RESULTS AND DISCUSSION
Calcium Carbonate Concentrations
Concentrations of CaCO3 are quite variable in the Unit I sediments
(Table 2). The ranges of concentrations are: Site 897, 2% to 78%; Site 898, 2% to 61%; Site 899, 3% to 48%; Site 900, 2% to 67%. This variability reflects the intermix of pelagic and hemipelagic
sediment types in these turbidites, which ranges from nannofossil ooze to fine sand (Shipboard Scientific Party, 1994a-d).
The results of the shipboard and shorebased organic carbon analyses of individual samples are compared in
Table 2. About one-third of the comparisons agree within 0.1%, which is excellent for sediments with relatively low TOC concentrations (<0.5%). The postcruise concentrations determined by analysis of Carbonate-free samples tend to be systematically higher than concentrations determined onboard ship by difference between total carbon and carbonate carbon concentrations
(Fig. 2). The difference between the two procedures is not related to CaCO3 content. This comparison shows that (1) the difference method slightly underestimates TOC concentrations in sediments having low amounts of organic matter and (2) washing Pleistocene and older sediment samples with 3N HCl and water does not mobilize and remove significant amounts of organic matter. These results agree with a similar comparison of the shipboard TOC by-difference procedure with a shorebased direct analysis in the sense that little difference was found, but they disagree in that the shorebased procedure gave slightly lower values (Katz, 1992). Two factors are important to the differences between the results of our comparison and those of Katz (1992). The primary factor is probably that he used a LECO CS analyzer instead of a Carlo Erba CHNS analyzer, and the secondary one is that the sediments he studied contained between 1% and 8% TOC. The two comparisons nonetheless show that little error is introduced into TOC determinations over a wide range of concentrations by the shipboard by-difference procedure.
Averaged shorebased organic carbon concentrations of the Unit I samples are: Site 897, 0.78%; Site 898, 0.68%; Site 899, 0.44%; Site 900, 0.36%. These values are slightly higher than the average organic carbon content of 0.2% of deep-sea sediments and rocks from DSDP Legs 1 through 33 compiled by McIver (1975). Many of the samples, however, are poor in organic carbon, and their low concentrations are not related to their CaCO3 concentrations
(Table 2). Of particular note, the uppermost lithologic units drilled at Sites 897, 898, and 899 seem to be parts of a single, continuous turbidite unit
(Fig. 1; Table
1), yet the TOC concentrations at Sites 897 and 898 are significantly higher than those at Site 899, which is located between the other two. In addition, TOC concentrations are variable with depth in Unit I as it is expressed at each of the four drill sites. The accumulation of organic matter at the landward edge of the Iberia Abyssal Plain evidently has varied both spatially and temporally during the Pliocene and Pleistocene. Part of this variability may result from the turbidites originating from different locations on the Iberia Margin, and part
may be caused by the greater thicknesses of the turbidites at Sites 897 and 898 relative to Sites 899 and 900, which affects the degree of postdepositional preservation of organic matter (Shaw and Meyers, this volume).
Atomic C/N ratios calculated from the shipboard and shore-based TOC analyses are compared in
Table 2. Nearly half the comparisons agree within ±1, and only one sample (149-898A-13H-6, 95-96 cm) has widely divergent ratios. In general, the two procedures give comparable results, particularly when considering that C/N ratios should not be over-interpreted. The ratios based on shorebased TOC measurements tend to be systematically higher than those based on the shipboard TOCs
(Fig. 3). This difference cannot be caused by differences in TOC concentrations, which are higher in the shorebased results
(Fig. 2). Instead, it is likely that errors in quantifying the nitrogen concentrations are larger in the shipboard procedure, inasmuch as these concentrations are determined by NCS elemental analysis of whole sediment samples in which the total carbon peak is much larger than the nitrogen peak. A likely consequence of this procedural difference is that the variability of the shorebased C/N ratios is less than that of the shipboard ratios
(Table 2). For this reason, the results of the shorebased analyses will be the basis for discussing the significance of the C/N ratios.
The averaged shorebased atomic C/N ratios of the Unit I samples are: Site 897, 4.6 ± 1.6; Site 898, 3.8 ± 3.9; Site 899, 3.7 ± 1.8; Site 900, 2.9 ±2.1. These ratios indicate that marine contributions dominate the organic matter in these turbidite layers and that the organic matter has been at least partially degraded within its host sediment. Fresh algal organic matter typically has C/N ratios between 4 and 10 (Meyers, 1994). Degradation of organic matter in marine sediments tends to lower C/N ratios as nitrogenous compounds break down to produce ammonia, which is retained by clay minerals, and the CO2 released by oxidation of organic carbon escapes
(Müller, 1977). Some of the samples have C/N ratios of 1 or 2; these are too low to represent undegraded organic matter. A few samples have C/N ratios between 10 and 20; these samples probably contain elevated proportions of land-derived organic matter. Furthermore, the higher average C/N ratio of 4.6 in the Site 897 Unit I samples suggests that the marine organic matter at this location has experienced less degradation than at the other sites. This is consistent with the greater average thickness of the turbidite units at this site (Shipboard Scientific Party, 1994a).
