Elsevier

Marine Geology

Volume 153, Issues 1–4, January 1999, Pages 161-176
Marine Geology

Cyclic variations in the chemical composition of eastern Mediterranean Pliocene sediments: a key for understanding sapropel formation

https://doi.org/10.1016/S0025-3227(98)00083-8Get rights and content

Abstract

Two Pliocene Mediterranean sediment cores from Ocean Drilling Program (ODP) Leg 160, Sites 964 (Pisano Plateau, Ionian Basin) and 967 (lower northern slope of Eratosthenes Seamount, south of Cyprus) have been analyzed for major and minor elements (Si, Ti, Al, Fe, Mg, Ca, K, Ba, Cr, Ni, V, Zn, Zr) by X-ray fluorescence analysis at a depth resolution of 3 to 4 cm (a total of 430 samples). Core 964C-9H comprises five sapropels with up to 9.7% total organic-carbon (TOC), one 2.5-m-thick turbidite, and a 50-cm segment of a second turbidite, whereas Core 967B-9H contains eight sapropels with TOC contents of up to 6.7%. Cyclic variations in carbonate content and detrital-matter composition are evident at both sites. At Site 964 (Pisano Plateau) terrigenous detrital-matter chemistry reflects periods of stronger Saharan dust input (higher Si/Al and Zr/Al ratios) alternating with fluvial input from the northern borderlands (higher Mg/Al ratios). Cyclicity is not well-expressed at this location because turbidites and winnowing occur. Site 967 (Eratosthenes Seamount) sediments are strongly influenced by discharge from the Nile River during humid periods (low K/Al and Mg/Al ratios). At Site 967 sapropels occur only during periods of enhanced Nile discharge with correspondingly lower Si/Al, Ti/Al, Mg/Al, K/Al, and Zr/Al ratios. Accumulation rates of carbonate and terrigenous detrital matter were 30% lower during such episodes because of the reduction in eolian input and carbonate production and/or carbonate dissolution. Periodical Ba enrichments, which are not only seen in the sapropels, but in every K/Al minimum, document the association of enhanced bioproductivity and sapropel formation. As calculated from excess Ba concentrations, productivity increased by up to a factor of five during intervals of intensified Nile runoff. Fluviatile discharge seems to be responsible for an enhanced input of nutrients and suspended matter containing low Mg/Al and K/Al ratios. Based on organic-carbon concentrations, episodes of sapropel formation lasted at least from 1000 to 4000 years at Site 964, and from 2000 to 6000 years at Site 967. Periods of enhanced bio-productivity, as defined by Ba enrichments, lasted from 8000 to 12,000 years at Site 967. Episodes of high river discharge and corresponding nutrient input, as well as conditions of sapropel formation, seem to have lasted longer in the eastern part of the basin (Site 967) than at the Pisano Plateau (Site 964).

Introduction

Pliocene to Holocene Mediterranean sediments contain numerous dark-colored layers, termed sapropels, which are characterized by high total organic-carbon (TOC, ≥2%), S, and Fe contents and significant enrichments in various trace elements (Calvert, 1983; Pruysers et al., 1991; Thomson et al., 1995). Different approaches have been taken to elucidate the paleoceanographic scenario that led to their formation.

The occurrence of sapropels is cyclic, and seems to correspond closely with minima in the precessional index when perihelion occurs in the Northern Hemisphere summer (Rossignol-Strick, 1983Rossignol-Strick, 1985; Prell and Kutzbach, 1987; Hilgen, 1991). Rossignol-Strick (1983)demonstrated that the temporal distribution of recent eastern Mediterranean sapropels correlates with maximum potential strength of the African monsoon, a function of orbital precession, which would have resulted in maximum discharge from the Nile River. Rohling and Hilgen (1991)postulated that increased precipitation along the northern borderlands of the eastern Mediterranean, because of increased activity of Mediterranean summer depressions, could have been an additional precession-related factor for reduced sea surface salinities during sapropel formation. δ18O-signatures of planktonic foraminifers reveal that surface water salinities must have been lower during times of sapropel formation (Williams and Thunell, 1979; Vergnaud Grazzini et al., 1986).

