Carbon isotope record of trace n-alkanes in a continental PETM section recovered by the Bighorn Basin Coring Project (BBCP)

Continental sedimentary records of early Paleogene hyperthermals are typically limited to weathered, often discontinuous, outcrop exposures. In 2011, the Bighorn Basin Coring Project collected the first continuous terrestrial records of the Paleocene-Eocene Thermal Maximum (PETM) in the Bighorn Basin, Wyoming. Organic matter preservation was poor during the PETM, even in core material. Concentrations of leaf waxes during the PETM are too low for compound-specific carbon isotope analysis by conventional means. However, the recent development of picomolar-scale compound-specific isotope analyses (pico-CSIA) has reduced sample requirements and enabled measurements of carbon isotope ratios of n-alkanes across the PETM in the Basin Substation core. While the prominent, negative carbon isotope excursion (CIE) in total organic carbon that typically identifies the PETM in the sedimentary record is absent from the core, lithostratigraphic, biostratigraphic, and chemostratigraphic data suggest that the most likely position of the PETM is from ~87.82 to ~50 MCD. This ~40 m interval coincides with the lowest weight percent organic carbon, n-alkane abundances, and n-alkane d13C values and the highest n-alkane average chain lengths. Comparison of the n-alkane isotope record from the core with that from organic-rich rocks exposed in the SE Bighorn Basin suggests that n-alkanes in the core fail to express the full magnitude of the CIE. We hypothesize that floodplain sediments at Basin Substation contain a mixture of PETM and reworked fossil n-alkanes. Low total organic carbon suggests that PETM climate accelerated organic matter decay rates and floodplains may have acted as a carbon source during the PETM.

早古近纪高温事件的陆相沉积记录通常局限于风化严重且多不连续的露头剖面。2011年，大角盆地取心项目（Bighorn Basin Coring Project）在怀俄明州大角盆地采集到了古新世-始新世极热事件（Paleocene-Eocene Thermal Maximum, PETM）的首个连续陆相记录。即使在岩心材料中，PETM期间的有机质保存状况也较差。PETM期间叶蜡的浓度过低，无法通过常规方法进行化合物特异性碳同位素分析（compound-specific carbon isotope analysis）。然而，皮摩尔级化合物特异性同位素分析（picomolar-scale compound-specific isotope analyses, pico-CSIA）的最新发展降低了样品需求，使得能够对盆地变电站岩心中跨越PETM的正构烷烃（n-alkanes）碳同位素比值进行测量。尽管岩心中缺失了沉积记录中通常用于识别PETM的、显著的总有机碳负碳同位素漂移（carbon isotope excursion, CIE），但岩石地层学、生物地层学和化学地层学数据表明，PETM最可能的位置在约87.82至约50米复合深度（MCD）之间。这一约40米的区间与最低的有机碳重量百分比、正构烷烃丰度、正构烷烃δ¹³C值以及最高的正构烷烃平均链长相一致。将该岩心的正构烷烃同位素记录与大角盆地东南部出露的富有机质岩石中的记录进行对比，发现岩心中的正构烷烃未能完全反映CIE的幅度。我们推测，盆地变电站的泛滥平原沉积物中混合了PETM时期的正构烷烃和再沉积的化石正构烷烃。低总有机碳含量表明，PETM气候加速了有机质的分解速率，且泛滥平原在PETM期间可能充当了碳源。