Polycyclic aromatic hydrocarbons (PAHs) in late Eocene to early Pleistocene mudstones of the Sylhet succession, NE Bengal Basin, Bangladesh: Implications for source and paleoclimate conditions during Himalayan uplift

• Published in: Organic geochemistry Vol. 56; pp. 25 - 39
• Main Authors: Zakir Hossain, H.M., Sampei, Yoshikazu, Roser, Barry P.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01-03-2013; Elsevier
• Subjects: Earth sciences; Earth, ocean, space; Exact sciences and technology; Geochemistry; Paleobotany; Paleontology; Soil and rock geochemistry; Stratigraphy; Asia; Bangladesh; Himalayas; minerals; Neogene; Paleogene; lower Pleistocene; upper Tertiary; decomposition; paleoclimate; phenanthrene; Oligocene; upper Eocene; Cenozoic; correlation coefficient; clastic rocks; Thallophyta; mudstone; Quaternary; gymnosperms; Tertiary; biomarkers; Plantae; Eocene; weathering; organic minerals; climate; low temperature; polycyclic aromatic hydrocarbons; karpatite; Pliocene; high temperature; sedimentation; sedimentary rocks; combustion; fungi; uplifts; Spermatophyta; Miocene; Phanerozoic; monsoons; Pleistocene
• DOI: 10.1016/j.orggeochem.2012.12.001
• EISSN: 1873-5290
• ISSN: 0146-6380

► We separated PAHs into five groups indicating land environment. ► Pyrene showed negative correlations to perylene and coronene. ► Ratios of 4-ring to 5-/6-ring PAHs can be indicators for wildfire scale. ► In early stage of Himalayan uplift climate was arid and small wildfires occurred.
Distribution and possible sources of polycyclic aromatic hydrocarbons (PAHs) have been investigated in 23 late Eocene to early Pleistocene mudstones from the Sylhet succession of the northeastern Bengal Basin, Bangladesh. Paleoclimatic conditions in the southern Himalaya region throughout the Himalayan uplift were reconstructed, based on combustion derived PAHs and aromatic land plant derived biomarkers. Phenanthrene, fluoranthene (Fla), pyrene (Py), benz[a]anthracene (BaAn), chrysene/triphenylene (Chry+Tpn), benzofluoranthenes (Bflas), benzo[e]pyrene (BePy), benzo[a]pyrene (BaPy), perylene (Pery), indeno[1,2,3-cd]pyrene (InPy), benzo[ghi]perylene (BghiP), coronene (Cor) and retene (Ret) were the identified PAHs. Fla/(Fla+Py) ratios>0.5 and InPy/(InPy+BghiP)>0.2 from almost all Sylhet samples suggest occurrence of natural wildfires. Low contents of BaAn and BaPy indicate decomposition by long exposure to sunlight before sedimentation, or early diagenetic weathering. Increased Cor, InPy and BghiP contents suggest occurrence of larger, high temperature wildfires. Correlation coefficients of the PAHs and p-values for statistical hypothesis testing showed that the positive and negative correlations within the PAHs may be indicative of high or low temperatures in wildfires. Fungi derived Pery showed negative correlations with Py (r=−0.67, p=4.6×10−4) and Fla (r=−0.56, p=5.0×10−3), but not with Cor, Bflas, InPy and BghiP. Based on the correlation coefficients for all PAHs and their p-values, five statistical groups ([Py, Fla], [Cor, Bflas, InPy, BghiP], [BaAn, Chry+Tpn, BaPy], [Pery] and [Ret]) were recognized. These groups are probably correlated with origins and depositional processes. According to the results, the Sylhet succession was deposited in three differing paleoclimatic regimes: (1) First phase (late Eocene to early Miocene, early to middle stage of Himalayan uplift): High contents of combustion derived PAHs (Fla, Py and BePy), significant gymnosperm derived Ret, and low Pery abundances in the Jaintia and Barail groups indicate arid climatic conditions. Although wildfires could often occur, 5- or 6-ring combustion PAHs (Cor, InPy and BghiP) contents are low, suggesting that the wildfires were relatively low temperature. (2) Second phase (middle to late Miocene: middle to late stage of Himalayan uplift): Combustion derived PAHs and fungi derived Pery were dominant in the Surma Group. The climate was arid to humid and seasonal, with a dry season giving conditions suitable for combustion. Abundant Fla, Bflas, BePy, Cor, InPy and BghiP imply high temperatures in large wildfires. However, frequency of the wildfires decreased because of wet climate. (3) Third phase (late Miocene to Pleistocene: late stage of Himalayan uplift): Moderate to high Pery contents and low Fla, Py and BePy abundances in the Tipam and Dupitila groups indicate establishment of more humid climate. InPy, BghiP, Cor, Bflas and BaPy were predominant. Intensified humid and seasonal climate arising from the Himalayan monsoon decreased the incidence and frequency of general wildfires, but increased the ratio of large to small wildfires.