RESULTS FOR SITE SURVEY SAMPLES

Leg 199 site survey samples were cored in December 1997-January 1998 aboard the Ewing and are labeled as EW9709. Thirty-three samples from three cores, representing ages spanning from 9 to 50 Ma, were analyzed twice: (1) in a 2-M Na₂CO₃ solution and (2) in an independently weighed sample digested in a 2-M KOH solution. Thirteen samples (39%) were run as replicate analyses to quantify the precision of the analytical method. Note that we do not correct our data for an assumed water content of the biogenous opal fraction. The water content of opal can range between 2 and 15 wt% and depends upon many factors including age, species, and sample handling. Our data is reported simply as SiO₂ or SiO_{2 biogenous} and probably underrepresents the total amount of hydrated biogenous opal in a given sample. However, correcting the reported silica data using an assumed scalar value for water content does not change the general conclusions drawn from this study. A comparison of the measured SiO₂ for the two digestion methods is illustrated in Figure F3. Two striking facts are noted. In most cases, the amount (weight percent) of digested silica for a given sample is higher after the KOH leach vs. the Na₂CO₃ leach, and in some cases, this amount exceeds the Na₂CO₃ value by more than a factor of two. The discrepancy in measured SiO₂ for the two methods increases with increasing SiO₂ in the sample. This is in contrast to the results of the composite standard (Fig. F2) whose SiO₂ value of 30 wt% is relatively constant for both alkaline extractions. Assuming both extractions produce accurate results for samples containing up to 30 wt% SiO₂ (based on the Site 1098 standard), one would expect no difference in dissolved SiO₂ for the site survey samples based on opal content alone. Yet, the KOH digestion consistently results in higher SiO₂ values, by a factor of 1.2-1.8, relative to the Na₂CO₃ extraction of identical samples.

This discrepancy is explained by examining smear slides made of the solid residue after digestion with Na₂CO₃ and KOH. Fourteen smear slide pairs were made, providing a direct comparison of the microscopic examination of the residue with the SiO₂ previously measured in the supernatant. Figure F1A and F1C illustrate a typical visual comparison between the two residue types. For nearly all EW9709 samples, a common feature of the solid residue remaining after a 9-hr Na₂CO₃ digestion (Fig. F1A) is a high abundance of undigested radiolarians and other opal fragments. In contrast, such fragments are generally absent after the KOH digestion (Fig. F1C). However, in several cases, we noted the presence of rare to occasional highly resistant opal fragments, which remained even after a 9-hr KOH digestion at 85°C. Generally, the amount of undigested siliceous fossils was essentially 0- after 9-hr KOH treatment.

To underscore the inefficiency of using Na₂CO₃ digestion, data are presented in Table T2 for six test samples from EW9709-3PC, which were allowed to digest for up to 14 hr at 85°C. Measurements were made at 6.5 and 14 hr, and the residue was examined afterward. Three of the six samples contained abundant undissolved radiolarians even after the 14-hr digestion (750, 800, and 850 cm), and all samples resulted in very low opal contents when compared to a 9-hr KOH extraction (Table T2). Generally, the KOH extraction residue contained very few opal fragments, indicating complete digestion of the biogenous opal. We caution against assuming that higher SiO₂ values resulting from a KOH leach relative to the Na₂CO₃ leach is simply the result of undigested silica fossils remaining after the Na₂CO₃ treatment. For example, Figure F4 illustrates this point when volcanic glass is present. The measured SiO₂ in this sample (EW9709-7PC; 122-124 cm) is 9.9% and 17.7 wt% SiO₂ for the Na₂CO₃ and KOH leach, respectively. However, the difference between the two values cannot be accounted for by the presence of biogenous silica remaining after the Na₂CO₃ leach. No opaline fossils were found upon a smear slide examination of the sediment residue remaining after the Na₂CO₃ leach (Fig. F4A). However, we did notice abundant glass in this residue and proportionally less glass after the KOH leach (Fig. F4B), which suggests that the near twofold difference in SiO₂ reflects a greater dissolution of the glass in the KOH solution relative to Na₂CO₃ solution. We tested whether the alkaline digestions attack amorphous volcanic glass by leaching a young volcanic ash from the eastern Pacific (ME0005-24JC; 981-982 cm, Ash Layer D; ~84.2 k.y.). The SiO₂ leached by the KOH digestion was 2.5x the measured result for the Na₂CO₃ extraction (25 vs. 10 wt% SiO₂, respectively) (Table T3). Nevertheless, both treatments strongly but incompletely dissolved the volcanic ash as well as the small amount of biogenic silica in the initial sample. An interesting result is the difference in the SiO₂ weight percent (of the weighed sediment sample) between the results from an initial extraction that is the "the opal-bearing ash," and the results from a second extraction of the recovered residue, the "the opal-free ash." For both the Na₂CO₃ and the KOH treatments, this difference yields a calculated biogenic opal content of 4 wt%, a value consistent with smear slide estimates of the amount of opal fragments in the undigested sample (<5 wt%). The potential for obtaining accurate opal values for ash samples using this double-extraction technique deserves further investigation.

