Abstract

Long-chain alkenones, haptophyte-derived biomarkers, is a powerful tool for reconstructing sea surface temperature. Despite increasing reports of non-marine occurrence of alkenones, application of alkenone proxies in the settings are still challenging. Because the temperature calibrations of the alkenone unsaturation indices (U^K₃₇, U^K’₃₇) vary depending on the taxonomic differences of the alkenone producers, identification of the producer is a key to apply the calibrations for the coastal/lacustrine settings. The phylogeny of the alkenone producers is largely divided into three family-level groups, namely the Groups I – III (Theroux et al., 2010). In Japanese lakes, the Group I alkenone producer is identified in Lake Toyoni (Hokkaido) and Ichi-no-Megata (northeastern Japan) (Longo et al. 2018). Here we report the third example of lacustrine alkenone in Japan, which revealed from the analysis of ca. 1.8 m sediment core taken from Lake Nakaumi, western Japan. The L. Nakaumi is a coastal brackish lake and lagoonal in origin. Its water changed oligohaline to polyhaline by intrusion of seawater from Japan Sea. evaluated the source species of alkenones in Lake Nakaumi from the composition of alkenone and related compounds, and water temperature were preliminary reconstructed by selecting applicable calibrations.

The long-chain alkenones were detected in all samples from the 1.8 m core taken from near the center of L. Nakaumi (17NK-3 core). The alkenone compositions are clearly different between the samples above and below the horizons around 20 cm depth. corresponding to the horizon in about 1960s. Below 20 cm depth , the di-unsaturated alkenones such as C37:2 alkenones are dominant, and the U^K’₃₇ are higher. Meanwhile, above 20 cm depth, the C37:3 alkenones are dominant and C37:4 and C40 alkenones are relatively abundant, resulting contrastingly lower U^K’₃₇ and U^K₃₇ values. By comparing with chemotaxonomic features of known alkenone producers as reported previously , we estimate that the main alkenone producers in sediments below and above the horizons in about 1960s are Group III species (including marine producers: e.g. Emiliania huxleyi and Gephyrocapsa oceanica ) and Group II species (coastal and lacustrine producers: e.g. Ruttnera lamellosa and Isochrysis garbana), respectively.

The Cretaceous is an interval of great interest for understanding the ecological evolution in terrestrial areas under hothouse condition. The reconstruction and ecological interpretation of the Cretaceous vegetation were performed by paleobotanical and palynological investigations, and recently, by organic geochemical analysis using terrestrial plant-derived biomarker. Records based on plant fossils and biomarkers are dominated by those of vascular plants, especially gymnosperm and angiosperm. In our works, we focus on the other members of terrestrial ecosystem, such as lichens and fungi to cover better scope of paleoecosystem on land. In addition, such organisms include pioneer species, which can grow in extreme environments. Hence retrieving paleontological information on such organisms has potential to piece out paleoecosystem reconstruction especially at the timing after severe environmental disturbance and paleoecology under extreme environments. Thus, we analyzed terrestrial biomarkers related to non-vascular plant, thallophyte, in sediments across the Cenomanian/Turonian boundary (CTB) of the Yezo Group distributed in the Tomamae area, northern Hokkaido, to reconstruct paleoenvironment and paleovegetation, and to examine the applicability for these indicators.

The CTB is known as an interval of the oceanic anoxic event (OAE) 2. In the study area, the OAE2 level reached about 700 m. Each phase of OAE2 was determined by δ¹³C of wood. By the sterane and hopane analyses, it shows immature of organic matter in the sediments. Perylene and dibenzofurans could be detected from all samples across the CTB. The perylene is thought to be mainly originated from terrestrial fungi. The dibenzofurans are known to be produced by plants, fungi and lichens, while the dibenzofurans having alkyl groups, especially at positions 1 and 9, are thought to be derived from lichens. Thus, the 1-methyl dibenzofuran ratio (1-MDBF ratio) was proposed as an indicator of lichen vegetation. The 1-MDBF ratios vary in the sediment across the CTB; the ratios increase during the 1st build up stage, consistently low values during the Trough, and tend to increase from the 2nd build up to Plateau stages. It is presumed that the variations are attributed to environmental and climatic changes in paleo-Hokkaido across the CTB.

Alkenones are long-chain unsaturated ketones produced by a few number of haptophyte algae of the order Isochrysidale. The degree of unsaturation of C₃₇ alkenones has been shown to vary as a function of the environmental temperature, and these lipids have been thus used for decades as a proxy for sea surface temperatures thanks to the unsaturation indices U^K₃₇ ([C_37:2 – C_37:4]/[C_37:2 + C_37:3 + C_37:4]) and U^K’₃₇ ([C_37:2]/[C_37:2 + C_37:3]). More recently, alkenones have been globally reported in freshwater and saline lakes, but their use as a proxy for terrestrial temperatures is hampered by the rather limited knowledge about the environmental factors controlling alkenone distribution and the haptophyte species that produce them. Here, 106 lakes from the Northern Great Plains in Saskatchewan, Canada were investigated for alkenone producers and composition.

The study of surface sediments indicate that 55% of the Canadian lakes contain alkenones, with very high concentrations (up to 2.3 mg/g of sediment) in 7% of the lakes. Statistical analyses reveal that salinity and stratification of the water column play key roles in determining alkenone presence and abundance, which is certainly linked to the ecology of the producing haptophyte species. Moreover, genomic analyses based on surface sediments from some lakes evidence the presence of four distinct species from Group I and Group II haptophytes. Even though calibration of the U^K₃₇ index was not possible from the study of the surface sediments, seven haptophyte strains were isolated from three Canadian lakes, and characterised by genomic analyses as belonging to the clade Isochrysis. The culture of these new species shows that not only the U^K₃₇ and U^K’₃₇, but also alternate indices based on alkenoates (esters with structure close to the alkenones), are temperature dependent.Taken together, these data bring new information on the production of lacustrine alkenones, and allow us to use these lipids for palaeoenvironmental reconstructions. A short core from one of the survey sites (Humboldt Lake) is being analysed to calibrate the alkenone proxy with historical climate records from the 20^th century, a process which that will allow a longer temperature reconstructions in the Northern Great Plains.

Geosciences ’09. Geological Society of New Zealand, New Zealand Geophysical Society joint conference, 23-27 Nov, Oamaru: Programme & Abstracts