ALBERT

Description: The geologic time scale for the Cenozoic Era has been notably improved over the last decades by virtue of integrated stratigraphy, combining high-resolution astrochronologies, biostratigraphy and magnetostratigraphy with high-precision radioisotopic dates. However, the middle Eocene remains a weak link. The so-called "Eocene time scale gap" reflects the scarcity of suitable study sections with clear astronomically-forced variations in carbonate content, primarily because large parts of the oceans were starved of carbonate during the Eocene greenhouse. International Ocean Discovery Program (IODP) Expedition 369 cored a carbonate-rich sedimentary sequence of Eocene age in the Mentelle Basin (Site U1514, offshore southwest Australia). The sequence consists of nannofossil chalk and exhibits rhythmic clay content variability. Here, we show that IODP Site U1514 allows for the extraction of an astronomical signal and the construction of an Eocene astrochronology, using 3-cm resolution X-Ray fluorescence (XRF) core scans. The XRF-derived ratio between calcium and iron content (Ca/Fe) tracks the lithologic variability and serves as the basis for our U1514 astrochronology. We present a 16 million-year-long (40-56 Ma) nearly continuous history of Eocene sedimentation with variations paced by eccentricity and obliquity. We supplement the high-resolution XRF data with low-resolution bulk carbon and oxygen isotopes, recording the long-term cooling trend from the Paleocene-Eocene Thermal Maximum (PETM - ca. 56 Ma) into the middle Eocene (ca. 40 Ma). Our early Eocene astrochronology corroborates existing chronologies based on deep-sea sites and Italian land sections. For the middle Eocene, the sedimentological record at U1514 provides a single-site geochemical backbone and thus offers a further step towards a fully integrated Cenozoic geologic time scale at orbital resolution.

