IDS Oral Presentation Award to Kotaro Hoshi
by Kotaro Hoshi |
Diatom-derived biomarker analysis in the northern North Pacific off the Gulf of Alaska
Biomarker analysis is somewhat close to microfossil analysis, the difference is we use molecular fossils instead of actual fossils. Biomarkers of diatoms are very important for both paleoenvironment and palaeoceanographic research, because of its significance in primary production especially after the Miocene, where diatom diversity shows a major increase. However, there aren’t much we know about diatom biomarkers compared to other phytoplankton such as haptophytes. Sterols: a compound which has a different side chain depending on its producer, is one of the few known biomarkers to be widespread among diatoms. Ostreasterol (C28Δ5,24(28)sterol) is the most common sterol among diatoms, especially bi-polar centrics. Other diatom sterols to note is Brassicasterol (C28Δ5,22sterol) and 22-dehydrocholesterol(C27Δ5,22sterol). Brassicasterol is common among pennates and rarely detected from centrics. 22-dehydrocholesterol is a sterol detected from centrics which doesn’t have Ostreasterol as their major sterol.
Long chain alkyl diols (LCDs), on the other hand is a biomarker specific to the genus Proboscia. The ratio of Proboscia derived 1,14-diols to Eustigmatophytes derived 1,13-diols or 1,15-diols is known as the diol index, which is an upwelling proxy developed from surface sediments and sediment trap data from upwelling regions such as the Arabian Sea. The basic logic behind the diol index is that during weak upwelling periods, Proboscia biomass remains low relative to other phytoplankton which thrives at stratified conditions such as Eustigmatophytes. However, once upwelling intensifies and more nutrients gets supplied to the surface, Proboscia rapidly increases due to its weakly silicified frustule and therefore increases the ratio of 1,14-diols in sediments.
The aim of this research is to conduct diatom-derived biomarker analysis in the Gulf of Alaska to investigate the linkage between diatom accumulation and biomarkers, and the controlling factors of primary production at the Gulf of Alaska over the past 10 million years. The samples were collected by Akemi Fukumura at IODP Expedition 341 conducted in 2013 and this research focuses on Site U1417: the most distant site from the mainland and located at the edge of the submarine fan. The interesting point about Site U1417 and nearby Site U1418 is the poor preservation of biosiliceous fossils. Diatom fossils are barren except for the top 200m and the interval between 300m and 420m, which is about ~2Ma and 2.7Ma ~ 5Ma. The poor preservation in probably caused by the changes in oceanographic conditions at the sea floor. So, biomarker analysis is especially important and effective to fill in the gaps of poor fossil records.
LCDs including 1,14-diols were detected from all 32 samples analyzed, indicating that Proboscia diatoms thrived even at intervals where no fossil were found. Diol index indicates that upwelling conditions varied from weak to very strong upwelling at the Gulf of Alaska. Linkage between cooling/warming events and upwelling strength were observed. Intervals of cooling events such as Northern Hemisphere Glaciation (NHG), MIS-M2 cooling, and late Miocene cooling shows a major enhancement in upwelling. Warming events such as the mid Piacenzian warm period (MPWP) and the early Pliocene warm period (EPWP) shows an opposite trend where upwelling is weakened. Furthermore, diol index showed a similar trend with diatom accumulation records from a nearby ODP Site 887. Major diatom sedimentation changing events described as event B and D in Barron et al., 1998 were also observed in Site U1417 as an abrupt increase and decrease in diol index values. Event B marks a major increase in diatom sedimentation, probably caused by enhanced upwelling due to high-latitude cooling. Event D marks a major decrease in diatom sedimentation, probably caused by stratification of surface water due to glacial freshwater input. The time lag of event D might have been caused by different timings of mass freshwater input, where at Site U1417 it happened much later at the end of NHG.
Diol index values spikes correspond with intervals of diatom and biosiliceous ooze accumulation at Site U1417. This indicates that the high diol index values correspond with high diatom productivity in terms of lithology. However, if we compare diol index with the accumulation of other diatom biomarkers, such as sterols and 1,14-diols we see a completely opposite trend at Miocene and Pliocene. Where high accumulation of diatom biomarkers corresponds with low diol index values. But after the NHG, which ends around 2.4 Ma, trends of diatom biomarkers and diol index values matches well. The hypothesis is that during the Miocene & Pliocene, nutrient supplied via local tectonics such as glacial erosion, orogeny and subduction had major influence on diatom production over upwelling. Then, during the Pleistocene, upwelling became the driving force of diatom production. We will carry on further research using both biomarkers and fossils to verify the hypothesis.
Kotaro Hoshi, Hokkaido University, Graduate School of Science, Division of Natural History Sciences won the third prize in the oral presentation category of the International Diatom Symposium in Yamagata.