11:45 AM - 12:00 PM
[MIS16-21] C40-alkenone unsaturation index from the Cretaceous and paleothermometry in southern high latitude of Proto-Indian Ocean
Keywords:alkenone, Cretaceous, biomarkers, paleothermometry, OAE
What would happen on regional environments on the Earth with the worst scenario of global warming? Geologic archives memorized the results of “grand experiments of global warming” during greenhouse period of the past, like Cretaceous. As greenhouse periods are deep past older than 4000 Myr, geochemical signals are lost or seriously affected by diagenesis. In addition, because such periods are much warmer than the Quaternary, some paleoceanographic proxies cannot be applied with the same manner.
Sea surface temperature (SST) and its fluctuation (cyclicity, magnitude and their rate of changes etc.) is the most important environmental information. Only collecting SST information of greenhouse periods as same resolution as Quaternary can answer the question that we mentioned at the start.
Alkenones (strait chain alkyl ketone) are potential tool for this purpose. Alkenone molecules from marine sediments are known to be products of haptophyte algae and are often employed for paleothermometry for Quaternary and Neogene. Di- and Tri-unsaturated alkenones with 37 carbon atoms (C37 alkenones) are the target molecules for this purpose. On the other hand, only di-unsaturated one has been reported from Paleocene or Cretaceous. Thus alkenone paleothermometry cannot be applied as same manner for the greenhouse periods.
This study detected alkenones with 37-40 carbon atoms in the late Cretaceous sediments from southern margin of proto-Indian Ocean (IODP Exp. 369 Site U1516). Di-unsaturated C40 alkenone (C40:2Et) was the most dominant species among the alkenones. In addition to C40:2Et, tri-unsaturated C40 alkenone (C40:3Et) was observed by this study for the first time. Then we calculated C40 alkenones unsaturation index (UK’40) and discussed its stratigraphic characteristics through the Albian to uppermost Cenomanian up to OAE2 interval.
Content of C40:2Et was highest at the top of black clay layer (BCL: sediment associated with OAE2). It is relatively low through the interval below BCL. It decreased above BCL and reach to substantially zero at 30 cm above the top of BCL, then no detection above.
UK’40 showed characteristic fluctuation in the Upper Cenomanian. Maximum value near 1.0 were detected from the lowest sample of BCL, as well as from samples of Albian/Cenomanian boundary interval. In the BCL, it showed dramatic decrease toward 0.85 within the interval of 15 cm. Not only C37 alkenones, C38, C41 and C42 alkenones also show their temperature-dependent nature of the unsaturation ratio based on in vitro experients (Araie et al., 2019). This fact strongly suggests that C40 alkenones produced during Cretaceous also had similar temperature dependence. If it is accepted, then following interpretation is possible for interval near the OAE2: (1) on the half way of the 1st build-up of the carbon isotope excursion (CIE), SST reached maximum. It is as same SST as that of Albian/Cenomanian boundary; (2) through the later phase of 1st build-up of CIE, SST dropped considerably and reached minimum just above the 1st peak of CIE. It could be comparable event with “Plenus Cold Event”.
UK’40 cannot be converted into SST value so far, however, some approaches are possible to improve UK’40 as a proxy of SST for Cretaceous. Oxygen isotope study on individual planktonic foraminifera with excellent preservation from identical sample with C40 alkenones can provide SST information comparable with UK’40. Collecting haptophyte strain that produce C40 alkenones from present saline lake and cultural experiments using them could help discussion on temperature dependence of UK’40. Thus UK’40 is a promising proxy for quantitative paleothermometry for high temperature regime (>28 C).
Citation: Araie et al. (2018) Org. Geochem. 121, 89-103
Sea surface temperature (SST) and its fluctuation (cyclicity, magnitude and their rate of changes etc.) is the most important environmental information. Only collecting SST information of greenhouse periods as same resolution as Quaternary can answer the question that we mentioned at the start.
Alkenones (strait chain alkyl ketone) are potential tool for this purpose. Alkenone molecules from marine sediments are known to be products of haptophyte algae and are often employed for paleothermometry for Quaternary and Neogene. Di- and Tri-unsaturated alkenones with 37 carbon atoms (C37 alkenones) are the target molecules for this purpose. On the other hand, only di-unsaturated one has been reported from Paleocene or Cretaceous. Thus alkenone paleothermometry cannot be applied as same manner for the greenhouse periods.
This study detected alkenones with 37-40 carbon atoms in the late Cretaceous sediments from southern margin of proto-Indian Ocean (IODP Exp. 369 Site U1516). Di-unsaturated C40 alkenone (C40:2Et) was the most dominant species among the alkenones. In addition to C40:2Et, tri-unsaturated C40 alkenone (C40:3Et) was observed by this study for the first time. Then we calculated C40 alkenones unsaturation index (UK’40) and discussed its stratigraphic characteristics through the Albian to uppermost Cenomanian up to OAE2 interval.
Content of C40:2Et was highest at the top of black clay layer (BCL: sediment associated with OAE2). It is relatively low through the interval below BCL. It decreased above BCL and reach to substantially zero at 30 cm above the top of BCL, then no detection above.
UK’40 showed characteristic fluctuation in the Upper Cenomanian. Maximum value near 1.0 were detected from the lowest sample of BCL, as well as from samples of Albian/Cenomanian boundary interval. In the BCL, it showed dramatic decrease toward 0.85 within the interval of 15 cm. Not only C37 alkenones, C38, C41 and C42 alkenones also show their temperature-dependent nature of the unsaturation ratio based on in vitro experients (Araie et al., 2019). This fact strongly suggests that C40 alkenones produced during Cretaceous also had similar temperature dependence. If it is accepted, then following interpretation is possible for interval near the OAE2: (1) on the half way of the 1st build-up of the carbon isotope excursion (CIE), SST reached maximum. It is as same SST as that of Albian/Cenomanian boundary; (2) through the later phase of 1st build-up of CIE, SST dropped considerably and reached minimum just above the 1st peak of CIE. It could be comparable event with “Plenus Cold Event”.
UK’40 cannot be converted into SST value so far, however, some approaches are possible to improve UK’40 as a proxy of SST for Cretaceous. Oxygen isotope study on individual planktonic foraminifera with excellent preservation from identical sample with C40 alkenones can provide SST information comparable with UK’40. Collecting haptophyte strain that produce C40 alkenones from present saline lake and cultural experiments using them could help discussion on temperature dependence of UK’40. Thus UK’40 is a promising proxy for quantitative paleothermometry for high temperature regime (>28 C).
Citation: Araie et al. (2018) Org. Geochem. 121, 89-103