Description: A series of basaltic flows and associated dolerite dykes were cored at depths of 〉680m below the seafloor, at the base of IODP Site U1513. These cores were obtained from the Naturaliste Plateau, offshore SW Australia as part of IODP Expedition 369. Quartered sections of the core were selected during the expedition to establish the age, composition, and mineralogy of these basaltic rocks. This data collection includes the results of X-ray fluorescence (XRF), X-ray diffraction (XRD) and 40Ar/39Ar isotopic analyses of eight basaltic rocks cored at the site. The XRF measurements were collected using a Spectro Ametek XEPOS III energy dispersive XRF spectrometer at the University of Wollongong. The XRF results are presented as a spreadsheet/table containing the major and minor elemental composition of each sample, together with the reference materials that were run during the same analytical session. The XRD and argon isotopic analyses were collected at the Research School of Earth Sciences at the Australian National University. The XRD analyses were obtained using a Malvern PanAnalytical Empyrean Series 3 x-ray diffractometer equipped with a Bragg-BrentanoHD divergent beam optic and a PIXcel3D detector (1D scanning mode, 3.347 degrees active length), with a CoK⍺ radiation source. The samples were spiked with 20 wt.% corundum (Baikalox, 1 μm), suspended on a low-background holder (Si or quartz) and analysed over a range of 4-85° 2𝛳, with a step width of 0.0131303° 2𝛳 and a total dwell time of 71 s per step. Phase identification was carried out with the software Match! and the Crystallographic Open Database (Inorganic, Revision 248644, 03.03.2020). Phase quantification was performed using the direct derivation method within the Match! software package. The 20 wt.% corundum spike was used as a reference value to quantify the wt.% of other mineral phases, the percentage of unidentified phases and the amorphous content of each sample. The XRD data are presented as a series of worksheets within a spreadsheet and consist of two columns (2-theta angle, intensity). The calculations and phase identification data are also included as a separate worksheet within the same Excel file. The argon analyses were conducted on eight whole-rock samples that were processed in the Australian National University Mineral Separation Laboratory. These samples were crushed and sieved, with the 250–420 µm size fraction being retained for argon dating. Each sample aggregate was placed in a 5L beaker, the beaker was continually filled with water and occasionally stirred over a ~30-minute period, leading to the fine and low-density material being flushed from the aggregate. The sample was then air-dried and later washed in deionised water before being air-dried once more. The air-dried material was then hand-picked using a binocular microscope and wrapped in aluminium packets. Care was taken to ensure the hand-picked material did not include alteration minerals. The aluminium packets were placed into a quartz irradiation canister together with aliquots of the fluence monitor GA1550. Packets containing K2SO4 and CaF2 were placed in the middle of the canister to monitor 40Ar production from potassium and other reactions. The samples were then sent for irradiation at the UC Davis MNRC nuclear reactor in California, USA prior to analysis. The samples were irradiated between the 2nd to 4th October, 2018 (inclusive). The irradiated samples were unwrapped on their return to ANU, weighed and rewrapped in tin-foil ready for analysis in the mass spectrometer. Samples were dropped into a furnace that was designed and constructed at ANU. This consists of dual, temperature-controlled furnaces that function independently in the extraction line. The furnace temperature is calibrated using the melting-point of five different metals, with video monitoring of the melting point to assess accuracy. A thermocouple is located immediately adjacent to each sample within the furnace for accurate temperature readings. The furnace maintains the selected temperature for the entire duration of the heating step. During this study, the furnace was heated to 400°C to melt the tin. The contaminated gas from the tin and sample was then pumped away prior to the analysis of the sample. Backgrounds were measured prior to each step analysis and subtracted from each step analysis. The basaltic samples were analysed with 29 steps and with temperatures of the overall schedule rising from 450°C to 1450°C. The furnace was degassed four times at 1450°C for 20 min and the gas was pumped away prior to a new sample being dropped into the furnace. Temperature steps in the schedule were increased in small increments to minimize mixing of different gas populations on each step. Contamination and erroneous data were recorded at the start and end of the experiments as these data can reflect 39Ar recoil from clay and Ca derived 37Ar recoil. This can be seen in the spectra due to the low temperatures at which the experiments start, and these data are recorded so as not to lose potential information that aids the interpretation of the results. Fluence monitors, GA 1550, were analysed using a CO2 continuous wave laser and an ARGUS VI Mass Spectrometer at the Research School of Earth Sciences at the Australian National University. Samples were analysed using the temperature-controlled step heating method. Gas released from laser analysis and each step of the furnace experiments were exposed to three different Zr-Al getters to remove active gases for 10 min, the purified gas then being isotopically analysed in the mass spectrometer. Corrections for argon produced by interaction of neutrons with K and Ca were calculated using the following correction factors: (36Ar/37Ar)Ca: 2.297E-04, (39Ar/37Ar)Ca: 7.614E-04, (40Ar/39Ar)K: 5.992E-02, (38Ar/39Ar)K: 1.158E-02 and (38Ar)Cl/(39Ar)K: 8.170E-02. 40K abundances and decay constants are taken from standard values recommended by the IUGS subcommission on Geochronology. Steps with low radiogenic argon were not used in the age interpretation. Stated precisions for 40Ar/39Ar ages include all uncertainties in the measurement of isotope ratios and are recorded at the one sigma level in the data files.

Description: We present a new method for reconstructing flood basalt lava flows from outcrop data, using terrestrial laser scanning (TLS) to generate three-dimensional (3D) models. Case studies are presented from the Faroe Islands and the Isle of Skye (UK), both part of the North Atlantic Igneous Province (NAIP). These were analyzed to pick out lava flow tops and bases, as well as dykes, lava tubes, and sedimentary layers. Three-dimensional surfaces were then generated using modeling software, and 3D geological models constructed. Finally, the models were interrogated to give data on flow thickness and crust-to-core ratio. The aim of this research is to obtain quantitative data on the internal heterogeneity of a sequence of flood basalt lava flows, and to provide high-resolution information about flow geometries and volcanic facies variations in 3D. Lava flow sequences display complex stacking patterns, and these are difficult to understand from photos or outcrop observations. Laser scanning allows us to study inaccessible outcrops, while avoiding the perspective distortion in conventional photography. The data from this study will form parts of larger models of flood basalt provinces, which will be used to improve seismic imaging in areas of basalt cover, and aid our understanding of facies architecture in flood basalts.