WP-5 Abrupt environmental transitions (AETs) and tephras in marine sediment cores
Marine sediment cores provide excellent records of ocean/climate change which, in general outline, frequently match those reported from ice-core records and high resolution lake sequences. A number of climate proxy records from the Mediterranean and Red Sea, for example, suggest regional equivalents of D-O (Dansgaard-Oeschger) and H (Heinrich) events as well as more recent climate oscillations, such as the Younger Dryas (YD) and 8.2 kyr event. The ‘big picture’ is now well resolved; it suggests a degree of ‘teleconnection’ between the marine, terrestrial and polar ice sheet realms. However, what is not well resolved at present are: (a) The magnitude of any leads and lags in the system; (b) Whether the AETs had equal impact in different geographical sectors; and (c) The causal mechanisms behind this complex series of climatic perturbations. Key intervals of interest, such as the H events or YD, are often associated with 14C “plateaux”, possibly reflecting major reorganisations of ocean circulation. Together with the issue of variable marine reservoir ages, this seriously compromises the precise synchronization of records.
The new chronological schema proposed by RESET offers a means of addressing this problem, and should resolve issue (a) while contributing new insights to (b) and (c). With better synchronization of AETs throughout the Mediterranean and Red Seas, we will be in a better position to answer such fundamental questions as whether oceanic changes lead atmospheric changes, and the extent to which the various AETs were synchronous with continental (lake) and ice-core records, with a centennial precision.
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Aims and Research Objectives
The aim of this work package is to provide a more secure palaeoenvironmental context for assessing the link between archaeological events and AETs in the North Atlantic-Mediterranean-Red Sea regions. This will be addressed through a series of research objectives:
- To resolve more fully the sequence of AETs that affected the Mediterranean and Red Seas during the past 140 kyr and assess their regional impacts within the wider North Atlantic context
- To optimize RESET’s application of tephrostratigraphical (WP-4) and age modeling (WP-7) protocols for the synchronization of marine records
- To test hypotheses concerning the driving mechanisms behind the AETs that affected the Mediterranean-Red Sea region during the last c.140 ka
Research focus
The key hypotheses to be examined are whether:
- Global sea-level/ice-volume variations reconstructed from continuous sediment-core records from the Red Sea were synchronous with Antarctic climate cycles. (Important for WP-2)
- Changes in the Mediterranean hydrological cycle (mostly ascribed to African monsoon variability) approximated, but were not precisely synchronous with, humidity cycles along the basin margins. (Important for WP-1). Differences in timing may reflect time needed for redistribution of regional and North Atlantic moisture within the Mediterranean catchment area
- North Atlantic ocean/climate shifts in marine records were truly synchronous with those in Greenland ice cores, as well as with those in Europe and around the western and eastern Mediterranean. (Important for WP-3). A more complicated scenario has been deduced for a well documented climatic anomaly around 8 ka BP (Rohling & Pälike, 2005), a caution against straightforward assumptions of synchroneity for earlier events.
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Detailed (centennial-scale) tephrochronological frameworks will be developed for the last 140 thousand years in highly-resolved marine proxy series characterizing surface temperature, salinity, ice rafting, and aeolian dust fluxes, along a transect throughout the Red Sea, Mediteranean and into the North Atlantic. Hypothesis 1 requires detailed tephrochronological investigation of central Red Sea cores KL11 and especially KL09, to complement the extensive centennial-scale proxy data series already being generated for these cores within a previous research grant. Hypothesis 2 requires a detailed tephrochronology of core LC21 from the SE Aegean Sea (held at BOSCORF, NOCS), coupled with centennial-scale palaeoclimate proxy record generation, as well as detailed tephrochronology of a core from the western Mediterranean / Alboran Sea. Suitable cores with available high-resolution proxy data have been reported in the literature, and only some strategic resolution enhancement will be required. Regarding Hypothesis 3, we will develop a detailed tephrochronology of a core from the Iberian margin, such as MD95-2042 for which a comprehensive suite of proxy data exists already. More central to the testing of hypothesis 3, notably with respect to the role of changes in the thermohaline circulation in causing AETs, we will investigate the composite sequence of cores D298-P1 and P3, from Eirik Drift (south of Greenland), obtained by Rohling and Bacon in September 2005 within the framework of RAPID project and held at BOSCORF, NOCS. Millennial-scale pilot records exist for the D298 cores, and the work for RESET would concern centennial-scale proxy record development as well as tephrochronology.
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