An obvious limitation of our results is the lack of independent analyses of the EW9709 samples, including the biogenic silica, calcium carbonate, and clay mineral components. Such data would provide a better gauge of the accuracy of the two extraction methods, including the amount of presumed clay dissolution, which occurs during digestion. (We are the first to measure biogenic opal for EW9709 cores. Other laboratories are not using KOH to digest the opal fraction of marine sediments, and there is a widespread belief that even the less harsh Na₂CO₃ treatment leaches excess silica from clay minerals.) Alternatively, we approached the problem by focusing on sediments that are similar to those from Leg 199 site survey sites where biogenic opal data are available and measured by an independent method. A total of 16 samples were provided by ODP's West Coast Repository for this part of the study (Deep Sea Drilling Project [DSDP] Leg 16 Site 162) and analyzed for SiO₂ and calcium carbonate. Site 162 samples are from the central equatorial Pacific Ocean, are Eocene in age, and the biogenic opal fraction is dominated by radiolarians. The 16 samples we analyzed are from a group of 116 composite samples studied by Leinen (1976, 1979), who estimated the amount of biogenous silica in each sample using a normative calculation calibrated to an X-ray determination of the opal in the sediment. The robustness of the normative method is based upon accurately measuring the relative proportions of the four clay mineral groups (montmorillonite, illite, chlorite, and kaolinite) such that one can estimate a bulk SiO₂:Al₂O₃ ratio for the sample. Leinen's SiO₂:Al₂O₃ ratio was obtained by employing X-ray diffraction (XRD) analysis for a subset of representative samples. This ratio is subsequently used to calculate the nonbiogenous silica fraction in each sample, which is finally subtracted from the bulk sediment to yield the biogenous silica fraction. The 16 samples we requested and analyzed correspond to the same 16 sampled intervals in Leinen's (1976, 1979) study. She organized these samples by age into five groups, each spanning a 1-m.y. interval between 30 and 50 Ma.

Two sediment samples from each of the 16 samples were weighed and digested separately: the first in a 2-M Na₂CO₃ solution and the second using a 2-M KOH solution. The solid residue remaining after the extractions was saved for later smear slide analysis. A summary of results for both the wet-digestion methods and the normative analysis calculations are given in Table T4 and illustrated in Figure F5. In general, the Na₂CO₃ extractions underestimate the amount of biogenous silica as predicted from the normative calculation. Samples whose normative biogenic opal is <10 wt% are in the best agreement, but above this value, the Na₂CO₃ treatment underestimates, by one-third to one-half, the normative opal values. Conversely, results from the KOH extractions are very good overall. They are slightly higher than the normative calculation for values <10 wt% biogenic opal, and beyond this division, the measurements from the KOH treatment are within 80%-90% of the normative value. This discrepancy between the two treatments for samples with opal >10% is significant because Leg 199 recovered many stratigraphic intervals with very high opal contents (>50 wt%). Smear slide examination of the postdissolution residues further supports using a KOH extraction for opal measurements of Leg 199 sites. In general, opal fragments were common to abundant in the residue after the Na₂CO₃ extraction but rare to occasional in the KOH residue. Many were very small rod-shaped spicules, frustules, and fragments, which are nearly impossible to physically separate from the residue matrix (e.g., to do a simple mass balance to compare the two techniques). Figure F6A and F6B illustrates the differences in the dissolution residue for Sample 162-14-4: 80-81 cm (~46 Ma). Note the abundance of silica fossils in the Na₂CO₃ digestion residue (Fig. F6A) compared with the occasional fragments remaining after the KOH leach (Fig. F6B). Corresponding opal values are consistent with the assertion that the KOH extraction is a better estimator of this component over Na₂CO₃. This yields 37.1 wt% vs. 19.2 wt%, respectively (Table T4), which indicates that half of the fossil opal remains undigested in the Na₂CO₃ treatment.

