RESULTS

TOC concentrations vary between 0 and 11.7 wt% in the Cretaceous sequence (Table T1). Virtually all samples with TOC >2 wt% are from the OAE-related black shale intervals.

C_org/N_tot ratios range between 1.5 and 91.7 (Table T1), with most samples having values <40 (Fig. F1). C_org/N_tot ratios are often used as indicators of the origin of organic matter, with lower values (4–10) being typical of marine algae and higher values (>20) considered to be typical of land-derived material (Meyers, 1994). However, many Cretaceous black shales (Meyers et al., 1984; Meyers, 1987) and Mediterranean sapropels (Bouloubassi et al., 1999; Meyers and Doose, 1999; Nijenhuis and de Lange, 2000) have high C_org/N_tot ratios even though the organic matter appears to be marine-derived. The correspondence of high C_org/N_tot ratios with high TOC percentages, as observed in the samples from Site 1276 (Fig. F1), has been postulated to result from a coupling between higher fluxes of organic matter and improved preservation of its carbon content relative to its nitrogen content (Twichell et al., 2002). This diagenetic process is promoted by water-column suboxic conditions and appears to be associated with denitrification (Van Mooy et al., 2002).

Values of ¹³C_org vary from –28.3 to –19.6 in the sequence (Table T1), although almost all samples show ¹³C_org at less than –22 (Table T1). Values of ¹³C_org are commonly used as an origin indicator of organic matter with higher values (from –25 to –20), typical of modern marine phytoplankton, and lower values typical of terrestrial plants (from –30 to –27). However, the mid-Cretaceous was a greenhouse world with high pCO₂. The availability of dissolved CO₂ impacts the carbon isotopic composition of algal organic matter because biological discrimination in favor of ¹²C increases when pCO₂ is high and decreases when it is low (e.g., Fogel and Cifuentes, 1993). For this reason, marine organic matter produced in the Cretaceous commonly has ¹³C values from –28 to –25 (e.g., Rau et al., 1987), which is similar to modern C₃ land plants. Nonetheless, organic carbon isotopic compositions can still be used to reconstruct past productivity rates because during episodes of enhanced productivity algal discrimination decreases, producing higher ¹³C_org values. These higher ¹³C_org values are common in OAEs, and most of our samples within the OAE-related black shale interval present somewhat higher ¹³C_org values (greater than –26).

Values of ¹⁵N range between –3.2 and 2.6, with most samples having ¹⁵N values <0 (Table T1; Fig. F2). Like carbon, nitrogen isotopes can also be used as a source and/or paleoproductivity indicator. Marine algae in the modern ocean are commonly characterized by higher ¹⁵N values (from +7 to +10), whereas land plants average ~0. Values of ¹⁵N can reflect variable nutrient uptake because of higher nitrogen fractionation (lower ¹⁵N values) with high nitrate availability (Calvert et al., 1992). More importantly, ¹⁵N values of sediment organic matter are also sensitive to processes that deeply affect the nitrogen cycle, such as nitrogen fixation. Low ¹⁵N values, common in mid-Cretaceous black shales, can reflect shifts in the mode of primary marine production from algae to microbes (Rau et al., 1987), in which nitrogen fixers become the dominant producers. Values of ¹⁵N are inversely related to TOC concentrations in our samples. Samples with higher TOC contents are characterized by negative nitrogen isotopes, which is especially evident for TOC > 5 wt% (Fig. F2). These samples are within the OAE-related black shales and their low ¹⁵N values suggest that these layers were deposited during periods characterized by an altered nitrogen cycle when nitrogen fixation rates were significantly higher in the surface ocean.

Rock-eval analyses help to identify the type (marine, bacterial, or terrigenous) and maturity of sedimentary organic matter (e.g., Meyers, 1996). We report a suite of parameters and indexes obtained from this analysis in Table T2. Land-plant organic matter tends to be rich in woody components and consequently has lower hydrogen indexes (HIs) and higher oxygen indexes (OIs) than found in lipid-rich and cellulose-poor algal organic matter. Diagenesis, however, can cause marine organic matter to gradually acquire HI and OI values similar to those of land plant material or cause its degradation to detrital, Type IV organic matter (Meyers, 1996). The T_max of all samples from Site 1276 are <435°C (Table T2), which indicates that the organic matter is thermally immature with respect to petroleum generation (Espitalié et al., 1977). Additionally, a Van Krevelen–type plot of the HI and OI values of all samples (Fig. F3) indicates that their organic matter is a mixture of continental and oxidized marine material (Meyers, 1996). In particular, the samples from the OAE 2 and OAE 1b–related black shales (low OI and high HI) seem to contain Type I or II marine-derived organic matter (Table T2).

Major and minor element compositions are useful to evaluate changes in paleoproductivity rates, preservation of organic matter, and paleoenvironmental conditions. A normalization of the elemental concentrations to Al is commonly used (e.g., Wehausen and Brumsack, 1999; Warning and Brumsack, 2000; Rinna et al., 2002) in order to compensate for clastic or carbonate dilution. Aluminum is not influenced by biogenic activity, authigenic enrichment, or diagenetic dissolution (Rinna et al., 2002).

Mild enrichments of redox-sensitive and chalcophile elements (U, Re, Cu, Mo, Cr, V, Cd, and Zn), such as the ones observed in our samples (Tables T3, T4), likely indicate deposition in an oxygen-poor environment (van Santvoort et al., 1997; Nijenhuis et al., 1998; Schenau et al., 1999; Hofmann et al., 2001; Arnaboldi and Meyers, 2003). These elements are precipitated from seawater and their accumulation and immobilization in sediment is associated with dysoxic/anoxic conditions either in the water column or in the sediment. Finally, Ba, which is generally a good proxy for paleoproductivity, especially in the deep ocean (e.g., Dymond et al., 1992), presents moderate enrichments in our samples (Tables T3, T4).