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.Highlights► 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.

The Sylhet Basin of Bangladesh is a sub-basin of the Bengal Basin. It contains a very thick (up to 22 km) Tertiary stratigraphic succession consisting mainly of sandstones and mudstones. The Sylhet succession is divided into the Jaintia (Paleocene–late Eocene), Barail (late Eocene–early Miocene), Surma (middle–late Miocene), Tipam (late Miocene–Pliocene) and Dupitila Groups (Pliocene–Pleistocene), in ascending order. The origin of the organic matter (OM) and paleoenvironment of deposition have been evaluated on the basis of C, N, S elemental analysis, Rock-Eval pyrolysis and gas chromatography–mass spectrometry (GC–MS) analysis of 60 mudstone samples collected from drill core and surface outcrops. Total organic carbon (TOC) content ranges from 0.11% to 1.56%. Sulfur content is low in most samples. TOC content in the Sylhet succession varies systematically with sedimentation rate, with low TOC caused by clastic dilution produced by high sedimentation rates arising from rapid uplift and erosion of the Himalaya.The OM in the succession is characterized by systematic variations in pristane/phytane (Pr/Ph), oleanane/C30 hopane, n-C29/n-C19 alkane, Tm/Ts [17α(H)-22,29,30-trisnorhopane/18α(H)-22,29,30-trisnorhopane] and sterane C29/(C27 + C28 + C29) ratios during the middle Eocene to Pleistocene. Based on biomarker proxies, the depositional environment of the Sylhet succession can be divided into three phases. In the first (middle Eocene to early Miocene), deposition occurred completely in seawater-dominated oxic conditions, with abundant input of terrestrial higher plants, including angiosperms. The second phase (middle to late Miocene) consisted of mainly freshwater anoxic conditions along with a small seawater influence according to eustasic sea level change, with diluted OM derived from phytoplankton and a lesser influence from terrestrial higher plants. Oxygen-poor freshwater conditions prevailed in the third phase (post-late Miocene). Planktonic OM was relatively abundant in this stage, while a high angiosperm influx prevailed at times. Tmax values of ca. 450 °C, vitrinite reflectance (Ro) of ca. 0.66% and methylphenanthrene index (MPI 3) of ca. 1 indicate the OM to be mature. The lower part (middle Eocene to early Miocene) of the succession with moderate TOC content and predominantly terrestrial OM could have generated some condensates and oils in and around the study area.

Abnormally abundant n-C37 and n-C38 alkanes, together with a significant amount of the n-C39 alkane, were found in the Miocene Onnagawa siliceous mudstones. The relative abundance of these alkanes resembles that of ΣC37, ΣC38 and ΣC39 alkenones from Emiliania huxleyi and Gephyrocapsa oceanica. The stable carbon isotope ratio of the n-C37 alkane is −23.5‰, supporting their planktonic origin. The n-C37-C39 alkanes are likely to be derived from alkenones produced by the family Gephyrocapsaceae during the Middle to Late Miocene period. Total organic carbon concentration of the samples rich in the alkenone-derived n-alkanes is in the range of 1.5–2.3%, and the samples are characterized by a low pristane/phytane ratio and a high homohopane index, suggesting anoxic depositional conditions. These sediments are relatively immature (Ro=0.33–0.45%), suggesting that the generation of the long-chain n-alkanes from alkenones took place during early diagenesis. The mudstones rich in n-C37-C39 alkanes are generally poor in CaO (<1.9%) and rich in SiO2 (77.9–85.3%). There is no relationship between CaO concentration and abundance of the n-C37-C39 alkanes. Since the Onnagawa siliceous mudstones were formed in a deep-sea environment, the low concentration of CaO is most probably due to the removal of calcareous skeletons by dissolution during deposition below the carbonate compensation depth. These alkenone-derived n-alkanes are useful biomarkers for detecting contributions of calcareous nannoplankton to deep-sea sediments.

•Bolgoda sediments contain vascular plants and sapropels deposited at low rates.•Organic carbon contents increased dramatically at ca. 2.5 cal ky B.P.•This change was due to sea-level regression.•Reworked OMs and aquatic plants increased after ca. 2.5 cal ky B.P.•PAH contents rise from anthropogenic activity after European settlement in the 15th C.The tropical Bolgoda Lake is one of the largest lagoonal estuaries in Sri Lanka. Organic constituents in this lake were investigated using 28 surface sediments, three cores from the lake, and four short cores from mangrove mud. Elemental and molecular compositions of sediments and living organic matter (OM) were determined using CNS elemental analysis and gas chromatography–mass spectrometry, respectively. Chronology of events was determined using accelerated mass spectrometry (AMS) 14C data for bulk OMs, woods and mollusk shells. The history of Bolgoda Lake can be divided into two major chrono-stratigraphic divisions, namely a lower sedimentary succession (from ca. 7.0 cal ky B.P. to 2.5 cal ky B.P.) and an upper sedimentary succession (from ca. 2.5 cal ky B.P. to the Recent), according to the sapropelic depositional system. Concentrations of the mangrove biomarker taraxone increased significantly at ca. 7.0 cal ky B.P. The lower part of the sediment sequence had moderate productivity, and was mainly deposited under marine-terrestrial influence in oxygen-poor/anoxic conditions during mid-Holocene sea-level highstands. A drastic increase in total organic carbon (TOC) content, reaching 29%, was found in one of the Bolgoda cores. This increase occurred because of sea-level regression at about 2.5 cal ky B.P. This change was probably caused by development of a beach rock/sandbar, and the estuary system changed to a semi-closed lake due to the sea-level regression. As a result, aquatic vascular plants predominate in this depositional system. After ca. 2.5 cal ky B.P., TOC content decreased slightly, but relatively high TOC contents were maintained in the muddy sediments. In moderately-sized and shallow (1–2 m) tropical estuaries/lagoons such as Bolgoda Lake, drastic changes in TOC contents may be the most sensitive indicator of sea-level variations. Increase in higher plant OM with higher n-C29/n-Call and n-C37/n-Call alkane ratios also suggest a climatic transition from wetter to somewhat drier after the middle Holocene. Accumulations of pyrogenic polycyclic aromatic hydrocarbons (PAHs) in the modern sediments record anthropogenic activity after European settlement in the 15th century.