The clay mineral fraction of samples used in the normative study is generally dominated by smectite (60%-80% smectite) (Leinen, 1976; Heath, 1969). Leinen (1976) selected one sample from Site 162 and determined the clay mineral content by XRD analysis (162-17-4: 125-126 cm). This sample, which we also obtained and analyzed, contains 30 wt% nonbiogenic components, of which 93 wt% is smectite, based on replicate analyses (Leinen, 1976). For both extractions, our digested silica values for this sample are in very good agreement with her normative calculation (~3.5% biogenic silica) (Table T4). The fact that both extractions did not produce excess silica from a matrix containing ~30 wt% smectite indicates that the presence of clay minerals does not obviate accurate measurements of biogenic silica, and that widely-held beliefs about clay mineral solubility during alkaline digestions required reexamination. Although the KOH extraction produced better biogenic silica results overall when compared to the normative analysis, the differences between our samples limits our ability to refine the comparisons. First, Leinen's samples (1976) represent 2-cm-thick intervals, whereas our sampled intervals are 1 cm thick, due to sample availability from the repository. Thus, we are missing up to 50% of her sampled interval. Second, Leinen (1976) mixed her samples together and did one analysis for each million-year time interval. We analyzed all samples individually within each million-year time interval. Therefore, we can compare results only for the average of the interval, rather than by a sample-by-sample comparison of the opal results. In addition to missing half of her sampled interval, one of our samples from the time interval 37-38 Ma (162-4-6: 60-61 cm) is 40 cm below the sample she used for her composite value for this time interval (Table T4). These differences, along with the inherent errors associated with each of the two types of analyses (normative vs. wet alkaline), probably account for some of the observed discrepancies in our respective results.

Finally, we report SiO₂ results for relatively young sediments from the Pacific to test whether or not the harsher KOH extractions can produce acceptable biogenic opal values compared to the Na₂CO₃ digestion. A total of 40 analyses, including replicates, were performed for 9 samples ranging in age from 0 to 84 ka. The results of these analyses are summarized in Table T3 for the samples taken from cores W8709A-5BC, ME0005-24JC and K7905-42BC. For all samples except two, there is little difference between the measured SiO₂ weight percent of the two alkaline techniques. Additionally, smear slide analyses show no evidence of residual opaline fossil fragments after both digestions. As was the case for the Site 1098 Composite Standard discussed previously, this result is significant because dissolution of aluminosilicates does not appear to be a significant source of excess SiO₂, particularly for the KOH extraction. The two exceptions to these results are samples which contain volcanic ash and glass: W8709A-5BC, 5-20 cm, and Ash Layer D from Site ME0005A-24 JC, discussed previously. The first sample shows a twofold difference in SiO₂ weight percent for the KOH digestion relative to the Na₂CO₃ digestion. However, smear slide analysis shows no evidence of undigested silica shells remaining in the residue after the Na₂CO₃ extractions, again suggesting that the source of the excess silica is volcanic, not biogenic. We did observe occasional volcanic glass fragments in the residue, but a quantitative estimate of the glass was not adequately determined for this sample. The presence of volcanic ash and glass will likely result in erroneously high estimates of the biogenic silica for both digestions, but the KOH digestion will produce a much greater error. In summary, these results suggest that there is no disadvantage to routinely using a 2-M KOH solution to estimate the biogenic opal in pelagic sediments of both young sediments and others spanning the Cenozoic.