RESULTS AND DISCUSSION

TOC concentrations are bulk sedimentary parameters that represent the fraction of organic matter that has survived degradation during sinking to the seafloor and early diagenesis in the sediments. TOC concentrations are influenced by both initial primary production and subsequent exposure to degradation, so they integrate the different origins of organic matter, delivery routes, depositional processes, and consequent degrees of preservation. TOC concentrations are expressed in weight/weight ratios and are therefore influenced by other components of the sediments. For example, TOC concentrations can be diluted by clastic sediment particles in some settings or concentrated by dissolution of carbonate minerals in others.

Most of the Pliocene-Pleistocene sediments sampled in the six sites cored during Leg 161 contain organic-carbon concentrations that are approximately the same as the average of 0.3% compiled by McIver (1975) from Deep Sea Drilling Project (DSDP) legs 1 through 33. The organic-carbon-poor Leg 161 sediments represent conditions of low productivity and poor preservation of organic matter that evidently typify most of the post-Messinian history of the Mediterranean Sea. The exceptions are the dark-colored sapropel layers in which TOC concentrations exceed 0.5% and reach as high as 6.3% (Comas, Zahn, Klaus, et al., 1996). Comparison of TOC concentrations at the five sites from which continuous sedimentary records were obtained shows two general patterns: (1) the differences between the TOC content of background sediments and sapropels is greatest in sediments from Site 974 in the Tyrrhenian Basin and diminishes in more westward locations, and (2) the occurrence of sapropels is limited to Pleistocene sediments (Fig. 2).

The first of these patterns is a general extension of results obtained from the eastern Mediterranean Sea during Leg 160, wherein TOC concentrations in Pleistocene sediments reach 20% on the Pisano Plateau in the Ionian Basin (Bouloubassi et al., in press). This trend may reflect significant paleoceanographic differences between the locations that roughly correspond to their distances from the Nile River and the Black Sea, the largest present-day sources of nutrient-enriched fresh- or low-salinity water to the Mediterranean Sea. The TOC concentrations in the seafloor may record geographical differences in surface paleoproductivity. It is more likely, however, that the pattern of decreasing TOC is produced by differences in sedimentation rates, which in general increase from east to west. Organic carbon concentrations are diluted more in the western locations by clastic and biogenic sediment components than in the eastern sites. If this is true, then production and subsequent burial of organic matter has been periodically elevated to the same or similar levels throughout the Mediterranean Basin since the beginning of the Pleistocene. Until sediment chronology is refined sufficiently to allow calculation of organic-carbon-mass-accumulation rates, this hypothesis remains untested.

An important difference exists between the histories of organic carbon burial in the western Mediterranean sites that were sampled during Leg 161 and those in the eastern Mediterranean sampled during Leg 160. Sapropel occurrences begin during the middle Pliocene in the eastern Mediterranean Sea (Emeis, Robertson, Richter, et al., 1996), whereas they do not start to appear in the western sites until the beginning of the Pleistocene (Fig. 2). Furthermore, TOC concentrations are higher in the Pliocene sapropels than in their Pleistocene equivalents, reaching as high as 32% at Site 969 on the Mediterranean Ridge south of Crete (Bouloubassi et al., in press). In contrast, chronologically equivalent sediments from the Leg 161 sites contain only a few tenths of a percent TOC (Fig. 2). Unlike the Pleistocene TOC concentrations, the difference in onset of sapropel deposition indicates real paleoceanographic differences between the eastern and western Mediterranean locations.

C/N values help to distinguish between algal and land-plant origins of sedimentary organic matter. Algae typically have atomic C/N values between 4 and 10, whereas vascular land plants have C/N values of 20 and greater (Premuzic et al., 1982; Emerson and Hedges, 1988; Jasper and Gagosian, 1990; Meyers, 1994; Prahl et al., 1994). This distinction arises from the absence of cellulose in algae and its abundance in vascular plants. The difference between algal and land-plant C/N source signatures is largely preserved in sedimentary organic matter (Jasper and Gagosian, 1990; Meyers, 1994).

