Biomarker analysis of invertebrate fossils preserved in concretions from the Lower Cretaceous Allaru Mudstone of Queensland, Australia

The Lower Cretaceous Allaru Mudstone (Albian, ~104–102 Ma) of northeast Australia was deposited within the epeiric Eromanga Sea and hosts a diverse fossil fauna, sometimes exhibiting exceptional morphological preservation. Two calcium carbonate concretions from this stratum—one containing a labeceratid ammonite (Myloceras sp.), the other a torynommid crab (Torynomma quadrata)—were investigated by organic and inorganic geochemical techniques. Biomarkers were extracted from each fossil and its surrounding matrix and separated into saturated, aromatic, and polar fractions by column chromatography. Saturate fractions were analysed via gas chromatography – mass spectrometry (GC–MS) and GC – multiple reaction monitoring – mass spectrometry (GC–MRM–MS). Mineral phases were characterized by X-ray diffraction. With respect to biomarker distributions, the ammonite showed a higher relative abundance of C27 regular steranes and diasteranes and a lower thermal maturity, reflected by C29 ααα steranes and Ts/(Ts+Tm), relative to the surrounding matrix (Ts = 18α(H)–22,29,30–trisnorneohopane; Tm = 17α(H)–22,29,30–trisnorhopane). The fossilized crab showed a similar maturity trend to the ammonite—a possible indicator of rapid encapsulation of fossil regions—but there was little difference in sterane and hopane distributions between this fossil and its matrix. Biomarker data complements previous foraminiferal and isotopic studies with a marine phototroph-dominant source of n-alkanes, extended hopanes and methylhopanes in a suboxic to oxic water column, as well as sporadic terrigenous input evidenced by a pentacyclic triterpenoid derived from higher land plants. Incongruence of the C27 ααα 20S and C28–29 αββ 20R sterane isomer abundances with thermal maturity proxies suggests co-elution with 5β 20R steranes, a product of microbial alteration of steroids during early diagenesis.