Date Published: January 27, 2017
Publisher: Public Library of Science
Author(s): Werner Ehrmann, Gerhard Schmiedl, Sarah Beuscher, Stefan Krüger, Shang-Ping Xie.
North Africa experienced dramatic changes in hydrology and vegetation during the late Quaternary driven by insolation-induced shifts of the tropical rain belt and further modulated by millennial-scale droughts and vegetation-climate feedbacks. While most past proxy and modelling studies concentrated on the temporal and spatial dynamics of the last African humid period, little is known about the intensities and characteristics of pre-Holocene humid periods. Here we present a high-resolution record of fine-grained eastern Saharan dust from the Eastern Mediterranean Sea spanning the last 180 kyr, which is based on the clay mineral composition of the marine sediments, especially the kaolinite/chlorite ratio. Minimum aeolian kaolinite transport occurred during the African Humid Periods because kaolinite deflation was hampered by increased humidity and vegetation cover. Instead, kaolinite weathering from kaolinite-bearing Cenozoic rocks was stored in lake basins, river beds and soils during these periods. During the subsequent dry phases, fine-grained dust was mobilised from the desiccated lakes, rivers and soils resulting in maximum aeolian uptake and transport of kaolinite. The kaolinite transport decreased again when these sediment sources exhausted. We conclude that the amount of clay-sized dust blown out of the Sahara into the Eastern Mediterranean Sea is proportional to the intensity of the kaolinite weathering and accumulation in soils and lake sediments, and thus to the strength of the preceding humid period. These humid periods provided the windows for the migration of modern humans out of Africa, as postulated previously. The strongest humid period occurred during the Eemian and was followed by two weaker phases centred at ca. 100 ka and ca. 80 ka.
The North African environments underwent major changes in the intensity of precipitation during late Quaternary time. These changes include both monsoon-paced pluvials [1, 2] of the African Humid Periods (AHPs) and millennial-scale droughts that were induced by cold intervals of the northern high-latitudes and occurred during the last glacial period . Proxy data and model results indicate a differential hydrological response in the western versus eastern parts of North Africa [4–6]. To date, there is little knowledge of the intensities and characteristics of pre-Holocene humid periods. There is an on-going debate about abrupt versus gradual transitions between the AHPs and the subsequent dry phases, which likely involved significant vegetation-climate feedbacks and non-linear ecosystem responses [2, 7–9].
The studied sediment core M40/4_SL71 was retrieved in 1998 from the SE Ionian Sea, ca. 50 km SW of Crete (Fig 1; 34°48.67’ N, 23°11.65’ E, 2788 m water depth) during cruise M40/4 of the German research vessel “Meteor”. Core retrieval was carried out in compliance with international law and regulations of the neighbouring countries as mediated by the “Control Station German Research Vessels” (formerly “METEOR Operations Control Office”) of the University of Hamburg. The sediments consist mainly of yellowish, brownish and greyish foraminifera-nannofossil ooze . Five sapropel layers occur within the sequence (Fig 2) and are characterised by their olive grey to olive black colour indicating preserved organic material, yellowish colour indicating post-sedimentary oxidation, and/or laminations. The sapropels are S1 (15–19 cm), S3 (182–184 cm, oxidised 178–182 cm), S4a (215.5–219 cm, oxidised), S4b (227–228 cm), S5 (261.5–277 cm), and S6 (387–414.5 cm, oxidised 384.5–387 cm). The latter shows two interruptions at 391–392 cm and 395.5–397 cm. Furthermore, three volcanic ash layers were recovered. The upper one at 84–88 cm represents the Y5-Tephra. The middle one at 173–176 cm occurs shortly above sapropel S3 and probably can be assigned to the X1-Tephra (Y9; cf. ). The lower one occurs within S6 at 394.5 cm and therefore could be identified as the V1-Tephra .
The results of the clay mineral analyses on sediments of core M40/4_SL71 are presented in Figs 2 and 3, which also show the position of the sapropel and tephra layers. The raw data presented in this paper are available at https://doi.pangaea.de/10.1594/PANGAEA.869095.