Quantitative X-Ray Diffraction (QXRD) Proxy for Determining Biogenic Content

Research results from diatomaceous-rich sediment drifts (Schollaert Drift) within the Gerlache Strait (western Antarctic Peninsula - AP) have shown that the concentration of biogenic silica (x-ray amorphous) can be determined from QXRD analyses using discrete sediment samples. The percentage of diatomaceous material in the drift was determined using two methods; the dissolution method and from mineral mass balance determined from QXRD techniques. The QXRD determination yielded values that were within 5% of the biogenic silica determined by the dissolution method.

Two cores from SHALDRIL were similarly analyzed for biogenic silica using these two methods - 1) a 108-meter core in Maxwell Bay (NBP0502-1B), a fjord in the South Shetland Islands of the AP and 2) a 75-meter core from Firth of Tay (NBP0602A-8B) northeastern AP. Biogenic Si content was determined using the NaOH leaching analysis procedure of DeMaster, 1981 and duplicates were done at random intervals having reproducibility within 1.1%. QXRD analyses were done on the same samples. For the Maxwell Bay core, QXRD overestimated the percent of biogenic silica by ~50% to 60%. This is most likely due to the volcanic glass in the sediment which the QXRD does not recognize as a mineral due to its lack of crystalline structure and is therefore attributed to biogenic content. The QXRD method from Firth of Tay core showed a better relationship to the solution method due to its higher percentage of diatomaceous material within the sediment. Using QXRD as a proxy to determine biogenic content, although possible in some environments, is not appropriate in an environment with volcanic and volcaniclastic rocks. Finally biogenic bulk accumulation rates and grain size analysis showed distinct climate stages within these two cores allowing for comparison between eastern and western sides of AP.