Portable X‐ray fluorescence spectroscopy as a tool for cyclostratigraphy

Cyclostratigraphic studies are used to create relative and high‐resolution timescales for sedimentary successions based on identification of regular cycles in climate proxy data. This method typically requires the construction of long high‐resolution datasets. In this study, we have demonstrated the efficacy of portable X‐ray fluorescence spectroscopy (pXRF) as a non‐destructive method of generating compositional data for cyclostratigraphy. The rapidity (100 samples per day) and low cost of pXRF measurements provide advantages over relatively time‐consuming and costly elemental and stable isotopic measurements that are commonly used for cyclostratigraphy. The non‐destructive nature of pXRF also allows other geochemical analyses on the same samples. We present an optimized protocol for pXRF elemental concentration measurement in powdered rocks. The efficacy of this protocol for cyclostratigraphy is demonstrated through analysis of 360 Toarcian mudrock samples from North Yorkshire, UK, that were previously shown to exhibit astronomical forcing of [CaCO3], [S] and δ13Corg. Our study is the first to statistically compare the cyclostratigraphic results of pXRF analysis with more established combustion analysis. There are strong linear correlations of pXRF [Ca] with dry combustion elemental analyzer [CaCO3] (r2=0.7616), and pXRF [S] and [Fe] with dry combustion elemental analyzer [S] (r2=0.9632 and r2=0.9274 respectively). Spectral and cross‐spectral analysis demonstrates that cyclicity previously recognized in [S], significant above the 99.99% confidence level, is present above the 99.92% and 99.99% confidence levels in pXRF [S] and [Fe] data respectively. Cyclicity present in [CaCO3] data above the 99.96% confidence level is also present in pXRF [Ca] above the 98.12% confidence level

A Southern Hemisphere record of global trace-metal drawdown and orbital modulation of organic-matter burial across the Cenomanian–Turonian boundary (Ocean Drilling Program Site 1138, Kerguelen Plateau)

Despite its assumed global nature, there are very few detailed stratigraphic records of the late Cenomanian to the early Turonian Oceanic Anoxic Event 2 from the Southern Hemisphere. A highly resolved record of environmental changes across the Cenomanian\u2013Turonian boundary interval is presented from Ocean Drilling Program Site 1138 on the central Kerguelen Plateau (southern Indian Ocean). The new data lead to three key observations. Firstly, detailed biostratigraphy and chemostratigraphy indicate that the record of Oceanic Anoxic Event 2 is not complete, with a hiatus spanning the onset of the event. A decrease in glauconite and highly weathered clays after the onset of Oceanic Anoxic Event 2 marks the end of the hiatus interval, which can be explained by a relative sea-level rise that increased sediment accommodation space on the Kerguelen Plateau margin. This change in depositional environment controlled the timing of the delayed peak in organic-matter burial during Oceanic Anoxic Event 2 at Site 1138 compared with other Oceanic Anoxic Event 2 locations worldwide. A second key observation is the presence of cyclic fluctuations in the quantity and composition of organic matter being buried on the central Kerguelen Plateau throughout the latter stages of Oceanic Anoxic Event 2 and the early Turonian. A close correspondence between organic matter, sedimentary elemental compositions and sediments recording sea-floor oxygenation suggests that the cycles were mainly productivity-driven phenomena. Available age-control points constrain the periodicity of the coupled changes in sedimentary parameters to ca 20 to 70 ka, suggesting a link between carbon burial and astronomically forced climatic variations (precession or obliquity) in the Southern Hemisphere mid-latitudes both during, and after, Oceanic Anoxic Event 2: fluctuations that were superimposed on the impact of global-scale processes. Finally, trace-metal data from the black-shale unit at Site 1138 provide the first evidence from outside of the proto-North Atlantic region for a global drawdown of seawater trace-metal (Mo) inventories during Oceanic Anoxic Event 2

Early Jurassic Palaeoenvironmental Change – A North African Perspective

Extreme palaeoclimate change events from Earth history can be used to further our understanding of the Earth’s potential responses to anthropogenic climate change. The geological record of the Toarcian Oceanic Anoxic Event (T-OAE; ~183 million years ago) provides an opportunity to study a period of extreme palaeoclimate change that was associated with global carbon release and global warming. This research presents: evidence for the effects of T-OAE climate change at tropical latitudes; a more efficient technique for high-resolution proxy dataset production, and an astronomical timescale for the T-OAE. In this study, protocols for elemental analysis using portable X-ray fluorescence spectroscopy (pXRF) have been optimised for the production of high quality, high-resolution proxy data from sedimentary successions. The new protocols have been used to demonstrate that these data are suitable for cyclostratigraphic studies, and that they provide the opportunity to produce astronomical timescales more quickly, cheaply and more easily than commonly-used combustion analyses. Three published and two previously unstudied successions of early Toarcian strata along a proximal to distal profile in the Middle Atlas Basin, Morocco were investigated and recorded. Sedimentological and geochemical analyses show widespread deposition of storm beds and increased sedimentation rates within the T-OAE, providing evidence for acceleration of the tropical hydrological cycle due to global warming associated with the T-OAE. High-resolution sedimentological and geochemical records of a T-OAE succession reveal an -8‰ carbon isotope excursion over 66.6 m of strata coincident with evidence for enhanced storm activity. Cyclostratigraphic analysis of the sedimentological and geochemical data show that T-OAE enhanced storm activity lasted for >950 kyr and was modulated by 100 kyr and 21 kyr astronomical cycles in seasonality. Furthermore, it is demonstrated that intense tropical storm frequency increased in response to increasing temperature, and that the T-OAE carbon cycle perturbation lasted ~1.3 Myr

Geochemical data from the Cenomanian-Turonian interval of ODP Hole183-1138A

Geochemical data from Ocean Drilling Program Site 1138A, core 69, spanning the Cenomanian–Turonian boundary interval (~94 Ma). The data were generated from marine sediment in the archive half of the core, which was opened and sampled for this project following approval from the IODP curatorial advisory board. The data consist of: (i) Bulk organic geochemistry (total organic carbon, hydrogen and oxygen indexes, total inorganic carbon) measured by Rock Eval pyrolysis and coulormat titration; (ii) Bulk organic carbon isotopes, measured on decarbonated sediments; (iii) Bulk sediment elemental concentrations measured by ICP-MS; (iv) Bulk sediment molybdenum isotopes, measured by MC-ICP-MS; (v) n-alkane compound-specific carbon isotopes measured by GCMS-IRMS; (vi) Maceral assemblage compositions, measured by visual petrography