Unusual distribution of hydrocarbons in a hydrothermally altered phosphorite nodule from Kusu Basin, northern Kyushu, Japan

Lacustrine phosphorite nodules occur in diatomaceous mudstone from the Kusu Basin, northern Kyushu, Japan. Most of these phosphorite nodules have been hydrothermally altered. Features of the extractable hydrocarbons in an altered nodule are n-alkanes with an even-carbon-number predominance and unusual suites of dimethyalkanes and alkylcyclopentanes. Maturity indicators based on hopanes show that this non-altered sample is much more mature than a hydrothermally altered one. The hydrocarbons in the altered phosphorite were produced during the alteration process and dimethylalkanes and alkylcyclopentanes can be regarded as specific biological markers for hydrothermal environments.