The secrets of past climate are locked within sapropels, dark sludge composed of plant remains found at the bottom of the sea. Scientists were surprised to discover some of this sludge in the western Mediterranean. They are studying it to piece together a 5-million-year history of climate and environmental change in the Mediterranean.
by the Leg 160 and 161 Ocean Drilling Project Shipboard Scientific Parties
Since marine sediments reveal lots of information about climate change, retrieving sediment sequences from climate-sensitive areas is an important aspect of paleoclimate research. In the spring of 1995, over 7000 meters of sediments that reach back more than 5 million years were recovered in the Mediterranean Sea by the Ocean Drilling Program (see figure 1). Because the Mediterranean is a semi-enclosed, landlocked basin, it's water doesn't mix much with sea water. The composition of Mediterranean sediments is especially sensitive to climate change, and environmental signals are preserved within them in great detail.
Fig. 1. ODP Legs 160 and 161 drill sites in the Mediterranean.
One of the most important results was the discovery in the westernmost Mediterranean of dark, organic, carbon-rich layers known as sapropels (see figure 2). These unconsolidated deposits are composed of the remains of algae, and they also contain mud and spores. They are commonly found in sediment of the eastern Mediterranean basin and in southern Italy, Sicily, and Crete, but until their recent discovery they were never before detected in the western Mediterranean except in the Tyrrhenian Sea. Sapropels have been deposited periodically over the past 5 million years and are believed to reflect distinct changes in Mediterranean climate and marine biological production. Traditionally, scientists have thought that sapropels developed during episodes of oxygen-deprived bottom water in the eastern Mediterranean. New models correlate sapropel formation with hotter periods in the Earth's history, when the Northern Hemisphere received stronger summer insolation, which made for more intense summer monsoons throughout Asia and Africa and enhanced moisture transport to the Mediterranean catchment area. When the ratio of evaporation to precipitation in the eastern Mediterranean changes, the water column structure stabilizes, and the cold deep water cannot form. Dissolved nutrients, which usually accumulate in deep waters, remain in the euphotic zone to stimulate biological production in the water. Ultimately, the combined effects of biological production and oxygen deprivation in the water may lead to sapropel formation.
Fig. 2. Typical example of sapropels recovered in the eastern Mediterranean. Sapropels are dark, organic, carbon-rich layers of sediment up to 30 cm thick that are commonly found in the eastern Mediterranean basin and southern Italy, Sicily, and Crete. They were deposited periodically over the past 5 million years and are believed to reflect distinct changes in Mediterranean climate and marine biological production.
In the eastern Mediterranean, more than 80 sapropels up to 50 cm thick were recovered. They were found grouped together between sites up to several hundred kilometers apart. At site 974 in the Tyrrhenian Sea, improved coring techniques enabled us to obtain a more complete recovery of the sedimentary sequence, which included 36 sapropels. Thirty-eight sapropels in the southern Balearic Sea and 40 organic-rich layers farther west in the Alboran Sea were also recovered. Most of these layers are dispersed over intervals as thick as 3 m due to higher sedimentation rates, but some form discrete layers up to 30 cm thick with sharp upper and lower boundaries and resemble sapropels found in the eastern Mediterranean.
In the eastern Mediterranean, the sapropels can be extraordinarily rich in organic carbon, up to 30%. In the western Mediterranean, they contain between 0.8 and 2.5% organic carbon. Color banding directly above and below the sapropels probably marks the return of oxygen to the water and the possible burn-down of organic matter after it was deposited.
This discovery of sapropels across the western Mediterranean challenges the idea that the paleoceanographic conditions that cause sapropels to form occur only in the eastern Mediterranean. Sapropels formed in the two parts of the sea at different times, though. In the western Mediterranean, few sapropels formed during the Pliocene, about 7 million years ago, but increased in the earliest Pleistocene, at about 1.8 million years ago. In the eastern Mediterranean, sapropel deposition started in the earliest Pliocene but became less frequent after 2.4 million years ago.
Several factors may account for this asymmetric time distribution. The eastern Mediterranean sapropel record is more susceptible to changes in freshwater input, deep water formation, and nutrient supply. Since the development of major ice sheets on the Northern Hemisphere around 3 million years ago, the additional ice may have caused a decrease in the moisture reaching the eastern Mediterranean. At the same time, greater seasonal contrasts in temperature and precipitation may have caused more deep water to form while preventing oxygen from leaving the water. Accordingly, the frequency of sapropel formation has decreased considerably since the Pliocene. The western Mediterranean is more affected by fluctuations of North Atlantic climate. With the onset of larger-scale, glacial-interglacial cycles during the Pleistocene, the Atlantic inflow likely started to vary at a rhythm imposed by ice sheet growth and decay. Recent studies have shown that ice sheet instabilities occur during maximum ice growth, causing meltwater to surge into the North Atlantic even during full glacial periods. Meltwater pulses or surges entering the western Mediterranean with the Atlantic inflow would change its circulation and density structure. Simultaneous sea level changes would alter the flow through the Straits of Gibraltar and Sicily, further contributing to changes in the western Mediterranean's water cycle.
Continuing studies of the sediments recovered will concentrate on three goals. First, by establishing the temporal relationship between sapropel formation in the western and eastern Mediterranean, we will be able to determine the paleoceanography of the entire Mediterranean. The second goal is to document the cycle of inflowing Atlantic waters and their potential links to marine environmental changes in the western Mediterranean Sea during the last 5 million years. The third is to study physical, chemical, and biological properties in the entire Mediterranean Sea during periods of sapropel formation. By deciphering the pattern of sedimentation for the entire Mediterranean, we can reconstruct the region's climatic and environmental history and examine the dynamics of water circulation in a sea that has gone through cycles of high biological productivity and bottom water oxygen deprivation.
Source: Eos, Jan. 16, 1996, p. 19.
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