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How is black shale formation in the Early Eocene Arctic Ocean influenced by export of terrestrial organic matter? Details from an organic petrological approach on marine sediments from IODP Hole 302 (Lomonosov Ridge)

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Boucsein, B. , Knies, J. and Stein, R. (2008): How is black shale formation in the Early Eocene Arctic Ocean influenced by export of terrestrial organic matter? Details from an organic petrological approach on marine sediments from IODP Hole 302 (Lomonosov Ridge) , IODP/ICDP-Kolloquium, Hannover.-14.3.2008. .
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In 2004 the IODP Expedition 302 (ACEX) recovered a 430m thick sequence of upper Cretaceous to Quaternary sediments on the Lomonosov Ridge in the central Arctic Ocean (Backman et al. 2006). For the first time insights in the environmental Pre-Pleistocene history of the Arctic Ocean are possible (see e.g. Brinkhuis et al. 2006, Moran et al. 2006). Our results of the organic geochemical basis parameters (total organic carbon (TOC), stable carbon isotopes (&#948;13C), total organic carbon/total nitrogen (C/N) ratios, total organic carbon/total sulphur (C/S) ratios, Hydrogen indices) and first maceral data on the entire ca. 200m thick Paleogene organic carbon (OC) rich section have been published recently (Stein et al. 2006).Here, we will focus on the black shales formed during the global &#948;13C-events Paleocene/Eocene Thermal Maximum (PETM) and Elmo. New detailed organic petrographical data (maceral analysis) are compared with the results of organic geochemistry (basis parameter, organic and inorganic nitrogen fraction). Such combined petrographical and organic geochemical approaches were established during the last decades, especially to solve questions concerning the paleoenvironmental conditions of recent and ancient marine deposits.During the Paleocene/Eocene the Early Arctic Ocean was an enclosed basin influenced by warm surface-water temperatures as indicated by TEX86 data (Sluijs et al. 2006). Data on e.g. radiolarians, terrestrial palynomorphs (Backman et al. 2006) and maceral data (Boucsein and Stein, subm.) give evidence for river run-off causing low-surface salinity. Therefore, fluvial nutrient supply may have induced primary productivity as it is also suggested from the abundances of marine diatoms and diatom resting spores (Backman et al. 2006). The isolated position of the Arctic Ocean during that time combined with freshwater discharge support the idea of OC accumulation in an anoxic basin with a stratified water column. We found abundances of finely dispersed and small sized pyrite framboids (<5µm, Fig.1) which indicates in-situ formation of the framboids in an euxinic water column. This is also supported by the observed low C/S ratios and the occurrence of finely laminated sediments. Euxinic conditions up into the photic zone are described by Sluijs et al. (2006) who found the biomarker isorenieratene in the sediments of the PETM, which is related to green sulfur bacteria. Accordingly, this black shale like sediments are compared with deposits from euxinic environments as fore example the modern Black Sea (Stein et al. 2006).The organic petrographical characteristics of the studied sediments show distinct temporal and spatial variations as shown in Figure 1 and can be summerized as follows:While during the Campanian the particulate OM in the sediments is characterized by a dominance of terrigenous macerals (vitrinite, inertinite and detritus (20-40%)) the maceral composition changes during the Paleocene, and is probably related to erosion and rised sea levels. Typically, increased portions of recycled vitrinite, liptinite and detritus occur. To some extent, high amounts of inertinite (>20%) are found and correlate with increased OI values (200-400mg CO2/gC). Moreover, we found pyrofusinite in the inertinite fraction which is interpreted as an indicator for vegetation fires in the hinterland.Drastic environmental changes are supposed for the depostion of the ACEX black-shales during the Early Eocene. Especially during the PETM and Elmo event a significant increase in the 'aquatic/marine group' is found. High amounts of alginitic material (40-45%) and bituminite (up to 50%) together with increased HI values (250-300 mgHC/gC) are found. Characteristically are the finely laminated sediments of bituminitic layers, including well preserved alginite bodies of freshwater and marine origin. Here, the major aquatic macerals are lamalginite and liptodetrinite (fragmented liptinitic particles <10µm in size). Additionally, dinoflagellate cysts are found and also telalginites which are originated from Botryococcus, Pediastrum and Tasmanales. The observed amounts of up to 5% are similar to organic petrographical data from surface sediments