Chapter 28 - The Paleogene Period

The initial aridification of mid-latitude Asia is thought to be one of the most prominent Cenozoic environmental changes in Asia. However, precisely dated, fine-grained sediments documenting initial aridity beyond central Asia are extremely rare. Here, we reconstruct the history of Middle-Late Eocene (47.8–37.8 Ma) hydroclimate change in northeast China using continuous palynological records from a lacustrine sedimentary sequence in the Fushun Basin. Our results indicate that the onset of aridification occurred concurrently with increased climatic variability at 41.2 Ma, as expressed by a sharp increase in xerophytic Ephedra and mountainous coniferous forest at the expense of an abrupt decrease in broadleaved forest, along with frequent alternations between semi-desert and swamp forest vegetation after 41.2 Ma. The climate changed markedly from a stable, warm-wet climate during the Middle Eocene to a cooler, drier climate with pronounced warm-dry and cold-wet oscillations during the Late Eocene. This finding challenges the previously prevailing idea that the East Asian monsoon was significantly enhanced in northeast China after 41–40 Ma. A comprehensive comparison of records from eastern and central Asia demonstrates that the onset of aridification and enhanced climate variability after 41 Ma were widespread across mid-latitude Asia. This may be explained by reduced moisture transport by the westerlies and strengthening of high- and low-latitude forcing linked to the coeval retreat of the proto-Paratethys Sea.

The short-term climate and environmental consequences (<∼1.0–10 Kyr) immediately following the end-Cretaceous extinction have been attributed to transient effects originating from Deccan volcanism and the Chicxulub impact. These two events have received significant attention for their role in precipitating extreme climate conditions. In contrast, the long-term (> ∼100 Kyr) climate and environmental changes of the post-extinction have been extensively studied in the marine systems, yet their ramifications within terrestrial ecosystems are less well-characterized and require further elucidation. This study presents a carbon isotope analysis of pedogenic carbonates from matured calcisols in the Nanxiong Basin, South China, to reconstruct carbon cycles and atmospheric CO₂ concentrations (pCO₂) spanning from 76.0 Ma to 62.0 Ma, aiming to decipher the long-term terrestrial environmental and climatic conditions. Results show that δ¹³C values range from −12.4‰ to −5.0‰ (mean − 9.0‰) and pCO₂ varies between ∼250 ppmV and ∼ 2200 ppmV (mean 920 ppmV). When combined with δ¹³C data from Songliao Basin, NE China, the carbon cycle variation exhibits a ¹³C collapse and smooth towards subsequent rebound, indicating a process of ∼600 Kyr deterioration, ∼600 Kyr stabilization and ∼ 400 Kyr recovery in the terrestrial ecosystem. The fluctuation of estimated pCO₂ aligns with the change pattern of sea surface temperatures, attesting to the strong perturbation of climate and environment at the same pace as the carbon cycle variations. It is proposed that ecosystems and environments in both terrestrial and surface ocean experienced a more unstable, difficult and erratic recovery process and were much more sensitive to climatic changes than in deep ocean for ∼1.5 million years in the aftermath of the end-Cretaceous mass extinction. In addition, the divergence of proxy variations from expected effects implies the Deccan eruption and Chicxulub impact could not have played a long-term role in governing the climatic and environmental perturbations following the Cretaceous-Paleogene Boundary.

The Late Cretaceous–early Paleogene interval is globally associated with transient to long-term changes in the stable carbon isotopic composition of marine carbonates (δ¹³C_carb). Based on biostratigraphic reconstruction, this critical period of Earth's history is thought to coincide with the deposition of world heritage Paleocene phosphate deposits (phosphorites) in northwestern Morocco. However, the detailed stratigraphy of the Gantour basin, one of the most important Moroccan phosphate deposits, has not yet been constrained. For instance, the former “Montian” Stage has been used to tentatively approximate the Danian, whereas the succeeding Selandian Stage remains to be identified. Here, we develop a detailed organic carbon isotopic (δ¹³C_org) curve from phosphorus-rich horizons of the western Gantour sedimentary sequence in an attempt to constrain their stratigraphic placement and depositional age model. Upsection, these strata host long-term negative and positive δ¹³C_org trends that tend to correlate with global δ¹³C_carb records of the Cretaceous–Paleogene and mid-Thanetian transitional boundaries. The data support the presence of Danian and Selandian rocks in the Gantour basin, which are succeeded by strata containing characteristic signatures of the well-known Cenozoic δ¹³C maximum at 58–57.5 Ma (the Paleocene Carbon Isotope Maximum). Our results shift the previously proposed Cretaceous–Paleogene transition in the Gantour basin further down into the older sediment C2M layer without interfering with recorded massive biological turnover in faunal diversity and abundance. Moreover, the refined stratigraphy suggests that the deposition of the Gantour phosphorites spanned ∼8.5 Myr. Our results confirm the utility of δ¹³C_org chemostratigraphy for dating and correlating phosphate-bearing deposits of the Tethyan province. They have important implications for deciphering Paleocene phosphogenesis, the co-evolution of associated vertebrate groups, and for prospecting phosphorus-rich mineral deposits.

