Erhaltung geochemischer Proxies in Foraminiferen - Karbonatdiagenese im natürlichen Labor des IODP Pacific Equatorial Age Transect (PEAT)
Final Report Abstract
The elemental and isotopic composition of calcitic foraminiferal tests, deposited in marine carbonate sediments, is widely used to reconstruct oceanic and climatic conditions of the geologic past. However, ancient tests are normally altered after deposition by recrystallisation, during which the original biogenic calcite is replaced by secondary, inorganic calcite. The impact of this recrystallisation process on foraminiferal proxies was still unclear prior to this study. The project comprised a multi-component study of pore waters, bulk carbonates and foraminiferal tests from sediments from the Integrated Ocean Drilling Program Expedition 320/321 Pacific Equatorial Age Transect (PEAT) and investigated the consequences and processes involved in carbonate diagenesis. These sediments are of similar age and initial composition but have been subjected to different diagenetic histories and thus form a natural diagenesis laboratory. Several Sr parameters of bulk carbonates and associated pore waters, including Sr 2+ concentration, Sr/Ca ratios, radiogenic 87Sr/86Sr and stable δ88/86Sr ratios, indicate that the sediments of Site U1336 were extensively altered compared to the other PEAT sites, most likely caused by an inferred higher geothermal gradient driving recrystallisation. A benthic foraminiferal stable isotope record of Site U1336 was established for the middle Miocene (13-16 Ma) and compared to existing high resolution records from Sites U1337 and U1338 nearby. Despite the extensive recrystallisation of Site U1336 bulk carbonates, the carbon isotope events of the Monterey Excursion can clearly be identified in our record and the absolute values and amplitudes of the δ 13C and δ18O signatures agree well with those from the neighbouring Sites U1337 and U1338. Laser ablation ICP-MS derived element/Ca ratios of the planktonic foraminifera Dentoglobigerina venezuelana from specific target intervals reveal that ancient tests exhibit intratest Mg/Ca and Mn/Ca heterogeneity comparable to modern foraminifera. However, decreasing Sr/Ca ratios across the tests, in particular in tests > 20 Ma, indicate open system recrystallisation. The highest proportion of altered tests is found at Site U1336 reflecting the most intense recrystallisation. Nevertheless, Mg/Ca heterogeneity is preserved in the tests and average Mg/Ca values are comparable to modern foraminifera suggesting that much of the original Mg/Ca signal is retained. By comparing benthic and planktonic foraminifera of the middle Miocene, our data demonstrate that benthic foraminifera can be reliably used for palaeoceanographic studies of carbonate-rich sediments, whereas planktonic foraminifera are more prone to recrystallisation, despite good preservation of Mg/Ca ratios.
Publications
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(2012) Carbonate diagenesis in the Pacific Equatorial Age Transect (PEAT) Sites and the preservation of geochemical signals in foraminifera. In: IODP/ICDP Colloquium 2012, 07.03.-09.03.2012, Kiel
Voigt J., Hathorne E. C. and Frank M.
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(2013) A middle Miocene benthic foraminiferal stable isotope record from extensively recrystallised carbonate sediments of IODP Site U1336 in the Equatorial Pacific. In: AGU Fall Meeting 2013, 09.-13.12.2013, San Francisco, USA
Voigt J., Hathorne E., Holbourn A. and Frank M.
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(2013) Reliable geochemical proxy data from recrystallised foraminifera. In: 11th International Conference on Paleoceanography (ICP11) 2013, 01.-06.09.2013, Sitges - Barcelona, Spain
Voigt J., Hathorne E. and Frank M.
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(2014) Cenozoic record of stable Sr isotope composition in seawater. In: IODP/ICDP Colloquium 2014, 17.-19.03.2014, Erlangen
Voigt J., Hathorne E., Eisenhauer A. and Pälike H.
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(2015) Preservation of geochemical proxies in foraminifera during carbonate diagensis in sediments of the eastern equatorial Pacific. Ph. D. thesis, Christian-Albrechts-Universität zu Kiel
Voigt J.
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(2015) Variability of carbonate diagenesis in equatorial Pacific sediments deduced from radiogenic and stable Sr isotopes. Geochimica et Cosmochimica Acta 148, 360-377
Voigt J., Hathorne E. C., Frank M., Vollstaedt H., and Eisenhauer A.
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(2016) Minimal influence of recrystallization on middle Miocene benthic foraminiferal stable isotope stratigraphy in the eastern equatorial Pacific. Paleoceanography 31
Voigt J., Hathorne E. C., Holbourn A., and Frank M.
