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<p>Hydrocarbon seepage is a widespread phenomenon at continental margins around the world. The composition of the ascending fluids can be variable, consisting of short- and long-chain hydrocarbons as well as crude oil. A prominent site of oil seepage is the Bay of Campeche in the southern Gulf of Mexico. The seepage of petroleum is known to have an inhibiting effect on life at seeps, but short- and long-chain hydrocarbons have been shown to be degraded by a range of heterotrophic sulfate-reducing bacteria. Here we present lipid biomarker and carbon isotope data from authigenic carbonates from the Campeche Knolls in the southern Gulf of Mexico. The Campeche carbonates display d<sup>13</sup>C values in the range of -31.3‰ to -21.9‰, which is in accord with carbon derived from oil-derived hydrocarbons. Interestingly, the Campeche carbonates contain particularly high amounts of bacterial non-isoprenoidal ether lipids (DAGEs) with a wide variety of alkyl chain lengths. The bacterial biomarkers show heavier carbon isotopic signatures than their counterparts at methane seeps. These data allow for the characterization of bacterial oxidation of oil-derived hydrocarbons in modern and, in cases of moderate to good biomarker preservation, ancient environments, permitting the assessment of the influence of different fluid chemistries on the composition of chemosynthesis-based communities at seeps.</p>

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High-magnesium calcite (HMC) and aragonite are metastable minerals, which tend to convert into low-magnesium calcite (LMC) and dolomite. During this process, primary compositions are frequently altered, resulting in the loss of information regarding the formation environment and the nature of fluids from which the minerals precipitated. Petrological characteristics have been used to recognize primary LMC, however, neither the element distribution within primary LMC nor the effect of diagenetic alteration on element composition have been studied in detail. Here, two mostly authigenic carbonate lithologies from the northern Gulf of Mexico dominated by primary LMC were investigated to distinguish element compositions of primary LMC from LMC resulting from diagenetic alteration. Primary LMC reveals similar or lower Sr/Ca ratios than primary HMC. The lack of covariation between Sr/Ca ratios and Mg/Ca ratios in the studied primary LMCs are unlike compositions observed for LMC resulting from diagenetic alteration. The Sr/Mn ratios and Mn contents of the primary LMCs are negatively correlated, similar to secondary, diagenetic LMC. Element mapping for Sr and Mg in the primary LMC lithologies revealed no evidence of conversion from aragonite or HMC to LMC, and a homogenous distribution of Mn is in accordance with the absence of late diagenetic alteration. Our results confirm that Sr/Ca ratios, Mg/Ca ratios, and element systematics of primary LMC are indeed distinguishable from diagenetically altered carbonates, enabling the utilization of element geochemistry in recognizing primary signals in carbonate archives.