from the Laptev Sea shelf and indicate the influx of freshwater/brackish water outflow. During the Elmo event a more proximal position to the paleo-coast than during the PETM is indicated by higher portions of land derived material such as corpohuminite and cross sections of rootlets in combination with increased amounts of sporinite.A well-known problem of petrographical analysis is the occurrence of amorphous OM (bituminite) and the determination of its biological origin. The observed bituminite appears in form of lenses or fine laminations and shows a weakly to occasionally non-fluorescence and contains in part inclusions of liptodetrinite, strong fluorescing lamalginite and single algae cysts. Whether the precursors of the bituminitic material is of terrigenous or aquatic origin is difficult to define using light microscopy techniques only. The observed inclusions of well preserved algae cysts and lamalginite inside the bituminitic layers suggest that it is originated as a bacterial decomposition product of alginitic material. Nevertheless, our new data on inorganic and organic nitrogen indicate, that a terrigenous source for the bituminite is more reliable. As discussed from Pennsylvanian oil source rocks (Oklahoma, USA) by Littke (1993) bituminite could be also a product from the precipitation of humic acids which are originated from peat deposits underlying these sediments. Similar to our studied sediments the Pennsylvanian source rocks are characterized by low HI values (200-400mgHC/gC), Rm values of 0,57%, a dominance of non-fluorescing bituminite and lamalginite and are deposited in a shallow marine environment. In our case of the ACEX source rocks the widespread North Siberian peat deposits are considered as a possible source area for terrestrial humic acids. During times of rised sea-levels, humic acids probably were released due to erosion of peat deposits, further transported onto the shelf by rivers and, though may be accumulated in the studied sediments.Summarizing, we suggest that preservation under anoxic conditions has been favorable for the accumulation of aquatic OC during the early Eocene events. Despite the events of increased primary produced OM during that time organic petrological data show a dominance of terrestrial material with fluxes of enhanced fluviatil material. Among the PETM and Elmo events environmental conditions have changed from anoxic to oxic/suboxic conditions. This is probably related to increased vertical mixing by water masses entering the basin and is resulting in a reduced stratification of the water column. Moreover, increased portions of detritus indicate a stronger fragmentation of the particles due to high energy transport by e.g., bottom currents.References:Backman, J.K., Moran, K., McInroy, D.B., Mayer, L.A., and the Expedition 302 Scientists (2006): Proc. IODP, 302, College Station TX.Brinkhuis, H., Schouten, S., Collinson, M.E., Sluijs, A., Sinninghe Damsté, J.S., Dickens, G.R., Huber, M., Cronin, T.M., Onodera, J., Takahashi, K., Bujak, J.P., Stein, R., van der Burgh, J., Eldrett, J.S., Harding, I.C., Lotter, A.F., Sangiorgi, F., van Konijnenburg-van Cittert, H., de Leeuw, J.W., Matthiessen, J., Backman, J., Moran, K., and the Expedition 302 Scientists, 2006. Episodic fresh surface waters in the Eocene Arctic Ocean, Nature, vol. 441, doi: 10.1038/nature04692. Boucsein, B. and Stein, R. (subm.): Early Eocene black shale formation and paleoenvironment in the Central Arctic Ocean (Lomonosov Ridge): Results from a detailed organic petrological study. Marine and Petroleum GeologyLittke, R. (1993): Deposition, diagenesis, and weathering of organic matter-rich sediments: Lect. Earth Sci. 47, Berlin, Springer Verlag, 217 p.Moran, K., Backman, J., Brinkhuis, H., Clemens, S.C., Cronin, T., Dickens, G.R., Eynaud, F., Gattacceca, J., Jakobsson, M., Jordan, R.W., Kaminski, M., King, J., Koc, N., Krylov, A., Martinez, N., Matthiessen, J., McInroy, D., Moore, T.C., Onodera, J., ORegan, A.M., Pälike, H., Rea, B. Rio, D., Sakamoto, T. Smith, D.C., Stein, R., John, K., Suto, I., Suzuki, N., Takahashi, K., Watanabe, M., Yamamoto, M., Frank, M., Jokat, W., Kristoffersen, Y. (2006): The Cenozoic paleoenvironment of the Arctic Ocean. Nature 441, 601-605.Sluijs, A., Schouten, S., Pagani, M., Woltering, M., Brinkhuis, H., Sinninghe Damsté, J.S., Dickens, G.R., Huber, M., Reichart, G.J, Stein, R., Matthiessen, J., Lourens, L.J., Pedentchouk, N., Backman, J., Moran, K. , the Expedition 302 Scientists, 2006. Subtropical Arctic Ocean temperatures during the Paleocene Eocene thermal maximum, Nature, 441, 610-613.Stein, R., Boucsein, B. and Meyer, H. (2006): Anoxia and high primary production in the Paleogene central Arctic Ocean: First detailed records from Lomonosov Ridge, Geophys. Res. Lett., 33, L18606. doi: 10.1029/2006GL026776

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