The Miocene Carbonate crash: Shifts in carbonate preservation and contribution of calcareous plankton

Abstract

This thesis documents sedimentary changes in the middle to late Miocene of the Atlantic and Pacific Ocean basins. This time interval known as the carbonate crash interval (12-9Ma) displays a severe perturbation of the carbonate system in the framework of the major Cenozoic cooling accompanied by changes in circulation mode, global nutrient shifts, plankton size changes and stratification of the ocean basins. These developments led to modern patterns of biogenic sediment distribution and ecological niches. The main goal of this work was to investigate control mechanisms on Carbonate-Crash-events (CC-events) and to find hints to major discrepancies concerning timing and strength of these events. For this purpose carbonate preservation proxies and carbonate budgets were investigated and evaluated in the Atlantic at the Ceará Rise in a depth transect (ODP sites 926, 927 and 928). The data show that the dissolution occurred in a broad zone between the foraminiferal lysocline at 3300 m depth and the carbonate compensation depth (CCD) at about 4000 m water depth. Detailed mass losses of coccoliths and foraminifer carbonate were calculated among sites. Dissolution is evident throughout the record however preservation seems to increase in correspondence to Northern Component water formation (precursor of North Atlantic Deep water). Productivity decreases of calcareous plankton productivity here (centered at about 9.5 Ma) seem to be as well a factor controlling CC-events during the late Miocene. Furthermore the evaluation of preservation proxies from the coarse calcareous silt fraction (CSmean and CS percent) showed that the fragmentation of foraminifera is probably a more suitable indicator of carbonate dissolution. CSmean and CSpercent did not reproduce the depth dependant carbonate dissolution, which was evident in all other parameters (carbonate content, coarse fraction content, foraminiferal fragmentation). The comparison of Ceará Rise coarse fraction records to Caribbean Site 999 showed in contrast to earlier results preservation in phase. The contrasting results obtained from comparison of Ceará Rise records with the Caribbean led to a revision concerning CC-events especially in key locations of the Pacific and Indian Ocean. The productivity decreases in the beginning of the CC-can be assigned to a shift from La Niña-like to El Niñolike conditions, imprinted in the sediments off Baja California and in the Eastern Equatorial Pacific (EEP). Own data from the SE Pacific (ODP Site 1237) in offshore Peru/Chile provided evidence for enhanced dissolution starting at about 10.5 to 10.3 Ma.. This trend is possibly indicating the influx of corrosive southern sourced waters coupled to better carbonate preservation in the Atlantic. Hence significant basin to basin fractionation is evident. The end of the CC is characterized by a change to better preservation at the same time in the low latitude Caribbean, the Pacific and the Indian Ocean at about 9.5 Ma at similar II water depths (~3000 m). Nannofossil productivity decreases were at least important in the Atlantic in the late phase of the CC. Restructuring of plankton communities driven by the paleoceanographic revolution during the CC-events may have resulted quite often in productivity decreases, which however are confirmed only by few case studies. Budgeting of calcareous nannoplankton seems desirable. Especially during carbonate crash events bigger nannofossils the discoaster nannoliths comprise an important part of the sediments, in order to budget these nannoltihs 3-d models of 11 Neogene discoaster morphologies as well as Sphenolithus nannoliths based on morphometric measurements were set up and applied in a test studie on samples of Ceará Rise ranging from 8.6 to 3.3 Ma to. The significance of carbonate contribution exceeds by far their abundance. A rather abrupt abundance decrease of discoaster nannoliths was found in low latitude sediments of ODP sites from the Indian Ocean and the equatorial Pacific associated with the small Reticulofenestra umbilicus- interval (starting at 8.85 Ma). This event is associated with rising MAR in these key locations initiating the transition from the carbonate draught period of the CC-events to the period of the biogenic bloom .