Carbonate cycles found in Pliocene to Holocene Mediterranean sediments have been examined by use of micropaleontological (De Visser et al., 1989; Howell et al., 1990) and magnetostratigraphical (Langereis and Hilgen, 1991; Hilgen, 1991) as well as geochemical (Van der Weijden, 1993; Van Os et al., 1994) methods. The consensus is that carbonate cycles, including intercalated sapropels, correspond closely with minima in the precessional index (e.g. Rossignol-Strick, 1983; Prell and Kutzbach, 1987; Hilgen, 1991; Lourens et al., 1992).

Few published studies deal with sources and distribution of terrigenous detrital matter in Mediterranean sediments. Investigations by Venkatarathnam and Ryan (1971)and Dominik and Stoffers (1978)are based on clay mineralogical studies of short cores. Foucault and Mélières (1995) analyzed the mineralogy of cyclic sediments from the Narbone Formation in Sicily. They found varying amounts of eolian- and fluvially-derived minerals, which, according to their interpretation, were controlled by alternation of humid and dry periods on both the northern and southern borderlands of the western Mediterranean.

In this paper high-resolution geochemical studies of Pliocene sedimentary sequences from Ocean Drilling Program (ODP) Sites 964 and 967 in the eastern Mediterranean are presented. This is one of the first attempts to investigate longer sedimentary sequences which contain several sapropels from the eastern Mediterranean by high-resolution major- and minor-element geochemistry. The basic idea behind such an approach is that any change in paleoenvironmental or climatic conditions should be reflected in major- and minor-element abundances. In particular, the question will be addressed whether cyclic chemical changes are only restricted to sapropel containing intervals, or are also seen in the non-sapropelic parts of the sedimentary sequences.

Section snippets

Sampling and methods

ODP Site 964 (Fig. 1) is located at a water depth of 3670 m in the Ionian Basin at the foot of the Calabrian Ridge on a small bathymetric high, the Pisano Plateau. Site 967 (Fig. 1) is located south of Cyprus on the lower northern slope of the Eratosthenes Seamount at a water depth of 2560 m. For our investigation we selected an interval that, according to ODP Leg 160 shipboard biostratigraphic studies (Emeis et al., 1996), belongs to nannofossil zone MNN17-16b and is 2.51 to 2.82 Ma old. This

Bulk chemistry

The sediments from both sites investigated may be regarded as marls or calcareous marls with mean CaCO3 contents (calculated from CaO) of 62% for Site 964 and 51% for Site 967 (Table 1). In order to compensate for carbonate dilution the chemical composition of the sediments is discussed here on the basis of element/Al ratios. Significant differences in the lithogenic sediment fraction of the two sites are evident: Ti/Al ratios are consistently higher at the Eratosthenes Seamount, whereas Si/Al,

Conclusions

The chemical composition of the terrigenous detrital component of Pliocene sediments from the eastern Mediterranean, ODP Sites 964 and 967, exhibits cyclic changes, which are caused by precession-induced climatic variations. Periodicity in major-element abundances reflects episodes of either enhanced eolian or fluviatile input. At ODP Site 964 (Pisano Plateau, Ionian Basin) terrigenous detrital matter chemistry reflects periods of stronger Saharan dust input alternating with fluvial input from

Acknowledgements

We thank the crew and scientific party of ODP Leg 160 for their generous support. Thanks to B. Schnetger and H. Schale for their help during sample preparation and analysis. A. Eisenhauer kindly provided the preliminary Sr-isotope measurements. We are grateful to G.J. de Lange, I.A. Nijenhuis and H.F. Passier for helpful discussions and critical comments. We further would like to acknowledge the helpful reviews by J. Thomson and G.J. de Lange. This project was financially supported by the

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