Depositional History of Miocene-Age Sediments from the Weddell Sea, Antarctica, Using Sedimentological and Geochemical Proxies

Abstract

Understanding the stability of Antarctic Ice Sheets is a key to understanding past climates and the response of ice-sheets to warming. In this thesis, I examined the middle through upper Miocene marine sedimentological and geochemical record in the Weddell Sea. Sediments were recovered at Ocean Drilling Program (ODP) Site 694 located in 4653 m of water in the northern Weddell Sea Abyssal Plain, but recovery was poor and many of the recovered sediments were disturbed. Seven lithological units were identified based on composition and sedimentary structures. Preliminary ages were assigned based on diatom datums. Turbidites are present throughout most of the section. Between about 17 and 14.3 m.y. diatoms formed 10 to 40% of the sediment ice-rafted detritus (IRD) was rare. After an interval of glacialmarine sediment with few diatoms, IRD is common. Between 14.2 and 9 m.y. diatoms are absent or present in lower abundance. The Mid-Miocene Climatic Optimum (MMCO) occurred between ~17 and 14.5 Ma, when d 18 O o / oo lightened to 1.6. From the late Oligocene through the MMCO, the EAIS is assumed to have been ephemeral based on the light isotope values. After the late MMCO, d 18 O values got heavier by ~0.8 o / oo and by13.5 m.y. the EAIS became a permanent covered by ice. We postulate that diatomaceous sediments were deposited in the warmer interval of the MMCO, that the glacial marine dominated sediment (Lithologic Unit VI) marked the end of the MMCO, and that the sediments above it were deposited after the EAIS became a permanent feature of the crysophere. During this time sediments of a mixed provenance of clays, silts and gravels were observed for sediments with ages 10.44, 11.35, 12.51, 14.08, 14.14 and 14.23 Ma. Considerable work was expended on geochemical proxies to determine provenance and its compositional characteristics. The primary analytical method that was chosen is XRF core scanning. This method is nondestructive, relatively inexpensive, and provides high-resolution data. Data is expressed as elemental intensity. Two different methods were used to convert elemental intensity to weight percent oxide, the Data Reduction Method and the Log Ratio Calibration Equation (LRCE) model. Both methods require actual sample analysis for ground truthing and gave similar weight percent oxide results. In addition to the XRF core scanner, select samples were analyzed for major, minor, trace and rare earth elements. The geochemical proxies were used to assess the composition, tectonic activity, elemental enrichment and the degree of chemical weathering. All of these proxies indicate a felsic provenance with zircon enrichment most likely due to sediment recycling. The samples displayed similar Upper Continental Crust and Chondrite patterns, which indicates that these sediments have similar compositions and possibly share the same provenance, this being geologically varied regions within East Antarctic. This is expected because the West Antarctic Ice Sheet had not form until ~ 8 m.y.

XRF-Derived Cyclicity in Pliocene and Pleistocene Sediments from ODP Site 693, Dronning Maud Land, Antarctica

“…The TAM formed during the early Cenozoic from the opening of the West Antarctic Rift. The peaks are composed of various Triassic/Jurassic sedimentary rocks whereas the depths are composed primarily of gneisses made from metamorphosed East Antarctic rock and granite intrusions (Harley, 2005;Chin, 2013).…”

“…Aluminum was another possible normalization factor, but Ti was deemed a more suitable choice in accordance with previous studies (e.g. Jorgensen, 2012;Chin, 2013 To prepare the datasets for wavelet analysis, they were interpolated and subsequently detrended. The XRF datasets were interpolated (that is, expanded to intervals not captured in the actual XRF scanning procedure due to the presence of gaps, etc.)…”