Qualitative and quantitative changes of organic and carbonate carbon in sedimentary records are frequently used to reconstruct past environments, paleoproductivity and sediment provenance. Amongst the most commonly used proxies are Total Organic Carbon (TOC), Mineral Carbon (MinC), as well as Hydrogen (HI) and Oxygen Indices (OI) of organic matter (OM). Rock Eval pyrolysis enables the assessment of these quantitative and qualitative parameters with a single analysis. This is achieved through transient pyrolysis of the samples up to 650 °C followed by combustion up to 850 °C, with hydrocarbons, CO and CO2 measured during the thermal decomposition of both OM and carbonate minerals. Carbonate minerals with low thermal cracking temperatures, such as siderite (<400 °C), can induce significant matrix effects which bias the TOC, MinC and OI Rock-Eval parameters. Here we assess the applicability of End-Member Analysis (EMA) as a means of correcting Rock-Eval thermograms for siderite matrix effects. For this, we performed Rock-Eval pyrolysis on sideritic sediments of Lake Towuti (Indonesia). New thermal boundaries were constrained in Rock-Eval thermograms using EMA to limit siderite matrix effects and improve TOC, MinC and OI calculations. Our approach allowed us to: (1) evaluate the influence of siderite matrix effects on Rock-Eval thermograms; (2) properly exploit a Rock-Eval dataset to characterize the type and sources of OM in siderite-rich sediments and (3) identify the OM behind degradation and mineralization processes. The Rock-Eval dataset revealed sediments with a substantial amount of refractory OM, especially in those where TOC is high and HI characteristic of autochthonous biomass. These results, associated to alternative indices used to assess OM preservation, suggest that refractory OM is residually enriched following strong degradation of labile compounds. Finally, relatively labile and refractory organic fractions may be consumed in the formation of siderite during this sequential process of OM mineralization.
Bibliographical noteFunding Information:
The Towuti Drilling Project was partially supported by grants from the International Continental Drilling Program , the US National Science Foundation , the German Research Foundation , the Swiss National Science Foundation ( 20FI21_153054/1 and 200021_153053/1 ), Brown University , Genome British Columbia , and the Ministry of Research, Technology, and Higher Education (RISTEK). PT Vale Indonesia, the US Continental Drilling Coordination Office, the GeoForschungs-Zentrum Potsdam and DOSECC Exploration Services are acknowledged for logistical assistance to the project. This research was carried out with permission from the Ministry of Research and Technology (RISTEK), the Ministry of Trade of the Republic of Indonesia, and the Natural Resources Conservation Center (BKSDA) and Government of Luwu Timur of Sulawesi. We thank the valuable comments and suggestions of F. Baudin and Ch. Giguet-Covex to an early version of this manuscript as well as co-Editor in Chief S. Rowland.
© 2019 The Authors
Copyright 2019 Elsevier B.V., All rights reserved.
- End-Member Analysis
- Mineral carbon
- Organic matter
- Rock-Eval pyrolysis
- Thermal analysis
- Tropical lake