C/N values in Leg 161 Pliocene-Pleistocene sediments are variable (Comas, Zahn, Klaus, et al., 1996). The C/N values of some samples low in TOC are very low (<5). These values are lower than those observed in fresh algal matter. They are probably an artifact of the low carbon contents combined with the tendency of clay minerals to absorb ammonium ions generated during microbial degradation of organic matter (Müller, 1977). The C/N values in samples especially low in organic carbon consequently are not accurate indicators of organic-matter source; they instead indicate continued breakdown of organic matter after burial.

In contrast to the organic-carbon-poor background sediments, the C/N values of sapropels range between 10 and 20 (Comas, Zahn, Klaus, et al., 1996), which is intermediate between unaltered algal organic matter and fresh land-plant material. Because diagenesis rarely depresses land-plant C/N values to values as low as those found in the sapropels (Meyers et al., 1995), it is likely that the organic-carbon-rich sediments contain algal material that has been partially degraded during or soon after settling to the seafloor. Preferential loss of nitrogen-rich, proteinaceous matter can elevate the C/N values of algal organic matter by this process.

In support of the concept of diminished degradation of the organic-matter content of the sapropels, a well-developed correspondence between higher TOC concentrations and greater C/N values exists in the sapropels from all five Leg 161 sites that were continuously cored (Fig. 3). This correspondence suggests two factors that are important to elevating the TOC levels of the sapropels. First, the organic matter that was delivered to the seafloor and accumulated in the sapropel layers experienced less degradation during sinking than the organic matter that was delivered during times when sapropels did not form. Nitrogenous components of biological organic matter are typically recycled more effectively than other forms of organic matter under conditions of partial degradation. The organic matter in the sapropels consequently retained a greater proportion of its carbon content, and C/N values were therefore enhanced. This process accompanies the elevated fluxes of organic matter to sediments under upwelling zones, where Suess and Müller (1980) document enhancement of C/N values in surficial bottom sediments. Second, organic carbon experienced less remineralization after being incorporated into the bottom sediments, so that organic matter retained the elevated C/N values that developed during transport to the seafloor.

Rock-Eval pyrolysis was originally developed to characterize the organic matter present in oil source rocks, which typically is more thermally mature and at higher concentration than commonly found in non-lithified sediments obtained by scientific ocean drilling. Rock-Eval analyses have nonetheless proved valuable in helping to determine organic matter sources in DSDP and ODP samples. The hydrogen index (HI) and the oxygen index (OI) relate to the origin of the total organic matter and are commonly plotted against each other in a Van Krevelen-type diagram in which a comparison of elemental H/C and O/C values is approximated. In the HI-OI plot, three main types of organic matter and their thermal alteration pathways are defined. Type I organic matter is especially rich in aliphatic hydrocarbons and hydrocarbon-like components and is derived from algae and microbial biomass. Type II organic matter is moderately rich in these aliphatic components and originates from the waxy coatings of land plants and from partially degraded algae. Type III organic matter is poor in hydrocarbon-like materials but rich in carbohydrates. Land-plant organic matter is usually rich in woody components and consequently has lower hydrogen indices and higher oxygen indices than found in lipid-rich and cellulose-poor algal organic matter. Type III organic matter therefore usually typifies woody land-plant matter, but it may also represent poorly preserved algal organic matter.

The source-distinction between continental and marine organic matter that can be made from the results of Rock-Eval pyrolysis becomes blurred by diagenesis. Oxidation of organic matter affects both HI values and OI values. As hydrocarbon-rich organic matter (Type I or II) is oxidized, its hydrogen content decreases while its oxygen content increases, and it takes on the HI-OI characteristics of Type III vascular plant organic matter. A further constraint on the use of Rock-Eval pyrolysis for determination of organic-matter source is that samples should contain at least 0.5% TOC to yield meaningful results (Katz, 1983; Peters, 1986).

