Palaeogeography, Palaeoclimatology, Palaeoecology
Trans-Mediterranean comparison of geochemical paleoproductivity proxies in a mid-Pleistocene interrupted sapropel
Introduction
Multiple layers of sapropels, which are colored greenish-brown to black, enriched in organic carbon, and laminated, are characteristic of the Late Miocene to Quaternary sediment record of the Mediterranean Sea (e.g., Cramp and O'Sullivan, 1999). The sapropels contrast markedly against the light-colored, organic-carbon-poor homogeneous marls that comprise most of the sediment record and that typify modern sediments. Their repetitive appearance corresponds to precessional minima, which are times of northern hemisphere maximum seasonality that recur at ∼21 ky intervals. This feature indicates that the sapropels record times of globally forced paleoclimate and paleoceanographic changes that were amplified in the Mediterranean region. Of particular importance, the high concentrations of organic carbon in the sapropels imply some combination of elevated marine productivity and enhanced preservation of organic matter, neither of which exists under the modern conditions in this sea.
Its near-isolation from the world ocean and predominantly arid climate make the modern Mediterranean Sea a highly oligotrophic system. The anti-estuarine circulation of the Sea is driven by evaporation of its surface waters and seasonal cooling that combine to form dense water masses that sink from the surface to carry nutrients away from the euphotic zone and to ventilate deep basins. Primary production is weakly sustained by delivery of fresh nutrients from continental runoff (Bethoux, 1989). The precessional minima with which sapropels coincide were periods of wetter climate in the Mediterranean region (Rossignol-Strick, 1983, Rohling and Hilgen, 1991, Wehausen and Brumsack, 2000). The wetter climate increased river runoff, which delivered more soil-derived nutrients to the Mediterranean Sea (e.g., Sarmiento et al., 1988), thereby initiating greater primary production, and which simultaneously diluted its surface waters (e.g., Bernasconi and Pika-Biolzi, 2000), thereby increasing near-surface stratification and slowing the anti-estuarine circulation. Increased primary production would have been sustained by enhanced regeneration of phosphate from organic matter in bottom sediments during sulfate reduction (Slomp et al., 2002, Slomp et al., 2004). Equally important, the near-surface stratification could have encouraged cyanobacterial nitrogen fixation, which would have provided a ready supply of bioavailable nitrogen (e.g., Struck et al., 2001).
This scenario of climate-induced eutrophication would potentially be sensitive to place-to-place variations in increased precipitation, enhanced land run-off, and strengthened surface stratification. Because some sapropel layers exhibit multi-century interruptions that record excursions from the conditions that favored their formation (Milder et al., 1999, Myers and Rohling, 2000, Krishnamurthy et al., 2000, Mercone et al., 2001, Arnaboldi and Meyers, 2003, Casford et al., 2003), temporal variations in climatic conditions were also relatively common. Such interrupted sapropels—sometimes also called “composite” sapropels (Calvert and Fontugne, 2001)—present special opportunities to explore temporal and spatial variations in sapropel formation. In this contribution, we examine elemental and isotopic proxies of organic matter production and preservation in same-age mid-Pleistocene sapropel layers from four locations, and we discuss the results in terms of regional differences in the paleoclimatic processes that magnified marine productivity.
Section snippets
Settings and sampling
We obtained sediment sequences from Ocean Drilling Program (ODP) Sites 967, 969, 974, and 975 (Fig. 1). These sites represent different deepwater depositional settings in the Mediterranean Sea. Site 967 is on the flank of the Eratosthenes Seamount in the Levantine Basin. Although the nearest land is Cyprus to the north, this location is strongly influenced by variations in Nile River outflow and is hence sensitive to the paleoclimate fluctuations driven by the West Asian monsoon. Site 969 is on
Calcium carbonate, organic carbon, and total nitrogen concentrations
The sediment samples were freeze-dried and then ground to a homogeneous powder in an agate mortar and pestle. Concentrations of calcium carbonate were measured using the “Carbonate Bomb” procedure of Müller and Gastner (1971) in which a known weight of sediment is treated with 3N HCl to release a volume of CO2 that is proportional to its carbonate content. The carbonate-free residue from this procedure was recovered by centrifugation, rinsed to remove chlorides, and dried for subsequent
Results and discussion
The results of our analyses of same-age sapropels from the four locations show differences that have important implications about the paleoclimatic and paleoceanographic processes involved with sapropel formation.
Summary and conclusions
The sapropel layers corresponding to insolation cycle 90 at ODP Site 967 in the Levantine Basin, Site 969 south of Crete, Site 974 in the Tyrrhenian Basin, and Site 975 in the Balearic Sea display similar increases in organic carbon mass accumulation rates and decreases in δ15N values. The dramatic excursions to light δ15N values in the sapropel layers indicate that nitrogen-fixing photoautotrophic bacteria were very important during times of sapropel deposition. The changes to wetter climate
Acknowledgments
We thank Caroline Slomp and John Thomson for their thoughtful and very useful suggestions to improve this manuscript. Stephan Kim, Amy Boetcher, and Laurie Cotsonika measured the inorganic and organic carbon concentrations. The Ocean Drilling Program, funded by the National Science Foundation and IPOD countries, provided the samples and the special opportunity for PAM to spend two months in the western Mediterranean Sea during Leg 161.
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