CONCLUSIONS

For sediments with a significant radiolarian content, the commonly used Na₂CO₃ method is not adequate to efficiently digest the opaline shells. The KOH digestions produced much better results overall. Complete digestion normally did occur with a 2-M KOH solution and the operating conditions we used (85°C for 8.5-10 hr in a shaking water bath). In some cases, rare opal fossil fragments were still visible on smear slides even after this harsher treatment. Except for the case where volcanic glass is present, the KOH treatment does not seem to result in the leaching of excess silica from the matrix material (e.g., aluminosilicates and quartz). For samples containing volcanic ash and glass, significant discrepancies in the measured silica were found for the two treatments; the KOH digestion resulted in a twofold difference in dissolved SiO₂ relative to the Na₂CO₃ treatment. However, both the Na₂CO₃ and the KOH extractions dissolved a significant fraction of the volcanic ash and overestimate biogenic opal in ash-bearing samples. A possible method for measuring opal in ash-bearing samples utilizes a double extraction treatment. The biogenic opal is calculated as the difference between the SiO₂ value obtained from the opal-bearing sediment and the value obtained on the opal-free residue, which is recovered and digested a second time using the same operating conditions of the first extraction. In summary, our study suggests that the commonly used Na₂CO₃ method is inadequate and that KOH digestions will give more accurate biogenic silica values for Eocene and Miocene sediments, primarily because Na₂CO₃ is not nearly as effective at dissolving radiolarians. More generally, our results indicate that using a 2-M KOH solution to measure biogenic silica is an acceptable treatment for marine sediments throughout the Cenozoic.