The results of Rock-Eval analyses of Leg 161 sapropels show that their organic-matter content appears to be dominated by varying mixtures of Type II algal material and Type III land-plant-material (Fig. 4). The indication of large proportions of land-derived organic matter in many of the sapropels conflicts, however, with C/N values in these sediments (Fig. 3) that are too low (<20) for land-plant organic matter. The contradiction between the Rock-Eval source characterization and the elemental source characterization is evidence that the marine organic matter has been heavily oxidized, because well-preserved Type II organic matter has high HI values (Espitalié et al., 1977; Peters, 1986).

Comparison of TOC concentration with Rock-Eval HI values shows that sapropels with higher TOC concentration also have higher HI values (Fig. 5), which is a pattern also found in sapropels in the eastern Mediterranean (Bouloubassi et al., in press). Higher TOC concentrations also correspond to higher C/N values (Fig. 3). Sapropels having higher HI values consequently also have higher C/N values, which is an indication of partial but not extensive alteration of marine organic matter during sinking and incorporation into bottom sediments. The relationship between higher HI values and elevated TOC concentrations is therefore consistent with preservation of marine organic matter being important in elevating the organic-matter content of Mediterranean sapropels (e.g., Ten Haven et al., 1987; Calvert et al., 1992; Cheddadi and Rossignol-Strick, 1995).

Rock-Eval T_max values are below 425°C in most of the Pliocene-Pleistocene sediments from the locations sampled by Leg 161 (Comas, Zahn, Klaus, et al., 1996). These values indicate that the organic matter in most of the basins of the western Mediterranean is thermally immature with respect to petroleum generation (Espitalié et al, 1977; Peters, 1986). Because organic matter is sensitive to temperatures that are only slightly elevated (>60°C), the thermal immaturity of the Pliocene-Pleistocene organic matter is evidence of low heat flows in the western Mediterranean basins since the times these sediments were deposited.

Two of the Leg 161 sites, however, exhibit T_max values that deviate from being relatively low. The deviations suggest that local geothermal gradients may have been sufficiently elevated at some former time to have matured the organic matter buried at these locations. A well-developed pattern of increasing T_max values with depth is present in the upper 150 m of sediment at Site 974 in the Tyrrhenian Basin (Fig. 6). Because of the relatively shallow burial depth, the increase in thermal maturity with depth is probably a consequence of the elevated geothermal heat flow (157 mW/m²) measured at this location (Comas, Zahn, Klaus, et al., 1996).

Rock-Eval T_max values are relatively low in the Pleistocene sediments of Site 977 in the eastern Alboran Basin, but they become elevated to 475°C-495°C in the deeper, Pliocene sediments (Fig. 7). The higher T_max values indicate that organic matter is overmature with respect to petroleum generation in the older sediments (Fig. 8). The thermal maturity indicated by the T_max values suggests that temperatures were between 125° and 150°C for sediments of this age, which is substantially greater than the present bottom-hole temperature that the measured heat flow (101 mW/m²) would yield (Comas, Zahn, Klaus, et al., 1996). The thermal maturity of the organic matter in the Pliocene sediments consequently indicates that heat flows were formerly higher in the Eastern Alboran Basin, which is a tectonically active part of the greater Mediterranean Basin.

Other organic geochemical evidence of formerly higher geothermal gradients is present in the sediments of the eastern Alboran Basin. Higher molecular-weight thermogenic gases are present at low levels in sediments from Site 977. Concentrations of propane, iso-butane, and iso-pentane exceed those of ethane in sediments from about 200 mbsf to 450 mbsf (Comas, Zahn, Klaus et al., 1996). A general correspondence between elevated Rock-Eval T_max values and occurrence of these heavier gases suggests that the C₃, C₄, and C₅ gases were produced by thermal degradation of sedimentary organic matter during a period of elevated heat flow or hydrothermal activity in the Eastern Alboran Basin. Because only organic matter in sediments deeper than 200 mbsf exhibits thermal alteration, the interval of elevated heating must have occurred during the Pliocene.

