Voigt, Janett (2015) Preservation of geochemical proxies in foraminifera during carbonate diagenesis in sediments of the eastern equatorial Pacific. (PhD/ Doctoral thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 202, X pp.

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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 is still unclear. In this thesis, 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) was conducted to investigate 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 Sr2+ concentration, radiogenic 87Sr/86Sr ratios, Sr/Ca 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 (16-13 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 δ13C 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 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 paleoceanographic studies of these carbonate-rich sediments, whereas planktonic foraminifera are more prone to recrystallisation, despite good preservation of Mg/Ca ratios.

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