Assemblages of multiple organic matter (OM)-related sedimentary energy systems are common in the Eocene nonmarine basins of Northeast China. During the Middle Eocene Climatic Optimum, oil shale developed in large quantities in the Yilan Basin. Numerous scientific papers have concentrated on the sedimentary cycle and industrial quality of the coal- and oil shale-bearing intervals, whereas few studies have studied the genetic environmental transformation mechanism of the coal and oil shale deposits. The Dalianhe Formation cover thick oil shale member and coal unit, which is a natural archive for studying the environmental evolution of lake swamp systems. In the present paper, the thermal maturity of organic matter (OM), oil shale industrial quality, coal rank, source of OM and paleoenvironmental conditions associated with OM accumulation in the Yilan Basin are investigated through bulk geochemical analyses, organic maceral analyses and biomarker composition analyses. The coal in the Yilan Basin is classified as subbituminous coal with high total organic carbon (TOC) values, high oil yields, low ash yields, and type II OM. The quality of the low-maturity oil shale in the Coal-Bearing Member is better than that in the Oil Shale Member. The oil shale in the Yilan Basin has type I-II OM and relatively high oil yields, which corresponds to the considerable content of liptinite with a predominantly alginite composition. Based on the higher Pr/Ph values, coal was deposited under oxic–dysoxic conditions, and the OM in the coal mainly originated from terrestrial plants, whereas the oil shale accumulated in a dysoxic environment with lower Pr/Ph values, and featured mixed OM sources, including terrestrial plants and low aquatic organisms. Affected by lake level fluctuations and wide peat swamping, the thickest and highest-quality coal and oil shale developed in swamps under a dysoxic–oxic water column in freshwater environments. Subsequently, the lake basin expanded rapidly, and the sedimentary environment changed to a dysoxic–oxic semideep–deep lake, in which extremely thick mudstone and a large amount of inferior oil shale with low oil yields accumulated under the influence of gravity flows. The basin continued to rapidly subside, the water column became anoxic–dysoxic, and the deposition of mudstone, siltstone and fine sandstone predominated.

The dating of continuous Eocene sediments in the Fushun Basin, NE China, provides a good opportunity to study the evolution of the land paleoclimate during extreme high-temperature events in mid-latitude regions. Here, the lower part of the Eocene Xiloutian Formation in the Fushun Basin was sampled, and isotope geochemistry, element geochemistry, organic geochemical analysis, magnetic susceptibility, and sediment color analysis were conducted to determine the evolution of paleoclimate in the continental lake basin during the Middle Eocene Climatic Optimum (MECO) period. The oxygen (δ¹⁸O) and carbon (δ¹³C) isotope data indicated the presence of phases during the MECO, with 401.5–374.3 m (41.2–40.42 Ma) representing the main stage and 374.3–371.2 m (40.42–40.32 Ma) the recovery stage. In addition, evaluation of the opening and closing of lakes using δ¹⁸O and δ¹³C showed that the lakes in the lower part of the Xiloutian Formation in the Fushun Basin are currently in a transition period between open and closed. The reconstructed paleotemperature and precipitation in the lower part of the Xiloutian Formation were characterized by an initial high trend followed by a low trend, with the highest values of both occurring during the main MECO stage. The lower part of the Xiloutian Formation is characterized by an oxidative oxygen environment of brackish water, and MECO events did not affect the salinity and redox state of the water. However, the total organic carbon (average 5.24%) in the dark mudstone during the main MECO stage was significantly higher than that during the cooling period (average 4.67%), indicating that the MECO promoted carbon fixation and enrichment of organic matter. Climate comparison showed that the climatic types in eastern and western regions of China during the Eocene were similar. Furthermore, climatic evolution may be influenced by the uplift of the Tibetan Plateau, and the retreat of the inland proto Paratethys Sea.

Biomagnetostratigraphic studies carried out at the Ypresian/Lutetian (Eocene) Gorrondatxe section solved major chronostratigraphic calibration discrepancies that had existed for several decades. The new calibration resulted in the definition of the Global Stratotype Section and Point (GSSP) for the Lutetian Stage at the Gorrondatxe layer containing the lowest occurrence (LO) of the calcareous nannofossil Blackites inflatus (base of Subzone CP12b). However, subsequent studies cast some doubt on the reliability of the Gorrondatxe biomagnetostratigraphic data and, consequently, the appropriateness of the criterion used for the definition of the Lutetian GSSP. In order to address the issues raised, the biomagnetostratigraphy of the Gorrondatxe section was revisited and an additional cyclostratigraphic analysis was undertaken using magnetic susceptibility data series. The present study shows that the Gorrondatxe biomagnetostratigraphy is reliable. The spectral analyses showed the dominance of precession (∼20 ky) and short (∼100 ky) eccentricity forcing on sedimentation, but the influence of obliquity (∼38 and ∼ 55 ky) cycles was also detected and modulation by long (405 ky) eccentricity cycles was deduced. The succession was tuned to the 49.122–48.084 Ma interval and the astronomical ages obtained for a significant number of biomagnetostratigraphic events compared well with those derived from other studies. One of the exceptions is the LO of B. inflatus, which seems to be older in Gorrondatxe than in the Mediterranean and Western Atlantic areas. This diachrony most likely also delayed the LO of B. inflatus in historical reference sections of the North Sea area, rendering previous claims that the Lutetian GSSP caused some historical Lutetian sections to be Ypresian in age invalid. In fact, the definition of the base of the Lutetian Stage by the LO of B. inflatus at Gorrondatxe, now astronomically age dated at 48.455 Ma, best guarantees that the original concepts of the Ypresian and Lutetian historical stratotypes are maintained. Combined with secondary biomagnetostratigraphic marker events and an astronomically tuned cyclostratigraphy, the Ypresian/Lutetian Gorrondatxe section provides the globally significant chronostratigraphic reference model expected of any GSSP.