Milankovitch-cycle changes in the earth's orbit about the sun have pervasive impacts on global climate. In particular, precessional changes, which affect summer-winter climate differences at cyclicities of 19 and 23 k.y., appear to be strongly expressed in the sapropels and their shallow-water equivalents, the laminites, of the Mediterranean Basin (Fontugne and Calvert, 1992; Berggren et al., 1995; Lourens et al., 1996; Hilgen et al., 1997). Sapropels are expressed differently at different locations and at different times of post-Messinian Mediterranean paleoclimate history. Local and regional factors evidently overprint the global Milankovitch forcing factors. Moreover, these organic-carbon-rich sediments often occur as bundles—regular successions of light/dark layers—interspersed with intervals in which the dark layers are absent. This is a common pattern and suggests repetitive variations in the intensity of precessional climate change.

Nutrient-enhanced productivity appears to be involved in the deposition of the deep-water sapropels. Upper Pliocene-lower Pleistocene sapropels from the eastern Mediterranean Sea are especially rich in organic carbon, some exceeding 30% (Bouloubassi et al., in press). Exceptionally high rates of organic-matter delivery had to contribute to such elevated organic-carbon concentrations. Rock-Eval analyses of sapropels from Leg 161 sites indicate that variable oxidation of marine organic matter has occurred in the western Mediterranean (Fig. 4, Fig. 5). The range in degree of organic matter preservation, from good to moderate, suggests that elevated productivity ultimately contributed to improving organic matter preservation in the seafloor. When the delivery of organic matter was great enough to exhaust benthic oxygen availability, sediments rich in organic carbon accumulated. Several lines of evidence support this concept. The compositions of nannofossil assemblages in eastern Mediterranean sapropels suggest that the triggering mechanism for enhanced preservation was increased productivity (Castradori, 1993). Molecular and isotopic organic geochemical compositions confirm that the organic matter in sapropels originated from enhanced marine productivity (Ten Haven et al., 1987; Calvert et al., 1992; Rullkötter et al., in press, Bouloubassi et al., in press). These generalizations about the origin and preservation of the organic matter in the sapropels are likely to apply in the western, as well as in the eastern, Mediterranean Sea.

Fontugne and Calvert (1992) noted that the ¹³C values of bulk organic matter in upper Pleistocene deep-water sapropels in the eastern Mediterranean are consistently more negative than in intercalated oozes. They interpret the more negative values to reflect periodic freshwater flooding of the photic zone of the eastern Mediterranean. Enhanced delivery of soil-derived nutrients to oceanic surface waters commonly accompanies periods of increased river flow. Calvert et al. (1992) note that ¹⁵N values of the organic matter in the sapropels are exceptionally low, which is evidence that nitrogen-fixing microbes were probably important primary producers during the intervals of sapropel formation. The existence of nitrogen-fixing microbes is indirect evidence for density stratification that extended into the photic zone of the Mediterranean Sea during these intervals.

Precessional (19 to 23 k.y.) regional changes in climate have been postulated to be central to formation of the Mediterranean laminated sediments (Hilgen et al., 1997). The basic premise is that summer-winter differences greater than at the present (the Earth is now farthest from the sun at northern summer solstice, and modern laminites/sapropels are not being deposited) increased precipitation in the Mediterranean region (Rossignol-Strick, 1985). The periods of increased seasonal differences correspond to Milankovitch-cycle precessional minima. During the intervals of greater summer-winter contrast, increased continental runoff delivers more nutrients to seawater, thereby enhancing algal and microbial production and the delivery of marine organic matter to the seafloor. Benthic oxidation of the increased flux of organic matter overwhelms the supply of dissolved oxygen at the seabottom, and preservation of subsequently delivered organic matter is enhanced. In this scenario, deep waters need not stagnate to become anoxic, even though the increased continental runoff would probably dilute surface waters and create salinity stratification in large areas of the Mediterranean Sea.