superficial and bottom sediment samples from 83 European mountain lakes, ranging from Norway to the Pyrenees and East Europe, were tested for estrogenic compounds by the recombinant yeast assay. The results showed widespread potential estrogenic activity arriving at remote lakes. Tatra Mountains (Slovakia) and Scotland Highlands were the regions with the highest prevalence of lakes with high estrogenic values. Comparison of the estrogenic activity in the superficial layer of sediments with pre-industrial age sections showed that estrogenic compounds were predominantly deposited in recent times. Chemical analysis showed that highly estrogenic sediments were significantly enriched in both polycyclic aromatic hydrocarbons (PAH) and organochlorine compounds. For PAH, enrichment ratios in highly estrogenic samples versus nonestrogenic ones were inversely correlated with the vapor pressure value for each compound, indicating a significant relationship between estrogenicity and accumulation of less volatile PAH. Two PAH of predominantly diagenetic origin, retene and perylene, did not show specific enrichment in estrogenic samples. Principal component analysis revealed a strong correlation between estrogenic activity and the presence of contaminants of anthropogenic origin. These data reveal significant amounts of estrogenic compounds in remote lakes, relate them to the overall human activity, and suggest that they may affect organisms inhabiting these ecosystems.

Abrupt temperature changes in the Western Mediterranean over the past 250,000 years.

A continuous high-resolution Western Mediterranean sea surface temperature (SST) alkenone record spanning the past 250,000 years shows that abrupt changes were more common at warming than at cooling. During marine isotope stage (MIS) 6, SST oscillated following a stadial-interstadial pattern but at lower intensities and rates of change than in the Dansgaard/Oeschger events of MIS 3. Some of the most prominent events occurred over MISs 5 and 7, after prolonged warm periods of high stability. Climate during the whole period was predominantly maintained in interglacial-interstadial conditions, whereas the duration of stadials was much shorter.