Pliocene to Recent development of carbonate systems from the central and southern Indian Ocean (Maldives and West Australian Shelf) : primary cycles versus diagenetic influence
Aachen (2020) [Dissertation / PhD Thesis]
Page(s): 1 Online Ressource (XI, 106 Seiten) : Illustrationen, Diagramme, Karten
Cyclic changes in environmental conditions can be recorded in sediment by variations in their geochemistry or mineralogy. However, diagenetic processes within the sediment play an important role in altering theses cycles; either modifying them or producing a new cycle. Paleoceanographic proxies such as δ18O or Mg/Ca ratios in foraminifers are often used to reconstruct paleoceanographic variations or to build age models with orbital resolution. Both of these proxies are susceptible for diagenetic alteration. This is even more apparent for stable oxygen and carbon isotopes values of bulk rock samples, which are directly influenced by the variable carbonate mineralogy and diagenetic phases. These diagenetic modifications can potentially mask the primary relationship between sea-level change and its sedimentary response. More intense diagenetic alteration can even lead to a complete reversal of the original isotope signal. It is therefore very important to use proxies that are inert against carbonate diagenetic processes such as e.g. organic biomarkers. The quantitative analysis of calcitic foraminiferal assemblages is another excellent tool to resolve primary environmental changes on orbital timescales. The preservation of the pteropod Heliconoides inflatus (synonym: Limacina inflata) can be used to track aragonite dissolution within the sediment during shallow burial below the seafloor at locations far above the aragonite lysocline. Using these methods can help to resolve problems that hamper the interpretation of geochemical proxy records and fill the gaps in previous studies on the origin of cyclic carbonate successions. This thesis focuses on the development of two different carbonate platforms from the central and southern Indian Ocean (ODP Leg 115, Site 716, Maldives and IODP Leg 356, Site U1460, Southwest Shelf of Australia). At the ODP Site 716 (Maldives), chlorin, a degradation product of chlorophyll-a and a potential indicator of primary productivity, was previously shown to record paleoclimate variations. However, chlorin like all proxies that are measured as concentrations are influenced by variations in the background sedimentation rates. In order to confirm that chlorin is a proxy for primary productivity and to better understand the controlling factors of primary productivity in the central Indian Ocean in times of global warmth, a quantitative analysis of planktic foraminiferal assemblages on orbital timescales in an early Pliocene time window was conducted. Cross wavelet analysis demonstrates the in-phase relation between thermocline species and chlorin, indicating upwelling related to a shallower thermocline, in which it is proven, that the chlorin is a proxy for primary productivity. The out of phase relationship between thermocline species and aragonite content indicates that the productivity was higher during sea-level lowstands. At IODP Site U1460 (SW Shelf of Australia), located at the transition from the ramp/shelf edge to the temperate carbonate platform slope, cemented fecal pellets are an important sedimentary component. Usually, hardened fecal pellets are known to form in shallow, tropical settings. However, the data presented in this thesis shows with the help of modal analysis and scanning electron microscopy, that pellets can also form in situ in deeper waters than typically assumed. They occur more abundantly during interglacials, when the site was below swell wave base. The near seafloor cementation by calcite, dolomite and pyrite related to bacterial sulfate reduction driven aragonite dissolution is essential for the hardening of pellets and their preservation in the fossil record. Cool-water environments provide important modern analogues for ancient carbonate deposits. However, the development of temperate carbonate systems over glacial-interglacial timescales is discussed controversially and there is no universally accepted model. The Middle Pleistocene to Holocene sequence at IODP Site U1460 contains a record of sea-level controlled sedimentary cycles. The primary mineralogical differences in the sedimentary cycles are modified by more intense diagenetic alteration in the interglacials. In order to evaluate the primary sedimentary versus diagenetic influence on the sedimentary record, a combination of core analysis with porewater geochemistry is used to identify the primary sedimentary record and its diagenetic overprint. Using the TEX86, as an independent sea surface temperature proxy, it can be demonstrated that the primary signal in the oxygen isotope ratios is preserved in the studied cores and can be used for paleoclimatic reconstructions with more confidence. A comparison of the porewater profile with core data such as XRD derived mineralogy, SEM observations, grain size analysis and shell preservation of the aragonitic pteropod Heliconoides inflatus indicates that significant aragonite dissolution starts close to the seafloor and that the extent of dissolution varies based on glacial to interglacial fluctuations in marine organic matter (alkenone) concentrations. Overall, the study on the ODP Site 716 (Maldives) provides the first proxy record that is able to resolve precessional changes in productivity in the central Indian Ocean during the early Pliocene. It gives insight into climate mechanisms controlling fluctuations of primary productivity on the processional band in the central equatorial Indian Ocean during the early Pliocene (chapter 2). The study on the subtropical carbonate ramp off western Australia (IODP Site U1460) documents the occurrence of fecal pellets in deep water, subtropical settings (chapter 3) and evaluates the primary sedimentary versus diagenetic influence on the sedimentary record (chapter 4).