Abstract
Maoming oil shales are typical low-altitude lacustrine sediments that were deposited during the late Paleogene. The hydrocarbon composition and compound-specific stable carbon isotopic composition (δ13C) of organic matter in the profile samples of the oil shales have been analyzed. The results show that algae and aquatic plants are major parent sources of the organic matter in the oil shales associated with a small portion of terrestrial higher plant input. The δ13C composition of the bulk organic matter and the n-alkanes varies greatly on the profile from −26.9 to −15.8‰ and −31.7 to −16.2‰, respectively. While a good positive correlation among the δ13C composition of individual n-alkanes implies that these n-alkanes were originated from the similar source input. The δ13C composition of n-alkanes on the profile displays a positive excursion trend from the bottom to the top, and this excursion was likely related to the general decreasing trend of the partial pressure of atmospheric CO2 (pCO2) during the late Paleogene. The δ13C composition of the C30-4-methyl steranes ranges from −11.9 to −6.3‰, which is suggestive of Dinoflagellates-related source input. Coincidently, the high abundance C33-botryococcanes were detected in the samples on the top section of the profile and display an extremely positive carbon isotopic composition of −4.5 to −8.4‰, suggesting that the lower partial pressure of atmospheric CO2 had triggered a bicarbonate consumption mechanism for Botryococcus braunii B. Therefore, the δ13C composition of n-alkanes and C33-botryococcanes and their profile variation suggest that a general declining process associated with fluctuation in the partial pressure of atmospheric CO2 is likely the major reason for the rapid climatic changes toward the end of the late Paleogene.
Highlights
The paleoclimate had experienced rapidly significant variation during the late Paleogene, as the global climate was switched from the warm “greenhouse” into the “glaciation and interglacial periods” from the Eocene to the Oligocene (Liu et al, 2009; Pearson et al, 2009; Colwyn and Hren, 2019)
By studying the composition and distribution of hydrocarbon compounds including biomarkers and their δ13C composition and profile variations in the oil shale samples symmetrically collected from a full drill core from Maoming Basin, this study aims to correlate organic geochemical profile of the oil shales with the paleoenvironment variation during the geological period of the Maoming oil shale sedimentation, and to provide better understanding of the climate changes of low-latitude continent region during the late Palaeogene
The characteristics of organic matter in the Maoming oil shales of Youganwo Formation suggest that the organic matter were mainly derived from the aquatic algae input associated with a small portion of aquatic/terrestrial high plant inputs
Summary
The paleoclimate had experienced rapidly significant variation during the late Paleogene, as the global climate was switched from the warm “greenhouse” into the “glaciation and interglacial periods” from the Eocene to the Oligocene (Liu et al, 2009; Pearson et al, 2009; Colwyn and Hren, 2019). The evidence of these significant climate changes including the fast formation of the Antarctic ice sheet, the carbonate compensation depth (CCD) rapid deepened and the increase of the oxygen isotope ratio (δ18O) of deep-sea benthic foraminifera (Tripati et al, 2005; Liu et al, 2009; Galeotti et al, 2016; McKay et al, 2016; Passchier et al, 2017). The systematic research on more terrigenous records, especially the records of the low latitude lacustrine sediments, is much needed to the full understanding of the climate change in the late Paleogene
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