Organic 
13C values of Unit I sediment samples from the four sites average ~-23‰, which indicates that most of the organic matter originated from marine production (Meyers, 1994). The range of these values, from -19.0‰ to -26.8‰
(Table
2), suggests, moreover, that the origins have not been identical and that the proportion of land-derived material may become important in some turbidite layers. For example, Sample 149-898A-7H-6, 63-64 cm, has a
13C value of -25.1‰ and a shorebased C/N value of 13.3. The organic carbon in this sample appears to be as much as 50%
terrigenous. The 13C values of other samples, in contrast, are significantly heavier than the average. Sample 149-897C-19R-3, 61-62 cm, for example, has a value of-19.0‰, which is somewhat heavier than typical marine organic matter (-20‰ to -22‰; Meyers, 1994). Isotopically heavy organic matter can be produced by marine algae during times of diminished availability of dissolved CO2, such as accompany spring blooms or
upwellings. The organic matter in the isotopically heavy samples may have originated from coastal areas of high productivity 200 km to the east of the abyssal plain and was delivered to the deep-sea by turbidity flows of continental shelf sediment.
A Van Krevelen-type plot of the HI and OI values suggests that the Leg 149 turbiditic sediments contain type III (land-derived) organic matter
(Fig. 4). This source assignment for the organic matter conflicts, however, with the low C/N ratios and the
13C values for these samples
(Table
2), which suggest that the organic matter is predominantly marine. The contradiction between the
Rock-Eval source characterization and the elemental and isotopic source characterizations indicates that the marine organic matter has been heavily oxidized, probably by microbial reworking. The microbial reworking of the organic matter in the turbiditic units at Sites 899 and 900 evidently happened prior to redeposition of these sediments on the Iberia Abyssal Plain, inasmuch as little sulfate reduction has occurred at these locations (Shipboard Scientific Party, 1994c, 1994d). In contrast, in situ microbial reprocessing of organic matter is indicated at Sites 897 and 898 by the disappearance of interstitial sulfate and accompanying appearance of pore-water methane with increasing sediment depth at these locations (Shipboard Scientific Party, 1994a, 1994b). The HI values of samples from Sites 897 and 898 are in general higher than those of samples from Sites 899 and 900
(Fig. 4), indicating that the marine organic matter at Sites 897 and 898 has in fact not been as severely microbially reworked.
The amounts and types of organic matter present in Pliocene-Pleistocene Unit I sediments at Sites 897, 898, 899, and 900 differ. Both Site 897 and Site 898 have sediments that contain higher amounts of organic matter that has experienced less microbial reworking than present at Sites 899 and 900. The lithologic contents of Sites 897, 898, and 900 seem to be the same, although Unit I sediments at Site 900 are clearly different from sediments obtained at the other three sites
(Table 1). The four sites are in close proximity to each other, and it is unlikely that they experienced significantly different paleoceanographic conditions since the sediments were emplaced. Instead, it is likely that two factors associated with the turbidity flows that created the turbidite sequence on the Iberia Abyssal Plain influenced accumulation of organic matter in the sediments at these sites.
The first possibility is that the turbidity flows may have obtained their entrained organic matter from different environments during the Pliocene-Pleistocene. A variety of possible initial settings exits on the Iberia Margin. The locations of the Leg 149 drill sites are surrounded by topographic highs to the south and the north and are not very far from the continental margin of Iberia
(Fig. 5). Turbidites could have originated from all of these locations, and they would carry their entrained organic matter to the Iberian Abyssal Plain locations where the sediments would only partially intermix. Considerable geographic heterogeneity would remain, as would the downcore differences in the various turbidity layers (Shipboard Scientific Party, 1994a-d).
The second possibility is that differences in the thicknesses of turbidites layers present at the four sites affected post-depositional degradation of the entrained organic matter. Turbidite sequences are thicker at Sites 897 and 898 (Shipboard Scientific Party, 1994a, b) and probably protected organic matter from oxic early degradation. In contrast, turbidites at Sites 899 and 900 are thinner (Shipboard Scientific Party, 1994c, d) and organic matter could be oxidized soon after deposition. An analogous relationship between rates of turbidite accumulation and degree of organic matter preservation has been described in Quaternary sediments on the Cape Verde Abyssal Plain (Thomson et al., 1993). The more distal Iberia Abyssal Plain sites in particular, have thicknesses (up to 1 m) that approximate those studied on the Cape Verde Abyssal Plain. Nonetheless, both differences in delivery of organic matter and in its subsequent preservation could be involved at the Leg 149 sites.
