A. Hayes

Eastern Mediterranean sapropel S1 interruption: an expression of the onset of climatic deterioration around 7 ka BP

We discuss the palaeoclimatic interpretation of unprecedented high-resolution micropalaeontological studies of shortterm (2 to 4 centuries) interruptions within early Holocene organic-rich layer (sapropel) S1 from the eastern Mediterranean. Results for cores from the Adriatic and Aegean seas that contain ‘double’ S1 sapropels indicate that these interruptions, which are centred roughly around 7000 years 14 Cnc BP, are genuine and related to climatic deterioration. This interpretation is endorsed by a coeval dry event recorded in terrestrial records and indications of climatic deterioration affecting human migration patterns and early societies in Egypt. The presence of sapropel interruptions in the two major source areas of deep water for the entire eastern Mediterranean likely implies that similar intervals may be found throughout the basin, provided that sedimentation rates and sampling resolutions allow the detection of events with a duration of only several centuries. Moreover, our results show that the ‘sapropel mode’ of circulation comprises a delicate balance between reduced ventilation and enhanced productivity, which is easily disturbed through surface water cooling triggering a short time of improved deep water ventilation.

Mediterranean planktonic foraminiferal faunas during the last glacial cycle

This paper highlights the planktonic foraminiferal abundance variations during the last glacial cycle from six new cores along a roughly west–east transect in the Mediterranean Sea, together with results from previous studies. Multivariate statistical analysis describes a first significant axis that indicates a general sea surface temperature (SST) gradient from west to east. As expected lower SST values are recorded in glacial times but the eastern and western basins seem to have reacted differently to glacial conditions. The western basin shows a SST decrease from the Alboran Sea to the central Mediterranean, whereas the eastern basin records a west to east increase that is similar to the present-day eastern Mediterranean gradient. These new results show that: (1) few almost basin-wide faunal trends may be recognised, but these are interrupted by local anomalies with opposing trends; and (2) major abundance variations are distinctly diachronous in some species and virtually synchronous in others. The implication is that over the glacial–interglacial timescale concerned, biostratigraphy should only be used on a local rather than basin-wide scale, unless it is supported by other independent dating methods.

Colour logging as a tool in high-resolution palaeoceanography

Abstract Colour and diffuse reflectance records can be used to develop astronomically tuned age models for long sediment cores. Here, we present high-resolution (1 mm) colour records from a sediment core from the western Gulf of Cadiz of SW Spain (D13892), spanning the last deglaciation, in parallel with stable isotope (δ18O) and sea surface temperature (SST) proxy data. The age model is based on δ18O stratigraphy complemented by five atomic mass spectroscopy (AMS) radiocarbon datings. We find good comparison between the colour record of D13892 and the GISP2 oxygen isotope series (R2 = 0.81), which strongly suggests that the sediment colour reflects the state of the climate. As sediment colour variability has previously been found to be diagnostic of changes in mineralogical/chemical composition, we relate the causes of the colour variability in D13892 to changes in the local particle flux, and support these observations with data from core-logging X-ray fluorescence (XRF) analyses. The colour and XRF logger records for D13892 suggest that the last glaciation and Younger Dryas were characterized by an enhanced supply of terrigenous detritus into the western Gulf of Cadiz. Cyclicities with wavelengths of 607 and 1375 years are recognized in the colour records for the Holocene. This cyclicity also relates to variability in detrital supply, with an important eolian component implied by enrichment in hematite during cycle maxima.

Deposition of sapropel S1 sediments in oxic pelagic and anoxic brine environments in the eastern Mediterranean: differences in diagenesis and preservation

Sediments from a boxcore in the previously anoxic brine-filled Poseidon Basin, eastern Mediterranean, have been studied and compared to sediments deposited in a ‘normal’ eastern Mediterranean environment. The boxcore can be divided into three main sedimentary intervals based on AMS-radiocarbon ages, foraminiferal and geochemical zonations. From the base of the core upwards these are: (1) 12.3‐31.2 cm, organic-rich sediments redeposited from within the brine; (2) 6.6‐12.3 cm, sediment containing a ‘cold’ foraminifera fauna redeposited from above the brine into the basin while the brine was still present; (3) 0‐6.6 cm, oxic pelagic sediment accumulated since the reoxygenation of Poseidon Basin which occurred1800 yrs BP. Near the base of the latter unit, a Mn-oxide peak has formed and it marks the present boundary between oxic and suboxic environments. A progressive downward oxidation front, which is usually found in ‘normal’ sapropel S1 sediments, has never formed in Poseidon Basin sediments. This has resulted in the preservation of the relationship between organic carbon and organic-related trace elements, e.g. Se, in the organic-rich sediments of Poseidon Basin, whereas such a relationship has been obliterated in ‘normal’ sapropel S1 sediments. On the basis of the carbonate content as well as the Sr=Ca ratio, preservation of carbonates appears to be better in the brine sapropel sediments of BC15 than it is in ‘normal’ sapropel S1 sediments. The high opal content of BC15 shows that biogenic opal is also much better preserved. The overall lower Corg=Ba ratio in BC15 suggests a better preservation of barite relative to that of organic carbon in shallow brine sediments, but is as yet inconclusive for the organic carbon preservation potential of brine relative to ‘normal’ unoxidised sediments.